## 3 The Influence of Ultrasound on the Physical and Chemical Properties of Juices
However, importantly, compared to the fresh sample (control, untreated), the amount of malic acid, ascorbic acid and other beneficial compounds was increased in the sonicated sample but decreased in the pasteurized sample. This study highlights the feasibility of replacing traditional pasteurization with ultrasonic processing to improve the quality of noni juice on an industrial scale. Several other studies have shown a greater reduction in microbes in juice samples treated with ultrasound at higher temperatures. For example, carrot juice was thermosonicated (24 kHz) at 50 \({}^{\circ}\)C, 54 \({}^{\circ}\)C and 58 \({}^{\circ}\)C for 10 min to evaluate quality parameters and microbial growth after processing and during storage at 4 \({}^{\circ}\)C. Control and samples sonicated at 50 \({}^{\circ}\)C and 54 \({}^{\circ}\)C had 10-, 12- and 14-day shelf lives, respectively. However, juices subjected to ultrasound at 58 \({}^{\circ}\)C had the best microbiological quality. In this case, microbial growth remained low at approximately 3 log for mesophiles, 4.5 log for yeasts and moulds and 2 log for Enterobacteria after 20 days of storage. What is more, the juice prepared in this way retained >98% of its carotenoids and 100% of its ascorbic acid. The content of phenolic compounds increased in all processed and stored juices. In this way, the planned experiment confirms that thermosonication is a promising technology ensuring the stability of carrot juice by retarding the growth of microorganisms during storage with minimal changes in the number of bioactive compounds [61]. Bermudez-Aguirre and Barbosa-Canovas [62] subjected pineapple, grape and cranberry juices to sonication (24 kHz) at 40 \({}^{\circ}\)C, 50 \({}^{\circ}\)C and 60 \({}^{\circ}\)C for 10 min in continuous and pulsed modes. They observed a reduction in _Saccharomyces cerevisiae_ by 5-7 log units (depending on the product to be fixed) during treatments at 60 \({}^{\circ}\)C, with the continuous mode being more effective. In general, the authors of this paper stated that this processing technique seems to be a viable technology for preserving juices, but the process parameters should be controlled, because changes in the pH and colour of products may occur during cavitation. Ultrasound has been reported to increase the sensitivity of microorganisms to heat, high osmotic pressure and low pH due to cavitation and other changes in the outer membrane of the cellular structure. Basumatary et al. [63] examined the effect of thermal sonication on the microbiological quality of pomelo (_Citrus maxima_) juice, which they treated for 15, 30, 45 and 60 min at 30, 40 and 50 \(\pm\) 2 \({}^{\circ}\)C. Two ultrasound frequencies, 33 and 44 kHz, were used. Very good test results were obtained, because the total number of microorganisms, yeasts and moulds decreased significantly in relation to the control samples (fresh juice). With the increase in treatment time and temperature, the number of microorganisms under consideration decreased. Subsequent studies therefore indicated that a properly conducted process with the use of ultrasound can be used as a substitute for heat treatment in the processing of pomelo juice due to its positive effect on the quality of the final product (significant increase) in terms of antioxidant activity, with a lower decrease in ascorbic acid content compared to pasteurization. Subsequent analyses confirmed that ultrasonic treatment may not only be an alternative to pasteurization, but also improve the nutritional value and safety of freshly pressed plum juice. Research by Oladunjoye et al. [64] consisted of pasteurization of the product at 90 \({}^{\circ}\)C for 60 s and thermal sonication of the drink for 5, 10, 20 and 30 min at the temperatures of 40, 50 and 60 \({}^{\circ}\)C at an ultrasound frequency of 40 kHz, and then determination of the microbiological properties.