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Alcohol and vinegar P 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 allergen?  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.