pH and acidity in lactic-fermenting cereal gruels: effects on viability of enteropathogenic microorganisms

Survival of Bacillus cereus, Campylobacter jejuni, enterotoxigenic Escherichia coli, Salmonella typhimurium and Shigella flexneri during lactic acid fermentation of cereal gruels prepared from low-tannin (white) and high-tannin (red) sorghum varieties was studied. A previously fermented gruel (starter culture, SC) recycled daily or stored for 7, 14 or 28 days, germinated cereal flour (power flour, PF), or a combination of PF and SC (PF+SC) were used as starters. At 24 h, the pH of all gruels with added starter was ≤4; the pH in control gruels without starter was ≥5.2. pH decrease was significantly faster in gruels made with PF+SC than with either PF or SC alone (P<0.05). A daily recycled SC resulted in a significantly faster decrease in pH (P<0.05) than SC stored for more than 7 days. Acid production was correlated with pH decrease (r=-0.94; P<0.01). In control gruels, the enteropathogens remained at the inoculation level or increased in number. Their growth was inhibited within 24 to 48 h in the fermented gruels, in the order: Bacillus > Campylobacter > Escherichia coli > Salmonella > Shigella. The inhibition rate was significantly faster in fermenting gruel with PF+SC (P<0.05) than in gruel with PF or SC alone and correlated with pH development (r=0.71; P<0.01). Both white and red sorghum gruels gave similar results. Using PF+SC as a starter resulted in a faster decrease in pH as well as a more rapid inhibition of enteropathogenic microorganisms. The effect is optimal if the SC is transferred daily.