There are a number of freezing methods and equipment were developed for speed, quality, or for specific types of food. The conditions surrounding the product during freezing are maintained by an enclosure and a refrigeration system.
Freezing systems can be classified in two groups: 1. Direct Contact Systems 2. Indirect Contact Systems.
In direct contact system there is a direct contact between the food product and medium used for reduction of product temperature is used.
The most important types of direct freezing systems are:
i. Air blast freezing system.
ii. Fluidized-bed freezing system.
iii. Immersion freezer.
There are numerous systems used to freeze food products without direct contact between the product and the medium used for the product temperature. Most frozen food are the result of using indirect contact types of freezing systems, where food is separated from the refrigerant by some barrier.
The important types of indirect freezing systems are:
I. Plate Freezers:
It is an indirect freezing system in which the food product does not have any direct contact with the medium used for the product temperature. The basic system of plate freezers consists of flat hollow plates, refrigeration coil these plates to cool the surface in contact with the food products. The plates are made up of aluminum or mild steel.
Aluminum plates are formed from a number of hollow, extruded sections, butt-welded together along their length. The sections are connected at each end to header, allowing ammonia to flow to and fro, each section of the plate. The food products are placed between stacked parallel plates and then pressure applied to the overall stacks to minimize the thermal contact resistance between the plates and product.
The product should be in planar geometry, so unpackaged meat and fish products are suited for this method. Other irregularly shaped products including vegetables such as cauliflower, spinach, broccoli and shrimp can be frozen using this method by packaging the product in brick shaped container prior to freezing. The refrigerated temperature of this method is around -30°C and it takes less than 24 hours to freeze cartons of chilled and boned meat to below -10°C. There are two types of plate freezer; horizontal plate freezers and vertical plate freezers.
a. Horizontal Plate Freezers:
A horizontal plate freezer consists of a set of parallel, refrigerated plates within an insulated enclosure. The stack of plates is placed inside a steel frame and each plate connected to the adjacent plate allowing the plates to be moved apart to form a gap for cartons to be placed between two adjacent plates.
Hydraulic rams are used to move the plate and to apply the pressure on the cartons between the plates during freezing. The freezers are operated either batch or continuous modes. In the batch mode, the spacing in the plates is expanded to allow the product to be loaded on the large tray.
Once the products loaded, the plates are hydraulically closed between the refrigerated plate and the food product. Once the freezing process is over, the plates are opened and the product is removed. In the continuous mode the plates are moved through the enclosed system as the product freezes and then the product between the plates removed one at a time.
It is primarily used to freeze unpacked food such as fish. Here the food product is placed directly between the plates and then pressure is applied to assure proper thermal contact. Once the process has been completed, the plates can be opened to remove the product.
Advantages of Plate Freezing Methods:
The main advantages of plate freezing method are mentioned below:
(i) Faster temperature reduction and freezing time particularly suited to hot offal and hot boned meat.
(ii) Better stowage density in container.
(iii) Lower refrigeration capacity requirements due to the absence of high capacity fans in the freezer.
(iv) High quality of the products.
(v) Speed of freezing is high.
(vi) Significant reductions in energy, packaging and resource consumption.
Main disadvantages of plate freezing methods are:
(i) Higher capital cost.
(ii) A larger charge of refrigerant is required.
(iii) All cartons should be the same height to simplify loading.
II. Air Blast Freezers:
Air blast freezers are used for both direct and indirect freezing applications. In this method, the product is exposed to a low temperature and high velocity air steam. Air blast freezers typically are operated at temperatures of -30 to -45°C with the air velocities of 10-15 m/sec.
Air-blast freezers have package that acts as the barrier, for product with unusual shapes, short freezing times are possible by maintaining high air velocities, low air temperature, good contact between the package and the product surface. This process can be continuous or batch, most systems are continuous.
In this method a refrigeration system first cools an air streams to a minimum of -40°C. The air flows through an enclosed system in which the food product is moved through the system on a conveyor belt. The configuration of the conveyor belt system can vary depending on the food products.
In the simplest design of conveyor belt is that the product is conveyed along a straight belt through on tunnel freezer. But in some other design includes multi-tiered and spiral conveyor belts which are used to minimize the space requirements of the system. The most important factor of the air blast freezer is to ensure uniform air flow over the product.
The direction of the air flow is also an important factor. The best method is parallel air flow (the air flows parallel to the product in the same direction). The other methods of the air flows are counter flow (air flows in the opposite direction to product) and cross flow (air flows perpendicular to the product).
The counter flow methods are effective methods than the other two. The batch air blast freezers are usually consists of a well-insulated container with an air cooler and fans. The product is loaded on a movable belt with stacked trays and the belt is moved into the freezing container. Maintaining uniform air flow is very important, so suitable facilities are required to ensure uniform airflow over all the trays for proper freezing.
III. Immersion Freezers:
This is another important method for freezing which uses direct contact systems. Here the freezing is done with the help of refrigerant. In this method the refrigerant is come in contact with unpackaged food. So this refrigerant must be non-toxic, pure, clean, free from foreign taste, odour, color or bleaching agents and so on.
The refrigerants are used for the immersion freezing are classified in to two classes:
1. Low freezing point liquids – These are chilled by indirect contact with another refrigerant.
2. Cryogenic liquids – Compressed liquefied nitrogen which gives their cooling effect to their own evaporation.
The low freezing point liquids that have been used for the non-packaged food includes solutions of sugars, sodium chloride and glycerol. These must be used at sufficient concentration to remain at -18°C or lower to be effective. In the case of NaCl brine, this requires a concentration of 21%.
NaCl brine cannot be used with unpackaged food that should not become salty, especially fish and fish products. The disadvantage of NaCl brine is that the possible accumulation of the salt brine on the surface of the product. Sugar solutions have been used to freeze fruits at -18°C and glycerol – water mixtures have been used to freeze fruits.
Cryogenic liquids are liquefied gases of low boiling point such as liquid nitrogen and liquid carbon dioxide with boiling point -196°C and – 79°C respectively. Today liquid nitrogen is the most commonly used cryogenic liquid in immersion freezing of food.
(i) It undergoes slow boiling at -196°C, which provide a great driving force for heat transfer.
(ii) Liquid nitrogen contacts all portion of irregularly shaped food, thus minimizing resistance to heat transfer.
(iii) Since the cold temperature results from evaporation of liquid nitrogen, there is no need for a primary refrigerant to cool this medium.
(iv) Liquid nitrogen is nontoxic and inert to food constituent. By displacing air from the food it can minimize oxidative changes during freezing and through packaged storage.
(v) The speed of liquid nitrogen freezing produces frozen food with a quality unattainable by non-cryogenic freezing methods. Some products such as mushroom cannot be frozen by other methods without excessive tissue damage. But such kinds of products can be effectively frozen by means of liquid nitrogen.
Disadvantages of Liquid Nitrogen:
Cost is high.
There is intimate contact between the food or package and refrigerant, so resistance to heat transfer is minimized. This is important with irregularly shaped food pieces to be frozen very rapidly such as loose shrimp, mushrooms and other food.
Although loose food pieces can be frozen individually by immersion freezing and air blast freezing, immersion freezing minimizing their contact with air during freezing, which can be desirable for food sensitive to oxidation. The speed of immersion freezing with cryogenic liquids produces quality unattainable by any other freezing method.