Microwave vacuum drying of fermented dairy products: Effect of processing on drying kinetics and powder quality

Fermented dairy products have gained widespread popularity globally due to their bioactive compounds and nutritional value. While these dairy products are typically consumed fresh, their preservation as shelf stable powders can bring utilization, transportation, and supply chain advantages. This research investigates the use of microwave vacuum drying (MVD) as an effective technology to produce shelf-stable dehydrated fermented dairy products of high quality and viable lactic acid bacteria. The microwave vacuum drying kinetics for commercial plain yogurt, Greek style yogurt, and kefir were evaluated and successfully modeled using the Page model (0.93 < R² < 0.96). Greek yogurt and kefir dried faster than plain yogurt, likely due to differences in matrix structure and EPS-associated water binding by the plain yogurt matrix. MVD processing reduced viable LAB counts by approximately 1.5 log. All 3 products achieved shelf stability (a_w < 0.4) within 120 min of MVD, corresponding to 113.6 W/g cumulative energy input, and 2 kPa pressure. Selected reconstitution conditions include mixing with water at 4°C for 2 min and 12,000 rpm. Reconstitution delivered reconstituted products with similar viscosity and density as the original products, for all tested products. The dried and reconstituted products showed minimal quality differences compared with fresh samples, which was possible since the product temperature was <50°C at all times during MVD. A good retention of viable lactic acid bacteria was achieved, as indicated by the <2 log reduction in the concentration of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in the reconstituted MVD products. These findings lay the groundwork for the manufacture of high-quality, shelf-stable fermented dairy powders, which positions MVD technology as an important innovation driver in the dairy industry.