Effect of vacuum and microwave concentrating techniques on Color stability and antioxidant properties of black and red raspberry juice

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.