Although what you do in your kitchen is technically processing, processed food refers to the products of food manufacturers or "processors." These products are literally manufactured in industrial conditions. They include almost any food packaged in a box, bag, jar, or can, including foods such as bread, pizza, ice cream, cake, or frozen dinners.
I mentioned foods that don't seem like they should be industrial: bread, pizza, ice cream. If those foods were made at home, they could be simple and natural with minimal processing. The ingredients of bread can be as simple as whole wheat flour, yeast, and water. Pizza is a similar dough with a topping made from cheese, tomatoes, olive oil, and herbs. Ice cream can be made from milk, sugar, and eggs.
But in the supermarket, these foods are complicated, with a long list of ingredients. These foods are not simply "baked," "made," or "cooked"—they are manufactured, or industrially processed.
(See this list for examples of foods that are industrially processed when they appear in the supermarket.)
These foods are manufactured under high temperatures and high pressures that degrade the ingredients. Today's food is manufactured in large quantities (for economy of scale) and shipped long distances (for market share). The foods are "products" that need a long shelf life to survive the mass production and long-distance shipping.
If any part of an ingredient might cause the final product to spoil, that part is deliberately removed or transformed during processing. Unfortunately, those spoilable food parts contain most of the original food's nutrients and taste! Typically, spoilable food parts are replaced with ultrarefined or synthetic ingredients. In fact, today's food products often have a negative nutritional value, because the body needs to pull from its own mineral stores in order to break down and eliminate the ingredients.
Processed foods often consist primarily of refined grains, sweeteners, refined salt, low-quality artificial fat, and food additives.Others begin with whole foods that are then cooked and/or treated or mixed with additives. Even a piece of raw meat from the supermarket can contain processed ingredients as additives, according to this article in the Washington Post.
The food additives serve many purposes. Additives substitute for traditional ingredients, such as butter or eggs, that give texture to food. They allow the unfinished product to be mixed or baked, and give the final "product" the look and feel of traditional food. They replace the taste lost in processing and are engineered to satisfy tastes for richness, sweetness, saltiness, and fat. A few select nutrients, in synthetic form, are sometimes added back to the product. Additives are at best manufactured from food sources; frequently, additives are synthetic.
Most of today's food products are derived from just a handful of crops. Wheat, corn, soy, and rice supply two-thirds of American calories, almost entirely through ingredients of processed foods.
Processed food is linked to "Western diseases" such as heart disease, diabetes, and cancer, and is probably the primary source of much ill health. "Western diseases" were rare before these foods were widespread, and were unknown in areas in the world that cooked and ate in traditional ways.
Dirty Secrets of the Food Processing Industry
by Sally Fallon, Weston A. Price Foundation
This is an important and very readable article! It describes the abuse done to food by industrial processing: what happens when breakfast cereal, milk, orange juice, soup, and vegetable oil are manufactured by food processors. The details are shocking, and very unappetizing.