Freezing tunnel and its types
Freezing tunnel is a salonon with a cooling capacity of -30 to -60 degrees Celsius which can reduce the temperature of the product (meat, fish, etc.) and pass it through the maximum crystallization step at the desired speed.
When the product has to be kept in its original state for a long time, it is usually frozen and stored at -18 or less. These products not only include products such as vegetables, fruits, juices, beers, meats, poultry, seafood and eggs (without box) existed naturally but also include other products such as bread, butter, ice cream and various foods that are prepared or cooked:
The following factors determine the final quality factors and life of maintenance frozen products:
1-The nature and composition of the frozen product
2- Careful selection, transportation and preparation of the product for freezing
3- Freezing method
4- Maintenance conditions
Products that are frozen must have a high quality. In the case of vegetables and fruits, it is very important to choose the suitable type of product for freezing, as some of them are not suitable for freezing and become a low quality product after freezing.
Frozen vegetables and fruits should be harvested after complete ripening and frozen immediately to prevent undesirable chemical changes by enzymatic and microbial reactions. Before the freezing of vegetables and fruits, operations are performed on them so that after cleaning and washing these products, to remove external substances such as leaves, dirt, insects and sap from their surface and to eliminate natural enzymes in vegetables, they are washed in hot water or steam at 100 degrees Celsius.
It should be noted that the enzymes do not remove at low temperatures and continue to operate mildly, even at -18 and below. Therefore, the above method, which destroys the enzyme, increases the life of vegetables. The time required for washing depends on the type of vegetable and varies from one to one and a half minutes for green beans to eleven minutes for large clusters of cereals. Although most microbial contaminants are eliminated during the steam washing process with the enzymes, but most bacteria survive and must be cooled to 10 ° C after washing and before packaging to prevent the decaying of vegetables by these bacteria…
Fruits also need to be cleaned and washed, such as vegetables, to remove external substances and reduce microbial contamination. Although fruits are more susceptible to enzymatic degradation than vegetables, they are never vaporized to eliminate natural enzymes because doing it will result in the loss of fresh fruit quality.
The enzymes involved in freezing fruits are those that speed up oxidation and cause rapid decay of the center of fruit. To control the oxidation rate of frozen fruit, it is covered by diluted sugar syrup. In some cases, ascorbic acid, citric acid or sulfur dioxide are also used.
Meat products generally do not require any operation before freezing, but according to consumer demand, large quantities of meat and meat products are prepared and frozen in specific ways. This issue applies to poultry and seafood as well.
Fish due to the relative instability of their adipose tissue usually freeze immediately after cooling, but the beef is refrigerated for several days before freezing and during this time, it becomes somewhat brittle with enzymatic activities. Especially if this action exceeds six or seven days, it shortens the life of the meat. Tests show that if the hen is frozen 1 to 2 hours after slaughter, it will be more brittle than the hen that was frozen immediately after slaughter, but a delay of more than 24 h in freezing, without significantly increasing the crispness of the chicken, shortens its maintenance life.
Foods may freeze slowly or quickly. In slow freezing, the product is stored in a low-temperature room and is usually frozen in calm air. In this method, the space temperature is between -18 to -40. Because the airflow in the freezing process is as normal convection, the heat transfer time from the product varies from three hours to three days, depending on the volume and conditions of the freezing room. An example is the use of slow-freezing, beef-freezing, boxed chicken, fish, boxed fruit and eggs (white, yolk or all) in packages of 5 to 15kg. Quick freezing is accomplished by one or a combination of the following methods: 1) immersion; 2) indirect contact; 3) wind
Freezing with wind
In this method, the combined effect of low temperature and high air velocity is used to create high heat transfer intensity. Although in different applications, the method used varies considerably, but they are all accomplished by high-speed airflow and low temperatures around the product. Regardless of the method used, the arrangement of the freezer is such that air can flow freely around all parts of the product.
Integrated freezers with high-speed winds are available in ground and air models. When the amount of frozen product is higher, freezing is done in insulated tunnels. In some cases, the product moves slowly through the conveyor belts in the freezing tunnel and freezes until it reaches the other side of the tunnel. Another way is to freeze the product on carts and direct them to the freezing tunnel. The products enter the warehouse after freezing. Freezing method is used to freeze all products by wind, and it is particularly suitable for freezing products with irregular size and shape such as chicken and meat.
Freezing with Indirect Contact
Indirect freezing is usually done in plate freezers. In these freezers, the product is placed on metal plates that refrigerant is passed through them. In this method, since the product is in direct thermal contact with the cooled plate, the major heat transfer from the product is carried out as the conductor and the freezing efficiency will largely depend on the contact surface. This method is especially suitable in cases where the amount of frozen product is low.
