Introduction: These days, most of the disinfectants used in the food industry such as chlorinated compounds are dangerous and harmful. Common methods of removing all types of pollution have many disadvantages for human health and the environment. It is possible to help preserve the environment and human health by replacing these methods with new ones such as ultrasound technology. Ultrasonic waves are non-thermal technology that helps increase microbial safety and prolong the shelf life of heat-sensitive foods with nutritional value and functional characteristics. Ultrasonic waves are known as one of the most effective disinfection methods for all forms of microbial and fungal contamination. These waves reduce the resistance of microorganisms by physically damaging them. Therefore, this study deals with the effect of high-power ultrasound waves on the population of two type of pathogenic microorganisms in the washing effluent of tomatoes. The selected bacteria included one type of gram-positive bacteria (Staphylococcus. aureus) and one type of gram-negative bacteria (Escherichia coli) to compare the effect of ultrasound waves on the two different types of bacteria with different cell walls.
Materials and Methods: In this research, irradiation of high-power ultrasound waves were applied to the water after washing the tomatoes. In this washing effluent, the impacts of ultrasonic power (100, 300, 500 W), radiation time (300, 750, 1200 s), and water temperature (0, 30, 60 °C) were examined on the survival of the S. aureus and Ecoli. The data analysis was done for each experimental runs, using the response surface methodology (RSM), to find the best model for estimating the difference in bacterial population (CFU) before and after irradiation.
Results and Discussion: The lack of fit was not significant in the analysis of variance and also the value of the explanation coefficient in the model for S. aureus and Ecoli were 0.9721% and 0.9206% respectively. This indicated the appropriate accuracy of the quadratic model in estimating the number of S. aureus and Ecoli remaining in the water after washing tomatoes (for the mentioned independent variables). Gram-negative bacteria (E coli), are composed of an inner thin peptidoglycan cell wall, surrounded by an outer lipopolysaccharide membrane. Gram-positive bacteria (S. aureus), lack an outer membrane but are made up of a multi-layered and very complex structure layers of peptidoglycan many times thicker than is found in the Gram-negatives. In general, the application of ultrasound waves causes to destruction of the mentioned bacteria. The main disinfection effect of ultrasonic waves on the population of S. aureus was power, while for Ecoli the main variable was temperature (based on the highest coefficient of quadratic equations/ 99% confidence level). Through physical, chemical and mechanical effects caused by acoustic cavitation, ultrasound is able to affect the bacterial suspension without producing a side product. The antimicrobial effect of ultrasound is achieved by a combination of chemical effects such as the production of active free radicals and thermal effects such as the production of local hot spots. The observations showed that increasing the temperature first increased and then decreased the effectiveness of ultrasound waves in the inactivation of bactetria. The negative effect of increasing temperature can be related to the decrease in the intensity of bubble explosion.
Conclusion: According to the results of the experimental tests in the average time (750 s), with the simultaneous decrease in temperature (from 60 to 0 ºC) and increase in power (from 100 to 500 W), the destruction effect of ultrasound waves on S. aureus and Ecoli was increased. In the perturbation curves, the simultaneous effect of all three parameters (temperature, time and power), were investigated at the middle points (30 ºC, 750 s and 300 W). At these points, power changes were more effective in reducing S. aureus population, while temperature changes were more effective on the reduction of E coli. The population of S. aureus and E. coli decreased by increasing power of ultrasonic waves. Temperature and power had a synergistic effect, that is, the increase of both parameters led to the decrease of bacteria population. Finally, the tested variables were optimized by desirability in the RSM to minimize the population of microorganisms (S. aureus and E coli simultaneously), and parameters (in the range) obtained for the ultrasonic power, time, and temperature were 300 W, 1200 s, and 0 °C respectively.