Microbial Growth Responses In Fermented Maize Dough Systems

ABSTRACT Food safety and the occurrence of diarrhoea are a challenge in the management of food systems in Africa. Infections arising from diarrhoea can be very devastating on the population, especially children. The traditional fermented maize dough systems have been identified as being able to reduce considerably the growth of diarrhoeal causing organisms and improve safety The study was set up to investigate: 1. The survival of selected diarrhoeal causing bacteria (Salmonella, Shigella and Escherichia coli) in maize dough fermenting systems (steeping water, maize dough, Ga kenkey water and maize dough porridge koko) to determine the safety of the products; 2. The survival of four Escherichia coli strains in synthetic medium containing lactic and acetic acids; 3. Tolerance of the dominant yeasts (Candida krusei and Saccharomyces cerevisiae) involved in maize fermentation for lactic acid. 4. Changes in short-term intracellular pH of single cells of Candida krusei and Saccharomyces cerevisiae in the presence of high and low concentrations of lactic acid to explain their tolerance for lactic acid. Five Salmonella species, three Shigella species, five pathogenic Escherichia coli strains and two non-pathogenic strains were inoculated into fermented maize dough systems at a concentration of 106 - 107 cfu/ml at 28 °C. Viable cells were recovered on selective and non-selective media. Almost all the bacteria survived in maize steeping water for 48 h without reduction in numbers. All the salmonellae and shigellae and two E. coli strains were completely inhibited in fermenting maize dough after 48 hours whilst three E. coli strains (026 (VTEC), 0157 (VTEC II) and 03 EAggEC) survived for 48 h but were significantly reduced by more than 3 log units. None of the Salmonella strains survived in koko for 24 h but Sh. flexneri 2a II and four pathogenic E. coli strains (0157 (VTEC II), 026 (VTEC), 03 (EAggEC), 0111 (EPEC)) and one laboratory strain E. coli K12, survived in koko for 48 h with less than 2 log reductions. All the bacteria were completely inhibited in kenkey water after 24 h. The decrease in populations of the bacteria in the different fermentation systems was observed at pH of < 4. Greater numbers of survivors were recovered with non-selective medium than with a selective medium for Gram-negative bacteria. In Trypticase Soy Broth Yeast Extract (TSBYE) supplemented with lactic and acetic acids in concentrations found in fermented maize dough systems, E. coli strains 0111 (EPEC), 03 (EAggEC), K12 and M23 were inhibited to various extents. Concentrations of 31 - 62 mM, undissociated lactic acid had only a bacteriostatic effect on the four pathogens, while above 62 mM, a bactericidal effect was noted after 24 h. Concentrations of > 17 - 33 mM undissociated acetic acid were required to completely inhibit the four E. coli strains. These results confirm that fermented maize dough systems have antimicrobial properties which may inhibit the survival of some pathogenic and non-pathogenic bacteria. The extent of inhibition varied among the species investigated, namely Salmonella, Shigella and Escherichia coli; and also among the maize dough systems. The presence of lactic acid at low pH was found to be the main anti-microbial property of the fermented maize dough systems.