Chair of Food and Bioprocess Engineering, Technical Univ. of Munich, 85354, Freising, Germany.
J Food Sci. 2019 Jul;84(7):1682-1691. doi: 10.1111/1750-3841.14698. Epub 2019 Jul 9.
Microwave vacuum drying as compared to conventional vacuum drying has evinced advantages regarding drying time, while comparable product characteristics were achieved when drying sensitive biological material. Due to the volumetric microwave input, a time reduction of up to 90% is possible. When drying viscous liquids, a foamed structure that remains stable during drying exhibits further advantages as the diffusion-limited third drying step is enhanced by the porous structure. As foams not only have to be thermally resistant during microwave vacuum processing, but also withstand the vacuum, a specific process for foam drying by microwaves under low pressure conditions was developed. Foam formation and stabilization was achieved by using a synergistic mixture of proteins and carbohydrates; Lactobacillus paracasei ssp. paracasei F19 (L. paracasei) served as a model sensitive substance. Investigation of surface activity and foaming properties as a function of L. paracasei concentration revealed a significant positive contribution of the bacterial cells. It was shown that L. paracasei directly adsorbed at the air-water interface. Besides, a structuring of the liquid lamellae was assumed. Moreover, drying time was reduced to at least 50% compared to microwave vacuum drying without foaming. It was further observed that the slight loss in survival was mainly due to the relatively high moisture content and high vacuum levels at the beginning of the process. However, foaming, vacuum application, and final drying, respectively, did not affect viability of the bacterial cells. Thus, by incorporation of lactic acid bacteria into foam structures, drying can be carried out in a fraction of time, and further results in high-product quality. PRACTICAL APPLICATION: The application of continuous foam drying offers an efficient and energy-saving alternative to the currently applied techniques for the processing of sensitive material. The process could be applied for the preservation of starter cultures and probiotics as well as in the pharmaceutical industry, when sensitive material such as therapeutic proteins is dried. This process is especially suitable for freezing-sensitive and thermolabile substances.
与传统真空干燥相比，微波真空干燥在干燥时间方面具有优势，同时在干燥敏感生物材料时可达到可比的产品特性。由于采用体积微波输入，干燥时间可缩短高达 90%。在干燥粘性液体时，泡沫结构在干燥过程中保持稳定，具有进一步的优势，因为多孔结构增强了扩散限制的第三干燥步骤。由于泡沫不仅在微波真空处理过程中必须具有耐热性，而且还必须承受真空，因此开发了一种在低压条件下通过微波进行泡沫干燥的特定工艺。通过使用蛋白质和碳水化合物的协同混合物来实现泡沫形成和稳定；Lactobacillus paracasei ssp. paracasei F19（L. paracasei）用作模型敏感物质。研究了表面活性和起泡性能与 L. paracasei 浓度的关系，结果表明细菌细胞有显著的积极贡献。结果表明，L. paracasei 直接吸附在气-水界面上。此外，假设液体薄片的结构发生了变化。此外，与不发泡的微波真空干燥相比，干燥时间至少缩短了 50%。进一步观察到，存活率的轻微损失主要是由于过程开始时相对较高的水分含量和高真空度。然而，发泡、真空应用和最终干燥分别不会影响细菌细胞的活力。因此，通过将乳酸菌纳入泡沫结构中，可以在很短的时间内完成干燥，并且进一步实现高产品质量。实际应用：连续泡沫干燥的应用为处理敏感材料的当前应用技术提供了一种高效节能的替代方案。该工艺可用于保存发酵剂和益生菌，以及在制药行业，当干燥治疗性蛋白质等敏感物质时。该工艺特别适用于对冷冻敏感和热敏的物质。
