FREEZING SYSTEMS
The environment maintained during freezing of the product is established by a physical structure and a refrigeration system.
Freezing systems can be classified into two groups: direct-contact systems and indirect-contact systems. This classification is based on the type of contact between the product surface and the refrigeration medium.
DIRECT-CONTACT SYSTEMS
Any freezing system that brings a refrigeration medium into direct contact with the product surface would be classified as a direct contact system.
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These systems attempt to bring the cold medium into contact with the maximum product surface area. In general, these types of freezing systems would be expected to be highly efficient, since barriers to heat, transfer are reduced to a minimum. It should be noted that the product surface may be covered by package film.
The refrigeration medium used for these systems would include low-temperature air moving over the product surface at high air speeds, as well as selected liquid refrigerants that may allow for phase change during the product freezing process.
Here are some direct contact freezing systems.
An air-blast freezing system.
In an air-blast freezing system, cold air is blown over the product to cool and freeze it. The product is typically placed on trays or racks, and a high-velocity air stream is circulated over and around the product. The cold air acts as the cooling medium, and heat is transferred from the product to the air, gradually cooling and freezing it. The air is usually chilled using a refrigeration system and is continuously circulated to maintain a consistent temperature. Air-blast freezing is commonly used for freezing a wide range of products, including fruits, vegetables, meat, seafood, and bakery items.
The fluidized-bed freezing system.
In a fluidized-bed freezing system, the product is suspended or fluidized in a stream of cold air or another cooling medium. The cold medium is blown upward through a perforated bed or mesh, causing the product to float and circulate within the bed. This fluidization allows for efficient heat transfer as the cold air surrounds the product, rapidly cooling and freezing it. Fluidized-bed freezing is particularly suitable for small, individually quick-frozen (IQF) products, such as berries, peas, and other small food items. It offers advantages like high freezing rates, minimal product clumping, and uniform freezing.
Immersion freezer.
An immersion freezer involves submerging the product in a cold liquid, such as a refrigerant or a chilled brine solution. The product is typically placed in a container or basket and immersed in the cold liquid. The cold liquid extracts heat from the product, causing it to cool and freeze. Immersion freezing is commonly used for various food products, including seafood, meat, and some processed foods. It allows for efficient heat transfer and can achieve rapid freezing, making it suitable for certain industrial freezing applications.
INDIRECT-CONTACT SYSTEMS
Most frozen foods are the result of using indirect-contact types of freezing systems, the food is separated from the refrigerant by some type of barrier. These barriers would include product package surfaces as well as structural components of the freezing system.
Plate Freezing:
In plate freezers, the product is placed between parallel plates that are chilled using a refrigerant. The plates act as the indirect contact surface, and the refrigerant absorbs the heat from the product, cooling the plates and freezing the product.
Air Blast Freezing:
In this method, cold air is blown over the product to cool and freeze it. The product is typically placed on trays or racks, and the cold air is circulated over and around the product. The indirect contact between the product and the cold air allows for gradual cooling and freezing.
Fluidized Bed Freezing
In fluidized bed freezers, the product is suspended or fluidized in a stream of cold air or another cooling medium. The cold medium indirectly cools the product by transferring heat through the surrounding air or fluid. This method provides efficient heat transfer and uniform freezing.
Immersion Freezing
Immersion freezers involve submerging the product in a cold liquid, such as a refrigerant or a chilled brine solution. The product is placed in a container or basket and immersed in the cold liquid, allowing for indirect contact and gradual freezing.
Cryogenic Freezing
Cryogenic freezing methods utilize extremely low temperatures achieved through the use of liquefied gases, such as liquid nitrogen or carbon dioxide. The cold gas or liquid is circulated around the product, providing indirect contact and rapid freezing.
Texture and Quality Effects in freezing food
When food undergoes freezing, there are inherent texture and quality changes that occur due to the formation of ice crystals and other molecular transformations. Mostly the texture and quality of food during freezing changes because of the size of ice crystals produced during freezing i.e. rapid freezing help to food to maintain its quality because of smaller ice crystals produced in it, Where are slow freezing can damage the texture and quality of food because of the larger ice crystals during freezing.
Here are some key points about texture and quality changes in freezing food:
Ice Crystal Formation: During freezing, water molecules in the food form ice crystals. These ice crystals can disrupt the cellular structure and lead to changes in texture. The size and distribution of the ice crystals can affect the final texture of the frozen food.
The expansion of ice crystals during freezing can cause physical damage to the cells in the food. This can result in a loss of structural integrity and changes in texture, such as softening or a loss of crispness.
Moisture Redistribution: Freezing can cause the migration of moisture within the food. When frozen and thawed, this can lead to the formation of ice crystals, resulting in moisture loss and changes in texture, such as dryness or sogginess.
Freeze-Thaw Effects: Repeated freezing and thawing cycles can further impact the texture and quality of frozen food. The formation and subsequent melting of ice crystals during each cycle can lead to additional cell damage and moisture loss, affecting the overall texture and taste.
Packaging and Freezer Burn: Inadequate packaging can result in freezer burn, which occurs when the food is exposed to air and loses moisture. Freezer burn can cause changes in texture, resulting in a dry, tough, or off-flavored product.
DESIGN OF FROZEN FOOD STORAGE
The storage facilities for frozen foods have significant influence on the quality of the frozen product. In order to maintain maximum quality, the facilities must be maintained at the optimum temperature, and any fluctuations in product temperature should be minimized.
Maintaining the temperature of the frozen food storage environment at the appropriate temperature depends on several factors including:
(a) Negligible heat transfer through the walls of the facility.
(b) Controlling the movement of ambient temperature air into the facility through openings.
(c) Minimizing the movement of elevated temperature product into the facility.
(d) Continuous operation of the refrigeration system.
ADVANTAGES OF FREEZING
Extended shelf life of perishable foods.
Retention of nutritional value.
Preservation of sensory attributes and quality.
Convenience and availability of ready-to-eat or ready-to-cook foods.
Preservation of seasonal availability.
Industrial applications beyond food preservation.
Reduction of food waste.
DISADVANTAGES OF FREEZING
Potential loss of texture in certain foods.
Risk of freezer burn if not properly packaged.
Requires adequate storage space and freezing equipment.
Quality deterioration over extended periods of time.
Limited shelf life after thawing.
Some foods may undergo flavor changes or lose certain qualities.
Freezing may not be suitable for all types of foods due to their specific properties.
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