Quality characterisation and estimation of phytochemicals content and antioxidant capacity of aromatic pigmented and non-pigmented rice varieties

Food Research International 46 (2012) 334–340

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Quality characterisation and estimation of phytochemicals content and antioxidant capacity of aromatic pigmented and non-pigmented rice varieties Sangeeta Saikia, Himjyoti Dutta, Daizi Saikia, Charu Lata Mahanta ⁎ Department of Food Engineering and Technology, School of Engineering, Tezpur University, Assam, India

a r t i c l e

i n f o

Article history: Received 19 April 2011 Accepted 22 December 2011 Keywords: Pigmented rice Nutritive value Physicochemical properties RVA Phytochemicals Antioxidant capacity

a b s t r a c t Two pigmented and two non-pigmented aromatic rice varieties were analysed for physical dimensions, physicochemical properties, colour, viscosity changes while cooking, phytochemicals content and antioxidant capacities. The rices were also evaluated for the effect of cooking on phytochemicals content and antioxidant capacities. While Bakul joha (BJ) and Keteki joha (KJ) varieties are non-pigmented, Poreiton chakhao (PC) is purple coloured and Chak-hao-amubi (CA) is red coloured. The varieties differed in size and shape of the kernels; varying from tiny to big in size, short to long in length, and round to bold in shape. KJ contained the highest amount of protein (9.9%) and PC had the maximum amount of fat (2.1%). Non pigmented rices were of high amylose and of high pasting profile parameters and the pigmented ones were of vice-versa nature. Highest levels of total phenolics, total flavonoid content and ferric reducing antioxidant property (FRAP) were observed in CA (579.00 mg GAE 100 − 1 g, 220.5 mg QE 100 − 1 g and 5.45 mM 100 − 1 g respectively); PC recorded maximum levels of anthocyanin content (35.87 mg cyanidin-3-glucoside equivalent 100 − 1 g). Both PC and CA possessed good DPPH (2, 2-diphenyl-1-picrylhydrazyl) radical scavenging activity with 94.19% and 96.43% abilities, respectively. Cooking drastically reduced the phytochemicals and antioxidant capacities. These properties further decreased with keeping time of the cooked rices; PC, however, retained maximum amounts compared to others. The study revealed wide differences in the properties of the aromatic rices. PC (purple) was superior to CA (red) for cumulative phytochemicals content and antioxidant properties. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Rice (Oryza sativa L.) is an important cereal crop consumed both as staple food as well as processed products. In developing countries, the quality of rice in terms of nutritional quality has been greatly emphasised as rice is the staple and main cereal consumed. Consumption of a single cereal may cause deficiency diseases because of the lower concentration of essential amino acids, minerals and vitamins (Bouis, Chassy, & Ochanda, 2003). The rice varieties commonly have whitish kernels. There are also rice varieties with a coloured testa (black, purple, or red), that give slightly coloured kernels on milling (Finocchiaro et al., 2007). The proportion of amylose (and amylopectin) in the starch is predominantly responsible for the different physicochemical and cooking properties of the rice kernel and rice has been categorised into high amylose, intermediate amylose, low amylose and waxy rice types (Bhattacharya, Sowbhagya, & Indudhara Swamy, 1982; Juliano, 1979). The content of amylose in rice is considered the principal determinant of rice quality. However, rice varieties with similar amylose content have shown to possess different rice characteristics on cooking which indicated that secondary differences exist among varieties with similar

⁎ Corresponding author. Tel.: + 91 3712 267008 5702; fax: + 91 3712 267005. E-mail address: [email protected] (C.L. Mahanta). 0963-9969/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2011.12.021

