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Varietal influence on antioxidant properties and glycemic index of pigmented and non-pigmented rice
Accepted Manuscript Varietal influence on antioxidant properties and glycemic index of pigmented and nonpigmented rice Meera K, Smita M, Sundaramoorthy Haripriya, Soumya Sen PII:
S0733-5210(18)30825-7
DOI:
https://doi.org/10.1016/j.jcs.2019.03.005
Reference:
YJCRS 2730
To appear in:
Journal of Cereal Science
Received Date: 25 October 2018 Revised Date:
15 February 2019
Accepted Date: 5 March 2019
Please cite this article as: K, M., M, S., Haripriya, S., Sen, S., Varietal influence on antioxidant properties and glycemic index of pigmented and non-pigmented rice, Journal of Cereal Science (2019), doi: https:// doi.org/10.1016/j.jcs.2019.03.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Varietal influence on antioxidant properties and glycemic index of pigmented and non-
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pigmented rice
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Meera. K1, Smita M1, Sundaramoorthy Haripriya1*, Soumya Sen1
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Department of Food Science and Technology, Pondicherry Central University, Pondicherry 605014 India
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*Corresponding author:
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Dr. Sundaramoorthy Haripriya, Department of Food Science and Technology, Pondicherry
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Central University, Pondicherry- 605014, India
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Ph: +91 9443701906
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Fax: +91 413 2654621
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E-mail: [email protected]
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Abstract
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Diabetes is a chronic metabolic illness characterized by hyperglycemia, mainly affected by staple
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diet pattern as consumption of rice which makes population more vulnerable to this condition.
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On the contrary, some traditional pigmented rice varieties are proven to be beneficial in
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managing different metabolic disorders as diabetes, aging etc. Nutrient composition and
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glycemic potential of pigmented rice cultivars namely kattuyanam, red kavuni, black kavuni and
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karudan samba were studied and found to have immense nutrient potentials. Kattuyanam had
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comparatively higher content of fibre, protein, amylose, total phenol, total flavonoid, DPPH and
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reducing power followed by red and black kavuni. Amylose content (27.28-30.03%) correlated
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inversely with the glycemic index (r = -0.713; p ≤ 0.01) and glycemic load (r = -0.574).
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Kattuyanam, an indigenous pigmented rice variety, with 30.03% amylose and 5.35% fibre was
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eliciting low GI of 47.19 and glycemic load of 8.80 whereas karudan samba recorded high GI of
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69.74 and glycemic load of 13.84. Hence, kattuyanam with high amylose and low GI can serve
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as a source of functional food and could play a vital role in the management and prevention of
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diabetes and related disorders.
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Keywords: Glycemic index; Glycemic load; Kattuyanam; Antioxidant; Amylose.
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1. Introduction Diabetes mellitus, a chronic endocrine-metabolic disorder, indicated with high blood
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glucose level with multiple etiologies. In addition to genetic predisposition and lifestyle pattern
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contributing to a higher percentage in diabetes, it is widely known that the diet that is consumed
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also substantially attributes to this condition as it is well proven that metabolism is greatly
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affected by food intake. Across the globe, the trend of increasing diabetes mellitus is an alarming
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condition. The report of International Diabetes Federation (2017), records that 72.9 million
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people were affected with diabetes in 2017, and the numbers are estimated to increase up to
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134.3 million by the year 2045 in India. Diabetic population in urban areas have risen to 472.6
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million in 2015 owing primarily to global urbanization. The vulnerability of rice (Oryza sativa.
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L) consuming population of the world, for developing diabetes (type II) has been on the steady
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increase. Albeit, among the cereal, rice stands second after wheat in consumption pattern
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throughout the world. Rice is one of the world’s leading food crops, which supplies directly
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more than 42% of calories to the human population and India is ranked as the second largest rice
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producer in the world. The rise of diabetes (type II) among rice consumers in the Asian countries
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has increased manifold. Comparatively, rice has a high glycemic index (GI) among high starch
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sources. Regular consumption of rice with high GI is increasingly associated with elevated risk
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of type II diabetes, obesity, coronary heart disease and other chronic conditions. Hence,
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adherence to low GI food or reduced intake of high GI food has also been unveiled as a leading
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mitigation strategy to curb the blood glucose level inflation in people with type II diabetes and to
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those for weight management. It is in this context the function of amylose present in rice merits a
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detailed investigation.
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Amylose in rice plays a crucial role in controlling the rate of digestion of starch in the
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gastrointestinal system. Further, amylose is used as a critical indicator to predict the rate of
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starch digestion, insulin response in rice and blood glucose. Myriad studies have established that
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starches with high amylose are of nutritional interest as they contribute to slower emptying time
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which is associated with beneficial physiological effects (Prasad, Hymavathi, Babu, & Longvah,
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2018). Rice with high amylose content exhibited lower glycemic index values than low amylose
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cultivars (Kumar et al., 2018a). Many researchers have reported that GI for rice ranged from 54
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and 121 (Foster-Powell, Holt, & Brand-Miller, 2002; Kumar et al., 2018b; Panlasigui et al.,
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1991). Thus, it can be construed that, the foods containing a low GI and high antioxidant content
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have immense potential in reducing the risk of diseases related to cardiovascular disease,
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impaired glucose metabolism, metabolic syndrome and so on (Kumar et al., 2018b). In quest of
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such foods with a low GI and high antioxidant properties, certain native pigmented rice varieties
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were arbitrarily investigated and results were surprisingly found positive in terms of nutrition
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potentials compares to commonly consumed rice.
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In Southern India, though some rice cultivars have been cultivated traditionally, research
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on the glycemic index and bioactive phytochemicals, especially on these varieties, have not been
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conducted. Given the circumstances of the alarming rise of diabetes with increased intake of rice
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consumption, this research makes an earnest attempt for rice breeders and consumers to study the
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glycemic and antioxidant potentials of different pigmented rice cultivars of South India which
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are inclined to provide immense health benefits to the human population in general and the
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people with diabetes in particular. The study aims to pave the ground that certain rice varieties
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could mitigate diabetes mellitus and hence a blanket conception casting aspersion on the very
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consumption of rice needs a relook.
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2. Materials and methods
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2.1. Rice varieties and sample preparation Rice samples were procured from rice farmers from the January 2016 harvest in
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Tamilnadu, India. Four indigenous rice cultivars namely kattuyanam (brown), red kavuni (red),
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black kavuni (black) and karudan samba (white, unpolished) were used in the study. All four
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analyzed cultivars are indigenous upland rice cultivars grown in subsistence oriented farming
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systems. The paddy samples were collected directly from the field and dried at 40°C to acquire a
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moisture content of about 10%. Stored paddy samples were dehusked using a laboratory de-
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husker (Model THU-34A, Satake Engineering Co., Japan) to obtain brown rice. The rice grains
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were milled into flour using a laboratory mill (A11B, IKA Inc, India). Then the flour samples
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were sieved through 100 µm sieve-size and stored in an airtight container for further biochemical
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analysis.
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2.2. Nutrient composition of rice varieties
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The moisture (934.01), ash (942.05) and crude fat (963.15) contents of rice samples were
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determined according to Official methods of analysis (AOAC, 1984). Protein content (954.01)
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was estimated by micro kjeldhal method, multiplied with a conversion factor of 6.25 (AOAC,
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1984). Available carbohydrate of each rice variety was determined by difference method (FAO
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& WHO, 1998). Crude fibre content (FIWE Fiber Analyzers, VELP Scientifica, Shanghai) was
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determined by the method of AOAC (1984). Amylose content of rice was determined using the
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standard iodine calorimetric method by calculating from the standard curve for standard amylose
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solution at 20–100 g/ml from potato starch (Reddy, Luan, & Xu, 2017).
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2.3. Preparation of cooked rice
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The rice samples were cleaned manually to remove dirt, stones, weed seeds, foreign
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particles etc., washed with tap water and cooked in pressure cooker in the ratio of 1:8 (rice:
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water), respectively. The rice samples were cooked for around 40 mins until the water was fully
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absorbed into rice grains (no solid loss). Longer cooking time and more water was required for
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these rice cultivars in order to attain rice consistency that made the food more palatable and
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acceptable for consumption. Then the cooked rice samples were left for about 45 mins before
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commencement of the study.
