In general, there are three kinds of foods in our kitchens: perishable, semi-perishable, and shelf-stable or “nonperishable.”
Contrary to popular conception, it is not necessary for a food to be highly processed and full of chemical preservatives for it to be shelf-stable.
Basic Difference Between Shelf-Stable and Perishable Foods
The basic difference between shelf-stable and perishable foods is that perishable foods tend to have high moisture and a high pH, while shelf-stable foods tend to have low moisture, or low pH, or a combination of both.
Some foods are naturally shelf-stable because of their very low moisture content. These are foods such as dried pastas, cereal grains, nuts, certain candies or other confections, etc. Other foods, like canned foods, can be rendered shelf-stable by heat sterilization, or by use of preservatives, such as canned or bottled drinks.
As well, food can be processed or prepared in a way that reduces its water content to the point that it becomes shelf-stable. Examples are dry baking mixes (such as cake mixes), dried fruits like raisins, and any type of very dry baked product, like saltine crackers.
Most shelf-stable foods must remain sealed in their package to have the longest possible shelf-life. However, the term nonperishable causes confusion. Nonperishable is generally used as a term to encompass foods that have both medium to long shelf-lives. These are opposed to any foods that spoil quickly and would be considered perishable or semi-perishable.
Shelf-Stable Foods are Not Completely Nonperishable
So, in other words, shelf-stable or “nonperishable” foods do go bad and can never be considered completely nonperishable. All foods are subject to innate chemical reactions that cause them to deteriorate and this can happen even without the influence of bacteria or molds. Also, the environment can affect the shelf-life and quality of the food, since odors, gases, and moisture can sometimes permeate the packaging. While some shelf-stable foods, such as nuts, may only last for months, others, such as many canned foods will last for years.
Some foods are treated in such a way that they are able to be kept for slightly longer periods of time, but that still does not make them shelf-stable. They may still require refrigeration and will still only last a short time compared to a shelf-stable food. An example is pasteurized milk, the way we normally buy it. Milk, through pasteurization, is made semi-perishable. Otherwise, it would spoil within days, rather than weeks.
Semi-Perishable versus Shelf-Stable
On the other hand, a less gentle heat treatment can make even milk shelf-stable. An example of a product made shelf-stable by heat-treatment, but that still has a finite shelf-life is shelf-stable milk such as Parmalat. This type of milk uses ultra-high temperature or UHT pasteurization to increase its storage life at room temperature along with special aseptic packaging and aseptic processing. These products commonly come in aseptic paperboard containers such as Tetra Pak, which were introduced in the 1960s. These cartons are a lamination of paper, aluminum foil, and plastic. All three together give very good protection against gases, water vapor, with the inner plastic forming a hermetic seal. Lightweight and easily printable, this type of packaging is used widely also in fruit juices and other beverages and is starting to be used for soups, broths, and similar products. Visions of these containers completely replacing canned food, however, have been slow to materialize.
To describe aseptic processing simply, it involves pre-sterilizing the product before filling the sterile container in a sterile environment with sterile equipment and then closing the container in a way that is sterile. While conditions being sterile may seem like an obvious solution, it requires a much more expensive plant, and more energy and resources. Most traditionally canned products are actually further processed with heat after filling.