Vacuum cooling technology for the food processing industry: a review

Journal of Food Engineering 45 (2000) 55±65

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Vacuum cooling technology for the food processing industry: a review Karl McDonald, Da-Wen Sun * FRCFT Group, Department of Agricultural and Food Engineering, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland Received 28 October 1999; accepted 31 January 2000

Abstract Vacuum cooling is a rapid evaporative cooling technique, which can be applied to speci®c foods and in particular vegetables. Increased competitiveness together with greater concerns about product safety and quality has encouraged some food manufacturers to use vacuum cooling technology. The advantages of vacuum cooling include shorter processing times, consequent energy savings, improved product shelf life, quality and safety. However, the cooling technique has a limited range of application. Traditionally, products such as lettuce and mushrooms have been cooled under vacuum. Recent research has highlighted the possible applications of vacuum cooling for cooling meat and bakery products, fruits and vegetables. This paper comprehensively reviews the current state of the technology. It is concluded that while vacuum cooling remains a highly specialised cooling technique, with continuing research its application may make its use in the food and vegetable processing industries more competitive and widespread. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum cooling; Rapid cooling; Chilling; Refrigeration; Foods; Vegetables; Meats; Ready meals

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Applications of vacuum cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Fruit and vegetables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Meat and meat products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Fish and ®sh products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Sauces, soups and particulate foods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Bakery products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Ready meals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 57 58 59 59 59 60

3.

Advantages and disadvantages of vacuum cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.

Factors a€ecting vacuum cooling rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5.

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

1. Introduction Vacuum cooling is a batch process whereby moist products containing free water are cooled by evapora*

Corresponding author. Tel.: +353-1-706-7493; fax: +353-1-4752119. E-mail address: [email protected] (D.-W. Sun).

tion of moisture under vacuum (Mellor, 1980). Products to be cooled are loaded into a vacuum chamber and the system put into operation (Anon, 1994) as shown in Fig. 1. The vacuum cooling process is as follows: As the boiling point changes as a function of saturation pressure as shown in Fig. 2, for a boiling temperature of 0°C, the saturation pressure will be 6.09 mbar. As the pressure in the vacuum chamber is reduced, the energy

0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 4 1 - 8

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K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

Fig. 1. Schematic representation of a laboratory scale vacuum cooler.

required to evaporate the water is furnished by the product itself in the form of latent heat of evaporation. A driving di€erential due to the vapour pressure di€erence between water in the product and the surrounding causes vapour to escape into the surrounding atmosphere so that the sensible heat of the product is reduced (Everington, 1993). Prior to the saturation temperature having been reached, vacuum pumps used to evacuate air from the vacuum chamber were not achieving cooling of the product but were solely evacuating air from the chamber (Anon, 1986). Thus, it is necessary to reduce the pressure in the vacuum chamber to the ¯ash point as quickly as possible for both quality and economical reasons. The pressure in the vacuum chamber is allowed to decrease until the product has reached its desired temperature or the temperature drop ceases at a point where there is no free water available for evaporation (Mellor, 1980; Sun, 1999a). The pumps used to evacuate vacuum chambers are of two main designs, with one employing mechanical rotary pumps and the other using steam jets and baro-

Fig. 2. Saturation pressure of water.

metric condensers (Decker, 1993; DiRisio, 1994). The water vapour given of by a product in a vacuum chamber during evaporation must be removed to prevent saturation of the air, which would prevent further evaporation and ecient cooling. In mechanical vacuum cooling, water vapour is passed over refrigerated coils where it condenses on the cold surface and passes to drain. If the water vapour were not removed, very large pumps would be required to remove the vapour, as each kilogram of water expands to approximately 2000 m3 of vapour at the low pressures used in vacuum cooling (Barger, 1961). In steam jet vacuum coolers, some of the water vapour is lique®ed by using barometric condensers and exhausting to the atmosphere (Decker, 1993; DiRisio, 1994). Vacuum cooling is a highly product speci®c process. It has been traditionally used as a precooling treatment for products such as leafy vegetables (Harvey, 1963; Cheyney, Kasmire & Morris, 1979; Shewfelt, 1986; Anon, 1981a; Turk & Celik, 1993; Tambunan, Morishima & Kawagoe, 1994; Varszegi, 1994; Shewfelt & Phillips, 1996; Sullivan, Davenport & Julian, 1996) and mushrooms (Gormley & MacCanna, 1967; Gormley, 1975a; Noble, 1985; Burton, Frost & Atkey, 1987; Frost, Burton & Atkey, 1989) to remove ®eld heat and thus extend shelf life and quality. Vacuum cooling has been applied to horticultural products in the US for over 50 years (Barger, 1961) with the ®rst commercial vacuum cooling plant built in Salinas, California in 1948 to cool lettuce (Thompson & Rumsey, 1984). The most important characteristics for commercial application of vacuum cooling are that products should have a large surface to mass ratio, be able to lose a percentage of their moisture content and have a structure not excessively damaged by the removal of water thus avoiding loss of quality (Noble, 1985). The removal of water vapour from a product during vacuum cooling results in the loss of heat, approxi-

