Freezing
Freezing
Friday, 10 April 2009
Initially, freezing at Langford was carried out in an uncontrolled manner just to provide raw material for the thawing investigations. As more pilot plant and instrumentation became available specific studies were set up to freeze products under controlled conditions either as freezing studies in their own right or as part of combined investigations into freezing and thawing. To convert the free water in lean meat into ice a large amount of heat (300 kJkg-1) has to be extracted. However, in contrast to thawing, the freezing process is aided by a three fold increase in the thermal conductivity of the frozen meat.
Many of the early investigations looked at commercial performance (Cutting & Malton, 1974; Cutting, 1974) and the problems in meeting European Economic Community (EEC) freezing requirements (Cutting, Cox & Malton, 1972). One problem common to many operations was that of measuring the time taken for the centre of products to pass through the latent heat plateau. The only reliable method was to use a multi-point probe inserted through the thickest section of the product. However, it was not possible to remove the probe without thawing the material.
There were a number of commercial reasons for determining accurately the freezing time at the centre of a food carton. If freezing times are under-estimated then cartons may be transferred from the blast freezer to the cold store with the inside of the block still unfrozen. This, at worst, may create microbiological problems in the subsequently thawed material, or at best, will impose a product load on the cold store that it is not designed to take. On the other hand, if blocks are left in the blast freezer beyond the required freezing time the whole process becomes inefficient.
Mathematical modelling indicated one possible solution to the problem (James, Bailey & Ono, 1976). The model was used to produce a set of time temperature curves, at various depths, through an infinite slab of lean meat as it was being frozen. An analysis of these curves indicated that the end of the freezing plateau at the centre of the block should be reflected in a change in the temperature-time gradient at or near the surface. This change was subsequently demonstrated experimentally, and a method was devised which enabled the freezing time at the centre of a block to be determined using a probe inserted just under the surface of the block. The same principal also applied to thawing.
A number of equations, most based on the initial work of Plank, have been developed for the prediction of freezing times of slabs and cylinders. With experience they can be used to estimate the freezing time of ‘homogeneous foods’, under constant conditions, with a high degree of accuracy. They also provide very quick methods of indicating the effect on freezing time of small changes in product thickness, media temperature, etc. As already mentioned, complex cases in which the ambient temperature, the surface heat transfer coefficient and the physical properties of the solid may vary can be handled using finite difference techniques based on the numerical method described by Dusinberre (1949).
Using modelling and experimental verification a predicted relationship between freezing time and air temperature for meat blocks at low (0.5 ms-1) and high (5 ms-1) air velocities was produced for different packaging configurations (James, Creed & Bailey, 1979). The relationships were obtained using a modification to Dusinberre’s method. Increasing the air velocity from 0.5 to 5 ms-1 reduces the freezing time by an average of 29%. Freezing times of blocks in cardboard cartons with lids are between 25 and 82% longer than in metal trays, for any given air temperature and velocity. Investigations showed that to freeze 15 cm thick meat blocks in corrugated cardboard cartons in less than 24 h air temperatures below -30°C and air velocities exceeding 5 ms-1 are required. A survey of industrial performance carried out at the same time indicated that only 58% of industrial throughput was frozen within ±20% of the required freezing time (Creed & James, 1981).
Further data were required for intervention freezing of beef quarters and mutton and offal processing.
A literature survey (James & Bailey, 1987) reported that although brine spray and liquid nitrogen immersion systems had been used to freeze beef quarters, most investigations had used air. Temperature in air systems ranged from -11 to -40°C and weight loss from 0.3 to 1.19%. In Langford investigations (James & Bailey, 1987) beef quarters ranging in weight from 40 to 140 kg were frozen in air at -32°C, 1.5 ms-1. On average hindquarters below 50 kg and forequarters below 75 kg could be frozen in a 24 h operation.
Mutton production is seasonal and continuity of supply for processing can be achieved by frozen storage and subsequent thawing and boning. Again, limited experimental investigations were used to verify a predictive program for freezing of mutton carcasses (Creed & James, 1984). The predictions indicated that any condition more severe than -20°C, 0.5 ms-1 would achieve a 24 h freezing operation for unwrapped carcasses. To guarantee an overnight (15 to 16 h) freezing cycle for wrapped carcasses, conditions more severe than -30°C, 4 ms-1 would be required.
Although edible offal comprises from 3 to 4% of the cold weight of a carcass there were little published data on its refrigeration. Investigations (Creed & James, 1983) showed that liver was amenable to plate freezing and the freezing time to -7°C (Y) was related to the initial temperature (I) and (R). Where R = 1 /( -1.5°C - refrigerant temperature). The studies were extended to examine the effect of different packaging materials on freezing time (Creed & James, 1985).
Other investigations showed that both the type of packaging and the tray used to contain the packs during freezing, substantially affected freezing time (Malton & Bailey, 1982). The use of trays with holes or slots in the sides did not reduce freezing time. However, trays that stacked so that a gap of 3 to 5 cm was maintained between the trays markedly reduced the freezing time of packs of sausages.
In the 1990s with the growing understanding of the potential of CFCs for environmental damage we focused our attention at Langford at alternative refrigeration cycles. One of he most exciting, and one of the most environmentally friendly, alternative refrigerants identified was air. The use of air as a refrigerant was not new. In the 19th Century air was extensively used in cold store applications and ship based chilling systems. However, the development of vapour compression cycles, based initially on ethyl ether, ammonia or sulphur dioxide, but then superseded by CFCs produced systems with much higher efficiencies. However, since that time air cycle had been extensively used in aircraft air conditioning and development of rotary compressors and expanders had greatly improved its efficiency and reliability. Advances in turbine technology, together with the development of air bearings and ceramic components offered further increases in efficiency. Combining this with newly available compact heat exchangers with greatly improved heat transfer characteristics made competition with existing vapour compression and certainly liquid nitrogen systems, potentially quite feasible.
The use of air as a refrigerant is based on the principle that when a gas expands isentropically from a given temperature, its temperature is reduced (Gigiel, 1996). The resulting cold air can then be used as a refrigerant, either directly in an open system or indirectly by means of a heat exchanger in a closed system. In such systems the efficiency is limited because heat transfer is over a range of temperatures and the efficiencies of compression and expansion have an important effect.
Some of the first work on air cycle refrigeration at Langford (Gigiel et al., 1994; Gigiel et al., 1995) was carried out as part of an EU Joule II funded project with TNO (Holland), FKW (Germany), University of Strathclyde (UK) and Trinity College Dublin (Ireland). As part of this we constructed an open cycle pilot plant (Figure 16) to study refrigeration and heat pump applications, and their associated safety and control aspects. In operation the pilot plant far exceeded expectations and was shown to achieve minimum temperatures of -84°C and hot temperatures of between +40 to +70°C. To look at the practical application of air cycle a 0.3 m3 domestic freezer cabinet was connected up to a small air cycle unit salvaged from a decommissioned military aircraft (interestingly in the early days of air cycle research at Langford decommissioned military aircraft were found to be an ideal source of components). In a further project a fluidised bed food freezing system was developed employing air cycle technology (Russell, Gigiel & James, 2000).
Due to the depth of interest in air cycle by the refrigeration and food industry at the time we set up a special interest group that was sponsored by MAFF. Out of this many more air cycle projects were initiated. Other research has looked at using this technology for retail display cabinets, and research is currently continuing at Langford with defra LINK funded project looking at the “Development of integrated, rapid heating and cooling systems for the food industry” using air cycle technology.