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Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products
Food Chemistry xxx (2016) xxx–xxx
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Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products Oladejo Thomas Adepoju a,⇑, Oluwatosin Boyejo a, Paulina Olufunke Adeniji b a b
Department of Human Nutrition, Faculty of Public Health, College of Medicine, University of Ibadan, Ibadan, Nigeria Department of Hospitality and Tourism Management, Redeemer’s University, Ede, Osun State, Nigeria
a r t i c l e
i n f o
Article history: Received 23 March 2016 Received in revised form 14 September 2016 Accepted 16 October 2016 Available online xxxx Keywords: Yellow yam Processing methods Nutrient retention Antinutrients
a b s t r a c t There is dearth of documented information on nutrient retention of Dioscorea cayenensis products. This study was carried out to evaluate effects of processing methods on nutrient and antinutrient retention of yellow yam products. Fresh Dioscorea cayenensis tubers were purchased from Bodija market in Ibadan, peeled, cut into small pieces, divided into nine portions. One portion was treated as raw sample, and others processed into local delicacies. All nine samples were analysed for proximate, mineral, vitamin and antinutrient composition using AOAC methods. Data were analysed using ANOVA at p = 0.05. Raw yam contained 66.79 g moisture, 2.62 g crude protein, 0.27 g lipid, 0.17 g fibre, 0.63 g ash, 29.69 g carbohydrates, 262.30 mg potassium, 61.53 mg magnesium, 0.79 mg iron, 0.39 mg zinc, and yielded 108.26 kcal energy with insignificant vitamin content/100 g edible portion. Processing significantly improved macronutrients and energy content with significant reduction in all antinutrients of products (p < 0.05). The yam products can serve as staple source of energy to consumers. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The nutritional value of yams lies in their potential ability to provide one of the cheapest sources of dietary energy in the form of carbohydrates in developing countries (Ofosu, 2012; Otegbayo, Achidi, Asiedu, & Bokanga, 2001). Yam (Dioscorea spp.), an annual or perennial climbing plant with edible underground tuber is a multi-species crop that originated principally from Africa and Asia before spreading to other parts of the world, being native to warmer regions of both Southern and Northern hemispheres (Abiodun, Adegbite, & Oladipo, 2009; IITA, 2006). Yam belongs to the family Dioscoreaceae within the genus Dioscorea (Otegbayo, 2004) and serves as a staple crop in West Africa (Asiedu, Vuylsteke, Terauchi, & Hahn, 1992). It is a high value crop (IITA, 2006) that forms about 10% of the total roots and tubers produced worldwide (FAO, 2002), and is the third most important tropical root and tuber crop after cassava and sweet potato (Fu, Huang, & Chu, 2005). About 48.7 million tonnes of yams were produced on five million hectares in about 47 countries worldwide in 2005, and 97% of this was in sub-Saharan Africa (FAO, 2008). West and Central Africa account for 94% of world production, Nigeria
⇑ Corresponding author.
being the leading producer with 34 million tonnes and an average per caput consumption of 258 kcal/day (IITA, 2009). In West Africa the most economically important yam species are the White yam (Dioscorea rotundata), Yellow yam (Dioscorea cayenensis) and Water yam (Dioscorea alata) (Otegbayo, Aina, Asiedu, & Bokanga, 2006). Yam tubers have been used as traditional food in the home with little industrial use; however the traditional uses are diverse and the crop has more utilization potentials. Yam is consumed in different forms, mainly boiled, fried, or baked. Tubers are often dried and milled into flour for various products. Boiled yam could also be pounded and eaten with sauce. Yam can be fried or roasted as snacks. Another processed product is pottage which is usually prepared with other ingredients such as onions, pepper, a protein source, oil, and so on (Baah, 2009). Boiled yam, pounded yam and Amala are the forms of yam most consumed in West Africa, especially in Nigeria and Benin (Akissoe et al., 2001) Most domestic cooking methods have effects on nutritional quality of foods by reducing the level of antinutrients and some nutrients, while enhancing other nutrients as well (Adepoju, Adekola, Mustapha, & Ogunola, 2010; Kataria & Chauhan, 1988; Oste, 1991). Despite the fact that yellow yam (Fig. 1) serves as a staple which can be prepared into different menus, it has been less studied compared to other root and tuber crops (Hoover, 2001). This might be
E-mail address: [email protected] (O.T. Adepoju). http://dx.doi.org/10.1016/j.foodchem.2016.10.071 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Fig. 1. Raw yellow yam.
due to its lesser popularity compared with white yam varieties. Intra and inter varietal diversity of foods is being promoted as a means of dietary diversity to tackle malnutrition in all its ramifications, and providing nutrition information about yellow yam may promote its popularity and importance. This study was carried out to determine the nutrient composition of raw yellow yam and effects of processing methods on nutrient and antinutrient content of some of its products.
2. Materials and methods 2.1. Sample collection and preparation Fresh yellow yam tubers (D. cayenenesis) were purchased from Bodija market in Ibadan, Oyo State, Nigeria. The tubers were cut into pieces and divided into two sections. The first section was divided into two portions. The first portion was peeled and 500 g of it was used as raw yam, labelled as Sample 1. The second portion was roasted unpeeled with coal fire on iron wire mesh, the burnt skin scraped with knife to remove the black burnt portion and 500 g portion of the scraped yam used and labelled Sample 2. The second section of the fresh yam was peeled and cut into small round pieces, mixed thoroughly, washed with distilled water, and then divided into five portions of 500 g each. One portion was further sliced into smaller pieces, washed with distilled water and drained. Five hundred grammes (500 g) of the washed portion was taken, 1 g of salt was added, thoroughly mixed and fried with vegetable oil, a little water being added to allow the yam to get done and also to reduce drying of frying oil. This was labelled sample 3. To 500 g of the second portion, 1 g of salt was added and boiled to dryness with distilled water at 100 °C (about 30minutes) and then divided into two smaller portions treated as follows: the first smaller portion served as boiled yam and labelled as Sample 4, while the remaining smaller portion was boiled with palm oil and a cube of magi, mashed when soft to yield porridge and labelled as Sample 5. The third portion (500 g) of the peeled, sliced raw yam was washed with distilled water and boiled at 100 °C for 30minutes without addition of salt, and then divided
