Influence of storage conditions and packaging materials on some quality attributes of water yam flour

The study investigated some quality attributes of water yam flour stored in three packaging materials [high and low density polyethylene and plastic container] under different storage conditions [relative humidity (36, 56, 75 and 96%), temperature (25 ± 2, 35 ± 2 and 45 ± 2°C)] for 24 weeks. The functional properties, proximate composition and microbial load of the samples were evaluated at 4 weeks interval. Significant differences (p < 0.01) were observed for proximate composition, functional properties and microbial load of the samples during storage. The interactive effect of storage conditions and packaging materials was significant (p < 0.01) on proximate composition and pasting properties (except trough viscosity). The yam flour samples were still shelf stable after the 24 weeks of storage. Subjects: Food Chemistry; Food Engineering; Packaging


Introduction
Water yam, Dioscorea alata, is an important staple in West Africa. It is an important energy source, of many people (Ekwu, Ozo, & Ikegwu, 2005). The nutritional value of yam varies greatly between different species but water yam has a crude protein content of 7.4%, starch content of 75-84%, and Vitamin C content ranging from 13.0 to 24.7 mg/100 g (Djeri et al., 2015), but it is underutilized compared to white yam (D. rotundata) possibly because of its loose watery texture (Adegunwa, Alamu, & Omitogun, 2011).
Water yam tubers have been utilized traditionally in homes with little or no industrial application; however the traditional uses are diverse (Baah, 2009). Yams are eaten boiled, fried or baked. Yams are also processed into yam chips and flour that is used in the preparation of a paste (Karim, Kayode, Oyeyinka, & Oyeyinka, 2013;Osunde, 2008).
Yam suffers considerable postharvest losses which can be as high as between 10 and 15% within the first 3 months after harvest (Ansah, Tetteh, & Donkoh, 2017;Osunde, 2008). To minimize these losses yam is converted to local flour to prevent spoilage in the form of gbodo, which have limited industrial use. It is a well known fact that storage stability of food systems depends on the storage conditions, packaging materials and the water activity of the food material. Information on the influence of storage conditions on storage stability of yam flour meant for industrial applications is scanty. A number of deteriorative reactions which affect the quality attributes of foods (nutritional composition and functional properties) are initiated during processing; these reactions continue during storage at a rate proportional to variation in the storage conditions, therefore variation in storage conditions could affect the storage stability of many processed foods. Deterioration of flour products are usually attributed to the type of packaging materials and spoilage organisms such as bacteria and fungi (Ilouno, Ndimele, Adikwu, & Obiekezie, 2016;Okigbo, 2003). Hence, it is important to determine optimal storage conditions that would best maintain the flour quality attributes during storage. Therefore, the objective of this study was to determine the effect of storage conditions and packaging materials on some quality attributes of water yam flour.

Material and method
Freshly harvested wholesome water yam tubers were procured from Kuto market in Abeokuta, Ogun state, Nigeria.

Processing of water yam flour
The yam tubers were cleaned, hand peeled with stainless steel knife, washed and sliced into chips (using a vegetable slicer) inside water. The yam slices were pre-treated by soaking in 0.28% potassium metabisulphite solution for 15 min. The pre-treated slices were then drained and dried in a cabinet dryer at 60°C for 48 h. The dried chips were milled into flour using a laboratory hammer mill. The flour was packaged in plastic (PP) containers, high density polyethylene (HDPE) and low density polyethylene (LDPE) and stored in incubators at 4 relative humidities (36, 56, 75 and 96%) over three temperatures (25 ± 2, 35 ± 2 and 45 ± 2°C) for 24 weeks to study the effect of storage conditions on the flour. The functional properties, proximate and microbiological analyses of the samples were conducted at interval of 4 weeks during storage.

