21 High pressure processing Indrawati, A. Van Loey and M. Hendrickx Katholieke Universiteit, Leuven 21.1 Introduction Food quality, including colour, texture, flavour and nutritional value, is of key importance in the context of food preservation and processing. Colour, texture and flavour refer to consumption quality, purchase and product acceptability whereas the nutritive values (i.e. vitamin content, nutrients, minerals, health related food components)refer to hidden quality aspects. In conventional thermal processing, process optimisation consists of reducing the severity of the thermal process in terms of food quality destruction without compromising food safety. Due to the consumer demand for fresher, healthier and more natural food prod ucts, high pressure technology is considered as a new and alternative unit opera tion in food processing and preservation 21.2 High pressure processing in relation to food quality and safety The effect of high pressure on food microorganisms was reported for the first time by Hite in 1899, by subjecting milk to a pressure of 650MPa and obtaining a reduction in the viable number of microbes. Some years later, the effect of high pressure on the physical properties of food was reported, e.g. egg albumin co- angulation(Bridgman, 1914), solid-liquid phase diagram of water(Bridgman, 1912)and thermophysical properties of liquids under pressure(Bridgman, 1923). A more extensive exploration of high pressure as a new tool in food technology started in the late 1980s(Hayashi, 1989). Recently, extensive research has been conducted and is in progress on
21 High pressure processing Indrawati, A. Van Loey and M. Hendrickx, Katholieke Universiteit, Leuven 21.1 Introduction Food quality, including colour, texture, flavour and nutritional value, is of key importance in the context of food preservation and processing. Colour, texture and flavour refer to consumption quality, purchase and product acceptability whereas the nutritive values (i.e. vitamin content, nutrients, minerals, healthrelated food components) refer to hidden quality aspects. In conventional thermal processing, process optimisation consists of reducing the severity of the thermal process in terms of food quality destruction without compromising food safety. Due to the consumer demand for fresher, healthier and more natural food products, high pressure technology is considered as a new and alternative unit operation in food processing and preservation. 21.2 High pressure processing in relation to food quality and safety The effect of high pressure on food microorganisms was reported for the first time by Hite in 1899, by subjecting milk to a pressure of 650 MPa and obtaining a reduction in the viable number of microbes. Some years later, the effect of high pressure on the physical properties of food was reported, e.g. egg albumin coagulation (Bridgman, 1914), solid–liquid phase diagram of water (Bridgman, 1912) and thermophysical properties of liquids under pressure (Bridgman, 1923). A more extensive exploration of high pressure as a new tool in food technology started in the late 1980s (Hayashi, 1989). Recently, extensive research has been conducted and is in progress on
434 The nutrition handbook for food processors possible applications of high pressure for food preservation purposes or for changing the physical and functional properties of foods. The potentials and limit ations of high pressure processing in food applications have become more clear. A number of key effects of high pressure on food components have been de- monstrated including (1) microorganism inactivation; (ii) modification of bio- polymers including enzyme activation and inactivation, protein denaturation and gel formation;(ii) quality retention (e.g. colour, flavour, nutrition value)and (iv) modification of physicochemical properties of water( Cheftel, 1991; Knorr, 1993). One of the unique characteristics of high pressure is that it directly affects non-covalent bonds(such as hydrogen, ionic, van der Waals and hydrophobic bonds)and very often leaves covalent bonds intact(Hayashi, 1989). As a conse- quence, it offers the possibility of retaining food quality attributes such as vita- mins(Van den Broeck et al, 1998), pigments( Van Loey et al, 1998)and flavour components, while activating microorganisms and food-quality related enzymes, changing the structure of food system and functionality of food pro- teins(Hoover et al, 1989; Knorr 1995: Barbosa- Canovas et al, 1997; Messens et al, 1997; Hendrickx et al, 1998). Furthermore, by taking advantage of the effect on the solid liquid phase transition of water, some potential applications in food processing such as pressure-assisted freezing(pressure shift freezing), pres ssure assisted thawing(pressure shift