Effects of waxing materials and storage periods on physicochemical properties of trifoliate yam (Dioscorea dumetorum)

Applied Science Reports www.pscipub.com/asr E-ISSN: 2310-9440 / P-ISSN: 2311-0139 App. Sci. Report. 4 (2), 2013: 219-224 PSCI Publications Effects of waxing materials and storage periods on physicochemical properties of trifoliate yam (Dioscorea dumetorum) Ezeocha, V.C. and Oti, E. National Root Crops Research Institute, Umudike, Nigeria Corresponding author email: avezeocha@yahoo.com Key words Chemical properties Functional properties Nutrient retention Storage Waxing A B S T R A C T The effect of different waxing materials (bee wax, palm wax and paraffin wax) and storage period (0, 2, 4, 6, 8 weeks) on some physico-chemical parameters of Dioscorea dumetorum tubers was assessed. Harvested tubers of the yellow variety of D. dumetorum were grouped into four. Three of these were differently treated with bee wax, palm wax and paraffin wax while the fourth group was not treated (control sample). The four samples were then stored under the same condition (26.5 + 4.0 o C and RH of 55.2+15.5%) for 8 weeks. Tubers were evaluated bi-weekly for their chemical and functional properties using the recommended standard methods. Weight loss varied with storage time and waxing material; tubers waxed with palm wax had the least weight loss (9.09%) while the un-waxed tubers had the highest weight loss (26.67%) at the end of the 8 weeks storage. Chemical analysis showed that moisture content, starch yield and crude protein content significantly (p<0.05) reduced with increase in storage period while the dry matter, fat, crude fibre and ash contents increased with storage period. Functional properties such as water and oil absorption capacity as well as swelling index also varied with both storage period and waxing material. The water and oil absorption capacities decreased significantly with storage period. All the waxing materials retained the nutrient contents more than the un-waxed tubers however, palm wax performed better than the other waxing materials in terms of weight after storage, nutrient and functional properties after storage. 2013 PSCI Publisher All rights reserved. Introduction Yam (Dioscorea spp. L) is a vegetatively propagated tuber food crop. It belongs to the family Dioscoreaceae and produces underground tubers and or aerial tubers, bulbils and rhizomes which act as a source of food, feed and drugs or medicines. Out of the over 600 species of the genus Dioscorea, six (6) are cultivated for food in the tropics (Hahn et al, 1987). The six (6) edible yam species are Dioscorea alata (water yam), Dioscorea rotundata (white guinea yam), Dioscorea esculanta (Chinese or lesser yam), Dioscorea cayenensis (yellow guinea yam), Dioscorea bulbifera (aerial or bulbils yam) and Dioscorea dumetorum (trifoliate yam) (Degras, 1993). Trifoliate yam ( D. dumetorum) is a lesser known yam among the cultivated yam species and is underutilized. Its Cultivation is widespread in the south-eastern part of Nigeria. D. dumetorum possesses relatively higher protein content than other Dioscorea species currently cultivated and a favourable nitrogen/energy balance (Agbor and Treche, 1983). The storage ability of this yam is however restricted by the severe changes which occur in the tubers after harvest and renders them unpalatable for human consumption. Therefore, only freshly collected tubers are usually consumed locally and technological transformation of D. dumetorum must be carried out promptly after harvest. These changes limit their production and commercialization outside production zones thereby hampering their economic and nutritious value as food (Afoakwa and Sefa-Dedeh, 2001). The extent of changes associated with D. dumetorum warrants the need for improved methods for their storage. There is always the need for adequate and efficient storage method to save the excess crop that is produced from deterioration and waste. Storage serves