Determination of functional properties
The bulk density of the samples was determined using the method described by Abiodun, Adegbite, and Oladipo (2010), water binding capacity (WBC) was carried out using the Medcalf and Gillies (1965) method, oil absorption capacity (OAC) by the method of Sosulki (1962) while wettability was determined by the method described by Nwosu, Onuegbu, Kabuo, and Okeke (2010).

Determination of pasting properties
Pasting characteristics was determined with a Rapid Visco Analyser (RVA), (Tecmaster TCW3, Perten Instrument, Australia). A portion (3 g) of flour sample was weighed into a dried empty canister; 25 ml of distilled water was dispensed into the canister containing the sample. The solution was thoroughly mixed and the canister was well fitted into the RVA. The slurry was heated from 50 to 95°C with a holding time of 2 min followed by cooling to 50°C with a 2 min holding time. Heating and cooling were done at a constant rate of 11.25°C/min. Peak viscosity, trough, breakdown, final viscosity, set back, peak time, and pasting temperature were read from the pasting profile from a computer connected with the RVA (Newport Scientific, 1998).

Determination of proximate composition
The moisture, protein, fat and ash contents of the samples were determined using AOAC (2000) method.

Microbiological assay
Total bacteria and fungi counts were determined using the pour-plate procedure as described by ICMSF (1988). One gram from each sample was weighed into 9 ml of 0.1% (w/v) peptone water in a beaker and allowed to stand for 5 min with occasional stirring using a sterile glass rod. Aliquots (1 ml) of serial dilutions of 10 −6 were aseptically inoculated on Nutrient Agar. This was used for the determination of total viable bacteria count. The determination of mould counts was enumerated on Potatoes Dextrose Agar supplemented with 0.01% chloramphenicol. Dilutions of 10 −6 were incubated at 37°C for 24 h (total viable bacteria count) and 30°C for 4 days (mould count). The colonies were enumerated and expressed as colonies forming unit per gram (cfu/g).

Statistical analysis
All experimental data obtained were subjected to the general linear model (GLM) procedure of SPSS (version 21) for the analysis of variance and Pearson's correlation was also determined. In all cases, α = 0.05.

Functional properties of water yam flour during storage
The effect of storage conditions and packaging materials on functional properties of water yam flour is presented in Figures 1-4 and Table 1. The mean values of bulk density, WBC, OAC and wettability ranged between 0.67 and 0.80 g/mls, 62.03 and 128.00%, 0.13 and 0.53 and 53.67 and 80.37 s, respectively. The bulk density, water absorption index, WBC and OAC decreased with storage period. The bulk density of food product is affected by particle size and it is an important parameter in determining packaging materials and material handling during food processing (Adebowale, Adegoke, Sanni, Adegunwa, & Fetuga, 2012;Adebowale, Sanni, & Awonorin, 2005). The decrease in bulk density was more prominent in samples stored in LDPE and HDPE, which could be due to increase in moisture content, which affects the flour particle size. Wettability of the flour was affected by storage temperature and packaging materials (p < 0.01).Wettability of flours is an important indicator of instant characteristics of dried flours. The slight increase in wettability of the flour samples irrespective of packaging materials and storage condition indicates a slight reduction in instant characteristics of the flour (Udensi, Oselebe, & Iweala, 2008). The decrease in WBC could be due to the loose association between amylose and amylopectin in the native granules of starch and weaker associative forces maintaining the granules structure Adebowale, Sanni, & Onitilo, 2008). The reduction in OAC as storage progresses could probably be due to the reduced ability of the flour to entrap fat to its apolar end of its protein chain as a result of decrease in its protein content (Adeleke & Odedeji, 2010).

