EFFECTS OF BLANCHING ON GRAPES (VITIS VINIFERA) AND CHANGES DURING STORAGE IN SYRUP

Abstract

ABSTRACT This paper studies the effects of conventional and microwave blanching on grapes that are being processed for storage in syrup. An analysis was made of the blanching effects on macro- and micronutrients, as well as on color and mechanical properties. To test the effectiveness of these blanching treatments, enzymatic residual activity was measured for polyphenol oxidase, peroxidase and pectinmethylesterase. Both blanching treatments reduced enzyme activity in the grapes, especially polyphenol oxidase (99%). Blanching treatments also caused significant decrease in tartaric acid (TA) also as in color and mechanical changes, more marked in conventional-treated samples. As for the effect of storage, microwave treatment supposed a greater stability of TA and total phenols than conventional one, also as a greater antioxidant activity of grapes. From this point of view, microwave treatment of samples immersed in water can be proposed as a good alternative to conventional heating in boiling water for grapes blanching. PRACTICAL APPLICATIONS Blanching is a relatively mild treatment, which aims to inactivate enzymes that would cause a decrease of final product quality. The microwave energy has attracted considerable interest because of the penetration capacity of waves, heating not only the food surface but also the inner part. This speeds up the drying process and contributes to improve the quality of the product. The results of this study showed that microwave blanching of grapes when immersed in water has advantages as compared with conventional treatment. Enzyme inactivation achieved with this kind of energy was very similar in both cases, but microwave treatment supposed a greater stability of tartaric acid and total phenols than conventional one, also as a greater antioxidant activity of grapes.