Refrigeration in the food trades—meats and fish

Refrigeration in the food trades—meats and fish

16 Refrigeration in the food trades – meats and fish 16.1 Meat industry applications In the meat industr y, the main applications of mechanical ref...

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16 Refrigeration in the food trades – meats and fish

16.1

Meat industry applications

In the meat industr y, the main applications of mechanical refrigeration are: Chilling of carcases directly after slaughter and dressing Cooling of meat-handling rooms such as butcheries Chilled water and brine for cooling poultry Chill storage of edible meats and offal Chilling of brine and pickling vats Meat and poultry freezing Animals when slaughtered, are at a body temperature of 39°C. The carcase cools slightly as it is being dressed, but must be put into refrigerated chambers as soon as possible [41, 42]. The speed of cooling depends on the thickness of the joint, so the larger carcases are usually halved into sides. While there is a need to remove body heat to check deterioration, if this process is too quick with beef or lamb, the resulting meat may be tough. A general rule for lean meat such as beef is that no part should be cooled below 10°C for at least 10 hours after slaughter, although this limit may be varied by the local producer. The total time in this chiller stage will be about 24 hours for a beef side [43]. Meat-cooling curves are shown in Figure 16.1. During the initial cooling stage, the surface of the meat will be quite warm, and careful design of the coolers and their operation is needed to reduce weight loss by evaporation from the surface. A good air circulation is required at a humidity level of 90–94%, so as to keep the surface dry without too much dehydration. In order to maintain a good and steady air circulation around the carcases at this time, they are hung from rails (see Figures 14.1 and 16.2).

Refrigeration in the food trades – meats and fish 189

% Weight loss to 10°C deep leg

1.5

Cooling time to 10°C 33.9

26.8

24.8 4°C

30.5 1.0

27.3 24.9

21.9

20

0°C

0.5 0

0.5

1.0

2.0 Air velocity (m/s)

3.0

Figure 16.1 Effect of air velocity and temperature on the weight loss of beef carcases [43]

Storage conditions in terms of air movement and humidity will be different to those used when initially chilling the carcase. Chilled meat on the bone is stored at about 0°C, up to the point of sale. The humidity of the surrounding air is also critical in the case of fresh meats – too dry and the meat will lose weight and discolour, too humid and a slime will form on the surface.

16.2

Boned, boxed and processed meats

A lot of meat is now boned or produced as the final cuts, in the factory. For this, the meat needs to be at 0°C or just below, i.e. just above the temperature at which it starts to freeze hard. This work must be carried out under hygienic and cool conditions. The air temperature is usually not lower than 10°C, for the comfort of the butchery staff, but some establishments work down to 2°C or 3°C. Air movement in the working area must be diffused and not too fast, to give an acceptable environment to the operators. Cut meats are usually wrapped or vacuum packed directly after cutting. The viscera, bones and other parts not going for human consumption have a byproduct value, and will probably need to be stored at chill temperature before disposal. Cut meats may be frozen or kept at ‘chill’ temperatures. If the latter, the shelf life is comparatively low and the product will be despatched almost immediately for sale. In ‘protein economy’ processes, parts of the carcase which are

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190

not to be sold as joints or cuts are made up in moulds into artificial joints, ‘gigots’ or meat loaf, in a pre-cooking operation. The madeup product must then be cooled to about 0°C, and may then be sliced and vacuum packed, these operations taking place in airconditioned rooms kept at temperatures of 10°C or lower. Most such items are for ‘chill’ storage and immediate distribution for sale. There are many variations in the manner of handling and processing meats, and these will be known only to specialist companies in the trade. The principles of cooling are the same for all. Meat may be frozen on the bone, but this is not a very convenient shape for packing and handling. It is more usually boned, vacuum wrapped, boxed and then frozen. Boxed meat sizes are about 635 × 350 mm and 100, 125 or 150 mm thick, the largest of these holding some 25 kg. The freezing may be in a cold air blast and the speed of cooling will depend on the thickness of the slab (see [1–7]) and the insulation effect of the box or wrapping (Figure 16.2). Thinner pieces of meat can be frozen between refrigerated plates (see Figure 7.9a) [44]. Predicted Experimental Heat Metal tray Box without lid

