Refrigeration is a science that deals with absorption of heat at a temperature from surroundings at lower temperature and rejecting it to a relatively higher temperature at the cost of some external work following Clausius statement of second law of thermodynamics. Refrigeration sector is one of the largest economic sectors, covering precooling, cooling, air conditioning, and freezing applications for various commodities and people and appears to be an energy intensive sector by consuming about 20 per cent of the total electricity generated worldwide. The refrigeration sector includes air conditioners, household fridges, coolers, pre-coolers, refrigerators, heat pumps, and freezers for various applications, ranging from food cooling to space cooling. Thus, refrigeration has found its importance in many engineering and industrial sectors. One of them is processing, preservation and nourishment of food by storing them in places whose temperature is lower than that of the surroundings. Figure 1 shows the energy consumption of different preservation processes over storage time together with the energy used for the agricultural production.
Figure 1: Energy consumption of different preservation processes over storage time together with the energy used for the agricultural production
Storage Life of Frozen Foods
According to the International Institute of Refrigeration, storage life is defined as the length of duration of a food product stored under specified conditions so that it is liable for consumption. Different food products have different storage lives. Also, it has to be considered that storage life of a particular food product may vary according to its location and other ecological aspects. The ranges of indicative practical storage lives of various food products are given in Table 1.
Different Refrigeration Processes Involved in Food Processing
The principle behind preservation of foods by refrigeration is to reduce and maintain the temperature of the food in order to mitigate any detrimental or undesirable changes occurring in the food. These changes can be microbiological, physiological, biochemical or physical. This in turn can help in improving the nutritional quality of the food. There are several refrigeration processes involved in the processing and nourishment of food. The refrigeration techniques that are implemented one after the other have been illustrated below:
Pre-cooling: Rapid cooling is necessary to retard the metabolism of food products and to increase their life span. The lesser the temperature is, the better the quality and longevity of the cuisines. Based on the type of the food product, different refrigeration techniques have been proposed. i.e. hydrocooling for small fruits, vacuum cooling for green leafy vegetables, blast-air system for cooling the surfaces of meat products, cooling of milk and other dairy products in specialised tanks, pulsed air in some other products and so on. Chilling: After precooling, the food product has to be chilled and maintained at the favourable temperature. Chilling is usually done in classic cold rooms equipped with sufficient ventilation. The chilling and conservation temperatures depend on the sensitivity of the product i.e. for high-sensitivity products such as mangoes, ginger, sweet potatoes temperatures below 8-12°C are not recommended, as they can endure metabolic disturbances that shorten the life of the goods. For medium-sensitivity products such as tangerines, green beans, potatoes, it is possible to lower the temperature not less than 4-6°C. For low-sensitivity products, a temperature of 2-3°C, down to just above the freezing point is recommended. In slaughterhouses, low temperatures that can be achieved by using blast chillers or cold storage rooms are recommended in order to inhibit the growth of microorganisms. For eggs, conservation at higher temperature could be enough as long as the shell remains unbroken and for dairy products an adequate temperature of nearly 8-10°C would be enough to inhibit the growth of pathogenic germs.
Freezing: Freezing is a process of lowering the temperature of a product below its solidification point. Freezing hinders the metabolism of the fruit and vegetable products. Longer the product remaining frozen, longer is its durability for storing.
Super-cooling and Super-chilling: These are new processes implemented for the storage of food products. Supercooling is lowering down the temperature of a food product just below its freezing point without any formation of ice and superchilling is partial freezing of a food at a temperature just below its freezing point.
Apart from the above-mentioned techniques, some other techniques are also adopted such as crystallisation of fat, cryoseparation of undesirable components; cryoconcentration and so on.
Refrigerants Used in Food Processing Industries
Refrigerants are the working fluids used in refrigeration systems. Selection of refrigerant plays a vital role in any refrigeration industry, so it has been a real challenge to develop and implement refrigerants having desirable thermodynamic properties and high energetic and exergetic performances. Various refrigerants used in food processing sectors and their percentage shares have been presented in figure 2.
Figure 2: Percentage distribution of refrigerants used in food processing
Earlier, the refrigerants were selected based on their thermodynamic properties only, but eventually focus has also been made over the ozone depletion potential (ODP) and global
warming potential (GWP) of refrigerants. Some of the desirable properties of refrigerants needed in food processing industry have been listed below:
It should have low flammability and toxicity.