thawing), non-frozen storage under pressure at subzero temperature and formation of different ice polymorphs can be offered while keeping other food quality properties(Kalichevsky et al, 1995). Beside pressure can also induce increased biochemical reaction rates with effect on bio- conversions and metabolite production(Tauscher, 1995). Based on these effects of high pressure on food systems, several potential applications can be identified such as high pressure pasteurisation of fruit and vegetables products(Parish, 1994: Yen and Lin, 1996), tenderisation of meat products(Elgasim and Kennick, 1980; Ohmori et al, 1991; Cheftel and Culioli, 1997), texturisation of fish pro- teins, applications in the dairy industry(Messens et al, 1997) and high pressure freezing/thawing(Kalichevsky et al, 1995) With regard to food safety, the effect of combined high pressure and tem- perature on microorganisms has been investigated extensively (Sonoike et al 1992: Hashizume et al, 1995; Knorr, 1995: Heinz and knorr 1996: Hauben, 1998; Reyns et al, 2000). The number of vegetative cells can be remarkably reduced by applying pressures up to 400 MPa combined with moderate temperatures up to 40C for 10-30 minutes(Knorr, 1995). On the other hand, exposing the sur- viving fraction of vegetative cells to repeated pressure cycles can also increase their pressure resistance, e.g. Escherichia coli mutants resistant to high pressure inactivation were created (Hauben, 1998; Alpas et al, 1999: Benito et al, 1999) Microbial spores can be inactivated by exposure to high pressure but a pressure treatment at room temperature may not be sufficient for substantial reduction of viable spore counts. Most studies show that pressure can induce spore germina- tion and the extent of spore inactivation can be increased by increasing pressure and temperature(Knorr, 1995; Wuytack, 1999). However, tailing phenomena for germination and inactivation curves can occur for 'super dormant spores after
possible applications of high pressure for food preservation purposes or for changing the physical and functional properties of foods. The potentials and limitations of high pressure processing in food applications have become more clear. A number of key effects of high pressure on food components have been demonstrated including (i) microorganism inactivation; (ii) modification of biopolymers including enzyme activation and inactivation, protein denaturation and gel formation; (iii) quality retention (e.g. colour, flavour, nutrition value) and (iv) modification of physicochemical properties of water (Cheftel, 1991; Knorr, 1993). One of the unique characteristics of high pressure is that it directly affects non-covalent bonds (such as hydrogen, ionic, van der Waals and hydrophobic bonds) and very often leaves covalent bonds intact (Hayashi, 1989). As a consequence, it offers the possibility of retaining food quality attributes such as vitamins (Van den Broeck et al, 1998), pigments (Van Loey et al, 1998) and flavour components, while inactivating microorganisms and food-quality related enzymes, changing the structure of food system and functionality of food proteins (Hoover et al, 1989; Knorr, 1995; Barbosa-Cànovas et al, 1997; Messens et al, 1997; Hendrickx et al, 1998). Furthermore, by taking advantage of the effect on the solid liquid phase transition of water, some potential applications in food processing such as pressure-assisted freezing (pressure shift freezing), pressureassisted thawing (pressure shift thawing), non-frozen storage under pressure at subzero temperature and formation of different ice polymorphs can be offered while keeping other food quality properties (Kalichevsky et al, 1995). Besides, pressure can also induce increased biochemical reaction rates with effect on bioconversions and metabolite production (Tauscher, 1995). Based on these effects of high pressure on food systems, several potential applications can be identified such as high pressure pasteurisation of fruit and vegetables products (Parish, 1994; Yen and Lin, 1996), tenderisation of meat products (Elgasim and Kennick, 1980; Ohmori et al, 1991; Cheftel and Culioli, 1997), texturisation of fish proteins, applications in the dairy industry (Messens et al, 1997) and high pressure freezing/thawing (Kalichevsky et al, 1995). With regard to food safety, the effect of combined high pressure and temperature on microorganisms has been investigated extensively (Sonoike et al, 1992; Hashizume et al, 1995; Knorr, 1995; Heinz and Knorr, 1996; Hauben, 1998; Reyns et al, 2000). The number of vegetative cells can be remarkably reduced by applying pressures up to 400 MPa combined with moderate temperatures up to 40°C for 10–30 minutes (Knorr, 1995). On the other hand, exposing the surviving fraction of vegetative cells