Vol. 67, Nr. 5, 2002—JOURNAL OF FOOD SCIENCE 1827 © 2002 Institute of Food Technologists Food Engineering and Physical Properties JFS: Food Engineering and Physical Properties Effects of a Combined Process of High-Pressure Carbon Dioxide and High Hydrostatic Pressure on the Quality of Carrot Juice S.-J. PARK, J.-I. LEE, AND J. PARK ABSTRACT: A combined treatment of high-pressure carbon dioxide (HPCD) and high hydrostatic pressure (HHP) was investigated as a non-thermal processing technique to enhance the safety and shelf life of carrot juice. Aerobes were completely inactivated by a combined treatment of 4.90 MPa-HPCD and 300 MPa-HHP. A com- bined treatment of 4.90 MPa-HPCD and 600 MPa-HHP effectively inactivated enzymes. The residual activities of polyphenoloxidase, lipoxygenase, and pectinmethylesterase were less than 11.3%, 8.8%, and 35.1%, respec- tively. Cloud and color were considerably affected by HPCD, but not by HHP. Enzyme activities and the total color difference showed a strong correlation with pH, which was dependent on the pressure of carbon dioxide. Keywords: high-pressure carbon dioxide , high hydrostatic pressure, carrot juice, enzyme inactivation, cloud Introduction T RADITIONALLY, HEAT PROCESSES HAVE BEEN USED TO ENSURE THE safety of food against many pathogenic microorganisms. Thermal energy, however, inevitably leads to destruction of heat- sensitive nutrients, texture, color, and flavor (Balny and others 1992). Non-thermal processes, such as high hydrostatic pressure (HHP), are being used as alternatives for food preservation. High-pressure is used commercially in the United States for the production of guacamole, tomato-based salsas, and fresh oys- ters, and for other products in Europe and Japan (Neil 1999). To secure safety against some pressure-resistant microorganisms, however, HHP should be used in combination with another pro- cess. High-pressure carbon dioxide (HPCD) is a candidate for such a combination because of its ability to inactivate microbes and its ease of use. Carbon dioxide has been used in modified atmosphere pack- aging (MAP) to improve the shelf life of food by inhibiting bacte- rial growth. Fraser (1951) first tried gas pressurization with N 2 , NO 2 , Ar, and CO 2, reporting that CO 2 could inactivate 95% to 99% of Escherichia coli at 37 °C and 3.40 MPa. The principle of HPCD treatment is based on gas dissolution in a cell by pressurization that, when rapidly decompressed to atmospheric pressure, caus- es fatal functional damage to the cell (Balaban and others 1991). Carrot juice is a natural source of â-carotene (Senti and Rizek 1975). Since canned carrot juice is a low-acid food of approxi- mately pH 6.0, it has a higher risk of bacterial contamination than other acidic foods (orange juice). Hence, it requires severe heat treatment (115 to 121 °C) for protection from spoilage (Des- rosier 1976; Kim and Gerber 1988). However, high temperatures (especially retorting temperatures) can destroy carotenes (Khan and others 1975; Kim and Gerber 1988). Hong and others (1999) reported that microbial inactivation by HPCD is governed essen- tially by penetration of carbon dioxide into cells, the effective- ness of which can be improved by increasing pressure and de- creasing the pH of the suspension. Because dissolved carbon dioxide can lower the pH of carrot juice, HHP and HPCD can be combined for a synergistic effect. We, hence, have demonstrated the effectiveness of HPCD as a combined process with HHP in processing carrot juice. The effects of the combined treatment on aerobes, the activities of food quality-related enzymes, and the physical properties of carrot juice were examined and changes in the quality parameters were observed for a 4-wk storage period at 4 °C. Materials and Methods Preparation of carrot juice Carrots purchased in a local market were washed with tap wa- ter and peeled. Carrot juice was obtained with a juice extractor (JM-511, Samsung Co., Korea), producing approximately 50% of the weight of the fresh carrots as juice. The juice was filtered through filter paper (No. 4, Whatman TM International Ltd., U.K.) and then used as a sample. Measurement of concentration of dissolved CO 2 The concentration of dissolved CO 2 was measured using a method described by Ishikawa and others (1995). Ten mL of pressurized distilled-water was spread to an airtight container containing 20 mL of 1 M NaOH. The concentration of dissolved CO 2 was calculated from an amount of acid that was needed to neutralize to pH 7.0 with 0.5 M HCl . HPCD treatment Figure 1 shows the HPCD treatment equipment set-up (Kodam Engineering Inc., Seoul, South Korea) using stainless steel tubes, a pressure gauge, and a thermometer connected to a thermocouple (HI 93530, Hanna Instruments (Singapore) Pte Ltd., Singapore). The vessel (inside volume of 500 mL) and the screw type cover were made of stainless steel. To prevent direct contact of the sample with the inside wall of the vessel, a Pyrex ® glass vessel was located inside the pressure vessel. A magnetic stirrer was used to mix carbon dioxide gas thoroughly with the sample (100 mL). The inner temperature of the processing cham- ber was maintained at approximately 5 °C by placing the pres- sure vessel in an ice jacket. It is well known that the solubility of CO 2 gas into water phase increases with temperature decrease