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How to Make Salad Safe (If Not Low-Calorie)
The question often is, "If I don't have time to buy and cook vegetables, what's wrong with picking up something at the deli?" The two most commonly consumed deli salads in the United States are coleslaw (cabbage with mayonnaise and/or a vinaigrette) and potato salad. While cabbage protects against cancer, for unknown reasons, coleslaw does not. Coleslaw does contain large quantities of vitamin K that help bones use calcium and ensure that blood clots.
Mayonnaise-based dressings "suffocate" most of the bacteria and yeasts that would spoil coleslaw. This is the reason a container of creamy coleslaw does not develop a bad odor on the store shelf, and it also helps the salad retain some of its antioxidant value.
The most common contaminant of coleslaw is E. coli. This bacterium does not grow in coleslaw. An unusual characteristic of this bacterium in coleslaw is that it dies off, even if the coleslaw is held at room temperature. However, if you have enough E. coli in your slaw to make you sick when you buy it, enough bacteria will survive to make you sick several days later if you do not keep in the refrigerator. Although you definitely should keep coleslaw cold, acidity is more important than temperature in determining whether E. coli multiplies.
A small change in pH, from 4.5 to 4.3, that is doubling the amount of vinegar added to a mayonnaise-based slaw or using oil and vinegar to make the slaw, increases the rate at which E. coli dies off more than 10-fold. If you have healthy bacteria in your system from eating yogurt or by taking probiotics, the chances of being affected by E. coli contamination are even lower.
Listeria is a slightly different story. Listeria does not grow on coleslaw unless it is contaminated and then temperature-abused, that is held at over 77º F/25º C for over 48 hours. Oddly, coleslaw that has spoiled in the bag and then is put in the coldest part of the refrigerator grows more Listeria than coleslaw that has spoiled in the bag and is put in a higher and warmer shelf. Just as in the case of E. coli, however, having beneficial bacteria in your system greatly reduces the likelihood of ever developing symptoms of listeriosis.