three important purposes. These are to ensure steady availability of produce and stable prices of produce, thereby reducing the seasonal fluctuations of market prices; to enable farmers and producers sell off their produce at strategic times for best market prices; and to eliminate or reduce quantitative and qualitative losses, thereby ensuring that healthy seeds are available for use in the next planting season. Some improved methods have been developed for yam storage such as the use of γ-radiation (Lawal et al, 2011), low temperature storage (Afoakwa and Sefa -Dedeh, 2001) etc. However, these technologies are restricted in commercial application because they are technically complex and quite expensive for adoption by small scale farmers who are the major producers of yam. Various types of skin coating materials (Bee wax, paraffin wax, carnauba wax, shellac etc.) have been used t o restrict moisture loss from the surface of crops (Ahmad et al., 1997; Farooqi et al., 1981), through evaporation, transpiration and respiration (Malik., 1994).Waxes are the esters formed from a fatty acid and a high molecular weight alcohol. At present, wax have been used as an effective technology to increase the postharvest quality of fruits and vegetables (Qiuping and Wenshui, 2007; Fan et al., 2009; Tzoumaki et al., 2009; Tietel et al., 2010). Waxing is currently the most common way of preserving cassava for export. The reasons for their use are: they extend product shelf life (Park et al., 1994), control oxidation and respiration reactions (McHugh & Krochta, 1994), add to texture and sensory characteristics and are environmentally friendly (Guilbert et al., 1996).Waxed cassava can be stored for 1-2 months (Rickard and Coursey, 1981). Bemul-wax coating combined with calcium chloride has been reported to prevent spoilage in sweetpotato (Afolabi and Oyelede, 2011). Storage methods that limited the contac t of the tubers with the external environment were found to slow down the severe changes in D. dumetorum (Afoakwa and Sefa-Dedeh, 2002). Eka (1998) reported that there is dearth of information on nutrient changes during storage of most root and tuber crops in Nigeria. This study was undertaken to determine the effect of different waxing materials (bee wax, palm wax and paraffin wax) and storage period on some physico-chemical parameters of Dioscorea dumetorum tubers. The study aims to provide information for the processor so that he will know the changes that occur in D. dumetorum during storage in order to help them in their choice of raw materials for processing thereby providing consumers with products of desirable food quality. Information from this work will also help farmers select best waxing materials that will help to preserve D. dumetorum tubers during storage for a longer period. Materials and Methods D. dumetorum (yellow cultivar) tubers were obtained from the yam programme of National Root Crops Research Institute, Umudike. The D. dumetorum tubers were sorted to remove tubers with injuries after which they were cured for 48 hours and cleaned off sand. The tubers were then divided into four portions; one portion was dipped for 2 seconds inside bee wax, the second portion was dipped for 2 seconds inside palm wax, the third portion was dipped inside paraffin wax while the forth portion which was the control was left without waxing. Both the waxed and the un-waxed tubers were kept under the sun for 48hrs. The tubers of each group were gently arranged in plastic crates to avoid them from bruising each other prior to storage. Each portion including the control was stored in wooden boxes measuring 75cm x 75cm x 60cm. The average temperature and relative humidity of the storage environment were 26.5+ 4.0 o C and 55.2+15.5% respectively. Some marked tubers were used for the determination of biweekly variation