amylose contents. Gel consistency, viscogram indices, gelatinisation temperature and equilibrium moisture content attained by milled rice when soaked in water at room temperature (EMC-S), among others, have been offered as secondary indices but, no one method explains the secondary differences (Sowbahagya, Ramesh, & Bhattacharya, 1987). Interestingly, the North-Eastern region of India is rich in germplasm of all the four types. Some of these are rich in the rice aromatic compound, 2-acetyl-1-pyrroline (Bradbury, Fitzgerald, Henry, Jin, & Waters, 2005; Buttery, Ling, Juliano, & Turnbaugh, 1983). In addition to that, few of them are pigmented varieties. Usually pigmented rice varieties are red, black or purple. These are traditionally known to have health benefits and are particularly valued in local markets. Great interest has been shown in the polyphenols in rice for their multiple biological activities. These phenolic compounds include ferulic acid and diferulates, anthocyanins, anthocyanidins and polymeric proanthocyanidins (condensed tannins) (Chun et al., 2005). Phenolics have the ability to donate hydrogen and act as reducing agents. Phenolics also act as singlet oxygen quenchers and free radical hydrogen donors and because of these properties, phenolics have protective effect on cell constituents against oxidative damage. Such antioxidant characteristics of phenolics have been shown in epidemiological studies to prevet cancer, cardiovascular and nerve diseases (Kehrer, 1993). In the pigmented rice varieties, anthocyanin is present in abundance that has notable antioxidant and anti-inflammatory properties (Hou, Qin,

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Zhang, & Cui, in press; Jing & Giusti, 2007). However, the heat labile bioactive components are often lost during the high heat processing treatments (Pascual et al., in press). Attention is currently being given to the antioxidative and radicalscavenging properties of coloured rice cultivars because of their potential to provide and promote human health by reducing the concentration of reactive oxygen species and free radicals (Nam, Choi, Kang, Koh, & Friedman, 2006; Oki et al., 2005). The objective of this study was to compare the physical parameters, physicochemical properties, phytochemical and antioxidant potentials of the four aromatic rice varieties native to the North-East India viz., Keteki joha (KJ), Bakul joha (BJ), Poreiton chakhao (PC), and Chak-hao-amubi (CA), and determine the effect of cooking and subsequent keeping on total phenolic and flavonoid contents and the antioxidant activities. 2. Materials and methods Non pigmented Keteki joha (KJ) and Bakul joha (BJ) paddy varieties were purchased from Regional Rice Research Station, Assam Agricultural University and the pigmented Poreiton Chakhao (PC) and Chak-hao-amubi (CA) from farmers of Manipur. All the paddy samples were from the recent harvest of December, 2010. The paddy samples were cleaned of foreign material, packed in polyethylene bags and placed inside cloth bags, and stored at 4 °C. The paddy samples were then taken out from cold and brought to room temperature and were shelled (6% polishing, weight basis) in Satake rice dehusker and emery type polisher, respectively with polishing times of 4–4.5 min depending on the variety. Milled PC was deep purple in colour; milled CA was mild red in colour. The kernels were then ground in a laboratory grain mill (Fritsch Pulverisette 14) to 100 μ size and stored at 4 °C for further experiments. Results of proximate analysis and physicochemical properties have been rounded off to one decimal point. 2.1. Physical properties Weight of 1000 kernels of each sample was determined. The bulk density was determined using the mass/volume relationship. A tared empty plastic measuring cylinder was filled with the kernels from a constant height and excess kernels were struck off the top level and then cylinder was again weighed along with kernels. The true density defined as the ratio between the mass of paddy and the true volume of the grain, was determined using the kerosene displacement method (Selvi, Pinar, & Yesiloglu, 2006). The length (L) and breadth (B) were measured with vernier callipers and L/B ratio was calculated. 2.1.1. Colour values The L*, a* and b* colour values of the different rice flour samples were measured using Colour Measurement Spectrophotometer (Hunter ColorLab Ultrascan Vis) where ‘L*’ indicates degree of lightness or darkness (L* = 0 indicates perfect black and L*= 100 indicates most perfect white); ‘a*’ indicates degree of redness (+) and greenness (−); whereas ‘b*’ indicates degree of yellowness (+) and blueness (−). Hue angle and Chroma were determined.

