It’s because it comes pre-contaminated, reader Glen. Rye is odd stuff as cereal grains go. So odd in fact that it wasn’t widely cultivated until about 400 A.D., many thousands of years after wheat was first domesticated. The reason, probably, was because rye didn’t (doesn’t) make especially good bread, even though it’s a very close cousin of wheat.
Rye has other virtues, however. It grows well in poor soil. It has big thick roots which help it resist drought. It can withstand temperatures that would kill most other cereal grains, it’ll even stay alive when it’s covered with snow. That’s a handy thing, and indeed many North American farmers like to plant rye in the fall after other crops have been harvested, because a covering of rye helps prevent soil erosion and compaction in the off-season.
But where was I? Ah yes, resilience. You can see why rye has always made a handy “plan B” crop for poor farmers. It may not make an especially good boule, but you can grow it on bad, nutrient-poor land, and odds are it’ll still be there in the bad years when the more toothsome wheat harvests have failed.
But rye has some unusual behaviors. One of the most unusual is that its berries will often germinate while they’re still on the ear, before they’ve fallen off the stem into the soil where they can take root. Strange. And because these germinated berries are still on the ear at harvest time, they end up being collected along with all the other un-germinated rye berries and eventually milled into flour.
What difference does that make? Well as you may recall from previous discussions about germination, it’s a process whereby a berry or seed sends out a shoot. The outer shell cracks open so the shoot can emerge. At the same time enzymes in the berry activate and begin the process of breaking down the long-chain starches stored in the endosperm into the simple sugars the shoot needs for fuel.
And that, friends, presents a big opportunity for invading yeasts, molds and bacteria. Prior to germination, all those micro-critters were waiting patiently on the outside of the berry, unable to get in to the banquet. That very same film is on wheat (indeed it’s on most fruit and berries…grapes and plums for instance). However the key difference with rye is that all the little bugs aren’t just waiting around on the surface of the berry for the dinner bell to ring. They actually inside many of the berries, feeding and reproducing.
So you can see where I’m going here. Rye flour has all this stuff in it when you buy it at the grocery store. Sure it looks like flour, but what it really is is a riot of starch, sugars, enzymes, yeasts and bacteria. No wonder it makes a starter so readily — just add water! In fact many bread starter-makers like to begin their white or wheat flour starters with rye flour because it takes off so readily. Once the culture is growing they just switch to another type of flour. Not a bad plan, actually. Works great.