It should zero ODP value and GWP value as minimum as possible.
It should be easy leak detectable.
Selection of refrigerants has significant impact on safety, reliability, energy consumption, system performance and also on the environment. Some of the refrigerants involved in food processing industries have been listed below in table 2. However, some alternative refrigerants have now been replaced in place of earlier used refrigerants.
Earlier, the use of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were in vogue for a long time due to their excellent thermodynamic properties. But as per recommendations of the Montreal and Kyoto protocols, they were being phased out due to their significant ODP and GWP values. As an immediate replacement of CFC-12, R-134a was adopted due to its low ODP value. Afterwards, alternative for R-134a was also needed to overcome the high GWP of this refrigerant. Figure 3 shows the relative performances of other 12 alternative refrigerants with respect to R134a in terms of COP, volumetric displacement, mean temperature difference between the air and the refrigerant in the evaporator and compressor discharge temperature. The compositions of 12 alternative refrigerants have also been presented in table 3.
Figure 3: Figure showing the performance of alternative refrigerants relative to R134a
Different types of refrigeration systems implement different types of refrigerants by taking their thermodynamic properties into consideration. It has been observed that the usage of R404A is used in most of the sectors due to its excellent food conservation and freezing applications and widespread use in split type and centralised refrigeration systems. Figure 4 shows the percentages of different refrigerants used in different food processing sectors.
Figure 4: Various refrigerants used in different food processing sectors
The term cold chain refers to a temperature-controlled supply chain that a refrigerated product passes through, which is then either until it is removed by a customer in a retail environment or unloaded from a delivery vehicle in the vicinity of its destination. For consumers, the cold chain is often related to transport, retail and household refrigerators. But refrigeration is also used in the agri-food industry for the storage of raw materials and final products, as well as for food processing. An efficient and effective cold chain is designated to provide the best conditions for inhibiting any undesirable changes for as long as is practical. Effective refrigeration produces safe food with a long and quality life. A schematic diagram of a simple food chain has been presented in figure 5.
Figure 5: Figure showing an overall cold chain and its components
As food moves along the cold chain, it becomes a herculean task to control and regulate its temperature. This is because the temperatures of bulk packs of refrigerated products in large storerooms are far less sensitive to small heat inputs than single consumer packs in open display cases or in a domestic refrigerator.
Refrigeration Cycle in Food Processing Industry
In food processing industries, the refrigeration systems in general operate with single-stage, direct expansion vapour compression refrigeration (VCR) cycle. These systems are very simple, reliable and also cheap. A VCR cycle consists of four basic components: compressor, evaporator, expansion valve and a condenser. A low-pressure cold liquid refrigerant is allowed to evaporate to a gas within the evaporator coil. This process requires heat, which is extracted, thus, cooling any medium surrounding the evaporator. The low-pressure hot gas from the evaporator is compressed in the compressor to a high-pressure hot gas. This high-pressure hot gas is then passed through another coil, where it condenses back to a high-pressure cold liquid. This process releases heat into any medium surrounding the condenser coil. This high-pressure cold liquid refrigerant then passes through the expansion valve where throttling process takes place, to a lower pressure section. It may be noted that the throttling process occurring in the expansion device is irreversible in nature. The low-pressure liquid then passes back to the evaporator. The schematic diagram of a simple mechanical VCR system and the corresponding T-s diagram have been shown in figure 6 (a) and 6 (b) respectively. In general, factors influencing the nutrient content of refrigerated food include storage temperature, storage length, humidity and light.
Figure 6: (a) Schematic and (b) T-s diagram of a VCR system
Figure 7: Schematic diagram of a centralized DX system
The different processes undergoing in a VCR cycle with respect to the T-s diagram are:
Process 1-2: Isentropic work input to the compressor.
Process 2-3: De-superheating and condensation at constant pressure.
Process 3-4: Throttling in the expansion valve. As throttling process is irreversible in nature, it is represented by dotted line in the T-s diagram.
Process 4-1: Evaporation at constant pressure.
