Agriculture Reference
In-Depth Information
metabolites, such as carotenoids, flavonoids, volatiles and
lipids (Davies & Albrigo 1994). Although citrus fruit are
very low in proteins and fats, they are a good source of
fibre and pectins. Citrus fruit and juices are rich in several
types of bioactive compounds and their antioxidant activity
and related benefits are derived not only from vitamin C
but also from the other phytochemicals which are mainly
flavonoids (Martí
et al
. 2009). Citrus provides essential
components required for human health, nutrition and well-
being (Fallik 2004, Bijzet 2006) and is globally considered
an important functional food.
Flavour and aroma
In citrus, volatile terpenoids, the principal components of
essential oils are responsible for the aroma and flavour
of the fruit. Citrus volatile compounds consist mostly of
mono-(C10) and sesquiterpenes (C15) which are the major
components of citrus essential oils, including derivatives
such as alcohols, aldehydes, ketones and acids (Rodrigo &
Zaccarias 2006; Tadeo
et al
. 2008).
Pectins
Pectins are high-molecular-weight carbohydrates composed
of chains of anhydrogalacturonic linkages. They serve as
intercellular bonding materials in many fruit and vegeta-
bles. During maturation of citrus fruit, insoluble pectins
are converted to water-soluble pectins and pectinates
(Nagy & Attaway 1980). Total pectic substances decrease
in the peel and pulp over the season, and water-soluble
pectins increase as percentage of the total pectins. This
change in pectin composition signals fruit softening or
over-maturity.
Sugar and acid
The sugar and organic acid concentration affect citrus
fruit taste characteristics and organoleptic quality. Sugars,
consisting of sucrose, glucose and fructose, are the major
components of citrus juice soluble solids (70-80%) and
responsible for sweetness (Rodrigo & Zaccarias 2006).
The main organic acids of citrus fruit are citric (70-90%)
and malic acids with trace amounts of tartaric, benzoic,
oxalic and succinic acids (Karadeniz 2004). Total acidity
of citrus juice is an important factor in overall juice quality
and in determining time of fruit harvest (Harding
et al
.
1940). During ripening, a decline in titratable acidity (TA),
due to catabolism of citric acid and an increase in sugars, is
observed.
Maturation
Pre-harvest conditions and the degree of harvest maturity
significantly affect the fruit quality after harvest. In tem-
perate or subtropical regions, the fruit of most citrus varie-
ties are generally harvested at the fully mature stage, at
which colour is fully developed, except for the acid citrus
group such as lemons and limes. Partly green citrus fruit
are harvested after reaching full size in these areas (Murata
1997) and are often conditioned through de-greening. The
sugar content in the rind and juice of the fruit tend to
increase throughout fruit development. The acid content
of the juice on the other hand tends to decrease after
fruit reaches full size. The flavoured acid citrus group,
including lemons and limes, should contain a high content
of citric acid in the juice and should be harvested before
reaching full fruit size, since the matured fruit has low
acidity. Lemons that are harvested according to fruit size
rather than colour or maturity require de-greening.
Since citrus is nonclimacteric (ripens on the tree), the
fruit is harvested once physiological maturity is reached.
Mature, sound citrus fruit show no respiratory peak and lit-
tle ethylene production under normal harvesting conditions.
The maturation curve for citrus is influenced by varietal
characteristics, environmental factors such as air tempera-
ture, solar radiation, water availability in the soil and geo-
graphical factors including topography, elevation and
exposure surface in the plant (Ortolani
et al
. 1991). In order
to relate temperature and plant development, thermal sum-
mation or accumulative growing degree days (GDD) are
Ascorbic acid and mineral composition
Fresh citrus and its juice are an important source of vitamin C.
Ascorbic acid functions as a coenzyme and is an essential
part of the human diet (Nagy & Attaway 1980). Levels of
ascorbic acid are quite variable among citrus fruit and tend
to decrease seasonally. Ascorbic acid levels are expressed
as mg (100 ml juice)
−1
and range from 18 to 20 in some
tangelos to over 70 for 'Pineapple' sweet orange.
Limonin
The bitterness in citrus fruit is affected by limonin and
naringin, which are generally recognized as the major two
bitter compounds (Suwanna & Ratiporn 2009). Limonin is
the bitter limonoid found in major citrus cultivars such as
grapefruit and naringin is the bitter flavonoid. Limonin
is an anti-oxidant and a member of the furanolactones
subclass and can be found in the albedo, flavedo, segment
membranes, juice and pit in different concentrations
depending on the citrus cultivar. Most citrus varieties also
contain flavonoids (naringin, rhoifolin, lomcerin, hesperi-
din, neohesperidin, citronin and tangeretin) in the rind and
juice segments. Naringin (flavanone-neohesperidose), which
tastes bitter, is found in grapefruit.
