Songklanakarin J. Sci. Technol. 36 (3), 317-324, May - Jun. 2014 http://www.sjst.psu.ac.th Original Article Effect of indirect ohmic heating on quality of ready-to-eat pineapple packed in plastic pouch. Hoang Pham, Weerachet Jittanit, and Tanaboon Sajjaanantakul* Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, Chatuchak, Bangkok, 10900 Thailand. Received 20 February 2013; Accepted 25 March 2014 Abstract Ready-to-eat fruits packed in sealed containers are highly perishable due to their intrinsic characteristics and lack of full thermal process. Ohmic heating has the advantages of rapid liquid heating through electrical current. The aim of this research was to investigate the effect of indirect ohmic heating on pH, total soluble solids, polyphenol oxidase activity, color and texture of ready-to-eat pineapple packed in a polypropylene pouch with 1% calcium chloride and 0.3% ascorbic acid packing solution. The pre-packed sample in a pouch was placed in the ohmic heating jar filled with 0.5% sodium chloride ohmic heating solution which was then ohmic heated at different voltage gradients (20, 30, 40 V/cm), to different packing solution temperatures (60, 70, 80°C) for 60s. Samples were kept at 4°C for quality measurement. It was found that browning index of ready-to-eat pineapple treated with 20 V/cm at 80°C, 30 V/cm at 70°C and 80°C, 40 V/cm at 80°C did not change during 12 days cold storage (p>0.05). Polyphenol oxidase was inactivated when the temperature of the pineapple was 62°C or higher. After 10 days at 4°C, the pineapple heated with 30 V/cm at 70°C had much higher firmness than the un-heated sample kept at the same storage condition. Indirect ohmic heating of pre-packed ready-to-eat pineapple in polypropylene pouch with 30 V/cm at 70°C packing solution temperature for 60s could be used as minimal heating methods to maintain the quality of ready-to-eat fruits in 12 days at 4°C. Keywords: ohmic heating, ready-to-eat pineapple, quality, minimally process, plastic pouch 1. Introduction ing of the fresh pineapple market is linked to recent develop- ment of fresh-cut and other pineapple products (FAO, 2009). Fresh-cut fruit products are fruits that have been Cut pineapple ranked the highest of all tropical fruit in washed, peeled, sliced, chopped or cut prior to being packed quantity and dollars sold in value-added category for US for consumption (Barry-Ryan and O’beirne, 1998). Minimal fresh fruit in supermarkets (Cook, 2007). Rapid deterioration process can be applied to this fresh-cut or fresh produce to of fresh-cut fruit limits its shelf life. Peeling and cutting create ready-to-eat (RTE) fruit product which has fresh-like exacerbate the problems as metabolic activity increases and quality and appearance with extended shelf life. Modern cellular compartmentation is lost. These bring about enzyme customer demand for RTE products is increasing as a means and substrate interactions causing product browning, soften- of saving time without abandoning a healthy diet. The grow- ing, microbial deterioration, and off-flavor and odor develop- ment (Ahvenainen, 1996). Consequences of mechanical injuries to RTE fruits accelerate enzymatic browning from a * Corresponding author. group of enzymes such as polyphenol oxidases (PPO). PPO Email address: [email protected] have been reported to occur in all plants and exist in particu- . Peer-reviewed paper selected from “Food Innovation Asia larly high amounts in mushroom, banana, apple, lettuce, and Conference 2013” pineapple (Yoruk and Marshall, 2003; Wuyts et al., 2006; 318 H. Pham et al. / Songklanakarin J. Sci. Technol. 36 (3), 317-324, 2014 Gawlik-Dziki et al., 2008, Manzocco et al., 2009). Minimal ness of 95 mm, and cored by 25 mm diameter stainless steel heat treatment can be applied to RTE product to control cylindroid cutter, then cut into 1 cm thickness slices, and enzymatic browning, firmness changes, and decay (Brecht, divided into six 10-g wedges per slice. A portion of 150-g 1993). RTE pineapple wedges was placed in plastic zip lock pouch Ohmic heating is a process in which heat is internally filled with 150 ml of packing solution at ratio 1:1 to weight of generated within food by passing an alternating electric RTE pineapple and volume of packing solution. Polypropy- current through the food (Wang and Sastry, 1993). Most foods lene (PP) pouches (180 x 280 mm), 0.055 mm thickness contain ionized molecules such as salts and acids, hence supplied by Hungchor Supplier, Bangkok, Thailand, were electric current can move through the food and generate the used. The packing solution was 1% calcium chloride (Fisher heat inside (Palaniappan and Sastry, 1991b). The advantage Scientific, Germany) + 0.3% ascorbic acid (Aldrich, Germany) of ohmic heating for particulate foods is a more uniform solution (adapted from Martinez-Ferrer et al., 2002). The heating of both liquid and solid phases as compared to ohmic treated sample was packed with this solution (1% conventional heating. In ohmic heating, heating rate of food calcium chloride + 0.3% ascorbic acid). All procedures were expressed as the temperature increase per unit of time (°C/s) conducted in a sanitary processing room at 18°C. depends on electric current and voltage applied to the food. Electrical conductivity (ó) of food has been pointed out as 2.2 Experimental setup a critical parameter to the understanding of food heating rate under ohmic heating (Palaniappan and Sastry, 1991b). Electri- Indirect ohmic heating is an ohmic heating process cal conductivity of most food materials subjected to ohmic applied to fresh pineapple wedges that were packed in PP heating is a linear function of temperature (Palaniappan and plastic pouch. The RTE fruits were ohmic heated through the Sastry, 1991b). Research on ohmic applications in fruits pouch. A diagram of the indirect ohmic heating setup and and vegetables has been undertaken by several authors data acquisition system is shown in Figure 1. The ohmic (Palaniappan and Sastry, 1991a; Sastry and Palaniappan, heating system consisted of a transducer, a data logger (DX 1992; Castro et al., 2004b; Legrand et al., 2007), but little has 1012, Yokogawa, Japan) and multimeter (Fluke 8088A, Fluke, been done on RTE fruit (Meldrum et al., 2006). Application of USA) connected to a computer for temperature, voltage, and ohmic heating to solid or large fruit particles packed in a electrical current data collection. The system included fuses pouch is not common due to the impairing effect of plastic and an uninterruptible power supply (UPS) to stabilize the pouch on electrical current passing to the pre-packed fruit. power supply. The circuit was linked to ohmic heater However, heating of pre-packed RTE fruit in an ohmic system electrodes. The setup of indirect ohmic heater is shown in can be employed to take the advantage of rapid ohmic heat- Figure 2. The ohmic heater included a glass jar (Norita Glass, ing effect on the heating medium which in turn rapid heats Thailand) 300 mm in height and 200 mm in diameter with the pre-packed sample. Advantages of indirect ohmic heating an acrylic cover attached. The 500 mm diameter curve 316L over conventional heating on microbial growth of RTE stainless steel electrodes were attached to the acrylic cover pineapple packed in plastic pouch, and the energy consump- with 64 mm gap between the 2 electrodes. Ohmic heating tion was demonstrated by Hoang et al. (2013). This indirect medium was a liter of 0.5% sodium chloride solution adjusted ohmic heating of pre-packed RTE fruits offers an alternative to pH 5.5 with 0.2M citric acid. method to minimal process technique. The aim of this study The pre-packed RTE fruit pouch was placed in the was to determine the effect of voltage gradient and tempera- center between the electrodes, with 15 mm distance from ture of packing solution on pH, total soluble solids, PPO each electrode. The ohmic heating medium whose level was activity, color, and texture of RTE pineapple packed in plastic matched to the level of packing solution inside the pouch was pouch treated by indirect ohmic heating and then kept at 4°C. 2. Materials and Methods 2.1 Sample preparation Fresh pineapple, smooth cayenne cultivar, was bought from wholesale fruit market in Rungsit district, Bangkok, Thailand, and stored at 10±1°C overnight prior before pro- cessing. The fruits had been harvested on the previous day with not more than 15% of the eyes were yellow. All fruits were within the range of 13.8±0.3 °Brix and a pH of 3.8±0.1. Cutting boards, knives, working surface were cleaned with 200 ppm sodium hypochlorite solution before use (Montero- Calderon et al., 2008). Fruits were peeled with a pouch thick- Figure 1. Schematic diagram of the ohmic heating unit. H. Pham et al. / Songklanakarin J. Sci. Technol. 36 (3), 317-324, 2014 319 pany Ltd., Japan). pH was directly measured using a pH meter (model 210A, Orion Research, USA). 2.4.2 PPO assay PPO activity was determined according to the methods used by (Rojas-Grau et al., 2008; Montero-Calderón et al., 2009) with some modification. A 50 g of pineapple was mixed with a Mcllvaine buffer solution (1:1) at pH 6.5 containing 1% vinylpolypyrrolidone (Fluka, Sigma Aldrich, Germany). The mixture was blended and homogenized for 30s and centrifuged at 9,100 x g for 30 min at 4°C (Sorvall RC-6 Plus centrifuge, roto SLA 1500, Thermal Scientific, Germany). The supernatant was collected and filtered through Figure 2. Schematic diagrams of indirect ohmic heating system (a), Whatman No.1 paper, and the resulting solution constituted and RTE pineapple packed in plastic pouch (b).