Evaluation of Hybrid Fermented Milk-Soy Products and Their Potential Microbial Contamination

Hybrid milk/soy products with milk-to-soy protein ratios of 75:25, 50:50, and 25:75 (w/w) were fermented using a yogurt culture, and their microbial and physicochemical properties were compared with those of conventional dairy yogurt after 16 h of incubation at 30 °C. In the sample containing 75% soy protein, differences in Streptococcus thermophilus counts, active and titratable acidity, and lactic acid content were observed compared with the milk control. All samples differed in water-holding capacity, viscosity, and gel strength. The higher soy content led to a change in the color and visual appearance. Inoculation with three purposely added fungal contaminants resulted in greater radial growth in hybrid products compared to dairy yogurt. The growth of inoculated contaminating yeast Rhodotorula mucilaginosa was significantly higher in hybrid products; a cell count of Kluyveromyces marxianus CCDM 269 was higher only in the products containing 50 and 75% soy protein.