to repeated pressure cycles can also increase their pressure resistance, e.g. Escherichia coli mutants resistant to high pressure inactivation were created (Hauben, 1998; Alpas et al, 1999; Benito et al, 1999). Microbial spores can be inactivated by exposure to high pressure but a pressure treatment at room temperature may not be sufficient for substantial reduction of viable spore counts. Most studies show that pressure can induce spore germination and the extent of spore inactivation can be increased by increasing pressure and temperature (Knorr, 1995; Wuytack, 1999). However, tailing phenomena for germination and inactivation curves can occur for ‘super dormant’ spores after 434 The nutrition handbook for food processors
High pressure processing 435 long exposure times. As a consequence, to achieve sterility with minimal impact on nutrition value, flavour, texture and colour, high pressure processing using multiple high pressure pulses and achieving an end temperature above 105oC under pressure for a short time has been proposed(Meyer et al, 2000; Krebbers etal,2001) 21.3 High pressure technology and equipment for the food industry High pressure technology has been used in the industrial production process of ceramics, metals and composites in the last three decennia. As a result, today, high pressure equipment is available for a broad range of process con- ditions, i.e. pressures up to 1000MPa, temperatures up to 2200C, volumes up to several cubic meters and cycling times between a few seconds and several Since high pressure technology offers advantages in retaining food quality attributes, it has recently been the subject of considerable interest in the food industry as a non-thermal unit operation. High pressure equipment with pressure levels up to 800 MPa and temperatures in the range of 5 to 90C(on average)for times up to 30 minutes or longer is currently available to the food industry The actual high pressure treatment is a batch process. In practice, high pres- Ire technology subjects liquid or solid foods, with or without packaging, to pres- sures between 50 and 1000 MPa. According to Pascals principle, high pressure acts instantaneously and uniformly throughout a mass of food and is independent of the size and shape of food products. During compression, a temperature increase or adiabatic heating occurs and its extent is influenced by the rate of pressurisation, the food composition and the( thermo)physical properties of the pressure transfer medium. The temperature in the vessel tends to equilibrate towards the surrounding temperature during the holding period. During pressure release(decompression), a temperature decrease or adiabatic cooling takes place In high pressure processing, heat cannot be transferred as instantaneously and uniformly as pressure so that temperature distribution in the vessel might become crucial. During the high pressure treatment, other process parameters such as treatment time, pressurisation/decompression rate and the number of pulses have to be considered as critical L. Two types of high pressure equipment can be used in food processing: con- entional batch systems and semi-continuous systems. In the conventional batch systems, both liquid and solid pre-packed foods can be processed whereas only pumpable food products such as fruit juice can be treated in semi-continuous systems. Typical equipment for batch high pressure processing consists of a cylin drical steel vessel of high tensile strength, two end closures, a means for restrain ing the end closures(e g a closing yoke to cope with high axial forces, thread pins),(direct or indirect) compression pumps and necessary pressure controls and
long exposure times. As a consequence, to achieve sterility with minimal impact on nutrition value, flavour, texture and colour, high pressure processing using multiple high pressure pulses and achieving an end temperature above 105°C under pressure for a short time has been proposed (Meyer et al, 2000; Krebbers et al, 2001). 21.3 High pressure technology and equipment for the food industry High pressure technology has been used in the industrial production process of ceramics, metals and composites in the last three decennia. As a result, today, high pressure equipment is available for a broad range of process conditions, i.e. pressures up to 1000 MPa, temperatures up to 2200°C, volumes up to several cubic meters and cycling times between a few seconds and several weeks. Since high pressure technology offers advantages in retaining food quality attributes, it has recently been the subject of considerable interest in the food industry as a non-thermal unit operation. High pressure equipment with pressure levels up to 800 MPa and temperatures in the range of 5 to 90°C (on average) for times