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30 min. The crucible was cooled and final weight was taken. The crude fibre was calculated as Loss in weight of sample=sample weight
100: The carbohydrate content (estimated total carbohydrate content) was determined by difference from the content of moisture, protein, ash and fats. 2.3. Physicochemical properties Four rice varieties were estimated for amylose content by the method of Sowbahagya and Bhattacharya (1979); equilibrium moisture content on soaking at room temperature by the method of Indudhara Swamy, Ali, and Bhattacharya (1971); gel consistency test according to Cagampang, Perez, and Juliano (1973); and expansion ratio on cooking of whole kernels by the method of Umadevi, Veerabadhiran, Manonmani, and Shanmugasundaram (2010). The varieties were classified on the basis of gel consistency (gel length) as hard (27–40 mm), intermediate (41–60 mm), and soft (over 60 mm) gel types. 2.3.1. Aroma detection Aroma of the rice samples was detected using a qualitative test developed by Singh, Bhattacharya, and Mahadevappa (1986). A 200 mg brown rice sample was taken in a glass tube and 0.5 mL of 0.1 N potassium hydroxide was added to it. The tube was kept closed with a wood cork. After 15 min, the aroma was identified by opening the cork and bringing the tube mouth near to the nose. In case aroma was very mild, the tube was corked again for another 15 min and aroma was sensed. 2.3.2. Pasting properties The pasting profiles of the rice flour samples were recorded using a Rapid Visco Analyser (RVA Starchmaster2, Newport Scientific Instruments). Viscosity profiles were recorded using rice flour suspensions (12% w/w; 28.5 g total weight). The Rice1 profile of Newport Scientific was used, where the sample holding initially was at 50 °C for 1 min, heating was from 50 °C to 95 °C in 3:45 min, a second holding phase was at 95 °C for 2:40 min, a cooling phase from 95 °C to 50 °C in 4 min and a final holding phase at 50 °C for 1 min. The pasting point (PP); the corresponding gelatinization temperature (GT); peak viscosity (PV); hot paste viscosity (HPV); cold paste viscosity (CPV); breakdown (BD); and total setback (SBt) were recorded. HPV is the minimum viscosity at 95 °C and CPV is the final viscosity at 50 °C. BD = PV–HPV and SBt = CPV–HPV. 2.4. Phytochemicals and antioxidant activities

2.2. Nutritive quality

2.4.1. Sample extraction The method of Atala, Vásquez, Speisky, Lissi, and Lopez-Alarcon (2009) was followed. Briefly, 10 g of each sample that was powdered in a grinder was extracted with 100 mL of extraction solvent (75:25 v/ v, acetone: water). Extracts were shaken in a water bath at 25 °C for 90 min, and centrifuged at 950 g for 15 min (Hettich-Zentrifugen). The supernatant was then stored at −20 °C until further analysis of total phenolics, flavonoids, anthocyanins and antioxidant activities.

The milled rice samples of 6% polish were analysed for protein, fat and ash, and crude fibre as per AOAC (1995) and expressed on dry weight basis. The crude fibre was determined by digestion of 2 g sample initially in 0.255 N sulphuric acid for 30 min. After filtration, residue was suspended in 0.313 N sodium hydroxide and boiled for another 30 min followed by washing with 1.25% boiling sulphuric acid, three 50 mL volume of distilled water and 25 mL of ethanol. The residue was then transferred to a preweighed crucible and dried at 130 °C, weighed after cooling and then ignited at 600 °C for

2.4.2. Total monomeric anthocyanin pigment content Total monomeric anthocyanin pigment content of the rice samples was determined with slight modifications of the pH differential methods of Giusti and Wrolstad (2001) and Wolfe, Wu, and Liu (2003). Briefly, 0.5 mL of the sample extract was mixed thoroughly with 3.5 mL of 0.025 M potassium chloride buffer (pH 1). The mix was vortexed and then allowed to stand for 15 min. The absorbance was then measured at 515 and 700 nm against distilled water in a UV–vis spectrophotometer (Cecil, Aquarius 7400). The extract was also mixed