K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

mately equivalent to the latent heat of vaporisation of water (Longmore, 1971; Anon, 1971). Products with high water contents in excess of 90% such as mushrooms and lettuce would experience a cooling e€ect of approximately 5.5±6°C for every 1% loss in weight due to water removal (Frost et al., 1989; Barger, 1963). It is possible to actually freeze products using vacuum cooling. However, in most cases this is not desirable as freezing can cause cellular and structural damage due to formation of ice crystals. An advantage of vacuum cooling is that it is possible to stop the cooling process at a predetermined pressure and temperature (Anon, 1971). Current applications of vacuum cooling technology are almost exclusively restricted to the horticultural industry and within this to only a limited number of vegetables and fungi. However, research has indicated that vacuum cooling may have wider applications throughout the agriculture and food industry. Current research indicates that vacuum cooling may have potential for use with a wider variety of horticultural produce (Hayakawa, Kawano, Iwamoto & Onodera, 1983; Ishii & Shinbori, 1988; Sun, 1998; Sun, 1999a,b), and in the meat (Burfoot, Self, Hudson, Wilkins & James, 1990; Morgan, Radewonuk & Scullen, 1996; Mc Donald, 1999; Mc Donald & Sun, 1999a,b; Mc Donald, Sun, Desmond & Kenny, 1999, Mc Donald, Sun & Kenny, 2000; Sun, Kenny, Mc Donald & Desmond, 1999; Sun & Wang, 2000) and bakery industries (Khromeenkov, Uteshev & Surashov, 1975; Bradshaw, 1976; Lehrke, 1976; Anon, 1978a,b; Dawson, 1982; Kratochvil & Holas, 1984a,b). Research has also been carried out to extend the application to the ¯oricultural industry (Sun & Brosnan, 1999). Recent trends in the food industry and non-conventional processing techniques have heightened interest in vacuum cooling in applications such as the safety of cook chill meats (Gaze, Shaw & Archer, 1998). This paper presents the current position of vacuum cooling application in the food and vegetable industries and examines the relative advantages and disadvantages of the technique and prospects with respect to further research and understanding of the e€ects on products. 2. Applications of vacuum cooling 2.1. Fruit and vegetables Fresh items such as fruit and vegetables are respiring, transpiring, senescing and dying all at the same time (Shewfelt & Phillips, 1996). It is generally accepted that the quality of fruits and vegetables begins to deteriorate upon harvesting and continues to decline quickly thereafter (Anon, 1981a). Field heat can cause rapid deterioration of some horticultural crops such as lettuce

57

and it is necessary to remove this heat as quickly as possible after harvesting (Gormley, 1975b). The e€ect of temperature on shelf life and decay is dramatic. Reducing temperature from 10°C to 5°C approximately doubles shelf life (Cheyney et al., 1979). Vacuum cooling can rapidly and conveniently reduce temperature due to ®eld heat (Gormley, 1975b). Lettuce stored at ambient temperatures will normally have a shelf life of 3±5 days. However, storage at 1°C with 90% relative humidity can increase shelf life by up to 14 days (Artes & Martinez, 1994, 1996). Using vacuum cooling vegetables such as lettuce can be cooled from about 25°C (®eld temperature) to 1°C in less than 30 min, after which they can be distributed by refrigerated vehicles into cold storage depots and retail outlets (Everington, 1993). Vacuum cooling is the standard commercial procedure used for lettuce in many European countries, as well as in the US. It is applied commonly to iceberg lettuce prior to wrapping in PVC ®lm or after packaging in perforated polypropylene bags (Artes & Martinez, 1994, 1996). The bene®cial e€ect of vacuum cooling and packaging in prolonging shelf life and reducing weight loss of lettuce has been extensively reported (Barger, 1961; Aharoni & Yehosua, 1973; Cheyney et al., 1979; Stanley, 1989; Wang, Hinsch & Kindya, 1984; Artes & Martinez, 1994, 1996). Research into vacuum cooling of other vegetables is limited. However, indications are that the technique has a practical application for a wide variety of fruit vegetables and some grains including strawberries (Gormley, 1975b; Eccher & Borinelli, 1974; Anon, 1981a), blackcurrants (Anon, 1981a), melons (Chambroy & Flanzy, 1980), cabbage (Greidanus, 1971; Shiina, Kawano, Yamashita & Iwamoto, 1994), spinach (Sun, 1998), broccoli (Sun, 1999a,b; Perrin, 1981), rice (Murata, 1974), peppers (Sherman & Allen, 1983; Sun, 1998), turnips (Ishii & Shinbori, 1988), aubergine (Hayakawa et al., 1983), cucumber (Hayakawa et al., 1983), and carrot (Hayakawa et al., 1983). Research has shown that for fruits and vegetables with high water contents such as those mentioned, the resulting weight loss in cooling from an ambient temperature of 25°C to 1°C would be approximately 4%. This weight loss is a signi®cant problem in vacuum cooling of fresh vegetables. However, pre-wetting of produce with water prior to cooling can reduce weight loss (Chen, 1988; Sun, 1999b). In some cases the uptake of water can result in a net weight gain after vacuum cooling (Everington, 1993). Some research has indicated that the time of the pre-wetting interval will a€ect weight losses (Sun, 1999b; Barger, 1961; Sun, 1998). Another main application of vacuum cooling for vegetables is the pre-cooling of mushrooms. Mushrooms have a short shelf life of 3±4 days at ambient temperatures, although this can be signi®cantly increased using cold storage and refrigerated distribution techniques

