Tom here today...
Two years ago for Christmas I got Elizabeth a vinegar crock, and we've successfully made a couple of different batches of vinegar using it -- both red wine and white wine vinegar.
We've also attempted to make apple cider vinegar using apple cores and peels, with absolutely no success – the process we used involved submerging the peels and cores in a manner similar to fermenting pickles, and all we wound up with was some moldy liquid.
Then, about a month or two ago, Elizabeth read Acid Trip: Travels in the World of Vinegar, by Michael Harlan Turkell, and it gave us insight into what's really going on with the creation of vinegar. Armed with this insight and a little more internet research, I set out to try making apple cider vinegar.
Vinegar is sometimes referred to as a twice-fermented liquid. First, a sugary liquid is fermented with yeast to produce alcohol. Next, the alcohol is consumed by acetobacter (acetic acid-forming bacteria) to form vinegar. This was the fatal flaw in our earlier apple cider vinegar attempt – you've got to start with an alcoholic beverage.
Fortunately, I have some experience fermenting sugary liquids into alcohol. I got a gallon of unpasteurized, no-additive apple cider from the store, added some yeast, popped on an airlock, and started it fermenting. I used some dry wine making yeast from the homebrew store, and did a little bit of measurement using a hydrometer, which measures the specific gravity, or sugar content of the cider. Measuring the change in the specific gravity tells you how much sugar has been converted to alcohol.
Science!
After about a week the yeast had done the trick – the gravity had fallen from about 1.052 to just over 1.0, so the alcohol percentage was right around 6.8% abv, perfect for making vinegar. Now it was time to introduce the acetobacter and think about the next process. I put some of the cider into the vinegar crock, along with some unpasteurized cider vinegar, and that's happily sitting on top of the fridge – in about 3-6 months we'll give that a try and see how it goes.
I was also inspired by this vinegar-making series by Nordic Food Lab, in which they explored the science of making vinegars. They noted that the acetobacter fermentation is different than lactic acid fermentation (the one for pickles and sauerkraut). Lactic acid fermentation is an anaerobic process, where you want to keep oxygen out – this is why you use a weight and make sure the food you're fermenting is completely submerged. In contrast, acetobacter fermentation is aerobic – so not only do you need air, you can speed up the process by introducing a supply of air. Commercial operations have a variety of different ways of introducing air. We're not a commercial operation, but we did have the fish tank pump that Elizabeth got for last spring's compost tea. After a thorough cleaning and some new hose lines, I used that.
The fish tank pump worked like a charm. Again, I added some unpasteurized cider vinegar to introduce the acetobacter. The cider foamed up quite a bit during the process, and I set it outside in the garage so that we didn't have to hear the hum. I put a little cheesecloth around the top of the jug to keep the fruit flies out, and let it sit out there for about a week.
As you can see, the vinegar is fairly opaque, a result of the opaque cider I started with. In a side-by-side comparison with cider vinegar from the store, it's not quite as sweet – likely a result of the initial alcohol fermentation. Next time, I might try stopping that fermentation before the specific gravity gets all the way down, so that there's some residual sugar. The other next thing to try is to make vinegar out of other alcoholic liquids. In addition to wine vinegar, starting with beer would make malt vinegar.
One final note – we'll use this vinegar for salads and cooking, but not for canning. While the vinegar is quite tart, and it seems comparable to the tartness we've got from store vinegar, we don't have the equipment to test the acidity, and so there's no way to be sure it's got the 5% acidity needs to ensure safe canning.