Efficient process freezing is crucial for maintaining food quality and enhancing productivity in the food industry. At Linde, we offer advanced temperature-reduction equipment utilizing liquid nitrogen and liquid carbon dioxide, designed to optimize food processing and preservation. The importance of freezing or cooling products cannot be overstated, as various temperature-reduction systems effectively eliminate processing heat. Below are key considerations to keep in mind when implementing freezing processes in food production.
Cooling Time and Temperature
The relationship between cooling time and temperature is a fundamental aspect of process cooling, much like our experiences with home refrigeration. For instance, a product at 70°F (21°C) placed in a freezer at 0°F (-17°C) overnight will likely freeze solid, while the same product in the freezer for just an hour may only see a minor temperature decrease. Additionally, a product in a 0°F freezer will cool more rapidly than if it were placed in a refrigerator set at 37°F (3°C).
For large-scale process cooling, the operating temperature of the equipment can be set significantly lower than the desired final product temperature. This allows for a much quicker cooling process compared to traditional chilled storage methods. Process cooling is a deliberate and controlled temperature adjustment, contrasting with the unintended temperature fluctuations that often occur during transportation or inadequate initial cooling.
Convection and Conduction
The principles of convection and conduction are vital for effective cooling. Convection can be observed in everyday situations; for example, a breeze on a 50°F (10°C) day will make you feel colder than still air. Similarly, exposure to 50°F (10°C) air does not pose the same risks as contact with 50°F water due to water’s superior conductive properties.
In industrial process cooling, enhancing heat transfer through convection can be achieved by increasing air velocities around products. While chilled air circulation has limited potential for improving conduction, utilizing liquids can significantly enhance this heat transfer. Cryogenic methods, such as contact with carbon dioxide snow or direct impingement with liquid nitrogen, leverage conduction for more efficient cooling.
Practical Limitations
The effectiveness of process cooling is inherently limited by the product’s ability to conduct heat internally. For items with low internal conduction, when the product’s surface reaches the freezer temperature, further heat transfer ceases. The internal heat continues to conduct upward, causing the surface temperature to rise incrementally. Linde’s Food Technology Laboratory technicians assess the product’s dimensions—height, width, and depth—to determine the necessary amount of cryogen and ensure optimal cooling performance. By balancing temperature reduction, processing time, improved convection, and optimized conduction, the desired final product can be achieved efficiently.
Products Require Time to Equilibrate
Most foods do not achieve perfect internal conduction during cooling. When a product is placed in an environment below the desired temperature, it experiences temperature gradients within its mass. If the cooling parameters—time, temperature, convection, and conduction—are correctly established, the product will complete its cooling cycle once the appropriate amount of BTUs has been removed, rather than achieving a uniform temperature throughout.
For example, a chicken breast at 30°F (-1.1°C) placed in a -15°F (-26°C) freezer may have temperature variations from about -15°F at the surface to 30°F in the center. The larger mass of the exterior will equilibrate with the warmer center over time, resulting in the desired final temperature of 0°F (-17°C).
Freezing Equipment Design
In the food industry, freezing is generally associated with water transitioning from a liquid to a solid state. However, many food products do not undergo a state change when subjected to cold temperatures; they may become firmer or more brittle. For instance, fats can crystallize, and rapid cooling in industrial settings allows further processing without delays.
The duration required to cool a product from its initial temperature (Ti) to its final temperature (To) can vary significantly. Most food freezing systems are specifically designed to accommodate the narrow ranges of food product weights, temperatures, and dimensions. Typically, food mass is around 60 lb./ft3, and systems such as the Linde CRYOLINE® tunnel, spiral, and cabinet freezers are engineered around this standard, with typical freezing temperatures hovering around 0°F, ±10°F (5.5°C).
Contraction Must Be Considered
Certain food products can react adversely to rapid exterior freezing, causing stress and pressure on the interior. A prime example is empanadas, where the quick freezing of the outer shell can lead to cracks, while the warm, moist filling takes longer to freeze. By optimizing temperature settings and exposure times, empanadas can be frozen successfully without compromising their integrity.
Improving Further Processing
Freezing or chilling significantly benefits a variety of food products by preserving product integrity during processes such as grinding or shredding. This method is particularly effective for animal proteins, spices, and herbs. Chilling spices prior to grinding retains their flavor, while freezing fresh herbs before shredding and drying helps maintain their taste and expedites dehydration. Additionally, meats are chilled to prevent the melting of essential fats during processing.
By understanding these critical factors, food producers can optimize their freezing processes. The experts at Linde are equipped to demonstrate how our advanced freezing and chilling technology, alongside our cryogens, can be tailored to your specific products and processing needs.