into two sub-portions. The first sub-portion was pounded with mortar and pestle with ordinary water and labelled as Sample 6, while the second sub-portion was pounded with the water used in cooking the yam, and labelled as Sample 7 (Adepoju, 2012). The fourth portion of the peeled, washed and sliced raw yam (500 g) was washed with distilled water, mashed with a warring blender with addition of distilled water and mixed with 1 g of salt and 5 g of powdered dry pepper. The slurry paste obtained was moulded into balls and fried with vegetable oil to produce fried yam cake (Ojojo), and then labelled as Sample 8. The fifth portion of peeled washed and sliced raw yam (500 g) was parboiled at 60 °C for 10minutes, left overnight in the warm water (18 h), drained and sun dried for 4 days. The dried parboiled yam was then grinded to powder, prepared with boiling water to form a thick paste called Amala (Ihekoronye & Ngoddy, 1985) and labelled as Sample 9. 2.2. Chemical analyses 2.2.1. Proximate composition analysis The moisture content of the samples was determined by air oven method (Gallenkamp, Model OV – 440, England) at 105 °C (AOAC, 2005 (967.08)). The crude protein of the samples was determined using micro-Kjeldahl method and the amount of crude protein calculated by multiplying percentage nitrogen in the digest by 6.25 (AOAC, 2005 (988.05)). Crude lipid was determined by Soxhlet extraction method (AOAC, 2005 (2003.06)). The ash content was determined by muffle furnace at 550 °C for 4 h, and ash calculated as g/100 g original fresh sample (AOAC, 2005 (942.05)). Crude fibre was determined using the AOAC 958.06 method while the carbohydrate content was obtained by difference. Gross energy of the samples was determined using ballistic bomb calorimeter (Manufacturer: Cal 2 k – Eco, TUV Rheinland Quality Services (Pty) Ltd, South Africa). 2.2.2. Mineral composition analysis Potassium and sodium content of the samples were determined by digesting the ash of the samples with perchloric acid and nitric acid, and then taking the readings on Jenway digital flame
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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photometer/spectronic20 (AOAC, 2005: (975.11)). Phosphorus was determined by vanado-molybdate colorimetric method (AOAC, 2005: (975.16)). Calcium, magnesium, iron zinc, manganese, copper and selenium were determined spectrophotometrically, by using Buck 200 atomic absorption spectrophotometer (Buck Scientific, Norwalk) and compared with absorption of standards of these minerals (AOAC, 2005: (975.23); Adepoju & Etukumoh, 2014; Bamigboye & Adepoju, 2015). 2.2.3. Vitamin analysis 2.2.3.1. b-Carotene determination. The b-carotene content of the yam was determined through ultraviolet absorption measurement at 328 nm after extraction with chloroform. Calibration curve of bcarotene standard solutions was made and the sample b-carotene concentration estimated as microgram (lg) of b-carotene/100 g sample (AOAC, 2005 (1893)). 2.2.3.2. Thiamine (vitamin B1) determination. Thiamine content of the samples was determined by measuring the absorbance of the sample solution and that of the standards on a fluorescent UV Spectrophotometer (Cecil A20 Model) at a wavelength of 285 nm (AOAC, 2005, (1893)). 2.2.3.3. Riboflavin (vitamin B2) determination. Riboflavin content of the sample was determined by measuring the absorbance of the sample solution on the fluorescent spectrophotometer at a wavelength of 460 nm and compare the absorbance with that of standard solutions of riboflavin at 460 nm (AOAC, 2005, (1893)), and the sample riboflavin obtained through calculation. 2.2.3.4. Ascorbic Acid Determination. Ascorbic acid in the sample was determined by titrating its aqueous extract with solution of 2,6-dichlorophenol-indophenol dye to a faint pink end point (AOAC, 2005, (1893)).
2.2.4. Antinutrient analysis Oxalate was determined by extraction of the samples with water for about three hours and standard solutions of oxalic acid prepared and read on spectrophotometer (Spectronic20) at 420 nm (Meena, Umapathy, Pankaja, & Prakash, 1987). The absorbance of the samples was also read and amount of oxalate estimated. Phytate was determined by titration with ferric chloride solution (Sudarmadji & Markakis, 1977). The tannin content was determined by extracting the samples with a mixture of acetone and acetic acid for five hours, measuring their absorbance and comparing the absorbance of the sample extracts with the absorbance of standard solutions of tannic acid at 500 nm on spectronic20 (Adepoju & Etukumoh, 2014; Bamigboye & Adepoju, 2015; Griffiths & Jones, 1977). Saponin was also determined by comparing the absorbance of the sample extracts with that of the standard at 380 nm (Makkar & Becker, 1996). The % apparent retention (AR) of nutrients was calculated by the formula of Murphy, Criner, and Gray (1975):
% AR ¼
Nutrient content per g of cooked food on dry basis 100 Nutrient content per g of raw food on dry basis
Data were analysed using analysis of variance and level of significance set at p < 0.05.
3. Results The result of proximate nutrient composition of raw yellow yam and its processed products are shown in Table 1. Raw yellow yam was high in moisture content, moderate in carbohydrate, ash and gross energy, but low in crude protein, lipid, and fibre. There was significant reduction in moisture content of samples 2, 3, and 8 (p < 0.05), significant increase in samples 6 and 7 (p < 0.05), and slight insignificant increase in samples 4, 5, and 9
Table 1 Proximate nutrient composition of fresh and processed (As consumed) yellow yam products (g/100 g)*. Sample
MC
CP
CL
CF
Ash
CHO
GE
1 2 3 4 5 6 7 8 9
66.79 ± 0.44 57.85 ± 2.31 46.88 ± 1.82 69.50 ± 0.20 69.20 ± 0.20 75.37 ± 0.47 75.39 ± 0.47 54.09 ± 2.09 68.63 ± 0.29
2.62 ± 0.05 3.02 ± 0.15 6.25 ± 0.12 2.18 ± 0.04 6.37 ± 0.31 1.62 ± 0.01 1.73 ± 0.03 6.83 ± 0.58 1.21 ± 0.05
0.27 ± 0.01 0.46 ± 0.02 7.12 ± 0.18 0.27 ± 0.01 4.76 ± 0.18 0.19 ± 0.01 0.19 ± 0.01 8.59 ± 0.65 0.04 ± 0.01
0.17 ± 0.01 0.20 ± 0.02 0.42 ± 0.01 0.13 ± 0.01 0.30 ± 0.02 0.08 ± 0.01 0.19 ± 0.53 0.53 ± 0.04 0.07 ± 0.01
0.63 ± 0.02 1.43 ± 0.06 2.07 ± 0.07 1.39 ± 0.03 2.29 ± 0.09 1.07 ± 0.01 1.11 ± 0.01 3.03 ± 0.22 0.41 ± 0.03
29.52 ± 0.82 37.04 ± 1.66 37.26 ± 1.79 26.53 ± 0.20 17.08 ± 0.09 21.67 ± 0.20 21.19 ± 0.20 26.93 ± 0.25 29.64 ± 0.22
108.26 ± 0.03 144.81 ± 0.15 208.51 ± 0.05 100.48 ± 0.07 100.32 ± 0.07 80.77 ± 0.35 81.89 ± 0.30 196.49 ± 0.20 106.30 ± 0.49
MC = Moisture Content, CP = Crude Protein, CL = Crude Lipid, CF = Crude Fibre, CHO** = Total Carbohydrates, GE = Gross Energy. Sample 1: Raw yam, Sample 2: Roasted yam, Sample 3: Fried yam, Sample 4: Boiled yam, Sample 5: Porridge, Sample 6: Pounded yam with ordinary water, Sample 7: Pounded yam with cooking water, Sample 8: Ojojo, Sample 9: Amala. * Values are means ± SD of triplicate determinations.
Table 2 Mineral composition of fresh and processed (As consumed) yellow yam products (mg/100 g)*.
