Dec 18, 2024

VACUUM MICROWAVE DRYING TECHNOLOGY: A FASTER AND LOWER ENERGY ALTERNATIVE TO FREEZE DRYING

PART 1 – PRESERVE MATERIAL QUALITY & APPEARANCE

NuWave Research’s vacuum microwave drying is a dehydration method for food materials similar to freeze drying, but much faster and better at preserving material quality and appearance. In part 1 of our series discussing freeze and vacuum microwave drying, we look at their drying techniques and end products.

DRYING TECHNIQUES

Both techniques dry material in an oxygen-free environment with help from vacuum to lower the boiling point of water, but their overall process and temperatures differ.

VACUUM MICROWAVE DRYING

Vacuum microwave drying is a fast one step process, where vacuum is created and then heat is applied. Total processing time can be as little as 30 minutes. It combines the benefits of vacuum and microwave’s uniform volumetric heating (60°C or below) to remove water from material. The process starts with waveguides directing microwaves into a chamber, while a vacuum pump decreases the chamber pressure. Polar molecules in the material absorb the microwaves and try to align to their electric field, causing rapid vibration and friction between molecules. This friction converts microwave energy into thermal energy, increasing the temperature of the material and evaporating water.

FREEZE DRYING

Freeze drying is an often lengthy three-step tray-based process (with total processing time ranging from 10hours to multiple days). It starts by freezing the material to temperatures between -45°C to -35°C and lowering the chamber pressure with a vacuum pump (a process called lyophilization). Then, the primary drying phase removes water by very slowly adding heat to the bottom of the trays, causing the ice in the material to change directly from a solid to a vapor (a process called sublimation). This sublimation stage is a very slow process that requires precise control of temperature and pressure to prevent the ice from turning to liquid water. Finally, the secondary drying phase (sometimes referred to as desorption) reduces the residual moisture content as much as possible by raising the temperatures higher than during the primary drying phase.

Compared to freeze drying, vacuum microwave drying is a relatively simple process that applies microwave energy directly to the material and does not require freezing the material prior to drying. Microwave vacuum drying has the flexibility to process material at various starting temperatures depending on what’s best for the application (from frozen, to room temperature, to pre-heated).

END PRODUCT

Both techniques can be ideal for drying fragile or heat sensitive materials because their relatively low temperatures and oxygen-free environment are ideal to retain material qualities (like nutrients, vitamins, flavors, and antioxidants in food). Although, vacuum microwave drying is often better at preserving original quality like colors compared to freeze-drying. This is because vacuum microwave drying is much faster, so there’s usually less heat exposure overtime, leading to much less color degradation. For instance, freeze-dried strawberries are a dull bleached pink, whereas vacuum microwave dried strawberries are a natural vibrant red.

The photo on the left is a visual comparison of fresh strawberries in a bowl next to vacuum microwave dried sliced strawberries on a plate. Compared to the freeze dried strawberries by our R&D team in the photo on the right, vacuum microwave dried strawberries retain their bright, fresh appearance.

The final moisture percentage of products can vary between the two methods, with freeze-drying achieving between 4-1% and vacuum microwave between 40-0.01% and below (depending on the material and end-product goal). In addition, texture often differs between the two methods, with vacuum microwave drying creating crunchy or chewy bites and freeze-drying sometimes causing spongy textures.

CONCLUSION

Freeze drying and vacuum microwave drying are great at preserving original material quality, like nutrients in food products, but vacuum microwave drying is better at retaining original material appearance, like colors. This makes vacuum microwave drying ideal for fresh looking single ingredient dried fruit snacks. Although, the choice between drying methods depends upon multiple factors like the material, processing parameters, and end-product goal.

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