Graduate Thesis Or Dissertation
 

The effect of cultivar maturity and frozen storage time on the cell wall polysaccharide composition of muskmelon(Cucumis melo)

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  • 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.
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