Ultrasound and malic acid treatment of sweet lemon juice: Microbial inactivation and quality changes

Abstract

The objective of this study was to compare the impact of ultrasound (US; 30 kHz; 2, 4, 6, and 8 min) and malic acid (MA; 0.5 and 1%), individually and in combination, on inactivation of foodborne pathogens (Salmonella Typhi, Shigella dysenteriae, Listeria monocytogenes, and Shigella flexneri) and the effect of US treatment on β-carotene and total phenolics content in sweet lemon juice. The extent of cell membrane damage and inactivation of intracellular enzymes were investigated as a bactericidal mechanism of the treatments. Results indicated that reduction levels of the four pathogenic bacteria enhanced with the increase of MA concentration from 0.5% to 1.0%. The effectiveness of combined treatment of US and MA on microbial inactivation was significantly higher than that of each treatment alone. The highest bactericidal effect was observed in the combination of US treatment and 1% MA, which reduced the initial bacterial value (7.2–7.6 log CFU/ml) to 5, 4.6, 3.5, and 3.8 log CFU/ml for S. Typhi, S. dysenteriae, L. monocytogenes, and S. flexneri, respectively. The greatest damage to the cell membrane and inactivation of enzyme was observed in the combined treatment, which was related to their synergistic bactericidal impacts. Total phenolic compounds and β-carotene did not undergo any change in samples treated by US. The results provide evidence that US treatment in combination with MA can be used as a potential method to enhance the microbiological safety of sweet lemon juice. Practical applications Numerous diseases prevalence has been recorded in many countries related to fresh fruit juices. Although thermal processing is recommended as an appropriate technique for killing microorganisms and increasing the shelf life of fruit juices, this traditional processing leads to the loss of nutrients and quality of juices. Consequently, due to the tendency of consumers to healthy and nutritious foods, the use of non-thermal methods including US has been considered for processing of food products.