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similarly with 0.025 M sodium acetate buffer (pH 4.5), and the absorbance was measured at the same wavelength after standing for 15 min. Results were expressed as mg of cyanidin-3-glucoside equivalent/100 g of sample. Total anthocyanin content ¼ ðA
MW
DF
1000Þ=ε
C Where, A (absorbance) = [(A515–A700) pH 1.0–(A515–A700) at pH 4.5]; MW is equal to 449.2 (molecular weight of cyanidin3-glucoside); DF is the dilution factor of sample; ε is the molar absorbtivity of cyanidin-3-glucoside, equal to 26,900; C is concentration of the buffer in mg/mL. 2.4.3. Determination of total phenolic content A modified version of the Folin–Ciocalteu assay (Slinkard & Singleton, 1977) was used to determine the total phenolic content in the extracts from the rice samples. Gallic acid standard curve was made with appropriate concentrations of aqueous gallic acid solution. For the analysis, 20 μL each of extract, gallic acid standard or blank were taken in separate test tubes and to each 1.58 mL of distilled water was added, followed by 100 μL of Folin–Ciocalteu reagent, mixed well and within 8 min, 300 μL of sodium carbonate was added. The samples were vortexed immediately and allowed to incubate in dark for 30 min at 40 °C. The absorbance was measured at 765 nm. The phenolic content was expressed in mg GAE/100 g. 2.4.4. Determination of total flavonoid content The method of Dewanto, Wu, Adom, and Liu (2002) with slight modification was followed. Briefly, to 0.5 mL of the extract in a test tube, 2.25 mL of distilled water and 0.15 mL of 5% sodium nitrite solution were added and the contents were mixed well and kept for 6 min. To this, 0.3 mL of a 10% aluminium trichloride hexahydrate solution was added and contents were mixed well and allowed to stand for 5 min followed by addition of 1 mL of 1 M sodium hydroxide. The mixture was vortexed and absorbance was measured immediately at 510 nm. Results were expressed as quercetin equivalent (mg QE/ 100 g) of sample. 2.4.5. Determination of ferric reducing antioxidant property (FRAP) The method of Benzie and Strain (1999) was followed. For this method, a 40 μL aliquot of extract was thoroughly mixed with 3 mL of FRAP solution and the mixture was incubated at 37 °C for 4 min. The absorbance was determined at 593 nm against distilled water as blank. FRAP solution was pre warmed at 37 °C and prepared freshly by mixing 2.5 mL of a 10 mM 2,4,6-TPTZ [2,4,6-tri(2-pyridyl)-1,3,5-triazine] solution in 40 mM hydrochloric acid with 2.5 mL of 20 mM ferric chloride and 25 mL of 0.3 M acetate buffer (pH 3.6). A calibration curve was prepared, using an aqueous solution of ferrous sulphate (1–10 mM). FRAP values were expressed as μM of ferrous equivalent Fe (II) per 100 g of sample. 2.4.6. Determination of DPPH activity The DPPH (2, 2-diphenyl-1-picrylhydrazyl) radical scavenging activity of the extracts was measured according to the method of Brand-Williams, Cuvelier, and Berset (1995). Precisely, 100 μL of extracts were added to 1.4 mL DPPH radical methanolic solution (10 − 4 M). The absorbance at 517 nm was measured at 30 min against blank (100 μL methanol in 1.4 mL of DPPH radical solution). The results were expressed in terms of radical scavenging activity using the following equation: Radical scavenging acitivityð% Þ ¼ ½ðAo −As Þ=Ao 
100; where, Ao is absorbance of control blank, and As is absorbance of sample extract.

2.5. Rice cooking Twenty kernels of each variety were taken in beakers and 40 mL distilled water was added to each. After soaking for 30 min at room temperature (27 ± 1 °C), the samples were cooked by open steaming in an autoclave (Equitron 7407ST) at 100 °C for 10 min in the water used for initial soaking. Complete cooking was indicated by loss of opaque uncooked portions when cooked kernel was pressed between glass slides. Cooked samples were immediately cooled by putting the beakers in cool water and analysed for moisture. The cooked rice was kept in room temperature and analysed after 15 min, 45 min and 90 min respectively for total phenolics, flavonoids, DPPH and FRAPS activities. 2.6. Statistical analysis All experiments were carried out in triplicates and presented as mean ± standard deviation of mean using SPSS version 11.5. The data were statistically analysed by Duncan's multiple range tests at 5% significance level. 3. Results and discussion 3.1. Physical properties The results for the physical parameters of the four rice varieties are given in Table 1. True density was lowest for BJ. CA recorded the highest kernel weight. The higher weights of the pigmented varieties can be related to their larger kernel sizes. The L/B ratio clearly indicated that the pigmented PC and CA kernels were longer and slender than the non-pigmented BJ and KJ. The varieties were found to vary in their size and shape class (Table 1). 3.1.1. Colour values The L*, a*, b* colour values of the four rice flour samples are given in Table 1. Whiteness was more prominent in BJ followed by KJ. CA and PC exhibited much lower L* values due to the pigmented external layers of the kernels that existed even after 6% polishing. The positive a* values for redness were the highest for the CA sample due to reddish brown coloured external layers followed by PC and was least

