ABSTRACT
Ultrasound has found wide application as an advanced technology in processing technologies, including food technology. It is used both in measurements and diagnostic tests, and, as a processing aid, with the other technologies in food processing. The objective of this study was to determine the effects of combined caustic soda and ultrasound on reducing the drying time of grapes in raisin production and the possibility of substituting the ultrasound technology for conventional methods. Two samples of Thompson seedless grapes, one intact and one pre-treated with caustic soda [0.5% W/V, at 80°C for 30 sec.], were sonicated in an ultrasonic bath [28 KHz frequency and 150 W intensity] for 0, 10, 20, 40 and 60 minutes and immediately dried by conventional sun-drying for 7 days. The moisture content, weight, and water activity of the samples were determined daily. The ultrasound treatment of short durations alone had no affect. The treatment was statistically significantly effective only when the exposure time was 40 or 60 minutes or longer. Further analysis of the data showed that the combined treatment of ultrasound with caustic soda significantly reduced the length of time required for dehydration of grapes and raisin production. The findings indicate that the ultrasonication technology can be used economically, reducing the length of time for drying grapes and, thus, increasing productivity in the process of raisin production. However, more studies are needed to determine whether ultrasound alone can be substituted for caustic soda technology in raisin production