Microbiological and physical evaluation of nonfat set-type hybrid yogurts formulated with soy and rice proteins

• Hybrid yogurts represent a sustainable strategy to diversify protein sources • High-protein hybrid yogurts blending dairy and plant-based proteins were developed • Plant protein source impacted hybrid yogurts’ texture and stability • Soy protein accelerated fermentation, maintaining texture and microbial viability • Rice protein reduced gel strength, increased syneresis, and weakened structure This study aimed to investigate the impact of incorporating soy protein isolate (SPI) and rice protein isolate (RPI) on the microbiological, physicochemical, and structural properties of high-protein, nonfat, set-type hybrid yogurts. Yogurts were formulated to contain 6% total protein, consisting of 3% protein from skim milk powder and an additional 3% from whey protein concentrate, and/or the selected plant proteins in varying proportions. The fermentation process was driven by Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, and the yogurt samples were monitored over a 14-day refrigerated storage period. Yogurts containing SPI exhibited the fastest acidification (3.5 h to reach pH 4.5) and the highest bacterial counts at day 0 (∼9.8 log CFU/g). Conversely, RPI-containing yogurts required a longer fermentation time (∼5 h) and exhibited lower microbial viability (∼8.6 log CFU/g). After 14 days, all samples remained above 7 log CFU/g, confirming starter stability during storage. Physicochemical and structural analyses showed that Y-S developed a branched, less dense and more porous protein network than the dairy control (Y-W), whereas Y-WS (with 1.5% WPC + 1.5% SPI added proteins) exhibited an intermediate microstructure. SPI-containing formulations displayed texture parameters (firmness, consistency, cohesiveness) closest to the control, while RPI led to a weaker gel and higher syneresis (up to 15% spontaneous and 64% forced). Overall, hybrid yogurts containing soy proteins showed superior texture and stability, and faster fermentation. Although both plant proteins can be incorporated into dairy matrices, further optimization is required to achieve the desired texture and stability in high-protein hybrid yogurts.