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(Murr & Morris, 1975). However, it is not always appreciated that the time taken to cool mushrooms is an important factor (Frost et al., 1989). Mushrooms are over 90% water and their porous structure allows water to escape readily (Noble, 1985), which makes them suitable for vacuum cooling. Manufacturers of vacuum coolers claim that vacuum cooling can extend shelf life of mushrooms (Barnard, 1974; Anon, 1977) but experimental work still has to fully quantify this extension (Noble, 1985). Burton et al. (1987) indicated that the advantage of vacuum cooling over conventional cooling is equivalent to approximately 24 h of extended shelf life after 102 h storage. However, this could not be explained entirely by the faster cooling rates (Burton et al., 1987). The technique has been adopted commercially in the US, UK, Ireland and other parts of Europe (Lane, 1972), and has been found to cool mushrooms uniformly within a stack (Barger, 1963). However, research has indicated that while vacuum cooling is a rapid and ecient method of cooling it can have adverse e€ects on mushroom quality. Barnard (1974) found that weight losses increased with increasing mushroom surface area. In these experiments open cup mushrooms su€ered greater weight losses than closed cup mushrooms. It was also pointed out that only high quality mushrooms could be subjected to vacuum cooling as discoloration of damaged or wet mushrooms were accelerated during the process (Barnard, 1974). These ®ndings are signi®cant because they indicate that a proportion of any mushroom harvest will not be suitable for vacuum cooling (Anon, 1986). In addition it has been found that weight loss as water vapour during storage at 5°C is greater in vacuum cooled mushrooms (Frost et al., 1989). Reasons for this are inconclusive, but it is postulated that it may be due to increased mushroom hyphal surface area (Frost et al., 1989). However, scanning electron microscopy examinations have indicated no apparent damage or change in hyphal structure (Frost et al., 1989). Enzymatic browning of mushroom caps caused by the enzyme polyphenol oxidase is a major criterion of mushroom quality (Gormley & MacCanna, 1967). Research has indicated that while there is no signi®cant di€erence in whiteness between vacuum and conventional cooled mushrooms kept at 5°C for 102 h, there is a signi®cant di€erence if mushrooms are stored at 18°C after cooling. Vacuum cooled mushrooms were less brown at the end of the experimental period than those conventionally cooled (Frost et al., 1989). Gormely (1975a) indicated that vacuum cooling of slightly deteriorated mushrooms accelerated browning compared to non-vacuum cooled mushrooms held at 1°C for 8±10 days, in comparison to no evidence to suggest that vacuum cooling increases enzyme activity as reported by Burton et al. (1987). However, pre- and post-packaging of similar type mushrooms will reduce activity of the

polyphenol oxidase and lower the incidence of browning (Gormley, 1975a). 2.2. Meat and meat products The main objective of a cooked meat producer is to produce an economically viable product that is microbiologically safe and consumer acceptable (Mc Donald et al., 1999). However, industrial-cooking operations cannot be relied upon to destroy all pathogenic microorganisms that may be present. If cooling rates following cooking are suciently slow, microbial spores surviving cooking can germinate, grow and form toxins (Mc Donald, 1999; Mc Donald et al., 1999, 2000; James, 1990a; Blankenship, Craven, Le‚er & Custer, 1988; Juneja, Snyder & Marmer, 1997). Therefore, for safety reasons, a minimum temperature-time treatment should be achieved during cooking followed by suciently rapid cooling to minimise growth of any surviving pathogens (Burfoot et al., 1990; Sun & Wang, 2000). As the e€ectiveness of vacuum cooling is dependent on surface to volume ratio its application to large meat products such as hams would appear limited (Mc Donald, 1999). However, some research has indicated the possible use of vacuum cooling to rapidly cool cooked meats and meat products. James (1990b) showed that it was possible to cool large hams (6.8±7.3 kg) from 70°C to 10°C in only 30 min using vacuum cooling in comparison to 624 min for more conventional air blast cooling. Burfoot et al. (1990) also demonstrated the effectiveness of vacuum cooling in cooling large joints of turkey, beef and ham (Mc Donald, 1999). Hofmans and Veerkamp (1976) investigated the e€ect of vacuum cooling on broiler carcasses and found cooling times from 40°C to 10°C to be similar to air blast chilling at 2.5 m/s. The research showed that vacuum cooling was not suitable for commercial use due to the slow cooling e€ect (Veerkamp & Hofmans, 1977). Everington (1993) investigated the e€ect of surface pasteurisation followed by vacuum cooling on o€al and found that the cooling time was 3 min from 95°C to 40°C, after which time cooling became progressively longer. The time taken to reach 10°C was approximately 20 min. However, despite the rapid cooling, mass losses due to water evaporation in vacuum cooled meats can be as high as 10% and this is a major economic loss to producers. Some meat products such as minced, diced or sliced meats can be cooked in trays by steam injection and cooled under vacuum. The accumulation of steam condensate in the trays during cooking will o€set most of the evaporative losses in vacuum cooling. Losses were comparable to those obtained in air blast chilling of 2±3% (Everington, 1993). In addition, loss of water has signi®cant e€ects on meat quality particularly texture. Beef products cooled by vacuum cooling were found to be tougher

