Rye isn’t a wheat at all, though it is a grass (Secale cereale) that produces a grain. That grain is useful in many of the same ways that wheat is useful, but truth be told, you’ve really gotta love rye if you want to make things out of it.
Why? Because unlike rye’s housebroken cousin, wheat, it resists coming to heel. For one, rye grain has the nasty habit of germinating (sprouting) before it can even be harvested, which means that the enzymes stored in the germ of the seed have already become activated. Why is this a problem? Because enzymes digest starch. Their job is to break down the long-chain starch molecules stored in the endosperm into energy-giving sugars that the sprout can use to grow. This isn’t a big deal from the standpoint of endosperm consumption, since there’s still plenty left for us humans when we finally get around to harvesting it. However it does mean that the flour that’s eventually made from the grain contains copious amounts of those same active enzymes. Mix that flour with water, and the enzymes continue their work, digesting starch at an accelerated rate, undermining a dough’s ability to rise.
Rye bread’s rising ability is further compromised by the fact that rye’s protein profile is different from ordinary wheat. Whereas wheat contains ample amounts of both gliadin and glutenin (the two components of gluten), rye flour has gliadin and glutelin, a protein that lacks glutenin’s ability to form end-to-end bonds. No end-to-end bonding means no long, stretchy gluten networks, no stretchy gluten networks means no (or limited) bubble-holding ability, no bubble-holding ability means all the gas and steam escapes, and well…you get the idea. This is why most rye breads are made from both rye and wheat flour, a mixture known as maslin.
All is not lost in the dicey world of rye, however, for rye grain does contain one unique ace-in-the-hole: a gummy cell wall goo called pentosan gum. And while one of pentosan gum’s effects is to even further undermine gluten formation, the gum itself traps and holds gas bubbles, contributing to rise. Pentosan gum also traps and holds water molecules, creating a bread that is at once moist and less inclined to staling. Thus even oddballs have their virtues (ask my mother, she’ll tell you).
One word of warning about pentosan gum: if you knead a rye dough for too long the gum leaches out of the rye granules and turns the whole dough into a giant sticky mess. No matter how much more flour you add you can never un-stick it, because more rye flour is just more fuel for the proverbial fire, as it were.
Look for rye flour in light, medium and dark varieties. As with wheat flours, the darker the shade, the more of the bran and germ that’s been left in it. Organic rye flour is rare, but can be found, and is excellent for getting bread starters going. The reason has to do with the premature germinating that rye is famous for. Sprouted grain is a magnet for yeasts, mold and bacteria (all of which are seeking a place at the table for endosperm banquet), and are very handy when it comes to beginning a culture.