Starches of two water yam (Dioscorea alata) varieties used as congeals in yogurt production

Abstract The physicochemical properties of water yam (Dioscorea alata var. Akaba and Matches) starches were determined prior to their use as congeals for yogurt production. The moisture content ranged from 9.34% to 15.8% for A100 (100% Akaba) and M100 (100% Matches), respectively, indicating lower moisture content in the Akaba variety compared to Matches variety. Similar trend was observed for their water activity. The pH ranged from 5.88 to 6.93 indicating low acidity of the water yam starches. The water absorption capacity (WAC) ranged from 4.10 to 4.89 g/g, seemingly restricted reflecting protein–moisture interaction of the starches. Although the swelling power did not differ significantly (p > 0.05) ranging from 10% to 14%, they were quite restrictive as the WAC. The L* values of the starches were predominantly lightness in color, highest for A100 sample. The pasting temperatures of Akaba (A100), Matches (M100), and A50:M50 were not significantly different (p > 0.05). Peak viscosity of the water yam starches was in a range of 509–528 BU. The highest attributes were for taste (6.4), mouthfeel (5.4), flavor (5.4) sourness (4.6) and consistency (5.9), which were obtained from 1.5 % Matches, 0.5 % Akaba + 0.5 % Matches, 1.0 % Akaba + 1.0 % Matches samples. The overall acceptability (5.8) was higher than the control yogurt (4.7), indicating sample 0.5% Akaba + 0.5% Matches as the best‐bet yogurt.

acts as a digestive aid, encourages the growth of beneficial bacteria and inhibits the growth of harmful bacteria in the gut, stimulates intestinal immunity and is an excellent food for lactose intolerant people (El-Abbadi et al., 2014).
In addition, other lactobacilli and bifidobacteria are also sometimes added during or after culturing yogurt. However, due to the expanding regime of functional foods and nutritional needs often accompanying species of live lactic acid bacteria and bioactive compounds, the composition of bacteria starter culture varies at industrial production (Birollo, Reinheimer, & Vinderola, 2000;Park et al., 2005).
The fermentation of lactose by these bacteria produces lactic acid, which acts on milk protein to give yogurt its texture and characteristic flavor (Lahtinen et al., 2012). The fermentation process results in partial hydrolysis of fat, protein, and lactose resulting in yogurt been easily digestible compared to milk and suitable for people suffering from lactose intolerance.
Congeals have been added to yogurts to improve the texture and consistency especially without substantially changing its other properties whenever yogurt production end result was thin instead of thick yogurt. Food congeals are based on either polysaccharide (starch, vegetable gums, and pectin) or proteins. Different congeals may be more or less suitable in a given application, due to differences in taste, clarity, and their responses to chemical and physical conditions (Deven, Glassburn, Jodelle, & Deem, 1998). Subsequently, flavorless powdered starches such as arrowroot starch, cornstarch, potato starch, cassava and yam and their derivatives have been used as congeals. Food congeals from vegetable gums included alginin, guar gum, locust bean gum, and xanthan gum. Proteins used as food thickeners included collage, egg white, and gelatin, whereas sugar included agar and carrageenan (Deven et al., 1998). Generally, starch congeals in yogurt improve the viscosity, texture, and mouthfeel and prevent wheying-off. Starch congeals are popular due to their advantage to thicken yogurts without adding fat and give the food a transparent, glistening sheen, creamy texture as well as ease processing at a lower cost compared to other hydrocolloids (Koegh & O'Kenedy, 1998). Starch granules imbibe water and swell to many times their original size, resulting in increased viscosity of the solution (Basim, Hazim, & Ammar, 2004). The gelatinization results in changes of the granular structure, swelling and hydration, and solubilization of starch molecules. Swelling is accompanied by leaching of granule constituents, mostly amylose. In a mixture of milk and starch, during heat, treatment may lead to different rheological characteristics in the final yogurt gel product compared to that made from milk alone (Narpinder, Jaspreet, Lovedeep, Navdeep, & Balmeet, 2003). Starch behavior in a system like that one of yogurt will also depend on their physical and chemical characteristics, such as mean granule size distribution, amylase/amylopectin ratio, and mineral content.
Yam (Dioscorea spp.) is a tropical tuber crop and a major source of income for farmers and traders in sub-Sahara Africa (Tortoe, Dowuona, Akonor, & Dziedzoave, 2017). Yam is not only a good source of starch and vitamin C, but it is also an important sociocultural crop that is prominent in the cultural and religious festivals of the people of West Africa (Deutsche Gesellschaft fur Technische Zusammenarbeit, 1995). There are more than 200 species of yam in cultivation (Amusa, Adegbite, Mohammed, & Baiyewu, 2003).
Water yam (Dioscorea alata) is noted for its bulkiness, high moisture, and starch content, although underutilized compared to the popular variety of Dioscorea rotundata. Alternative food uses of water yam as a source of suitable congeal in yogurt production would improve its cultivation and increase incomes for farmers and traders and expand its food forms for consumers. In expanding the food uses of Dioscorea alata, this study was aimed at analyzing some physicochemical properties of starches obtained from two varieties of Dioscorea alata (Akaba and Matches) in Ghana prior to evaluating their sensory acceptability as congeals in yogurt production.

