Why is it that so many lists of forbidden foods put a taboo on vinegar? You read about it in some national groups' food guides. You hear about it at your local meetings. Club newsletters contain warnings to always check with the manufacturer. Friends will tell you to stick with certain kinds of vinegar: rice, wine, apple cider... what gives?
The process by which vinegar is made is theoretically quite simple (liquid ingredients are fermented until they turn acidic, and then the resulting liquid is filtered and diluted) but the steps actually taken are many and complex. You almost need to be a scientist to understand what's going on. This complexity can be intimidating to lay people worried about the possibility of letting gluten sneak into their diet, and when you look at even the simple steps, it's easy to see the Ghost of Gluten Possible haunting every step. If wheat is the source of the original liquid, will gluten be in it? What's in the stuff they add to start the fermentation process? Add to this the fact that many celiacs have other sensitivities (unrelated to celiac disease), and all it takes is a few people saying, "Well when I use vinegar, I get sick..." to start a sort of Urban Legend That Will Not Die.
But the fact is that vinegar in most forms is safe (avoid malt vinegar and check on any flavored vinegar that has had other ingredients added afterward). The following article on the production of distilled vinegar was written by Richard Abrams, a celiac himself and a retired biochemistry professor (University of Pittsburgh). The research and interviews presented here, along with the thorough description of the processes should lay to rest all doubts about consuming distilled vinegar in a gluten-free diet.
Concerning gluten in vinegar, I personally do not think there is any, at least not enough to have any effect on a celiac. There is a wide-spread belief that because a small part of the alcohol made in this country may come from the fermentation of wheat starch, some surviving gluten or gluten peptides will somehow be carried over in the distillation process to contaminate the final product. Since I do not feel that the celiac diet should be restricted any more than is necessary, I posted a rather long statement on this subject about two years ago. If I can find a copy in my files and if I do not get into trouble in trying to transfer it, I will add it to this note.
ALCOHOL AND VINEGAR - SOURCES OF GLUTEN? It has been common practice among celiac support groups and others offering advice on gluten-free diets to proscribe white (distilled) vinegar and alcohol as potential sources of gluten. The assumption is made that any product manufactured from a gluten-containing grain is probably contaminated with gluten proteins or peptides. But to those with some chemical training (myself plus others who have posted comments on this subject in the past), distillation is a highly effective method for separating volatile substances such as alcohol from non-volatiles such as proteins or peptides.
Vinegar, especially white vinegar, is a common ingredient in commercially prepared foods. Because I object to dietary restrictions that are unnecessary and that further limit an already restricted diet, I have tried to learn something about the manufacturing process. The following summary is for others who might like to know how white vinegar comes into being and what the level of gluten contamination, if any, is likely to be. My sources of information have been the biotechnology literature and telephone contacts with knowledgable people in the industry. I personally have had no hands-on experience in vinegar making.
FROM GRAIN TO STARCH TO FERMENTABLE SUGAR: White (distilled) vinegar is of course not distilled, but the ethyl alcohol from which it is made is distilled from a yeast fermentation mixture. (In the UK, however, I believe that 'distilled vinegar' has a different meaning, that it is made from malt and that it is in fact, distilled.) In most of the world, molasses, which can be fermented directly by yeast, is the major source of alcohol. Alcohol is also made synthetically from petroleum products but I do not believe that alcohol from this source is much used in the food industry. In the U.S., starches derived from grains are the major source, mostly (about 85%) from corn.
Starches are mixtures of large straight and branched chain polymers of the simple sugar, glucose. Since yeast is incapable of fermenting starches, whether from corn, wheat, potatoes or any other source, the grain starches must be pre-digested with amylases, enzymes that are capable of splitting the starch molecules into smaller fragments. Depending upon the nature of the amylases, the end products are usually maltose (a disaccharide of two linked glucoses), some free glucose, and small amounts of assorted dextrins (small polymers of more than two glucoses). In the brewing industry, the source of the amylases is usually barley malt, barley that has been allowed to sprout until the amylases and proteases needed to digest the nutrient stores in the seed have developed and then heated enough to stop the sprouting without inactivating the enzymes. For industrial alcohol, including the food industry, it is common to replace malted barley with cheaper, partially purified amylases prepared from bacteria (several species of Bacillus can be used) and/or a mold such as Aspergillus niger or Aspergillus oryzae.
ALCOHOLIC FERMENTATION: After the starch is largely transformed by the amylases, yeast is added and the temperature adjusted to initiate fermentation. The small polymers in the digest such as maltose (glucose-glucose) and maltotriose (glucose-glucose-glucose) are hydrolyzed by the yeast cells to glucose. Glucose is then converted by fermentation to ethanol and carbon dioxide, releasing energy in the process which is used by the yeast for growth. Fermentation results in conversion of roughly 90% of the original starch to alcohol plus smaller amounts of other volatile products such as aldehydes, ketones, fusel oils (higher alcohols), phenol derivatives, and esters. These volatile contaminants which vary with the yeast strains used and with the bacterial contaminants in the fermentation mixture contribute significantly to the flavors of alcoholic beverages but must be removed in the production of purified neutral spirits.
