Sugar co-fermentation by lactic acid bacteria. The effect of end products on Saccharomyces cerevisiae

DALLAGNOL, A.M.; GEREZ, C. L.; FONT DE VALDEZ, G.; G. ROLLÁN

Madrid. España

Congreso; 13th ICC Cereal and Bread Congress. Cereals in the 21st century: present and future; 2008

Lactic acid bacteria (LAB) greatly influence the sensorial, textural, nutritional, and shelf life characteristics of breads (De Vuyst et al., 2005). Due to their Food Grade status, the interest on biopreservation by LAB has increased in the last years. The antifungal activity is an important selection criterion for using LAB as starter culture for packed bakery products where fungi spoilage is the main cause of substantial economic losses. In a previous work, LAB with ability to inhibit Aspergillus, Fusarium, and Penicillium strains were selected. The antifungal activity (related to organic acids) depends on the availability of soluble carbohydrates (maltose, glucose, sucrose and fructose) during dough fermentation. The LAB end products may also inhibit the yeasts growth, which are traditionally used as leavening agents in bakery. The aim of this study was to evaluate the co-metabolism of carbohydrates by antifungal LAB strains, and its effect on growth of Saccharomyces cerevisiae. L. plantarum CRL 778, L. reuteri CRL 1100 and L. brevis CRL 772 were grown at 30ºC for 24h in LAPT broth containing glucose, fructose and maltose (LAPTm), and glucose, fructose and sucrose (LAPTs). Sugar catabolism and end products (lactic acid, acetic acid and ethanol) were determined by HPLC. The effect of the metabolites [LAB cell-free extracts (CFE)] on the growth of S. cerevisiae was determined by colony counts (CFU/ml) and optical density (OD580). L. plantarum CRL 778 (facultative heterofermentative LAB) produced the highest (64 mM) amount of lactic acid compared to the other strains (obligated heterofermentative LAB). At this concentration, yeast growth was reduced by 25% after 24h incubation. Production of lactic acid by L. reuteri CRL 1100 and L. brevis CRL 772 was dependent on the carbon source in contrast to the production of acetic acid which was not affected. With respect to LAPTs broth, L. reuteri CRL 1100 produced twice more (27 mM) lactic acid and similar amount of acetic acid (17 mM) in LAPTm medium. Cultures in LAPTm reached a lower pH (4.6); this situation increased the un-dissociated fraction of acetic acid, which was the main factor for the yeast growth inhibition (80%). This harmful effect was less pronounced (38%) with the LAPTs-CFE. On the other hand, L. brevis CRL 772 produced ethanol in high amounts (22 mM); however, this concentration did not inhibit significantly (p>0.05) yeast cells growth. The inhibitory effect displayed by the antifungal LAB strains on S. cerevisiae growth was dependent on the type and concentration of the organic acid (lactate and acetate) produced as well as pH reached during fermentation. Further studies are needed to determine whether this deleterious effect is significant during bread manufacture.