Abstract
Traditionally, food microbiologists have suggested the ability of a fatty medium to increase bacterial survival during thermal treatments. However, only theories have been suggested to describe the physiology of bacterial survival, one of which describes the encapsulation of bacteria within the oil droplet. The aim of this work was to measure the extent of survival in milk and model emulsions comprising increasing concentrations of dispersed oil and to understand the mechanisms of bacterial survival within these food emulsion systems. The inactivation kinetics of Salmonella enteritidis Phage type 4 was investigated at a minimal heating temperature of 55°C within a number of model emulsions and UHT milks and cream. Survival of S. enteritidis was measured by calculating the D-value from survivor curves obtained during heat challenge. A heat challenge method was developed which, as far could be ascertained, was free from methodological artefacts influencing the nature of survival curves. Cells were heated in model emulsions comprising increasing concentrations of hexadecane or sunflower oil emulsified using three different surfactants, and in UHT milks and cream. Each emulsion type used was characterized using laser sizing apparatus and microscopic techniques. Overall, an increase in survival was observed as the dispersed oil phase within the model emulsions used increased from 5 to 15% v/v. Tailing of survivor curves was observed during heat challenge, the extent of which was increased as the dispersed phase was also increased. This was measured by the decrease in r2 coefficient which measured the degree-of-best-fit for the linear plot of the survivor curve from which the D-value was calculated. Survival of S. enteritidis at 55°C in UHT milk and cream was also enhanced by the increase in milk fat concentration. Again, pronounced tailing was observed with cells suspended in cream that contained 18% v/v milk fat. Microscopic examination of S. enteritidis grown in the model emulsions showed the close association of cells with the dispersed phase of the emulsion, suggesting a relationship between adhesion of cells to the oil droplets and the increase in cell survival. Bacteria were not encapsulated within dispersed phase oil droplets as suggested by previous workers. Cell Surface Hydrophobicity of a number of Enterobacteriaceae identified bacteria possessing higher and lower CSH than S. enteritidis. Heat challenge of bacteria possessing higher CSH suggested that the influence on CSH and the adhesion of cells to dispersed oil droplets during heat challenge did not significantly increase D-values. However, detection of cells following heat challenge showed that cells possessing high CSH were being recovered long after those cells which possessed low CSH. The comparison of D55-value of S. enteritidis in broth alone and broth containing surfactant identified that the surfactant Tween 60 increased the cells sensitivity to heat. As dispersed phase was added the sensitivity to heat decreased, suggesting a ‘mopping up’ effect by the dispersed phase. This removal of surfactant or toxic components of surfactant from the aqueous phase of the emulsion reduced cell sensitivity to heat resulting in an increase in D55-value. Further investigations revealed that partitioning of surfactant between the oil and aqueous phase occurred, resulting in differing levels of sensitivity to heat within each separated phase. The influence of casein on the survival of S. enteritidis in milk was also investigated. D55-values were performed in modified milks and casein solutions to determine their influence on heat sensitivity. Casein concentration was measured using the Kjeldhal protein determination and Lowry protein assay. Overall, cells were found to be more sensitive to heat in skimmed milk than in modified milk and casein solutions.