Evaluation of yeast extracts as growth media supplements for lactococci and lactobacilli by using automated spectrophotometry.

An automated spectrophotometric (AS) method was used to evaluate the growth-promoting ability of yeast extracts (YE) on cultures of Lactobacillus acidophilus and Lactococcus lactis subsp. cremoris. The AS data were compared to that obtained from classical shake flask fermentations and from 250 ml bioreactors equipped with pH control. In assays involving the evaluation of 26 different commercial YE, maximum growth rate (&mgr;(max)) values determined with the AS unit ranged from 0.25 to 0.45 h(-1) for Lb. acidophilus and from 0.10 to 0.40 h(-1) for Lc. cremoris. Good correlations were obtained between AS data and manual sampling from the shake flasks or the bioreactors for mmax, as well as maximum optical density (OD(max)). The AS method is thus useful as a screening tool for the selection of YE lots in media formulation. Species reacted differently to the 26 YE, but less variation was observed between strains of the same species. This suggests that a producer of various lactococci or lactobacilli can expect a relatively constant response to a given YE lot between strains of the same species. However, it should not be assumed that the YE having the best growth-promoting properties for Lb. acidophilus will also be the best media supplements for the growth of Lc. cremoris.