Table 1 Physical properties of milled aromatic rice varieties. Properties Physical Bulk density (g/cm3) True density (g/cm3) 1000 kernel weight (g) Length (mm) Breadth (mm) L/B

Colour L* a* b* Hue angle (°) Chroma

Bakul joha

Keteki joha

Poreiton chakhao

Chak-haoamubi

0.48 ± 0.12a

0.56 ± 0.02d

0.54 ± 0.03c

0.53 ± 0.01b

0.94 ± 0.05a

1.19 ± 0.01c

1.09 ± 0.01b

1.03 ± 0.01b

11.64 ± 0.04a (Tiny)⁎ 4.21 ± 0.01a (short) 2.31 ± 0.05b 1.84 ± 0.09a (Round)

13.42 ± 0.01b (Small) 5.82 ± 0.03b (Medium) 2.02 ± 0.07a 2.91 ± 0.01c (Quasislender)

16.56 ± 0.02c (Small) 6.51 ± 0.03d (Long) 2.02 ± 0.09a 3.25 ± 0.06d (Slender)

22.32 ± 0.01d (Big) 6.21 ± 0.08c (Long) 2.60 ± 0.1c 2.38 ± 0.05b (Bold)

81.14 ± 0.04d 0.31 ± 0.10a 5.71 ± 0.07b 86.89 5.72

78.36 ± 0.05c 0.63 ± 0.05b 6.69 ± 0.03c 84.62 6.72

48.58 ± 0.09a 2.47 ± 0.04c 1.77 ± 0.14a 35.62 3.04

62.12 ± 0.11b 4.41 ± 0.07d 8.79 ± 0.03d 63.35 9.83

Means with different letters in the same row indicate that there is significant difference between samples (p ≤ 0.05) from Duncan's multiple range test. ⁎ The term within brackets indicates the size and shape class determined as per Bhattacharya and Sowbhagya (1980).

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3.2. Nutritive quality The data are presented in Table 2. Protein was maximum in the non pigmented KJ (9.9%, db) followed by CA (8.8%, db). Fat content was in general high in all the samples with the highest amount present in PC (2.1%, db). Higher values of fat and protein content in pigmented rice varieties were also reported by Sompong, SiebenhandlEhn, Linsberger-Martin, and Berghofer (2011). The aromatic rice varieties were found to be good sources of protein and fat. 3.3. Physicochemical properties The values for different physicochemical parameters studied are shown in Table 3. Based on Juliano (1979) and Bhattacharya et al. (1982) classification, BJ and KJ varieties were high amylose types and PC and CA were waxy varieties. PC and CA, due to their waxy nature recorded higher EMC-S than BJ and KJ. From gel consistency values, the four rice varieties can be grouped into two categories (Cagampang et al., 1973). BJ and KJ are hard gel types and PC and CA are soft gel types indicating that cooked rice of BJ and KJ will have hard texture and that of PC and CA will have soft texture. Highest expansion ratio of cooked rice was recorded by BJ, closely followed by CA. 3.3.1. Aroma detection CA and PC produced strong aroma after treatment with potassium hydroxide for 15 min. Aroma was detected in BJ and KJ after a further treatment time of 15 min. 3.3.2. Pasting properties The effect of rice types was also clearly reflected in the RVA curves of the samples (Fig. 1). GT of the waxy varieties were higher than the high amylose varieties. However, high amylose BJ and KJ showed distinctly higher PV (3232 cP and 2687 cP), HPV (1489 cP and 1360 cP) and CPV (3540 cP and 3268 cP respectively) than the waxy CA and PC (Table 4), which was contrary to the findings of Suwannaporn, Pitiphunpong, and Champangern (2007). Amongst the two waxy varieties, PC showed comparatively less viscosity than CA on heating and cooling. The two waxy varieties (CA and PC) also showed very minimal BD of 25 and 6, and SBt of 849 and 357 respectively as also obtained by Tukomane and Varavinit (2008). The patterns were similar to patterns exhibited by cross-linked starch (Cooreman, Rensburg, Van, & Delcour, 2003; Koh & Singh, 2009). The higher GT and minor breakdown in the waxy pigmented rices may be due to greater resistance to loss of molecular arrangement on cooking

