3.1. Colour quality of the yoghurt
Colour is considered as one of the most important quality parameters in yoghurt production. The result presented in Table 1 revealed a significant differences (p < 0.05) in the lightness parameter of the samples, with sample from cow milk (CM) having the highest value (54.40 ± 041) followed by yoghurt from oat milk (OM) sample (50.98 ± 0.73) and African yam beans (AYB) sample having the least value (49.66 ± 0.34). Besides, a significant (p < 0.05) difference was also recorded in the redness parameter with sample OM having the highest value (3.71 ± 0.04) and sample AYB with the second value (3.53 ± 0.04) while sample CM had the least value of redness (2.90 ± 0.12). Unlike the first two parameters, there is no significant (p > 0.05) difference between sample OM (8.99 ± 0.69) and sample CM (8.51 ± 0.14) in the yellowness parameter whereas sample AYB (6.68 ± 0.18) is significantly lower when compared to samples CM and sample OM (Table 1). The colour results obtained from this study showed that sample CM, which is produced from cow milk, had the typical yoghurt (light-white) colour. The implication of having a colour that is different from this could affect the marketability of the yoghurt. That is, when such yoghurt product deviated from the consumers’ choice for yoghurt colours, which they were used to. Sample OM, which is the yoghurt produced from oats milk, had the closest value in terms of lightness, hence making oat seeds a close substitute to cow milk-based yoghurt when compared to sample AYB with least lightness value.
Table 1
Colour qualities of the yoghurt samples
Samples | L* | a* | b* |
CM | 54.40 ± 0.41a | 2.90 ± 0.12c | 8.51 ± 0.14a |
OM | 50.98 ± 0.73b | 3.71 ± 0.04a | 8.99 ± 0.69a |
AYB | 49.66 ± 0.34c | 3.53 ± 0.04b | 6.68 ± 0.18b |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |
3.2. pH of the yoghurt
The pH is a measure of the hydrogen ion concentrations in any compound. Study (Hwang and Lee, 2006) reported that the optimum pH of thick fermented milk coming into the market ranged from 3.27 to 4.59. It is interesting to note that the pH of the yoghurt samples (3.50–3.87) in this study, as presented in Table 2, fell into this optimum range. The pH values for the three samples are not significantly (p > 0.05) different from one another (Table 2). Notably, the lower pH affected the casein (milk protein), causing it to coagulate and precipitate, thereby forming the solid or thick curd that made up the yoghurt (Falade et al., 2015).
Table 2
Samples | pH | |
CM | 3.69 ± 0.00b | |
OM | 3.50 ± 0.00c | |
AYB | 3.87 ± 0.00a | |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |
3.3. Total soluble solids of the yoghurt
The total soluble solids of the yoghurt samples from CM, OM and AYB were in the range of 13.55 to 14.50% as presented in Table 3. Literature reported that the total soluble solid is the mass fraction of substances remaining after completion of the heating process (Osundahunsi, Amosu and Ifesan, 2007). It is important because it has effect on the viscosity of the yoghurt. Past finding had shown that an increase in the total solids led to an increase in viscosity, which was good for the rheological property of a yoghurt sample (Falade et al., 2015). It is worthwhile to note that the current yoghurt samples produced from the vegetable sources (OM and AYB) had higher total solids than those produced from animal source (CM). The result obtained in this study is similar to the 12.4–14.50% previously reported for soy yoghurt (Osundahunsi, Amosu and Ifesan, 2007).
Table 3
Total Soluble Solids of the yoghurt samples.
Samples | Total Soluble solids (%) |
CM | 13.55 ± 0.30b |
OM | 13.85 ± 0.02b |
AYB | 14.50 ± 0.90a |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |
3.4. Viscosity of the yoghurt
The viscosity of the yoghurt samples were shown in Table 4 and were between 11.45 and 18.40 mPa−ls. The yoghurt from CM had the highest value (18.40 mPa−ls), while AYB sample had the least value (11.45 mPa−ls). Obviously, viscosity is affected by the strength and number of bonds between casein micelles in yoghurt, as well as their structure and spatial distribution (Izadi et al., 2014). The result obtained for sample CM (18.40 mPa−ls) was closely related to 19.40 mPa−s reported (Salami et al., 2020) for Bambara groundnut yoghurt. The viscosity of yoghurt has been found to be dependent on the lactic acid production. For instance, there existed a symbiotic or proto-cooperative relationship between Streptococcus thermophilus and Lactobacillus bulgaricus bacteria during yoghurt production. This is because the coagulation of milk proteins was induced by thermophilic bacteria (Streptococcus thermophilus and Lactobacillus bulgaricus), which propagated at high temperatures. Therefore, as the concentration of lactic acid increased, the proteins present in milk formed gel to give the end result as the viscous yogurt. Besides, the high protein content obtained for the CM sample (Table 5) might be responsible for its high viscosity (Table 4).
Table 4
Viscosity (mPa−s) and antioxidant activity of yoghurt samples.
