Microwave Vacuum Drying – Opportunities and Challenges with a special reference to Dairy Products
From Leto Solla
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Abstract:
Microwave Vacuum Drying (MVD) has evolved as an alternative to traditional drying methods. MVD is a relatively gentle drying process that leads to low Maillard browning, heat damage, or off-flavors in the dried product. However, the interaction of multiple processing parameters (microwave power, vacuum level, dielectric properties, shape, size, product structure, and texture) have contributed to the limited application of this promising and versatile drying technology in the food and dairy industry to date. To explore its potential for dairy applications, the use of VMD (nutraREVTM, Enwave, BC, Canada) for drying concentrated skim milk, cheese, and heavy cream was investigated.
Microwave Vacuum Drying (MVD) has evolved as an alternative to traditional drying methods. MVD is a relatively gentle drying process that leads to low Maillard browning, heat damage, or off-flavors in the dried product. However, the interaction of multiple processing parameters (microwave power, vacuum level, dielectric properties, shape, size, product structure, and texture) have contributed to the limited application of this promising and versatile drying technology in the food and dairy industry to date. To explore its potential for dairy applications, the use of VMD (nutraREVTM, Enwave, BC, Canada) for drying concentrated skim milk, cheese, and heavy cream was investigated.
Concentrated skim milk was obtained by reconstituting non-fat dried milk to 37.5% total solids. A custom factorial design was used to optimize drying parameters with respect to product properties and drying efficiency. Drying conditions that maximized yield and minimized drying time while maintaining a good product quality were identified as 2 mm layer thickness, 60 mbar, and specific microwave power input of 1.29 W g-1. Regression analysis indicated that layer thickness was the most important parameter (P < 0.01) to limit the product temperature to below 55 °C in the final drying stage. This allowed good solubility indices of the powder (< 0.3 mL) to be achieved. Specific energy input and vacuum level did not affect solubility (P > 0.1), but significantly affected foaming, as did layer thickness (P ≪ 0.01).
Samples of part-skim Mozzarella cheese with an initial moisture content of ~50% w.b. were cut into 3/8” cubes and microwave vacuum dried. In each trial, the cheese was dried for 20 min at 1.5 W g-1 and 30 min at 1 W g-1 to reduce temperature gain. After MVD, all cheese samples puffed into porous spheres, with a final moisture content ranging from 5 - 9% w.b.. The pressure in the microwave vacuum chamber had a significant effect (p < 0.05) on volumetric expansion, which varied from 75 - 300%, with maximum expansion occurring at 8 kPa. Pressure also had a significant effect (p < 0.05) on the texture of the final product and showed an inverse relationship to that of the volumetric expansion because expansion defines the internal structure.
Heavy cream was adjusted to 30% fat, left unhomogenized and homogenized at 65 °C at 1000 psi and in two passes at 1000 psi and 500 psi. The cream was microwave vacuum dried at 2 and 3 mm layer thickness, 1.5 W g-1, and 27 – 37 mbar or 60 – 67 mbar. A lower pressure and thicker layer resulted in faster drying while overall the drying conditions had a small effect on reconstitution as compared to the pretreatment and the reconstitution conditions. Moisture content, color, solvent extraction, and SEM were used to characterize dehydrated heavy cream. Particle size, color, whipping properties, microscopy, and confocal microscopy were used to characterize the reconstituted dehydrated heavy cream. Homogenization conditions significantly affected dehydrated cream structure, free fat content (P < 0.05), fat globule size, and cream microstructure after reconstitution as well as whipped cream properties.
These findings can be useful for dairy processors interested in adopting MVD as a versatile drying process for long life dairy product innovations with novel functionalities and structures.
Speaker Biography:
Joseph Dumpler studied Food and Bioprocess Engineering at the Technical University of Munich (TUM), Weihenstephan, in Germany. He received his Ph.D. in Dairy Science and Technology with a special focus on UHT treatment of concentrated skim milk. For his work on the reaction kinetics of heat-induced aggregation of proteins in concentrated skim milk, he was awarded the J.T.M. Wouters Young Scientist Award, the Julius Maggi Research Award (2018), and the award for the best Ph.D. thesis granted by the Association of Dairy, Food, and Biotechnologists at TUM. Currently, he is a postdoctoral associate in the Department of Food Science at Cornell University, working on vacuum microwave drying of dairy products such as milk, cheese, and cream.
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