Combining sanitizers with ultrasound, optimizing oxygen conditions and not reusing washing water can reduce bacterial contamination of lettuce and leafy greens, according to a summary of recent USDA laboratory research.
The findings are published in the July issue of the US Department of Agriculture's (USDA) science monthly, Agricultural Research.
The research is likely to be of interest to growers, processors and retailers, as an outbreak of a foodborne disease can be extremely costly, as well as potentially fatal for the consumer.
The USDA said that researchers at its Produce Quality and Safety Laboratory (PQSL) in Maryland have been focusing on ways to keep salads safe at processing plants before and after bagging.
Microbiological safety is a key issue for ready-to-eat prepared vegetable tissues, because they are intended for consumption raw, without further preparation or cooking.
Bacteria can contaminate crop plants in situ during the growth of the plant or during harvesting, handling, processing, distribution or preparation.
PQSL technologist, Yaguang Luo, in an examination of wash waters and sanitizers, simulated washing methods to analyse how processing practices could affect safety and quality of precut lettuce, stated the ARS article.
For the study, according to Agriculture Research, romaine lettuce leaves were sliced and then rinsed in either fresh wash water or various types of reused wash waters. The washed leaves were then dried, placed in bags made from special oxygen-permeable films, and stored at 5°C.
The article stated that microbial growth and product quality were monitored at various intervals over 14 days of storage and the results at the end of storage time showed that unwashed control leaves and leaves washed with reused water had higher bacterial counts than those washed with clean water.
The study was first published in HortScience.
Luo, in partnership with Illinois University, has also evaluated the effectiveness of combining a sanitizer with ultrasound treatment for industrial-scale produce washing, with her study demonstrating that this approach could cut bacterial contamination from about 300,000 colony forming units to 10, stated the article.
Another PQSL study by microbiologist Arvind Bhagwat analysed bacterial resistance to different temperatures and its findings were originally published in the April 2008 issue of the Journal of Food Science.
Bhagwat's aim, according to the article, was to discover if a lack of oxygen would hinder a bacterium's survival in the human gut.
The study was prompted by the fact that manufacturers have been using a type of low-oxygen based modified atmosphere packaging (MAP) to extend the time that bagged salad appears fresh, according to Agriculture Research.
Bacteria were applied to fresh-cut lettuce and stored under various MAP conditions for eight days for the study.
Low-oxygen conditions were defined as between 0.5 per cent and one per cent oxygen in a sealed package and regular oxygen conditions were defined as 20 percent oxygen in a sealed package.
The article stated that the results showed that "when stored under extremely low-oxygen conditions and at temperatures of 15°C or above, bacteria became more resistance to synthetic gastric juice. In comparison, no resistance was induced among bacteria stored under extremely low-oxygen conditions and at temperatures of 10°C or below"
"The findings also highlight the importance of responsible use of MAP. Proper storage temperature is important to minimizing bacterial adaptability," said Bhagwat.
The PSQL research was published in the USDA's Agricultural Research magazine (July 2008, Vol. 56, No. 6).