Dissimilation of organic acids by dairy lactic acid bacteria
F. Z. HEGAZI and I. G. ABO-ELNAGA One hundred and eleven different strains of lactic acid bacteria, belonging to the genera Leuconostoc, Streptococcusand Luctobucillus, wereexamined for their ability to degrade 10 organicacids by detecting CO, production, using the conventional DURHAM tube method. All the strains did not break down succinate, glutarate, 2-0x0glutarate, ormucate. Malate, fumarate, citrate, gluconate, tartrate and pyruvate werevariably attacked. A malolactic fermentation was brought about by two thirds of S. cremoris and S. luctis strains as well as L. cusei but not by L. buchneri or L. brevis. One third of the lactic streptococci examined, i.e. S. crernoris AM,, ML, and SK,, were capable of diacetyl production. They required glucose for cleavage of citrate, whereas Leuconostoc citrovorum and S. fueculis did not. S. cremoris differed from S. luctis in not producing CO, from gluconate. From all the lactic acid bacteria examined, only L. plantarum dissimilated tartrate. Production of acetoin and diacetyl is a more reliable evidence for assessing the degradation of pyruvate, compared to detection of evolved C 0 2 . Facultatively heterofermentative lactobacilli and Leuconostoc cirrovorum produced acetoin and diacetyl from pyruvate, whereas the obligately heterofermentative lactobacilli did not, a character that would be of taxonomic value. Glucose fermented by both the facultatively and obligately heterofermentative lactobacilli did not prove to be a metabolic source of acetoin and diacetyl. The facultative-heterofermenters degraded pyruvate in the presence of glucose with lactate as the major product together with a mean of acetate of 4.1 %, ethanol of 7.9 %, acetoin of 1.7 % and diacetyl of 2.6 % yield on a molar basis after 60 days at 30 "C. L. brevis produced acetate and lactate. The role of S. cremoris and the facultatively and obligately heterofermentative lactobacilli in flavour development during the ripening of many cheese varieties has been confirmed.
'umarate plus
Citrate Citrate plus Pyruvate plus Gluconat % glucose 1 % glucose 1 % glucose