Food can be acute (slowly) frozen or quick-freezing. Abrupt cooling is carried out by placing food at low temperature of the room, still air and let them freeze slowly. Freezing room temperature is maintained in the range from 0 to 40F (17.7 to 40C). Since air circulation in this process occurs due to natural convection heat transfer from die product takes from three hours to three days, depending on the product in bulk and freezer conditions. Products that are acute usually frozen, half-carcasses of beef and pork, boxed poultry meat fillet and all the fish, fruit, stored in large barrels and eggs (whites, yolks, whole or in large banks.
Quick freezing is done by using one or more of the following ways:
Air Freezing Air freezing uses the combined effect of low temperature and high air velocity to cause a high coefficient of heat transfer from a food product. Aldiough used technique can vary significantly depending on the application, shock freezing is due to the circulation of high-speed, low air temperature around die product.
Heat transfer in this quick freezing process, first of all, by forced convection. Regardless of method, it is important that the design of the freezer and download power will be frozen such diat air can circulate freely around all sides of the product. These smaller units are the same size for commercial access and a walk-in freezers.
- air shock freezing
- contact freezing
- immersion freezing
These modules are used where large volumes of products are frozen. In the first type, the product is loaded in the roll-in shock freezing with multi-level aollies, which are passed in the block. After the product is in frozen condition, truck, pushed out the opposite side of the device directly into a frozen storage. The conveyor, or tunnel systems, products are dirough a freezing tunnel to the slow-moving mesh conveyor belt. The unfrozen product is placed on the conveyor at the warm end of the tunnel and was frozen by the time it reaches the opposite end. Although shock freezing used to freeze almost all kinds of products, it is particularly suitable for freezing products inhomogeneous or irregular shapes and sizes such as poultry meat.
Spiral belt freezer is one of the most popular types of shock freezing currendy used. They have a capacity in the range from 1000 to 12000 pounds of product per hour, with freezing times varied from 10 minutes to three hours. In spiral belt freezer consists of an endless belt is wrapped around a large rotating cylinder, as shown in Fig. 3-6. The number of levels can be varied to accommodate different capacity requirements. Two or more towers can be configured dieir conveyor belts work in series for products that require long term freezing time. Typical products, frozen in a spiral tape, freezers raw and cooked meat cutlets, fish fillets, chicken parts, pizzas and a variety of packaged products.
Contact Freezing Contacts freezing process, through which the food placed on the metal plate freezer dirough which the refrigerant circulates. With die product is in thermal contact with refrigerating plates, heat transfer occurs mainly due to the heat. The effectiveness of the freezing process depends on the die quantity of the product, the surface area, which remains in contact with the plate heat exchanger. Thus, this type of freezing block is especially useful when preparing small, flat, rectangular, consumer-size packages for frozen storage.
In multi-freezer consists of a series of horizontally, parallel, refrigerating plates, three to four inches apart, which are driven by hydraulic pressure, so that they can be opened to get the product between diem, and then closed the product, at the right pressure. Die when the plates are closed, packages are strongly held between the plates. So both the top and bottom of the packages are in good thermal contact widi die refrigerating plates, speed of heat transfer by conduction, is high and the product quickly frozen. Stoves also limit die number of expansion packs, what happens when water-based products are frozen.
Immersion Immersion Freezing freezing is carried out by immersing the product under conditions of low temperature brine (sodium chloride or glucose solution. In refrigerated liquid is a very good conductor of heat, because it at the molecular level thermal contact with the product. Thus, the heat transfer is fast and the product will be completely frozen in a very short time. The advantage of immersion freezing is that the product was frozen in individual units and merge in the collection.
The main disadvantage of immersion freezing is that juices, as a rule, extracted from food product by osmosis. Osmosis is a process in which a molecule travel through the membrane (food surface of the skin) due to differences in fluid pressure through a membrane. Where sodium chloride brine is used as a coolant, salt penetration into the product can sometimes be overkill, thereby reducing the die quality of frozen foods. Conversely, when the fruit frozen in a solution of sugar, sugar penetration in die fruit entirely beneficial. The products most of the frozen by immersion of fish and shrimp. Immersion is particularly suitable for freezing of fish and shrimp in the sea, as immersion in the freezer relatively compact and space aboard a ship is limited. In addition, dive produces freeze "glaze" (the thin layer of ice on the surface of the product, which helps to prevent dehydration unpackaged products to die during storage period.
Quick Freezing Against the Dramatic quick-Freeze Products-frozen immersion in water or multi-freezing methods almost always exceed those that cutting (slowly), frozen air methods for shock freezing. The fundamental difference between freezing and cold in the size, number and location of ice crystals that form in the product, as the cellular fluid hardens. When the goods are slowly frozen, large ice crystals are formed, which cause serious damage to the tissues of some products by destroying cells. After thawing of the product that experienced significant damage to cells exposed to lose too much fluid through a drip or bleeding, significantly reducing product quality. And Vice versa, quick freezing produces less ice crystals which form almost completely inside the cell, so diat destruction of the cells is significantly reduced. Therefore, the thawed product recalls fresh product in the hardness and texture.
Temperatures range between 30 and 25F (1.1 3.8C) often is the range of the maximum ice-crystal education. Therefore, fast heat through this zone is desirable from the point of view of maintaining high product quality. This is especially true for fruit and vegetables because, as pass through serious tissue damage at a slow frozen. Ice crystal formation begins in most products, when their temperature is reduced by approximately 30 F (1.1 C). Most liquids solidify the die temperature of the product reduced to 25 F (3.8C), aldiough some extremely high concentration of liquid still remain unfrozen even at temperatures below -50F (-10C).
Animal tissue is much harder and much more flexible than in the plant tissue. Thus, the speed of freezing of meat and meat products is not as important as the prices for fruits and vegetables. Recent experiment indicates that birds and fish suffer cellular damage, when slowly frozen. This does not mean that frozen meat, poultry and fish are not higher than those which are slowly frozen, only that in terms of cellular damage, quick freezing is not so important in the frozen meat, poultry and fish, as well as in fruits and vegetables. For example, birds, slowly frozen takes on a dark look, making it much less attractive to the consumer. This alone is enough to justify the quick freeze birds. Similarly, quick freezing reduces processing time, and hence the amount of bacterial deterioration. This is particularly useful in the processing of fish because of its tendency to quickly ruin...