K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

59

(Mc Donald et al., 2000). However, Self, Nute, Burfoot & Moncrie€ (1990) indicated that pressure change rate during vacuum cooling did not a€ect cooked chicken breast tenderness. Vacuum cooling has been shown to signi®cantly reduce microbial load (2.0 log10 CFU/g) in cooked beef joints in comparison to other commonly used cooling methods such as air blast cooling (Mc Donald & Sun, 1999a). Burfoot et al. (1990) in contrast found that bacterial condition of large meats was not signi®cantly a€ected by cooling methods. In the production of many processed meats such as ham, the raw meat following brine injection is often tumbled under vacuum to assist in increasing the rate of myosin release from the meat which improves bind and meat quality. However, this tumbling action creates heat, which is detrimental to the eciency of the process, and refrigeration must be provided to keep the product cool during the tumbling. A patent has proposed the use of the vacuum itself to both cool and increase the rate of myosin extraction from the raw meat without the use of external refrigeration. However, it has a high initial cost as well as high running costs (Franklin, Goembel & Hahn, 1990).

the products are normally placed in a jacketed sealed vessel cooked under pressure and then vacuum cooled (Anon, 1981b). Scraper blades are sometimes used in the vessels to ensure that viscous product does not adhere to the vessel walls. Weight loss within these closed systems are more readily controlled than with other vacuum cooling systems by adjusting composition and water content of the sauces (Everington, 1993). Some companies have indicated diculties in cooling meat slurries by conventional methods (James, 1990a). Research has shown that large 1100 kg batches of meat sauce can be cooled from 85°C to 10°C in less than 30 min using vacuum cooling in comparison to air blast cooling which can take in excess of 6 h (James, 1997). Di Risio (1990) reported that it was possible to cook 3785 l of tomato sauce to 93°C in 18 min and cool to 7°C in 14 min using vacuum cooling. The process provides fast, uniform cooling of these products. However, considerable problems are faced in designing systems that can be cleaned easily and operated continuously (James, Burfoot & Bailey, 1987).

2.3. Fish and ®sh products

Cooling under vacuum can accelerate the cooling of bakery products. For example, delicate products such as panetonni (fermented Italian cake) can be cooled in 4 min under vacuum in comparison to 24 h in air. This has led to many Italian producers of this product switching over to vacuum cooling (Everington, 1993). However, due to structural changes resulting from a build up of vapour pressure in product areas of low vapour permeability such as the crust on bread, specialised vacuum cooling techniques are required. Use of a modulated vacuum cooler (MVC) allows rapid cooling of bakery products without adverse e€ects on volume and texture (Bradshaw, 1976). Rather than applying a continuous vacuum to the product, the pressure is modulated during cooling. The MVC system allows the speed at which the vacuum is drawn to be controlled, bringing about a reduction in cooling times and improved quality. Vacuum cooling of bakery products typically takes place in a temperature range 98±30°C which results in a weight loss of about 1% for every 10°C drop in temperature or 6.8% from 98°C to 30°C. However, conventional air blast cooling normally results in a 3±5% weight loss depending on air velocity (Everington, 1993). Therefore, weight loss di€erences between the two methods of cooling are small with weight losses due to vacuum cooling being further compensated by reducing baking times to increase moisture retention in the product (Acker & Ball, 1977). Products will then undergo cooling at a higher weight thus compensating for water loss during vacuum cooling (Bradshaw, 1976). Wheaten breads (2 kg loaves), French bread sticks, meat pies, sausage rolls, pastries and cakes cooled

Vacuum cooling has had limited application in the ®sheries industry. However, industrial processing of tuna has made some use of the cooling technique. Normally, when tuna are caught at sea they are immediately frozen in brine until transported to canning plants on the mainland. Here, they are ®rst thawed and then steam cooked to 65°C in cylindrical vessels. Following this the tuna is cooled to between 35°C and 40°C using vacuum cooling, which typically results in a 3±4% weight loss. Due to the high ®nal cooling temperature (35±40°C), cooling times of greater than 20 min are required in order to prevent damage to the delicate ¯esh of the tuna (Everington, 1993). Other research has indicated the possible application of vacuum cooling at sea to cool small ®sh such as whiting or crustacean such as shrimp with the energy to power equipment coming from waste stack gases (Carver, 1975). Rolfe (1963) demonstrated the use of vacuum cooling to freeze trays of cooked haddock ®llets with tray loading of 12.75 kg/m2 . However, weight losses were as high as 21%. 2.4. Sauces, soups and particulate foods Vacuum cooling of meat sauces, meat slurries or meat-requiring texturisation is a common practice in modern frozen and chilled ready meal production. Use of vacuum cooling provides an opportunity to develop a batch cooling process for liquid systems that eciently and e€ectively reduces the temperature of the mass (Shaevel, 1993). In this type of vacuum cooling system

