Power ultrasound as a tool to improve the processability of protein-enriched fermented milk gels for Greek yogurt manufacture.

One approach to avoid production of acid whey during the manufacture of high-protein yogurt and related products is to concentrate the milk before fermentation. However, the resultant gels are firm so that stirring in the tank and further processing are difficult on an industrial scale. We hypothesize that power ultrasound (US) during fermentation softens the gel because sound waves cause cavitation and strong shear forces in the fluid. Skim milk was standardized to different protein contents up to 12%, heated (85°C, 30 min), and acidified with thermophilic or mesophilic starter cultures. An excessive increase in gel firmness as a function of protein content was detected. In the next series of experiments, US was applied during fermentation. Milks (10% protein) were acidified at 43.5°C and sonicated from pH 5.8 to 5.1 with a sonotrode (20 kHz, 20 W). Immediately after fermentation, gels were agitated using a rheometer with a vane geometry. The maximum torque required to break the gel was reduced by 75% following US, and gel firmness was reduced by 80%. Gels were then processed into stirred yogurt and analyzed. Sonicated samples were smoother with fewer large aggregates. Confocal laser scanning microscopy images suggested a less cohesive structure and more compact microgel particles, resulting in reduced viscosity. We concluded that US is a promising tool to weaken the gel and facilitate further processing. This enables new approaches for the manufacture of Greek yogurt, particularly in regard to avoiding production of acid whey and developing products with novel textures.