## 3 The Influence of Ultrasound on the Physical and Chemical Properties of Juices
When evaluating the microflora of pasteurized plum juice, a decrease in all assessed microorganisms to an undetectable level was found, which could be related to cell wall/membrane rupture and nuclear components leading to cell death. However, after thermal sonication at 40 \({}^{\circ}\)C, the total number of microorganisms decreased from 3.75 to 1.01 log CFU/mL, and the number of yeasts and moulds decreased from 4.17 to 3.64 log CFU/mL. Raising the ultrasonic treatment temperature to 50 and 60 \({}^{\circ}\)C allowed the number of bacteria to be eliminated to undetectable levels, while the number of yeasts and moulds at 50 \({}^{\circ}\)C decreased from 3.36 to 1.93 log CFU/mL, with no detectable increase at 60 \({}^{\circ}\)C. The incomplete inactivation of yeasts and moulds at 40 \({}^{\circ}\)C and partially at 50 \({}^{\circ}\)C may be due to the formation of spores that are resistant to these treatment conditions. The inactivation of microorganisms via thermal sonication mainly concerns the acoustic cavitation taking place in the cellular structure, which in turn causes the implosion of bubbles in the liquid and/or the generation of free radicals with antimicrobial properties. In addition, the high acidity level of plum juice and induced osmotic pressure, together with other reactions, may intensify the effect of cavitation on the structure of microorganisms, resulting in the release of intracellular components, such as protein lipids and nuclear compounds [57; 65; 66; 67; 68]. Not without significance is the high quality of ultrasound-treated juice, which is characterized, among other properties, by a higher content of ascorbic acid, carotenoids (2.19-4.30%) and flavonoids (10-16%), as well as antioxidant activity (32, 52-48.5%) compared to the control (untreated juice). Only elevated temperature (60 \({}^{\circ}\)C) was not conducive to most of these quality features, which means that the thermosication process may require further optimization of treatment parameters in order to maintain the quality of the final product to the maximum extent [64]. Due to the biological diversity of individual food products, the structure of ultrasonic generators and the methods of setting and expressing process parameters, it is difficult to clearly indicate the optimal processing conditions for juices pressed from various fruits or vegetables. Based on our own research, we found that the ultrasound intensity needed to inactivate microorganisms ranges widely from 28 to 90 W/cm\({}^{2}\) and depends on the initial microbiological condition and the type of juice processed. In the case of tomato juice, we showed that ultrasound sonication at an intensity of 40 W \({}^{-2}\) for 5 min and 28 W cm\({}^{-2}\) for 10 min yielded a microbiologically pure product devoid of microorganisms involved in spoilage even after 10 days of storage [68]. Other research results relating to the effective minimization of the presence of microorganisms contaminating fruit and vegetable juices through the action of ultrasounds were collected, as presented in Table 2.  
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Koda et al. [76] studied the effectiveness of ultrasound with a frequency of 500 kHz on the inactivation of microorganisms (Gram-negative bacteria _Escherichia coli_ IAM 12058 and Gram-positive bacteria _Streptococcus mutans_ JCM 5175) and concluded that the mechanism  
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of their action is based mainly on chemical effects. Transmission electron microscopy (TEM) studies of bacteria confirmed lethal damage resulting from the interaction of bacterial cells with cavitation bubbles. A significant number of empty cell envelopes as well as their cytoplasmic content have been observed. According to other scientists, the mode of action of ultrasound on microorganisms in liquids includes hydrodynamic effects (the phenomena of intracellular cavitation and microflux) and the formation of radicals that disrupt the structure of the cell.