Correlation of the sensory attributes of thick yam paste (amala) and the functional and pasting properties of the flour as affected by storage periods and packaging materials

Yam (Dioscorea species) is one of the extremely regarded food crops in tropical countries of West Africa; it is tightly integrated into the social, cultural, economic, and religious life of the people (Diop, 1998; Ferraro, Piccirillo, Tomlins, & Pintado, 2016). Yam is the second most essential tuber crop in Africa, with Nigeria producing about 34 million tonnes (Food and Agriculture Organization, 2000). Yam tuber is an essential source of carbohydrate and it is also a significant source of income in countries where they are planted. In 2007, 96% of the worldwide production of yam (52 million tons) was from Africa, while 94% of the yam was from West Africa, with Nigeria alone producing 71% (Djeri et al., 2015; Obadina, Babatunde, & Olotu, 2014). Fresh Received: 22 April 2020 | Revised: 29 May 2020 | Accepted: 16 June 2020 DOI: 10.1111/jfpp.14732


Practical applications
This research shows the relationship between the sensory attributes of thick yam paste (amala) and the functional and pasting properties of the flour as affected by the storage periods and packaging materials. The use of polypropylene woven sack and polyvinyl chloride container in the storage of yam flour for 20 weeks shows different functional and pasting properties, and thus varying relationships with the sensory attributes of the amala. Yam flour should be properly packaged in polyvinyl chloride containers and stored for 4 months, to keep the sensory attributes of the amala, with good pasting properties. yam tubers are challenging to preserve and are subject to postharvest physiological deterioration during storage due to their moisture content (Afoakwa & Sefa-Dedeh, 2001;Nwaigwe, Okafor, Asonye, & Nwokocha, 2015).
Yam tubers are processed into flour in West African countries like Nigeria, Ghana, and the Republic of Benin to reduce postharvest physiological deterioration due to the high moisture content and seasonal nature (Akissoe et al., 2001;Asiyanbi-Hammed & Simsek, 2018;Babajide & Olowe, 2013;Mestres, Dorthe, Akissoe, & Hounhouigan, 2004). Yam flour is a shelf-stable form of conserving yam tubers to make it available during the off-season (Abiodun & Akinoso, 2014;Oni, 2006), thereby decreasing transportation costs (Iwuoha, 2004). Yam flour is produced by harvesting, sorting, peeling, slicing, blanching, drying, and milling. This is an ancient traditional method of processing yam into dry yam (gbodo), and subsequently, yam flour. The quality of the gbodo and yam flour differs from the processor location, yam species, processing methods, and conditions, as well as the type of packaging material used for storage (Adejumo, Okundare, Afolayan, & Balogun, 2013;Akissoe, Akissoes, Hounhouigan, & Nago, 2004;Hounhonigan, Kayode, Bricas, & Nago, 2003). Adebowale et al. (2017) reported that the best packaging material with less quality losses in the storage of water yam flour at 25°C and 36% relative humidity for 24 weeks was the plastic container, attributed to its good barrier properties. Lawal, Olaoye, Ibrahim, Sanusi, and Oni (2014) on their own part reported that the Hessian bag should not be used for the storage and preservation of yam flour but that yam flour should be packaged in less permeable plastic packaging materials such as polyethylene and polypropylene bags for shelf-life extension. However, there is presently dearth of information on the effect of packaging materials on the functional and pasting properties of stored yam flour and as it affects the sensory attributes of the cooked thick paste known as amala.
Amala is a traditional thick paste prepared from blanched, fermented or unfermented dried yam flour with a specific texture (Abiodun & Akinoso, 2014;Akissoe, Akissoes, Hounhouigan, & Nago, 2006;Awoyale, Maziya-Dixon, Sanni, & Shittu, 2010). Color and taste are the permanent attributes of amala obtained from yam flour (Abiodun & Akinoso, 2014;Akissoe et al., 2004). The decline of these sensory attributes (texture, color, and taste) in the amala may be due to the packaging materials used for the storage of the yam flour and the activities of spoilage organisms on the yam flour (Ilouno, Ndimele, Adikwu, & Obiekezie, 2016;Jinadasa, Galhena, & Liyanage, 2015;Okigbo, 2003). Amala is usually consumed with preferred soup immediately after preparation or may be wrapped in low-density polyethylene/polypropylene sheet and kept warm in a food flask until ready to serve within 24h or 48h (Fetuga, Tomlins, Henshaw, & Idowu, 2014), while other consumers in Southwest Nigeria may prefer wrapping the amala in special local leaves known as Ewe Eran (Thaumatococcus Daniellii) and Ewe Gbodogi (Megaphrynium Macrostachyum) (Akinfenwa, 2018), and kept warm in a food flask until ready to serve. The knowledge of the functional and pasting properties of yam spp could be used to predict and interpret their behavior under actual cooking and cooling conditions (Hariprakesh & Bala, 1996;Jimoh, Olurin, & Aina, 2009). Therefore, this study aims to evaluate the relationship between the sensory attributes of amala and the functional and pasting properties of the yam flour as affected by storage periods and packaging materials.

