Temperature Processing of Meat Products: Heat Treatment and Thermal Properties
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The thermal state of a physical body is described by its temperature, one of the fundamental parameters of a body's state. The International System of Units (SI) provides two temperature scales for measuring temperature: the thermodynamic temperature scale and the International Practical scale. Temperatures on each of these scales may be expressed in degrees Kelvin (°K) or in degrees Celsius (°C), depending on the zero point (the position of zero) on the scale.
What units measure heat and how much energy is a joule?
In the SI, the joule (J) is the accepted unit for measuring energy of all kinds and quantities of heat. A joule is the mechanical work performed by a force of 1 Newton when a body is moved a distance of 1 metre in the direction of the force. To heat equal quantities of different physical bodies of the same mass by the same number of degrees, different amounts of heat must be supplied. This is explained by the differing heat capacities of bodies.
Heat capacity is the ratio of the quantity of heat transferred to a body to the corresponding change in its temperature: C = ΔQ/ΔT.
How does the state of matter relate to refrigeration?
The state of aggregation of a substance (solid, liquid, gaseous) depends on external conditions — temperature and pressure. Phase transitions of homogeneous bodies occur at a constant temperature that depends on the external pressure.
To carry out a phase transition, heat must be supplied or removed; this is called the heat of phase transition. It is spent on internal work to rearrange the molecules of the body, and a step-like change in the body's density occurs without any chemical change.
To produce cold, those phase transitions are used that proceed at low temperatures while absorbing heat from the cooling medium: melting, boiling, and sublimation.
- Melting is the transition of a substance from the crystalline to the liquid state, accompanied by the absorption of heat. Vapour formation can occur through two processes: evaporation and boiling.
- Evaporation is vapour formation above an open liquid surface, occurring at any temperature.
- Boiling is vapour formation throughout the entire liquid, with vapour bubbles rising into the space above the liquid. Boiling of a liquid occurs at a boiling temperature defined for a given boiling pressure p₀. Besides melting and boiling, refrigeration machines also exhibit the reverse processes, which are accompanied by the removal of heat: solidification and condensation.
- Solidification is the transition of a substance into the solid state from the liquid at a constant melting temperature corresponding to a given pressure. Condensation is the transition of a substance from the vapour state to the liquid, accompanied by the removal of heat.
- Sublimation is the process of a body passing from the solid state directly into the vapour state, bypassing the intermediate liquid state. The most common refrigerants are ammonia, Freon-12, and Freon-22.
Ammonia
Ammonia (NH₃, R717) is a colourless gas with a sharp, suffocating odour that is harmful to the human body. The permissible concentration of ammonia in air is 0.02 mg/L. At high concentrations it causes severe irritation of the mucous membranes of the eyes and respiratory tract, and a person's stay for 60 minutes in a room with an ammonia volume fraction of 1.5–2.7% leads to death.
In a mixture with air at an ammonia volume fraction of 15 to 28%, ammonia is explosive. Ammonia vapours are lighter than air. It does not act on ferrous metals, aluminium, or bronze, but in the presence of moisture it corrodes other non-ferrous metals: zinc, bronze, copper and its alloys; it dissolves readily in water. By its thermodynamic properties, ammonia is one of the neutral refrigerants.
The pressure in the condenser under normal conditions is no higher than 1.175 MPa and only at very high water temperatures reaches 1.470 MPa. The normal boiling temperature (at atmospheric pressure) is −33 to −35 °C, the critical temperature +132.4 °C. The freezing temperature is −77.7 °C.
Freon
Freon-12 (R 12) is a colourless gas with a faint specific odour, imperceptible at a volume fraction of less than 20%, and 4.18 times heavier than air. R 12 is one of the least hazardous refrigerants. It is entirely non-explosive; at temperatures above 400 °C with an open flame it decomposes, forming hydrogen chloride, hydrogen fluoride, and traces of the poisonous substance phosgene, so smoking and working with open flame in a room containing Freon installations is strictly prohibited.
It dissolves without limit in oil, and its solubility increases as temperature rises. R 12 is practically insoluble in water. Dehydrated R 12 is neutral toward all metals. It serves as a solvent for many organic substances.
It can leak through the smallest gaps in the system and even through the pores of ordinary cast iron, so compressors for R 12 use castings only of dense, fine-grained cast iron. The condenser pressure does not exceed 1–1.2 MPa. The normal boiling temperature of R 12 is −29.8 °C, the critical temperature +112 °C, the freezing temperature −155 °C.