Table 3 Physicochemical properties of the milled aromatic rice samples. Parameters

Bakul joha

Keteki joha

Poreiton chakhao Chak-hao-amubi

Amylose (%, db) 28.8 ± 0.02d 26.4 ± 0.11c 2.2 ± 0.04a EMC-S (%, wb)1 29.7 ± 0.03b 26.7 ± 0.04a 36.4 ± 0.03d Gel consistency 27 ± 0.00a 29 ± 0.00b 144 ± 0.00d (mm) ER2 4.7 ± 0.03 c 3.6 ± 0.06d 3.3 ± 0.02a

above GT with a higher retention of absorbed water in the waxy varieties. On cooling, however, comparatively lower yet distinct setbacks representing retrogradation were observed for the samples. Such minor setbacks for waxy rice varieties were also observed by Traore, McClung, Chen, and Fjellstrom (2011). Though the two waxy varieties had very low levels of amylose, the possibility behind this setback pattern may be that the lengthiest chains of amylopectin molecules have acted as linear molecules that precipitated on cooling in a manner similar to amylose chains. CA had almost double the viscosity at PV and HPV and more than double at CPV compared to PC.

3.4. Phytochemicals and antioxidant activities 3.4.1. Raw rice samples The phytochemicals namely, total phenolic, flavonoid content and anthocyanin content that were studied in the four rice samples are reported in Fig. 2. The results showed that CA (red) variety contained significant amount of total phenolics (579.00 mg GAE100 − 1 g) as compared to purple coloured PC (245.00 mg GAE 100 − 1 g), and non-pigmented KJ (41.00 mg GAE 100 − 1 g) and non-pigmented BJ (39.00 mg GAE 100 − 1 g). Goffman and Bergman (2004) in their study on coloured rice have reported different phenolic contents. The phenolic contents in the white, red and purple rice ranged from 25 to 246, 34–424, and 69–535 mg GAE 100 − 1 g, respectively. Similarly, the flavonoid content in CA was more (220.50 mg QE 100 − 1 g) than PC (123.75 mg QE 100 − 1 g), KJ (26.50 mg QE 100 − 1 g) and BJ (27.75 mg QE 100 − 1 g). However, the anthocyanin content was high in PC (35.87 mg cyanidin-3-glucoside equivalent 100 − 1 g) as compared to CA (1.81 mg cyanidin-3-glucoside equivalent 100 − 1 g), KJ (0.89 mg cyanidin-3-glucoside equivalent 100 − 1 g) and BJ (0.73 mg cyanidin-3-glucoside equivalent 100 − 1 g). The dark purple colour of black rice results from the high content of anthocyanins in the pericarp layers (Hiemori, Koh, & 4000

100

Bakul joha

Keteki joha

Poreiton chakhao

Chak-haoamubi

Moisture (%, wb) Protein (%, db) Carbohydrate (%,db) Fat (%,db) Crude fibre (%, db) Ash (%, db)

13.7 ± 0.12d 7.7 ± 0.03b 76.6 ± 0.02b

12.1 ± 0.02 b 9.9 ± 0.11d 76.4 ± 0.06a

13.5 ± 0.03c 6.6 ± 0.02a 77.2 ± 0.01c

11.6 ± 0.04a 8.8 ± 0.05 c 78.0 ± 0.03d

1.6 ± 0.11b 0.2 ± 0.01a 0.7 ± 0.02b

1.0 ± 0.07a 0.2 ± 0.06a 0.8 ± 0.04c

2.1 ± 0.08c 0.2 ± 0.12a 0.5 ± 0.09a

1.0 ± 0.07a 0.3 ± 0.06b 0.5 ± 0.07a

Means with different letters in the same row indicate that there is significant difference between samples (p ≤ 0.05) from Duncan's multiple range test.

