High pressure and ultrasonification technologies for manufacturing yogurt

Nonthermal processing is a rapidly growing field of research and industry use for production of safe foods and modification and/or improvement of quality. It is expected that this trend will grow, as consumers want minimally processed foods of natural flavor that are free from additives and preservatives. High hydrostatic pressure (HHP) and ultrasonification are two promising nonthermal processing technologies studied in this research for manufacturing low fat probiotic yogurt and improving the viability of probiotics in yogurt.; Yogurt was manufactured using heat, HHP, and a combined treatment of HHP and heat. The effect of ultrasonification on the physicochemical, rheological, textural, and microstructure of low fat probiotic yogurt were studied. The combined application of HHP and thermal treatment resulted in yogurt gels with improved physicochemical characteristics and water holding capacity over heat or HHP alone. The HHP and heat combined treatment resulted in yogurt gels with improved consistency indices over gels obtained from thermally treated milk. The starter and inoculation rate that provided different fermentation pathways also affected the consistency index and texture properties. Rheological behavior differences of yogurts varied according to the treatment used, and were attributed to structural phenomena of casein micelles. The combined HHP and heat milk treatments exhibited small rounded micelles that tended to fuse and form small irregular aggregates in association with clumps of dense amorphous material, which resulted in improved gel texture and viscosity.; Ultrasonification was used to rupture yogurt bacteria to improve the viability of probiotics in yogurt. The probiotics grew better in sonicated culture yogurt compared to unsonicated culture yogurt, indicating increased availability of nutrients for the probiotics, which can be attributed to beta-galactosidase availability. Sonicated starter yogurts presented lower syneresis compared to the control yogurts during storage. Ultrasonification improved the viability of probiotics by two log cycles at the end of storage period. The reduction of viability beyond the 24th day can be attributed to the lowering of pH. Overall, the results suggest that ultrasonification can possibly improve the viability of probiotics and quality of yogurt. Finally, both HHP and ultrasonification are potentially promising nonthermal processing technologies that can be selected for manufacturing yogurt to improve quality and viability of probiotics.