in weight of the tubers. The percentage weight loss for each week was computed based on the initial tuber weight using the formulae: Percent weight loss = Weight before storage Weight after interval (g) Weight of fresh fruits (g) X 100 Three tubers were drawn from each portion for physic-chemical analysis at bi-weekly interval till the end of the eight weeks storage period. The oven method was used to determine the moisture content (Anderson and Igram, 1989), the crude protein, ash, crude fat, crude fibre, phosphorus, calcium and carbohydrate contents were evaluated based on the A.O.A.C methods of analysis (AOAC, 2000). The water absorption capacity was determined with the method of Abbey and Ibeh (1988), the method of Beuchat (1977) was used for the determination of oil absorption capacity while the swelling index of the samples was determined using the method of Lin et al. (1974). Statistical analysis The experiment had a 4 (waxing materials) x5 (storage period) factorial struct ure, the analyses were replicated three times. The effects of the factors on each parameter were determined by analysis of variance using SPSS 20 (Version, 2011). Also, Duncan s Multiple Range Test (DMRT) at 5% probability (P 0.01) was performed to compare the means of different treatments. Results and Discussion Figure 1 shows the effect of waxing material on the weight of D. dumetorum tubers during storage. Waxing significantly reduced the weight loss throughout the storage periods. Weight loss is mainly associated with respiration and moisture evaporation through the skin. Coatings act as barriers, thereby restricting water transfer and protecting peel from mechanical injuries, as well as sealing small wounds and thus, delaying dehydration (Hernandezmunoz et al., 2008). The highest percentage weight loss was observed in the un-waxed tubers. The lower percentage weight loss in the waxed tubers might be due to wax coating, which acts as barrier between inner and outer environment of the tubers. This shows that 220

waxing during storage period slows down the rate of weight loss probably by reducing the rate of transpiration and respiration. Minimum weight loss was observed in palm waxed and paraffin waxed tubers. 35 30 Percentage weight loss 25 20 15 10 5 Unwaxed Bee waxed Palm wax Parafin wax 0 2wks 4wks 6wks 8wks Storage period Figure 1. Effect of waxing and storage period on the weight loss of D. dumetorum tubers Table 1 shows the effect of storage period and waxing material on the proximate composition of D. dumetorum. The moisture content ranged from 54.03% (in un-waxed tubers after 8 weeks storage) to 75.73% (in the freshly harvested tubers). Moisture content reduced significantly with storage period, a similar observation was made in cocoyam (Enomfon and Umoh, 2004). These moisture losses are high compared to 31% in D. rotundata and 35% in D. dumetorum reported by Treche and Agbor-Egbe (1996) after 110 days of storage. The reduction in moisture during storage can be attributed to physiological processes that the yam underwent such as respiration, transpiration and sprouting. Afoakwa and Sefa-Dedeh, (2001) has related this high moisture loss observed in D. dumetorum to the hardening phenomenon. He suggested that the rapid water removal in the tubers may cause the cell wall polysaccharides to shrink permitting greater interactions by means of hydrogen bonding and Van der waals forces, resulting in increased cell rigidity during storage. Waxing significantly reduced the rate of moisture loss compared to the un-waxed tubers. The bee waxed tubers had the highest moisture retention in the 2 nd, 4 th and 6 th week of storage however on the 8 th week of storage the moisture content reduced with palm wax having the highest retention. Moisture loss was highest in the un-waxed tubers. moisture content (%) 80 70 60 50 40 30 20 10 0 0 2 4 6 8 storage period unwaxed bee waxed palm waxed parafin waxed Figure 2. Effect of storage period and waxing materials on the moisture content of D. dumetorum. The dry matter portion of yam tubers is mostly composed of carbohydrates, which exist primarily in the form of starch and sugars (Ikediobi and Oti, 1983). Dry matter of the D. dumetorum samples ranged from 21.45 45.98% with the un-waxed tubers having the highest dry matter after eight weeks storage. Dry matter increased significantly (p>0.05) with 221