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2.4. Determination of in-vivo glycemic response of rice varieties
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Glycemic response measurement protocol was approved by human ethical committee
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(Approval No. HECPU/2018/01/190218) of Pondicherry University, Puducherry, India. Twelve
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healthy individuals aged between 20-31 years with a normal blood glucose tolerance (fasting
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blood glucose level of <7mmol/L) and normal body mass index (BMI) were selected for the
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study after anthropometric screening among 20 healthy volunteers. Subjects were asked to report
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at 7.30 am in the laboratory after an overnight fast of 12-14 hrs. Fasting blood glucose level was
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measured by using a standardized glucometer (Sugarchek Advance, Wockhardt). Subjects were
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asked to consume a cooked rice portion containing 50g of available carbohydrate with 250ml of
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water within 10-12 mins. Then, finger prick blood glucose test was obtained at 15, 30, 45, 60, 75,
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90, 105 and 120 mins after consumption of the test food. For standard, a 50g of glucose in 250ml
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of water was given and blood glucose responses were taken at similar intervals. The glycemic
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index (GI) of rice varieties for each subject was calculated using the following formula according
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to FAO and WHO (1998).
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The glycemic index of each rice variety was calculated on the basis of the average of 12
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glycemic index values of the 12 participated subjects, who consumed the rice samples. Subjects
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were asked to consume each rice variety for 3 days and minimum 3 days gap was given for each
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rice variety during which standardization of glucose response was measured. Meanwhile,
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glycemic load of each rice variety was also calculated by multiplying its glycemic index to its net
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available carbohydrate content in a serving size (g), and then divided by 100.
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2.5. Antioxidant activity
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2.5.1. Extraction of rice flour
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Antioxidant activity of the rice flour extract was prepared according to the method
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described by Mir, Bosco, Shah, & Mir (2016) with little modifications. One gram of each rice
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flour was weighed accurately and extracted for 4 hours with 25 ml HPLC grade methanol
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(Himedia) in the dark at ambient temperature (28 ± 1° C) using a magnetic stirrer (IKA C-MAG
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HS 4 Stirrer, India). The mixture was centrifuged at 3000 × g for 15 mins and the supernatant
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extract was stored in sealed polypropylene tubes at 4° C until further analysis.
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2.5.2. Total phenolic content
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Total phenolic content of samples was determined using the Folin-Ciocalteau
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spectrophotometric method (Reddy, Kimi, & Haripriya, 2016).
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2.5.3. Total flavonoid content
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Total flavonoid content of the pigmented rice varieties was determined by the method designed by Reddy et al. (2016). 7
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2.5.4. DPPH radical scavenging activity assay 2, 2-Diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity of pigmented rice was determined using the method described by Reddy et al. (2016). %&&"
2.5.5. Reducing power assay
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The method of Yen & Duh (1993) was followed for determining the reducing power of
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rice flour extract and the absorbance was measured at 700nm using a UV-Vis Spectrophotometer
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(Shimadzu, UV-1800, Japan).
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2.6. Statistical analysis
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Values of nutrient compositions were expressed as mean ± standard deviation and the
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results of GI as mean ± standard error by using SPSS 22 software (SPSS Institute Inc., Cary,
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USA). Single factor ANOVA was employed to compute the significance of differences between
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mean values at p<0.05 by Duncan multiple range test (DMRT). Pearson correlation coefficient
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(r) studies were carried out to find the interrelationship between the nutrient composition,
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antioxidant properties and the corresponding glycemic index and glycemic loads of indigenous
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pigmented rice cultivars using bivariate significance at p < 0.01 and p < 0.05 levels. All
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observations were means of triplicates.
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3. Results and discussion
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3.1. Nutritional composition of rice cultivars
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The results of the nutrient composition of indigenous pigmented rice cultivars differed
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significantly and are presented in Table 1. The moisture content, crude protein, crude fibre, 8
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available carbohydrate, fat and ash were significantly different (p<0.05) for all the rice cultivars.
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These results are consistent with Kraithong, Lee, & Rawdkuen (2018) who reported the results of
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crude protein (6.51-7.27%), moisture (5.47-9.87), fat (1.13-3.60%), ash (0.47-1.57%) and
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carbohydrate (77.06-85.58%) in four rice cultivars (Riceberry, Phitsanulok, Brown jasmine and
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Red jasmine) of Thailand. The moisture content of kattuyanam rice exhibited the highest i.e.
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11.93 and that of red (10.90%) and black kavuni (9.00%) rice were minimal among the cultivars
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analyzed. The moisture content of rice cultivars was lower than that of the recommended (<20%)
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and also within the admissible range (<15%) for food product development (Reddy, Luan, & Xu,
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2017). The variation in the moisture content could be due to the varied initial moisture content of
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the rice grains and processing conditions. The ash and fat content of black kavuni rice were
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found to be highest than other rice cultivars studied. Ash content was highest in black kavuni
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(1.75%) and lowest in karudan samba (1.28%), which the fact implies that black kavuni can be
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utilized as a better source of essential mineral nutrition. The crude protein value ranged from a
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minimal of 10.22% in karudan samba to a maximal of 14.01% in red kavuni. In this study, the
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crude protein content was substantially higher >10% in all tested indigenous pigmented rice
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cultivars whilst the mean crude protein content of rice reported by other researchers were in the
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range of 7-12% (Sompong et al., 2011; Panlasigui et al., 1991). Protein content was found
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negatively correlated to the available carbohydrate among the rice cultivars (r = -0.838; p ≤
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0.01). Various factors such as irrigation, genotype, edaphic, genetic variation and agro-climatic
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factors modify the protein content of rice. The carbohydrate content of rice cultivars were in the
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range of 72.98-75.41%. Crude fibre which measures the amount of indigestible sugars present in
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the food was highest in kattuyanam rice (5.35%). The value of crude fibre content in rice
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cultivars studied is of the order kattuyanam > red kavuni > black kavuni > karudan samba. In this
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study, higher amount of crude protein, ash, crude fibre and crude fat were reported than the
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results of the pigmented rice varieties studied so far which could be owed to the presence of
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greater outer layer (Kraithong et al., 2018). From Table 2. amylose content showed significant
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difference (p<0.05) between the cultivars and ranged from a maximum of 30.03% in kattuyanam
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to a minimum of 27.28% in red kavuni. All the four cultivars used in this study belonged to high
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amylose content (>25%) and showed a positive correlation with the crude fibre (r = 0.855; p ≤
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0.01). Prasad, Hymavathi, Babu, & Longvah (2018) reported a similar positive association
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between crude fibre and amylose content. High amylose rice generally offers crispness and
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firmness to food products owing to a 3-dimensional network formation. The pigmented rice
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varieties studied could be a potential functional ingredient in preparation of food products that
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requires a hard texture including noodles, snacks, and other extruded products.
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3.2. Total phenolic content
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Table 3 presents the total phenolic content of four indigenous pigmented rice cultivars
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and the results were expressed as mg gallic acid equivalent per g. The total phenolic content of
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the rice cultivars were in the range of 1.91-5.99 mg GAC eq/g. Karudan samba exhibited the
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lowest total phenolic content of 1.91±0.03mg GAC eq/g while kattuyanam rice recorded the
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highest total phenolic content of 5.99±0.11mg GAC eq/g. Boue et al. (2016) have reported
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polyphenolic compounds like phenolic acids, anthocyanins, flavonoids were substantially present
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in higher amounts in black and red pigmented rice varieties than non- pigmented rice. Phenolic
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compound plays a vital role in combating chronic diseases such as cardiovascular diseases and
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type II diabetes. Phenols are the major contributors in determining the antioxidant potential of
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cereal grains. The total phenolic content of four rice cultivars were found to be significantly
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higher than that of pigmented rice varieties of Taiwan (0.24-1.78g GAE/kg) and China (0.246-
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0.563 mg GAE/g) as reported by Huang and Ng (2012) and Pang et al. (2018). Many researchers
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have reported that phenolic content of black rice was higher than that of red rice (Boue et al.,
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2016; Somaratne et al., 2017). However, in our study red kavuni presented higher phenolic
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content (5.89 mg GAC eq/g) when compared with black kavuni rice (3.33 mg GAC eq/g).
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Cultivation techniques, growing conditions, genotype, degree of maturity, extraction, storage and
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ripening process could impact phenolic constituents of rice.
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3.3. Total flavonoid content
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The highest flavonoid content of rice cultivar was observed for red kavuni (84.40 mg
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catechin eq/g), followed by kattuyanam (71.10 mg catechin eq/g) > black kavuni (44.08 mg
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catechin eq/g) > karudan samba (42.33 mg catechin eq/g) (Table 3). Flavonoids are the most
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treasured phytochemicals of the secondary metabolites that impart its role in regulating various
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biological functions comprising anti-inflammatory, antimicrobial, antitumor, antioxidant and
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anti-allergic properties (Bhat & Riar, 2017). The total flavonoid content of pigmented rice used
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in this study were higher than values of other pigmented rice varieties reported earlier (Mir et al,
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2016; Pengkumsri et al, 2015; Reddy et al, 2016). Shen et al. (2009) stated that black rice
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possesses higher flavonoid content than brown and red rice cultivars which in turn contradicts
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the statement of this study owing to the presence of higher flavonoid content in red kavuni rice
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cultivar. Red and black pigments present in the rice cultivars were due to the deposition of
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bioactive compounds in the aleuronic layer of the rice grain, thus leading to the coloration of rice
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and owes to the quantity of phenols and flavonoids content of rice grains. On the other hand, rice
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should be consumed as whole grains enriched with abundant phytochemical compounds to
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ensure the maximum intake of bioactive compounds (Bhat & Riar, 2017). Hence, it is
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noteworthy that pigmented rice varieties are a prodigious source of functional foods.
