Development of probiotic soy-soursop yoghurt

Abstract

The increasing demand for functional foods due to their health benefits has created a need for products that can be produced from local sources. This study developed probiotic yoghurt from a blend of soy and soursop using a mixture of Lactobacillus rhamnosus yoba 2012 and Streptococcus thermophilus as the starter culture. Soursop pulp was added to the extracted soy milk in portions of 0%, 5%, 10% and 15%. The blend was then further processed to make soy soursop yoghurt by first pasteurizing at 85℃ for 15 s. The blends were cooled, inoculated with the starter culture and incubated at 45C for 24h. The yoghurt samples were analyzed for pH, acidity, Lb. rhamnosus yoba counts at intervals of 0h, 2h, 4h, 6h and 24h. Consumer acceptability and willingness to purchase were determined after 24h of fermentation. The most acceptable yoghurt sample was also analyzed for its nutritional composition. Lactobacillus rhamnosus yoba was able to grow in the soy-soursop yoghurt up to 8.1-9.3 log cfumL-1 . The highest cell growth was observed in the yoghurt containing 5% soursop whereas the lowest was observed in yoghurt containing 15% soursop. There was no significant difference (P>0.05) in the cell counts of all the yoghurt samples apart from that containing 15% soursop, which was however not significantly different from that containing 10% soursop. The pH reduced with the growth of the Lb. rhamnosus yoba up to 3.9-4.4, with 15% soursop having the lowest pH. The pH values were significantly different (P<0.05). Titratable acidity increased with fermentation to values of 0.62%-0.85% lactic acid. All the yoghurts were accepted although the yoghurt containing 15% soursop had the lowest scores (5 = neither like nor dislike). The yoghurt containing 5% soursop was the most liked and was thus analyzed for nutritional composition. The addition of soursop to the soy increased the fiber content (2.1%) of the yoghurt with barely any effect on moisture (86.4%), proteins (4.6%), carbohydrates (6.47%), fat (0.3%), and ash content (0.4%). There were no yeasts and moulds and total coliforms detected during the 21 days of cold storage (4℃) of all the yoghurt samples. The pH reduced slightly during the 21 days of storage. The study showed that an acceptable probiotic soy-soursop yoghurt can be produced by adding soursop to soy milk to produce a yoghurt with increased fiber content.