Coring Lettuce Increases E. coli Risk
Recent research from the University of Illinois and the U.S. Department of Agriculture’s Agricultural Research Service has determined that field coring lettuce – the practice of removing the lettuce core in the field – dramatically increases the risk of the lettuce harboring E. coli contamination.
Many prepackaged salad producers core lettuce in the field as it is being picked by farm workers. The two 2012 studies found that because the workers maintain contact with the soil while picking and coring, both the coring knives and the coring ring can maintain and transfer Escherichia coli microorganisms from the soil onto the leaves of lettuce after coring. Once there, the hardy bacteria can continue to grow, even sometimes surviving the washing process.
Removing the core from lettuce allows the leaves to be easily packaged. Coring is typically done in the field for prepared salad mixes packaged into plastic bags. The coring is done in the field because it increases the efficiency of the process that takes the lettuce off the farm. At the plant, the loose leaves are then cut, washed and packaged into bags.
The University of Illinois at Urbana-Champaign research found that frequently washing the coring rings and knives with chlorine during the coring process reduces the risk of contamination. They also developed a new coring device that also cuts down E. coli contamination.
Over the past decade a number of E. coli outbreaks have resulted from cored and bagged lettuce or spinach packaged into plastic bags. Theories ranged from the farms being too close to dairy farms to organic farming methods – both of which have since been debunked.
Buying whole heads of lettuce uncored not only reduces the risk of contamination. It also reduces carbon emissions used for the extra manufacturing processes.
Coring lettuce at home is simple and fun. Iceberg lettuce can be decored simply by smashing the head core-first onto a cutting board. Romaine and other lettuces can be decored by simply pulling off the leaves.
Refrigeration is vital for storing lettuce whether cored or not. Another study – this from Korea’s Chung-Ang University – found that maintaining temperatures below 5 degrees C (41 degrees F) deters the growth or kills off Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Listeria monocytogenes, and E. coli. Temperatures above 15 degrees C (60 degrees F) allowed most of these microorganisms to grow.
Because the plant’s immune system typically defends against E. coli contamination when growing, maintaining the core and the lettuce head through to the time of salad preparation in the kitchen is an easy and less expensive way to reduce the risk of contamination. And under no conditions should a bagged packaged lettuce or even a fresh head of lettuce be left out of the refrigerator for very long.
Yang Y, Luo Y, Millner P, Turner E, Feng H. Assessment of Escherichia coli O157:H7 transference from soil to iceberg lettuce via a contaminated field coring harvesting knife. Int J Food Microbiol. 2012 Feb 15;153(3):345-50.
Tian JQ, Bae YM, Choi NY, Kang DH, Heu S, Lee SY. Survival and growth of foodborne pathogens in minimally processed vegetables at 4 and 15 °C. J Food Sci. 2012 Jan;77(1):M48-50.
Bezanson G, Delaquis P, Bach S, McKellar R, Topp E, Gill A, Blais B, Gilmour M. Comparative Examination of Escherichia coli O157:H7 Survival on Romaine Lettuce and in Soil at Two Independent Experimental Sites. J Food Prot. 2012 Mar;75(3):480-7.
Zhou B, Luo Y, Millner P, Feng H. Sanitation and Design of Lettuce Coring Knives for Minimizing Escherichia coli O157:H7 Contamination. J Food Prot. 2012 Mar;75(3):563-6.