Measuring and modelling the glucose-fermenting activity of Lactobacillus plantarum

Effect of added carbohydrates on membrane phase behavior and survival of dried Lactobacillus plantarum

The relation between the protective effect of externally added carbohydrates on Lactobacillus plantarum cells during air-drying and the phase behavior of cell membranes was studied. The residual activity after drying could be improved from 44% in the control to 79 and 66% after the addition of sorbitol and maltose, respectively, whereas trehalose addition resulted in a residual activity of 30%. Membrane phase transition temperatures (Tm) were determined in intact hydrated and dry cells, using Fourier transform infrared spectroscopy. The Tm of hydrated cells was 4 degrees C, increasing to only 20 degrees C after drying. Because endogenous soluble sugars were absent, this phase behavior is attributed to the structure of the predominant phospholipids, PG and lysyl-PG. The restricted increase of Tm is held responsible for the survival of part of the cells. The added maltose, trehalose, and sorbitol did not influence Tm in vivo. We suggest that the effective carbohydrates act through their free radical scavenging activity and not by direct interaction with the polar lipid headgroups.

Glucose and sucrose fermenting capacity of homofermentative lactic acid bacteria used as starters in fermented salads

Salads, i.e., vegetables and/or meat in an oil-in-water emulsion with a low pH due to the addition of acids, are popular in western countries. The shelf life of these salads is influenced mainly by storage temperature and the use of preservatives, i.e., sorbic and benzoic acid. Lactic acid fermentation could be an interesting novel approach to salad preparation. As part of an investigation on fermented salads, lactic acid bacteria from the genera Lactobacillus, Pediococcus and Streptococcus were screened for their capacity to ferment glucose and sucrose in a model system and in a standard salad. The temperature optimum of these strains was in the range of 34-48 degrees C. Most of these strains produced about equal amounts of L(+)- and D(-)-lactic acid. To determine the specific sugar fermenting capacity (mmol lactic acid produced per min per g dry weight) of non-growing cells of lactic acid bacteria, a semi-automated model system was used, based on monitoring the pH decrease in a phosphate buffer during conversion of sugars to lactic acid. The strains tested showed acid production rates on glucose varying between 0.03 and 0.39 mmol of acid produced per min per g dry weight (average 0.23). These glucose-grown cells showed varying, but lower fermentation rates on sucrose (0-0.25 mmol/min/g d.w., average 0.04) than sucrose-grown cells (0-0.33 mmol/min/g d.w., average 0.19). Results obtained with the model system were in good agreement with those of fermentation tests, carried out with a standard salad. Inhibition of spoilage bacteria in the standard salad could be achieved by using starter cultures with high fermenting capacity.