| Abstract: | Glucose, fructose, sucrose and starch are naturally present in cereals. Fermentation of different combinations of these carbohydrates by Lactobacillus fermentum Ogi E1, a sourdough heterofermentative lactobacillus, was investigated to determine effects on fermentation kinetics, growth energetics and alpha-amylase production. Irrespective of the substrate combination, the strain was able to simultaneously produce alpha-amylase and consume starch, glucose, fructose and sucrose. In mixtures of starch with either sucrose or fructose or with both fructose and glucose, yields of alpha-amylase from biomass (Y(amy/x)) were similar to those observed for starch. However, for starch and glucose or starch, glucose, fructose and sucrose mixtures, both Y(amy/x) and the specific rate of alpha-amylase production decreased markedly. In fructose- or sucrose-containing mixtures, mannitol was formed stoichiometrically indicating that fructose served as electron acceptor, and acetate was produced at constant yield from biomass (Y(ac/x)) (1 g acetate g biomass(-1)). Acetate production was expected to confer to the strain a competitive advantage during natural fermentation by improving biomass formation and growth through an increase in the ATP gain. Y(ATP) varied depending on the carbohydrate mixture, indicating different effects of substrate mixtures on the efficiency in ATP coupling to biomass formation. Compared to starch fermentation, the highest value of Y(ATP) (29 g biomass mol ATP(-1)) was estimated for the starch/fructose mixture but no increase in mu(max) was observed. The lowest value (16 g biomass mol ATP(-1)) was obtained for the starch, glucose and fructose mixture, whereas for the mixture of all carbohydrates, Y(ATP) was similar to that obtained with starch alone (20 g biomass mol ATP(-1)) and it was intermediary for the starch and sucrose mixture (17 g biomass mol ATP(-1)). It is concluded that competitiveness of the strain cannot be based on expected energy gain in mixed substrate fermentation involving fructose and sucrose with glucose and starch, but rather on its ability to simultaneously use carbohydrates while producing alpha-amylase and to produce acetic acid. Acetic acid production could enhance the strain capacity to inhibit nonacid-tolerant, competitive microflora at the earlier stage of natural fermentation. |
