Discolouration of bottled garlic in vinegar
Users 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.
All plant 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.
These pigments may be involved in some colour changes observed in preserved garlic but American scientists have identified another more general explanation.
Certain 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.
This excerpt taken from
Food Science Australia Fact Sheet