*
Sample
Na
K
Ca
Mg
P
Fe
Zn
1 2 3 4 5 6 7 8 9
8.53 ± 0.05 6.36 ± 0.09 6.55 ± 0.11 4.86 ± 0.07 12.36 ± 0.08 4.03 ± 0.52 3.78 ± 0.07 17.19 ± 0.09 9.53 ± 0.31
262.30 ± 0.25 294.59 ± 0.12 326.99 ± 0.14 217.81 ± 0.08 293.15 ± 0.05 173.01 ± 0.06 176.09 ± 0.07 428.03 ± 0.31 206.53 ± 0.35
22.53 ± 0.13 17.79 ± 0.15 12.74 ± 0.14 13.68 ± 0.05 22.05 ± 0.10 9.98 ± 0.05 10.92 ± 0.07 32.24 ± 0.25 23.43 ± 0.25
61.53 ± 0.25 10.92 ± 0.14 9.29 ± 0.11 8.48 ± 0.05 18.95 ± 0.09 6.58 ± 0.07 6.81 ± 0.09 27.51 ± 0.12 24.80 ± 0.30
19.50 ± 0.10 26.99 ± 0.16 22.74 ± 0.14 19.95 ± 0.10 23.10 ± 0.13 15.75 ± 0.07 6.81 ± 0.09 33.40 ± 0.22 28.83 ± 0.30
0.79 ± 0.02 0.63 ± 0.01 0.59 ± 0.02 0.47 ± 0.01 1.04 ± 0.01 0.35 ± 0.01 0.37 ± 0.03 1.52 ± 0.02 1.02 ± 0.03
0.39 ± 0.01 0.24 ± 0.02 0.18 ± 0.02 0.22 ± 0.06 0.74 ± 0.01 0.16 ± 0.01 0.17 ± 0.01 1.07 ± 0.02 0.19 ± 0.02
Values are means ± SD of triplicate determinations.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Table 3 Selected vitamin composition of fresh and processed (As consumed) yellow yam products (mg/100 g)*.
*
Table 4 Antinutrient composition of fresh and processed (As consumed) yellow yam Products (mg/100 g).*
Sample
b-Carotene (lg/)
Thiamine
Riboflavin
Ascorbic acid
Sample
Phytate
Oxalate
Tannin
Saponin
1 2 3 4 5 6 7 8 9
7.65 ± 0.03 11.32 ± 0.02 31.20 ± 0.02 7.53 ± 0.01 40.67 ± 0.01 5.13 ± 0.01 5.61 ± 0.01 95.21 ± 0.02 6.18 ± 0.03
0.14 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.07 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.12 ± 0.01 0.05 ± 0.01
0.06 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.05 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.09 ± 0.01 0.00 ± 0.00
14.38 ± 0.03 2.13 ± 0.01 2.07 ± 0.02 1.44 ± 0.01 3.76 ± 0.01 1.09 ± 0.01 1.15 ± 0.01 3.81 ± 0.02 3.27 ± 0.02
1 2 3 4 5 6 7 8 9
0.05 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.02 ± 0.00 0.04 ± 0.00 0.04 ± 0.00 0.03 ± 0.00 0.02 ± 0.00
48 ± 0.00 0.39 ± 0.01 0.34 ± 0.00 0.38 ± 0.00 0.13 ± 0.00 0.36 ± 0.00 0.37 ± 0.00 0.12 ± 0.00 0.11 ± 0.00
0.01 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.00 ± 0.00
0.06 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.02 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.03 ± 0.00 0.01 ± 0.00
Values are means ± SD of triplicate determinations.
compared with raw sample (sample 1). Significant reduction occurred in crude protein content of samples 4, 6, 7, and 9 (p < 0.05) while significant increase was observed in samples 2, 3, 5, and 8 compared with sample 1 (p < 0.05). Significant decrease also occurred in crude lipid content of samples 6, 7 and 9 while increase was observed in samples 2, 3, 4, 5 and 8 (p < 0.05). Roasting, frying and preparation of the yam to porridge and Ojojo (Samples 2, 3, 5 and 8 respectively) significantly increased the protein, lipid, fibre and ash content of the products (p < 0.05), the increase more pronounced in porridge (Sample 5) and Ojojo (Sample 8). Boiling and pounding resulted in significant reduction in value of crude protein, lipid and fibre content of the products. Roasting, frying and preparation of the yam to Amala brought significant increase in the carbohydrate and gross energy content, while boiling and pounding reduced the carbohydrate content of products compared with the raw yam sample (sample 1). Table 2 shows the mineral composition of raw and processed yellow yam samples. Raw yellow yam (Sample 1) was very low in sodium, calcium, magnesium and phosphorus, low in iron and zinc, but moderate in potassium compared with daily human requirements of these minerals. Processing yam into various products resulted into highly significant decrease in mineral content of the products compared with the raw sample (p < 0.05). However roasting, frying, and processing yam into porridge and Ojojo (Samples 2, 3, 5 and 8) resulted in significant increase in potassium content of the products compared with raw yam sample (p < 0.05). Table 3 shows the vitamin composition of raw and processed products of yellow yam. Raw yellow yam was very low in beta carotene, thiamine and riboflavin, but contained substantial amount of ascorbic acid. Roasting and frying significantly increased the beta carotene content of the products (Samples 2 and 3) but completely removed the thiamine and riboflavin while drastically reducing the ascorbic acid content of the products (p < 0.05). Boiling significantly reduced the beta carotene and ascorbic acid content of the products (Samples 4, 6, 7) while completely removing the thiamine and riboflavin from the products (p < 0.05). Preparing the yam to porridge and Ojojo improved the beta carotene content of the products. Table 4 shows the antinutrient composition of raw and various processed yam products. The raw yam sample (Sample 1) was very low in antinutrients. All the processing methods further resulted in reduction in content of the antinutrients studied. The percent apparent nutrient retention in products (Samples 2, 4, 6, 7 and 9) are shown in Table 5. Roasted yam (Sample 2) had the highest level of retention for most of the nutrients followed by boiled yam (Sample 4). Pounded yam with the cooking water (Sample 7) retained more nutrients compared with pounded yam with ordinary water (Sample 6), while Amala (Sample 9) had the lowest nutrient retention among all samples. The apparent nutrient retention of the samples (Samples 3, 5, and 8) could not be
*
Values are means ± SD of triplicate determinations.