A kind of plate freezers used in large commercial businesses to produce small, flat and cubic packages are multi-page freezers which consist of a set of horizontal and parallel plates. The plates are hydraulically opened to enter the product and after entering the product, the plate is pressed on the product, after the plates are closed, the products are completely enclosed between them and because they have good thermal contact with the cooled plates from top to bottom, the heat transfer intensity is high and the product is frozen quickly.
In this method, freezing is done by immersing the product in a low-temperature brine solution, usually sodium chloride or sugar. Because the cooled liquid is a good conductor and has good thermal contact with the product, heat transfer is rapid and the product is frozen in a short period of time.
Another advantage of immersion freezing is that the products are frozen in separate units and not stacked.
The main disadvantage of immersion freezing is the tendency for the extract of the product due to osmotic property, resulting in contamination and dilution of the freezing solution. Also, when sodium chloride solution is used, excess salt may penetrate the product. But when the fruit freezes in the sugar solution, the sugar penetration into the fruit is quite beneficial. Products like fish and shrimp are most frozen by immersion. This method is especially suitable for freezing fish and shrimp in the sea because the immersion freezing freezers are relatively compact and, in addition, the immersion freezing method provides a thin layer of ice at the product surface that prevents the drying of unpacked products.
The Individual Quick Freezer (IQF), or IQF, is used in food processing units for instant freezing of products such as vegetables, fruits, chicken, shrimp, fish, ice cream, meat and ready foods. The products that are frozen in these tunnels retain their novelty and initial properties completely and the grains or components of the product do not stick together and are completely separate. These tunnels are in two types of spiral and in-line. Compliance with world valid health standards such as FDA, USDA, CFIA, UL, CSA, OSHA is observed in the construction of these tunnels.
Types of freezing tunnels
IQF Linear Freezing Tunnel
This type of tunnel makes possible the quick and single freezing of products with the immersion property of product in cold air (Fluidization). These tunnels have a capacity of 200 kg / h to 12000 kg / h and are used for immediate freezing of the following products. Vegetables such as peas, green beans, beans, sweet corn, mushrooms, fries, carrots, cauliflower, onions, tomatoes, broccoli, Brussels sprouts and a variety of other vegetables.
Spiral Freezing Tunnel
These tunnels are manufactured with a freezing capacity of 100 kg / h to 3000 kg / h and more. These tunnels are mainly continuous and used in the production line for immediate freezing of the following products.
– Types of poultry including chicken, pate and chicken fillets, fried chicken and other products
– Red meat such as pate, meat pieces, meatball, raw hamburger and similar products
The difference between slow freezing and quick freezing
Quick freezing products are always better than slow freezing products. In 1932, D.K Tressler summarized the theories of R.Plank, H.F.taylor, C.Birdseye, and G.A.Fitzgerald on the types of freezing, the difference between slow freezing and quick freezing, and listed the following cases as the main advantages of the freezing method:
- In quick freezing, ice crystals are less and therefore, less damage occurs to the product cells.
• In quick freezing, the freezing time is very short and there is little time for salt penetration and water separation.
- In quick freezing, the product quickly reaches temperatures below the growth temperature of the germs, molds and yeasts and therefore prevents product decomposition during freezing.
The main difference between quick freezing and slow freezing is the size, number and location of forming ice crystals from cellular fluids. When the product is frozen slowly, large ice crystals are formed which, through cellular degradation, can cause severe damage to the tissue of some products, but in rapid freezing because the ice crystals are small and fully formed in the cell, cellular destruction is significantly reduced. Products that suffer significant cell damage are prone to losing large amounts of their water or blood and their quality reduce.
The formation of ice crystals in most products begins from -1° C, and although even at temperatures below -45, there are still large amounts of very compact non-frozen fluids; most fluids make crystal by reducing product temperature to -4. The temperature range between -1 to -4 is known as the maximum ice crystal formation range, and in terms of product quality, rapid heat capture is desirable in this area. This is especially true about fruits and vegetables because their texture is severely damaged by slow freezing.
Because animal tissue are more elastic and stiffer than plant tissue, the freezing rate of meat and meat products will not be as critical as fruits and vegetables. Recent experiments show that if the chickens and fish become frozen, they suffer less cellular damage. This does not mean that quick freezing meats are not superior to slow freezing meats, but they are less important in terms of cellular damage than quick freezing fruits and vegetables in the meats. For example, a slow-frozen chicken will have a dark appearance that is not favored by customers, and this would be a compelling reason to use the quick-freeze method to freeze chicken. Also, in all cases, quick freezing reduces the time of operation and thus the rate of decay by bacteria. This is especially important for fish that are more prone to quick decay.