Vacuum cooling is based on the rapid evaporation of moisture from the surface and within the products due to the low surrounding pressure. Water evaporation absorbs heat from the products. Water evaporation directly depends on the surrounding vapor pressure and causes the temperature decrease. When any free water containing product is placed in a closed chamber and the pressure is decreased, the vapor moves from the product to the surrounding atmosphere. For maintaining steady cooling process, it is necessary to evacuate the chamber continuously. Desired final temperature of the product can be controlled adjusting the final surrounding pressure.
Process of a vacuum cooling can be given as follows: Vacuum chamber is used to keep the food products. After placing the food into the vacuum chamber, the door is closed and the vacuum pump is switched on. When the pressure is reduced and water starts to evaporate, the food temperature begins to decrease. Cooling of the food continues until it reaches the desired product temperature. When the predetermined temperature is achieved, the pump is stopped, the ventilation valve is opened and atmospheric air is allowed into the chamber. After the process is finished, finally, the products are removed from the chamber.
Vacuum cooling is an established technique for rapid cooling processing which has been proven to be one of the most efficient cooling methods available (Brosnan and Sun, 2001, Sun and Wang, 2000, Wang and Sun, 2002a; McDonald and Sun, 2002). Vacuum cooling mainly depends on latent heat of evaporation to remove the sensible heat of cooled products. It can be considered a rapid and evaporative cooling method. Generally, vacuum cooling can be applied to any porous product which has free water (Wang and Sun, 2002b, Houska et al., 1996; McDonald and Sun, 2000; Dostál and Petera, 2003, Wang and Sun, 2001). It is one of the most effective methods to cool fresh fruits, vegetables, cut flowers, meat production, fish and sauces. The main components of a typical vacuum cooler are vacuum chamber, vacuum pump and vapor condenser. The function of the vacuum pumps and vapor condenser is to provide the vacuum in the chamber. The vacuum chamber is used to house the products to be cooled with vacuum cooling. When vacuum is established in the vacuum chamber, the pressure inside the chamber is reduced to the saturation pressure corresponding to the initial temperature of the product and some water evaporates away from the food until new equilibrium condition is achieved. During vacuum cooling, the large amount of vapor generated in the chamber is removed by the vacuum pump and/or through condensation when a vapor condenser is installed inside the chamber. Any food product with free water and whose structure will not be damaged by water removal from the product can be vacuum cooled (Wang and Sun, 2001). During evaporation, heat has been removed from the products through evaporation. To maintain the water evaporation, an ambient pressure (chamber pressure) reduction must be ensured until reaching the pressure that allows the attainment of a desired temperature. The latent heat required for the evaporation is supplied by product itself' reducing the sensible heat of the product and causing it to cool.
Fresh fruits, vegetables and ornamental crops are living tissues and can begin to deteriorate after harvesting. In order to keep the quality and extend shelf life, they need to be cooled immediately after harvest. This process is called as pre-cooling (Brosnan and Sun, 2001), which removes the field heat immediately after harvest (Nonnecke, 1989). Pre-cooling is a critical part of the proper temperature management process (Nowak and Rudnicki, 1990, Turk and Celik, 1993). When commodities are pre-cooled promptly after harvest, shelf life is extended, appearance is improved, pre-harvest freshness and flavor are maintained and deterioration is reduced. Vacuum cooling is one of the most effective rapid cooling methods for providing all of these.
There are two main requirements for using the vacuum cooling: (a) the product should have a large surface area for mass transfer, (b) product water loss should not represent an economic or sensory problem due to weight reduction and possible changes in structure or appearance (McDonald and Sun, 2000).
Any porous structured product can be used for vacuum cooling because the water vapor generated within the sample easily diffuses to the surrounding atmosphere. During vacuum cooling, heat and moisture transfer is a complicated process, which has been investigated by many researchers. Vacuum cooling is extensively used for cooling some agricultural and food products (Thompson and Rumsey, 1984, Varszegi, 1994, Sun and Wang, 2000; McDonald and Sun, 2001a). It has been widely applied in pre-cooling treatment for lettuce (Turk and Celik, 1993, Tambuna et al., 1994, Shewfelt and Phillips, 1996, Sullivan et al., 1996, Sun and Brosnan, 1999; Haas and Gur, 1987), mushrooms (Frost et al., 1989, Sun and Brosnan, 1999, Brosnan and Sun, 2001, Burton et al., 1987), meat production (McDonald and Sun, 2001a; McDonald and Sun, 2001b, McDonald et al., 2000, McDonald et al., 2002b), fish (Everington, 1993) and sauces (Thompson and Rumsey, 1984, McDonald and Sun, 2001b, Shaevel, 1993, Sun and Hu, 2003, Sun et al., 2003).
Martinez and Artes (1999) applied vacuum cooling for packed iceberg lettuce and monitored weight loss, wilting, physiological disorders, bacterial decay and quality. They found that levels of weight loss of unpacked lettuces and of heads packaged in perforated polypropylene film was similar to those found after 2 weeks of storage. The effects of different pressure reduction rates on the lettuce quality are investigated to determine if vacuum coolers with slower cooling rates were feasible (Rennie et al., 2001). Tao et al. (2006) carried out the study to investigate vacuum cooling and experiments on the effects of different storage conditions on weight loss, the respiration rate, soluble solid content, membrane permeability and degree of mushroom browning.
He et al. (2004) carried out a study to determine if the pressure reduction rate in a vacuum cooler would have an effect on the physical and chemical quality characteristics as well as the ultrastructure of iceberg lettuce after cooling and storage. Three different pressure reduction rates were taken to cool iceberg lettuce in a vacuum cooler.
Jackman et al. (2007) carried out experimental studies for the combined cooling methods of vacuum cooling and air blast cooling to compare the suitability in minimizing both the cooling loss and the cooling time for the large cooked beef. They showed that air blast cooling to an intermediate temperature and then vacuum cooling to the final temperature was more effective at minimizing the cooling loss compared to vacuum cooling first and then air blast cooling and both methods were similar in optimizing the cooling time.
Zhang and Sun (2006) experimentally investigated four different cooling methods, including vacuum, blast, cold room, and plate cooling for cooked broccoli and carrot slices. The cooling efficiencies of the four methods and the qualities of both vegetables after cooling were compared. They showed that vacuum cooling was the most efficient method among the four cooling methods. Their results show that the weight loss during vacuum cooling can be considerably reduced by spraying water onto the vegetables. The quality analysis revealed that vacuum cooling did not impose any negative effect on the vegetable qualities, in comparison with the other three cooling methods.
Cheng and Sun (2008) compared the mass loss of cooked meat product for the different cooling methods such as vacuum cooling, air blast cooling, slow air cooling, and water immersion cooling. They also compared the cooling rate, the weight loss, and the quality of large cooked ham joints. They indicated that despite the highest cooling loss, vacuum cooling significantly increases the cooling rate, and it is the only method that can meet the chilling requirements.
Sun and Zheng (2006) discussed the principles and equipment of vacuum cooling and analyzed the advantages and disadvantages of this technique. They presented an overview of the development of the vacuum cooling technology for the agri-food industry.
The aim of this study is to determine the effect of the pressure on the vacuum cooling of iceberg lettuce and comparison of the result with conventional cooling.
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