FEATURE ARTICLE
Safer Salads
Contaminated fruits and vegetables are more common than ever. Why? And what can consumers do to protect themselves?
Jorge M. Fonseca, Sadhana Ravishankar
After the Harvest
Although tainted water and soil splashed onto plants account for a share of contaminated produce, pathogens are often transmitted to produce by people whose hands or tools are dirty with human or animal feces or their own infection. Poor hygiene of food handlers is an important source of contamination for store-bought produce. Some agricultural workers assume that because plants grow in the soil, they don't need to follow hygienic practices. Moreover, consumers often forget or undervalue the importance of safe handling during food preparation.
Proper postharvest handling is more critical for produce with irregular or wounded surfaces. Any type of injury during any part of production and handling may permit the entry of pathogens such as E. coli O157:H7. Thus, it is critical to discard fruits or vegetables dropped on the floor. Even on unblemished surfaces, microorganisms can attach and form microcolonies, each colony walled behind a tough polysaccharide to form a biofilm. Many types or species of bacteria can occupy a biofilm, which may take hours or days to develop.
Biofilms can form on surfaces such as food-processing equipment and on the food itself. Indeed, most vegetables and some fruits provide ideal conditions for bacterial growth: high moisture content, high nutrient levels and near-neutral pH. Even before you bring them home, common salad vegetables (tomatoes, carrots, lettuce and mushrooms) purchased from grocery stores have been colonized by bacteria; the microcracks and rough surfaces of many types of produce are excellent sites for bacterial attachment and biofilm formation. Bacteria can enter through these cracks and be internalized by the plant tissue, a process that accelerates when warm produce is washed in cool water. Any bacteria in the water can enter the core of the product through the stem scar because of the pressure difference between cold water and warm core. We recommend washing in lukewarm water instead.
Stem scars and other inaccessible places on fruits or vegetables provide surfaces that can protect bacteria or biofilms from washing or sanitizing treatments. Irregular or rough surfaces, such as those found on leafy vegetables and cantaloupes, are ideal places for microbes. There, according to research, bacteria can resist our best efforts to get rid of them. As a result, the most effective strategy is to keep the produce free from harmful pathogens in the first place. If pre- and postharvest practices are stringent enough, we should be able to eat any produce with confidence.
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