Gluten is a huge molecule held together by smaller molecules linked together called amino acids. A very tiny part of the gluten molecule can initiate a response. If each amino acid that makes up gluten is represented as a single letter that very tiny part would be: SGQGSFQPSQQ. There are other sequences of amino acids that cause a reaction in gluten sensitive individuals, but the point is, as tiny as this fragment is with respect to the entire gluten protein, it is still HUGE with respect to the size of ethanol (the stuff you are drinking).
The alcohol you drink is ethanol. Ethanol is smaller than the size of the smallest amino acid in the smallest fragment of gluten that has been shown to initiate an autoimmune reaction. More specifically, ethanol is about 10 atomic mass units smaller than just the G in the sequence shown above.
The G is glycine, and by the way, each of these amino acids (represented by letters) by themselves is safe, and sold at most health food stores. For example Q = glutamine (yes, “L-glutamine,” the same amino acid mentioned in a recent post and used to heal intestinal damage). If the protein is viewed as beads on a string, then one of those beads might be good for you, but certain sequences strung together can initiate an allergic reaction of many types from acute peanut allergy to less-than-obvious gluten sensitivity.
When a distillation is performed, pure ethanol is separated away from all of the other “stuff” that forms as a result of fermentation. This is because ethanol is volatile (meaning it becomes a gas in the distillation process). Imagine a vat of fermentation products, you heat it, and only the volatile molecules like ethanol enter a tube attached to the vat. This tube is not just any tube - it is a curved condensation tube! Here is what it does: While the heated gas form of ethanol floats into it (because that is what gases do), the molecules are cooled and condense back into a liquid, and fall into a new sparkling clean vessel containing the stuff that intoxicates you and any other volatiles. So the fancier distillation columns that are actually used industrially also purify the ethanol away from other volatiles. Gluten does not stand a chance of “crossing over” because it is not volatile.
Here is a simplified analogy. Let's say you put some sand in the bottom of your tea kettle. If you take the spout off your tea kettle, and attach a condensing tube to the opening (a curved tube would be the simplest type of condensing tube but there are many elaborate types), you could distill your water away from the sand. The condensing tube would be curved so as to open into a new clean pot. Let us pretend that the sand is gluten and the water is ethanol. When you heat to the boiling point, the liquid becomes gas so it travels into the condenser, cools and becomes liquid, then falls into the clean pot.
Now having read that, is there any way that the new clean pot would contain any sand? No, and distilled alcohol (ethanol) does not contain any gluten. Remember, gluten is not volatile. Another non-volatile compound is table salt. So you could perform a distillation at home, with salt water. Has anyone ever inadvertently done this? Boiled a pot of salt water, perhaps to make some Tinkyada pasta, and walked away to do something else. You came back to find your pot almost empty with white crusty stuff (salt) all inside the pot.
So the gluten is left behind in a distillation process. If malt is added to the distilled product it will be disclosed on the ingredients label.
Vinegar is formed by fermentation in a similar way that ethanol is formed by fermentation. The process is to take ethanol and ferment it with bacteria. Later, there is a filtration to remove the bacteria. Rarely, vinegar is fermented from wheat-based alcohol. “Distilled vinegar,” gets its name from the fact that it was fermented from distilled alcohol.
The answer could be, perhaps, because so many people report a reaction to it and vinegar-based products. The never-ending fear is that cross-contamination during the fermentation process is leading to barely detectable amounts of gluten in the finished product (by barely detectable, I mean in terms of commercially available tests). Since the vinegar is rarely distilled post fermentation from the ethanol, the “messy” nature of the second fermentation step could pose a problem, especially for highly sensitive individuals. If the alcohol gets all used up by the bacteria, the bacteria go on to form carbon dioxide and water from the vinegar. So alcohol is periodically added in the fermentation process. Conceivably, one “shortcut” would be to just add beer at this juncture. Adding beer or some other form of cheap malted alcohol would keep the culture alive, and increase the “quality” and yield of the vinegar. Another fear is that the bacterial “mother” as it is called, contains trace gluten through cross-contamination. Claims that these practices actually take place are unsubstantiated by evidence.