Proximate composition of water yam flour during storage
The effect of storage conditions and packaging materials on the proximate composition of high quality water yam is presented in Figures 5-9 and Table 2. The mean values of moisture content, protein, fat, ash and carbohydrate ranged from 8.30 to 23.55%, 3.86 to 5.77%, 0.02 to 5.50%, 1.04 to 1.48% and from 68.11 to 84.37%, respectively. The steady but gradual change in the moisture content of the samples in all the packaging materials could be as a result of the relative moisture permeability of the packaging materials. For all the packaging materials, storage at 36 or 56% relative humidity produced little moisture increase while pronounced increases were noticed at 75 or 96% relative humidity (RH) especially at storage temperature of 35 and 45°C. Moisture content is an indicator of shelf stability; increase in moisture content can enhance microbial growth which leads to deterioration of foods (Adejumo, 2013). The recommended safe level of moisture content during storage of flours or food powder is between 12 and 14% Standard Organization of Nigeria, 2004). When storing in a plastic container (LDPE) (up to 24 weeks) the moisture content increased from 8.30 to 12.04%, which still falls within the permissible range (Daramola, Idowu, Atanda, & Oguntona, 2010) and as such attracts minimal mould and bacteria attack during storage. For storage in high density polyethylene (HDPE), up to 16 weeks of storage, moisture contents of samples, stored at 36, 56 and 75%, RH under all the storage temperatures were within acceptable level, which implies that the flour stored, within these conditions, will have little or no deterioration as a result of bacteria and mould attack. However, for low density polyethylene (LDPE), moisture content of samples was within permissible range, up to 16 weeks of storage, for samples stored at 36 and 56% RH under storage temperature of 25 and 35°C. Generally, the moisture content of samples stored in LDPE irrespective of storage temperature and relative humidity were higher than that of samples stored in HDPE and plastic containers, which are in agreement with earlier findings of Daramola et al. (2010) and Fasasi (2003) who reported higher moisture content for LDPE during storage for pupuru and African breadfruit seed flour, respectively. Hence, plastic container gave the best barrier protective ability under all the storage conditions studied. The protein and carbohydrate content of the samples decreased significantly with storage. Similar trends have been reported by Awoyale, Maziya-Dixon, and Menkir (2013) for Ogi stored under different storage condition. These reductions could be attributed to the rapid growth of microorganisms as a result of increased moisture content whose metabolic activities lead to production of enzymes (such as proteases and amylases) that catalyse biochemical reactions that breakdown nutrient in food (Achi & Akubor, 2000). Samples stored in plastic containers had the least protein and carbohydrate depletion at the end of storage. Protein content of food products has been related to final product quality such as texture and appearance (Adejumo, 2013). There were significant variations in the fat content of the samples with respect to storage period. The increase and subsequent decrease in fat content could be due to the activities of the enzymes lipase and lipoxidase during storage, which are enhanced by the moisture content of the samples (Nasir, Butt, Anjum, Sharif, & Minhas, 2003). Similar trend was reported by Awoyale, Maziya-Dixon, and Menkir (2013) for Ogi stored under different storage conditions. The ash content, an indication of the mineral contents of food, decreased significantly with storage. The reduction could be as result of biochemical activities of microorganisms.   There were gradual decrease in peak, trough, breakdown, final and setback viscosities with storage time. The decrease was more pronounced in samples stored at 96% relative humidity for all the packaging materials investigated. Peak viscosity is the maximum viscosity attainable during heating of starch. It is a measure of the ability of starch to swell freely before breakdown (Sanni, Kosoko, Adebowale, & Adeoye, 2004). The reduction in peak viscosity with storage time indicates loose association between amylose and amylopectin in the native granules of starch and weaker associative forces maintaining the granules structure which is an indication of WBC of the flour and degradation of starch granules during storage. The peak viscosities obtained at the end of storage were comparable with those reported by Wireko-Manu, Ellis, Oduro, Asiedu, and Maziya-Dixon (2011) for water yam flour. The trough viscosity, an indicator of the ability of the paste or gel to withstand mechanical stress at constant temperature as more starch granules and amylose leach out into the solution, decreased with storage time. Also, the breakdown viscosity of the samples decreased significantly with storage. It is the minimum viscosity attained during the constant temperature phase, a measure of the ability of paste to withstand breakdown during cooling (Maziya-Dixon, Adebowale, Onabanjo, & Dixon, 2005). The pronounced reduction in the breakdown viscosity of the samples at the end of storage indicate significant breakdown of starches during storage. The ability of paste to withstand heating and shear stress is an important factor for many processes especially those requiring stable paste. The final viscosity gives the ability of starchy foods to form viscous paste after cooking and cooling. Result obtained in this study indicates a significant reduction of final viscosity of the samples with storage. The values obtained at the end of storage are comparable to those reported by Wireko-Manu et al. (2011) for water yam flour. Setback viscosity is the stage at which retrogradation or re-ordering of starch molecules occurs. High setback is associated with syneresis or weeping, during freeze/thaw cycles. Low setback values obtained for the samples at the end of storage showed a low tendency to undergo retrogradation during freeze/thaw cycles (Maziya-Dixon et al., 2005). Peak

