Agriculture Reference
In-Depth Information
8
F
OOD
S
CIENCE OF
A
LLIUMS
This chapter reviews aspects of biochemistry that are special to alliums, all of
which are relevant to their properties as foods, both raw and processed, and also
to their well-known health benefits. Subjects reviewed are the biochemistry of
the flavour compounds, aspects of carbohydrate biochemistry and, thirdly, the
flavanoids responsible, among other things, for the colour of onion bulbs. The
evidence for health benefits from allium vegetables is then discussed. The
chapter ends with a brief consideration of the use of allium vegetables for various
processed foods and for making extracts for flavouring or medicines.
Allium vegetables are grown primarily for their distinct flavours. The nature,
biosynthesis and properties of the compounds that give rise to flavours have
been slowly uncovered by research spanning from the late 1940s to the present
day. The chemistry of allium flavour is complex, because the sulfur-containing
compounds responsible for flavour are labile and reactive. They are released
only when cells are damaged by slicing or crushing.
This sets in train a series of reactions starting with the release of an enzyme,
alliinase, present only in the vacuoles of undamaged cells, giving it access to
flavourless precursors that are stored in vesicles in the cytoplasm. The alliinase
converts the precursors to reactive and strongly flavoured thiosulfinates that are
the primary flavour compounds. The quantity and nature of the thiosulfinates
released depends in part on how thoroughly the tissue is macerated and for how
long the enzyme can act before being destroyed by cooking or processing.
What compounds are formed from thiosulfinates depend on cooking or
extraction conditions. More than 80 different compounds have been charac-
terized, formed by the interaction among the initial thiosulfinates and the
subsequent cascade of reaction products in fresh, and also steam-distilled,
extracts of alliums. These substances include polysulfides, thiosulfonates and
compounds with an S in ring structures, e.g. dithiins. The organic chemistry of