–30

Air temperature (°C)

Box with lid

0.5 m/s 5 m/s 0.5 m/s 5 m/s 0.5 m/s 5 m/s

7.3 18 6.8 12 5.1 8.7

–20

–10

0 20

40 60 80 Freezing time (h) from 4 to – 7°C

100

120

Figure 16.2 Freezing time for 150 mm wrapped boxed beef (Courtesy of AFRC Institute of Food Research, Bristol Laboratory)

16.3

Pork and bacon

Fresh pork has a shorter shelf life than beef, but is handled in the same way and at the same chill-room temperatures. Although no latent heat of the freezing of water content will be extracted at chill

Refrigeration in the food trades – meats and fish 191

temperatures, some heat will be removed when the fat ‘sets’ or crystallizes. The quantity of heat to be removed should be estimated and may be included in the sensible heat capacity in that temperature range. For example, the sensible heat capacity of pork meat averages 2.5 kJ/(kg K), but a figure as high as 3.8 may be used for carcase cooling to allow for this factor. A high proportion of pork is pickled in brine and smoked, to make ham or bacon. The original process was to immerse the meat in a tank of cold brine for a period. A quicker method is to inject the cold pickle with hypodermic needles into the cuts. Smoking is carried out at around 52°C, so the cured bacon must be cooled again for slicing, packing and storage.

16.4

Poultry

Poultry is immersed in hot water just after slaughter, in order to loosen the feathers for the plucking process. The carcases are then eviscerated and chilled as soon as possible by cold air blast or using iced water in the form of a bath or spray. Larger birds may be reduced to portions, so the flesh must be cooled to about 0°C to make it firm enough for cutting. Whole birds are prepared for cooking and then vacuum wrapped for hygiene. Poultry may be chilled for the fresh chicken market, or frozen. Chilling and freezing are mainly by cold air blast. Large birds such as turkeys are wrapped and immersed in low-temperature brine until the outside is well frozen, and then put into low-temperature storage to freeze right through. Some poultry is frozen by spraying with liquid carbon dioxide. Storage of chilled poultry is at –1°C. The shelf life is relatively short and the product will not remain in store for more than a couple of days.

16.5

Fish

Most fish is still caught at sea and must be cooled soon after it is taken on board, and kept cold until it can be sold, frozen or otherwise processed [45]. The general practice is to put the fish into refrigerated sea water tanks, kept down to 0°C by direct expansion coils or a remote shell-and-tube evaporator. The sea water must be clean and may be chlorine dosed. At this condition, fish can be kept for up to four days. Ice is also used on board, carried as blocks and crushed when required, carried as flake, or from shipboard flake ice makers. Artisanal fishermen in hot climates may take out crushed ice in

192

Refrigeration and Air-Conditioning

their small boats. Fresh fish is stored and transported with layers of ice between and over the fish, cooling by conduction and keeping the product moist. Fish kept at chill temperatures in this manner can travel to the final point of sale, depending on the time of the journey. Where refrigerated storage is used, the humidity within the room must be kept high, by using large evaporators, so that the surface of the fish does not dry. Most vessels can now freeze their catch at sea, enabling them to stay offshore without the need to run back to a port within the limited life of the chilled product. If the fish is to be cleaned and processed later, it is frozen whole, either by air blast or, more usually, in vertical plate freezers (see Figure 7.9b), followed by frozen storage. Some fishing vessels and the fish factory vessels will carry out cleaning, filleting and other operations on board and then freeze and store the final product. A limited amount of fish is frozen by immersing it in a cold concentrated sodium chloride brine. This is mainly tuna for subsequent canning, or crustaceans. Fish which is frozen in air blast will often be dipped into clean water afterwards, resulting in a layer of ice on the surface. This glazing process protects the fish from the effects of dehydration in subsequent storage. Some freezing of fish fillets and other processed fish is carried out between or on freezer plates, in an evaporator assembly similar to that shown in Figure 7.9a. Flat cartons of fish and fish fillets are frozen in these horizontal plate freezers. Health and safety requirements continue to become stricter in the maintenance of the cold chain and the latest regulations should be adhered to.