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In order to increase the shelf life, improve availability and reduce the storage and transportation costs, our study aimed at concentrating of two kinds of juices obtained from two raspberry cultivars (modified black and wild red) by two new techniques of vacuum and microwave in three pressure levels 12, 38/5 and 100 kPa. The influence of concentration process and operational parameters (concentrating duration and pressure) on color factors (a*, b* and L*) and antioxidant activity of the final product were assessed. Considering that the samples color is influenced by the presence of anthocyanins, relationship between color and antioxidant activity were determined by a high coefficient of determination (R²= 1). Results suggest color and anthocyanins deteriorate over time. Deterioration rate depends on the kind of concentration process and process pressure, being higher in evaporator-rotary technique and atmospheric pressure. Moreover, anthocyanin half-life times in the pressures 100, 38.5 and 12 kPa were 93.7, 97.7 and 103 min in evaporator-rotary technique and 146.5, 233 and 480 min in microwave technique for modified black raspberry, respectively while being 93.5, 115.5 and 117 min in evaporator-rotary and 118, 480 and 515 min in microwave technique for wild red raspberry, respectively.

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Abstract: Raspberry is a good source in terms of antocianines, polyphenols, vitamin C, fibers, proteins and minerals contents. Raspberry juice is susceptible to enzymatic and microbial reactions, because of its high moisture content (75-90%). Concentration of Raspberry juice increases its shelf life, while decreasing costs of storage and transport. In this study, extracts obtained from two raspberry cultivars of Amol (red) and Siyahkal (black) were concentrated by two methods, radiation (microwave) and convention (evaporator-rotary) applying three pressure levels: 12, 38.5 and 100 kPa. Changes in physicochemical properties including brix, acidity, pH, turbidity and color were assessed during the concentration process of two kinds of juice. Results showed decrease in acidity and color and increase in brix and turbidity of the juices, but rate and amount of changes were dissimilar in different methods and pressures. Samples concentrated by radiation using microwave in comparison with conventional method (evaporator-rotary) had superior quality. Furthermore, decrease in pressure led to better quality in product in both techniques.