Solution to blue garlic challenge

The winner of the blue garlic challenge (published in volume 406 issue 1) is:

Wai-Yin Lau, Department of Chemistry, University of Hong Kong, Hong Kong, China

The award entitles the winner to select a Springer book of her choice up to a value of €100.

Our Congratulations!

Solution

The explanation of the “greening” or “blueing” of garlic (Allium sativum L.) and “pinking” of onion (Allium cepa L.) given here will conclude with a new challenge of its own.

Color changes of Allium tissues have been studied by several investigators since Joslyn, in 1958 [1]. In the 1960s, the reactions involved in the formation of pink pigment in onion purée were investigated [2] and a three-step reaction scheme was proposed: (1) the formation of a colorless, ether-soluble substance [the color developer (CD)] by the catalytic action of alliinase (enzyme EC 4.4.1.4) upon then-unknown precursors in the fraction of neutral and basic amino acids; (2) the formation of a colorless, ether-insoluble pigment precursor (PP) from the color developer and an amino acid such as glycine; (3) the formation of a pink pigment from the pigment precursor PP and a naturally occurring carbonyl (NOC) substance such as formaldehyde. It was then assumed [3] that a blue pigment-forming “carbonyl” was present in garlic, and when reacted with the pigment precursor PP in place of NOC from onion, a blue color would form.

Subsequently, it was found [4] that the blue pigment-forming “carbonyl” was derived from isoalliin (a substrate for alliinase, a minor precursor in garlic flavor, but the major precursor of onion flavor), and the same reactions were responsible for “greening“ of garlic and of the mixture of garlic and onion.

But our understanding of the Allium chemistry advanced significantly when Eric Block and his colleagues [58] introduced methods for the study of Allium chemistry and produced a wealth of results based on such methods. Based on their work, it was established (see Fig. 1) that the pigment precursor was a 3,4-dimethylpyrrole derivative. It is thought to be formed by condensation of the amine group of Allium amino acids with the thial/thial S-oxide formed by [3]-sigmatropic rearrangement of bis-1-propenyl thiosulfinate, in turn formed by the action of alliinase on 1-PeCSO (isoalliin). The thial/thial S-oxide is an intermediate in the formation of zwiebelanes, and is closely related in structure to (Z,Z)-d,l-2,3-dimethyl-1,4-butanedithial S,S'-dioxide, a compound isolated from onion preparations, which could play a role in forming the pigment precursor. A second key aspect to the formation of colors in Allium preparation requires the intermediacy of thioacrolein.

Fig. 1
figure1

Formation of various visible light absorbing compounds from Allium bulbs (from [15])

Using such information, a positive correlation between thiosulfinate concentration and pink pigment formation was first observed [9], and 1-propenyl-containing thiosulfinates were confirmed to be the major color-developing compounds [10]. Then in 2005, Bai et al. [11] studied the mechanisms of the green color formation in “Laba” garlic, a preserve of garlic including vinegar and sometimes sugar: both alliinase and acetic acid are required for the color formation, and the decrease in the total thiosulfinates in garlic cloves is associated with the pigment formation.

The plant chemistry, however, is always wonderfully complex, so that other studies are not useless. Imai et al. [12] established a model reaction system that comprised only well-defined constituents and reported identifications of new substances, which were involved in the pigment formation along with various conditions that affected color development. Addition of glycine suggested that proteins in the garlic juice participated presumably in the pigment formation and that they were less reactive than 75 % MeOH-soluble free amino acids, such as glycine. The active compound that yielded blue color when combined with unheated onion juice was isolated from unheated garlic juice and was confirmed to be allicin, which derived from alliin by the action of alliinase. It was also confirmed that a vivid-blue color could be produced by using a highly defined model reaction system comprising only isolated alliin, pure glycine, and purified garlic alliinase. Later, Imai et al. [13, 14] isolated two pigment precursors and a reddish-purple pigment (PUR-1) and determined their chemical structures.

In 2007, Lee et al. [15] purified the green pigment, responsible for greening in crushed garlic cloves, and they analyzed it by liquid chromatography–electrospray ionization mass spectrometry (LC-ESI-MS), fast atom bombardment mass spectrometry (FAB-MS), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and nuclear magnetic resonance (NMR) spectroscopy. The purified green pigment was highly polar and slightly viscous, with a garlic odor, and easily turned to a yellow or brown color with exposure to room temperature. The absorption spectrum in methanol showed a crude methanolic green pigment-like profile with two absorbance maxima at 440 and 590 nm. Although complete isolation of the 411 Da compound for proper structure elucidation was not achieved in their experiments, the MS and NMR spectra of the 411 Da green pigment suggested the ambiguous structural assignment of one sulfur atom and odd number of nitrogen atoms, with 25–30 carbon atoms, including aromatic ring. Therefore, it was envisioned that the green pigment observed in crushed garlic cloves was a new sulfur-containing nitrogenous water-soluble compound differing significantly from all previously reported green pigments in plants.

Obviously, the story is not fully over, but the worst is when one tries to get this color, as I did. When I simply put peeled garlic cloves in vinegar, the color did not appear, whether I boiled the system or not, and even after 2 wk of maceration. According to Block [15], garlic heads should be aged before immersion in vinegar in order for the color to appear. Let’s meet again in 4 months from now!

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Correspondence to Hervé This.

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This article is the solution to the Analytical Challenge to be found at http://dx.doi.org/10.1007/s00216-013-7464-2

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This, H. Solution to blue garlic challenge. Anal Bioanal Chem 406, 2743–2745 (2014). https://doi.org/10.1007/s00216-014-7699-6

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