up to 30 minutes or longer is currently available to the food industry. The actual high pressure treatment is a batch process. In practice, high pressure technology subjects liquid or solid foods, with or without packaging, to pressures between 50 and 1000 MPa. According to Pascal’s principle, high pressure acts instantaneously and uniformly throughout a mass of food and is independent of the size and shape of food products. During compression, a temperature increase or adiabatic heating occurs and its extent is influenced by the rate of pressurisation, the food composition and the (thermo)physical properties of the pressure transfer medium. The temperature in the vessel tends to equilibrate towards the surrounding temperature during the holding period. During pressure release (decompression), a temperature decrease or adiabatic cooling takes place. In high pressure processing, heat cannot be transferred as instantaneously and uniformly as pressure so that temperature distribution in the vessel might become crucial. During the high pressure treatment, other process parameters such as treatment time, pressurisation/decompression rate and the number of pulses have to be considered as critical. Two types of high pressure equipment can be used in food processing: conventional batch systems and semi-continuous systems. In the conventional batch systems, both liquid and solid pre-packed foods can be processed whereas only pumpable food products such as fruit juice can be treated in semi-continuous systems. Typical equipment for batch high pressure processing consists of a cylindrical steel vessel of high tensile strength, two end closures, a means for restraining the end closures (e.g. a closing yoke to cope with high axial forces, threads, pins), (direct or indirect) compression pumps and necessary pressure controls and High pressure processing 435
436 The nutrition handbook for food processo instrumentation. Different types of high pressure vessels can be distinguished, 1.e (i)'monobloc vessel(a forged constructed in one piece);(ii)multi layer vessel consisting of multiple layers where the inner layers are pre-stressed to reach pressure or(iii)wire wound vessel cons sisting of pre-stressed vessels formed by winding a rectangular spring steel wire around the vessel. The use of monobloc vessels is limited to working pressures up to 600 MPa and for high pressure application above 600MPa, pre-stressed vessels are used. The position of high pressure vessels can be vertical, horizontal or tilting depending on the way of processing(Mertens and Deplace, 1993; Zimmerman and Bergman, 1993 Galazka and Ledward, 1995; Mertens, 1995: Knorr, 2001) 21.4 Commercial high pressure treated food products With regard to the large-scale application of high pressure technology in the food industry, a problem still to be solved today is the improvement of the economic feasibility, i.e. the high investment cost mainly associated with the high capital cost for a commercial high pressure system. The cost of a vessel is determined y the required working pressure/temperature and volume. Furthermore, once technically and economically feasible processes have been identified, one needs to evaluate whether the unique properties of the food justify the additional cost and to what extent consumers are willing to pay a higher price for a premium ity produc High pressure technology is unlikely to replace conventional thermal pro- tood g, because the second \echnique is a well-established and relatively cheap food preservation method. Currently, the reported cost range of high pressure processes is 0. 1-0.2S per litre(Grant et al, 2000) whereas the cost for thermal treatment may be as low as 0.02-0.04$ per litre. However, the technology offers commercially feasible alternatives for conventional heating in the case of novel food products with improved functional properties which cannot be attained by conventional heating Today, several commercial high pressure food products are available in Japan, Europe and the United States. A Japanese company, Meidi-Ya, introduced the first commercial pressure treated product(a fruit-based jam) on the market in April 1990, followed in 1991 by a wide variety of pressure-processed fruit yoghurts, fruit jellies, fruit sauces, savoury rice products, dessert and salad dress ings(Mertens and Deplace, 1993). Recently, there were more than 10 pressure treated food products available in Japan. In Europe, fruit juice was the first commercially available high pressure product in France followed by a pressurised delicatessen style ham in Spain and pressurised orange juice in the United Kingdom. In the United States, high pressure treated guacamole has been launched on the commercial market. In addition, pressure treated oysters and hummus are commercially available. A list of commercially available pressurised food products in Japan, Europe and the United States in the last decade is sum- marised in Table 21.1