Table 5 Percent nutrient retention by different cooking methods. Sample
Parameter Crude protein Crude lipid Crude fibre Carbohydrate Sodium Potassium Calcium Magnesium Phosphorus Iron Zinc b-Carotene Ascorbic acid
2
4
6
7
9
115.27 170.37 117.65 125.47 74.56 112.31 78.96 17.75 138.41 79.75 61.54 147.97 14.81
83.21 100.00 76.47 89.87 56.98 83.04 62.72 13.78 102.31 59.49 56.41 98.43 10.01
61.83 70.37 47.06 73.41 47.25 65.96 44.30 10.60 80.77 44.30 41.03 67.06 7.58
66.03 70.37 111.76 71.78 44.31 67.13 48.47 11.07 34.92 46.84 43.59 73.33 8.00
46.18 14.81 41.18 100.00 111.72 78.74 103.99 40.31 147.85 129.11 125.64 80.78 22.74
compared with that of the raw sample because of the added ingredients which influence the retention levels. 4. Discussion 4.1. Proximate composition Raw yellow yam was very high in moisture, high carbohydrate and gross energy, but low in crude protein, lipid, and ash content. The values obtained for the moisture content and crude protein of yellow yam is similar to the values reported by Ofosu (2012). The high gross energy content of yam explains why it serves as a staple source of energy in Nigeria. The values obtained for crude protein, lipid, and ash of raw yam sample were in agreement with the value reported by Adepoju (2012) for white yam (Dioscorea rotundata). The high moisture content of the raw yellow yaw predisposes it to easy spoilage and short shelf life. Roasting, frying, and processing the yellow yam to porridge and Ojojo (samples 2, 3, 5, and 8 respectively) significantly improved the proximate composition and energy content of the products. The significant improvement in the nutrient and energy content of the samples is believed to be partly due to reduction in the moisture content of the samples (as consumed) compared with the raw sample, and partly due to addition of ingredients such as vegetable oil, palm oil and salt to some of the products. Processing has been reported to enhance nutrient availability in processed foods (Kataria & Chauhan, 1988; Oste, 1991). The observed increase in fat and energy content of fried yam, porridge and Ojojo was a direct result of contribution from vegetable oil used in frying, and palm oil and other ingredients used in preparing porridge. Fat and oil contribute higher percentage of energy compared with carbohydrates and proteins.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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However, boiling resulted in increase in moisture content of yellow yam with attendant decrease in crude protein, lipid, fibre and carbohydrate and energy content of the products (Samples 4, 6, 7) compared with raw sample. Pounding the yam with the cooking water retained more nutrients and fibre than the sample in which ordinary water was used in pounding the yam (samples 7 and 6 respectively, Table 5). This was an indication that some of these nutrients leached into the cooking water which is usually discarded, and that the yam contained significant amount of soluble fibre. This observation was in line with the findings of Bradbury, Bradshaw, Jealous, Holloway, and Phimpisame (1998), Adepoju et al. (2010); and Adepoju (2012) who reported nutrient loss by cooking and soaking food items for a period of time through leaching. The significant reduction in all nutrients and gross energy content observed in Amala (sample 9) compared with all other samples might have resulted from the parboiling coupled with soaking in water overnight, leading to leaching of the nutrients into the soaking water. 4.2. Mineral composition The result of mineral composition of raw and processed yellow yam is shown in Table 2. Raw yellow yam (Sample 1) was very low in sodium, calcium, and phosphorus, low in magnesium, iron, and zinc but high in potassium. The very low level of sodium compared with potassium is a good advantage for the yam to be suitable for consumption by hypertension patients where sodium: potassium ratio is expected to be low (Ifon & Bassir, 1979). Generally, processing yellow yam into different products resulted in significant reduction in value for most of the minerals, the reduction being more pronounced when boiling was involved (Samples 4, 6, 7, 9). However, roasting (Sample 2) and frying (Sample 3) significantly increased the potassium and phosphorus content of the products compared with the raw sample. Also, preparing the yam into porridge (Sample 5) and Ojojo (Sample 8) significantly improved the sodium, potassium, calcium, phosphorus, iron and zinc content of the products compared with the raw sample (Sample 1). The observed increase in the mineral content of these products was believed to be contributed in part by the added salt and ingredients, and in part by bioconversion of components of the yam, leading to significant reduction in value of antinutrients such as phytates and oxalates which combine with these minerals. The observed pattern in mineral content variation of the yellow yam products is generally similar to that of white yam reported in the literature (Adepoju, 2012). There was a highly significant reduction in some mineral content of boiled and pounded yam samples (Samples 4, 6 and 7). This was believed to be due to the leaching of the minerals into the boiling water (Bradbury et al., 1998). Adepoju et al. (2010) reported that soaking of food items in water results in loss of minerals due to leaching into the soaking water. Yam pounded with its cooked water (Sample 7) retained more minerals compared with both boiled yam and yam pounded with ordinary distilled water (Samples 4 and 6, Table 5), thereby confirming leaching of these minerals into the boiling water, which is usually discarded. Processing the yam to Amala (Sample 9) led to significant increase in sodium, calcium, phosphorus, iron and zinc content of the product compared with the raw sample. This was similar to what was obtained for white yam Amala in the literature (Adepoju, 2012). The significant increase in the mineral content might have resulted from interconversion and release of these minerals from the chemically bound form as phytates to free form, with attendant decrease in phytate content of the product. Phosphorus is a constituent of phytic acid which combines with heavy metals such as calcium, iron and zinc,(Reddy & Salunkhe, 1982;
5
Makkar & Becker, 1996), thereby reducing their bioavailability. The release of these minerals from their bound state could have led to the observed increase in their values.
4.3. Vitamins composition Table 3 shows the some selected vitamin composition of raw and processed yam products. The raw yam sample (Sample 1) was very low in b-carotene, thiamine and riboflavin but moderate in ascorbic acid content. The low value of the b-carotene content of the yam was suggestive of the fact that the yellow colouration was not totally due to presence of b-carotene in the yam. Processing yellow yam into various products such as roasted yam (Sample 2), fried yam (Sample 3), porridge (Sample 5), and Ojojo (Sample 8) significantly increased the b-carotene content of the products but resulted in significant reduction in water soluble vitamins (p<0.05) compared to raw yam sample (Sample 1). The observed increase in the b-carotene content of the processed samples was believed to be due to either reduction in moisture content of the sample or addition of vegetable oil during the processing of the sample, while the significant reduction was believed to be due to either heat destruction or loss through leaching of these vitamins into the cooking water (Adepoju et al., 2010; Bradbury et al., 1998). Ojojo had the highest value of beta carotene, followed by porridge. The increase was believed to be due to contribution of additional ingredients used in preparing them.
4.4. Antinutrient composition The level of all antinutrients in the raw yellow yam (Sample 1) was very low (Table 4), and is unlikely to constitute any hindrance to digestibility of nutrients from other food sources in the human body. The different processing methods generally led to significant reduction in the levels of antinutrients in the processed samples. This finding supports the fact that processing significantly reduce the level of antinutrients in processed foods (Bhandari & Kawabata, 2006). The levels of antinutrients present in the products were very negligible, and are unlikely to constitute any hindrance to utilization or bioavailability of the nutrients.
5. Conclusion Raw yellow yam was high in moisture and carbohydrate content with moderate gross energy compared with its other nutrients. Processing the yam to different products led to improvement in the macronutrients with moderate gross energy, hence they are suitable for consumption by everyone. The low sodium, lipid and antinutrient content coupled with improved mineral and b-carotene values of the products qualify them for consumption by everyone. Pounding yam with the boiling water retained greater part of the nutrients. Yellow yam products are moderate in gross energy content, and can serve as good source of energy to consumers. It is advisable that the yam should be consumed in roasted form so as to retain greater part of the nutrients, or cooked with calculated amount of water when not being prepared into fried yam, porridge or Ojojo to prevent decantation which results in nutrient loss.
