Taxonomic Study of High Carbon Dioxide-Producing Lactic Acid Streptococci Isolated from Mixed-Strain Starter Cultures

Abstract

A taxonomic study was made of single strains isolated from commercial mixed-strain starter cultures with particular effort to determine the classification of isolates producing large volumes of CO2 in milk. The occurrence of these gassy types in mixed starter cultures was related to the floating curd defect in cottage cheese and to the slit-open defect in Cheddar cheese. Reference strains of Leuconostoc mesenteroides, L. citrovorum, L. dextranicum, Streptococcus lactis, S. cremoris and S. diacetilactis were included in the study. The high gas-producing lactic acid streptococci belong in Lancefield Serological Group N and were identified as S. diacetilactis. Both fast and slow lactose-fermenting strains of this organism were isolated from mixed strain starter cultures; the slow variety was encountered more often. The S. diacetilactis organisms also exhibited identifying cultural reactions and, along with S. lactis and S. cremoris, was distinguishable from Leuconostoc species by forming an acid curd in litmus milk after 48 hours incubation at 30[degree]C. Cultures of S. lactis were distinguished from S. cremoris primarily by the ability of S. lactis to liberate ammonia upon hydrolysis of arginine. The name S. cremoris var. aromaticus was suggested to identify 2 culture isolates which seemed intermediate between S. cremoris and S. diacetilactis. The reference strains of L. citrovorum and L. dextranicum were indistinguishable by any of the cultural tests used in the study.