Р. Д. Хип, Cambridge Refrigeration Technology
Chilled products are products that are cooled to a temperature higher than their freezing point and therefore must be stored at this temperature to avoid loss of quality. Almost always, such products lose their value during freezing, and in some cases the process of freezing can be completely damaged. However, in terms of freezing, the range of refrigerated products that can be subjected to this procedure is quite wide. In this chapter, we will look at fresh fruits and vegetables (both tropical and growing in mid-latitudes), all types of meat, fish and dairy products, as well as ready meals .
It is clear that refrigeration is an important part of the production, storage and supply of refrigerated products, but it should be noted that a large number of various refrigeration equipment is needed to freeze products. Consider, for example, the work of an enterprise for the supply of pre-cooked and chilled semi-finished products. Raw materials coming from different countries are cooled in refrigerators and transported to all corners of the world using high-tech refrigerated transport systems. Then they go to the port warehouse refrigerators and then are transported by refrigerated cars to the wholesale warehouses, from where they are either directly or through intermediaries sent to the consumer. This process is reflected in the diagram in fig. 4.1.
As we see, before using the product its quality is ensured in refrigerated warehouses. Some initial products may be frozen more deeply than chilled products, and therefore they require special equipment for defrosting. If we consider the cooling procedure, the products are first cooled by air (or, in some cases, by contact), and then they are stored in a refrigerating chamber or sold in a refrigerator before being sold.
Fig. 4.1. The chain of refrigeration
transport, and then kept in cold storage chambers or refrigerated display cases. Waste from the processing of basic raw materials can also be frozen. The main requirement in the refrigeration industry is the reliability of refrigeration equipment and the observance of the technological cycle. Cooling products is in fact always a part of the technological chain for preparing cooled products that is forgotten by the consumer, but it is precisely this that guarantees the quality and freshness of the product.
The process of cooling products is not new. For millennia natural ice and evaporation cooling have been used for these purposes, but only relatively recently mechanical cooling has been used to store food at low temperatures. In fact, storage of apples in low-temperature storage facilities in the United States dates back to 1870 . Refrigerated transport with chilled (non-frozen) meat connected the United States and the United Kingdom around 1875, and the start of trans-oceanic transport of refrigerated products between Australia, Asia and Europe dates from 1895 . To 1901, England imported more than 160 Ltd. tons of chilled beef annually.
The basic principles of cooling using water vapor compression were discovered in the 19th century, and this principle of cooling has reached our days almost unchanged. It is very simple, since the refrigeration system has only four interconnected elements (Fig. 4.2). The refrigerant in the vapor state is compressed to high pressure and, therefore, to a high temperature. Superheated steam is cooled and liquefied in a condenser, further flowing through a choke into a low pressure zone, where it condenses into a liquid that can be used to remove heat from storage chambers or a cooled zone, and this heat in the evaporator promotes evaporation.
Fig. 4.2. The main loop cooling system with vapor compression
chilled fluid at low pressure. cooled steam is supplied to the compressor to complete the cycle.
So, a compressor, a condenser, a choke and an evaporator are the main elements of a refrigeration unit of compression type. The heat removed during the evaporation process, as well as the heat removed from the cooling zone, plus the thermal equivalent of the energy expended to compress the refrigerant, must be compensated for in the condenser. This means that any refrigeration equipment must compensate for a certain part of the heat, which is greater, the greater the amount of heat taken from the product or refrigerated space. The energy consumed by compression-type refrigeration equipment depends primarily on the design of the equipment, but in general it depends on the temperature difference between the condenser and the evaporator. The larger this difference, the more energy is required for the compressor to perform its task, and the larger this temperature difference, the lower the cooling capacity of the refrigeration unit.
A theoretical analysis of refrigeration cycles and a full description of the details can be found in the numerous refrigeration manuals [1,7,11] and is beyond the scope of this book. Nevertheless, we will give a general overview of the principles of refrigeration systems, which can be useful for everyone who uses refrigeration equipment.