Pasting properties of water yam flour during storage
time, a measure of cooking time and time required to attain peak viscosity, increased with storage, indicating longer time is required to attain peak viscosity. The values obtained at the end of storage are comparable to those reported by Wireko-Manu et al. (2011) for water yam flour. The pasting temperature is the temperature at which the first detectable increase in viscosity is measured and it is an index characterized by the initial change due to the swelling of the starch granules. Lower pasting temperature observed for samples stored at 98% relative humidity indicates lower WBC and lower degree of association between starch granules Numfor, Walter, & Schwartz, 1996).

Microbiological stability of water yam flour during storage
The effect of storage conditions and packaging material on microbial stability of high quality water yam flour is represented in Figures 10-12 and Table 4, which are plots of counts (bacteria and fungi) against storage time at different temperature and relative humidity. In each Figure, the plots are represented in four relative humidity 36, 56, 75 and 96% at constant temperature.
The mean values of total viable bacterial and total viable fungal count ranged between 0.10 and 2.15 × 10 6 cfu/g, and 0.00 and 0.65 × 10 6 cfu/g, respectively. An increase in both bacterial and fungal load was observed for the samples during storage. The increase in microbial load could be due to increasing moisture content level in the samples and storage conditions (storage temperature and relative humidity) conducive for their growth. The largest increase was observed in samples stored at higher relative humidity (75 and 96%). Length of storage time led to an appreciable increase in bacteria and mould growth. Low storage temperature and relative humidity resulted in lower microbial counts during storage; therefore, the storage of the flour at 35 and 45°C under high relative humidity (75 and 96%) makes them vulnerable to microbial contamination as more colonies were  observed in samples stored at these storage conditions irrespective of the packaging materials. Similar trends have been reported for storage temperature and relative humidity effect on mould growth in corn flour by Samapundo et al. (2007). Relative humidity in this study had a profound influence than storage temperature for microbial growth. Microbial load in all packaging materials increased at the end of 24 weeks of storage. This could be associated with the relative permeability of the packaging materials to atmospheric gases such as oxygen, carbon dioxide and water vapour and also probably due to storage conditions employed in this study which are favourable for microbial growth and availability of nutrients (Akhtar, Anjum, Rehman, Sheikh, & Farzana, 2008). Plastic containers exhibited a better protection against bacteria and mould attack, acting as an effective barrier to moisture as it recorded the least bacteria and fungi load of 1.95 × 10 6 and 0.50 × 10 6 cfu/g, Figure 10. Effect of storage conditions and packaging materials on total aerobic bacterial and fungal count of water yam flour at 25°C.
respectively at the end of storage. The microbial contamination of foods requires sufficient moisture and mould generally portends to thrive at lower moisture content (Akhtar et al., 2008). Notes: TBC-total bacterial count; TFC-total fungi count.

Conclusion
Storage temperature and relative humidity significantly affected the quality attributes of the flour.
Minimal loss of quality during storage was recorded at temperature of 25°C and 36% relative humidity. The packaging material with the best barrier properties and hence less losses in quality of water yam flour during storage was plastic container.