Multiflash Drying: How Does It Work?

Multiflash drying (MFD) is an innovative food dehydration technique designed to preserve the quality of food products while reducing drying time and energy consumption. This technology is particularly suitable for processing sensitive materials like fruits and vegetables, which require careful drying to retain their nutritional value, texture, flavour, and appearance. The technique involves the repeated application of vacuum pulses to a food product, alternating between rapid evaporation and relaxation phases. The resulting rapid moisture removal ensures minimal damage to the food’s cellular structure, making it a preferred choice for high-quality dehydration applications. Look at the general review by Zhang et al., (2024).

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Principles of Multiflash Drying

At its core, MFD leverages vacuum drying principles with an intermittent process that alternates between vacuum application (low-pressure phase) and relaxation at atmospheric or slightly higher pressures. The process involves the following steps:

1. Heating Phase: The food material is heated at atmospheric pressure, often by conduction with hot air, heated plates or radiation including microwaves, to raise its temperature to a defined point which enhances moisture evaporation.
2. Vacuum Flashing: A vacuum pulse is applied to reduce the surrounding pressure, causing the moisture in the food to evaporate rapidly at a lower temperature. This is an adiabatic process because the product provides the latent heat of vapourization. There are two effects on the food: texturization and vapourization. This step is crucial for preserving heat-sensitive nutrients and volatile compounds.
3. Relaxation Phase: The vacuum is released, allowing the product to relax under atmospheric pressure. This phase prevents excessive shrinkage or collapse of the food’s structure.
4. Repetition: The cycle is repeated several times to gradually remove moisture without compromising the food’s physical and chemical integrity. Generally then, it is a method that relies on a sequence of vacuum-pulse cycles with the food, usually in the form of slices inside a vacuum chamber.

There are many different forms of multiflash drying that we have seen. These include the following:-

• microwave multi-flash drying (MWMFD)
• conductive multi-flash drying (KMFD).

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Advantages of Multiflash Drying

Multiflash drying offers several benefits over conventional drying methods like hot air drying, freeze drying, and spray drying:

1. Enhanced Product Quality: The low-temperature vacuum drying minimizes nutrient loss, preserves natural colours, and prevents oxidation of sensitive compounds.
2. Rapid Drying: By combining rapid evaporation with vacuum cycles, MFD reduces the overall drying time compared to traditional methods.
3. Energy Efficiency: Lower energy input is required due to the use of vacuum, which facilitates moisture removal at reduced temperatures.
4. Minimized Structural Damage: The intermittent vacuum cycles prevent excessive shrinkage and maintain the original shape and texture of the food product. In physical terms, there is a minimization of change in both porosity and apparent density.
5. Versatility: MFD can be tailored for a wide variety of food materials, including delicate fruits, vegetables, herbs, and even high-value ingredients like spices and extracts.

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Examples of Multiflash Drying Applications

1. Vegetable Slices and Chips

Carrot Chips:

• Process: Carrot slices are blanched to inactivate enzymes, then subjected to multiflash drying. The controlled vacuum pulses preserve the bright orange colour and carotenoid content while creating a crispy texture.
• Outcome: The final product is a vibrant, lightweight, and nutrient-dense snack with enhanced shelf life.

Potato Chips:

• Process: Raw potato slices are pre-treated (e.g., soaking in a sodium bisulfite solution to prevent browning) and then subjected to MFD. The technique produces chips with reduced oil absorption compared to conventional frying.
• Outcome: The chips are crunchy, uniformly dried, and retain their natural potato flavor.

Zucchini and Beet Slices:

• Process: Thinly sliced zucchinis and beets are dehydrated using MFD, maintaining their vivid green and deep red colours, respectively.
• Outcome: Nutritional value, especially antioxidants like betalains in beets, is preserved while achieving a desirable crispy texture.

Pumpkin & Squash:

• Process: Thinly sliced pumpkin are dehydrated to preserve their orange colour and texture.
• Outcome: Similar to zucchini by preserving nutritional value and compounds such as beta-carotenes.

Take a look at research by Monteiro et al., (2018a & b).

