I'm sure someone can be more specific for you, but generally speaking a processed food is any retail food product that contains artificial flavors, artificial colors, non-naturally derived preservatives, or other chemically derived additives. Artificial flavors and colors have been around since before the 1930's, and are a complex business. I must again quote the Atlantic Monthly article, which clearly illustrates how complex these organic compounds are: "Another example of a misleading ingredient label comes from Burger King. Its strawberry milk shake lists artificial strawberry flavor as one of its ingredients. By taking a closer look, we learn that the following ingredients make up the artificial strawberry flavoring: amyl acetate, amyl butyrate, amyl valerate, anethol, anisyl formate, benzyl acetate, benzyl isobutyrate, butyric acid, cinnamyl isobutyrate, cinnamyl valerate, cognac essential oil, diacetyl, dipropyl ketone, ethyl acetate, ethyl amyl ketone, ethyl butyrate, ethyl cinnamate, ethyl heptanoate, ethyl heptylate, ethyl lactate, ethyl methylphenylglycidate, ethyl nitrate, ethyl propionate, ethyl valerate, heliotropin, hydroxyphenol-2-butanone (10% solution in alcohol), a-ionone, isobutyl anthranilate, isobutyl butyrate, lemon essential oil, maltol, 4-methyllacetophenone, methyl anthranilate, methyl benzoate, methyl cinnamate, methyl heptine carbonate, methyl naphthyl ketone, methyl salicylate, mint essential oil, neroli essential oil, nerolin, neryl isobutyrate, orris butter, phenethyl alcohol, rose, rum ether, y-undecalactone, vanillin and solvent. Simply stating artificial strawberry flavoring is not educating consumers on what they are eating and what possible effects these food products could have on their bodies." Processed food became prevalant in the 1950's when artificial preservatives became available, and they showed up in canned milk, canned meat, TV dinners, etc. Soon the convenience snack food industry overtook the local markets, Supermarkets started to pop up, and things avalanched from there. It's difficult to find non-produce products that are not 'processed' in some way. Curiously, if one overlays the general per-capita cancer rate over the per-capita consumption of processed foods from 1950-current, the graphs are nearly identical.