With a bactericide in surface cleaning

One of the major long-established industrial applications of power ultrasound is in surface cleaning and it has proved to be an extremely efficient technology. Ultrasound is particularly useful in surface decontamination where the inrush of fluid that accompanies cavitational collapse near a surface is non-symmetric (Fig. 16.5). The particular advantage of ultrasonic cleaning in this context is that it can reach crevices that are not easily reached by conventional cleaning methods. Objects that can be cleaned range from large crates used for food packaging and transportation to delicate surgical implements such as endoscopes. This was recognised some years ago, see for example a general patent that relates to the use of ultrasound as a method of pasteurisation, sterilisation and decontamination of instruments and surfaces used within the medical, surgical, dental and food processing industries (Boucher, 1980). The use of ultrasound allows the destruction of a variety of fungi, bacteria and viruses in a much reduced processing time when compared to thermal treatment at similar temperatures. The removal of bacteria from various surfaces is of great importance to the food industry and can be efficiently accomplished with the combined use of sonicated hot water containing biocidal detergents (Quartly-Watson, 1998). Typical examples of items requiring repeated regular cleaning are plastic baskets, shackles (the hooks used for hanging poultry on a production line) and conveyer belts.

Chemicals such as chlorine are often used to clean and decontaminate food products and food processing surfaces. The effects of ultrasound on the surface contamination of poultry drumsticks have been examined (Sams and Feria, 1991). An increase in levels of bacteria, such as Salmonella, was observed during the pre-chilling stage possibly due to the dispersion of bacterial aggregates and their subsequent growth. Sonication post chilling also did not appear to reduce levels of bacteria. In this case sonication alone had little benefit in the preservation process, nevertheless, it was suggested that in combination with additional techniques it could prove to be beneficial. They postulated that the shape of the drumstick may have been detrimental to effective use of ultrasound with the formation of standing waves as a result of reflection from an uneven poultry surface resulting in reduced levels of effectiveness. Lillard examined the disinfection of flat poultry skin inoculated with Salmonella with a combination of ultrasound and chlorine (Lillard, 1993). The best results were obtained when employing ultrasound simultaneously with chlorine solution with levels reduced by 2.5 to 4.0log10. It was suggested that the enhancement arose through a combination of the release of Salmonella cells from the skin by bombardment with ultrasound and the ultrasonically increased penetration of chlorine into the cells.

In most countries the use of antibiotics in foods is strictly forbidden. Nevertheless, there have been several reports of the use of ultrasound to improve the effects of antibiotics in surface cleaning, particularly in the removal of biofilms. Thus ultrasound is able to promote the effectiveness of various antibiotics at concentrations that do not, on their own, reduce bacteria viability in biofilms (Qian et al., 1997). This synergy reduced the viability by several orders of magnitude for cultures of Pseudomonas aeruginosa, Escherichia coli, Staphylococcus epidenmidis and Staphylococcus aureus. Measurements of the bactericidal activity of gentamicin against P. aeruginosa and E. coli demonstrated that simultaneous application of 67 kHz ultrasound enhanced the effectiveness of the antibiotic. As the age of these cultures increased, the bacteria became more resistant to the effect of the antibiotic alone and the application of ultrasound appeared to reverse this resistance. The ultrasonic treatment-enhanced activity was not observed with cultures of gram-positive S. epidermidis and S. aureus. The synergy was confirmed in studies of biofilms of Escherichia coli (Peterson and Pitt, 2000). The biofilms were up to 300 pm thickness, far greater than those previously examined with Pseudomonas aeruginosa. Treatment with ultrasound alone did not appear to enhance killing rate even after two hours sonication. Antibiotic alone killed only 82% of the cells within two hours. A combination of ultrasound and antibiotic killed 99% of the cells within two hours with the best results obtained employing high power densities and low ultrasonic frequencies. Improvement in the killing rate may be due to the ability of ultrasound to introduce the antibiotic further into the biofilm than it would normally have reached on its own.

The use of ultraviolet light as an aid to disinfection and to aid food preservation is well known and well used within the food industry. Ultraviolet light has been used to inactivate Salmonella in thin aqueous and chocolate films (Lee et al., 1989). They compared the results obtained with those from the use of ultrasound as a method to inactivate the bacteria. Serotypes such as the heat-sensitive S. eastbourne to the less sensitive S. senftenberg were examined. They discovered that the thermal resistance of salmonella was greater in the chocolate than in the aqueous media. Decimal reduction times at 710C were 4.5 hrs, 4.6 hrs and 6.6 hrs for thermal treatment of S. eastbourne, S. senftenberg and S. typhimurium serotypes. Using ultrasound a 4log10 reduction was observed at 50C after only 10 minutes sonication in peptone water whilst a 0.78log10 reduction of the bacteria was observed in milk chocolate after 30 minutes. High temperatures were observed in the chocolate medium and this may have had some contributory lethal effect. This could be in some part due to the viscosity and thermal conductivity of the chocolate being detrimental to efficient sonication and cavitation production. Use of ultraviolet light killed 75% of the salmonella within 10 minutes when the chocolate thickness was 0.5 mm. Increasing chocolate film to 1 mm resulted in minimal destruction of the bacteria. They concluded that ultrasound produced high temperatures during the sonication process and that its bactericidal effect may be advantageous in the chocolate conching process.

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