| Materials
White yam tubers (Dioscorea rotundata) were purchased from the Bodija market, Ibadan, Oyo State, Nigeria. One hundred and fifty kilograms (150 kg) of the yam tubers were processed into yam flour according to the methods described by Awoyale et al. (2010). The packaging materials (PVC and PPS) were purchased from a local market (Aleshinloye) in Ibadan, Oyo State, Nigeria.

| Production of yam flour
Yam flour was produced using the method described by Awoyale et al. (2010). Fresh and healthy yam tubers were cleaned, washed properly with clean water to remove adhering sand particles. The head of each tuber (1 cm) was cut off and the remaining section peeled manually using a stainless steel knife. The peeled tubers were washed several times with potable water before slicing. The slices were spread in a single layer on drying trays and dried at 65°C for 48 hr in an oven. Dried yam slices were milled into flour using a hammer mill of 250 μm sieve size. The yam flour was then packaged into polypropylene bags and sealed for further study.

| Storage studies of yam flour
The yam flour samples (500 g) were weighed and packaged in polypropylene woven sack (PPS) (25 cm height × 13 cm breadth) and sealed with a stitching machine, and a polyvinyl chloride container (PVC) (6 cm height × 13 cm breadth), and covered with a lid (Awoyale, Maziya-Dixon, Alamu, & Menkir, 2016). These packaging materials were stored at ambient temperature (28-30°C) inside a cupboard without light interference for 20 weeks. The sensory attributes of the amala and the functional and pasting properties of the yam flour were evaluated every 4 week of the 20 weeks of storage periods.

| Preparation of yam amala for sensory evaluation
The yam flour was made into amala, as reported by Awoyale et al. (2010). The yam flour (1 part) was mixed with boiling water (approximately 1.5 parts v/v) in a stainless-steel cooking pot. Each sample of amala was prepared by pouring the yam flour into boiling water (100°C) in the stainless-steel cooking pot with continuous stirring until a homogenous thick paste was formed. The paste was covered and left on the electric cooker set at medium temperature for about 3 min to cook before another stirring to get a good textured paste. The amala was scooped adequately with a spoon and wrapped in polyethylene nylon before evaluation. The preparation of each of the amala samples was performed three times for each panelist. The sensory evaluation was carried out using trained panelists (twelve) from the staff and graduate students IITA, Nigeria, who consumed yam amala regularly based on attributes such as color/appearance, texture, stretchability, moldability, flavor, mouthfeel, and overall acceptability. The panelists were asked to rank the amala produced from the yam flour before and during storage (every 4 weeks) for 24 weeks, using a 9-point hedonic scale; 1-corresponds to disliked extremely and 9-liked extremely (Iwe, 2002;Nkama & Filli, 2006).

Bulk density
Flour samples (10 g) were measured into a 50 ml graduated measuring cylinder and gently tapped on the bench 10-times to achieve a constant height. The volume of the sample was recorded and expressed as grams per milliliter (Ashraf et al., 2012).

Swelling power and solubility index
The method reported by Afoakwa, Budu, Asiedu, Chiwona-Karltun, and Nyirenda (2012) was used for the determination of the swelling power (SWP) and solubility index (SI) of the samples. About 2.5% aqueous starch dispersion was put in centrifuge tubes, capped to prevent spillage, and heated in a water bath with shaker (Precision Scientific, Model 25: Chicago, USA) at a temperature of 85°C for 30 min. The tubes were allowed cooled to room temperature and centrifuged after heating (Thelco GLC-1, 60647: Chicago, USA) at 3,000 rpm for 15 min. The paste was separated from the supernatant and weighed. The liquid above the sediment was evaporated in a hot air oven (Mmert GmbH + Co.KG: D-91126, Germany) at a temperature of 105°C, and the residue weighed. All determinations were performed in duplicates, and the SWP and SI were calculated as:

Water absorption capacity
About 1 g of flour sample was weighed into a clean pre-weighed dried centrifuge tube and mixed adequately with distilled water (10 ml) by vortexing. The suspension was allowed to stand for 30 min and Centrifuged (Thelco GLC-1, 60647: Chicago, USA) at 3,500 rpm for 30 min. After centrifuging, the supernatant was decanted, and the tube with the sediment was weighed after removal of the adhering drops of water. The weight of water (g) retained in the sample was reported as the WAC .

