Why, when pressed garlic is put on roasting red pepper, does it turn bright turquoise?
This discoloration is the result of some complicated chemistry involving the garlic鈥檚 flavour compounds. The phenomenon is confusingly called 鈥済reening鈥�, and the food industry has encountered enough accidentally coloured batches of processed garlic for it to have generated some interest.
In the traditional Chinese pickle of garlic cloves in vinegar known as Laba garlic, the coloration is intentional. Chemists have speculated on its cause since at least the 1940s, and in the last few years Chinese and Japanese researchers have worked out what is going on.
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The flavour of garlic is generated when an enzyme called alliinase acts on stable, odourless precursors. These are normally in separate compartments in the cell but can combine if there is damage, including that caused by vinegar. The major flavour precursor in garlic is alliin () while a minor one is isoalliin ().
Key to the colour change is a product of these reactions called di-1-propenyl thiosulphinate. It can react at slightly acid pH with amino acids from the ruptured cells to form pyrrole compounds, which are then linked together by di-2-propenyl thiosulphinates to form dipyrroles. These are reddish purple, but as the cross-linking continues, molecules with deeper and bluer hues are formed. Among these are compounds called phycocyanins, which are related to chlorophylls and are found in some algae that are used as blue colouring by the food industry.
鈥淎s the reaction continues, the garlic produces compounds that have deeper and bluer hues鈥�
Keeping garlic somewhere cool increases the amount of isoalliin present, which is why the best Laba garlic is produced several months after harvest. It probably also explains the blue garlic halves in your questioner鈥檚 salad dressing taken from the fridge.
Isoalliin is also the major flavour precursor in onions. They smell different from garlic because they lack alliin and have a second enzyme that intercepts the product of the alliinase reaction to form onions鈥� characteristic tear-producing molecules. Onions do not turn blue because this second reaction leaves less thiosulphinate to be converted to coloured compounds. This explains why onions undergo 鈥減inking鈥� instead.
Meriel G. Jones
School of Biological Sciences
University of Liverpool, UK