Importance of bacterial surface properties to control the stability of emulsions.

In colloidal media such as emulsions or food matrixes, the stability results from physicochemical interactions. The same type of interaction is involved in the attachment processes of microorganisms, through their surface properties, to interfaces. When bacteria are present in a food matrix, it is probable that their surface interacts with the other constituents. In this paper, the involvement of bacterial surface properties of Lactococcus lactis subsp lactis biovar diacetylactis (LLD) on the stability of model emulsions has been studied. The hydrophobic and electrostatic cell-surface properties were characterized by the MATH method and by microelectrophoresis, respectively. The oil-in-water emulsions were stabilized by various surface-active compounds, CTAB, SDS or Tween 20, giving differently charged droplets. Two strains with different surface characteristics were added to the emulsion. Contrasting with emulsions made with the non-ionic surfactant, for which the stability was not modified by the addition of bacteria, the emulsions made with ionic surface-active compounds were unstable in the presence of bacteria when the bacterial surface charge was opposite to the one of the emulsion droplets. Moreover, aggregation and flocculation phenomena were observed for emulsions stabilized with the cationic surfactant, particularly for more negatively charged bacteria. The effect of bacteria on the emulsion stability depended on the strain which shows the importance of the choice of the microorganism according to of the characteristics of the colloidal media to obtain a stable system. In addition, these results suggest that the interactions between bacteria and other food components can influence the position of bacteria in food matrixes.