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Novel technologies for cleaning and decontaminating raw vegetables and fruits

Project Code: B02005

01/11/2012

Campden BRI
Seymour, I

This research aimed to demonstrates the feasibility of applying novel ultrasound and photodynamic washing technologies to eliminate the microbiological hazards associated with fruits and vegetables.

The research, as described in this report, is the first time that ultrasound and photodynamics have been investigated as viable technologies for the washing and decontamination of MPR fruits and vegetables. Current decontamination methods are crude, poorly understood and relatively ineffective. Assessment of bacterial attachment and fresh produce surface topography highlighted the difficulty in removing and killing attached or entrapped pathogenic bacteria. Even with a fully optimised washing system with excess disinfectant the best levels of microbial reduction are 10 to 100-fold at best.

Ultrasound methods focused on process optimisation in terms of microbiological effect whilst ensuring organoleptic quality acceptability to the consumer. In combination, chlorine and ultrasound offered an additional one log reduction in attached bacteria compared to chlorine or ultrasound alone. The mechanical cleaning action of cavitation appears to remove cells which are attached on the surface of fresh produce, rendering the pathogens more susceptible to the sanitiser. However, the decontamination efficiency was less pronounced when the small scale 2 litre trials were scaled up to 40 litres. A number of factors led to a reduction in the efficiency of cavitation, most notably the treatment temperature, the wash water quality and the dissolved gas status of the suspending medium. With the potentially high capital expenditure together with expensive process optimisation and water treatment, it is not known whether the fresh produce industry would be willing to take up this novel technology. An additional one log reduction does not completely eliminate the risk of pathogens on fresh produce and current methods may be sufficient to ensure “due diligence”.

The main conclusion from the photodynamic methods was the difference in singlet oxygen sensitivity between Gram-positive and Gram-negative bacteria. It was not possible to provide an effective photodynamic catalyst surface area sufficient to kill planktonic E. coli and S. Typhimurium. Decontamination and inactivation of attached L. monocytogenes by the RB catalyst was very much comparable to that reported for chlorine, a one to 2 log reduction at best. However, this technology is unlikely to significantly reduce the levels of attached Gram-negative bacteria. Ineffective light penetration coupled with the potentially high organic loading of dirty fresh produce in a large scale photodynamic washer may decrease the effectiveness of singlet oxygen generation. Photodynamic technologies are therefore unlikely to offer the fresh produce industry with a viable alternative to chlorine.

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