Safety and quality of products
The concept of food safety is directly related to the activities of pathogens and toxins. Food, of course, should not harm the consumer, causing disease or poisoning him. The quality of a food product is its nutritional value and taste, the nature of the texture and the appearance of a safe food product. Ideally, the issue of food safety is a matter of law, where product quality is a category determined by the market.
For chilled foods, safety and quality issues may or may not intersect. For fresh fruits and vegetables, the presence of microorganisms and dirt can lead to food being not only tasteless, but also leading to health risks. For many types of finished products (including semi-finished meat products), an increase in the number of pathogenic microorganisms that produce toxins largely depends on temperature and time. This results in damage to the product, which may look satisfactory in appearance and taste. For some dairy products, the development of pathogenic microorganisms can affect both the taste and the type of product. In any case, the quality and safety of the food product is based on the storage of the product at the lowest possible temperature, in order to exclude the possibility of the growth of microorganisms that could potentially lead to product deterioration. This principle runs through the whole process chain of preparing chilled foods (see fig. 4.1).
Refrigerants and the environment
Prior to the beginning of the 1990's. The question of choosing a refrigerant for refrigeration units did not care much for the consumer. Unfortunately, as it turned out at the present time, the chemical compounds used as refrigerant in refrigeration units, when released into the atmosphere, were able to lead to undesirable and unpredictable effects.
Reducing the ozone layer and global warming are two different environmental problems that humankind has faced today. The ozone layer, which protects the surface of our planet from excessive doses of ultraviolet radiation, can be damaged by stable chlorine and bromine compounds. These compounds - CFC (chlorofluorocarbon - chlorofluoro-carbon) and HCFC (hydrochlorofluorocarbon - hydrogen-chloro-fluorine-carbon) - are contained in refrigerants, and therefore the latter contribute to the destruction of ozone in the stratosphere and global warming of the climate.
Global warming is a natural phenomenon caused mainly by the reflection of sunlight from carbon dioxide and water vapor contained in the atmosphere. Fears of excessive global climate change are associated mainly with a large release of carbon dioxide into the atmosphere. The reason for this is seen in large emissions of smoke into the atmosphere, which mainly occurs when power plants operate on solid fuels and for a number of other reasons. There are more environmentally harmful gases, but, fortunately, their concentration is much less. These are primarily HFC (hydrogen-fluorine-carbon) gases, causing a "greenhouse effect".
Thanks to the Montreal Protocol , manufacturers around the world have stopped producing ozone-depleting compounds based on CFC compounds from 1990 and replaced them with less environmentally friendly HCFC compounds. The latter are likely to be the main refrigerants in 2010-2020 as well, since for most cases no more suitable compounds have been found. In Europe, the use of equipment that uses chlorofluorocarbon will be stopped, and the supply of new equipment based on hydrogen-chloro-fluorine carbon will be banned (however, during the preparation of this book, these restrictions have not yet entered into force). These substances strike two blows at producers and consumers of refrigeration units. Firstly, any change in technology costs money and can lead to an increase in operating costs as well as the cost of conversion. Secondly, the future replacement of environmentally dirty chlorine-fluorocarbons to more environmentally friendly ammonia and propane on a global scale will lead to higher equipment costs and retraining costs for service personnel. As an alternative, hydrogen-fluorine carbons have been developed that do not harm the ozone layer, but as some environmentalists have found out, they can contribute to enhancing the "greenhouse effect", and therefore they are on the list of substances prohibited under the Kyoto Protocol.
All these considerations must be considered by the potential buyer of refrigeration equipment. Ignorance of these things can lead to the fact that the equipment purchased for a lot of money will have to be seriously modernized long before its real life has expired. Also, ignorance of such facts can lead to financial losses, as well as loss of time and equipment. Reducing the use of CFC- and HCFC-compounds as sealing foams in refrigerated cabinets and vaults is described in detail in the literature, but so far has not been widely used.