Declation of conflict of interest The authors hereby declare that there is no conflict of interest whatsoever on this paper, as it was solely funded by them.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products Oladejo Thomas Adepoju a,⇑, Oluwatosin Boyejo a, Paulina Olufunke Adeniji b a b
Department of Human Nutrition, Faculty of Public Health, College of Medicine, University of Ibadan, Ibadan, Nigeria Department of Hospitality and Tourism Management, Redeemer’s University, Ede, Osun State, Nigeria
a r t i c l e
i n f o
Article history: Received 23 March 2016 Received in revised form 14 September 2016 Accepted 16 October 2016 Available online xxxx Keywords: Yellow yam Processing methods Nutrient retention Antinutrients
a b s t r a c t There is dearth of documented information on nutrient retention of Dioscorea cayenensis products. This study was carried out to evaluate effects of processing methods on nutrient and antinutrient retention of yellow yam products. Fresh Dioscorea cayenensis tubers were purchased from Bodija market in Ibadan, peeled, cut into small pieces, divided into nine portions. One portion was treated as raw sample, and others processed into local delicacies. All nine samples were analysed for proximate, mineral, vitamin and antinutrient composition using AOAC methods. Data were analysed using ANOVA at p = 0.05. Raw yam contained 66.79 g moisture, 2.62 g crude protein, 0.27 g lipid, 0.17 g fibre, 0.63 g ash, 29.69 g carbohydrates, 262.30 mg potassium, 61.53 mg magnesium, 0.79 mg iron, 0.39 mg zinc, and yielded 108.26 kcal energy with insignificant vitamin content/100 g edible portion. Processing significantly improved macronutrients and energy content with significant reduction in all antinutrients of products (p < 0.05). The yam products can serve as staple source of energy to consumers. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The nutritional value of yams lies in their potential ability to provide one of the cheapest sources of dietary energy in the form of carbohydrates in developing countries (Ofosu, 2012; Otegbayo, Achidi, Asiedu, & Bokanga, 2001). Yam (Dioscorea spp.), an annual or perennial climbing plant with edible underground tuber is a multi-species crop that originated principally from Africa and Asia before spreading to other parts of the world, being native to warmer regions of both Southern and Northern hemispheres (Abiodun, Adegbite, & Oladipo, 2009; IITA, 2006). Yam belongs to the family Dioscoreaceae within the genus Dioscorea (Otegbayo, 2004) and serves as a staple crop in West Africa (Asiedu, Vuylsteke, Terauchi, & Hahn, 1992). It is a high value crop (IITA, 2006) that forms about 10% of the total roots and tubers produced worldwide (FAO, 2002), and is the third most important tropical root and tuber crop after cassava and sweet potato (Fu, Huang, & Chu, 2005). About 48.7 million tonnes of yams were produced on five million hectares in about 47 countries worldwide in 2005, and 97% of this was in sub-Saharan Africa (FAO, 2008). West and Central Africa account for 94% of world production, Nigeria
⇑ Corresponding author.
being the leading producer with 34 million tonnes and an average per caput consumption of 258 kcal/day (IITA, 2009). In West Africa the most economically important yam species are the White yam (Dioscorea rotundata), Yellow yam (Dioscorea cayenensis) and Water yam (Dioscorea alata) (Otegbayo, Aina, Asiedu, & Bokanga, 2006). Yam tubers have been used as traditional food in the home with little industrial use; however the traditional uses are diverse and the crop has more utilization potentials. Yam is consumed in different forms, mainly boiled, fried, or baked. Tubers are often dried and milled into flour for various products. Boiled yam could also be pounded and eaten with sauce. Yam can be fried or roasted as snacks. Another processed product is pottage which is usually prepared with other ingredients such as onions, pepper, a protein source, oil, and so on (Baah, 2009). Boiled yam, pounded yam and Amala are the forms of yam most consumed in West Africa, especially in Nigeria and Benin (Akissoe et al., 2001) Most domestic cooking methods have effects on nutritional quality of foods by reducing the level of antinutrients and some nutrients, while enhancing other nutrients as well (Adepoju, Adekola, Mustapha, & Ogunola, 2010; Kataria & Chauhan, 1988; Oste, 1991). Despite the fact that yellow yam (Fig. 1) serves as a staple which can be prepared into different menus, it has been less studied compared to other root and tuber crops (Hoover, 2001). This might be
E-mail address: [email protected] (O.T. Adepoju). http://dx.doi.org/10.1016/j.foodchem.2016.10.071 0308-8146/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Fig. 1. Raw yellow yam.
due to its lesser popularity compared with white yam varieties. Intra and inter varietal diversity of foods is being promoted as a means of dietary diversity to tackle malnutrition in all its ramifications, and providing nutrition information about yellow yam may promote its popularity and importance. This study was carried out to determine the nutrient composition of raw yellow yam and effects of processing methods on nutrient and antinutrient content of some of its products.
2. Materials and methods 2.1. Sample collection and preparation Fresh yellow yam tubers (D. cayenenesis) were purchased from Bodija market in Ibadan, Oyo State, Nigeria. The tubers were cut into pieces and divided into two sections. The first section was divided into two portions. The first portion was peeled and 500 g of it was used as raw yam, labelled as Sample 1. The second portion was roasted unpeeled with coal fire on iron wire mesh, the burnt skin scraped with knife to remove the black burnt portion and 500 g portion of the scraped yam used and labelled Sample 2. The second section of the fresh yam was peeled and cut into small round pieces, mixed thoroughly, washed with distilled water, and then divided into five portions of 500 g each. One portion was further sliced into smaller pieces, washed with distilled water and drained. Five hundred grammes (500 g) of the washed portion was taken, 1 g of salt was added, thoroughly mixed and fried with vegetable oil, a little water being added to allow the yam to get done and also to reduce drying of frying oil. This was labelled sample 3. To 500 g of the second portion, 1 g of salt was added and boiled to dryness with distilled water at 100 °C (about 30minutes) and then divided into two smaller portions treated as follows: the first smaller portion served as boiled yam and labelled as Sample 4, while the remaining smaller portion was boiled with palm oil and a cube of magi, mashed when soft to yield porridge and labelled as Sample 5. The third portion (500 g) of the peeled, sliced raw yam was washed with distilled water and boiled at 100 °C for 30minutes without addition of salt, and then divided
