Direct Enzymatic Conversion of Lactose in Milk to Acid

Abstract

Commercial production of acidified dairy products depends on a fermentative conversion of lactose to lactic acid, a multi-enzyme process. This slow and complicated biological process naturally tends to have disadvantages such as susceptibility to contamination and variation in acid formation; and production difficulties like starter preparation, inflexible equipment requirements, and scheduling. In 1965, we originally suggested that some of these problems in milk acidification might be overcome by using an oxidoreductase enzyme (6). At the time several enzymes were known with the capability of converting monoand disaccharides in the presence of oxygen to a lactone, followed bv spontaneous hydrolysis to the corresponding aldobionic acid. Thus, it appeared feasible that enzymatic conversion of lactose in milk directly to acid could provide an alternative method for acidification. Enzymes capable of oxidizing lactose, galactose, or glucose have been isolated and studied, but applications to milk acidification have not been considered. For example, hexose oxidase (EC 1.1.3.5) was found in the marine red alga lridophycus flaccidum (1); a glucose dehydrogenase (EC 1.1.99.10) was isolated from Bacterium anitratum (3); a lactose dehydrogenase (EC 1.1.99) was separated from lactose-adapted cells of Pseudomonas graveolens (5); and glucose oxidase (EC 1.1.8.4) from Aspergillus niger has been available as a commercial food-grade enzyme for over 20 yr (8). Our initial studies with enzymatic conversion of lactose to acid indicated that the hydrolyzed sugar, as glucose and galactose, could be utilized to acidify milk to pH 4.5 to 5.0 (7). However, a source of oxygen to provide uniform distribution throughout milk was an essential factor to provide a uniform rate of reaction. The combination of hydrogen peroxide