| Natural starch extraction
The matured water yam varieties (Dioscorea alata var. Akaba and Matches) were washed under running water, peeled with a stainless steel knife, and washed again. The peeled tubes were immediately cut into thin slices (5 mm) into a plastic basin containing a solution of 1% sodium metabisulfite. The slices were removed after 10 min with a sieve to allow adhering water to drain and blended into a

| Determination moisture content
Five grams (5.0 g) of starch was measured into an Electronic Moisture Analyzer-Sartorius MA 45 (Sartorius GMBH, Gottingen, Germany) to measure the moisture content. The analysis was performed in triplicates.

| Determination of water activity
Using a Rotronic HygroLab 2 (Rotronic AG, Bassersdrof, Germany), the water activity (a w ) of the starch was measured in triplicates. The a w was calibrated using a saturated salt solution of relative humidity of 70% (0.75 a w ). In measuring the a w , 8.0 g of starch was weighed using Sartorius Portable, PT600, Sartorius GMBH, Gottingen, Germany, scale and transferred into the chamber of the Rotronic HygroLab 2. The a w was conducted in triplicates.

| Determination of pH
The pH of starches was determined using approved methods of the Association of Official Analytical Chemists (AOAC, 2000) in triplicates.

| Water absorption capacity
The water absorption capacity (WAC) of the starch was determined by a modified method of Afoakwa, Budu, Asiedu, Chiwona-Karltun, and Nyirenda (2012). Briefly, 1.0 g of starch was mixed with 10 ml of distilled water for 30 s. The sample was allowed to stand at room temperature (28°C) for 30 min, after which they were centrifuged at 1,050 g (Hermle Z 206A, Germany) for 30 min. The volume of the supernatant was recorded and the water absorption capacity calculated as the difference between the initial volume of water added to the starch and the volume of the supernatant. This was done in triplicates and mean values calculated.

| Swelling power
Starch dispersions of 2.5% were put in centrifuge tubes, capped to prevent spillage, and heated in a water bath with a shaker (GRANT

| Colorimetry
Color analysis of the starch was done in triplicates using a Minolta Chromameter (CR-310 Minolta, Japan) in triplicates. A reference white porcelain tile (L 0 = 97.63, a 0 = 0.31 and b 0 = 4.63) was used to calibrate the Chromameter before each determination. The starch color was described in L* a* b* notation, where L* is a measure of lightness, a* defines components on the red-green axis, and b* defines components on the yellow-blue axis.

| Starch pasting profile
The pasting properties of the starches were determined at 8% slurry using a Brabender Viscoamylograph (Viskograph-E, Brabender Instrument Inc., Duisburg, Germany) equipped with a 1,000 cmg sensitivity cartridge. The determinations were done for 100% Akaba, 100% Matches, and a combination of 50% Akaba: 50% Matches starches. The suspension 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 at a rate of 1.5°C/min, and held at this temperature for 15 min. The viscosity profile indices recorded included the following: pasting temperature, peak viscosity, viscosity at 95°C and viscosity after 15 min hold at 50°C (50°C-hold), breakdown and setback.

