ABSTRACT
Enzymes have an important impact on food quality, and they are present in many foods. They may originate from the raw material, for example, lipoxygenases, polyphenoloxidases, pectinases in plant material, proteases and lipases in milk, proteases in meat and fish, to name just a few. Enzymes account for ripening processes (leading to quality enhancement as in cheese and all kinds of other fermented products), they can be the cause of quality deterioration such as softening of fruits and vegetables, of flavor defects (such as soapy flavor, oxidized flavor), to name a few processes. Enzymes can also be present as a result of microbial contamination, such as proteases and lipases excreted by pschychrotrophic bacteria in milk. Furthermore, enzymes may be added as a processing aid (i.e., fermentation, biotechnology). The classical example is rennet added to milk to produce curd, which is essential for cheese manufacture. In the cheese ripening process, rennet is active again along with enzymes from various microorganisms as well as indigenous milk enzymes. Other processes in which enzymes are frequently used are bread making, brewing, fruit juice production, hydrolysis of proteins to produce hypoallergenic foods, and many more. Enzyme activity is thus very important for food quality and knowledge of enzyme kinetics is necessary
to understand and quantify the activity. However, there is nothing mysterious about enzyme kinetics: they obey the rules of thermodynamics, kinetics, and catalysis as we have discussed them before. Is it then necessary to spend a separate chapter on enzyme kinetics? Obviously, the answer is yes, and the reason is that enzymes as proteins are subject to all kinds of changes during food processing and storage. So, even though the kinetics is straightforward, the resulting action of enzymes may not be. And since they are active in so many foods, a separate chapter is indeed warranted. What we would like to show is that the combination of enzymes as catalysts and the intricate food matrix is a topic in its own right. The most important variables are the enzyme concentration, substrate concentration, the presence of
inhibitors and activators, and conditions such as pH, ionic strength, and temperature. Enzymes are also important in many other areas of science and technology, and the literature on enzymes is overwhelming. Many books and articles are devoted to classical enzyme kinetics, using Michaelis-Menten kinetics. We will also discuss Michaelis-Menten if only to show that it is based on normal kinetic principles and rate laws that were discussed in Chapter 4. However, we also would like to pay attention to more recent, computer-based possibilities to handle enzyme kinetics, such as progress curve analysis. Enzymes are catalysts and act according to ‘‘normal’’ kinetic principles as discussed in Chapter 4.
A complicating factor is that enzymes are proteins, i.e., distortion of their conformation causes inactivation (e.g., as a result of heat, pressure, change in pH, ionic strength, adsorption to surfaces). Also,
inhibitors and activators may be present. In raw materials, enzymes are sometimes physically separated from their substrates. As discussed in Chapter 4, there is homogeneous catalysis (catalyst is in the same phase as the reactants) and heterogeneous catalysis (catalysis takes place at the surface of a solid catalyst present in a solution). The action of enzymes can be regarded as a sort of microheterogeneous catalysis: catalysis takes place in the active site (‘‘microsurface’’) while the substrate and the enzyme are present in the same phase. This effect explains the very high specificity of enzymes. Enzymes work by
