Ultrasound pretreatment of fermented milk containing probiotic Lactobacillus plantarum AF1: Carbohydrate metabolism and antioxidant activity

Abstract The effect of ultrasound (100 W, 30 kHz, 25% amplitude for 5, 10, and 15 min; US) on growth, carbohydrate metabolism, and antioxidant activity of probiotic Lactobacillus plantarum AF1 was investigated during milk fermentation. The antioxidant activity of L. plantarum AF1 was measured by 2,2′‐azinobis‐(3‐ethylbenzothiazoline‐6‐sulfonate) (ABTS), 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH), inhibition of the autoxidation of ascorbate, reducing ability, and superoxide anion radical scavenging activity tests. The results indicated that the cell membrane permeability of sonicated samples was increased 88–94% compared to control. Additionally, β‐galactosidase activity and population of Lactobacillus strain increased with increasing ultrasound treatment time during the fermentation after the treatment. However, no significant differences were observed in the final counts of the control and the sonicated samples at the end of fermentation (24 hr). Although the lactose content was decreased significantly ( p ≤ .05) by extending sonication time, the amounts of glucose, galactose, and lactic acid enhanced significantly ( p ≤ .05) as the time of fermentation and sonication proceeded. The antioxidant activity was also enhanced significantly ( p ≤ .05) with increase in sonication time during 24 hr fermentation after the treatment. Therefore, the lowest and the highest antioxidant activities were seen in the control sample and the US‐15 min sonicated milk at the end of fermentation, respectively. Practical applications The dairy industry is seeking methods to increase nutritional and quality of dairy products. Recently, the ultrasound has attracted significant attention in food processing generally because of the recent consumers’ trend toward functional foods. The present study provides an investigation about the efficiency of ultrasound to improve the metabolism and growth of Lactobacillus plantarum during fermentation of milk and therefore increase the quality and antioxidant activity of milk. The results show that ultrasound is a potential technique for improve metabolism of Lactobacillus strain and thus increase antioxidant activity and quality of fermented milk. Consequently, the use of ultrasound will increase the health promoting effects and quality of fermented dairy products. These results will assist in designing ultrasound procedures for optimizing the general quality of fermented milk.