However, such systems have certain specifications in terms of the method for processing and the type of components used. For chilling, single-stage systems with flooded evaporators and screw compressors are recommended. Secondary refrigerants such as propylene glycol are preferred over to primary refrigerants as it significantly reduces the quantity of ammonia and promotes safety. For freezing, two-stage refrigeration systems with flooded evaporators and either of the piston or screw compressor is recommended. Refrigerants such as R134a or R717 are used in the high-temperature stage and R744 is used in the low temperature stage. As mentioned earlier, in most of the food processing industries, a centralised direct expansion (DX) system is established. Centralised systems provide the flexibility of installing the compressors and condensers in a centralised plant area, usually in the vicinity of any store. In the plant room, multiple refrigeration compressors, using common suction and discharge manifolds, are mounted on bases or racks normally known as compressor packs or racks which also contain all the necessary piping, valves, and electrical components needed for the operation and control of the compressors. The evaporators in the cold rooms are fed with refrigerant from the central plant through distribution pipework installed under the floor or along the ceiling of the sales area. Air-cooled or evaporative-cooling condensers used in conjunction with the multiple compressor systems are installed remotely from the compressors, usually on the roof of the plant room. Besides, separate compressor packs are used in chilling and freezing processes. The schematic diagram of a centralised DX system has been shown below in figure 7. However, the only major disadvantage associated with a centralised DX system is that a large amount of refrigerant is required, nearly 4-5 kg/kW of refrigeration capacity. To overcome such difficulty, a secondary or indirect system arrangement is used. With this arrangement, shown schematically in figure 8, a primary system can be located in a plant room or the roof and can use natural refrigerants such as hydrocarbons or ammonia to cool a secondary fluid, which is circulated to the coils in the display cabinets and cold rooms. Separate refrigeration systems and brine loops are used for the medium and low-temperature display cabinets and other refrigerated fixtures. The temperature of individual consumer packs, small individual items and especially thin sliced products responds very quickly to small amounts of added heat. All these products are commonly found in retail display cabinets and marketing constraints require that they have maximum visibility.
Figure 8: Schematic diagram of an indirect expansion system
Figure 9: Relationship between TDA and TEC for all chilled and frozen cabinets
Refrigeration System Performance in Food Processing Unit
To understand how efficiently a system works, it is very important to know the performance of a system. COP is one of the main performance parameters in a refrigeration system. The COP of the system can be defined as the ratio of refrigerating effect produced to the power input to the system. Evans et al expressed the power input in terms of total energy consumption (TEC) and total display area (TDA). TEC can be expressed by the following equation,
TEC = DEC + REC (1)
Where, DEC is direct electrical energy consumption, i.e. energy consumed by fans, heaters, defrost heaters, lighting, accessories and REC is the refrigeration electrical energy consumption and is calculated as:
Where, tr is defrost period in hr, Ø0 is the heat extraction rate in kilowatt based on EN441-12, Tc is the conventional condenser temperature at 308.18 K (35°C) for European comparisons, T0 is the refrigerant evaporator temperature in Kelvin based on EN44112. and 0.34 is taken to be the Carnot efficiency of refrigeration systems used in commercial refrigeration.
TDA can be evaluated by the following equation,
Where, Ho is the open and Hg is the glazed horizontal projection (m), Vo is the open and Vg is the glazed vertical projection (m), Tgh is the light transmission through glazing surface for horizontal projection (%), Tgv is the light transmission through glazing surface for vertical projection (%), L is the cabinet length (m), Loh is the horizontal open length (m), Lov is the vertical open length (m), Lgh is the horizontal glazing length (m), and Lgv is the vertical glazing length (m).
The relationship between TEC and TDA is established by Evans et al. by drawing regression lines as shown in figure 9.
Refrigeration techniques are vastly implemented in food processing industries for the storage, preservation and nourishment of food. For the storage, maintenance, transportation and other industrial parameters associated with food processing, a cold chain has been established. Industrial refrigeration is the first step in the cold chain where food is processed and stored before transport, retail and consumption. Performance of the system is analysed in terms of total energy consumption and total display area. The refrigeration sector faces many challenges with respect to reliability, energy consumption, environmental impact regulations and other economic concerns. Solutions for providing sustainable industrial refrigeration systems depend on the size of the facility and on the required temperature levels.