# Pasteurization of Foods with Ultrasound: The Present and the Future
## 4 Enzyme Inactivation  
During pasteurization, some enzymes should decrease or stop their activity from providing the product with stability during storage. Depending on the product, some necessary enzymes can be evaluated during pasteurization. For example, the relevant enzymes for milk pasteurization are alkaline phosphatase (ALP), lactoperoxidase (LP), lipase, and plasmin (alkaline protease). ALP indicates an adequate pasteurization process, while LP frequently checks for overprocessing [79]. Meanwhile, in fruit and vegetable products, the enzymes of interest are pectin methylesterase (PME), polyphenol oxidase (PPO), peroxidase (POD), lipoxygenase (LOX), and polygalacturonase (PG), among others. These enzymes affect the stability of the product in terms of cloudiness, color (discoloration, browning), viscosity, flavor, and nutritional value [80]. Like other novel technologies, ultrasound can increase the enzymatic activity when the proteins unfold because of the sonication effect. During this process, there is mass transfer and better contact between enzyme and substrate, boosting enzymatic activity [41]. However, if the sonication is combined with pressure and/or temperature, the proteins (enzymes) can be denaturized, and any enzymatic activity can be stopped [60]. Enzymes are denatured with ultrasound because of the effect of cavitation in hydrogen bonds and van der Waals forces between the polypeptide chains, but also when the free radicals produced during cavitation react with the amino acids of the enzyme modifying its functionality [5; 81]. Some examples of enzyme inactivation in pasteurized products are presented in Table 2. Like microbial inactivation, ultrasound is not enough to achieve important enzyme inactivation [82]. [PERSON] et al. [59] studied the inactivation of two target enzymes in milk pasteurization, ALP and LP. After 6 min of processing with ultrasound and thermal treatment (85 \({}^{\circ}\)C), both enzymes were fully inactivated. Both enzymes are often correlated with microbial inactivation during pasteurization; the complete inactivation of these enzymes is a good indicator of an effective pasteurization process. Usually, the inactivation of ALP is generally used as an indicator of the efficacy of the pasteurization treatment. In a different study, ALP was fully inactivated when ultrasound was applied with heat (20 kHz, 120 \(\upmu\)m, 150 W, 75.5 \({}^{\circ}\)C, 102.3 s) regardless of the type of milk. However, LP was only partially inactivated when using the cited conditions, remaining with 30% activity in whole milk and 47% in skim milk [82]. The case of pear juice in Table 2 shows three of the target enzymes to inactivate during pasteurization, Peroxidase (POD), Polyphenol Oxidase (PPO), and Pectin methylesterase (PME), almost inactivated (>95%) after 10 min of treatment. However, it is essential to highlight that ultrasound was applied together with thermal treatment (65 \({}^{\circ}\)C) [45]. Additional examples of enzyme inactivation with ultrasound include polygalacturonase in tomato juice and paste; PPO in apple juice and puree, strawberry puree, pineapple juice, and carrot juice; and POD in apple juice, carrot juice, watercress leaves, guava cubes, carrot slices, and green beans [81].