# Pasteurization of Foods with Ultrasound: The Present and the Future
## 3 Microbial Inactivation  
Pasteurization is commonly a thermal process that requires holding the product at a specific temperature for a certain period. Pasteurization aims to inactivate at least 5-log reductions or 99.999% of the pathogen of concern in the product [18,19]. This thermal process also reduces the enzymatic activity and spoilage microorganisms in the food providing stability during storage. Pasteurized products are kept under refrigerated conditions to support the slow growth of microorganisms. However, pasteurization is a strong thermal process that might affect the nutritional quality of some products (i.e., denaturation of vitamins or proteins) and affect the sensory quality of food (i.e., changes in color, texture, taste). Then, ultrasound has been evaluated as an alternative to pasteurization, using sound waves and the effects of cavitation to inactivate microorganisms and enzymes and preserve the quality of the food. Ultrasound can be applied alone in the product (i.e., juice, milk, or liquid food) or combination with heat and/or pressure. The combination of ultrasound and heat (thermosonication), ultrasound and pressure (mano-sonication), ultrasound, heat, and pressure (mano-thermo-sonication) has been evaluated in different products [20, 21, 22]. From the microbial point of view, ultrasound by itself is not strong enough to inactivate microorganisms at the pasteurization level, and it has been widely reported [2, 23, 24, 25, 26]. Though cavitation can produce cell permeabilization, known as \"sonoporation,\" this is a temporary and reversible increase in the permeability of the cell membrane allowing the transport of nutrients [3,27]. Hence, the combination of heat and/or pressure enhances the process, making the microorganisms more vulnerable to cavitation. In terms of microbial inactivation, many foodborne pathogens such as _Listeria monocytogenes_, _Escherichia coli_, _Salmonella_, and _Staphylococcus aureus_ have been studied under ultrasound with positive results [2, 28, 29, 30, 31, 32]. Furthermore, several studies deal with spoilage microorganisms such as yeast and molds [33, 34, 35, 36]. A few studies have also been conducted on spore-formers microorganisms [37, 38, 39]. The acoustic cavitation is responsible for the cellular wall's breakdown and further damage to the cell membrane. Stable cavitation promotes the erosion of the cell wall because of the formation of eddies. Transient cavitation is more aggressive in the cells creating orifices, breaking the cell walls, and releasing cytoplasmatic content [40]. Because ultrasound generates heat by itself, the cells also have thermal effects, such as the weakening of the cell wall and thermo-coagulation of proteins, leading to cellular death [41]. The most recent ultrasound reports for pasteurization are focused mainly on the microbial inactivation of aerobic microorganisms and yeast and molds in some novel fruit and vegetable juices and dairy products, as presented in Table 1. This table shows the need to use ultrasound plus temperature (at least 60 \({}^{\circ}\)C) to inactivate the microorganisms successfully. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Product** & **Microorganism** & **Processing** & **Log** & **Reference** \\ \hline FRUITS AND VEGETABLES JUICES & & & & \\ \hline Amora (_Sponius pimata_) & Aerobic microorganisms and yeast & 44 kHz, 500 W, 40 \({}^{\circ}\)C, 60 min & \(>\)1 and \(>\)1, & \\ juice & and molds & & respectively & [21] \\ Cloudy apple juice & Aerobic microorganisms and yeast & & 1125 W, 70 \({}^{\circ}\)C, 12 min & \(>\)3.5 and \(>\)3.5, & \\  & and molds & & respectively & [42] \\ Chinese bayberry juice & _Backills subtilis_ & 20 kHz, 400 W, 63 \({}^{\circ}\)C, 20 min & \(>\)5 & [43] \\ Mandarin juice & Aerobic microorganisms & 19 kHz, 750 W, 50 \({}^{\circ}\)C, 36 min & 1.3 & [44] \\  & Aerobic microorganisms and yeast & & 20 kHz, 750 W, 65 \({}^{\circ}\)C, 10 min & \(\sim\)4 and \(\sim\)3.5, & \\  & and molds & & & respectively & \\ \hline \hline \end{tabular}
\end{table}