2.5. Bakery products

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K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

conventionally in 1±3 h can be cooled in times ranging from 30 s to 5 min with MVC (Acker & Ball, 1977; Anon, 1978a). MVC systems are operated either in batch or continuous in-line systems. Choice of a particular system is dependent on production levels. Research on the use of MVC systems has illustrated notable advantages including extension of crust life in breads, improved product shape and moisture distribution with less collapse and contraction, increased shelf life due to absence of mould contamination during cooling, increased productivity due to rapid cooling times, elimination of the 24 h stabilisation period necessary for bread loaves intended for toasted rusk production, improvement of mechanical handling of soft exterior products such as sponge cake due to development of temporary external ®rmness, and reduction in ¯oor space required for cooling (Acker & Ball, 1977; Shipman, 1978; Chriastel, 1978). However, other research has indicated the e€ects of vacuum cooling on the aroma of bread. Vacuum cooling is demonstrated as reducing the aroma substance content of bread, especially more volatile substances as determined by headspace concentration gas chromatography and sensory analysis. However, signi®cant di€erences were not demonstrated between the taste of vacuum and conventionally cooled breads (Kratochvil & Holas, 1984a,b; Kratochvil, 1981). 2.6. Ready meals Vacuum cooling systems are suitable for wide spread integration with the cooking operations of many ready meal manufacturers. In these cases the same unit can be used for both cooking and cooling without the need or delay in physical transfer of product between vessels. Such an operation decreases processing times, increases throughput, maintains high quality of heat sensitive products and provides an ecient method of cooling high particulate sauces and slurries (James et al., 1987). However, the method and extend of vacuum cooling needs careful consideration. Use of high vacuum can draw cooked product into the vacuum pumping plant or splatter sauce onto the roof of the processing vessel (James et al., 1987). In these situations, extensive cleaning operations will be required to remove the product from the equipment and prevent microbial proliferation. Use of a low vacuum will increase cooling times (Carver, 1975). As with large cooked meat products the rate at which the vacuum is applied will a€ect the textural qualities of a sauce or slurry. As vacuum is applied, boiling will occur in the product causing expansion and rupturing within tissues. Development of these systems is constrained by safety concerns in operating over a range of positive and negative pressures (James et al., 1987).

3. Advantages and disadvantages of vacuum cooling Table 1 summarises the advantages and disadvantages of vacuum cooling to di€erent sectors of the food processing industry. Most vacuum cooling apparatus are operated in a batchwise process. That is, foodstu€s are placed in a vacuum chamber, the chamber is evacuated to predetermined level, the food is cooled and then removed. However, this production method is timeconsuming and inecient. In some incidences, it may be necessary to hold products temporarily until they are cooled using the vacuum cooler equipment. In this case the holding time can be variable. For example, some cooked product batches may have to be held at high temperature for longer periods than other batches, which can have negative e€ects on both product safety and quality. However, a recent development aims to make vacuum cooling a more continuous process. Products to be cooled are placed in a plurality of containers designed to hold products for cooling. The containers are inserted successively into a hollow cylinder from one end. The level of vacuum in the containers is then increased through a plurality of through holes formed in the cylinder in an axial direction, and through holes formed in the containers, thereby cooling the food in the containers. The containers are then removed from the cylinder from the other end one after another after being released from evacuation (Hokkaido, 1990). The major advantage of vacuum cooling over other techniques of cooling is the short time required to cool a suitable product to a given temperature (Figs. 3 and 4). Several studies have illustrated just how quick vacuum cooling is in comparison to more conventional forms of cooling such as air blast, water immersion and cold storage shown in Fig. 5, due to the large latent heat removed when moisture evaporates from the product surface (Burfoot et al., 1990; Mc Donald et al., 2000). Unlike other cooling methods such as air blast cooling, the cooling e€ect of vacuum cooling is generated within a product, which allows for uniform cooling of even tightly wrapped products (Barger, 1961; Noble, 1985). However, moisture must be allowed to evaporate from a product to facilitate vacuum cooling, thus adequate venting of containers or packaging is necessary (Barger, 1961). If a product is washed and if there is a need for a drying stage, vacuum cooling may be used not only to cool but also to remove the surplus moisture, which may be present on the surface of the product (Longmore, 1971). In normal circumstances, vegetables are cooled by air blast cooling or in cold storage. However, this requires a large storage surface area if vegetables are to be cooled correctly. In addition, in many cold storage installations vegetables are stacked in crates with a relatively small quantity of product per crate. This will increase cost to the producer. Vacuum cooling can help decrease this

K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

61

Table 1 Advantages and disadvantages of vacuum cooling to di€erent sectors of the food processing industry Applications

Advantages

Disadvantages

Fruit and vegetables

Increased shelf life Rapid cooling times resulting in quicker distributiona

Applicable mostly to large leafy produce Loss of moisture due to cooling technique resulting in product weight lossb High capital investmenta

Low running costs Accurate temperature controla Mushrooms

Normally a batch operationa Increasing moisture losses with increasing mushroom surface area Discoloration of poor quality mushrooms