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3.4. DPPH radical scavenging activity The capacity of indigenous pigmented rice cultivars to scavenge free radicals was
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evaluated by using DPPH (1,1-diphenyl-2-picryl hydrazyl) and it is considered as a critical
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indicator for in-vitro antioxidant activity. DPPH is a stable radical which transform its distinctive
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purple color to pale yellow by electron transfer into its non-radical (DPPH-H) form. Decrease in
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absorbance at 517nm defines the reaction between DPPH and phytochemicals and also its
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reducing ability of antioxidants in the pigmented whole grains (Bhat & Riar, 2017). The DPPH
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scavenging activity was greater than 90% for brown, red and black cultivars which varied from
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82.56 to 99.52% with karudan samba presenting lesser activity and the red kavuni and
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kattuyanam depicting higher DPPH radical scavenging activity as displayed in Table 3. Min et
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al. (2012) stated that increased DPPH activity in black and red colored rice cultivars could be
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due to the presence of increased anthocyanin and proanthocyanidins content in pigmented rice
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varieties. These values are found to be extremely higher than values reported by other
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researchers (Mir et al., 2016; Reddy et al., 2016; Bhat & Riar, 2017). The variations in the
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scavenging activity of rice cultivars may be associated to the concentration of phytochemical
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compounds such as phenols and flavonoids present in them. Phytochemical compounds have
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immense antioxidant potential and its impact on consumer’s nutrition and health are apposite
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(Mir et al., 2016). Saikia et al. (2012) reported DPPH activity of 94.19% for Chakhao Poreiton
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pigmented variety. In this study, all the four rice cultivars exhibited a positive correlation (r =
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0.910; p ≤ 0.01) between DPPH scavenging activity and total phenolic content as illustrated in
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Table 4. Mir et al (2016) also reported a strong positive correlation between DPPH and phenolic
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content of rice cultivars of Kashmir.
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3.5. Reducing power
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The reducing power of indigenous pigmented rice cultivars varied significantly (p < 0.05)
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from 0.83 to 6.38 mg AAE equivalent/g and the results were expressed as mg ascorbic acid
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equivalent per g (Table 3). Reducing power of phytochemical compounds depicts the capacity of
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natural antioxidant to donate electron so as to minimize the oxidized entities produced through
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various biological pathways. These oxidized entities i.e. free radicals are known to implicate
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various adverse effects on humans. The reducing power of kattuyanam (6.38 mg AAE/g) was
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found to be higher followed by red kavuni (5.78 mg AAE/g). The lowest reducing power was
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observed in karudan samba (0.83 mg AAE equivalent/g) which substantiate its decreased total
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phenolic content and DPPH radical scavenging activity. The reducing power of pigmented rice
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used in this study were higher than values of Thai pigmented rice varieties reported by
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Jiamyangyuen, Nuengchamnong, & Ngamdee (2017). The reducing power obtained in the
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present study followed the pattern of Bhat & Riar (2017) in which the colored rice cultivars
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(brown, red, black) were found to have higher reducing power than karudan samba (white rice).
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Food compounds with reducing power behave as primary or secondary antioxidants leading to
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the deactivation of lipid peroxidation process and radical chain reactions. High reducing power
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activity is an exhibitive of high phenolic content in the food ingredients. The higher absorbance
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of indigenous pigmented rice varieties signals their intense reducing and antioxidant potential
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(Bashir, Usmani, Haripriya, & Ahmed, 2017).
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3.6. Glycemic response of rice cultivars
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The mean weight, height, body mass index and age of subjects were 58.69±4.50 kg,
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1.60±0.06 m, 22.96±1.89 kg/m2 and 23±0.61 years old, respectively. The glycemic response
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curves of the four indigenous pigmented rice cultivars are presented in Fig.1 and the average
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(FBG) fasting blood glucose level of the subjects was 84.53±1.75 mg/dl. Glycemic index (GI)
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measures the ability of carbohydrate foods based on its postprandial glycemic effects. GI of food
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was categorized as high (≥ 70%), medium (56-69%) and low (≤ 55%) (FAO and WHO, 1998).
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The elicited glycemic index (GI) values ranged from 47.19 by kattuyanam to a maximum of
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69.74 by karudan samba (Table 2). According to FAO and WHO (1998) GI classification,
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kattuyanam was graded as low GI rice (47.19), while red and black kavuni was graded as
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medium GI rice (61.69 and 56.27) and karudan samba as high GI rice (69.74). Red and black
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kavuni with short bold and medium rice grains elicited medium GI values of 61.69±4.0 and
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56.27±4.3 respectively. Somaratne et al. (2017) reported the GI effect of 4 pigmented and 1 non-
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pigmented basmati rice varieties which ranged between 48-68%. Prasad et al. (2018) observed
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that GI of eleven rice varieties were 50.40±5.27 to 79.68±3.25 and long and slender rice grains
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exhibited lower glycemic response. Though, the amylose content was higher in all the four
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varieties, changes in starch digestibility and physicochemical properties may exhibit the
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difference in the GI values.
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In the current study, crude fibre displayed an inverse association between the GI (r = -
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0.904; p ≤ 0.01) and GL (r = -0.857; p ≤ 0.01) of indigenous pigmented rice cultivars. Crude
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fibre is a type of dietary fibre that retards starch digestibility and in turn alters the food glycemic
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response. Glycemic load (GL) is a strong indicator which predicts the response of insulin and
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postprandial glucose level than carbohydrate content and in fact glycemic load surpasses the GI
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for both composite meals and single foods (Rozendaal et al., 2018). The glycemic load (GL) of
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rice cultivars ranged from 8.80 (kattuyanam) to a maximum of 13.84 (karudan samba). Decrease
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in rate and quantum of starch digestion may contribute to lower GL in kattuyanam rice whereas
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greater extent of starch digestibility indicates higher GL in karudan samba rice. Glycemic load
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showed a strong positive correlation with glycemic index (r = 0.979; p ≤ 0.01).
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3.7. Pearson’s correlation coefficients Correlation analysis between glycemic index and antioxidant properties of indigenous
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pigmented rice cultivars were presented at p ≤ 0.05, and p ≤ 0.01 levels (Table 4). Amylose
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content was negatively correlated to glycemic index (r = -0.713; p ≤ 0.01) and glycemic load (r =
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-0.574) of indigenous pigmented rice cultivars. Glycemic load showed a strong positive
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correlation with glycemic index (r = 0.979; p ≤ 0.01). Fitzgerald et al. (2011) and Prasad et al.
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(2018) also noticed similar inverse correlation between predicted GI and amylose content in
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different rice cultivars. The difference in GI is not correlated with the percentage of amylose
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available. The physicochemical properties and starch digestibility could be the key factors in
318
determining the GI of rice. In addition to this, the resistance to digestion by the interaction
319
between amylose to protein or lipid in the formation of amylose-protein or amylose-lipid
320
complexes could also help to determine the GI of rice. Moghaddam, Vogt, & Wolever (2006)
321
have reported that elevated levels of crude fat and crude protein in the pigmented rice diminishes
322
glycemic responses via glucagon-like peptide (GLP-1) mediated effects on gastric emptying and
323
amino acid mediated effects on insulin secretion. Variations in the glycemic index of rice
324
genotypes depends on various factors including the amount of fibre, protein, degree of
325
amylopectin branch and amylose molecules present in rice kernel.
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Karudan samba (white rice) which exhibits high volume expansion during cooking
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showed high glycemic index (GI) value. Hence, karudan samba with high GI value, leads to
328
rapid rise in blood glucose level due to increased susceptibility of hydrolytic enzymes involved
329
in starch degradation and consequently the presence of high surface area of cooked rice grains
330
(Kumar et al., 2018b). The low GI of kattuyanam rice shows low rate of digestibility which
331
could be attributed to the composition and type of starch, its structure and above all the varietal
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difference exhibited. Studies have reported that lipids, ash and protein also have negative
333
correlation of GI of rice as the absorption of glucose from rice is reduced with increased lipid
334
and mineral content Somaratne et al (2017). The correlation between dietary fibre and GI of rice
335
are in consensus with Yusof, Talib, & Karim (2005) which also presents that dietary fibre is
336
independent to the GI of rice. A diet high in cereal fibre significantly lowers the glycemic index
337
and blood glucose response and consequently increasing polyphenolic content and antioxidant
338
capacity of food (Bhat & Riar, 2017). The correlations found in this research are in agreement
339
with the above findings. Glycemic index displayed negative correlation with total phenol (r = -
340
0.669*), total flavonoid (r = -0.362), DPPH (r = -0.760**) and reducing power (r = -0.740**). A
341
strong positive correlation was found between total phenolic content and reducing power (r =
342
0.993**). Several studies have reported that increased total phenol and flavonoid content in the
343
pigmented rice bran layer might affect the rate of starch digestion in rice by hindering the
344
activities of enzymes such as α-amylase, α-glucosidase and β-glucosidase (Donkor et al., 2012;
345
Somaratne et al., 2017). The bioactive compound of pigmented rice has proved to have aldose
346
reductase inhibitory activities which is proved beneficial in preventing diabetic complications.