storage time in all the samples. A similar observation was made by Otegbayo et al, (2011) on D. rotundata and D. alata and Sefa-Dedeh and Afoakwa (2002) on stored D. dumetorum tubers. The dry matter content of tubers coated with bee wax varied significantly only after six weeks of storage. The increase in dry matter content could be as a result of high loss of moisture from the tubers. Starch yield of the tubers ranged from 13.96% to 26.18%. Starch yield decreased significantly after 2 weeks of storage, in the subsequent weeks the decreases were not significant. The decrease in the starch yield of tubers with storage period may have been as a result of the conversion of starch to sugars by endogenous α-amylase during metabolic activities (Otegbayo et al, 2011). After 8 weeks of storage, the lowest starch yield (13.96%) was observed in the control while the highest starch yield was observed in the palm waxed tubers (17.05%). Starch yield also varied significantly with waxing as it was observed that the waxed tubers yielded more starch during storage than the un-waxed tubers. This may be due to the fact that waxing creates a modified atmosphere which slows down metabolism by reducing the amount of oxygen hence rate of metabolism during which starch is hydrolyzed to sugar. Palm waxed tubers and paraffin waxed tubers retained their starch better than bee waxed tubers. The fat content ranged from 0.03 1.85%. The fat content tended to increase with increase in storage period. The ash content of the tubers ranged from 2.00% - 5.43%. The ash content decreased significantly with storage period. It also varied significantly with waxing material. The crude fibre content of the tubers (which is composed of lignin, pectin, acid detergent fibre and cellulose) increased significantly with increase in storage period. A similar observation was made by Brillouet et al, (1981); Treche and Delpeuch, 1982; Treche and Agbor-Egbe, (1996); Sefa-Dedeh and Afoakwa (2002) and Otegbayo et al, (2011) on D. rotundata, D. alata and D. dumetorum. The increase in crude fibre content with storage may be responsible for the hardening phenomenon which develops few days after storage. The highest crude fibre content (8.10%) was observed in the control (un-waxed tubers) while the bee waxed tubers contain the least crude fibre content (4.26%) after 8 weeks of storage. Crude protein content decreased significantly with storage period. A similar observation was made by Otegbayo et al, (2011) on D. rotundata and D. alata. The decrease in crude protein content of the tubers with increase in storage period may have been as a result of the breakdown of protein in the tubers by endogenous proteinases (Osuji and Umezurike, 1985). After 8 weeks of storage, palm waxed tubers had the highest crude protein content (4.53%). Table 1. Effect of storage period and waxing material on the proximate composition of D. dumetorum. Nutrient Storage period Un-waxed Bee waxed Palm waxed Parafin waxed Dry matter (%) 0 24.78(d) 2 25.74a(d) 24.58ab(b) 21.45c(c) 23.60b(c) 4 32.70a(c) 28.30a(b) 29.55a(b) 30.35a(b) 6 41.34a(b) 27.96b(b) 30.05b(b) 30.07b(b) 8 45.98a(a) 42.56ab(a) 40.00b(a) 39.92b(a) Moisture content 0 75.73(a) (%) 2 74.27c(a) 75.45b(a) 75.70a(a) 75.40b(a) 4 67.30a(b) 71.70a(a) 70.45a(b) 69.65a(b) 6 58.67b(c) 72.05a(a) 69.95a(b) 69.93a(b) 8 54.03b(c) 57.45ab(b) 60.60a(c) 60.08(c) Starch yield (%) 0 26.34(a) 2 21.32c(a) 21.13c(a) 26.18a(a) 22.90b(a) 4 17.40c(b) 