| Preparation of yogurt
The methods described by Lee and Lucey (2010) were modified by homogenizing powdered cow's milk and water and heating at 85-90°C for 5 min in a boiling water bath during which sugar was added at a rate of 6.5% (w/v) to the mixture of custard-like consistency to denature the milk proteins to avoid the formation of curds. variety (0.5%, 1.0%, 1.5%); Matches variety (0.5%, 1.0%, 1.5%); and a combination of Akaba plus Match varieties (0.5%, 1.0%, 1.5%). The prepared yogurts were refrigerated at 4°C until subsequently used.
The control sample was a popular full milk dairy yogurt purchased from a supermarket in Accra.

| Sensory evaluation of yogurt
A sensory panel consisting of 20 semi-trained panelists who were familiar with sensory attributes of yogurt was assembled to assess the yogurts. A 7-point Hedonic scale was used to rate the yogurt for taste, mouthfeel, flavor, sourness, viscosity, and overall acceptability. A score of 1 represented "dislike extremely" and a score of 7 represented "like extremely" (Lawless & Heymann, 2010

| Data analysis
Data were analyzed for differences using ANOVA, and differences separated by Duncan's multiple range tests (SPSS 17.0.1, SPSS Inc. USA). Statistical significance was set at a level of 95% confidence interval. Results were reported as means ± SE, and descriptive results plotted out in the form of bar charts.
The water activity was lowest in A100 (0.41) and highest in for millet, corn, and cocoyam starches, which were considered to be within the acceptable range and beneficial in terms of storage and keeping quality of the starches (Mepba, Eboh, Eko, & Ukpabi, 2009;Suma & Urooj, 2015). According to Aguilera et al. (1995), high amounts of moisture in flour and starches may results in caking due to aggregation of particles into lumps, which subsequently lowers their quality and functionality.
The pH, which is also an indicator of the starch quality, ranged from 5.88 to 6.93 for A100 as the lowest and A10:M90 as the highest (Figure 3). Changes in the pH may affect the functionality of the water yam starches. Samples with a combination of Akaba and Matches water yam starches had an average pH range of 6.69-6.88 for A20:M80 and differ significantly (p < 0.05). The pH range suggested that the water yam starches are a low acid commodity (Thomas & Atwell, 1999) comparable to cocoyam and cassava starches. Generally, low acidic starches are necessary for the food manufacturing industry.

| Water absorption capacity and swelling power
The water yam starches were seemingly restricted in their water absorption capacity (WAC), which was observed in a range of 4.10-4.89 g/g for M100 as the lowest and A100 as the highest (Figure 4).
Clearly, the WAC conforms to the moisture content and the water activity as observed for M100 with highest moisture content (15.8%) F I G U R E 2 Water activity content of water yam starches F I G U R E 3 pH of water yam starches and highest water activity (0.68), whereas A100 had the lowest moisture content (9.34%) and lowest water activity (0.41; Figures 1 and 2). This indicated that the low moisture content and water activity of the samples had a high affinity for water, thus resulting in the highest WAC for A100 water yam starch. The WAC of the samples with a combination of Akaba and Matches starches was in the range of 4.20-4.80 g/g. The water yam starches were significantly different (p < 0.05) in their WAC behavior and were lower compared to studies reported by Bhupender, Rajneesh, and Baljeet (2013).
Interestingly, the WAC observed was lower than that observed by Osundahunsi, Fagbemi, Kesselman, and Shimoni (2003). The WAC is a reflection of the protein-moisture interaction of the yam starches.
Water yam starches with high amounts of proteins possess a lot of water-binding sites, which increases their WAC as observed by other authors (Bhupender et al., 2013). In other studies, the high WAC was attributed to the loosely associated amylose and amylopectin and the association of hydroxyl groups to form hydrogen and covalent bonds between starch chains (Das, Singh, Singh, & Riar, 2010).
Generally, differences in WAC may be attributed to differences in starch structure and morphology, amylose and amylopectin and the presence of salts, proteins and other granular components brought about by differences in genetic makeup.
The swelling power (SP) of the water yam starches was in the range of 10%-14%, quite restrictive and did not differ significantly (p > 0.05). The lowest SP (10%) was recorded for A100 and M100, whereas the highest (14) was observed for A80:M20 ( Figure 5).

