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

摘要

One approach to avoid production of acid wheyduring the manufacture of high-protein yogurt andrelated products is to concentrate the milk beforefermentation. However, the resultant gels are firm sothat stirring in the tank and further processing aredifficult on an industrial scale. We hypothesize thatpower ultrasound (US) during fermentation softens thegel because sound waves cause cavitation and strongshear forces in the fluid. Skim milk was standardizedto different protein contents up to 12%, heated (85°C,30 min), and acidified with thermophilic or mesophilicstarter cultures. An excessive increase in gel firmnessas a function of protein content was detected. In thenext series of experiments, US was applied during fermentation.Milks (10% protein) were acidified at 43.5°Cand sonicated from pH 5.8 to 5.1 with a sonotrode (20kHz, 20 W). Immediately after fermentation, gels wereagitated using a rheometer with a vane geometry. Themaximum torque required to break the gel was reducedby 75% following US, and gel firmness was reduced by80%. Gels were then processed into stirred yogurt andanalyzed. Sonicated samples were smoother with fewerlarge aggregates. Confocal laser scanning microscopyimages suggested a less cohesive structure and morecompact microgel particles, resulting in reduced viscosity.We concluded that US is a promising tool toweaken the gel and facilitate further processing. Thisenables new approaches for the manufacture of Greekyogurt, particularly in regard to avoiding production ofacid whey and developing products with novel textures.