Ultraviolet light (UV) and Ultrasound (US) can reduce four bacteria types on strawberries and lettuce, according to a study.
However, researchers found UV-C was less effective at reducing populations of bacterial types in strawberries when compared to lettuce.
The work looked at the efficacy of UV and US on the decontamination of inoculated lettuce and strawberries with four bacteria, E.coli, Listeria innocua, Salmonella Enteritidis and Staphylococcus aureus.
The non-thermal treatments for the inactivation of microorganisms were applied used spot inoculation.
The researchers noted that non-thermal technologies, such as ozonation, ultrasonication (US) and ultraviolet light (UV) have been applied before to food products to destroy microorganisms associated with spoilage and contamination.
E. coli and L. innocua were selected as indicators and surrogates of pathogens E. coli O157:H7 and Listeria monocytogenes. S. aureus and S. Enteritidis were chosen as they are common pathogens that can be found in ready-to-eat products, explained Birmpa et al.
Treatment of lettuce with UV reduced the population of E. coli, L. innocua, S.Enteritidis and S. aureus by 1.75, 1.27, 1.39 and 1.21 log CFU/g, respectively.
More than a 2-log CFU/g reduction of E. coli and S. Enteritidis was achieved with US in strawberries, UV treatment reduced bacteria only by 1–1.4 log CFU/g.
100 μL (10 drops) of bacterial cocktail corresponding to 107–108 of each bacteria type was spotted with a micropipette on the surface of each produce.
The maximum reductions of microorganisms, observed in strawberries after treatment with US, were 3.04, 2.41, 5.52 and 6.12 log CFU/g for E. coli, S. aureus, S. Enteritidis and L. innocua.
For UV treatment, a UV cabinet with four UV-C lamps were used with a peak emission of 254 nm. The inoculated samples were placed in sterile petri dishes and were left eight cm distance from the lamps and treated for 10, 20, 30, 45 and 60 minutes.
For the US treatment, a 5.75 litre ultrasound tank was filled with 3 litres of distilled water at an operating frequency of 37 kHz and a power up to 30 W/L. A glass beaker (600 mL) was placed in the US tank and filled with 9-fold dilution of buffered peptone water (BPW).
The effects of two non thermal disinfection processes, UV light 254 nm and US on the inactivation of bacteria and color in lettuce and strawberries were investigated.
Treatment with UV and US, for time periods (up to 45 min) did not significantly (p> 0.05) change the color of lettuce or strawberry, which could be good alternatives to other traditional and commonly used technologies such as chlorine and hydrogen peroxide solutions for fresh produce industry.
When lettuce samples were treated with ultrasound for 45 and 60 minutes, some parts of the leafy structure lost their green color.
As far as strawberry modifications are concerned, C* value, which shows the degree of saturation, purity and intensity of color changed for the samples treated with UV for 45 and 60 min and for those treated with ultrasound at the same times, compared to the control sample.
“The effectiveness of these two disinfection methods, were shown to be influenced by the dose, the exposure time and the surface of the food product. Some changes in the color of produce can be controlled if the exposure time is kept as low as possible, so as to inactivate effectively the microorganisms, but to still preserve the quality of the product,” concluded the study.
“Ultraviolet light and Ultrasound as non-thermal treatments for the inactivation of microorganisms in fresh ready-to-eat foods”
Authors: Angeliki Birmpa, Vasiliki Sfika, Apostolos Vantarakis