Temperature (oC)

Parameters

4.3 ± 0.01b

Means with different letters in the same row indicate that there is significant difference between samples (p ≤ 0.05) from Duncan's multiple range test. 1 EMC-S is equilibrium moisture content attained on soaking in water at room temperature. 2 ER is the expansion ratio on cooking of whole kernels.

BJ KJ

90

Table 2 Proximate composition of the milled aromatic rice samples.

2.9 ± 0.01b 35.2 ± 0.04c 119 ± 0.00c

3500 3000 2500

80

CA

2000 1500

70 PC

1000 500

60

0 50 00:03:18

00:06:38

00:09:58

--

Time (h:min:sec) Fig.1. Rapid Visco Amylography of four aromatic rice varieties.

Viscosity (cP)

for KJ and BJ flour. The yellowness (b* value) was also highest for CA. The non-pigmented varieties, KJ and BJ, gave higher b* values nearer to CA but purple coloured PC gave the lowest reading. Hue angle was lowest in PC indicating that PC was more red than others. Chroma, a measure of vividness of colour was very low in all the rice samples.

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Mitchell, 2009). It is known from previous investigation (Hiemori et al., 2009) that black and red rice contain a limited number of pigments, with the prominent anthocyanins being cyanidin-3-glucoside and peonidin-3-glucoside. PC (purple) can be exploited to be used as natural colourants as its anthocyanin content is twice that present in CA. Antioxidant activities were measured in acetone extract by DPPH and FRAP methods. The DPPH results showed similar radical scavenging activity in CA (96.43%) and PC (94.19%). The FRAP assay of the rice samples again showed significant FRAP values in CA (5.45 mM 100 − 1 g) than PC (2.59 mM 100 − 1 g). The FRAP values for KJ and BJ were 0.56 and 0.96 mM 100 − 1 g, respectively. Interestingly, CA, the red variety, showed higher antioxidant activity based on FRAP and DPPH results as compared to the PC (purple) variety inspite of it containing high amounts of anthocyanins. The reason

Table 4 Viscography parameters of the rice flour suspensions at 10% (w/w) concentration on dry basis. Samples

PP (cP)

GT (°C)

PV (cP)

HPV (cP)

CPV (cP)

BD (cP)

SBt (cP)

Bakul joha Keteki joha Poreiton chakhao Chak-haoamubi

62 78 114

72.9 74.1 83.7

3232 2687 621

1489 1360 615

3540 3268 972

1743 1327 6

2051 1908 357

134

78.1

1235

1210

2059

25

849

PP = peak point; PV = peak viscosity; HPV = hot paste viscosity; CPV = cold paste viscosity; BD = breakdown; SBt = total setback.

b) 240

600 575 550 525 500 475 450 425 400 375 350 325 300 275 250 225 200 175 150 125 100 75 50 25 0

220 200 180

TFC (mg QE/100g)

TPC (mg GAE/100g)

a)

140 120 100 80 60 40 20 0

BJ

c)

160

KJ

PC

CA

BJ

KJ

PC

CA

d)

40

5

30

FRAP (mM/100g)

Anthocyanin content (mg cyanidin-3-glucoside Eq/100g)

6 35

25 20 15

4

3

2

10 1

5

0

0 BJ

KJ

PC

CA

BJ

KJ

PC

CA

DPPH (%)

e) 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 BJ

KJ

PC

CA

Fig. 2. (a) Total phenolic content (mgGAE/100 g) (b) Total flavonoid content (mgQE/100 g) (c) Total anthocyanin content (mg cyanidin-3-glucoside Eq/100 g) (d) FRAP (μM/100 g) and (e) DPPH radical scavenging activity (%) of the four rice varieties.

