Few lunch boxes, school lockers, or backpacks are refrigerated, so it's important for the sandwich make to know a bit about food safety and food spoilage. The two most important principles are keeping temperatures cool and avoiding cross-contamination.

To minimize the growth of bacteria and other organisms that can cause food spoilage or food-borne illness, remember the basic formula 4-40-140: Perishable foods should spend no more than 4 hours at temperature between 40 and 140 degrees Fahrenheit. (Higher temperatures kill spoilage organisms, and lower temperatures keep them from growing.) By the end of four hours, bacteria may have multiplied to unsafe levels. The effect is cumulative, so food that has sat out at room temperature for two hours and then been returned to the refrigerator has only another two hours of room temperature shelf life left unless it has been cooked again.

What are perishable foods? Basically, those which are high in protein and moisture but low in acid, salt, or sugar (all of these are preservatives). Highly salted or dry-cured meats, such as prosciutto or dry salami, and most cheeses can stand room temperature much better than, say, home-cooked turkey breast or egg salad, Incidentally, mayonnaise has gotten a bad rap in terms of spoilage. Many cooks have an extreme fear of putting mayonnaise into sandwiches that will be out of refrigeration for more than a matter of minutes, fearing that it will spoil quickly. In fact, commercial mayonnaise contains enough vinegar to prevent spoilage for hours. (This is not necessarily true of homemade mayonnaise.) The meat will probably spoil sooner than the mayonnaise. In any case, observe the four hour rule and you should be safe.

In practical terms, a sandwich made in the morning from properly stored ingredients and taken to school or work should be fine at noon ~ but it is suspect by midafternoon.

The other principle to remember in sandwich making (as in all cooking) is to avoid cross-contamination, that is, reinfecting cooked foods with bacteria from raw foods (mostly meats). Prevention is simple. Tools, hands, and surfaces used for handling raw meats need to be thoroughly washed before being used for any food that will not be cooked. Most delicatessens do not sell raw meats, but if they do, they should have separate handling areas (including slicers, if appropriate) for raw and cooked items.

This information is from The Art Of The Sandwich by Jay Harlow.