How common is gluten?
Gluten is not usually present in fresh from the garden, fruits, vegetables and roots. In other words, if an apple is picked from a tree, the apple should be free from gluten.
Gluten can be introduced:
(1) during germination (e.g. from crop rotation)
(2) during preservation (e.g. waxing apples with a substance containing gluten)
(3) during food production (e.g. factory belts, shared processing equipment, flour dusting (often used in factories that process sticky foods, wooden production equipment, etc...)
(4) during processing. Cross contamination (shared utensils, shared surfaces, cast iron skillets, wooden cutting boards, flour dust in the air, shared steamers, shared toaster ovens, ovens, shared flour sifters, shared strainers, shared deep fryers, shared condiments, etc...) during food production.
Most factories produce products that contain gluten. For this reason, most packaged foods contain gluten. Many gluten-free labeled products are made in gluten-free facilities, are made on production lines that use allergen-friendly sterilization processes, or are tested to be below the International standard which is currently 20 parts per million (less than a bread crumb).