Agriculture Reference
In-Depth Information
quality control once the characteristics of a cultivar have been established, but
initial evaluations require taste-panel assessment. Interestingly, some cultivars
are perceived as 'sweet' although they have quite a high pyruvate score,
indicating a high ACSO content. Differences in sugar content could not explain
this anomaly (Crowther
et al.
, 2005). This presents the interesting possibility of
'sweet' onions that are likeable raw but which have a high content of the health-
promoting substances that derive from ACSOs.
A laboratory-based colorimetric method is routinely used for pyruvate
determinations in onion flavour assessment (Crowther
et al
., 2005). Disposable
biosensors for pyruvate concentration in macerated onion juice have been
developed (Abayomi
et al.
, 2006). These measure the electrical signal generated
by a reaction involving the enzyme pyruvate oxidase immobilized on an
electrode surface. The biosensors can be used to assess flavour strength on site
rather than in a centralized laboratory and can reduce the time and cost of
routine assessments for onion quality control 12-fold.
The biological function of flavour compounds
The flavour precursors give rise to many compounds with strong physiological
effects on other organisms (see below); this, and the fact that they are released
when cells are damaged, suggests that they are important in chemical defence,
both by deterring phytophagous animals (Hile
et al.
, 2004) and by being toxic
to invading fungi and bacteria.
In vitro
tests with extracts of onions and garlic
have shown them to inhibit the growth of more than 80 species of plant-
pathogenic fungi (Fenwick and Hanley, 1985b). Allicin has been tentatively
identified as fungicidal or fungistatic against a number of plant-pathogenic
fungi, and DADS was active against ten plant pathogenic fungi. Extracts of
onions and garlic are insecticidal and have also been shown to be toxic to a
number of plant-parasitic nematodes.
The major pests and diseases of alliums tend to be highly adapted to their
hosts and to use the characteristic volatiles as signals in locating their hosts,
e.g. the onion fly,
Delia antiqua
, the leek moth,
Didromus pulchellus
and sclerotial
germination and root invasion by the white rot fungus
Sclerotium cepivorum
(see Chapter 5).
ACSOs and their
-glutamyl derivatives can amount to 1-5% of the dry
weight of allium plants and seeds, and alliinase comprises about 6% of the
total soluble protein of onion bulb tissue. These substances probably have an
important role for nitrogen, sulfur and carbon turnover, storage and
transport within allium plants. The high quantity, vacuolar location and
tendency to aggregate in multimers of alliinase are typical features of storage
proteins.
