Growth of Leuconostoc citrovorum in Skimmilk Fortified with an Extract of Streptococcus lactis Cells

Abstract

Two strains of Leuconostoc citrovorum (CAF-B and Da 3) that form long chains and two (9 and 14) that produce short chains were grown at 22 and 30C in skimmilk which contained 0.0, 0.25, or 0.50mg protein per milliliter from a cell-free extract of Streptococcus lactis. Plating and microscopic examinations were done periodically during 30 hours of incubation. Growth of all strains was enhanced equally by both concentrations of cell-free extract regardless of incubation temperature. Strains CAF-B, 9, and 14 were stimulated more than Strain Da 3. Populations of most strains in fortified skimmilks usually exceeded those in plain milk by one log or more. Maximum populations in fortified milks, although higher, appeared 2 to 6 hours later than in plain milk. Generation times of all strains were reduced by the additive but the effect was greater at 30 than at 22C. All strains produced longer chains with added protein.Long chain formation was statistically significant and occurred earlier (<18 hr) at 30 than at 22C. Acid production by all strains increased when milk was fortified with the cell extract.