Comparison of Microbial Loads and Bioactive Compounds of the Grape Juice Samples Treated by Ultrasonication and Thermal Pasteurization

Pasteurization is one of the most widely used methods in decreasing of microbial loads in fruit juices. Unfortunately, high temperatures destroy anthocyanins and polyphenolic compounds. Nutritional status and quality of fruit juices can be improved using other decontamination methods such as ultrasound process. This study was carried out to investigate effects of ultrasound on contents of bioactive compounds and microbial loads of red-grape juice samples. Results achieved under the optimum conditions of ultrasonication were compared to those under thermal pasteurization.

Effects of three variables of ultrasound time (2, 6 and 10 min), temperature (0, 30 and 60 °C) and power (10, 105 and 200 W) on total phenol content, anthocyanin and total microbial count of the red-grape juices were studied. Design of experiments was carried out using response surface methodology (Box-Behnken design) followed by optimization. Quality of the optimized samples was compared to the quality of controls pasteurized at 90 °C for 30 s. Total phenol content, anthocyanin and total microbial count were assessed using Folin-Ciocalteu assay, spectrophotometry and total plate count method, respectively.

The maximum levels of phenols and anthocyanin compounds and the minimum microbial loads were achieved at 144.34 W for 2 min at 60 °C. Based on the results, contents of total phenol and anthocyanin in samples treated by ultrasound under optimal conditions were significantly higher than those in samples pasteurized by thermal process. No significant differences were seen between total microbial counts of the samples processed by ultrasonication and thermal pasteurization. These results indicated that more bioactive compounds of phenols and anthocyanins could be preserved in grape juices under ultrasonication than those under thermal processes. In addition, a similar safety scheme of the microbial load was achieved by optimizing the conditions of ultrasound treatment.