Oil absorption capacity
Flour sample (1 g) was suspended in vegetable oil (5 ml) inside a centrifugal tube. The slurry was shaken on a platform tube rocker for 1 min at room temperature and centrifuged at 3,000 rpm for 10 min. The supernatant was decanted and discarded. The adhering drops of oil were removed and reweighed. The OAC was expressed as the weight of the sediment/initial weight of the sample (g/g) (Akinwale, Shittu, Adebowale, Adewuyi, & Abass, 2017;Asouzu & Umerah, 2020;Niba, Bokonga, Jackson, Schlimme, & Li, 2001).

Dispersibility
A sample of 10 g was dispersed in distilled water in a 100 ml measuring cylinder, and distilled water was added up to 50 ml mark. The mixture was stirred vigorously and allowed to settle for 3 hr. The volume of settled particles was noted and the percentage was calculated (Asaam, Adubofuor, Amoah, & Apeku, 2018;Kulkarni & Ingle, 1991).

| Pasting properties of yam flour
The pasting properties of yam flour were measured using a Rapid Visco Analyzer (Model RVA 4500, Perten Instruments, and Australia) equipped with a 1,000 cmg sensitivity cartridge. Yam flour (3.5 g) was weighed into a dried empty canister and 25 ml of distilled water was added. The mixture was thoroughly stirred and the canister was fitted into the RVA as recommended. The slurry was heated from 50 to 95°C at a rate of 1.5°C/min, held at this temperature for 15 min, cooled to 50°C. Viscosity profile indices recorded from the pasting profile with the aid of Thermocline for Windows Software connected to a computer were peak viscosity, trough, breakdown, final viscosity setback, peak time, and pasting temperature (Akonor, Tortoe, & Buckman, 2017;Donaldben, Tanko, & Hussaina, 2020;Falade & Olugbuyi, 2010).

| Effect of packaging materials and storage periods on the sensory attributes of amala
Sensory evaluation is an expression of an individual likes or dislikes for a product as a result of biological variation in humans and how people perceive sensory attributes (Sharif, Butt, Sharif, & Nasir, 2017 Therefore, packaging yam flour in PVC may keep most of the properties preferred by the consumers when stored for up to 4 months.

