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It is now a relatively common practice to bottle vegetables and herbs and spices in either oil, vinegar or a mixture of both. This is done both commercially and domestically and the products should be refrigerated below 4 °C.
Products which are treated in this way include:
these products are safe if refrigerated, they represent a potential
food poisoning hazard unless certain basic precautions are taken in
This fact was unfortunately highlighted in Canada and the United
States in the 1980s when two serious outbreaks of botulism occurred in
which chopped garlic in oil was clearly identified as the source of
botulism toxin. Botulism is a rare disease, particularly in this
country, but because of its severe, debilitating symptoms and relatively
high mortality rate remains a major hazard in home preserves.
in Canada and the United States reacted to the above incidents by
preventing the sale of garlic-in-oil products in which refrigeration was
the only barrier to the growth of the bacterium, Clostridium botulinum the organism which causes botulism. These products are now required to contain an additional barrier to C. botulinum such as acidification.
1991 Australian authorities took similar precautions by regulating that
this class of product must not have a pH greater than 4.6. The pH of a
product is a measure of its acidity. Foods with a pH below 4.6 do not in
general support the growth of food poisoning bacteria including C. botulinum.
The necessary pH adjustment for these products can be achieved only by
adding acid to the vegetable material. Vinegar, which is a solution of
acetic acid, is the usual choice. Citric acid and lemon juice are other
possible acidifying agents.
to preserve these products without acidification seem to be based on
two false assumptions. The first of these is that the addition of oil
has a preservative effect. This is incorrect. The only function of the
oil is to prevent oxidation from the air in the container which can lead
to discolouration of some foods. By excluding air from the surface of
the vegetable, one is establishing anaerobic conditions which actually
favour the growth of some types of bacteria. Unfortunately, C. botulinum is one of these bacteria.
other incorrect assumption which is often made is that some herbs and
spices, and especially garlic, have significant anti-microbial
properties. The preservative effect of these materials including garlic
is slight and inconsistent as the botulism incidents in Canada and the
United States prove.
It is therefore essential that sufficient acid is added to the vegetable before oil is poured on so that any C. botulinum
or other potentially dangerous bacteria can not grow. Vinegar prepared
for domestic use contains 4 per cent acetic acid. Vinegar should be
added to the vegetable component of these preserves before any oil is
added so that the ratio of vegetable to vinegar by weight is not greater
than three to one. For example, to make 400 grams of preserved garlic,
one would mix 300 grams of garlic with 100 grams of vinegar. The
resulting mixture will then contain approximately one per cent acetic
acid which would ensure a final pH below 4.6. This will not guarantee
that the products will not spoil if not kept properly refrigerated, but
it will ensure they do not become toxic.
vegetable products e.g. tomatoes, are dried prior to being stored in
oil, a different set of circumstances applies. Correctly dried
vegetables and herbs will not support the growth of food poisoning
bacteria but they may still support the growth of spoilage organisms
such as yeasts and moulds. Moulds will usually only be a problem on
exposed surfaces but yeasts bring about fermentation in the absence of
Vegetables and herbs to be packed in oil without treatment
with vinegar should be dried almost to crispness. Tomatoes, including
sun-dried tomatoes, are a special case. The pH of fresh tomatoes is
normally just below 4.6. When the tomatoes are dried, the natural acid
components are concentrated and the pH is reduced. It will often be
close to 4.0 in the dry product and therefore the risk of food poisoning
No such safeguard exists with other vegetables,
however, and these must be either acidified or properly dried before
being covered with oil. This includes small quantities of garlic or
herbs which may be added to other preserved vegetables as flavourings.
of bottled garlic in vinegar are occasionally alarmed to find that the
product has turned green or blue-green. These colour changes do not make
the product unsafe but are obviously undesirable.
The problem of
garlic changing colour is associated with the addition of acid which
changes the normal pH of the product. This is precisely what is required
to ensure that the garlic remains safe but the change in acidity brings
about chemical changes in pigments in the garlic.
materials contain various pigments some of which change colour as the pH
of the plant tissue is changed by the addition of vinegar or other
acids. The most common of these pigments are the anthocyanins which may
be blue, colourless or red depending on the pH.
may be involved in some colour changes observed in preserved garlic but
American scientists have identified another more general explanation.
amino acids, natural components of foods, are responsible for many of
the pigment characteristics of the onion family which includes garlic.
The American scientists have shown that the outstanding difference in
composition between garlic which turns green and garlic which does not
is the presence of much higher levels of one particular amino acid in
the green garlic.
It is not possible to tell by looking at
untreated garlic whether it is likely to become green on crushing and
acidification. However the work reported indicates that if garlic bulbs
are stored for four weeks at a temperature above 23°C prior to
processing, the production of the green pigment is prevented.
This may not be practical at the domestic level, but could be a valuable precaution for commercial producers.
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Last updated: Last updated: 26 February 2015
Printed from: Vegetable preservation (http://csiroaucd2-cdc.it.csiro.au/en/Research/Health/Food-safety/Vegetable-preservation)