**1. Introduction**

Enzymes are globular proteins that catalyze biochemical reactions. This occurs due to the spatial configuration of the enzymes and catalytic site, which is determined by the quantity and sequence of amino acids and the organization of these chains, with folds and twists induced by attractions and repulsions among the near amino acids, resulting in a structure with minimal energy content [1].

The enzyme reaction occurs due to the interaction of the catalytic site and substrate, forming a complex enzyme-substrate following by the product formation [2]. The maximum velocity of this reaction occurs at specific pH, temperature and salt concentration and, at non-optimum conditions, the enzyme reaction is highly affected [3]. However, in many times, the desirable industrial process conditions are different of the optimum enzyme activity, making difficult the industrial application. Additionally, high costs and low stability limits the extensive use of enzymes in many processes.

Several chemical and physical methods were studied aiming to improve the performance of enzymes, withdrawing limitations and consequently increasing the range of application. Among the physical methods proposed, high isostatic pressure (HIP) and high pressure homogenization (HPH) are considered an important way to induce interesting changes on enzymes [4, 5]. HIP and HPH are emerging food processing technologies that involve the use of pressures up to 1000 MPa for HIP and up to 400 MPa for HPH to cause desirable changes in food and other products. The HIP process is based on the principle that the maintenance of a product inside vessels at high pressures induces changes in the molecules. To the contrary, for HPH process, the high shear and sudden pressure drop are the responsible phenomena for changes on the processed product.

The overall evaluation of the results obtained for many authors shows that mild conditions of both processes were able to improve the activity and the stability of several enzymes, whereas extreme process conditions (pressure, time and temperature) induce enzyme denaturation with consequent reduction of biological activity [4–9]. Considering the complexity and diversity involved in the enzyme structure and its ability to react, it is not possible to determine specific conditions that each process is able to promote increase and reduction of enzyme activity, being necessary to evaluate HIP and HPH on each enzyme.

In terms of molecular structure, the effect of HIP and HPH on enzymes can be explained by the alterations in the quaternary, tertiary and secondary of enzymes, which directly affects the enzymes active site configuration, inducing exposure of hydrophobic amino acids, exposure of SH groups due to unfolding of the protein, a reduction in the total SH content due to new disulfide bonds formation and changes in the α-helix, β-sheet and β-turn ratio composition due to alterations of the secondary structure [4–6, 10, 11]. However, the occurrence of these phenomena—sequence of occurrence, intensity and required pressure—might be different for HIP and HPH.

The impact of each process on enzymes was evaluated by few published revisions [4, 6, 12], however, no one dedicated to compare the effect of HIP and HPH on the main enzymes used in the food processing, aiming to describe the differences among the process parameters and its consequences in the performance and structure of processed enzymes. Moreover, no revisions have already evaluated the impact of these processes on enzyme in different matrixes. Therefore, this chapter aimed to evaluate comparatively the effects of high isostatic pressure and high pressure homogenization in the activity of enzymes currently applied in food industry and to identify the main structural changes induced by each process pressure on the enzymes. Additionally, this work will be useful to challenge the scientific community to fulfill the information gaps in this area and for the industry, that will have access to a comparative evaluation of these two technologies, being an important way to decide which technology is better to be applied in order to have satisfactory results for different enzymes.