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Thus, pigmented rice should be consumed along with its bran layer (Bhat & Riar, 2017).
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Conclusion
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Nutritional composition and biological properties of indigenous pigmented rice cultivars
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along with its contributing glycemic prospective have been assessed. Contrary to popular belief
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that rice consumption attributes to diabetes (type II), this study, established that native pigmented
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rice crops display a differential plethora of nutritive potentials packed in it. Kattuyanam with
353
lower GI and comparatively higher levels of protein, fibre and amylose appears to be a better
354
aspect for rice based dietetic management and provides beneficial physiological effects for the
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prevention of diabetes. Red and black kavuni which belongs to medium GI variety together with
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a nutritionally balanced diet can still bring relief in consumption of rice by diabetics and also
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consequently would serve as a goal for people wishing to reduce or maintain weight. These
358
polyphenols have correlated with increased antioxidant activity and reducing power activity in
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both kattuyanam and red kavuni varieties. Though all the pigmented rice varieties exhibited high
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amylose, the glycemic index was not reported to be low. This very finding brings in a negative
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correlation between amylose and GI. To substantiate the statement, high amylose red and black
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kavuni represented medium GI where, kattuyanam which also had high amylose reported low
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GI. Interestingly amylose had shown positive correlation with crude fibre and crude fibre had
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shown negative correlation with GI. Rice varieties of high amylose content, total phenol, total
365
flavonoid can serve as a source of functional food and variety like kattuyanam can serve as low
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GI. This information can benefit consumers, producers and food formulators where rice based
367
formulations can still be effective in addressing diabetic population where rice is considered as a
368
threat to increase blood glucose. The nutritional composition, biological properties and glycemic
369
potential of indigenous pigmented rice cultivars would serve as a better ingredient for its usage
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in the food industry. The current research would also enable useful information regarding rice
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based strategies and holistic approach to prevention and management of diabetes in India.
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Conflict of interest
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The authors have declared no conflict of interest.
Ethical approval
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This research work was carried out with the help of human participants. Thanks to the
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Institute Ethics Committee (Human studies), Pondicherry University, Puducherry for ethical
377
clearance (Approval No. HECPU/2018/01/190218).
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Acknowledgements The first author is thankful to the University Grants Commission, New Delhi, India for
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granting financial assistance in the form of Junior Research Fellowship (Grant No. 1538/NET-
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JUNE 2013). Our sincere gratitude to the volunteers who participated in the study.
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Yen, G.-C., Duh, P.-D., 1993. Antioxidative properties of methanolic extracts from peanut hulls. J. Am. Oil Chem.’ Soc. 70, 383-386. Yusof, B.N.M., Talib, R.A., Karim, N.A., 2005. Glycaemic index of eight types of commercial
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rice in Malaysia. Age (years) 25, 20-31.
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Table 1
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Proximate composition of indigenous pigmented rice cultivars (g/100g dry weight) Hue
Appearance Crude Protein (%)
483 484 485 486 487
Ash (%)
Moisture (%)
12.11±0.25
c
5.35±0.32
a
72.98±1.56
c
1.69±0.13
c
1.29±0.03
c
11.93±0.04
a
Red kavuni
Red
12.98±0.25
b
3.77±0.03
b
73.10±1.12
b
1.51±0.06
d
1.51±0.14
b
10.90±0.06
b
Black kavuni
Black
14.01±0.44
a
3.65±0.02
b
73.19±1.23
a
2.05±0.08
a
1.75±0.01
a
9.00±0.02
Karudan samba
White
10.22±0.25
d
2.95±0.07
a
1.74±0.12
b
1.28±0.09
c
11.35±0.23
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Brown
c
75.41±1.78
c a
Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
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(%)
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(%)
Fat (%)
Kattuyanam
479 480
Carbohydrate
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Crude fibre
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488 489 23
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490
Table 2
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Amylose content and glycemic index of indigenous pigmented rice cultivars Amylose (%)
Mean ± SD
496 497 498 499 500
Classification
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30.03±0.09
a
High
47.19±3.2
Low
8.80
Red kavuni
Red
343.78
27.28±0.26
c
High
61.69±4.0
Medium
11.22
Black kavuni
Black
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27.30±0.08
c
High
56.27±4.3
Medium
10.19
Karudan samba
White
315.10
27.71±0.04
b
High
69.74±4.5
High
13.84
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SE = Standard error; SD = Standard deviation. Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
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Mean ± SE
Glycemic load
Kattuyanam
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Classification
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Glycemic index (GI)
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Serving size, g equivalent to 50g of available carbohydrate
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Table 3
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Antioxidant properties of indigenous pigmented rice cultivars Total flavonoid
DPPH
Reducing power
(mg GAC eq/g)
(mg catechin eq/g)
(%)
(mg AAE/g)
Brown
5.99±0.11
a
71.10±2.36
b
99.45±0.14
a
6.38±0.15
a
Red kavuni
Red
5.89±0.19
a
84.40±0.95
a
99.52±0.00
a
5.78±0.11
b
Black kavuni
Black
3.33±0.02
b
44.08±0.38
c
95.48±0.24
b
2.95±0.07
c
Karudan samba
White
1.91±0.03
c
42.33±0.38
c
82.56±0.19
c
0.83±0.03
d
Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
507
512 513 514 515
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Kattuyanam
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Total phenol
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Rice cultivars
516 517
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Table 4
519 520
Pearson’s correlation coefficients between proximate composition, glycemic index and antioxidant properties of indigenous pigmented rice cultivars
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Parameter GI GL AML TPC TFC DPPH RP CP CF CHO F A M GI 1 GL 0.979** 1 ** AML -0.713 -0.574 1 TPC -0.669* -0.724** 0.457 1 TFC -0.362 -0.429 0.256 0.924** 1 ** ** DPPH -0.760 -0.859 0.293 0.910** 0.748** 1 ** ** ** ** RP -0.740 -0.786 0.510 0.993 0.888 0.925** 1 * Crude -0.463 -0.621 -0.225 0.418 0.239 0.741** 0.437 1 protein Crude -0.904** -0.857** 0.855** 0.773** 0.526 0.717** 0.815** 0.249 1 fibre CHO 0.749** 0.853** -0.251 -0.800** -0.602* -0.953** -0.822** -0.838** -0.689* 1 ** Fat -0.124 -0.120 -0.149 -0.525 -0.744 -0.190 -0.454 0.292 -0.132 0.047 1 Ash -0.084 -0.237 -0.567 -0.033 -0.146 0.327 -0.021 0.789** -0.216 -0.404 0.456 1 Moisture -0.075 0.058 0.659* 0.326 0.431 -0.074 0.306 -0.692* 0.396 0.197 -0.692* -0.901** 1 521 GI = Glycemic index; GL = Glycemic load; AML = Amylose (%); TPC = Total phenolic content (mg GAC eq/g of rice); TFC = Total 522 flavonoid content (mg catechin eq/g of rice); DPPH (%); RP = Reducing power (mg ascorbic acid eq/g of rice); CP = Crude protein 523 (%); CF = Crude fibre (%); CHO = Carbohydrate (%); F = Fat (%); A = Ash (%); M = Moisture (%). ** 524 Correlation is significant at p ≤ 0.01 level (2-tailed). * 525 Correlation is significant at p ≤ 0.05 level (2-tailed).
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Figure Captions
528
Fig.1. Mean blood glucose concentration (mg/dl) of indigenous pigmented rice cultivars
529
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530 531 532
534
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533
Fig.1.
