Structure-Function-Process Relationship for Microwave Vacuum Drying of Lactic Acid Bacteria in Aerated Matrices

Microwave vacuum drying (MWVD) of lactic acid bacteria is a promising alternative to conventional drying techniques. To further optimize this process and inhibit diffusion-limiting shrinkage in the last drying period, it was attempted to embed bacteria into a protective aerated matrix. As a foaming agent, whey protein isolate (WPI) was used. Besides, different carbohydrates (i.e., maltose (M), maltodextrin (MD), and sorbitol (S)) were evaluated for their ability to maintain structural stability as well as for their suitability as protectant during MWVD. Foam properties at atmospheric pressure as a function of carbohydrate type and concentration, WPI concentration and concentration of the model strain Lactobacillus paracasei ssp. paracasei F19 (L. paracasei) were related to MWVD behavior and product quality (e.g., survival rate). Therewith, it was aimed at specifying crucial characteristics as well as guiding values regarding foam design intended for MWVD. Overall, high foam stability combined with small and homogeneously distributed bubbles was defined as prerequisite. Further, M- and MD-based foams, in contrast to S-containing matrices, were effectively dried without foam collapse. The best drying performance regarding drying time, microwave energy demand, and survival rate was achieved with MD-stabilized foams. Besides, the general benefit of foam drying as well as the protective effect of MD during MWVD was demonstrated. By comparing MD-stabilized foams to non-foamed MD-containing suspensions as well as pure L. paracasei, the shortest drying time and highest survival rates resulted for the aerated matrices. Concluding, microwave vacuum foam drying displays an innovative approach for the preservation of lactic acid bacteria.