Quality and storage stability of yoghurt produced from pigeon pea milk supplemented with Propionibacterium freudenreichii

Abstract

Pigeon pea (Cajanus Cajan) is consumed in many parts of Africa as a source of protein and carbohydrate. It is underutilised and mainly grown for subsistence. Researching on pigeon pea may enhance value addition and increase its utilization. In this study, a non-dairy probiotic yoghurt was prepared from pigeon pea milk. Yoghurt samples were prepared, using 100% pigeon pea milk, pigeon pea/soy milk in the ratio 50:50 and 100% soy bean milk. The yoghurts were inoculated with yoghurt starter cultures and divided into two equal parts. One part inoculated with Propionibacterium freudenreichii was referred to as probiotic yoghurt, while the other part served as the control. The nutritional, sensory and some functional properties of the yoghurt were determined. The microbiological quality of yoghurt samples stored at 4, 10 and 21°C, respectively, for 4 weeks, were monitored and analysed for aerobic spores’ formers, E. coli, total plate counts, mould and Propionibacterium freudenreichii weekly. The protein contents of the yoghurt samples varied from 4.54-5.85% for 100% soymilk and 100% pigeon pea yoghurt respectively. The probiotic yoghurt showed slightly lower protein content than pigeon pea yoghurt alone. All the yoghurt samples had considerably high total solids (16.04-17.41%) and were fairly good sources of amino acids. Essential amino acids in the yoghurt samples were comparable to the FAO/WHO (2007) recommended amino acid requirement for adults. Anti-nutritional factors of yoghurt samples were significantly lower (P≤0.05) than their milk counterparts, which may be attributed to the fermentation process. Probiotic yoghurt samples showed higher firmness than non-probiotic samples. Total plate counts (log 7.01- 7.46 CFU/ml) samples stored for 2 weeks at 4° C were similar. Predominant organisms were LAB and Propionibacterium freudenreichii. Storage temperature of yoghurt samples had an influence on the total plate count and LAB. Total plate count and LAB significant increased approximately by log 2 CFU/ml for the first two weeks of storage. However, moulds and E. coli were not detected in all samples. Beyond 2 weeks of storage, there was significant decline in total plate counts and LAB, while mould grew and increased. Aerobic spore formers and moulds were observed in the control yoghurt. However, E. coli was not found in all yoghurt samples throughout storage period. The pH of the milk in which yoghurt mixtures were formulated, ranged from pH 7 to 6.8 for pigeon pea and soymilk declined significantly as a result of acidification. Decline in pH at 4, 10 and 21°C was significant (p≤0.05) with the rate higher at 21, 10 than 4° C. Decline in pH resulted in increased TTA values over storage temperatures and periods. Samples stored at 21°C and 10°C had significantly higher TTA values than samples stored at 4° C. The colour values evaluated were recorded as L*, b*, a* and ∆E* during 4 weeks storage at 4, 10 and 21° C. Significantly high values (p≤0.05) were recorded for L* yoghurt samples with soymilk. The colour scale defines positive (red) and negative (green) for a* and b* positive (yellow) and negative (blue). All a* values both positive and negative were less than 3. There was no negative value recorded for b*. Colour difference ∆E* values trends increased as storage time and temperature increased. There were significant (p≤0.05) differences between samples stored at same and different storage temperatures and periods. Water holding capacity was significantly different (p≤0.05) in all the yoghurt samples stored at 4, 10 and 21°C for 4 weeks. Formulation with 100% soymilk recorded higher values. Soy yoghurt and probiotic yoghurts (100 %) showed higher water holding capacity compared to pigeon pea yoghurt and pigeon pea/soymilk yoghurt. The addition of Propionibacterium freudenreichii did not significantly affect sensory properties of the yoghurts. Acceptable yoghurt was produced from pigeon pea with comparable quality to soy which serves as control. Proximate composition was comparable to previous reports. Microbial quality and profile of all the yoghurt samples were similar. The absence of pathogenic bacteria in all the yoghurt samples confirm their safety. Soy yoghurt was most acceptable amongst the yoghurt samples but all the samples had comparable ratings, and these ratings are within commercially acceptable range (4 to 9) for yoghurt. Storage at 4oC should be the most acceptable, as storage at 21oC encourage proliferation of contaminant