Freon-22 (R 22) is more toxic than R 12 but non-explosive. It dissolves without limit in oil only at high temperatures (in the condenser), whereas at low temperatures it has a limiting solubility, with the result that during boiling an oil-rich layer forms in the upper part of the evaporator.
It penetrates easily through gaps, is neutral toward metals, and is poorly soluble in water. The normal boiling temperature is −40.8 °C, the critical temperature +96 °C, the freezing temperature −100 °C. A refrigerant circulates in the closed system of the refrigeration machine (Fig. 1). The boiling temperature t₀ and condensation temperature t_K of the refrigerant depend on the pressure: the lower the pressure, the lower the temperature. 
How does a vapour refrigeration machine work?
An apparatus called the evaporator is placed inside the cooled room. In the evaporator the refrigerant boils at a low boiling pressure p₀ and the corresponding temperature t₀, cooling the room by absorbing the heat of vapour formation. To reuse the refrigerant, it must be converted back from the vapour state into the liquid.
For this, the vapour pressure must be raised and the vapour cooled to the condensation temperature. The vapour formed in the evaporator is drawn off by the compressor, compressed to the condensation pressure, and passes into an apparatus called the condenser, where it is cooled by water or air and condenses, releasing the heat of condensation, because the condensation temperature is higher than the air temperature (t_k > t_v).
Liquid refrigerant from the condenser is fed into the evaporator through a regulating valve, in which it is throttled down to the boiling pressure p₀. The theoretical process of a vapour refrigeration machine (Fig. 1 b, c) proceeds in the region of wet vapour, where line 4-1 represents the boiling of vapour in the evaporator; line 1-2 — the adiabatic compression of vapour in the compressor; 2-3 — the condensation process; 3-4 — the throttling of liquid in the regulating valve.
Which cooling systems are used in refrigeration chambers?
The required temperature and humidity regimes in refrigeration chambers are maintained by the combined operation of the refrigeration machine and the cooling devices. When designing a refrigeration installation and selecting a cooling system, it should be ensured that the cooling system is:
- reliable and flexible in operation;
- simple and convenient to operate;
- compliant with the rules of safety engineering and fire prevention;
- economical both in initial capital investment and during operation.
Depending on the cooling medium, two chamber cooling systems are distinguished: using a boiling refrigerant — direct cooling — and a circulating system — brine cooling. Depending on the heat-removal conditions and the design of the chamber cooling devices, tubular, air, and mixed cooling are distinguished. In tubular cooling, batteries are installed in the chambers and supplied with liquid refrigerant or heat-transfer fluid.
If air cooling occurs because of the refrigerant boiling in batteries placed directly in the cooling chamber, such cooling is called direct, and the chamber cooling devices are called direct-cooling batteries. Air cooling may also occur because of the heating of the heat-transfer fluid entering the batteries at a temperature 8–10 °C below the temperature of the air being cooled.
The most common heat-transfer fluids are brines (aqueous salt solutions — sodium chloride, calcium chloride), so such cooling is called brine cooling, and the chamber cooling devices brine batteries. With tubular cooling, natural air circulation is established in the chambers at a speed of 0.05–0.15 m/s, caused by the difference in densities between the warm air near the surface of the load and the cold air near the surface of the cooling devices.
Air cooling of chambers is carried out by air pre-cooled in a heat-exchange apparatus — the air cooler. Cold air from the air cooler is forced by a fan into the chamber, where it heats up and humidifies, comes into contact with the load, and again enters the air cooler, where it is cooled and dried, transferring heat to the boiling refrigerant or brine.
When chamber ventilation is required, outside air enters the air cooler. With air cooling there is forced air circulation, the speed of which reaches 2.5 m/s.
Mixed cooling is a combination of tubular and air cooling. At modern enterprises this method of cooling refrigerator chambers is rarely used. In the meat industry a significant amount of meat and meat products is frozen in blocks in freezing chambers — on shelves in metal and polyethylene basins, with subsequent thawing during unloading. 
Why is rapid freezing better than freezing in chambers?
All these drawbacks are eliminated by using freezing apparatuses designed for the rapid freezing of meat and meat products. Rapid freezing offers advantages over ordinary freezing in chambers:
- the nutritional value of products — vitamin content and taste qualities — is preserved most fully;
- the marketable appearance of the product and the sanitary conditions of production are significantly improved;
- the duration of the freezing process is shortened and the production area reduced.
