3496 J. Agric. Food Chem., Vol. 55, No. 9, 2007
Kubec and Vel´ısˇek
ABBREVIATIONS USED
of isoalliin is thus desirable and highly prized in these two
Allium species. Selecting onion/leek cultivars with a reduced
isoalliin content (and thus with a lower tendency to pinking)
would therefore cause lowering of both their pungency and their
pharmaceutical values. On the other hand, being only a minor
component in garlic [typically <5% of the S-alk(en)ylcysteine
sulfoxide pool] (15), the effect of isoalliin on organoleptic/
pharmaceutical properties of garlic is far less important. Hence,
selecting garlic varieties with suppressed isoalliin biosynthesis
may be an elegant way to completely prevent the greening
without the need of using any additives or special technological
treatment. The level of isoalliin may also be quite efficiently
governed by postharvest storage. For example, Kopsell et al.
(35) reported a significant increase in the isoalliin content in
onion stored at 5 °C. A several-fold increase was observed in
the formation of isoalliin-derived thiosulfinates (the color
developers) in homogenates prepared from garlic stored at 4
°C (36). As a consequence, Lukes (8) found that garlic stored
at 3 °C for 2-4 weeks was much more susceptible to greening
than that stored at regular temperature. Excessive low-temper-
ature storage of garlic and onion intended for industrial
processing should therefore be avoided to eliminate their
tendency for discoloration.
A, absorbance; m-CPBA, 3-chloroperoxybenzoic acid; GABA,
γ-aminobutyric acid.
Supporting Information Available: EI MS data of the newly
synthesized 1-alkenyl methyl disulfides, data showing the effect
of pH on pigment formation in model mixtures of alanine and
thiosulfinates, and data showing the resulting color of model
mixtures of various amino compounds and thiosulfinates. This
material is available free of charge via the Internet at http://
pubs.acs.org.
LITERATURE CITED
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Another important factor affecting the extent of discoloration
is the overall conversion of isoalliin into 1-propenyl-containing
thiosulfinates 1a/b-6a/b. The total content of these color-
developing thiosulfinates depends mostly on (i) the degree of
homogenization, (ii) the pH value during homogenization, and,
in the case of onion, (iii) the relative activities of alliinase and
lachrymatory synthase (37). The higher the lachrymatory
synthase activity is, the less 1-propenyl-containing thiosulfinates
are formed, with proportionally higher amounts of propanethial
S-oxide being generated instead. The latter compound (the onion
lachrymatory factor) was found not to possess the color-
developing ability (12). Such onion would therefore be more
pungent with a lower potential for pinking.
As shown in this and several previous studies, the pH value
can dramatically affect not only the formation of the color-
developing thiosulfinates (1a/b-6a/b) but also their subsequent
reactions with amino compounds. It was demonstrated that a
maximal amount of 1-propenyl-containing thiosulfinates in
homogenized garlic is formed between a pH of 4.5 and 5.0 (38).
It is also an optimal pH range for the pigment formation.
Therefore, lowering the pH of the homogenate below 4.0 as
quickly as possible is highly important to minimize the extent
of discoloration.
A lot of novel information about Allium discoloration has
recently been obtained. The natures of both garlic greening and
onion/leek pinking have been clarified, together with well-
proved identification of the key precursors and intermediates
(the color developers) (12). In this study, we have thoroughly
examined the color-forming potential of various amino com-
pounds and thiosulfinates. Moreover, Imai et al. (13) revealed
the structure of one group of colored compounds generated in
model mixtures simulating discoloration in garlic homogenates.
However, continuous research in our laboratory clearly shows
that the discoloration is an immensely complex process, yielding
hard to separate mixtures of many colored compounds even
when only simple model mixtures are studied. Further research
is aimed at identification of major colored compounds formed
during the reaction of thiosulfinates 1a/b-6a/b with amino
compounds and determination of the reaction pathways involved.