F I G U R E 4 Water absorption capacity of water yam starches
F I G U R E 5 Swelling power of water yam starches Swelling power is influenced by amylose and amylopectin content as well as their chain length distribution resulting in the similarities among the water yam starches observed. The SP is indicative of an intermolecular association between starches polymers associated with eating quality and is influenced by amylose that acts as both diluent and an inhibitor of swelling, which is responsible for retrogradation, whereas amylopectin is responsible for gelatinization behavior of starches (Tester & Morrison, 1990). It measures the hydration capacity of starches and is temperature dependent and accompanied by solubilization of starch granule constituents (Dorporto, Mugridge, Garcia, & Vina, 2011). The SP observed in this study was similar to that obtained for nonirradiated sweet potato starches as reported by Srichuwong, Sunarti, Mishima, Isono, and Hisamatsu (2005) and Ocloo, Bansa, Boatin, Adom, and Agbemavor (2010). In studies by Swinkles (1985), the author reported that yam bean starch had lower swelling power than cassava starch. The highest swelling power obtained for A80:M20 indicated the presence of high amylose content and probably indicate synergetic interactions between the different starches. Further, Sanni, Ikuomola, and Sanni (2001) reported that the high SP in potato was due to the high phosphate content that allows easier water entrance into the granules. According to Lindeboom, Chang, and Tyler (2004), amylose and amylopectin content are responsible for the properties of starch pastes, gels, and starchy food systems. Additionally, amylopectin is either short or long chains and starches with higher amounts of long chains result in gels with higher viscosity and stability compared to the short chain counterparts according to Wrolstad and Smith (2010).

| Color
The

| Pasting properties of the starches
The pasting properties of the best-bet water yam starches are presented in Table 2. The pasting temperatures of Akaba (A100), Matches (M100), and A50:M50 were similar without significant differences (p > 0.05) between them, which indicated that the swelling of the water yam starch granules commenced at similar temperature when subjected to heat (Table 2). This results in the formation of a viscous paste (Afoakwa, Adjonu, & Asomaning, 2010). The pasting temperature observed in this study was higher than that of yam bean starch paste reported in a range of 53-63°C, which was similar to that for cassava and sweet potato (Aprianita, 2010). However, the peak viscosity of the water yam starches in a range of 509-528 BU was lower than that reported by Sefa-Dedeh and Sackey ( showing that the water yam starches were able to withstand more heating and shear stress (Shimelis, Meaza, & Rakshit, 2006). The setback was in the range of 136-152 BU. These values indicated that the retrogradation tendency of water yam starches was mainly dominated by amylose gelation. Additionally, the high setback has been associated with a high degree of affinity among starch molecules caused by hydrogen bonding (Sefa-Dedeh & Sackey, 2002).
Generally, the setback viscosities significantly differed between the water yam starches. In other studies, the pasting viscosities were positively correlated and higher pasting viscosities corresponds to higher swelling power and higher water holding capacity of the samples (Singh et al., 2003).
overall acceptability highest score of the developed yogurts was 5.8, comparable to the maximum score of 7.0 and higher than the control yogurt sample (4.7). Overall acceptability was not significantly different (p > 0.05) for all the developed yogurts, although there were differences in rating for individual attributes. The yogurt developed from the combined samples of water yam starches of Akaba and Matches had higher sensory scores in a range of 5.4-5.8 compared to the single variety samples except for 0.5% Matches and 1.5% Matches samples.
Generally, clear trends were established for all the attributes of the developed yogurt samples compared to the control yogurt and the general overall acceptability for all the developed yogurts products was that of "like moderately" or better, in respect of the 7-point Hedonic scale used in the study (Lawless & Heymann, 2010).

| CON CLUS ION
Water yam starches of Akaba variety had lower moisture content compared to the Matches variety. The water activity of the two starches was lower, therefore, supported the quality and shelf life of the water yam starches, which were generally low acidic.
Additionally, the starches were restricted in their WAC. The SP of the starches was quite restrictive in their behavior but predominantly was lightness in color, which is a plus for new product development. The yogurt samples with combinations of water yam starches from Akaba plus Matches were sensory accepted more than the single variety yogurt samples. The best-bet yogurt sample was 0.5% Akaba + 0.5% Matches, with overall acceptability (5.8) higher than the control yogurt (4.7). This study established that water yam starches could be employed to thicken yogurts to produce transparent, creamy texture, sweet taste, flavor, consistency, and acceptable product.