instrumentation. Different types of high pressure vessels can be distinguished, i.e. (i) ‘monobloc vessel’ (a forged constructed in one piece); (ii) ‘multi layer vessel’ consisting of multiple layers where the inner layers are pre-stressed to reach higher pressure or (iii) ‘wire-wound vessel’ consisting of pre-stressed vessels formed by winding a rectangular spring steel wire around the vessel. The use of monobloc vessels is limited to working pressures up to 600 MPa and for high pressure application above 600 MPa, pre-stressed vessels are used. The position of high pressure vessels can be vertical, horizontal or tilting depending on the way of processing (Mertens and Deplace, 1993; Zimmerman and Bergman, 1993; Galazka and Ledward, 1995; Mertens, 1995; Knorr, 2001). 21.4 Commercial high pressure treated food products With regard to the large-scale application of high pressure technology in the food industry, a problem still to be solved today is the improvement of the economic feasibility, i.e. the high investment cost mainly associated with the high capital cost for a commercial high pressure system. The cost of a vessel is determined by the required working pressure/temperature and volume. Furthermore, once technically and economically feasible processes have been identified, one needs to evaluate whether the unique properties of the food justify the additional cost and to what extent consumers are willing to pay a higher price for a premium quality product. High pressure technology is unlikely to replace conventional thermal processing, because the second technique is a well-established and relatively cheap food preservation method. Currently, the reported cost range of high pressure processes is 0.1–0.2 $ per litre (Grant et al, 2000) whereas the cost for thermal treatment may be as low as 0.02–0.04 $ per litre. However, the technology offers commercially feasible alternatives for conventional heating in the case of novel food products with improved functional properties which cannot be attained by conventional heating. Today, several commercial high pressure food products are available in Japan, Europe and the United States. A Japanese company, Meidi-Ya, introduced the first commercial pressure treated product (a fruit-based jam) on the market in April 1990, followed in 1991 by a wide variety of pressure-processed fruit yoghurts, fruit jellies, fruit sauces, savoury rice products, dessert and salad dressings (Mertens and Deplace, 1993). Recently, there were more than 10 pressure treated food products available in Japan. In Europe, fruit juice was the first commercially available high pressure product in France followed by a pressurised delicatessen style ham in Spain and pressurised orange juice in the United Kingdom. In the United States, high pressure treated guacamole has been launched on the commercial market. In addition, pressure treated oysters and hummus are commercially available. A list of commercially available pressurised food products in Japan, Europe and the United States in the last decade is summarised in Table 21.1. 436 The nutrition handbook for food processors
High pressure processing 437 21.5 Effect of high pressure on vitamins Many authors have reported that the vitamin content of fruit and vegetable prod ucts is not significantly affected by high pressure processing. According to Bignon(1996), a high pressure treatment can maintain vitamins C, A, B,, B2, E and folic acid and the decrease of vitamin C in pressurised orange juice is ligible as compared to flash pasteurised juices during storage at 4C for 40 da Similar findings have been reported for red orange juice; high pressure (200- 500MPa/30C/1 min) did not affect the content of several vitamins(vitamins C, B1, B2, Bs and niacin)(Donsi et al, 1996) 21.5.1 Ascorbic acid he effect of high pressure treatment on ascorbic acid has been more intensively studied than on vitamins such asa.b.d.e and k studies on ascorbic acid stability in various food products after high pressure treatment are available Most authors have reported that the ascorbic acid content is not significantly affected by high pressure treatment. For example, in fruit and vegetables, about 82%o of the ascorbic acid content in fresh green peas can be retained after pres sure treatment at 900 MPa/20C for 5-10 minutes(Quaglia et al, 1996). Almost 95-99%o of the vitamin C content in strawberry and kiwi jam can be preserved by pressurisation between 400 and 600MPa for 10-30min(Kimura, 1992; Kimura et al, 1994). In freshly squeezed citrus juices, high pressures up to 600MPa at 23C for 10 min did not affect the