Glucose
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Kattuyanam
Red kavuni
150
110
50
EP
90 70
0
536
Karudan samba
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130
AC C
Blood glucose concentration (mg/dl)
170
Black kavuni
15
30
45
60
Time (mins)
537 538
27
75
90
105
120
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Highlights Kattuyanam with 30.03% amylose and 9.33% TDF elicited low GI of 47.19 Karudan samba with 27.71% amylose and 7.87% TDF recorded high GI of 69.74
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Amylose correlated inversely with the glycemic index and glycemic load
DPPH scavenging activity of rice cultivars was in the range of 82.56 to 99.52%
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Glycemic index and antioxidant activity are inversely proportional
S0733-5210(18)30825-7
DOI:
https://doi.org/10.1016/j.jcs.2019.03.005
Reference:
YJCRS 2730
To appear in:
Journal of Cereal Science
Received Date: 25 October 2018 Revised Date:
15 February 2019
Accepted Date: 5 March 2019
Please cite this article as: K, M., M, S., Haripriya, S., Sen, S., Varietal influence on antioxidant properties and glycemic index of pigmented and non-pigmented rice, Journal of Cereal Science (2019), doi: https:// doi.org/10.1016/j.jcs.2019.03.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Varietal influence on antioxidant properties and glycemic index of pigmented and non-
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pigmented rice
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Meera. K1, Smita M1, Sundaramoorthy Haripriya1*, Soumya Sen1
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Department of Food Science and Technology, Pondicherry Central University, Pondicherry 605014 India
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*Corresponding author:
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Dr. Sundaramoorthy Haripriya, Department of Food Science and Technology, Pondicherry
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Central University, Pondicherry- 605014, India
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Ph: +91 9443701906
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Fax: +91 413 2654621
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E-mail: [email protected]
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Abstract
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Diabetes is a chronic metabolic illness characterized by hyperglycemia, mainly affected by staple
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diet pattern as consumption of rice which makes population more vulnerable to this condition.
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On the contrary, some traditional pigmented rice varieties are proven to be beneficial in
24
managing different metabolic disorders as diabetes, aging etc. Nutrient composition and
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glycemic potential of pigmented rice cultivars namely kattuyanam, red kavuni, black kavuni and
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karudan samba were studied and found to have immense nutrient potentials. Kattuyanam had
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comparatively higher content of fibre, protein, amylose, total phenol, total flavonoid, DPPH and
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reducing power followed by red and black kavuni. Amylose content (27.28-30.03%) correlated
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inversely with the glycemic index (r = -0.713; p ≤ 0.01) and glycemic load (r = -0.574).
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Kattuyanam, an indigenous pigmented rice variety, with 30.03% amylose and 5.35% fibre was
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eliciting low GI of 47.19 and glycemic load of 8.80 whereas karudan samba recorded high GI of
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69.74 and glycemic load of 13.84. Hence, kattuyanam with high amylose and low GI can serve
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as a source of functional food and could play a vital role in the management and prevention of
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diabetes and related disorders.
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Keywords: Glycemic index; Glycemic load; Kattuyanam; Antioxidant; Amylose.
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1. Introduction Diabetes mellitus, a chronic endocrine-metabolic disorder, indicated with high blood
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glucose level with multiple etiologies. In addition to genetic predisposition and lifestyle pattern
45
contributing to a higher percentage in diabetes, it is widely known that the diet that is consumed
46
also substantially attributes to this condition as it is well proven that metabolism is greatly
47
affected by food intake. Across the globe, the trend of increasing diabetes mellitus is an alarming
48
condition. The report of International Diabetes Federation (2017), records that 72.9 million
49
people were affected with diabetes in 2017, and the numbers are estimated to increase up to
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134.3 million by the year 2045 in India. Diabetic population in urban areas have risen to 472.6
51
million in 2015 owing primarily to global urbanization. The vulnerability of rice (Oryza sativa.
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L) consuming population of the world, for developing diabetes (type II) has been on the steady
53
increase. Albeit, among the cereal, rice stands second after wheat in consumption pattern
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throughout the world. Rice is one of the world’s leading food crops, which supplies directly
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more than 42% of calories to the human population and India is ranked as the second largest rice
56
producer in the world. The rise of diabetes (type II) among rice consumers in the Asian countries
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has increased manifold. Comparatively, rice has a high glycemic index (GI) among high starch
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sources. Regular consumption of rice with high GI is increasingly associated with elevated risk
59
of type II diabetes, obesity, coronary heart disease and other chronic conditions. Hence,
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adherence to low GI food or reduced intake of high GI food has also been unveiled as a leading
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mitigation strategy to curb the blood glucose level inflation in people with type II diabetes and to
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those for weight management. It is in this context the function of amylose present in rice merits a
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detailed investigation.
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Amylose in rice plays a crucial role in controlling the rate of digestion of starch in the
65
gastrointestinal system. Further, amylose is used as a critical indicator to predict the rate of
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starch digestion, insulin response in rice and blood glucose. Myriad studies have established that
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starches with high amylose are of nutritional interest as they contribute to slower emptying time
68
which is associated with beneficial physiological effects (Prasad, Hymavathi, Babu, & Longvah,
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2018). Rice with high amylose content exhibited lower glycemic index values than low amylose
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cultivars (Kumar et al., 2018a). Many researchers have reported that GI for rice ranged from 54
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and 121 (Foster-Powell, Holt, & Brand-Miller, 2002; Kumar et al., 2018b; Panlasigui et al.,
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1991). Thus, it can be construed that, the foods containing a low GI and high antioxidant content
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have immense potential in reducing the risk of diseases related to cardiovascular disease,
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impaired glucose metabolism, metabolic syndrome and so on (Kumar et al., 2018b). In quest of
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such foods with a low GI and high antioxidant properties, certain native pigmented rice varieties
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were arbitrarily investigated and results were surprisingly found positive in terms of nutrition
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potentials compares to commonly consumed rice.
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In Southern India, though some rice cultivars have been cultivated traditionally, research
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on the glycemic index and bioactive phytochemicals, especially on these varieties, have not been
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conducted. Given the circumstances of the alarming rise of diabetes with increased intake of rice
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consumption, this research makes an earnest attempt for rice breeders and consumers to study the
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glycemic and antioxidant potentials of different pigmented rice cultivars of South India which
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are inclined to provide immense health benefits to the human population in general and the
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people with diabetes in particular. The study aims to pave the ground that certain rice varieties
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could mitigate diabetes mellitus and hence a blanket conception casting aspersion on the very
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consumption of rice needs a relook.
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2. Materials and methods
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2.1. Rice varieties and sample preparation Rice samples were procured from rice farmers from the January 2016 harvest in
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Tamilnadu, India. Four indigenous rice cultivars namely kattuyanam (brown), red kavuni (red),
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black kavuni (black) and karudan samba (white, unpolished) were used in the study. All four
92
analyzed cultivars are indigenous upland rice cultivars grown in subsistence oriented farming
93
systems. The paddy samples were collected directly from the field and dried at 40°C to acquire a
94
moisture content of about 10%. Stored paddy samples were dehusked using a laboratory de-
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husker (Model THU-34A, Satake Engineering Co., Japan) to obtain brown rice. The rice grains
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were milled into flour using a laboratory mill (A11B, IKA Inc, India). Then the flour samples
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were sieved through 100 µm sieve-size and stored in an airtight container for further biochemical
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analysis.
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2.2. Nutrient composition of rice varieties
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The moisture (934.01), ash (942.05) and crude fat (963.15) contents of rice samples were
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determined according to Official methods of analysis (AOAC, 1984). Protein content (954.01)
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was estimated by micro kjeldhal method, multiplied with a conversion factor of 6.25 (AOAC,
103
1984). Available carbohydrate of each rice variety was determined by difference method (FAO
104
& WHO, 1998). Crude fibre content (FIWE Fiber Analyzers, VELP Scientifica, Shanghai) was
105
determined by the method of AOAC (1984). Amylose content of rice was determined using the
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standard iodine calorimetric method by calculating from the standard curve for standard amylose
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solution at 20–100 g/ml from potato starch (Reddy, Luan, & Xu, 2017).
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2.3. Preparation of cooked rice
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The rice samples were cleaned manually to remove dirt, stones, weed seeds, foreign
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particles etc., washed with tap water and cooked in pressure cooker in the ratio of 1:8 (rice:
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water), respectively. The rice samples were cooked for around 40 mins until the water was fully
112
absorbed into rice grains (no solid loss). Longer cooking time and more water was required for
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these rice cultivars in order to attain rice consistency that made the food more palatable and
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acceptable for consumption. Then the cooked rice samples were left for about 45 mins before
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commencement of the study.
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2.4. Determination of in-vivo glycemic response of rice varieties
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Glycemic response measurement protocol was approved by human ethical committee
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(Approval No. HECPU/2018/01/190218) of Pondicherry University, Puducherry, India. Twelve
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healthy individuals aged between 20-31 years with a normal blood glucose tolerance (fasting
120
blood glucose level of <7mmol/L) and normal body mass index (BMI) were selected for the
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study after anthropometric screening among 20 healthy volunteers. Subjects were asked to report
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at 7.30 am in the laboratory after an overnight fast of 12-14 hrs. Fasting blood glucose level was
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measured by using a standardized glucometer (Sugarchek Advance, Wockhardt). Subjects were
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asked to consume a cooked rice portion containing 50g of available carbohydrate with 250ml of
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water within 10-12 mins. Then, finger prick blood glucose test was obtained at 15, 30, 45, 60, 75,
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90, 105 and 120 mins after consumption of the test food. For standard, a 50g of glucose in 250ml
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of water was given and blood glucose responses were taken at similar intervals. The glycemic
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index (GI) of rice varieties for each subject was calculated using the following formula according
129
to FAO and WHO (1998).