| Effect of storage periods and packaging materials on the functional properties of yam flour
The functional properties of a particular food describe the physical and chemical characteristics that impact the behavior of the protein in food systems during processing, cooking, storage, and consumption (Mahajan & Dua, 2002). The functional properties of yam flour as affected by storage period and packaging materials are shown in Table 2. The results revealed that all the functional properties of the yam flour were significantly affected by the storage periods (p < .001) except the swelling power, which was not significantly affected (p > .05). The packaging materials, moreover, have no significant effect (p > .05) on all the functional properties of the yam flour except the bulk density, which was significantly affected (p < .001).
The bulk density and the dispersibility of the yam flour were significantly affected (p < .001) by the interactions between the storage period and the packaging materials (Table 2).
Water absorption capacity (WAC) represents the ability of a product to associate with water under conditions where the water is limited (Singh, 2001). The WAC is desirable in food systems to improve yield, consistency, and give body to the food (Osundahunsi, Fagbemi, Kesselman, & Shimoni, 2003 (Table 2). However, a general reduction of the OAC was observed as the storage periods increased for the two packaging materials. The flavor of amala produced from the PPS (r = .76%, p > .05) and PVC (r = .81, p < .05) packaged yam flour had a positive correlation with the OAC (Tables 3 and   4). This implied that freshly produced yam flour would retain more flavor than the stored ones, notwithstanding the packaging materials used for storage (Seena & Sridhar, 2005). A negative but not significant correlation (p > .05) exists between the mouthfeel of the amala produced from the PPS (r = −.41) and PVC (r = −.03) packaged yam flour (Tables 3 and 4).
The swelling power (SWP) is an indication of the water absorption index of the starch granules during heating (Chinma, Ariahu, & Abu, 2013;Loos, Hood, & Graham, 1981 (Table 3). The correlation between the SI TA B L E 3 Pearson correlation between the sensory attributes of amala and the functional and pasting properties of yam flour packaged in polypropylene woven sack and stored for 20 weeks and these sensory attributes (texture, stretchability, moldability, mouthfeel, and overall acceptability) for the PVC packaged yam flour was negative (p > .05) (Table 4). This inferred that the type of packaging materials used for the storage of yam flour might affect the SI.
The bulk density (BD) is very critical to evaluate floury products regarding its weight, handling requirement, and the type of packaging materials suitable for storage and transportation of the food materials (Ohizua et al., 2017;Oppong, Arthur, Kwadwo, Badu, & Sakyi, 2015). The BD of the yam flour before storage was 86% (zero weeks), which significantly reduced to 75% (16 weeks) in PPS packaged yam flour and 70% (20 weeks) in PVC packaged yam flour ( Table 2). The values of the BD of this study were higher than the values (64%-76%) reported for different varieties of water yam flour by Udensi, Oselebe, and Iweala (2008) and Ogunlakin et al. (2013). Fagbemi (1999) and Adepeju, Gbadamosi, Adeniran, and Omobuwajo (2011) reported that high BD is desirable because it offers greater packaging advantage, as more quantity may be packed within a constant volume. This shows that proper packaging of yam flour in PPS and storing for 20 weeks will be more economical in terms of reduction in transportation cost compared to packaging in PVC. The BD of the PVC packaged yam flour had a positive but not significant (p > .05) correlation with all the sensory attributes (texture, color, stretchability, moldability, flavor, mouthfeel, and overall acceptability) of the amala (Table 4). The correlation was the same for that of the PPS packaged yam flour except for the stretchability and mouthfeel of the amala that was negatively correlated with the BD (Table 3). Besides, the BD of the PPS packaged yam flour had a significant positive correlation (r = .88, p < .05) with the flavor of the amala ( Table 3).
The measure of the reconstitution of flour in water is known as the dispersibility, and the more the dispersibility, the better the samples reconstitute in water (Adebowale, Sanni, & Onitilo, 2008;Kulkarni & Ingle, 1991). The dispersibility reduced from 74% (zero weeks) to 71.50% (8 weeks) in PPS packaged yam flour, and to 71.50% in PVC packaged yam flour (20 weeks). There was a sharp increase in the dispersibility of the yam flour at 12 weeks of storage in PPS (76%) and PVC (75.5%) packaging materials (Table 2).
This implied that yam flour packaged in PPS and PVC might reconstitute properly in hot water without lumps formation in the preparation of amala, when stored for 12 weeks (Adebowale et al., 2008), although there was no significant difference (p > .05) in the dispersibility of the yam flour stored at zero weeks and 12 weeks for the two packaging materials (Table 2) (Tables 3 and 4).

