Secondary chilling
Secondary chilling
Friday, 10 April 2009
Meat is chilled immediately after slaughter, ideally to between 0°C and 4°C. Most of the subsequent operations in the cold chain are designed to maintain the temperature of the meat. Cooking is a very common operation in the production of many meat products and operators appreciate the importance of rapidly cooling the cooked product. However, any unit operation such as cutting, mixing, tumbling will add heat to the meat and increase its temperature. A secondary cooling operation is then required to return its temperature to near to 0°C. Over the years, studies have been made at Langford of many aspects of secondary cooling of large and small individuals items, solid/liquid mixtures and sauces (James, 1990).
In many industrial cooking operations, whole hams and large meat joints are often cooked and cooled in an intact form and then supplied to restaurants or retail shops where they are sliced before sale. Langford surveys have shown that, in industry, many methods can be used for cooling whole hams. In these processes, cooling times can be long, up to 21 h, and final temperatures high at 15 to 20˚C.
Subsequent laboratory studies (Burfoot et al., 1990) compared three different cooling methods; forced air at 0˚C, 1.2 ms-1, immersion in water at 0˚C and cooling under vacuum, for large (6.8 to 7.3 kg) hams, and other meats. Although cooling times are much faster under vacuum, weight losses (typically 7.5 to 9.5%) are much higher than those in air (typically 2 to 3%).
A later study for the MLC identified the most efficient methods for cooling cooked meat joints available to retail butchers or caterers (James & James, 2001). Immersion of cooked meat joints in containers of iced water was identified as the most simple and practical method of obtaining the required cooling rates for large meat joints. Using this method, the temperature of joints up to approximately 9.5 kg (uncooked weight) will be reduced to below 12°C in less than 6 h. A final temperature of ≤5°C can be attained in 10 h if the ice water mixture is stirred at regular intervals and especially towards the end of the cooling process. This maintains a water temperature approaching 0°C at the surface of the meat. Without stirring stratification can occur and water over the surface will approach 4°C and cooling times to 5°C be substantially extended. To reduce the average temperature of meat from 70 to 5°C 0.8 kg of ice per kg of meat to be cooled will be required. If the average temperature after cooking is closer to 100°C, 1.2 kg of ice per kg of meat is needed. If smaller joints are to be cooled, other methods such as refrigerated air are acceptable. The dimensions of the joint, rather than its weight, was shown to govern the rate of chilling. For example joints up to 10 cm thick can be cooled to 5°C in less than 10 h in a chill room operating at 0°C. However, if the temperature of the room averages 3°C the time required will increase to 13 h. An initial cooling phase using immersion in, or spraying with, mains water will reduce the amount of ice or other refrigeration required in the cooling process. However, a two-stage process requires more control and double handling.
The importance of achieving a minimum required air velocity around small products was clearly demonstrated by data obtained from cooling pork pies (James, 1990). To guarantee that all the crust remained above -2˚C on the unwrapped 400 g (70 mm high, 95 mm diameter) pies an air temperature of -1.5 ±0.5˚C was used. At this temperature a small increase in air velocity from 0.5 to 1.0 ms-1 reduced the cooling time by 85 m (almost 30%). Even at very high velocities (>6.0 ms-1) appreciable reductions in cooling time were still being achieved. Raising the air velocity from 6 to 10 ms-1 reduced the cooling time by 10 minutes. In a high throughput baking line (>1000 items per h) this 10 minute reduction would produce a 7% increase in throughput thus justifying the higher capital and running costs of larger fans.
In the UK the Department of Health Cook-chill Guidelines (1989) recommend maximum cooling regimes and the use of special equipment to reduce product temperatures rapidly after cooking. Many other countries in Europe have similar guidelines or recommendations for the cooling of cooked products. FRPERC has carried out investigations into air blast cooling of Bolognese meat sauce in metal trays of different depths but the same lateral dimensions (Evans, Russell & James, 1996). These trials showed that, assuming surface freezing was to be avoided and a simple single stage operation used, only 10 mm depth of product could be chilled within the Guidelines.
A computer model was also developed which showed that other foods, such as beef curry and chicken Italian, will have a similar cooling response to those produced for Bolognese sauce. It was therefore concluded that the cooling response of most meat based convenience meal mixtures will be similar to that of Bolognese sauce and a series of cooling charts were produced.
In 1995 the UK Ministry of Agriculture Fisheries and Food (MAFF) funded a five year Advanced Fellowship in Food Process Engineering at Langford (the ‘MAFF Fellowship’), run by Steve James (FRPERC), Professor Joe Quarini (Mechanical Engineering) and Professor Koorosh Khodabandehloo (AMARC). As part of the Fellowship’s investigation of rapid cooling, work was carried out on immersion cooling of cooked meat sauces (James, Ketteringham & Evans, 1998; Ketteringham & James, 1999). This cooling method was shown to have great potential for use in the food industry as it provides a more effective system of temperature reduction than widely used air blast chilling. The rate of heat transfer is much greater when utilising a fluid to remove the thermal energy, than using air. Typical values for surface heat transfer coefficients for air blast chilling of food products are less than 50 Wm-2K-1 compared with values greater than 500 Wm-2K-1 for agitated water (James & James, 1996). Table 10 shows that there is a significant reduction in the cooling time of meat sauce from 70 to 3°C using immersion compared to air blast chilling. However, these preliminary results indicated that even though there was an improvement, it is still not possible to cool 40 mm thick meat sauce (in sealed lidded trays) from 70 to 3°C in less than 2.25 h under these conditions. Therefore, to meet UK guidelines, product thickness still needs to be less than 40 mm when cooled by immersion in water at -1°C.
The work on immersion chilling (Ketteringham & James, 1999) clearly demonstrated the reduction in cooling time that results from improved heat transfer. However, the poor conductivity of the food limits heat flow. High conductivity inserts, such as heat pipes, have been studied at Langford to overcome this problem. These studies showed that using heat pipes chilling of food products and meat joints can be reduced by between 20 and 45% (James, Ketteringham & James, 1998; Ketteringham & James, 2000; James et al., 2005b).