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The glycemic index of each rice variety was calculated on the basis of the average of 12
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glycemic index values of the 12 participated subjects, who consumed the rice samples. Subjects
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were asked to consume each rice variety for 3 days and minimum 3 days gap was given for each
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rice variety during which standardization of glucose response was measured. Meanwhile,
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glycemic load of each rice variety was also calculated by multiplying its glycemic index to its net
135
available carbohydrate content in a serving size (g), and then divided by 100.
136 137
2.5. Antioxidant activity
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2.5.1. Extraction of rice flour
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Antioxidant activity of the rice flour extract was prepared according to the method
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described by Mir, Bosco, Shah, & Mir (2016) with little modifications. One gram of each rice
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flour was weighed accurately and extracted for 4 hours with 25 ml HPLC grade methanol
142
(Himedia) in the dark at ambient temperature (28 ± 1° C) using a magnetic stirrer (IKA C-MAG
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HS 4 Stirrer, India). The mixture was centrifuged at 3000 × g for 15 mins and the supernatant
144
extract was stored in sealed polypropylene tubes at 4° C until further analysis.
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2.5.2. Total phenolic content
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Total phenolic content of samples was determined using the Folin-Ciocalteau
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spectrophotometric method (Reddy, Kimi, & Haripriya, 2016).
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2.5.3. Total flavonoid content
149 150
Total flavonoid content of the pigmented rice varieties was determined by the method designed by Reddy et al. (2016). 7
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2.5.4. DPPH radical scavenging activity assay 2, 2-Diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity of pigmented rice was determined using the method described by Reddy et al. (2016). %&&"
2.5.5. Reducing power assay
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The method of Yen & Duh (1993) was followed for determining the reducing power of
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rice flour extract and the absorbance was measured at 700nm using a UV-Vis Spectrophotometer
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(Shimadzu, UV-1800, Japan).
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2.6. Statistical analysis
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Values of nutrient compositions were expressed as mean ± standard deviation and the
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results of GI as mean ± standard error by using SPSS 22 software (SPSS Institute Inc., Cary,
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USA). Single factor ANOVA was employed to compute the significance of differences between
162
mean values at p<0.05 by Duncan multiple range test (DMRT). Pearson correlation coefficient
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(r) studies were carried out to find the interrelationship between the nutrient composition,
164
antioxidant properties and the corresponding glycemic index and glycemic loads of indigenous
165
pigmented rice cultivars using bivariate significance at p < 0.01 and p < 0.05 levels. All
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observations were means of triplicates.
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3. Results and discussion
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3.1. Nutritional composition of rice cultivars
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The results of the nutrient composition of indigenous pigmented rice cultivars differed
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significantly and are presented in Table 1. The moisture content, crude protein, crude fibre, 8
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available carbohydrate, fat and ash were significantly different (p<0.05) for all the rice cultivars.
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These results are consistent with Kraithong, Lee, & Rawdkuen (2018) who reported the results of
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crude protein (6.51-7.27%), moisture (5.47-9.87), fat (1.13-3.60%), ash (0.47-1.57%) and
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carbohydrate (77.06-85.58%) in four rice cultivars (Riceberry, Phitsanulok, Brown jasmine and
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Red jasmine) of Thailand. The moisture content of kattuyanam rice exhibited the highest i.e.
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11.93 and that of red (10.90%) and black kavuni (9.00%) rice were minimal among the cultivars
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analyzed. The moisture content of rice cultivars was lower than that of the recommended (<20%)
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and also within the admissible range (<15%) for food product development (Reddy, Luan, & Xu,
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2017). The variation in the moisture content could be due to the varied initial moisture content of
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the rice grains and processing conditions. The ash and fat content of black kavuni rice were
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found to be highest than other rice cultivars studied. Ash content was highest in black kavuni
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(1.75%) and lowest in karudan samba (1.28%), which the fact implies that black kavuni can be
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utilized as a better source of essential mineral nutrition. The crude protein value ranged from a
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minimal of 10.22% in karudan samba to a maximal of 14.01% in red kavuni. In this study, the
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crude protein content was substantially higher >10% in all tested indigenous pigmented rice
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cultivars whilst the mean crude protein content of rice reported by other researchers were in the
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range of 7-12% (Sompong et al., 2011; Panlasigui et al., 1991). Protein content was found
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negatively correlated to the available carbohydrate among the rice cultivars (r = -0.838; p ≤
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0.01). Various factors such as irrigation, genotype, edaphic, genetic variation and agro-climatic
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factors modify the protein content of rice. The carbohydrate content of rice cultivars were in the
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range of 72.98-75.41%. Crude fibre which measures the amount of indigestible sugars present in
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the food was highest in kattuyanam rice (5.35%). The value of crude fibre content in rice
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cultivars studied is of the order kattuyanam > red kavuni > black kavuni > karudan samba. In this
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study, higher amount of crude protein, ash, crude fibre and crude fat were reported than the
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results of the pigmented rice varieties studied so far which could be owed to the presence of
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greater outer layer (Kraithong et al., 2018). From Table 2. amylose content showed significant
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difference (p<0.05) between the cultivars and ranged from a maximum of 30.03% in kattuyanam
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to a minimum of 27.28% in red kavuni. All the four cultivars used in this study belonged to high
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amylose content (>25%) and showed a positive correlation with the crude fibre (r = 0.855; p ≤
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0.01). Prasad, Hymavathi, Babu, & Longvah (2018) reported a similar positive association
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between crude fibre and amylose content. High amylose rice generally offers crispness and
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firmness to food products owing to a 3-dimensional network formation. The pigmented rice
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varieties studied could be a potential functional ingredient in preparation of food products that
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requires a hard texture including noodles, snacks, and other extruded products.
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3.2. Total phenolic content
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Table 3 presents the total phenolic content of four indigenous pigmented rice cultivars
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and the results were expressed as mg gallic acid equivalent per g. The total phenolic content of
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the rice cultivars were in the range of 1.91-5.99 mg GAC eq/g. Karudan samba exhibited the
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lowest total phenolic content of 1.91±0.03mg GAC eq/g while kattuyanam rice recorded the
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highest total phenolic content of 5.99±0.11mg GAC eq/g. Boue et al. (2016) have reported
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polyphenolic compounds like phenolic acids, anthocyanins, flavonoids were substantially present
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in higher amounts in black and red pigmented rice varieties than non- pigmented rice. Phenolic
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compound plays a vital role in combating chronic diseases such as cardiovascular diseases and
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type II diabetes. Phenols are the major contributors in determining the antioxidant potential of
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cereal grains. The total phenolic content of four rice cultivars were found to be significantly
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higher than that of pigmented rice varieties of Taiwan (0.24-1.78g GAE/kg) and China (0.246-
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0.563 mg GAE/g) as reported by Huang and Ng (2012) and Pang et al. (2018). Many researchers
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have reported that phenolic content of black rice was higher than that of red rice (Boue et al.,
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2016; Somaratne et al., 2017). However, in our study red kavuni presented higher phenolic
220
content (5.89 mg GAC eq/g) when compared with black kavuni rice (3.33 mg GAC eq/g).
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Cultivation techniques, growing conditions, genotype, degree of maturity, extraction, storage and
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ripening process could impact phenolic constituents of rice.
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3.3. Total flavonoid content
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The highest flavonoid content of rice cultivar was observed for red kavuni (84.40 mg
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catechin eq/g), followed by kattuyanam (71.10 mg catechin eq/g) > black kavuni (44.08 mg
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catechin eq/g) > karudan samba (42.33 mg catechin eq/g) (Table 3). Flavonoids are the most
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treasured phytochemicals of the secondary metabolites that impart its role in regulating various
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biological functions comprising anti-inflammatory, antimicrobial, antitumor, antioxidant and
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anti-allergic properties (Bhat & Riar, 2017). The total flavonoid content of pigmented rice used
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in this study were higher than values of other pigmented rice varieties reported earlier (Mir et al,
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2016; Pengkumsri et al, 2015; Reddy et al, 2016). Shen et al. (2009) stated that black rice
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possesses higher flavonoid content than brown and red rice cultivars which in turn contradicts
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the statement of this study owing to the presence of higher flavonoid content in red kavuni rice
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cultivar. Red and black pigments present in the rice cultivars were due to the deposition of
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bioactive compounds in the aleuronic layer of the rice grain, thus leading to the coloration of rice
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and owes to the quantity of phenols and flavonoids content of rice grains. On the other hand, rice
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should be consumed as whole grains enriched with abundant phytochemical compounds to
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ensure the maximum intake of bioactive compounds (Bhat & Riar, 2017). Hence, it is
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noteworthy that pigmented rice varieties are a prodigious source of functional foods.