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could be the presence of oligomeric proanthocyanins or condensed tannins in red rice (Oki et al., 2005). 3.4.2. Effect of cooking on total phenolics, flavonoids and antioxidant activity The results of the effect of open steam cooking on the four rice samples are presented in Fig. 3. It was observed that the total phenolic and flavonoid contents decreased drastically on cooking when compared to raw (Fig. 2). The drastic decrease in TPC and TFC could be due to thermal degradation of phenol and flavonoid compounds (Randhir, Kwon, & Shetty, 2008; Zhang, Chen, Li, Pei, & Liang, 2010) as these compounds are very sensitive to heat treatments (Ismail, Marjan, & Foong, 2004). Thermal processing, in a few studies on cereals other than rice, has been reported to cause significant destruction of total phenolic content (Finocchiaro et al., 2007; Parra, Saldivar, & Liu, 2007; Zielinski, Michalska, Piskula, & Kozlowska, 2006). Zhang and Hamauzu (2004) attributed the losses in phenolics during cooking to be due to breakdown or conversion of phenolics to other products as well as into vapour during cooking. Likewise, FRAP and DPPH activity of the cooked rice samples decreased compared to the raw ones. The decrease was more drastic in case of CA than PC. On cooking, PC recorded the highest levels of TPC, TFC, FRAP, and DPPH than the other rices. Retention in the four cooked rice after 15 min of keeping time was between 6.1 and 68.3% for TPC; between 3.5 and 19.0% for TFC; between 3.6 and 17.8% for FRAP and between 26.0 and 64.4% for DPPH. Fig. 3 also revealed that there was a spike in the TPC and FRAP values after 90 min of keeping time from the values recorded after

45 min of keeping time. On exposure to heat or temperature they have a tendency of breaking down into smaller stable forms which may or/not show antioxidant activity. In case of rice, maximum amount of the phenolic compounds are present in the bound form. On cooking, the cellular breakdown fecilitates release of these bound phenolics (Gawlik-Dziki, 2008). There is possibility that these phenolics could replace the free phenolics, maximum of which got destroyed during the cooking process. Another possible reason could be that during cooking and then on storage, some kind of chemical degradation reactions may occur in the carbohydrate and protein molecules of the rice. These degradation reactions could lead to the formation of some reducing compounds that may exhibit antioxidant activities. Maillard degradation products with hydrophilic properties may be formed that reach high levels but their antioxidant activity and effects on health have not been verified (Giovanelli & Lavelli, 2002). 4. Conclusions In general, the non-pigmented aromatic rice was found to possess properties closer to each other and the pigmented varieties had properties closer to each other. Both the waxy PC and CA had lesser abilities to take up water during heating and cooking than the high amylose BJ and KJ. PC (purple coloured) was considerably rich in anthocyanins. Antioxidant capacity was highest in CA (red colour). PC was comparatively superior to CA as it was able to retain more amounts of phytochemicals and antioxidant capacities on keeping of the cooked rice. The non-pigmented rice also showed some

b) 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

14

TFC (mgQE/100g)

12

8 6

2

90 KJ 15 KJ 45 KJ 9 PC 0 1 PC 5 4 PC 5 90 C A1 C 5 A4 C 5 A9 0

45

BJ

15

BJ

BJ

BJ

BJ

BJ

45 90 KJ 15 KJ 45 KJ 90 PC 1 PC 5 45 PC 9 C 0 A1 C 5 A4 C 5 A9 0

0

c)

d)

0.55

70

0.50

65 60

0.45

55

0.40

50

0.35

45

DPPH (%)

0.30 0.25 0.20

40 35 30 25 20

0.15

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0.10

45 BJ 90 KJ 15 KJ 45 KJ 90 PC 15 PC 45 PC 90 C A1 5 C A4 5 C A9 0

BJ

90 KJ 15 KJ 45 KJ 90 PC 1 PC 5 45 PC 9 C 0 A1 C 5 A4 C 5 A9 0

BJ

BJ

BJ

45

0 15

5

0.00

15

10

0.05

BJ

FRAP (mM/100g)

10

4

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TPC (mg GAE/100g)

a)

339

Fig.3. Changes in (a) Total phenolic content (mgGAE/100 g) (b) Total flavonoid content (mgQE/100 g) (c) FRAP (μM/100 g) and (d) DPPH radical scavenging activity (%) after keeping at 15 min, 45 min and 90 min of the four rice varieties after cooking.

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