into two sub-portions. The first sub-portion was pounded with mortar and pestle with ordinary water and labelled as Sample 6, while the second sub-portion was pounded with the water used in cooking the yam, and labelled as Sample 7 (Adepoju, 2012). The fourth portion of the peeled, washed and sliced raw yam (500 g) was washed with distilled water, mashed with a warring blender with addition of distilled water and mixed with 1 g of salt and 5 g of powdered dry pepper. The slurry paste obtained was moulded into balls and fried with vegetable oil to produce fried yam cake (Ojojo), and then labelled as Sample 8. The fifth portion of peeled washed and sliced raw yam (500 g) was parboiled at 60 °C for 10minutes, left overnight in the warm water (18 h), drained and sun dried for 4 days. The dried parboiled yam was then grinded to powder, prepared with boiling water to form a thick paste called Amala (Ihekoronye & Ngoddy, 1985) and labelled as Sample 9. 2.2. Chemical analyses 2.2.1. Proximate composition analysis The moisture content of the samples was determined by air oven method (Gallenkamp, Model OV – 440, England) at 105 °C (AOAC, 2005 (967.08)). The crude protein of the samples was determined using micro-Kjeldahl method and the amount of crude protein calculated by multiplying percentage nitrogen in the digest by 6.25 (AOAC, 2005 (988.05)). Crude lipid was determined by Soxhlet extraction method (AOAC, 2005 (2003.06)). The ash content was determined by muffle furnace at 550 °C for 4 h, and ash calculated as g/100 g original fresh sample (AOAC, 2005 (942.05)). Crude fibre was determined using the AOAC 958.06 method while the carbohydrate content was obtained by difference. Gross energy of the samples was determined using ballistic bomb calorimeter (Manufacturer: Cal 2 k – Eco, TUV Rheinland Quality Services (Pty) Ltd, South Africa). 2.2.2. Mineral composition analysis Potassium and sodium content of the samples were determined by digesting the ash of the samples with perchloric acid and nitric acid, and then taking the readings on Jenway digital flame
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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photometer/spectronic20 (AOAC, 2005: (975.11)). Phosphorus was determined by vanado-molybdate colorimetric method (AOAC, 2005: (975.16)). Calcium, magnesium, iron zinc, manganese, copper and selenium were determined spectrophotometrically, by using Buck 200 atomic absorption spectrophotometer (Buck Scientific, Norwalk) and compared with absorption of standards of these minerals (AOAC, 2005: (975.23); Adepoju & Etukumoh, 2014; Bamigboye & Adepoju, 2015). 2.2.3. Vitamin analysis 2.2.3.1. b-Carotene determination. The b-carotene content of the yam was determined through ultraviolet absorption measurement at 328 nm after extraction with chloroform. Calibration curve of bcarotene standard solutions was made and the sample b-carotene concentration estimated as microgram (lg) of b-carotene/100 g sample (AOAC, 2005 (1893)). 2.2.3.2. Thiamine (vitamin B1) determination. Thiamine content of the samples was determined by measuring the absorbance of the sample solution and that of the standards on a fluorescent UV Spectrophotometer (Cecil A20 Model) at a wavelength of 285 nm (AOAC, 2005, (1893)). 2.2.3.3. Riboflavin (vitamin B2) determination. Riboflavin content of the sample was determined by measuring the absorbance of the sample solution on the fluorescent spectrophotometer at a wavelength of 460 nm and compare the absorbance with that of standard solutions of riboflavin at 460 nm (AOAC, 2005, (1893)), and the sample riboflavin obtained through calculation. 2.2.3.4. Ascorbic Acid Determination. Ascorbic acid in the sample was determined by titrating its aqueous extract with solution of 2,6-dichlorophenol-indophenol dye to a faint pink end point (AOAC, 2005, (1893)).
2.2.4. Antinutrient analysis Oxalate was determined by extraction of the samples with water for about three hours and standard solutions of oxalic acid prepared and read on spectrophotometer (Spectronic20) at 420 nm (Meena, Umapathy, Pankaja, & Prakash, 1987). The absorbance of the samples was also read and amount of oxalate estimated. Phytate was determined by titration with ferric chloride solution (Sudarmadji & Markakis, 1977). The tannin content was determined by extracting the samples with a mixture of acetone and acetic acid for five hours, measuring their absorbance and comparing the absorbance of the sample extracts with the absorbance of standard solutions of tannic acid at 500 nm on spectronic20 (Adepoju & Etukumoh, 2014; Bamigboye & Adepoju, 2015; Griffiths & Jones, 1977). Saponin was also determined by comparing the absorbance of the sample extracts with that of the standard at 380 nm (Makkar & Becker, 1996). The % apparent retention (AR) of nutrients was calculated by the formula of Murphy, Criner, and Gray (1975):
% AR ¼
Nutrient content per g of cooked food on dry basis 100 Nutrient content per g of raw food on dry basis
Data were analysed using analysis of variance and level of significance set at p < 0.05.
3. Results The result of proximate nutrient composition of raw yellow yam and its processed products are shown in Table 1. Raw yellow yam was high in moisture content, moderate in carbohydrate, ash and gross energy, but low in crude protein, lipid, and fibre. There was significant reduction in moisture content of samples 2, 3, and 8 (p < 0.05), significant increase in samples 6 and 7 (p < 0.05), and slight insignificant increase in samples 4, 5, and 9
Table 1 Proximate nutrient composition of fresh and processed (As consumed) yellow yam products (g/100 g)*. Sample
MC
CP
CL
CF
Ash
CHO
GE
1 2 3 4 5 6 7 8 9
66.79 ± 0.44 57.85 ± 2.31 46.88 ± 1.82 69.50 ± 0.20 69.20 ± 0.20 75.37 ± 0.47 75.39 ± 0.47 54.09 ± 2.09 68.63 ± 0.29
2.62 ± 0.05 3.02 ± 0.15 6.25 ± 0.12 2.18 ± 0.04 6.37 ± 0.31 1.62 ± 0.01 1.73 ± 0.03 6.83 ± 0.58 1.21 ± 0.05
0.27 ± 0.01 0.46 ± 0.02 7.12 ± 0.18 0.27 ± 0.01 4.76 ± 0.18 0.19 ± 0.01 0.19 ± 0.01 8.59 ± 0.65 0.04 ± 0.01
0.17 ± 0.01 0.20 ± 0.02 0.42 ± 0.01 0.13 ± 0.01 0.30 ± 0.02 0.08 ± 0.01 0.19 ± 0.53 0.53 ± 0.04 0.07 ± 0.01
0.63 ± 0.02 1.43 ± 0.06 2.07 ± 0.07 1.39 ± 0.03 2.29 ± 0.09 1.07 ± 0.01 1.11 ± 0.01 3.03 ± 0.22 0.41 ± 0.03
29.52 ± 0.82 37.04 ± 1.66 37.26 ± 1.79 26.53 ± 0.20 17.08 ± 0.09 21.67 ± 0.20 21.19 ± 0.20 26.93 ± 0.25 29.64 ± 0.22
108.26 ± 0.03 144.81 ± 0.15 208.51 ± 0.05 100.48 ± 0.07 100.32 ± 0.07 80.77 ± 0.35 81.89 ± 0.30 196.49 ± 0.20 106.30 ± 0.49
MC = Moisture Content, CP = Crude Protein, CL = Crude Lipid, CF = Crude Fibre, CHO** = Total Carbohydrates, GE = Gross Energy. Sample 1: Raw yam, Sample 2: Roasted yam, Sample 3: Fried yam, Sample 4: Boiled yam, Sample 5: Porridge, Sample 6: Pounded yam with ordinary water, Sample 7: Pounded yam with cooking water, Sample 8: Ojojo, Sample 9: Amala. * Values are means ± SD of triplicate determinations.
Table 2 Mineral composition of fresh and processed (As consumed) yellow yam products (mg/100 g)*.
