Refrigeration in food processing

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 applica-
tions for various commodities and people and appears to be an energy intensive sector by con-
suming about 20 per cent of the total electricity generated worldwide. The refrigeration sector in-
cludes 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 pro-
cesses over storage time together with the energy used for the agricultural production.

Figure 1: Energy consumption of different preservation processes over storage time to-
gether 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 dura-

tion 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 partic-
ular food product may vary according to its location and other ecological aspects. The ranges of in-
dicative 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 tempera-
ture 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 af-
ter the other have been illustrated below:
 Pre-cooling: Rapid cooling is necessary to retard the metabolism of food products and to in-
crease 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 pro-
posed. i.e. hydrocooling for small fruits, vacuum cooling for green leafy vegetables, blast-air sys-
tem for cooling the surfaces of meat products, cooling of milk and other dairy products in spe-
cialised 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-sensi-
tivity products such as mangoes, ginger, sweet potatoes temperatures below 8-12°C are not rec-
ommended, 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 re-
maining 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 tem-
perature 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 vi-
tal role in any refrigeration industry, so it has been a real challenge to develop and implement re-
frigerants having desirable thermodynamic properties and high energetic and exergetic perfor-
mances. 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 eventu-
ally 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 in-
dustries 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 recommenda-
tions 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 re-
spect 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 composi-
tions 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

Cold Chain

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 un -
loaded from a delivery vehicle in the vicinity of its destination. For consumers, the cold chain is of -
ten 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 un-
desirable 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 tem -
perature. This is because the temperatures of bulk packs of refrigerated products in large store-
rooms 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, expan-
sion 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 stor-
age temperature, storage length, humidity and light.
Figure 6: (a) Schematic and (b) T-s diagram of a VCR system

Refrigeration techniques are vastly implemented in food processing industries for the storage,
preservation and nourishment of food. For the storage, maintenance, transportation and other in-
dustrial 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 consump-
tion and total display area. The refrigeration sector faces many challenges with respect to reliabil-
ity, energy consumption, environmental impact regulations and other economic concerns. Solu-
tions for providing sustainable industrial refrigeration systems depend on the size of the facility and
on the required temperature levels.