Investigating the ability of Thermal Cycling to reduce the growth of Biofilms
The objective of this research was to determine if thermal cycling can reduce the growth rate of thermophilic bacteria during milk powder production. The test bacterium selected for the experiment was Geobacillus stearothermophilus, which has been found in the milk powder production plant as a contaminant. G. stearothermophilus persists in the manufacturing plants as biofilms on the various stainless steel and seal surfaces available in the production line. A biofilm can be defined as an aggregation of microbial cells and their associated extracellular polymeric substances (EPS), actively attached to, growing and multiplying on a surface (Johnstone, Ellar, & Appleton, 1980). Biofilm formation can cause film accumulation on food contact surfaces, and microbial colonization in milk storage tanks, fouling of heat exchangers and adhesion of contaminating spores on packaging material surfaces. The need to comply with permissible thermophilic spore count in the milk powder forces shut down of the plants for cleaning earlier than would be the case if non-microbial fouling of the heat exchangers were the only concern (Hinton, Trinh, Brooks, & Manderson, 2002). Consequently, the operational costs soar and profit decreases.
The species in question may not pose a threat to health but represent a continuing problem of spoilage and production of out-of-specification products. The bacterial culture was grown in tryptic soy broth (TSB) in a water bath at 55°C for 6 hours. The treatment was manipulated on three parameters:
• Step change temperature (55°C to 30°C or 35°C) • Step change duration • Period between step changes
The milk for the experiments was prepared by dissolving milk powder acquired from Fonterra into warm deionized autoclaved water. The experimental set-up consisted of a sterile plastic bottle (milk reservoir), two inlet tubes passing from either of two water baths maintained either at step-change temperature or control temperature. The tubes then entered a two-way solenoid valve through which the milk passed into a reactor tube having 10 coupons of 1 cm2 area inserted, and then to a pump tube, which finally drained the milk into a sealed collection vessel (a 5000 ml Schott-Duran glass bottle). The culture was introduced into the experimental set-up by two methods. In one method, the culture was inoculated into the sterile milk and in the other, the culture was inserted into the reactor tube and allowed to stand for 30minutes before being pumped out rapidly with sterile milk. The parameters for the variation were introduced using a Arduino-based software designed for the project, which switched the valve between the milk coming from the water bath at step-change temperature and control temperatures, according to the specific time pattern programmed into the software. To be sure of the temperature, a thermocouple measured the temperature of the milk as it exited from the valve and entered the reactor. The bacterial growth rate was measured by sampling both the coupons and outflowing milk at certain points of time during the trial. The results and data collected during the project clearly indicated a significant reduction in the growth rate of G. stearothermophilus owing to thermal cycling. Based on the conditions tested in this project, for control of both biofilm growth rate and contamination of the outflowing milk, a step-change at 35°C for 35 min with 15 min interval between two step-changes can be recommended as the best trial regime forpilot-scale experimentation for evaluation of effectiveness of thermal cycling.