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3.4. DPPH radical scavenging activity The capacity of indigenous pigmented rice cultivars to scavenge free radicals was
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evaluated by using DPPH (1,1-diphenyl-2-picryl hydrazyl) and it is considered as a critical
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indicator for in-vitro antioxidant activity. DPPH is a stable radical which transform its distinctive
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purple color to pale yellow by electron transfer into its non-radical (DPPH-H) form. Decrease in
245
absorbance at 517nm defines the reaction between DPPH and phytochemicals and also its
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reducing ability of antioxidants in the pigmented whole grains (Bhat & Riar, 2017). The DPPH
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scavenging activity was greater than 90% for brown, red and black cultivars which varied from
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82.56 to 99.52% with karudan samba presenting lesser activity and the red kavuni and
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kattuyanam depicting higher DPPH radical scavenging activity as displayed in Table 3. Min et
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al. (2012) stated that increased DPPH activity in black and red colored rice cultivars could be
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due to the presence of increased anthocyanin and proanthocyanidins content in pigmented rice
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varieties. These values are found to be extremely higher than values reported by other
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researchers (Mir et al., 2016; Reddy et al., 2016; Bhat & Riar, 2017). The variations in the
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scavenging activity of rice cultivars may be associated to the concentration of phytochemical
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compounds such as phenols and flavonoids present in them. Phytochemical compounds have
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immense antioxidant potential and its impact on consumer’s nutrition and health are apposite
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(Mir et al., 2016). Saikia et al. (2012) reported DPPH activity of 94.19% for Chakhao Poreiton
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pigmented variety. In this study, all the four rice cultivars exhibited a positive correlation (r =
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0.910; p ≤ 0.01) between DPPH scavenging activity and total phenolic content as illustrated in
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Table 4. Mir et al (2016) also reported a strong positive correlation between DPPH and phenolic
261
content of rice cultivars of Kashmir.
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3.5. Reducing power
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The reducing power of indigenous pigmented rice cultivars varied significantly (p < 0.05)
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from 0.83 to 6.38 mg AAE equivalent/g and the results were expressed as mg ascorbic acid
265
equivalent per g (Table 3). Reducing power of phytochemical compounds depicts the capacity of
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natural antioxidant to donate electron so as to minimize the oxidized entities produced through
267
various biological pathways. These oxidized entities i.e. free radicals are known to implicate
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various adverse effects on humans. The reducing power of kattuyanam (6.38 mg AAE/g) was
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found to be higher followed by red kavuni (5.78 mg AAE/g). The lowest reducing power was
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observed in karudan samba (0.83 mg AAE equivalent/g) which substantiate its decreased total
271
phenolic content and DPPH radical scavenging activity. The reducing power of pigmented rice
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used in this study were higher than values of Thai pigmented rice varieties reported by
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Jiamyangyuen, Nuengchamnong, & Ngamdee (2017). The reducing power obtained in the
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present study followed the pattern of Bhat & Riar (2017) in which the colored rice cultivars
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(brown, red, black) were found to have higher reducing power than karudan samba (white rice).
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Food compounds with reducing power behave as primary or secondary antioxidants leading to
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the deactivation of lipid peroxidation process and radical chain reactions. High reducing power
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activity is an exhibitive of high phenolic content in the food ingredients. The higher absorbance
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of indigenous pigmented rice varieties signals their intense reducing and antioxidant potential
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(Bashir, Usmani, Haripriya, & Ahmed, 2017).
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3.6. Glycemic response of rice cultivars
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The mean weight, height, body mass index and age of subjects were 58.69±4.50 kg,
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1.60±0.06 m, 22.96±1.89 kg/m2 and 23±0.61 years old, respectively. The glycemic response
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curves of the four indigenous pigmented rice cultivars are presented in Fig.1 and the average
285
(FBG) fasting blood glucose level of the subjects was 84.53±1.75 mg/dl. Glycemic index (GI)
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measures the ability of carbohydrate foods based on its postprandial glycemic effects. GI of food
287
was categorized as high (≥ 70%), medium (56-69%) and low (≤ 55%) (FAO and WHO, 1998).
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The elicited glycemic index (GI) values ranged from 47.19 by kattuyanam to a maximum of
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69.74 by karudan samba (Table 2). According to FAO and WHO (1998) GI classification,
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kattuyanam was graded as low GI rice (47.19), while red and black kavuni was graded as
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medium GI rice (61.69 and 56.27) and karudan samba as high GI rice (69.74). Red and black
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kavuni with short bold and medium rice grains elicited medium GI values of 61.69±4.0 and
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56.27±4.3 respectively. Somaratne et al. (2017) reported the GI effect of 4 pigmented and 1 non-
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pigmented basmati rice varieties which ranged between 48-68%. Prasad et al. (2018) observed
295
that GI of eleven rice varieties were 50.40±5.27 to 79.68±3.25 and long and slender rice grains
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exhibited lower glycemic response. Though, the amylose content was higher in all the four
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varieties, changes in starch digestibility and physicochemical properties may exhibit the
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difference in the GI values.
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In the current study, crude fibre displayed an inverse association between the GI (r = -
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0.904; p ≤ 0.01) and GL (r = -0.857; p ≤ 0.01) of indigenous pigmented rice cultivars. Crude
301
fibre is a type of dietary fibre that retards starch digestibility and in turn alters the food glycemic
302
response. Glycemic load (GL) is a strong indicator which predicts the response of insulin and
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postprandial glucose level than carbohydrate content and in fact glycemic load surpasses the GI
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for both composite meals and single foods (Rozendaal et al., 2018). The glycemic load (GL) of
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rice cultivars ranged from 8.80 (kattuyanam) to a maximum of 13.84 (karudan samba). Decrease
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in rate and quantum of starch digestion may contribute to lower GL in kattuyanam rice whereas
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greater extent of starch digestibility indicates higher GL in karudan samba rice. Glycemic load
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showed a strong positive correlation with glycemic index (r = 0.979; p ≤ 0.01).
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3.7. Pearson’s correlation coefficients Correlation analysis between glycemic index and antioxidant properties of indigenous
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pigmented rice cultivars were presented at p ≤ 0.05, and p ≤ 0.01 levels (Table 4). Amylose
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content was negatively correlated to glycemic index (r = -0.713; p ≤ 0.01) and glycemic load (r =
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-0.574) of indigenous pigmented rice cultivars. Glycemic load showed a strong positive
314
correlation with glycemic index (r = 0.979; p ≤ 0.01). Fitzgerald et al. (2011) and Prasad et al.
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(2018) also noticed similar inverse correlation between predicted GI and amylose content in
316
different rice cultivars. The difference in GI is not correlated with the percentage of amylose
317
available. The physicochemical properties and starch digestibility could be the key factors in
318
determining the GI of rice. In addition to this, the resistance to digestion by the interaction
319
between amylose to protein or lipid in the formation of amylose-protein or amylose-lipid
320
complexes could also help to determine the GI of rice. Moghaddam, Vogt, & Wolever (2006)
321
have reported that elevated levels of crude fat and crude protein in the pigmented rice diminishes
322
glycemic responses via glucagon-like peptide (GLP-1) mediated effects on gastric emptying and
323
amino acid mediated effects on insulin secretion. Variations in the glycemic index of rice
324
genotypes depends on various factors including the amount of fibre, protein, degree of
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amylopectin branch and amylose molecules present in rice kernel.
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showed high glycemic index (GI) value. Hence, karudan samba with high GI value, leads to
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rapid rise in blood glucose level due to increased susceptibility of hydrolytic enzymes involved
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in starch degradation and consequently the presence of high surface area of cooked rice grains
330
(Kumar et al., 2018b). The low GI of kattuyanam rice shows low rate of digestibility which
331
could be attributed to the composition and type of starch, its structure and above all the varietal
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difference exhibited. Studies have reported that lipids, ash and protein also have negative
333
correlation of GI of rice as the absorption of glucose from rice is reduced with increased lipid
334
and mineral content Somaratne et al (2017). The correlation between dietary fibre and GI of rice
335
are in consensus with Yusof, Talib, & Karim (2005) which also presents that dietary fibre is
336
independent to the GI of rice. A diet high in cereal fibre significantly lowers the glycemic index
337
and blood glucose response and consequently increasing polyphenolic content and antioxidant
338
capacity of food (Bhat & Riar, 2017). The correlations found in this research are in agreement
339
with the above findings. Glycemic index displayed negative correlation with total phenol (r = -
340
0.669*), total flavonoid (r = -0.362), DPPH (r = -0.760**) and reducing power (r = -0.740**). A
341
strong positive correlation was found between total phenolic content and reducing power (r =
342
0.993**). Several studies have reported that increased total phenol and flavonoid content in the
343
pigmented rice bran layer might affect the rate of starch digestion in rice by hindering the
344
activities of enzymes such as α-amylase, α-glucosidase and β-glucosidase (Donkor et al., 2012;
345
Somaratne et al., 2017). The bioactive compound of pigmented rice has proved to have aldose
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reductase inhibitory activities which is proved beneficial in preventing diabetic complications.
