Enhancing microbiological safety of fruit purées through synergistic heat treatment and ultrasonication

Abstract

The microbiological quality of fruit purée is determined by both microbiological purity and production hygiene. The risk of microbiological contamination is associated with the potential growth of various groups of microorganisms, even under refrigerated storage conditions. The predominant microflora contaminating fruit include acid-tolerant bacteria, molds, and yeasts. Yeasts metabolize carbohydrates, producing significant amounts of carbon dioxide and, under anaerobic conditions, ethanol, making them one of the primary spoilage agents of plant-based products. Raspberry (Rubus idaeus), strawberry (Fragaria xanassa), and aronia (Aronia melanocarpa) purée with a pH of ≤ 4.2 are susceptible to spoilage by molds, yeasts, and certain non-spore-forming bacteria, such as lactic acid bacteria. These microorganisms resist heat treatment at temperatures ranging from 80 to 100 °C. Applying a combined processing method - heat treatment and ultrasonication has been investigated for fruit purées. The microbiological analyses of products subjected to heat treatment in combination with ultrasonication demonstrated a reduction in the total microbial count, including yeasts and molds. The degree of inactivation of microorganisms was directly related to the temperature and duration of treatment, with conditions of 60-70 °C for 5-15 min being optimal. Ultrasonic treatment operates on the principle of cavitation, which involves the formation and subsequent collapse of microbubbles in a liquid medium. The collapse of these bubbles releases energy in the form of localized pulses of high pressure and temperature, resulting in mechanical disruption of microbial cells. The sudden pressure fluctuations and acoustic waves compromise the integrity of microbial cell membranes, facilitating their destruction. Furthermore, the combination of ultrasonication with moderate heat treatment at 60-70 °C enhances protein and enzyme denaturation within microbial cells, thereby increasing their susceptibility to inactivation. The cavitation induced by ultrasonication also favors uniform heat distribution within the product, thus effectively inhibiting spoilage-causing microorganisms even at relatively low temperatures. Consequently, thermal-ultrasonication treatment at 60-70 °C ensures efficient microbial elimination through synergistic mechanical and thermal effects and preserves the quality of fruit purées. This approach minimizes the degradation of heat-sensitive components such as vitamin C, in contrast to conventional high-temperature treatments, and helps the product retain the natural taste, aroma, and texture, which are critical for consumer acceptance.