The gravitational conveyor freezing apparatuses GKA-4, lines with membrane apparatuses FBM-1 and FBM-2, and the automated rotary freezing apparatuses MAR and ARSA have become most widespread at meat and dairy enterprises.
FBM-2 lines
Lines with the membrane apparatuses FBM-1 and FBM-2 are used to freeze meat and offal in blocks. The FBM-2 line (Fig. 1) consists of five membrane freezing apparatuses (3) with a capacity of 2 t/day each, a feeder-dispenser for charging raw material (2) into the block formers, a loading bucket (1) with a capacity of 350 L needed to fill one apparatus, two electric hoists (4) of the TE-0.5-511 type, a servicing platform (6), a bucket for supplying raw material (5), and a trolley (8) for receiving the meat blocks (7). The apparatus is a rectangular box with block formers. The box has a lowering bottom and a removable rubber lid. 
Inside the box, nine membrane chambers are installed vertically — these are internally hollow cooling plates made of corrosion-resistant steel sheets 3 mm thick. The plates are connected to one another by rubber corrugated pipe sections — compensators — made of frost-resistant soft rubber. The membrane chambers and compensators form a serpentine channel through which the heat-transfer fluid circulates, freezing the product through the metal surfaces of the membrane chambers, which move together and apart horizontally by means of a pneumatic cylinder.
Between the membrane chambers there are metal limiting partitions that serve as the walls of the block formers. The feeder is a rectangular welded box-frame in whose bottom there are 24 openings for the meat fed into the block formers. To the lower part of the openings, 48 half-forms are fastened freely on axles, designed to direct the meat into the package. The bucket for feeding meat into the feeder consists of two half-buckets, separated by a central partition and fixed on two axles.
Before loading the freezing apparatus, the membrane chambers are moved apart so that the distance between them is 120 mm. Then the packages are straightened and manually installed in the block formers of the apparatus. With the help of the electric hoists, the feeder, and the loading bucket, the raw material is loaded into the packages. The membrane chambers are moved together to a distance of 100 mm between them, and the circulation of the heat-transfer fluid is switched on.
After freezing is complete, the supply of heat-transfer fluid is switched off and the membrane chambers are moved apart. The blocks remain on the bottom of the apparatus, which is lowered and tilted by a pneumatic cylinder so that the blocks, under their own weight, slide off onto a conveyor or into a trolley.
FBM-2 lines are installed in rooms with t = 10–12 °C. The heat-transfer fluid temperature is −27 to −35 °C, and its pressure in the membrane chambers is 0.2–0.5 MPa. The duration of freezing meat and offal in blocks measuring 370 × 370 × 95 mm is 3–3.5 hours. In some technological processes — for example, in sausage making — food ice is used, and snow ice and flake ice generators serve to produce it. Snow ice is obtained in ice generators of vertical and horizontal types.
Snow ice generator
The vertical ice generator is a hollow cylinder (1) surrounded by a cooling jacket. Liquid refrigerant is fed into the jacket. Water is supplied by a pump to the inner surface of the cylinder through nozzles (5). As it flows down, the water freezes in a thin layer of ice, which is cut off by a knife (2) fixed on a vertical rotating axis (3).
The resulting ice, together with water, falls into a pan (4), from which it goes into production or for making briquettes. Vertical snow generators Л-250 have a capacity of 250 kg/h. 
The horizontal generator of snow and flake ice consists of a rotating steel cylinder (1) immersed in a bath (4) that is half filled with water. The cylinder is 900 mm long and has an external diameter of 700 mm. Through a seal by means of a hollow shaft (3), liquid refrigerant is supplied to the cylinder from one side, and the vapour formed is drawn off from the opposite side.
The cylinder is rotated by an electric motor through a gearbox at a rotation frequency of 8–12 per minute. An ice film 1–3 mm thick, in the form of dry flakes or a moist snow mass, is cut off by a stationary knife (2) and directed along an inclined plane into a hopper.
The ice generator is served by a refrigeration installation with a capacity of 46,600 W. At a boiling temperature of about −20 °C, the capacity of the AIL-200 ice generator is approximately 180 kg/h. Flake ice generators are also used that supply cold brine, pre-cooled in the evaporator of the refrigeration machine, into the cylinder.
