Each year Woodford Reserve releases a new limited edition product. In previous years it was a four-grain bourbon, then a bourbon finished in a Sonoma-Cutrer chardonnay cask. This year, it's a "sweet mash" bourbon.
Bourbon is usually made using the "sour mash" process. To make whiskey, you ferment grains with yeast to make a beer, then distill the beer to make whiskey. The sour mash process means you take some of the beer from one batch and add it to the next batch. This is similar to how sourdough bread is made with a "starter" that lives on from batch to batch.
For this special edition of Woodford, they skipped the sour mash part (so they're calling it "sweet"), but thinking about that made me realize I was missing a few details of the sour mash process. I wasn't sure if new yeast is added to the sour mash bourbon, or if there was enough yeast left from one batch to grow and ferment the entire next batch.
I asked Woodford's Master Distiller Chris Morris. He said, "We add new yeast to every mash at WR. The yeast is cultured fresh on a rye/malt mash which is added to the mash in the ferementer. So in this case the fresh culture was added to a sour-free mash instead of to a sour mash."
Also, in my understanding, it's not just about yeast. Bacteria or fungi carried over with the sour mash also influence the flavor, and all of these help ensure consistency from batch to batch.
But it turns out the sour versus sweet mash isn't the only difference between regular Woodford and this special edition. I sampled the sweet mash limited edition next to the standard Woodford and felt the biggest difference to my palate was a sharpness, or metallic taste that is more pronounced in the sweet mash version.
Unsure what to think about that, I looked up John Hansell's review from the Malt Advocate blog. That's where I learned the other difference- that regular Woodford is a combination of pot still and column still (most bourbon is all column still) whiskey, whereas the special edition Woodfords are all pot-still whiskey. Between these two products I was not so much tasting the difference between sour versus sweet as I was tasting the difference between the column and pot stills.
As usual in my experiments, I didn't learn what I thought I was going to, but I learned something new. Good enough.