The Reversibility of Oxidative Inactivation of Milk Lipase in Relation to its Activity in Cheddar Cheese

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Abstract
The study of the properties of milk lipase in relation to the development of rancid flavor in cheddar cheese is a problem of some practical importance. In a recent publication the authors showed that a higher amount of proteo- lytic enzymes (rennet and pepsin) can bring about a decrease in the intensity of rancid flavor in cheddar cheese which contained added commercial lipase (9). Freeman and Dahle (7) showed a similar improvement in the flavor of cheddar cheese by the use of proteolytic enzymes without added lipase. Considering the presence of a small amount of endogenous milk lipase, their results may be explained on the same basis as ours. Other methods of inhibiting lipase activity in cheese remain to be investigated. Copper in the presence of oxygen is an active lipase inhibitor. Davies (5) showed that the activity of milk lipase was depressed in unpasteurized butter to which 2-10 p.p.m. Cu had been added. Herrington and Krukovsky (8) confirmed this for raw milk. Later Krukovsky and Sharp (4) showed that dissolved copper caused no inactivation in the absence of oxygen. How- ever, oxygen alone was active, its activity being greatly accelerated by small amounts of copper. Oxidative inactivation of enzymes is common and, in general, reactiva- tion is possible with reducing agents. Thus it is stated that the activity of serum lipase and human milk esterase is augmented proportionally with the degree of reduction (10). Our interest lies in the properties of milk lipase in raw milk cheddar cheese where strongly reducing conditions prevail (6). It was decided, therefore, to investigate whether oxidative inactivation of milk lipase was reversible in the presence of reducing systems simulating those in cheese. Three systems were studied: 1. Anaerobic Milk under anaerobic conditions develops a strongly nega- tive potential. Aeration or copper cause the milk to maintain a high poten- tial. The range of Eh thus covered is approximately - .2 to .3 volts. It may be expected, however, that the same systems would be operative in cheese where the interior is anaerobic.
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