Scientific & Technical Report Removal of Heat-Resistant Bacterial Spores by Filters: Implications for Filter Use in Soft Drink Production

Introduction Soft drinks are subject to spoilage induced by bacterial spores resistant to high-temperature. To date, no effective sterilization method that also does not harm the taste profile of the final product has been developed. Filters with pore sizes slightly larger than conventional 0.2μm sterilizing-grade filters were evaluated for their ability to remove heat-resistant bacterial spores. In addition, these filters were evaluated with an eye on improving process parameters such as minimization of heat treatment on final products, limited alteration of flavor and ingredients, and energy saving and cost reduction in the soft drink manufacturing process. Spores from six bacterial species (Bacillus subtilis, Bacillus stearothermophilus, Bacillus coagulans, Clostridium thermaceticum, Clostridium thermosaccharolyticum and Alicyclobacillus acidoterrestris) were selected as test organisms because they are often isolated as spoilage bacteria in soft drinks.Spores, from each bacterial species, were suspended in 0.067M phosphate buffer solution (pH 7.0) at concentration estimated to be approximately 1x10 5 cfu/mL. Each 1 L suspension was filtered through a 47 mm test disk. The filtrate was assayed for the presence of viable spores. For filter membranes with absolute pore size ratings of 0.45μm, no spores were detected in the filtrate. For filters with pore size ratings of 0.65μm and 0.8μm, however, spores were detected. The study has shown that Pall filters with pore size ratings of 0.45μm are effective in removing heat-resistant bacterial spores, which suggests application of these filters for bacterial control in the soft drink manufacturing process. Non-carbonated soft drinks delivered in sealed containers such as PET or glass bottles or cans are usually treated by heat sterilization in the manufacturing process to remove spoilage microbes. Conditions of the heat sterilization are strict in some cases and mild in other cases, depending on the type and number of microbes in the soft drink. 1 Some types of microbes, that adversely impact quality of soft drinks, form spores when nutrients in the surrounding environment are depleted. The formed spores remain dormant until the surrounding environmental conditions are favorable for vegetative growth. Once spores are exposed to nutrient-rich environment, they germinate into vegetative cells and resume propagation (Figure 1).