Taking into account global warming, reduction of energy consumption and its efficiency, we can draw the following conclusion. New refrigerants may have lower efficiency and, as a result, require increased energy consumption, but it is likely that future environmental restrictions may also affect energy consumption reductions. In the near future, the potential consumer of refrigeration equipment will face a difficult choice dilemma - to face stricter requirements for refrigerant leaks, requirements to ensure efficient use of equipment and requirements to use only properly trained personnel. For more on this, see .
Chilled food and freezing
The main benefit of storing food in a cooled state is to increase the shelf life by reducing the possibility of product damage by microorganisms. Cooling, and this should be emphasized, cannot improve the quality of the defective product, cannot stop the deterioration process — it only slows it down (see chapters 7, 9,10).
For international land transport of refrigerated products, the Trade Agreement on the International Transport of Perishable Goods and the special equipment used in these shipments (UNECE)  lists many requirements. Food products are properly classified, and for them, the Agreement specifies the maximum storage temperature:
- offal - + 3 ° C;
- oil - + 6 ° С;
- game - + 4 ° С;
- milk for direct consumption - + 4 ° С;
- milk for further processing - + 6 ° С;
- yoghurts, kefir, cream, fresh cheese - + 4 ° С;
- fish, mollusks, crustaceans - 0 ° С (in an ice crumb);
- unstabilized meat products - + 6 ° С;
- meat (not offal) - + 7 ° C;
- bird, rabbits - + 4 ° C.
This list does not include cooked vegetable dishes with sauces or without them, as well as fresh fruits and vegetables.
There are two completely different refrigeration methods for refrigerated products - the cooling process itself, in which food products are cooled (from ambient temperature, for example, 30 ° C, or from cooking temperature — over 70 ° C), or stored refrigerated with tightly controlled temperature (from -1,5 ° С to + 15 ° С) depending on the type of products. The cooling system or refrigeration chambers can be very different from each other both in design and in characteristics. It should be noted that although some equipment for cooling can be used as a refrigerator, the cooling chambers themselves are not designed to cool products, but only to maintain the required temperature. Transport refrigerators are a special case of food storage, since the refrigeration equipment of these refrigerators is usually not designed for rapid cooling of the product.
The rate at which the product will be cooled depends on many factors. The size and shape of the containers can be important for the intensity of heat exchange with cooling air (or in some cases with water). Temperature and air velocity also affect the cooling process. A lot of factors: packaging, mass, density, water content, heat capacity, thermal conductivity, latent heat content, initial temperature - all this together and individually affects the cooling rate.
In the case of unpacked products, factors contributing to rapid cooling also lead to rapid loss of moisture, so it may seem that the best way is slow cooling. In fact, it is not. Increasing the cooling time increases the time during which the product loses moisture. Faster cooling is possible in the case of thin packaging, high speed cooling air flow and with the lowest air temperature, but this all leads to an increase in operating costs, and therefore the equipment is developed as a kind of compromise to obtain the most acceptable overall process system. This means that for different technological operations there are different equipment, and depending on the planned operation, you can choose the most appropriate equipment.
For most types of finished food products, refrigerators or air-cooled tunnels are used. For some vegetables, immersion in water (hydro-cooling) is used, and for fresh, leaf-covered foods, vacuum cooling can be used. For some products with relatively long storage periods, the cooling process can be carried out in cold rooms, but often the cooling process is accelerated by special air circulation measures. Any of these schemes should be carefully thought out.
Refrigerating air chamber cooling (air circulation)
Air circulation refrigerators are based on cooling the product with cold air that blows the product at high speed. For enterprises supplying semi-finished and refrigerated products and similar products there are special directories (such as English DHSS), where it is recommended to use equipment that should provide cooling products up to 50 mm thick with 70 0С to a temperature in the center of the product at 3 ° С or below more than 90 min. This requires an air speed of around 4 m / s and an air temperature of around -4 ° C.
