When researching gluten, you'll find the term has two commonly used definitions - one that describes the storage proteins common to most grains, and one that's relevant to those of us following a gluten-free diet.
Gluten, in its generic form, simply refers to the proteins grass plants build into their seeds (which we know as grains) to support the growth of the next generation of plants.
Almost all grains have gluten -- corn gluten, for example, works well as a fertilizer and a weed suppressor, while glutinous rice appears in many Thai dishes. However, those grains -- and the gluten in them -- are perfectly safe for people with celiac disease and gluten sensitivity (also known as gluten intolerance).
It's the gluten that occurs in a specific sub-group of grains -- the Pooideae subfamily of the Poaceae family of grasses -- that causes specific reactions in those of us who have celiac disease or are gluten-sensitive. The Pooideae subfamily includes wheat, barley, rye and oats.
Gluten Gives Dough Elasticity, Structure
The gluten in wheat, barley and rye actually consists of two proteins: gliadin and glutenin. When the two combine during the baking process, they form a thick, stretchy, glue-like substance that provides bread and other baked goods with elasticity and appealing texture.
Gluten also helps bread dough rise by trapping (literally gluing) bubbles from fermenting yeast within the dough itself, allowing the dough to rise into a light and airy loaf. Sadly, the gluten proteins in other grains don't provide this same ability, which is why it's so difficult to find decent gluten-free bread.
It's these qualities in baked goods that gave rise to the popularity of modern wheat (and to a lesser extent barley and rye). In fact, modern wheat has been bred to contain far more gluten than older varieties of wheat such as Einkorn wheat and spelt wheat, which makes modern wheat more toxic for those of us who react to it.
And it's no wonder gluten is so difficult to avoid: Wheat is the number three crop in the U.S. (behind only corn and soybeans), with 47.6 million acres in production in 2010 and 2.2 billion bushels grown (in comparison, only 3.6 million acres were used to grow rice in 2010).
When you combine the statistics for wheat, barley and rye, more farm acres are used to grow gluten grain crops than any other single crop -- 50.3 million acres produced close to 4 billion bushels of gluten grains in 2010.
'Gluten-Free' Means No Wheat, Barley or Rye Gluten
In most cases (certainly outside of agriculture) anyone referring to "gluten" or "gluten-free" means the gluten contained in the grasses wheat, barley and rye (and sometimes oats). These specific types of gluten proteins cause the reactions seen in celiacs and those with gluten sensitivity.
Unfortunately, because these grains -- especially wheat -- are so ubiquitous in our food chain, eating gluten-free involves far more than simply substituting gluten-free bread and other baked goods for the gluten-containing varieties. Gluten-containing ingredients appear in many processed foods, and anyone following a gluten-free diet needs to learn what terms mean 'gluten' on food labels.