initial(total and dehydro)ascor bic acid concentration(Ogawa et al, 1992). Similar findings are also reported in strawberry ' coulis'(a common sauce in French dessert) and strawberry nectar the vitamin C content was preserved after 400MPa/20C/30min(88.68% of the initial content in fresh sample) and in guava puree, high pressure(400 and 600MPa/15 min) maintained the initial concentration of ascorbic acid (Yen and Lin, 1996). Also, ascorbic acid stability in egg yolk has been investigated showing that high pressure treatment(200, 400, 600MPa) at 20oC for 30 min did not significantly affect the vitamin C content(Sancho et al, 1999). The evolution of the vitamin C content in high pressure treated food products during storage has also been investigated. Most studies show that storage at low temperature can eliminate the vitamin C degradation after high pressure treat- 2-3 months at 5C but a deterioration of vitamin C was noly unchanged for ment. For example, the quality of high pressure treated jam was unchanged for at 25C(Kimura, 1992; Kimura et al, 1994). Another study on strawberry nectar showed that ascorbic acid remained practically the same during high pressure processing(500MPa/room temperature/3 min) but decreased during storage(up to 75%o of the initial concentration after storage for 60 days at 3C)(Rovere et al, 1996). In valencia orange juice, the percentage of ascorbic acid in pressurised juice(500-700MPa/50-60.C/60-90s) was 20-45% higher than in heat treated juice(98 C/10s) during storage at 4 and 8C for 20 weeks(Parish, 1997) Studies on guava puree showed that different high pressure processes have a
21.5 Effect of high pressure on vitamins Many authors have reported that the vitamin content of fruit and vegetable products is not significantly affected by high pressure processing. According to Bignon (1996), a high pressure treatment can maintain vitamins C, A, B1, B2, E and folic acid and the decrease of vitamin C in pressurised orange juice is negligible as compared to flash pasteurised juices during storage at 4°C for 40 days. Similar findings have been reported for red orange juice; high pressure (200– 500 MPa/30°C/1 min) did not affect the content of several vitamins (vitamins C, B1, B2, B6 and niacin) (Donsì et al, 1996). 21.5.1 Ascorbic acid The effect of high pressure treatment on ascorbic acid has been more intensively studied than on vitamins such as A, B, D, E and K. Studies on ascorbic acid stability in various food products after high pressure treatment are available. Most authors have reported that the ascorbic acid content is not significantly affected by high pressure treatment. For example, in fruit and vegetables, about 82% of the ascorbic acid content in fresh green peas can be retained after pressure treatment at 900 MPa/20°C for 5–10 minutes (Quaglia et al, 1996). Almost 95–99% of the vitamin C content in strawberry and kiwi jam can be preserved by pressurisation between 400 and 600 MPa for 10–30 min (Kimura, 1992; Kimura et al, 1994). In freshly squeezed citrus juices, high pressures up to 600 MPa at 23°C for 10 min did not affect the initial (total and dehydro) ascorbic acid concentration (Ogawa et al, 1992). Similar findings are also reported in strawberry ‘coulis’ (a common sauce in French dessert) and strawberry nectar; the vitamin C content was preserved after 400 MPa/20°C/30 min (88.68% of the initial content in fresh sample) and in guava purée, high pressure (400 and 600 MPa/15 min) maintained the initial concentration of ascorbic acid (Yen and Lin, 1996). Also, ascorbic acid stability in egg yolk has been investigated, showing that high pressure treatment (200, 400, 600 MPa) at 20°C for 30 min did not significantly affect the vitamin C content (Sancho et al, 1999). The evolution of the vitamin C content in high pressure treated food products during storage has also been investigated. Most studies show that storage at low temperature can eliminate the vitamin C degradation after high pressure treatment. For example, the quality of high pressure treated jam was unchanged for 2–3 months at 5°C but a deterioration of vitamin C was noticed during storage at 25°C (Kimura, 1992; Kimura et al, 1994). Another study on strawberry nectar showed that ascorbic acid remained practically the same during high pressure processing (500 MPa/room temperature/3 min) but decreased during storage (up to 75% of the initial concentration after storage for 60 days at 3°C) (Rovere et al, 1996). In valencia orange juice, the percentage of ascorbic acid in pressurised juice (500–700 MPa/50–60°C/60–90 s) was 20–45% higher than in heat treated juice (98°C/10 s) during storage at 4 and 8°C for 20 weeks (Parish, 1997). Studies on guava purée showed that different high pressure processes have a High pressure processing 437