- The effect of frozen storage time on the composition
of the cell wall polysaccharide (CWP) of muskmelon (Cucumis
melo) cultivars at different stages of maturity was investigated.
Changes in composition, firmness, drip loss, and
color of Cantaloupe and Honey Dew melon flesh were determined
at three stages of maturity and for three periods of
storage at -23°C. Relationships between firmness, drip
loss, and other composition measurements, as well as the
total CWP sugar composition, were also determined.
Cell wall polyssacharides were isolated and purified,
and fractionations were performed using cyclohexane trans-
1,2-diamine tetraacetate (CDTA), Na₂C0₃, guanidinium thiocyanate
(GTC), and KOH. All fractions and residues were
dialysed and then freeze-dried. Following hydrolysis of
CWP fractions with trifluoroacetate (TFA), the alditol acetate derivatives of neutral sugars from each CWP fraction
were prepared and analyzed by gas chromatography,
using myo-inositol as the internal standard.
TFA insoluble fractions were analyzed colorimetrically
using phenol-sulphuric acid reagent. Uronic acid was determined
using 0.15% m-hydroxybiphenyl for absorbance at
520 nm with galacturonic acid as the standard. It was determined
that CDTA and Na₂C0₃ fractions were composed of
typical pectic materials, containing mostly galacturonic
acid with the neutral sugars arabinose, galactose, rhamnose,
and a smaller amount of xylose. As maturity increased,
CDTA fraction yields increased, though total neutral
sugar CWP compositions decreased. GTC and KOH fractions
were typical of hemicellulose, and contained principally
xylose, glucose, galactose, mannose, and fucose, with very
small amounts of uronic acid, arabinose, and rhamnose.
Residue fractions contained principally glucose and galactose,
with smaller amounts of mannose, xylose, arabinose,
and fucose. With the exception of xylose and glucose, all
neutral sugars decreased significantly (p < 0.01) as maturity
increased in both the Cantaloupe and Honey Dew melons.
Total uronic acid did not change as maturity increased,
except for Cantaloupe, where total uronic acid decreased
from the ripe to overripe stages. The CDTA fraction
yield increased and all neutral sugars decreased
significantly (p < 0.05) as storage time was increased.
Only the CDTA fraction yield was negatively correlated with the firmness of both melons, and was positively correlated
with drip loss as maturity and frozen storage time were increased.
Firmness was positively correlated with Na₂C0₃ and
GTC fraction yield in Cantaloupe, whereas for Honey Dew
there was no correlation between firmness and Na₂C0₃ or GTC
fraction yield as maturity increased. The KOH fraction was
negatively correlated with firmness in Cantaloupe, whereas
there was no correlation between firmness and KOH fractions
in Honey Dew existed as maturity increased. The residue
fractions increased in both melons only from the underripe
to the ripe stages, and did not change from ripe to overripe.
Firmness was positively correlated with total rhamnose,
arabinose, mannose, and galactose as maturity increased,
and the drip loss was negatively correlated with
all total neutral sugars as storage time was increased.
During frozen storage, there was a significant decreases
(p < 0.05) in total CWP sugars in relation to increased
storage time. The decrease in total sugars was
more dramatic during the 0 to 5 month period than the 5 to
10 month period of frozen storage. Galactose did not
change in the Cantaloupe, whereas in Honey Dew it decreased
34.3% from 0 to 5 months then decreased only 13% from 5 to
10 months of storage.