E.P.W. Kets

Mannitol-enhanced survival of Lactococcus lactis subjected to drying

Abstract Survival of Lactococcus lactis subjected to different drying conditions was investigated. Mannitol most remarkably enhanced the survival of dried cells to a level almost equalling that of viable cells [log10 (cfu ml−1) = 9.42] as was found prior to the drying process (log10 = 9.6). In the absence of mannitol, a survival was reduced by a factor of 104. Drying of cells at 20 °C led to higher survival rates than drying at 30 °C. Mannitol enhanced the survival rate at both temperatures, and at both 20 °C and 30 °C the highest reduction in survival occurred when cells were dried at a water activity of 0.76. In the presence of mannitol, differences in survival after drying at different water activities were less pronounced. Rehydration of cells dried in the presence of mannitol resulted in an extended lag phase of 4 h compared to fresh cells. No growth or acidification of the culture medium was observed for 12 h in the case of rehydrated cells dried in the absence of mannitol. It was hypothesized that a radical scavenging activity of mannitol could partly explain these observations.

Choline and acetylcholine: novel cationic osmolytes in Lactobacillus plantarum

Abstract The aim of this work was to study the physiological response of Lactobacillus plantarum subjected to osmotic stress in the presence of three structurally related compatible solutes. Either betaine, choline or acetylcholine was accumulated by osmotically stressed cells when provided in the chemically defined medium. Choline and acetylcholine were accumulated to maximum concentrations of 139 and 222 μmol g (dry weight) of cells−1 respectively and were not converted to betaine. Addition of 0.5 mM choline or 0.5 mM acetylcholine to the medium increased the growth rates of cells in media with various amounts of added sodium chloride. Both choline and acetylcholine are positively charged compounds; therefore, it was presumed that charged intracellular solutes could counterbalance the excess of positive charge. Intracellular inorganic ion levels (K+, SO2−4, PO3−4 and Cl−) of cells cultured under conditions of osmotic stress remained similar in the presence of either betaine, choline or acetylcholine. However, cells cultured in the presence of choline or acetylcholine accumulated an additional quantity of approximately 125 or 200 μmol.glutamate (dry weight) cells−1 respectively, as compared to cells grown in the presence of betaine. Hence glutamate appears to be the counterion for choline and acetylcholine. This is the first study demonstrating accumulation of choline and acetylcholine in lactic acid bacteria subjected to osmotic stress.

Combined influence of growth and drying conditions on the activity of dried Lactobacillus plantarum

The production of active dried starter cultures can be influenced at several levels in the production process. In this paper the following process factors are discussed: osmotic stress during growth and cell density prior to drying. Contradicting results are reported in the literature on the influence of osmotic stress during growth on the residual activity after drying. The combined approach in which two process factors were studied at a time resulted in an explanation for the discrepancy in earlier work. The cell density prior to drying had an important influence on the glucose fermenting activity after drying. Residual activities ranging from 0.10 to 0.83 were achieved using initial cell densities between 0.025 and 0.23 g of cell/g of sample, respectively. The drying tolerance of cells grown with osmotic stress of 1 M NaCl was low (residual activity = 0.06) and was not related to the cell density prior to drying. The influence of osmotic stress during growth on the drying tolerance of Lactobacillus plantarum was dependent on the cell density prior to drying.