Increased shelf life by up to 24 h Uniform cooling within a stack No mechanical damage due to coolinga

Meat products

Increased hygiene and product safetya Reduced microbial counts Very rapid cooling resulting in signi®cant ®nancial savings and compliance with current guidelines governing cooling of cooked meats Cooling units are compact and require little maintenancea,d

Sauces and soupse

Bakery products

Allowing development of closed systems where sauces are both cooked and cooled in the same vesselb Weight losses due to cooling more readily controlled than in other vacuum cooling systemsb Ecient cooling system for products such as food slurries which are dicult to coolb

Dicult to operate on a continuous basisa; b

Very rapid cooling of delicate confectionery items

Specialised modulated vacuum cooling (MVC) technology required for satisfactory results Some loss of product aroma due to vacuum cooling

Smaller weight losses than in other vacuum cooled product Use of MVC extending crust life of breads and improving product shape Increased shelf life of many products due to absence of moulds during cooling Increased productivity due to shorter cooling timesa Ready meals

Narrow product range, applicable to those products which can freely lose watera High loss of product yield due to moisture lossc Some loss of product quality due to moisture loss

Suitable for wide spread integration with many ready meal cooking operationsb No delay in physical transfer of product to separate cooling vessels with integrated systemsb Good for cooling heat sensitive foods such as cream based productsb

Sauces splattering on the roofs of processing vessels during vacuum cooling and dicult to cleanb

High vacuum can draw cooked product into the vacuum pumping plantb Low vacuum increasing cooling timesa Concerns about safety in systems which operate over a wide range of positive and negative pressuresb

a

Generally applicable to all food areas. Losses can be controlled by pre-spraying of water onto the produce prior to cooling. c Losses can be controlled or reduced by using MVC and increased brine injection levels in some products. d Units will vary in size depending on the application. e Applicable to both sauces/soups and ready meal products. b

cost (Greidanus, 1971). Furthermore, the type of packaging used to hold a product in cold storage will have an e€ect on the rate of cooling, unlike vacuum cooling where the e€ect is negligible (Longmore, 1973). Research has indicated that in contrast to vacuum cooling, air blast cooling is quicker for non-leafy vegetables and slower for leafy varieties. This is because in air blast cooling, heat is transferred by convection on the outer surface and by conduction from the surface to the centre. For leafy vegetables, the air between the leaves restricts the heat conduction process due to its low thermal conductivity and therefore cooling is slower (Sun, 1998).

The structure and indigenous characteristics of many products can present a number of problems in cooling. Mushrooms, for example, are traditionally dicult to cool properly due to their high moisture content and delicate physical structure. However, vacuum cooling has been shown to cool products such as mushrooms quickly, uniformly and e€ectively providing appropriate vacuum and processing times are applied (Longmore, 1971; Anon, 1971). As vacuum cooling is carried out in a static state, there is no mechanical damage (bruising or attrition of the product) as in some other cooling methods such as ¯uidised bed cooling (Longmore, 1971; Anon, 1971).

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Fig. 3. Temperature distribution of various products during vaccum cooling process.

Fig. 4. Temperature distribution of large (4±5 kg) cooked meat joints during a typical vaccum cooling process.

and subsequent damage (Longmore, 1971). In contrast, vacuum cooling rates can be as high as 0.5°C/min without causing freezing. The quicker cooling time allows for an increased shelf life due to decreased time in cold storage (Longmore, 1973). Precise temperature control is possible with vacuum cooling where product temperature can be brought within 1±3°C of freezing point by simply controlling the absolute pressure (Anon, 1971). The major disadvantage of vacuum cooling is the loss of weight due to moisture removal. Weight loss is an inevitable consequence of the vacuum cooling related to temperature reduction (Barger, 1961). Weight losses depend on a number of factors in particular, available surface area. Increasing surface area will facilitate an increased weight loss during vacuum cooling (Noble, 1985). Procedures are available which add water during cooling to prevent weight loss, but the equipment involved can be expensive and water tolerate packaging is needed (Chen, 1988; Sun, 1998). The vacuum cooling process and subsequent loss of moisture can also have detrimental e€ects on product quality such as texture, colour and sensory properties (Gormley, 1975a; Kratochvil & Holas, 1984a; Mc Donald et al., 2000). The equipment cost in vacuum cooling is high and the process is not applicable to all products with an additional cold store perhaps being required to keep produce cool (Longmore, 1973). However, the cost of vacuum cooling is comparable to the cost of other cooling processes even though the initial capital investment is high. The higher the capacity of a vacuum cooler the lower the initial cost per unit of throughput (Longmore, 1973). The energy consumption of vacuum cooling installations has been found to be much lower than other methods of cooling with high energy use coecients (sensible heat/electrical energy) of 2.65 in comparison to 0.52 and 1.2 for air-blast and hydrocooling (Chen, 1986). 4. Factors a€ecting vacuum cooling rate

Fig. 5. Typical cooling curves for large meat products (4±5 kg) cooled by di€erent methods.