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Thus, pigmented rice should be consumed along with its bran layer (Bhat & Riar, 2017).
348
Conclusion
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along with its contributing glycemic prospective have been assessed. Contrary to popular belief
351
that rice consumption attributes to diabetes (type II), this study, established that native pigmented
352
rice crops display a differential plethora of nutritive potentials packed in it. Kattuyanam with
353
lower GI and comparatively higher levels of protein, fibre and amylose appears to be a better
354
aspect for rice based dietetic management and provides beneficial physiological effects for the
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prevention of diabetes. Red and black kavuni which belongs to medium GI variety together with
356
a nutritionally balanced diet can still bring relief in consumption of rice by diabetics and also
357
consequently would serve as a goal for people wishing to reduce or maintain weight. These
358
polyphenols have correlated with increased antioxidant activity and reducing power activity in
359
both kattuyanam and red kavuni varieties. Though all the pigmented rice varieties exhibited high
360
amylose, the glycemic index was not reported to be low. This very finding brings in a negative
361
correlation between amylose and GI. To substantiate the statement, high amylose red and black
362
kavuni represented medium GI where, kattuyanam which also had high amylose reported low
363
GI. Interestingly amylose had shown positive correlation with crude fibre and crude fibre had
364
shown negative correlation with GI. Rice varieties of high amylose content, total phenol, total
365
flavonoid can serve as a source of functional food and variety like kattuyanam can serve as low
366
GI. This information can benefit consumers, producers and food formulators where rice based
367
formulations can still be effective in addressing diabetic population where rice is considered as a
368
threat to increase blood glucose. The nutritional composition, biological properties and glycemic
369
potential of indigenous pigmented rice cultivars would serve as a better ingredient for its usage
370
in the food industry. The current research would also enable useful information regarding rice
371
based strategies and holistic approach to prevention and management of diabetes in India.
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Conflict of interest
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The authors have declared no conflict of interest.
Ethical approval
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This research work was carried out with the help of human participants. Thanks to the
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Institute Ethics Committee (Human studies), Pondicherry University, Puducherry for ethical
377
clearance (Approval No. HECPU/2018/01/190218).
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Acknowledgements The first author is thankful to the University Grants Commission, New Delhi, India for
380
granting financial assistance in the form of Junior Research Fellowship (Grant No. 1538/NET-
381
JUNE 2013). Our sincere gratitude to the volunteers who participated in the study.
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rice in Malaysia. Age (years) 25, 20-31.
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Table 1
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Proximate composition of indigenous pigmented rice cultivars (g/100g dry weight) Hue
Appearance Crude Protein (%)
483 484 485 486 487
Ash (%)
Moisture (%)
12.11±0.25
c
5.35±0.32
a
72.98±1.56
c
1.69±0.13
c
1.29±0.03
c
11.93±0.04
a
Red kavuni
Red
12.98±0.25
b
3.77±0.03
b
73.10±1.12
b
1.51±0.06
d
1.51±0.14
b
10.90±0.06
b
Black kavuni
Black
14.01±0.44
a
3.65±0.02
b
73.19±1.23
a
2.05±0.08
a
1.75±0.01
a
9.00±0.02
Karudan samba
White
10.22±0.25
d
2.95±0.07
a
1.74±0.12
b
1.28±0.09
c
11.35±0.23
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Brown
c
75.41±1.78
c a
Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
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482
(%)
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(%)
Fat (%)
Kattuyanam
479 480
Carbohydrate
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477 478
Crude fibre
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Rice cultivars
488 489 23
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Table 2
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Amylose content and glycemic index of indigenous pigmented rice cultivars Amylose (%)
Mean ± SD
496 497 498 499 500
Classification
335
30.03±0.09
a
High
47.19±3.2
Low
8.80
Red kavuni
Red
343.78
27.28±0.26
c
High
61.69±4.0
Medium
11.22
Black kavuni
Black
345
27.30±0.08
c
High
56.27±4.3
Medium
10.19
Karudan samba
White
315.10
27.71±0.04
b
High
69.74±4.5
High
13.84
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Brown
SE = Standard error; SD = Standard deviation. Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
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Mean ± SE
Glycemic load
Kattuyanam
EP
494
Classification
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492 493
Glycemic index (GI)
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Serving size, g equivalent to 50g of available carbohydrate
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Rice cultivars
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Table 3
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Antioxidant properties of indigenous pigmented rice cultivars Total flavonoid
DPPH
Reducing power
(mg GAC eq/g)
(mg catechin eq/g)
(%)
(mg AAE/g)
Brown
5.99±0.11
a
71.10±2.36
b
99.45±0.14
a
6.38±0.15
a
Red kavuni
Red
5.89±0.19
a
84.40±0.95
a
99.52±0.00
a
5.78±0.11
b
Black kavuni
Black
3.33±0.02
b
44.08±0.38
c
95.48±0.24
b
2.95±0.07
c
Karudan samba
White
1.91±0.03
c
42.33±0.38
c
82.56±0.19
c
0.83±0.03
d
Values are means ± standard deviation with triplicates. Data followed by the different letter in the same column are significantly different (p < 0.05) by Duncan’s test.
507
512 513 514 515
EP
511
AC C
510
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Kattuyanam
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Total phenol
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Rice cultivars
516 517
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Table 4
519 520
Pearson’s correlation coefficients between proximate composition, glycemic index and antioxidant properties of indigenous pigmented rice cultivars
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526
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Parameter GI GL AML TPC TFC DPPH RP CP CF CHO F A M GI 1 GL 0.979** 1 ** AML -0.713 -0.574 1 TPC -0.669* -0.724** 0.457 1 TFC -0.362 -0.429 0.256 0.924** 1 ** ** DPPH -0.760 -0.859 0.293 0.910** 0.748** 1 ** ** ** ** RP -0.740 -0.786 0.510 0.993 0.888 0.925** 1 * Crude -0.463 -0.621 -0.225 0.418 0.239 0.741** 0.437 1 protein Crude -0.904** -0.857** 0.855** 0.773** 0.526 0.717** 0.815** 0.249 1 fibre CHO 0.749** 0.853** -0.251 -0.800** -0.602* -0.953** -0.822** -0.838** -0.689* 1 ** Fat -0.124 -0.120 -0.149 -0.525 -0.744 -0.190 -0.454 0.292 -0.132 0.047 1 Ash -0.084 -0.237 -0.567 -0.033 -0.146 0.327 -0.021 0.789** -0.216 -0.404 0.456 1 Moisture -0.075 0.058 0.659* 0.326 0.431 -0.074 0.306 -0.692* 0.396 0.197 -0.692* -0.901** 1 521 GI = Glycemic index; GL = Glycemic load; AML = Amylose (%); TPC = Total phenolic content (mg GAC eq/g of rice); TFC = Total 522 flavonoid content (mg catechin eq/g of rice); DPPH (%); RP = Reducing power (mg ascorbic acid eq/g of rice); CP = Crude protein 523 (%); CF = Crude fibre (%); CHO = Carbohydrate (%); F = Fat (%); A = Ash (%); M = Moisture (%). ** 524 Correlation is significant at p ≤ 0.01 level (2-tailed). * 525 Correlation is significant at p ≤ 0.05 level (2-tailed).
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Figure Captions
528
Fig.1. Mean blood glucose concentration (mg/dl) of indigenous pigmented rice cultivars
529
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530 531 532
534
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Fig.1.
Glucose
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Kattuyanam
Red kavuni
150
110
50
EP
90 70
0
536
Karudan samba
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130
AC C
Blood glucose concentration (mg/dl)
170
Black kavuni
15
30
45
60
Time (mins)
537 538
27
75
90
105
120
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Highlights Kattuyanam with 30.03% amylose and 9.33% TDF elicited low GI of 47.19 Karudan samba with 27.71% amylose and 7.87% TDF recorded high GI of 69.74
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Amylose correlated inversely with the glycemic index and glycemic load
DPPH scavenging activity of rice cultivars was in the range of 82.56 to 99.52%
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Glycemic index and antioxidant activity are inversely proportional