RESUMEN
The aim of the present study was to investigate the effect of addition of soymilk on physicochemical, microbial, and sensory characteristics of milk fermented with Lactobacillus acidophilus. Soybeans were blended 1:5 w/v with distilled water. The prepared soymilk was added to milk in combinations of 20%, 40%, and 60%. Milk was used as the control. All the samples were sterilized and fermented with Lactobacillus acidophilus LA-5 as probiotics. Then, they were kept at C for 14 days. Microbial count, titratable acidity, pH, syneresis, color parameters and sensory evaluation were performed during the storage time. Results showed that all the samples possessed minimum effective dose of LA-5 on day 14, although a significant decrease in LA-5 was observed in the sample with 60% soymilk. In each experimental day, there was a noticeable decrease in the pH of the samples. Addition of soymilk to milk also significantly increased syneresis, particularly in samples with 60% soymilk. Sensory evaluations showed that scores of texture, mouth sense, aroma, and flavor of the samples with 40% and 60% soymilk were significantly lower than other samples. With respect to color, "L" value decreased significantly in the fermented product with 60% soymilk and the decrease was more pronounced with longer storage times. In conclusion, addition of 20% soymilk did not substantially alter physicochemical and sensory characteristics of milk while providing an appropriate growth culture for LA-5. The mixture of milk-20% soymilk can be introduced as a good probiotic product with lower lactose content and additional nutritional benefits. Key words: Probiotics, Milk, Soymilk, Lactobacillus acidophilus