ALCOHOL IS PURIFIED BY DISTILLATION: After fermentation, alcohol and other volatiles are separated from the non-volatile components by distillation. There is often a first crude distillation in which the fermentation mixture is boiled and the vapors are condensed back to a liquid phase that contains about 50% alcohol plus other volatile products. While vigorous boiling can drive off microdroplets of the pot liquor (that contain non-volatile components) along with the alcohol containing vapors, industrial fractionating stills are built as tall refluxing columns with sections of transverse plate barriers that are designed to trap entrained droplets followed by multiple plates to rectify the vapors, i.e., to concentrate the alcohol, separate it from the other volatile constitutents (and purify them as byproducts), and to act as a further barrier against non-volatile material. The problem for the distiller is not the relatively simple task of avoiding carry over of the non-volatile material in the distilling pot, but rather the separation of the various volatile products from one another.
IS THERE NITROGENOUS CONTAMINATION IN THE DISTILLATE? I spoke to an official of a firm in Iowa that provides U.S.P. grade 95% ethanol to Heinz and other vinegar producers. I was informed that their product which undergoes repeated distillations contains no detectable nitrogen using an assay whose limit of detection is 0.1 parts per million. Assuming that all the hypothetical nitrogenous impurities are protein or peptide, this limit level corresponds to about 0.7 mg protein per liter of 95% alcohol. The alcohol undergoes about a 20-fold dilution in its conversion to vinegar (4 to 5% acetic acid), so that, if all of the hypothetical protein survives the acetous fermentation, its final concentration is less than 0.035 mg per liter of vinegar. This worst case scenario represents the limit of detection; the actual amount present may be far less, and if it exists, it may have originated from yeast protein (non-gluten) as well as from seed protein.
IS 0.035 mg/liter DANGEROUS? Assuming that the maximal daily consumption of vinegar by an average person is of the order of 30 ml ( 6 teaspoons), the daily dose of protein or peptide originating in the yeast fermentation mixture would be less than 0.001 mg (1 microgram). The significance of this maximum level can be judged by comparing it to the estimated gluten level in European GF diets that are based on wheat starch from which the gluten has been removed by washing (see the March 16 summary by Bill Elkus of a CelPro discussion of a possible gluten tolerance level). Estimates of residual gluten in these "gluten-free" diets are in the range of 4 to 40 mg per day. To reach the lower estimate of this possible "tolerance" dose would require a daily consumption of at least 100 liters of white vinegar. This might well be dangerous to one's health, but not because of its gluten content.
THE CONVERSION OF ALCOHOL TO VINEGAR: I should perhaps comment on the conversion of alcohol to vinegar, a process in which no distilllation or elaborate purification is involved. This process involves a second fermentation (the alcoholic yeast fermentation being the first) that uses bacteria, a species of Acetobacter, in place of the yeast. The procedure is basically the same whether the alcohol is U.S.P. grade or the crude alcoholic mixtures in fermented apple or grape juice. The bacteria in the vigouously aerated vinegar reactor may be in suspension or on the surface of wood chips, and the liquid phase contains in addition to the alcohol source, a nutrient mixture to keep the Acetobacter growing while they oxidize alcohol to acetic acid. The nutrient mixture is said to consist of a variety of salts and some carbon and nitrogen sources such as glucose, citric acid, ammonium phosphate, some yeast extract or dried yeast, and hydrolyzed soy flour. At the conclusion of the fermentation the vinegar is not distilled, but rather is filtered to remove microorganisms and particulate material and diluted to bring the acetic acid level down from values as high as 15 to 20% to roughly 5%.
CAN GLUTEN BE INTRODUCED DURING THE ACETOUS FERMENTATION? Since it is possible that traces of the nutrient mixture could persist into the finished product, one might wonder whether gluten-containing grains were ever used instead of or in addition to soy protein. I spoke to the president of one of the major suppliers of nutrient mixes to vinegar manufacturers. While he would not give me a detailed list of ingredients (trade secret!), he assured me that his product is gluten-free, and he further stated that he was familiar with the composition of other such products in use on both sides of the Atlantic, and that they were all gluten-free. The nutrient mixture is used in cider vinegar as well as in white vinegar, although in significantly smaller amount.
CAN VINEGAR BE A FOOD ANTIGEN? I have not searched the allergy or toxicology literature. It is evident that with any vinegar there is a finite content of dissolved solids that consists of inorganic salts, trace metals, and nitrogenous and other organic materials that could originate from the nutrient mix or as byproducts of the metabolism, death and lysis of the Acetobacter, or in the case of cider vinegar, from the variety of materials pressed out of the apple cores and peels and juice or produced in the yeast fermentation that yields alcohol for the subsequent acetous fermentation. Total solids in vinegars may approximate several percent, practically equal to the acetic acid concentration, and there may be constituents to which some people, celiac or not, are sensitive, but it is inconceivable to me that a gliadin or a gliadin peptide could be one of those constitutents.