18.05b(b) 18.03b(b) 21.24a(a) 6 14.28c(b) 16.55b(bc) 18.79a(b) 16.49b(b) 8 13.96b(b) 14.59b(c) 17.05a(b) 15.32ab(b) Fat (%) 0 0.13(d) 2 0.12b(d) 0.26a(c) 0.05c(d) 0.03c(d) 4 0.16c(c) 0.15c(c) 1.00a(c) 0.36b(c) 6 1.68a(a) 0.53c(b) 1.21b(b) 1.21b(b) 8 1.46b(b) 1.38b(a) 1.46b(a) 1.85a(a) Crude fibre (%) 0 2.02(d) 2 2.70b(d) 1.66c(b) 2.60b(c) 3.92a(b) 4 3.55b(c) 3.60b(a) 4.45a(b) 4.47a(ab) 6 4.45c(b) 4.21c(a) 7.42a(a) 4.98b(ab) 8 8.10a(a) 4.26c(a) 7.96a(a) 6.03b(a) Ash (%) 0 5.36(a) 2 5.19a(a) 4.38b(a) 5.19a(a) 5.43a(d) 4 4.14a(b) 3.64b(b) 3.27c(b) 4.39a(b) 6 3.40a(c) 2.87c(c) 3.16b(c) 3.43a(c) 8 3.38a(c) 2.73b(c) 2.00c(d) 2.82b(d) Crude protein (%) 0 7.23(a) 2 6.43d(b) 6.82c(a) 7.14b(a) 6.87c(a) 4 5.75d(c) 6.29c(b) 7.07a(a) 6.73b(a) 6 4.49c(d) 4.34c(c) 5.65a(b) 5.08b(b) 8 3.83c(e) 4.06b(d) 4.53a(c) 3.95b(c) Values with the same letter along the row are not significantly different (p > 0. 05) while values with the same letter inside brackets along the same column are not significantly different (p > 0. 05). 222

Table 2 shows the effect of storage period and waxing material on the functional properties of D. dumetorum. The water absorption capacity (WAC) ranged from 1.60 4.10. The highest WAC was observed after two weeks of storage with bee waxed tubers having the highest WAC of the treated tubers after 2 weeks of storage. Water absorption capacity of starches provides evidence of the degree of intermolecular association between starch polymers due to associative forces such as hydrogen and covalent bonding (Adebowale et al, 2006). The Water absorption capacity and oil absorption capacity decreased significantly (p>0.05) with storage period. After 8 weeks of storage, the un -waxed tubers had the highest WAC (2.25%) of the all treated tubers while palm waxed tubers had the least WAC (1.60%). The ability of starch to absorb water is an indication of its moisture stability more especially in the food industry (Adebowale et al, 2006). Water absorption capacity is a critical function of protein and is important in various food products like soups, gravies, dough and baked products (Adebowale et al, 2006). The un-waxed tubers had the highest water and oil absorption capacities. Swelling index of the tubers ranged from 2.77-1.14. It reduced significantly with storage period. It also varied with waxing material. Swelling index is largely controlled by the strength and character of the micellar network within starch granules. Swelling index is an indication of the water absorption index of the granules during heating and it reflects the extent of the associative forces within the granules (Ikegwu et al, 2010). Amylose is believed to restrict swelling by reinforcing the internal network of starch granules (Richardson et al, 2000; Hoover, 2001). The decrease in the swelling index of the flours during storage may be related to the decreases in starch content of stored tubers. The palm waxed tubers had the highest swelling index in the first 6 weeks of storage. Table 2. Effect of storage period and waxing material on the functional properties of D. dumetorum. Properties Storage period Un-waxed Bee waxed Palm waxed Parafin waxed WAC 0 2.75(ab) 2 3.05a(a) 4.10a(a) 2.50bc(a) 3.85b(a) 4 3.01a(ab) 2.08ab(b) 2.63c(a) 2.25cd(b) 6 2.05d(b) 2.01a(b) 2.15ab(b) 2.25a(b) 8 2.25a(ab) 1.95d(b) 1.60a(b) 1.78b(b) OAC 0 2.18(b) 2 2.59b(a) 2.61b(a) 2.88a(a) 2.93a(a) 4 1.45c(c) 1.50c(b) 1.63b(b) 1.28d(b) 6 1.40a(c) 1.13ab(d) 1.20ab(c) 1.10b(b) 8 1.30a(c) 1.25a(c) 1.00a(c) 1.25a(b) SI 0 2.77(a) 2 2.23ab(a) 2.10b(a) 2.44a(a) 2.05b(a) 4 1.81c(b) 2.01b(a) 2.12a(b) 1.75a(b) 6 1.71b(b) 1.94ab(a) 2.02a(b) 1.60c(b) 8 1.14c(c) 1.91a(a) 2.01b(b) 1.60b(b) Values with the same letter along the row are not significantly different (p > 0. 05) while values with the same letter insid e brackets along the same column are not significantly different (p > 0. 05). 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