Michael Johnson, professor of food science in the Dale Bumpers College of Agricultural, Food and Life Sciences, replies:
The vulnerability of a certain food type to spoilage depends upon factors both internal and external to the food itself. Some foods, such as pecans or bananas, have a shell or peel that acts as a natural barrier, keeping out microbes. However, in the case of the banana, a peel is not enough. Factors such as temperatures and even natural gases produced by the banana contribute to its eventual susceptibility to microbes. Meats such as ground beef have a high protein content ideal for microbes, and the temperature of the refrigerator is not enough to deter them for long – only the freezer has temperatures low enough to keep meat from spoiling. However, a high-sugar food such as jam does not spoil at room temperature because the sugar ties up the free water that molds and yeasts would need to grow. This principle applies to dried fruit like raisins and dried beef, like beef jerky, which you see unrefrigerated in the grocery store. Similarly, some foods do not spoil quickly if acid is added to them to lower the pH. This accounts for the general safety of properly fermented foods like pickles, cheeses and sausages. Today, an important control factor for spoilage is packaging, such as the plastic containers used for deli cold cuts and cheeses.