There are also small refrigerated cabinets capable of processing batches of up to 30 kg for stockpiling products, as well as for training and research. Also developed large models with a capacity of up to a quarter of a ton, designed for the use of carts or trays. A typical cart for such a system has a nominal capacity of 45 kg and is usually equipped with 20 food trays. The evaporator and the fan are, as a rule, at the inner wall of the chamber, and the compressor and the condenser can be located above the chamber (on or off it) - depending on whether the corresponding noise level from the compressor is acceptable or not in the room. Temperature control should provide storage of the product at 0-3 ° C or can use a cooling cycle based on the necessary control of the cooling air temperature, on the temperature control of product samples or on the use of an ordinary timer. At the end of the cooling cycle, the thawing cycle removes ice and frost from the evaporator. The power consumed by the 45 kg cooling unit is about 7 kW.
During the two-hour cycle “loading-cooling-thawing” it is convenient to use four loads per shift, where the last load of the cooled product remains in the refrigerator overnight. Sometimes, process recorders are used to control the process. In large systems, the doors can be located on each side so that carts with cooled product are rolled inside the refrigerated storage at temperature 0-3 ° С. There is an opportunity to combine refrigerating chambers with refrigerated cabinets, which allows customers to pick up chilled food products, cook, pack them and, as a result, pack already prepared packaged portions.
Other types of air circulation refrigerators have been developed to cool freshly whipped birds. They use a tunnel of dry ice from carbonic acid to ensure the cooling process. Although in such chambers and get the desired result, there is a risk of surface freezing, which is unacceptable for many foods. One of the promising refrigerants for chillers is liquid nitrogen, but at -196 ° C and atmospheric pressure, strict control and strict observance of safety precautions are necessary. An alternative is “synthetic liquefied air” (SLA - synthetic liquid air) , which eliminates the danger of suffocation inherent in other cryogenic substances.
All such systems depend on the ability to compress and liquefy gases. It can be noted that the total energy intensity of such systems (taking into account the energy costs for gas liquefaction) may be much higher than that of similar refrigeration systems, and operating costs may also be much higher, but in some cases, to reduce the total capital costs or speed up the process. cooling such systems may be quite acceptable.
Using chilled water, which is sprayed in a special chamber or fed into an immersion tank, provides very rapid cooling without the risk of freezing the product, but this method is only acceptable for vegetables and fruits that can withstand immersion in water. This method is hardly applicable to the main row of cooled products (except for ready-made meals packed in vacuum packaging). Water in such systems is periodically updated, and in order to avoid progressive water contamination, antifungal agents or other additives are added to it that may be necessary for certain types of products. Of course, it is possible to combine the process of hydrocooling with the usual measures for the purification of the product, such as the cleaning of root crops or plant roots.
Vacuum cooling plants
Vacuum refrigeration units are a highly specialized and extremely expensive type of equipment, well adapted for rapidly cooling packaged plants with a large number of leaves. Such systems operate at low pressure — the greens are placed in a sealed chamber, and low-temperature evaporation of moisture from the product occurs. The process is carried out in portions with the cooling time of one portion in 15-30 min, and typical equipment can process several tons of product at a time, which is usually placed on pallets or carts.
For a large number of "live" products, especially fresh fruits and vegetables, cooling may consist in placing the product packed in cardboard or boxes (baskets) in a cold storage and ensuring that air is circulated there at the right temperature. This extremely slow process, which requires several days to cool the product to the desired temperature, depends on the air circulation in the room and the options for laying the product. Many fruit storage facilities use a combination of exhaust ventilation, film rolls and product placement planning (Fig. 4.3). From the stacks of cardboard boxes of the same thickness, the air is drawn out, and to the boxes covered with foil, it comes to others. If necessary, the bottom of the pallet is covered from the air with mounting foam or other suitable material.