Optimal Conditions for Effective Use of Acid Urease in Wine

Abstract

ETHYL CARBAMATE (EC), commonly called urethane, is a well-known carcinogen in experimental animals (Mirvish, 1968 ; Inai et al., 1991). EC has been identified in various fermented foods and beverages (Walker et al., 1974 ; Canas et al., 1989 ; Dennis et al., 1989 ; Fauhl et al., 1993). In 1976, Ough suggested that EC occurred naturally during fermentation (1976a, b). Published reports indicate that the major pathway of EC formation in wine is the reaction of ethanol with urea (Ough et al., 1988a ; Monteiro et al., 1989), which has been detected at various concentrations in wine (Ough et al., 1990 ; Fujinawa et al., 1990). The urea in wine is produced by yeast metabolism of arginine from grape juice (Ough et al., 1988a). Our research group has reported that urea content in wine should be kept below 2 mg/L in order to control EC formation during aging and storage. This would also keep the concentration within target levels of wine industries in the US : 15 μg/L in table wines and 60 μg/L in dessert wines (weighed average basis, Kodama et al., 1994). Alcoholic beverage industries in U.S.A., Canada, Japan, and Taiwan have made efforts to reduce EC content. Both urea and alcohol are precursors of EC. Ethanol is essential in most commercial wines, therefore removal of urea is the most practical method for reducing EC formation (Ough and Trioli, 1988 ; Fujinawa et al., 1990). Urease, which hydrolyzes urea into ammonia and carbon dioxide, is one of the most extensively studied enzymes. It was the first protein molecule isolated and crystallized (Sumner, 1926). Commonly found types of the enzyme, such as Jack bean urease, function optimally at neutral pH. These types of urease are hardly active in low pH conditions such as in wines, which usually are pH 3 to 4. Acid urease derived from Lactobacillus fermentum, however, exhibits its highest rate of urea decomposition at acid pH conditions and can therefore be utilized for effective reduction of urea in wine (Kakimoto et al., 1989). The effectiveness of this enzyme in wine depends strongly on the type wine in which it is used and the conditions. Reports indicate the presence of inhibitors of urease in wines such as fluoride, malate, ethanol, and phenolic compounds (Trioli and Ough, 1989 ; Famuyiwa et al., 1991 ; Kodama et al., 1991). Acid urease has been affirmed as Generally Recognized As Safe by the Food and Drug Administration of the US for use in wine, including sake (1992).