*
Sample
Na
K
Ca
Mg
P
Fe
Zn
1 2 3 4 5 6 7 8 9
8.53 ± 0.05 6.36 ± 0.09 6.55 ± 0.11 4.86 ± 0.07 12.36 ± 0.08 4.03 ± 0.52 3.78 ± 0.07 17.19 ± 0.09 9.53 ± 0.31
262.30 ± 0.25 294.59 ± 0.12 326.99 ± 0.14 217.81 ± 0.08 293.15 ± 0.05 173.01 ± 0.06 176.09 ± 0.07 428.03 ± 0.31 206.53 ± 0.35
22.53 ± 0.13 17.79 ± 0.15 12.74 ± 0.14 13.68 ± 0.05 22.05 ± 0.10 9.98 ± 0.05 10.92 ± 0.07 32.24 ± 0.25 23.43 ± 0.25
61.53 ± 0.25 10.92 ± 0.14 9.29 ± 0.11 8.48 ± 0.05 18.95 ± 0.09 6.58 ± 0.07 6.81 ± 0.09 27.51 ± 0.12 24.80 ± 0.30
19.50 ± 0.10 26.99 ± 0.16 22.74 ± 0.14 19.95 ± 0.10 23.10 ± 0.13 15.75 ± 0.07 6.81 ± 0.09 33.40 ± 0.22 28.83 ± 0.30
0.79 ± 0.02 0.63 ± 0.01 0.59 ± 0.02 0.47 ± 0.01 1.04 ± 0.01 0.35 ± 0.01 0.37 ± 0.03 1.52 ± 0.02 1.02 ± 0.03
0.39 ± 0.01 0.24 ± 0.02 0.18 ± 0.02 0.22 ± 0.06 0.74 ± 0.01 0.16 ± 0.01 0.17 ± 0.01 1.07 ± 0.02 0.19 ± 0.02
Values are means ± SD of triplicate determinations.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Table 3 Selected vitamin composition of fresh and processed (As consumed) yellow yam products (mg/100 g)*.
*
Table 4 Antinutrient composition of fresh and processed (As consumed) yellow yam Products (mg/100 g).*
Sample
b-Carotene (lg/)
Thiamine
Riboflavin
Ascorbic acid
Sample
Phytate
Oxalate
Tannin
Saponin
1 2 3 4 5 6 7 8 9
7.65 ± 0.03 11.32 ± 0.02 31.20 ± 0.02 7.53 ± 0.01 40.67 ± 0.01 5.13 ± 0.01 5.61 ± 0.01 95.21 ± 0.02 6.18 ± 0.03
0.14 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.07 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.12 ± 0.01 0.05 ± 0.01
0.06 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.05 ± 0.01 0.00 ± 0.00 0.00 ± 0.00 0.09 ± 0.01 0.00 ± 0.00
14.38 ± 0.03 2.13 ± 0.01 2.07 ± 0.02 1.44 ± 0.01 3.76 ± 0.01 1.09 ± 0.01 1.15 ± 0.01 3.81 ± 0.02 3.27 ± 0.02
1 2 3 4 5 6 7 8 9
0.05 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.02 ± 0.00 0.04 ± 0.00 0.04 ± 0.00 0.03 ± 0.00 0.02 ± 0.00
48 ± 0.00 0.39 ± 0.01 0.34 ± 0.00 0.38 ± 0.00 0.13 ± 0.00 0.36 ± 0.00 0.37 ± 0.00 0.12 ± 0.00 0.11 ± 0.00
0.01 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.00 ± 0.00
0.06 ± 0.00 0.05 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.02 ± 0.00 0.03 ± 0.00 0.04 ± 0.00 0.03 ± 0.00 0.01 ± 0.00
Values are means ± SD of triplicate determinations.
compared with raw sample (sample 1). Significant reduction occurred in crude protein content of samples 4, 6, 7, and 9 (p < 0.05) while significant increase was observed in samples 2, 3, 5, and 8 compared with sample 1 (p < 0.05). Significant decrease also occurred in crude lipid content of samples 6, 7 and 9 while increase was observed in samples 2, 3, 4, 5 and 8 (p < 0.05). Roasting, frying and preparation of the yam to porridge and Ojojo (Samples 2, 3, 5 and 8 respectively) significantly increased the protein, lipid, fibre and ash content of the products (p < 0.05), the increase more pronounced in porridge (Sample 5) and Ojojo (Sample 8). Boiling and pounding resulted in significant reduction in value of crude protein, lipid and fibre content of the products. Roasting, frying and preparation of the yam to Amala brought significant increase in the carbohydrate and gross energy content, while boiling and pounding reduced the carbohydrate content of products compared with the raw yam sample (sample 1). Table 2 shows the mineral composition of raw and processed yellow yam samples. Raw yellow yam (Sample 1) was very low in sodium, calcium, magnesium and phosphorus, low in iron and zinc, but moderate in potassium compared with daily human requirements of these minerals. Processing yam into various products resulted into highly significant decrease in mineral content of the products compared with the raw sample (p < 0.05). However roasting, frying, and processing yam into porridge and Ojojo (Samples 2, 3, 5 and 8) resulted in significant increase in potassium content of the products compared with raw yam sample (p < 0.05). Table 3 shows the vitamin composition of raw and processed products of yellow yam. Raw yellow yam was very low in beta carotene, thiamine and riboflavin, but contained substantial amount of ascorbic acid. Roasting and frying significantly increased the beta carotene content of the products (Samples 2 and 3) but completely removed the thiamine and riboflavin while drastically reducing the ascorbic acid content of the products (p < 0.05). Boiling significantly reduced the beta carotene and ascorbic acid content of the products (Samples 4, 6, 7) while completely removing the thiamine and riboflavin from the products (p < 0.05). Preparing the yam to porridge and Ojojo improved the beta carotene content of the products. Table 4 shows the antinutrient composition of raw and various processed yam products. The raw yam sample (Sample 1) was very low in antinutrients. All the processing methods further resulted in reduction in content of the antinutrients studied. The percent apparent nutrient retention in products (Samples 2, 4, 6, 7 and 9) are shown in Table 5. Roasted yam (Sample 2) had the highest level of retention for most of the nutrients followed by boiled yam (Sample 4). Pounded yam with the cooking water (Sample 7) retained more nutrients compared with pounded yam with ordinary water (Sample 6), while Amala (Sample 9) had the lowest nutrient retention among all samples. The apparent nutrient retention of the samples (Samples 3, 5, and 8) could not be
*
Values are means ± SD of triplicate determinations.
