Isochoric Freezing

Isochoric freezing, also known as constant-volume freezing or pressure-shift freezing, is a process used in the food industry to preserve food products by rapidly reducing their temperature while maintaining a constant volume. This technique offers several advantages over traditional freezing methods which follow the isobaric (constant pressure) approach The benefits of isochoric freezing are shorter freezing times, improved product quality, and reduced energy consumption (Nida et al., 2021). In this essay, we will delve into the principles behind isochoric freezing, its applications in the food industry, and the benefits it offers.

At its core, isochoric freezing relies on the principle of thermodynamics, specifically the relationship between temperature, pressure, and volume of a substance. Unlike conventional freezing methods where the volume of the food product may change during the freezing process, isochoric freezing keeps the volume constant. This is achieved by applying pressure to the food product while lowering its temperature, preventing the formation of large ice crystals and minimizing damage to the cellular structure.

The process of isochoric freezing

The nature and thermodynamics of cryopreservation has been investigated in detail (Rubinsky et al., 2005). 

1. Precooling: The food product is pre-cooled to a temperature slightly above its freezing point. This helps prepare the product for the subsequent rapid freezing process.
2. Compression: The pre-cooled food product is subjected to high pressure, typically using a hydraulic system or mechanical compressor. The pressure applied depends on the specific properties of the food product and the desired freezing conditions.
3. Rapid cooling: Once the desired pressure is reached, the temperature of the food product is rapidly reduced, often using liquid nitrogen or another cryogenic fluid. This rapid cooling process prevents the formation of large ice crystals and ensures uniform freezing throughout the product.
4. Pressure release: After the food product has been sufficiently frozen, the pressure is released, allowing the product to return to ambient pressure. This step is crucial to prevent damage to the product when it is removed from the freezing chamber.

Isochoric freezing offers several advantages over conventional freezing methods, making it an attractive option for the food industry. 

1. Faster freezing times: By maintaining a constant volume and rapidly lowering the temperature, isochoric freezing can significantly reduce the time required to freeze food products compared to traditional methods. This is particularly beneficial for delicate or heat-sensitive products that require rapid preservation to maintain quality.
2. Improved product quality: The rapid freezing process of isochoric freezing results in smaller ice crystals and more uniform distribution throughout the product. This minimizes damage to the cellular structure and helps preserve the texture, flavor, and nutritional content of the food. Additionally, by maintaining a constant volume, isochoric freezing reduces the risk of mechanical damage to the product.
3. Energy efficiency: Isochoric freezing typically requires less energy compared to conventional freezing methods due to the shorter freezing times and more efficient use of cryogenic fluids. This can lead to significant cost savings for food manufacturers, especially for large-scale production operations.
4. Extended shelf life: The rapid freezing process of isochoric freezing helps preserve the freshness and quality of food products for longer periods, extending their shelf life and reducing food waste. This is particularly important for perishable products such as fruits, vegetables, seafood, and meat.
5. Versatility: Isochoric freezing can be applied to a wide range of food products, including both solid and liquid foods, making it a versatile preservation technique for the food industry. It can be used for individual items or in continuous processing systems for large-scale production.

In the food industry, isochoric freezing finds application in various sectors, including:

1. Seafood: Isochoric freezing is commonly used to preserve seafood such as fish, shrimp, and shellfish. The rapid freezing process helps maintain the quality and freshness of the seafood products, making them suitable for storage, transportation, and distribution to markets worldwide.
2. Fruits and vegetables: Fresh fruits and vegetables are often subjected to isochoric freezing to preserve their natural flavors, colors, and nutritional value. This technique is especially beneficial for delicate fruits and berries that are prone to damage during conventional freezing. There are some useful examples where the technology has been applied and investigated as in the freezing of sweet cherry (Bilbao-Sainz et al., 2019a;  spinach (Bilbao-Sainz et al., 2019b).
3. Meat and poultry: Isochoric freezing is employed in the meat processing industry to preserve the texture and juiciness of meat and poultry products. By minimizing the formation of ice crystals, isochoric freezing helps maintain the tenderness and flavor of the meat, enhancing its appeal to consumers.
4. Dairy products: Dairy products such as milk, cheese, and yogurt can benefit from isochoric freezing to preserve their quality and freshness. The rapid freezing process helps prevent the separation of milk solids and water, ensuring a smooth and creamy texture in dairy products.
5. Ready-to-eat meals: Isochoric freezing is increasingly being used in the production of frozen meals and convenience foods. By preserving the texture and flavor of individual ingredients, isochoric freezing helps maintain the overall quality of the finished meal, providing consumers with convenient and nutritious options.

Isochoric freezing is a valuable technique in the food industry for preserving the quality, freshness, and nutritional value of food products. By maintaining a constant volume and rapidly reducing the temperature, isochoric freezing offers advantages such as faster freezing times, improved product quality, energy efficiency, and extended shelf life. With its versatility and applicability to a wide range of food products, isochoric freezing continues to play a vital role in food preservation and production processes, contributing to the availability of high-quality food products for consumers worldwide.

References

Bilbao-Sainz, C., Sinrod, A., Powell-Palm, M. J., Dao, L., Takeoka, G., Williams, T., … & McHugh, T. (2019). Preservation of sweet cherry by isochoric (constant volume) freezing. Innovative Food Science & Emerging Technologies, 52, pp. 108-115.

Bilbao-Sainz C, Sinrod A, Powell-Palm M.J. et al. (2019b) Preservation of sweet cherry by isochoric (constant volume) freezing. Innov. Food Sci. Emerg. Technol. 52 pp. 108–115 (Article).

Nida, S., Moses, J.A. & Anandharamakrishnan, C. (2021) Isochoric Freezing and Its Emerging Applications in Food Preservation. Food Eng. Rev. 13, pp. 812–821 (Article)

Rubinsky, B., Perez, P. A., & Carlson, M. E. (2005). The thermodynamic principles of isochoric cryopreservation. Cryobiology, 50(2), pp. 121-138