Study of lipoxygenase and peroxidase as indicator enzymes in green beans: change of enzyme activity, ascorbic acid and chlorophylls during frozen storage K. Savas ß Bahc ßeci, Arda Serpen, Vural G€ okmen * , Jale Acar Department of Food Engineering, Hacettepe University, 06532 Beytepe, Ankara, Turkey Received 24 October 2003; accepted 1 March 2004 Abstract Effects of two blanching conditions using peroxidase (POD) and lipoxygenase (LOX) as indicator enzyme on residual enzyme activities, ascorbic acid and chlorophyll content in green beans during frozen storage were studied. No reactivation of both LOX and POD enzyme was observed during storage. The losses of ascorbic acid and chlorophylls during storage followed first order kinetics. Half-life of ascorbic acid in unblanched green beans was determined to be 1.89 months. It increased to 2.15 and 3.48 months by blanching at 70 °C for 2 min (for >90% LOX inactivation) and 90 °C for 3 min (for >90% POD inactivation), respectively. Half-lifes of chlorophyll a (Chl a) and chlorophyll b (Chl b) were determined to be 7.32 and 13.11 months in un- blanched green beans. Blanching green beans at 70 °C for 2 min decreased the half-lifes of Chl a and Chl b to 5.05 and 10.09 months while blanching at 90 °C for 3 min increased to 8.26 and 16.70 months, respectively. The results clearly showed that a blanching treatment to inactivate POD retains the quality attributes of green bean better during frozen storage. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Peroxidase; Lipoxygenase; Blanching indicator; Frozen storage; Ascorbic acid; Chlorophylls 1. Introduction Freezing is used to maintain product quality over long storage and results in a slower rate of most dete- riorative reactions such as senescence, enzymatic decay, chemical decay and microbial growth. However, freez- ing does not prevent off-flavor development, color and texture deterioration in frozen vegetables because en- zyme systems remain active even at sub-zero tempera- tures (Rodriguez-Saona, Barrett, & Selivonchick, 1995). In order to prevent enzymatic reactions during pro- cessing, most vegetables must first be blanched. Blanching can be carried out by different methods, but water blanching is most widely used techniques for this purpose. Blanching is a thermal process designed to inactivate the enzymes responsible for generating the off-flavors and off-odors. Apart from enzyme inactivation, blanching of vegetables prior to freezing has several advantages, but also a number of disadvantages. The advantages include stabilization of texture, flavor and nutritional quality, destruction of microorganisms and wilting of leafy vegetables, which assists in packaging (Cano, 1996). However, since blanching is a heat treat- ment, changes associated with mild thermal processing can be expected. These include loss of turgor in cells, due to thermal destruction of membrane integrity, and par- tial degradation of cell wall polymers. A further effect of thermal processing is the degradation of chromophores such as chlorophyll, resulting in color change. Pigment degradation will continue to take place through frozen storage. The extent of the color change and the rate of the reactions could be affected by the severity of heat treatment. For example, the greater the extent of con- version of chlorophyll in the initial product, the more rapid will be deterioration during the frozen storage. Blanching can lead to thermally induced degradation of nutrients such as ascorbic acid. It can also lead to leaching of nutrients. Hence, a determination must be made as to whether or not the unblanched product has adequate stability to retain structural integrity and quality attributes upon freezing and during frozen storage, and a decision must be reached as to the extent Journal of Food Engineering 66 (2005) 187–192 www.elsevier.com/locate/jfoodeng * Corresponding author. Fax: +90-312-2992-123. E-mail address: vgokmen@hacettepe.edu.tr (V. G€ okmen). 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.03.004