In today's debut of my occasional column in the New York Times Wednesday food section, I write about the strange, blue-green colors that can develop when garlic and onions are handled in certain ways. The information in the column comes from several recent papers on the subject from labs in Japan and China. For readers who would like to follow up, here are a few (slightly confusing!) chemical details, and the references.
Step 1: the garlic enzyme alliinase converts the special garlic compound alliin (an allyl cysteine sulfoxide) to a pungent garlic flavor compound, allicin.
Step 2: the garlic enzyme alliinase also converts the special onion chemical (a propenyl cysteine sulfoxide) to two substances: the "lachrymatory factor" that irritates our eyes and has nothing to do with color development, and a colorless "color developer" (a propene thiosulfinate).
Step 3: this color developer reacts with an amino acid to produce a colorless "color precursor."
Step 4: the color precursor reacts with allicin to make the pigment molecules, the pyrroles, which range from reddish to green.
Because enzymes are inactivated at temperatures above 140 degrees F or so, moderate heat moderately speeds all these reaction steps, while high heat stops the first two steps and greatly accelerates the last two.
When freshly harvested garlic is stored at cool temperatures, it slowly accumulates alliin, the precursor to pungent allicin. Stored garlic thus gets progressively more pungent and more prone to developing color.
Bai, B. et al. Increase in the permeability of tonoplast of garlic by monocarboxylic acids. J. Agric. Food Chem. 2006: 54, 8103-07.
Ichikawa, M. et al. Changes inorganosulfur compounds in garlic cloves during storage. J. Agric. Food Chem. 2006: 54, 4849-54.
Imai, S. et al. Identification of Two Novel Pigment Precursors and a Reddish-Purple Pigment Involved in the Blue-Green Discoloration of Onion and Garlic.
J. Agric. Food Chem. 2006, 54, 843-847.
Imai, S. et al. Model Studies on Precursor System Generating Blue Pigment in Onion and Garlic. J. Agric. Food Chem. 2006: 54, 848-852.