| Effect of storage periods and packaging materials on the pasting properties of yam flour
The pasting properties of flours are used in assessing the suitability of its application as a functional ingredient in food and other industrial products (Oluwalana, Oluwamukomi, Fagbemi, & Oluwafemi, 2011), and it also affects the sensory acceptability of the cooked starchy products (Adebayo-Oyetoro, Ogundipe, & Nojeemdeen, 2016). Gelatinization and pasting of starch are of importance to the food industry because they affect the texture, stability, and digestibility of starchy foods and, thus, control the application and use of flour in different products (Dosunmu & Bassey, 2003;Oke, Awonorin, & Workneh, 2013). The results of the pasting properties of yam flour as affected by storage periods and packaging materials revealed that the storage periods significantly (p < .05) affected the pasting properties, while the packaging materials have no significant effect on the pasting properties (Table 5). The combined interaction between storage periods and packaging materials had no significant (p > .05) effect on the pasting properties of the stored yam flour except for peak and trough viscosities ( Table 5).
The peak viscosity indicates the water binding capacity of the starch, which is often related to the final product quality and indi-  Table 3). The PVC packaged yam flour peak viscosity was positively correlated (p > .05) with all the sensory attributes of the amala except the flavor, mouthfeel, and overall acceptability, which were negatively correlated (Table 4). These variations may be attributed to the difference in the properties of the packaging materials used. The trough viscosity measures the ability of the starch paste to withstand breakdown during cooling. Thus, the trough viscosity is very important in describing the quality of the starch gel (Adebowale et al., 2008;Adegunwa, Omolaja, Adebowale, & Bakare, 2016;Madsen & Christensen, 1996;Mubaiwa, Fogliano, Chidewe, & Linnemann, 2018 Neale, 1999;Rasper, 1969). The texture ( (Table 4).
The breakdown viscosity significantly (p < .05) reduced from 296.96 RVU (zero weeks) to 209.75 RVU (12 weeks) in PPS packaged yam flour, and to 219 RVU (8 weeks) in PVC packaged yam flour (Table 5). There was no significant difference (p > .05) in the breakdown viscosity of the PPS packaged yam flour stored for 12 weeks, and the PVC packaged yam flour stored for 8 weeks with that of the 20 weeks of storage (Table 5). Breakdown viscosity describes the ability of the floury product to withstand heating and shear stress during cooking, and high breakdown viscosity is associated with a decreased ability of starch to withstand heating and shear tress (Adebowale et al., 2008Ohizua et al., 2017). This means that the storage of yam flour in PPS for 12 weeks and PVC for 8 weeks may increase the ability of the starch to withstand heating and shear stress during reconstitution to amala, due to the low breakdown viscosities (Adebowale et al., 2008;Falade & Christopher, 2015).
The final viscosity is the most commonly used parameter to determine the ability of starch-based materials to form a viscous paste or gel after cooking and subsequent cooling as well as the resistance of the paste to shear force during stirring (Adebowale et al., 2005;Maziya-Dixon et al., 2007;Sanni et al., 2015).  (Table 5).
This implied that the amala produced from the zero week yam flour might weep easily compared to that packaged in both PPS and PVC and stored for 20 weeks (Adebowale et al., 2005). The setback viscosities of the PPS and PVC (except color) packaged yam flour were positively correlated with all the sensory attributes (p > .05) (Tables 3 and 4).
The temperature at which the first detectable increase in viscosity is measured and which is an index characterized by the first change due to swelling of starch is called the pasting temperature (Chinma et al., 2013;Julanti, Rusmarilin, & Ridwansyah, 2015;Ohizua et al., 2017). The pasting temperature of the yam flour packaged in PPS and PVC remains statistically the same throughout the storage periods (approximately 83°C). The peak time, which is a measure of the cooking time of the flour (Adebowale et al., 2008;Ohizua et al., 2017), was approximately 5min for all the yam flour packaged in PPS and PVC and stored for 20 weeks (Table 5). However, there was a significant (p < .05) reduction in the peak time from 4.90 min (zero weeks) to 4.40 min (12 weeks) in PPS packaged yam flour, and to 4.37 min (12 weeks) in PVC packaged yam flour (Table 5). This implied that all the yam flour might be prepared into amala in approximately 5 min and less than the boiling point of water (83°C), thus, reducing the cost of energy consumption (Awoyale et al., 2016). The peak time of the PPS packaged yam flour was negatively correlated with all the sensory attributes of the amala (p > .05) except the flavor, which was positive and also not significant (p > .05) ( Table 3). In contrast, the peak time of the PVC packaged yam flour had a positive but not significant correlation with all the sensory attributes of the amala except for the mouthfeel, which was negative (p > .05) ( Table 4).
All the sensory attributes of the amala (except color and flavor) produced from the PPS packaged yam flour have a negative correlation with the pasting temperature of the yam flour (p > .05) (Table 3), while the correlation between the pasting temperature of the PVC packaged yam flour and all the sensory attributes were positive (p > .05) ( Table 4).

| CON CLUS IONS
This study revealed that the packaging materials and storage periods had no significant effect (p > .05) on the sensory attributes of the amala except the moldability, which was significantly affected by the storage periods. All the functional (except swelling power) and pasting properties of the yam flour were also significantly affected (p < .05) by the storage periods. The peak and trough viscosities of the yam flour packaged in polypropylene woven sack have a significant negative correlation with the texture, stretchability, moldability, and overall acceptability of the amala. The correlation of the peak and trough viscosities of the yam flour packaged in polyvinyl chloride container with the texture, stretchability, and moldability of the amala was positive but not significant, while that of the overall acceptability was negative, although the flavor of the amala produced from polyvinyl chloride container packaged yam flour was positively correlated with the oil absorption capacity and negative for the trough viscosity. The overall acceptability was higher in the polyvinyl chloride container packaged yam flour amala compared to that packaged in polypropylene woven sack, specifically at the 16 weeks of storage. Therefore, yam flour should be stored in polyvinyl chloride container instead of the polypropylene woven sack, to retain its sensory attributes for 12 weeks.

ACK N OWLED G M ENTS
This research was supported by Roots, Tubers, and Bananas Foods and carried out at the International Institute of Tropical Agriculture (IITA) Ibadan, Oyo State, Nigeria.

CO N FLI C T O F I NTE R E S T
The authors have declared no conflicts of interest for this article.