During malolactic fermentation (MLF) in grape must and wine, heterofermentative lactic acid bacteria may degrade arginine, leading to the formation of ammonia and citrulline, among other substances. This is of concern because ammonia increases the pH and thus the risk of growth by spoilage bacteria, and citrulline is a precursor to the formation of carcinogenic ethyl carbamate (EC). Arginine metabolism and growth of Lactobacillus buchneri CUC-3 and Oenococcus oeni strains MCW and Lo111 in wine were investigated. In contrast to L. buchneri CUC-3, both oenococci required a higher minimum pH for arginine degradation, and arginine utilization was delayed relative to the degradation of malic acid, the main aim of MLF. This allows the control of pH increase and citrulline formation from arginine metabolism by carrying out MLF with pure oenococcal cultures and inhibiting cell metabolism after malic acid depletion. MLF by arginine-degrading lactobacilli should be discouraged because arginine degradation may lead to the enhanced formation of acids from sugar degradation. A linear relationship was found between arginine degradation and citrulline excretion rates. From this data, strain-specific arginine-to-citrulline conversion ratios were calculated that ranged between 2.2 and 3.9% (wt/wt), and these ratios can be used to estimate the contribution of citrulline to the EC precursor pool from a given amount of initial arginine. Increasing arginine concentrations led to higher rates of growth of L. buchneri CUC-3 but did not increase the growth yield of either oenococcus. These results suggest the use of non-arginine-degrading oenococci for inducing MLF.The term MLF refers to the microbial conversion of L-malic acid to L-lactic acid in grape musts or wine by MLB. MLB are wine LAB belonging to three genera and include homo-and heterofermentative lactobacilli, homofermentative pediococci, and the heterofermentative species Oenococcus oeni (formerly Leuconostoc oenos) (15). MLF may be due to MLB naturally present in wine, but nowadays MLF is often induced with commercial starter cultures. Its main effects are to reduce the acidity of wines by converting dicarboxylic malic acid to monocarboxylic lactic acid and the modification of flavor properties (8). O. oeni is the preferred species for carrying out MLF, whereas most lactobacilli and pediococci are considered undesirable or spoilage bacteria because of flavor depreciation (6).Besides malic acid, some heterofermentative MLB degrade arginine, which is quantitatively one of the most important amino acids in grape musts and wines (25,27). Complete degradation of arginine by MLB occurs via the ADI pathway, leading to the production of ammonia, ornithine, ATP, and CO 2 (14). During the degradation of arginine, some citrulline is excreted (13). Arginine degradation by MLB has several enological implications (Fig. 1): the production of ammonia increases the pH and the risk of growth of spoilage microorganisms; formation of ATP may give arginine-positive MLB, including spoilage MLB, ...