Ultrasound processing of sweet lime juice: Effect of matrix pH on microbial inactivation, enzyme stability, and bioactive retention

The effect of pH on microbial, enzyme inactivation, and bioactive retention in ultrasound (US) treated sweet lime (Citrus limetta) juice was studied. The pH of the juice varied at 3.5, 4.0, and 4.5, whereas the US duty cycle (DC) ranged between 75% and 95% (0.07-0.7 W/cm(3)), and the treatment time varied between 15 and 30 min. More than 5-log reduction in Saccharomyces cerevisiae and Escherichia coli was achieved at DC 95% (25 min) while Listeria monocytogenes got inactivated at DC 85% (25 min). 95%-99% inactivation of peroxidase and pectin methylesterase was achieved at DC 95% (30 min), while 60% inactivation in polyphenol oxidase (PPO) was observed. The addition of ascorbic acid (1 g/L) as an EI resulted in the complete inactivation of PPO at DC 85% (25 min). A maximum color change (delta E*) of 2.6 was noticed at the highest US treatment condition (DC 95%, 30 min). Eventually, pH showed a marginal effect (< 5%) on retaining vitamin C and bioactive compounds, but US treatment enhanced them. US treatment at DC 95% (30 min) produced microbially safe (> 5-log cycle reduction) and enzymatically stable (99% enzyme inactivation) juice. To conclude, optimized US treatment increased total phenolic content (TPC), antioxidant capacity, and vitamin C in sweet lime juice by 16.7%, 10.7%, and 14.1%, respectively.Practical applications Consumer demand for superior quality sweet lime juice with minimal processing. This explores the opportunity of applying US technology as one of the nonthermal treatments for such a product. Furthermore, the treated juice should be microbially safe and enzymatically stable with better bioactive preservation. Typically, the pH of the fruit juice is one of the main influencing factors controlling microbiological spoilage. The current study explores the consequence of matrix pH and US conditions on the microbial inactivation, enzyme stability, and bioactive compounds of US-treated sweet lime juice. The hypothesis is that the ultrasonic power required to achieve microbial safety at a lower pH of sweet lime juice should be lower. This will eventually help the industry choose an optimal harvesting condition to obtain the desired pH during processing.