It is recognised in the vegetable industry that precooling and cold storage of products helps to maintain product quality and increase shelf life. However, the practice of pre-cooling in cold storage areas has limitations that can be overcome by using vacuum cooling. Cooling rates in cold storage can be slow depending on the type of product container, and location of product in a crate or pallet. Faster cooling rates could be obtained in cold storage by increasing air velocity and refrigeration. However, this may result in surface freezing of the product, irregular cooling of the inside of the product

As previously mentioned, the high speed at which vacuum cooling cools products is its major advantage over other cooling methods. Aspects of vacuum cooling equipment and the product itself will a€ect the vacuum cooling rate. The capacity of the vacuum pumps used in a vacuum cooler (Fig. 1) will a€ect the time required to reduce the pressure in the vacuum chamber from atmospheric to the ¯ash point of the product (Haas & Gur, 1987). In addition, it has been shown that regulation of pumping speed at speci®c times during cooling will increase or decrease cooling rates (Barger, 1961). The minimum pressure attained in a vacuum chamber will also a€ect the cooling rate of a product. Use of lower pressures in commercial vacuum coolers o€ers a

K. McDonald, D.-W. Sun / Journal of Food Engineering 45 (2000) 55±65

means of reducing ®nal product temperature within a shorter time than at a higher ®nal pressure (Harvey, 1963). Regulation of the condenser temperatures can also be used to increase or decrease cooling rates for products. However, careful control of both the ®nal pressure and the condenser temperature is needed to avoid damage to the product due to surface freezing, for example. The initial temperature of a product will have an insigni®cant e€ect on ®nal temperature when products of di€erent initial temperatures are cooled. However, products with higher initial temperatures such as a cooked meat joint will require longer to cool than lettuce, for example, due to their relative structure, porosity and geometry (Burfoot et al., 1990; Mc Donald et al., 2000). Furthermore, the package of a product will also a€ect the cooling rate in a vacuum cooler since the temperature reduction depends on the freedom of evaporation of moisture from the product (Barger, 1961). Some packaging can retard the movement of moisture from the product. Exposed heads of lettuce, or those wrapped in perforated packaging cool quicker than those packed in cartons or impermeable packaging (Harvey, 1963). Research has indicated the possible application of pre-wetting fruit and vegetables with water prior to vacuum cooling to reduce weight loss and increase cooling rates (Sun, 1998; Sun, 1999b). Pre-wetting is useful in some products such as sweetcorn, celery or iceberg lettuce which do not have a large surface area and do not give up moisture readily (Harvey, 1963; Sun, 1998). In addition, longer pre-wetting times will decrease weight loss in various products such as mushrooms. It has been shown that a pre-wetting time of 5 min with mushrooms will eliminate vacuum cooling weight loss and result in a mass gain of up to 0.6% (Sun, 1999b). Pre-wetting of products such as cooked meats with high initial temperature may also be useful, since less moisture has to be removed from the tissue to obtain the desired ®nal cooling temperature. However, dangers of cross contamination of product with the water used in pre-wetting are of serious concern. Analysis of the above factors which a€ect cooling rate and quality of product cooled by vacuum cooling has led to interest in mathematically modeling the process, so that predictions can be made about its e€ects on particular products (Burfoot, Hayden & Badran, 1989; Fejes, 1994; Houska, Zitny, Sestak, Jeschke & Burfoot, 1994; Houska et al., 1996; Varszegi, 1994). This may lead to better design of vacuum cooling equipment, and a greater understanding of the e€ects of the process on physical, chemical and sensory properties of food products cooled by the technique. However, modeling of the vacuum cooling process is still largely unexplored and requires further research. The Flair Flow initiative in Europe aims to make the prediction of the thermal

63

behavior in foods more accurate by collecting, measuring and validating data on processing techniques including vacuum cooling (Anon, 1995). 5. Conclusions The review has shown that vacuum cooling is an established cooling technique in the cooling of leafy vegetables such as lettuce and may have wider applications in cooling a variety of other foods and vegetable products in many parts of the world. The method has relatively predictable cooling rates and weight losses for particular products and associated temperature drops. The advantage of cooling under vacuum is that cooling times are reduced and temperature can be controlled to preserve or enhance product quality. The bene®ts to be gained in terms of increased shelf life for fruits and vegetables have been con®rmed by research. However, vacuum cooling remains a highly specialised cooling technique applicable only to products with a large surface to volume ratio and to products, which can lose a proportion of moisture without adverse e€ects on quality. It is apparent that vacuum cooling is only worthwhile in situations where it is included as part of an overall cold chain. As research continues on increasing the practical applications of vacuum cooling and improving the quality of vacuum cooled produce, the attractiveness of vacuum cooling in terms of lower capital and running costs than conventional cooling systems will make its use in the food and vegetable processing industries more competitive and widespread. References Acker, R., & Ball, K. M. J. (1977). Modulated vacuum cooling and vacuum treatment of bakery products. Getreide Mehl und Brot, 31, 134±138. Aharoni, N., & Yehosua, B. (1973). Delaying deterioration of romaine lettuce by vacuum cooling and modi®ed atmosphere produced in polyethylene packages. Journal of American Society of Horticultural Science, 98, 464±468. Anon, (1971). Vacuum cooling ± handling dicult products. Canadian Food Industries, 42, 17±18. Anon (1977). Vacuum cooling ± its application advantages and disadvantages. Report No. 4152. Camberley, Surrey, UK: Penwalt Stokes. Anon, (1978a). Bakery products cooled in minutes instead of hours. Modulated vacuum cooling is the key. Food Engineering International, 3, 33±34. Anon, (1978b). Modern methods for manufacture of marzipan, persipan and similar products. Kakao und Zucker, 30, 204±205. Anon, (1981a). Vacuum cooling for fruits and vegetables. Food Processing Industry, 12, 24. Anon, (1981b). Rapid vacuum cooling. Food Processing Industry, 9, 49. Anon (1986). Vacuum cooling. In C. W. Hall, A. W. Farrall, & A. C. Rippen, Encyclopaedia of Food Engineering (p. 817). Westport, Connecticut, USA: AVI Publishing Company.

