Microbial Inactivation

The fluctuating pressures induced by an ultrasonication process produce and break microscopic bubbles, creating micromechanical shocks to disrupt cellular structural and functional components up to the point of cell lysis [82]. Intracellular cavitations make ultrasound capable of inactivating microorganisms [83]. The inactivation effect depends on the control of critical factors including the amplitude of ultrasonic waves, the exposure or contact time with the microorganisms, the type of microorganism, the volume of food to be processed, the composition of the food, and the temperature of treatment [82]. The mechanism of inactivation of vegetative bacteria appears to be intracellular cavitations that lead to cellular lysis, but ultrasound alone has no effect on spores. Cavitations may play an auxiliary role and allow ultrasound to assist other methods in spore inactivation. This limits the singular use of ultrasound as a preservation method, requiring the use of a combination of ultrasound with other preservation processes (e.g., heat and mild pressure) for industrial applications.

Palacios et al. [84] examined the effect of ultrasound on the heat resistance of spores. After ultrasound treatment (20 kHz, 120 W, 12°C, 30 minutes), several substances were detected to be released from B. stearothermophilus spores to the surrounding aqueous medium, including calcium, dipicolinic acid, a glycopeptide of 7kDa, fatty acids, acyl glycerols, and glycolipids (but no phospholipids). The release of low-molecular-weight substances from the spore protoplast and the consequent modification of its hydration state led to the heat resistance reduction.

The presence of spoilage bacteria, yeasts, molds, and the occasional pathogen on fresh produce is not uncommon. Seymour et al. [85] examined the potential of ultrasound in cleaning minimally processed fruits and vegetables. Cut iceberg lettuce (100 g) inoculated with S. Typhimurium (106 CFU/g) was washed for 10 minutes with tap water (control), a 25 ppm free chlorine dip only, ultrasound (32 to 40 kHz, 10 to 15 W/l) only, and ultrasound combined with the 25ppm free chlorine dip. The control reduction was 0.7log10, while reductions of 1.6log10 and 1.7log10 were obtained from washing treatment by ultrasound and chlorine individually. Reductions obtained from the combined washing treatment were 2.6 to 2.7log10, corresponding to a 99.8% reduction in total bacteria. The cleaning action of cavitations appeared to remove cells attached to the surface of fresh produce, rendering the pathogens more susceptible to the sanitizer. The frequency of ultrasound (25, 32 to 40, 62 to 70 kHz) showed no significant effect on decontamination efficiency (P > 0.69).

Yeasts such as S. cerevisiae and Zygosaccharomyces spp., including Z. bailii and Z. rouxii, and pathogenic bacteria like L. monocytogenes can cause significant spoilage and affect the safety of nonpasteurized fruit juice products. Traditional thermal pasteurization methods can detrimentally affect the organoleptic properties when they are used to extend the shelf life for fruit juices. Mincz et al. [86] explored the potential use of ultrasound combined with refrigeration to extend the shelf life of fresh juice products. Freshly squeezed lemon and pineapple juices inoculated with S. cerevisiae, Z. bailii, Z. rouxii, and L. monocytogenes were immediately sonicated at 20 kHz at 45° C and an amplitude of 95 The treated samples were stored in sterilized glass containers (10 ml) at 7°C for 15 days. The microbial population and color of the inoculated samples was monitored at preset intervals during storage. The combined ultrasound and refrigeration treatment significantly suppressed microbial growth in fresh lemon and pineapple juices with improved color retention. No significant change in pH and aw was observed.

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