Table 5 Percent nutrient retention by different cooking methods. Sample
Parameter Crude protein Crude lipid Crude fibre Carbohydrate Sodium Potassium Calcium Magnesium Phosphorus Iron Zinc b-Carotene Ascorbic acid
2
4
6
7
9
115.27 170.37 117.65 125.47 74.56 112.31 78.96 17.75 138.41 79.75 61.54 147.97 14.81
83.21 100.00 76.47 89.87 56.98 83.04 62.72 13.78 102.31 59.49 56.41 98.43 10.01
61.83 70.37 47.06 73.41 47.25 65.96 44.30 10.60 80.77 44.30 41.03 67.06 7.58
66.03 70.37 111.76 71.78 44.31 67.13 48.47 11.07 34.92 46.84 43.59 73.33 8.00
46.18 14.81 41.18 100.00 111.72 78.74 103.99 40.31 147.85 129.11 125.64 80.78 22.74
compared with that of the raw sample because of the added ingredients which influence the retention levels. 4. Discussion 4.1. Proximate composition Raw yellow yam was very high in moisture, high carbohydrate and gross energy, but low in crude protein, lipid, and ash content. The values obtained for the moisture content and crude protein of yellow yam is similar to the values reported by Ofosu (2012). The high gross energy content of yam explains why it serves as a staple source of energy in Nigeria. The values obtained for crude protein, lipid, and ash of raw yam sample were in agreement with the value reported by Adepoju (2012) for white yam (Dioscorea rotundata). The high moisture content of the raw yellow yaw predisposes it to easy spoilage and short shelf life. Roasting, frying, and processing the yellow yam to porridge and Ojojo (samples 2, 3, 5, and 8 respectively) significantly improved the proximate composition and energy content of the products. The significant improvement in the nutrient and energy content of the samples is believed to be partly due to reduction in the moisture content of the samples (as consumed) compared with the raw sample, and partly due to addition of ingredients such as vegetable oil, palm oil and salt to some of the products. Processing has been reported to enhance nutrient availability in processed foods (Kataria & Chauhan, 1988; Oste, 1991). The observed increase in fat and energy content of fried yam, porridge and Ojojo was a direct result of contribution from vegetable oil used in frying, and palm oil and other ingredients used in preparing porridge. Fat and oil contribute higher percentage of energy compared with carbohydrates and proteins.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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However, boiling resulted in increase in moisture content of yellow yam with attendant decrease in crude protein, lipid, fibre and carbohydrate and energy content of the products (Samples 4, 6, 7) compared with raw sample. Pounding the yam with the cooking water retained more nutrients and fibre than the sample in which ordinary water was used in pounding the yam (samples 7 and 6 respectively, Table 5). This was an indication that some of these nutrients leached into the cooking water which is usually discarded, and that the yam contained significant amount of soluble fibre. This observation was in line with the findings of Bradbury, Bradshaw, Jealous, Holloway, and Phimpisame (1998), Adepoju et al. (2010); and Adepoju (2012) who reported nutrient loss by cooking and soaking food items for a period of time through leaching. The significant reduction in all nutrients and gross energy content observed in Amala (sample 9) compared with all other samples might have resulted from the parboiling coupled with soaking in water overnight, leading to leaching of the nutrients into the soaking water. 4.2. Mineral composition The result of mineral composition of raw and processed yellow yam is shown in Table 2. Raw yellow yam (Sample 1) was very low in sodium, calcium, and phosphorus, low in magnesium, iron, and zinc but high in potassium. The very low level of sodium compared with potassium is a good advantage for the yam to be suitable for consumption by hypertension patients where sodium: potassium ratio is expected to be low (Ifon & Bassir, 1979). Generally, processing yellow yam into different products resulted in significant reduction in value for most of the minerals, the reduction being more pronounced when boiling was involved (Samples 4, 6, 7, 9). However, roasting (Sample 2) and frying (Sample 3) significantly increased the potassium and phosphorus content of the products compared with the raw sample. Also, preparing the yam into porridge (Sample 5) and Ojojo (Sample 8) significantly improved the sodium, potassium, calcium, phosphorus, iron and zinc content of the products compared with the raw sample (Sample 1). The observed increase in the mineral content of these products was believed to be contributed in part by the added salt and ingredients, and in part by bioconversion of components of the yam, leading to significant reduction in value of antinutrients such as phytates and oxalates which combine with these minerals. The observed pattern in mineral content variation of the yellow yam products is generally similar to that of white yam reported in the literature (Adepoju, 2012). There was a highly significant reduction in some mineral content of boiled and pounded yam samples (Samples 4, 6 and 7). This was believed to be due to the leaching of the minerals into the boiling water (Bradbury et al., 1998). Adepoju et al. (2010) reported that soaking of food items in water results in loss of minerals due to leaching into the soaking water. Yam pounded with its cooked water (Sample 7) retained more minerals compared with both boiled yam and yam pounded with ordinary distilled water (Samples 4 and 6, Table 5), thereby confirming leaching of these minerals into the boiling water, which is usually discarded. Processing the yam to Amala (Sample 9) led to significant increase in sodium, calcium, phosphorus, iron and zinc content of the product compared with the raw sample. This was similar to what was obtained for white yam Amala in the literature (Adepoju, 2012). The significant increase in the mineral content might have resulted from interconversion and release of these minerals from the chemically bound form as phytates to free form, with attendant decrease in phytate content of the product. Phosphorus is a constituent of phytic acid which combines with heavy metals such as calcium, iron and zinc,(Reddy & Salunkhe, 1982;
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Makkar & Becker, 1996), thereby reducing their bioavailability. The release of these minerals from their bound state could have led to the observed increase in their values.
4.3. Vitamins composition Table 3 shows the some selected vitamin composition of raw and processed yam products. The raw yam sample (Sample 1) was very low in b-carotene, thiamine and riboflavin but moderate in ascorbic acid content. The low value of the b-carotene content of the yam was suggestive of the fact that the yellow colouration was not totally due to presence of b-carotene in the yam. Processing yellow yam into various products such as roasted yam (Sample 2), fried yam (Sample 3), porridge (Sample 5), and Ojojo (Sample 8) significantly increased the b-carotene content of the products but resulted in significant reduction in water soluble vitamins (p<0.05) compared to raw yam sample (Sample 1). The observed increase in the b-carotene content of the processed samples was believed to be due to either reduction in moisture content of the sample or addition of vegetable oil during the processing of the sample, while the significant reduction was believed to be due to either heat destruction or loss through leaching of these vitamins into the cooking water (Adepoju et al., 2010; Bradbury et al., 1998). Ojojo had the highest value of beta carotene, followed by porridge. The increase was believed to be due to contribution of additional ingredients used in preparing them.
4.4. Antinutrient composition The level of all antinutrients in the raw yellow yam (Sample 1) was very low (Table 4), and is unlikely to constitute any hindrance to digestibility of nutrients from other food sources in the human body. The different processing methods generally led to significant reduction in the levels of antinutrients in the processed samples. This finding supports the fact that processing significantly reduce the level of antinutrients in processed foods (Bhandari & Kawabata, 2006). The levels of antinutrients present in the products were very negligible, and are unlikely to constitute any hindrance to utilization or bioavailability of the nutrients.
5. Conclusion Raw yellow yam was high in moisture and carbohydrate content with moderate gross energy compared with its other nutrients. Processing the yam to different products led to improvement in the macronutrients with moderate gross energy, hence they are suitable for consumption by everyone. The low sodium, lipid and antinutrient content coupled with improved mineral and b-carotene values of the products qualify them for consumption by everyone. Pounding yam with the boiling water retained greater part of the nutrients. Yellow yam products are moderate in gross energy content, and can serve as good source of energy to consumers. It is advisable that the yam should be consumed in roasted form so as to retain greater part of the nutrients, or cooked with calculated amount of water when not being prepared into fried yam, porridge or Ojojo to prevent decantation which results in nutrient loss.
Declation of conflict of interest The authors hereby declare that there is no conflict of interest whatsoever on this paper, as it was solely funded by them.
Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071
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Please cite this article in press as: Adepoju, O. T., et al. Effects of processing methods on nutrient and antinutrient composition of yellow yam (Dioscorea cayenensis) products. Food Chemistry (2016), http://dx.doi.org/10.1016/j.foodchem.2016.10.071