Superoxide dismutase (SOD; EC 126.96.36.199) and catalase (EC 188.8.131.52; see Chapters 27 and 38 for detailed information) are present in milk and are able to remove reactive oxygen species generated by other (bio)chem-ical processes (90). SOD catalyzes the reduction of superoxide anion, as produced, for example, by XO, to H2O2 and O2. In turn, catalase is able to neutralize H2O2 to water and oxygen. A low level of exogenous SOD, coupled with catalase, is a very effective antiox-idant in dairy products (91).
Recently, SOD has been shown to protect beer against free radical damage (92). Obviously, the commercial feasibility of SOD as an antioxidant depends on cost, particularly compared to chemical antioxidants, if permitted. As far as is known, SOD is not used commercially as an antioxidant in food systems.
Catalase is used for the cold-sterilization of milk in regions lacking refrigeration and could in principle be applied in developed countries for the treatment of cheese milk (90). Good sources for catalase are beef, liver, Aspergillus niger, and sweet potato. There is also interest in using immobilized catalase reactors for milk pasteurization or for glucose oxidase-cata-lase reactions (93). Besides the removal of reactive oxygen species by SOD and catalase, other enzymes can be applied to remove the less but still reactive oxygen. Typical examples include glucose oxidase, D-amino acid oxidase, alcohol oxidase, and ascorbic acid oxidase. The disadvantage of these oxidases is that they produce H2O2, which by itself is a powerful oxidant. Catalase can be added to remove H2O2, but then oxygen is produced again. More recently, polyphenol oxidases such as laccase have been proposed as a deoxygenation tool for beer (94) and juices (95, 96). These enzymes have the advantage that they do not produce H2O2, and thus the combination with catalase is not necessary. As a result PPOs allow a more efficient oxygen removal.
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