2. Fruit Slices and Chips

Apple Chips:

• Process: Apple slices are pre-treated with ascorbic acid to prevent enzymatic browning. Multiflash drying is then applied, resulting in chips that retain their natural sweetness and crispiness.
• Outcome: The product has a light texture, preserved polyphenol content, and a shelf life of several months.

Mango Slices:

• Process: Fresh mango slices are dried using MFD at a controlled vacuum and temperature. The gentle drying ensures the retention of bright yellow colour and natural sugars.
• Outcome: The dried mango slices are pliable, intensely flavored, and retain high levels of vitamin C and carotenoids.

Banana Chips:

• Process: Banana slices are briefly blanched to reduce enzymatic activity before multiflash drying. The vacuum cycles remove moisture evenly, preventing the slices from becoming too tough or leathery.
• Outcome: The chips are light, sweet, and crunchy, with minimal sugar or oil required.

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Multiflash Drying for Fruit and Vegetable Pieces

Beyond slices and chips, MFD is also effective for processing diced or larger fruit and vegetable pieces.

3. Pineapple Chunks

• Process: Fresh pineapple chunks are subjected to multiflash drying after blanching and pre-treatment with citric acid to stabilize colour and pH.
• Outcome: The dried chunks retain their juicy sweetness and a chewy texture, making them ideal for snacking or use in trail mixes.

4. Bell Pepper Pieces

• Process: Red, yellow, and green bell peppers are diced and dried using MFD. The low-pressure environment prevents discolouration and maintains the peppers’ vibrant hues.
• Outcome: The pieces are lightweight, flavorful, and suitable for soups, stews, or seasoning blends.

5. Tomato Slices and Pieces

• Process: Tomato slices or wedges are pre-treated with calcium chloride to retain firmness, then multiflash dried.
• Outcome: The resulting dried tomatoes have an intense umami flavor and are ideal for salads, pastas, or as snacks.

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Comparison with Other Drying Techniques

1. Hot Air Drying: While effective for large-scale drying, hot air drying often results in nutrient loss, shrinkage, and colour fading due to high temperatures. MFD mitigates these issues by employing low-temperature drying.
2. Freeze Drying: Freeze drying is excellent for preserving quality but is energy-intensive and costly. MFD offers a cost-effective alternative with comparable quality retention for many products.
3. Spray Drying: Spray drying is unsuitable for solid or large particulate foods, whereas MFD can handle various shapes and sizes, including slices, chips, and pieces.

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Challenges and Future Prospects

Challenges

1. Initial Investment: Multiflash drying equipment requires a significant upfront investment compared to traditional dryers.
2. Technical Complexity: The process parameters must be carefully optimized for each food product to achieve consistent results.

Future Prospects

1. Automation: Integration of smart sensors and machine learning can further optimize MFD by automatically adjusting vacuum cycles based on real-time moisture content.
2. Sustainability: Advances in energy-efficient vacuum systems and renewable energy sources could reduce the environmental footprint of MFD.
3. Expanded Applications: MFD can be extended to new food categories, including high-value herbal extracts, plant-based proteins, and functional ingredients.

Multiflash drying represents a cutting-edge solution for food dehydration, striking a balance between quality preservation, efficiency, and versatility. Its ability to produce high-quality dried fruits, vegetables, chips, and pieces has made it a valuable tool for both industrial-scale production and niche markets. While challenges like initial costs remain, ongoing technological advancements are poised to make MFD more accessible and sustainable in the coming years especially for small food producers.

References

Monteiro, R. L., Link, J. V., Tribuzi, G., Carciofi, B. A., & Laurindo, J. B. (2018a). Microwave vacuum drying and multi-flash drying of pumpkin slices. Journal of Food Engineering, 232, pp. 1-10

Monteiro, R. L., Link, J. V., Tribuzi, G., Carciofi, B. A., & Laurindo, J. B. (2018b). Effect of multi-flash drying and microwave vacuum drying on the microstructure and texture of pumpkin slices. LWT, 96, pp. 612-619

Zhang, W. P., Chen, C., Ju, H. Y., Okaiyeto, S. A., Sutar, P. P., Yang, L. Y., … & Xiao, H. W. (2024). Pulsed vacuum drying of fruits, vegetables, and herbs: Principles, applications and future trends. Comprehensive Reviews in Food Science and Food Safety, 23(5), e13430.