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Franklin, D. L., Goembel, A. J., & Hahn, D. D. (1990). Vacuum chilling for processed meat. United States Patent Number 4942053. Frost, C. E., Burton, K. S., & Atkey, P. T. (1989). A fresh look at cooling mushrooms. Mushroom Journal, 193, 23±29. Gaze, J. E., Shaw, R., & Archer, J. (1998). Review of microbiological hazards, industry practices, and UK legislation, guidelines and codes of practice. In Identi®cation and Prevention of Hazards Associated with Slow Cooling of Hams and Other Large Cooked Meats and Meat Products Review No. 8 (pp. 1±12). Campden & Chorleywood Food Research Association, Gloucestershire, UK. Gormley, T. R. (1975a). A laboratory vacuum cooler. Irish Journal of Agricultural Research, 14, 211±213. Gormley, T. R. (1975b). Vacuum cooling and mushroom whiteness. Mushroom Journal, 27, 84, 86. Gormley, T. R., & MacCanna, C. (1967). Prepackaging and shelf life of mushrooms. Irish Journal of Agricultural Research, 6, 255±265. Greidanus, P. (1971). Economic aspects of vacuum cooling. Annual Report of Springier Institute, Wageningen, Netherlands. Project No. 644, 47±50. Haas, E., & Gur, G. (1987). Factors a€ecting the cooling rate of lettuce in vacuum cooling installations. International Journal of Refrigeration, 10, 82±86. Harvey, J. M. (1963). Improving techniques for vacuum cooling vegetables. ASHRAE Journal, 5, 41±44. Hayakawa, A., Kawano, S., Iwamoto, M., & Onodera, T. (1983). Vacuum cooling characteristics of fruit vegetables and root vegetables. Report of the National Food Research Institute (Shokuryo Kenkyusho Kenkyu Hokku). No. 43, 109±115. Hofmans, G. J. P., & Veerkamp, C. H. (1976). Vacuum cooling of broiler carcasses. In B. Erdtsieck, Proceedings of the Second European Symposium on Poultry Meat Quality. European Federation Branch of the World Poultry Science Association. Hokkaido, S. I. (1990). Vacuum cooling method and apparatus. United States Patent Number 5088293. Houska, M., Podloucky, S., Zitny, R., Gree, R., Sestak, J., Dostal, M., & Burfoot, D. (1996). Mathematical model of the vacuum cooling of liquids. Journal of Food Engineering, 29, 339±348. Houska, M., Zitny, R., Sestak, J., Jeschke, J., & Burfoot, D. (1994). Vacuum cooling process modelling. Potravinarske Vedy, 12, 1±15. Ishii, K., & Shinbori, F. (1988). E€ects of outer leaf trimming, precooling methods and delay in precooling on changes in quality of turnips. Journal of Japanese Society of Horticultural Science, 57, 544±548. James, S. J. (1990a). The cooling of cooked meat products. In Proceedings of the Future Meat Manufacturing Processes (pp. 1±9). London, UK: Institute of Mechanical Engineering. James, S. J. (1990b). Cooling systems for ready meals and cooked products. In R. W. Field, & J. A. Howell, Process Engineering in the Food Industry 2: Convenience Foods and Quality Assurance (pp.88±97). London: Elsevier. James, S. J. (1997). Secondary chilling of meat and meat products. In Meat Refrigeration ± Why and How? (pp. 1±4). UK: University of Bristol. James, S. J., Burfoot, D., & Bailey, C. (1987). The engineering aspects of ready meal production. In R. W. Field, & J. A. Howell, Process Engineering in the Food Industry: Developments and Opportunities (pp. 43±58). London: Elsevier. Juneja, V. K., Snyder, O. P., & Marmer, B. S. (1997). Potential for growth from spores of Bacillus cereus and Clostridium botulinum and vegetative cells of Staphylococcus aureus, Listeria monocytogens, and Salmonella serotypes in cooked ground beef during cooling. Journal of Food Protection, 4, 272±275. Khromeenkov, V. M., Uteshev, A. R., & Surashov, A. A. (1975). Vacuum cooling during biscuit production. Izvestiya Vysshikh Uchebnykh Zavedenii, Pishchevaya Tekhnologiya, 6, 144±145.

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