Samples | Viscosity (mPa−s) | Antioxidant activity (DPPH %) |
CM | 18.40 ± 0.12a | 27.99 ± 0.45a |
OM | 12.60 ± 0.06b | 23.25 ± 0.79b |
AYB | 11.45 ± 0.08c | 20.69 ± 0.24c |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |
Table 5
Proximate composition of the yoghurt samples in (%)
Samples | Moisture | Crude fats | Crude protein | Crude fiber | Ash | Carbohydrate |
CM | 86.45 ±0.03a | 2.95 ± 0.03a | 6.10 ± 0.01a | 0.02 ± 0.00 | 0.94 ± 0.04a | 3.56 ± 0.04b |
OM | 86.15 ± 0.01a | 0.25 ± 0.02c | 5.42 ± 0.04b | 1.18 ± 0.00a | 0.88 ± 0.01ab | 6.12 ± 0.06a |
AYB | 85.50 ± 0.03b | 1.25 ± 0.02b | 5.93 ± 0.00a | 0.09 ± 0.03b | 0.86 ± 0.00b | 6.37 ± 0.48a |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |
3.5. Antioxidant activity of the yoghurt
Antioxidant is a molecule that inhibited the oxidation of other molecules caused by free radicals, for example, the DPPH radicals. Antioxidant activity of a dairy food is important both for the shelf life of the product as well as for protection from oxidative damage in the human body. (Arise, Alashi, Nwachukwu, Ijabadeniyi, Aluko and Amonsou, 2016) Free radicals are compounds that caused harm if their levels become too high in the body system. They have been linked to multiple illnesses, including diabetes, heart disease, and cancer. From Table 5, the DPPH radical scavenging activity of sample CM was significantly (p < 0.05) higher (27.99%) when compared to sample OM (23.25%) and sample AYB (20.69%), respectively. The hydrolysis of milk protein or organic acid production might have also contributed to the antioxidant activity of the yogurt samples due to microbial metabolic activity during fermentation. For instance, a past study (Cho, Hwa, Yang and Lee, 2020) reported a strong relationship between the high oxidative stability of yogurt and the antioxidant peptides released during milk fermentation by Lactobacillus bulgaricus.
3.6. Proximate composition of the yoghurt samples.
The result of the proximate composition of the yoghurt samples is presented in Table 5. The moisture contents of the yoghurt samples AYB, OM and CM were 85.50,86.15 and 86.45%, respectively. The moisture content obtained for AYB sample is a bit higher than the 82.76% previously reported (Aderinola and Olanrewaju, 2014) for the yoghurt produced from 100% African yam bean milk. The result obtained for samples CM and OM in this study (86.45 and 86.15%) were also closed to the 88.34 and 85.00% previously obtained by Bristone et al. (2015) and Rashid, Salariya, Qureshi and Hassan (2012), respectively. Since typical yoghurt contained high moisture content, yoghurt samples from this study, therefore complied with this standard. Although, very high moisture content in yoghurt could affect the texture and mouth feel of the yoghurt but the high moisture content could also help to hydrate the body after consumption.
Sample CM has the highest fat content (2.95%) while the result obtained for the present sample AYB was a bit higher than the 1.07% reported (Aderinola and Olanrewaju, 2014) for AYB produced yoghurt. The high fat content of the cow milk could be as a result of the cholesterol content of the milk compared to the vegetable milk yoghurts, but which was being a disadvantage since high cholesterol was not nutritionally good for the body. However, sample OM had the lowest fat content, which could not cause increase in the cholesterol level of the blood; hence, sample OM would be a perfect vegetable milk-yoghurt alternative for sample CM in order to help control the cholesterol level of the blood. In addition, past finding (Mårtensson et al., 2001) reported that soluble fibre content and the beta-glucan in oats helped to lower the blood cholesterol level.
Moreso, sample CM had the highest protein content, which was expected since CM is from animal source. However, the protein content contents for AYB and OM are in line with the ranges from previous findings (Qureshi, Salariya, Rashid and Parveen, 2012; Aderinola and Olanrewaju, 2014), respectively. Sample AYB had the closest value to sample CM; hence, it could prove as a good alternative to animal source of milk for yoghurt production, most especially, to those who might be suffering from animal milk protein allergy or lactose intolerance. The sample AYB could also provide an easy source of protein to those suffering from malnutrition (kwashiorkor). The sample OM had the significant (p < 0.05) highest fiber content (1.18%) when compared to samples AYB and CM (with 0.09% and 0.02%), respectively. The high amount of fiber in sample OM could be as a result of its soluble fiber content (β-glucan), which helped in easy digestion and prevented colon cancer (Qureshi et al., 2012). Moreso, the ß-glucans possessed the prebiotic function in the gastrointestinal tract by supporting the growth of beneficial microbial groups, whereby the slowly digestible fraction of oat starch moderated the glycemic response (Angelov et al., 2018).
Ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provided a measure of the total amount of minerals within a food. The ash content of sample OM (0.88%) was not significantly different from sample CM (0.94%) and sample AYB (0.86%), respectively. The value obtained for sample OM in this study was a bit different from the 0.96% reported by Rashid et al. (2012). Moreso, the result recorded for sample AYB was closely related to 0.94% obtained by Aderinola and Olanrewaju (2014). However, the ash content (1.75%) reported by Bahareh, Hajirostamloo and Peiyman Mahastie, (2008) was higher than the present values (0.86–0.94%) obtained from this current research work. Meanwhile, a high ash content implied that the inorganic (mineral) composition in the yoghurt was high as well. Though the highest ash content was found to be in sample CM (yoghurt made from cow milk), we noticed that ash content value of sample OM was closed to that of sample CM, this simply meant that it could provide a good source of mineral component to the vegetarians.
There was a significant difference in the carbohydrate levels in each sample. For instance, the sample AYB was having the highest amount of carbohydrate (6.37%), while samples OM and CM had 6.12 and 3.56%, respectively. The result obtained for sample CM was a little different to the 4.56% reported by Bristone et al. (2015); whereas, the result recorded for sample AYB wass different from the 9.09% reported by Aderinola and Olanrewaju (2014). The carbohydrate in plain yoghurt occured mainly as simple sugars called lactose (milk sugar) and galactose. However, the lactose content of yogurt was lower than those in milk. This was because bacterial fermentation resulted in lactose breakdown. For instance, lactose broke down to form galactose and glucose; meanwhile, the glucose was mostly converted to lactic acid, the substance that contributed to the sour flavor of yogurt and other fermented milk products (Sanders et al. 1996).
3.5. Microbial count or storability of yoghurt samples
3.5.1. Total viable bacteria count
Total viable bacterial count was the most common microbiological test that gave a quantitative idea about the presence of microorganisms such as bacteria in a sample (Fung, 2002). Figure 4 showed the bacterial count results of the yoghurt samples stored at refrigeration temperature (4 oC). The result obtained on day zero of the storage showed that there was a significant difference (p < 0.05) between the yoghurt samples stored at 4 oC. For instance, the sample CM had the highest count 158 ± 0.30 cfu/ml, followed by sample AYB with 55 ± 0.60 cfu/ml and sample OM had the least count (19.50 ± 0.50 cfu/ml on the first day of storage. The high total viable count of the sample CM could be as a result of presence endogenous microorganism from the host cow. Meanwhile, the total viable bacteria count declined on day three (3) of the storage at 4 oC compared to the first day while a considerable increase was noticed on the seventh day of storage. A further decline in the total viable bacteria count also occurred on the fourteenth day of storage at 4 0C. The total bacterial count obtained from this study is similar to those reported by Aderinola and Olanrewaju (2014), where decline was also experienced as the days of storage increased, which could be as a result of the low temperature storage. Panoff, Thamma, vongs, Guéguen and Boutibonnes ( 1998) and Palova Charvat Masopust, Klapkova and Kvapil (2007) have previously reported the probable cause of death of bacterial cells to be as a result of damage in cell membranes and DNA denaturation during low temperature storage. The result (Fig. 4) further showed that the sample OM, which was the yoghurt produced from oats milk, would have a longer shelf life compared with other yoghurt samples due to its least bacteria count after 14 days of refrigeration storage.
3.5.2. Total fungal count
The result of the total fungal count of the samples stored at 4 oC on day zero, day seven and day fourteen were presented in Fig. 5. Sample CM was found to have the highest count of 78.50 ± 0.16 (cfu/ml). Meanwhile, a decline in the fungal count was also noticed as the storage days increased, which was similar to the result on the bacterial count (Fig. 4). The decrease in fungal population of the yoghurt could be as a result of the low storage temperature of the product. Past studies (Panoff et al., 1998; Nazina, Griroryan, Feng, Shestakova, Babich, PavlovaIvoilov, NiWang and She, 2007) had reported that the damage in the cell membranes and DNA denaturation were probable causes of bacterial cells death during low temperature storage. Contrarily, the implication of the high fungal count is short shelf life of the yoghurt. Hence, the sample OM have the tendency of longer shelf life (low fungal count) when compared to the other yoghurt samples.
3.6. Sensory attributes of the yoghurt samples
Interestingly, the sensory evaluation results revealed that the panelists generally accepted all the yoghurt samples as shown in Table 6. The current result is in line with the past reports that showed high consumers’ acceptability of yoghurt produced from other vegetable sources such as soymilk (Salami et al., 2020) and Bambara groundnut milk (Falade et al., 2015), respectively.
Table 6
Sensory attributes of the yoghurt samples.
Samples | Taste | Aroma | Appearance | Overall acceptability |
CM | 8.00 ± 0.16a | 7.50 ± 0.12a | 8.10 ± 0.09a | 8.30 ± 0.09a |
OM | 7.80 ± 0.12a | 7.50 ± 0.14a | 7.00 ± 0.13b | 7.40 ± 0.16a |
AYB | 7.30 ± 0.16a | 6.80 ± 0.18a | 6.80 ± 0.13b | 6.90 ± 0.17a |
Values are means ± SD of ten determinations. Means with different superscripts within the same column are significantly different (p < 0.05). *Sample keys: CM: Cow milk, OM: Oats milk, AYB: African yam beans milk |