Graduate Thesis Or Dissertation
 

Composition of pectic substances in Selva strawberry (Fragaria x ananassa Duch.)

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  • Pectic substances in the cell wall have long thought to be responsible for the textural changes in fruit during ripening and senescence. Compositional variation within the pectic macromolecules could affect the external characteristics of the fruit. Strawberries undergo adverse quality changes such as softening and juice loss after freezing and subsequent thawing. To obtain a desirable cultivar that retains sound textural qualities after processing has long been an objective of both strawberry breeders and food processors. The fruit of Selva is exceptionally firm and some may consider it to be too firm. The purpose of this study was to investigate the compositional factors that would account for the unusual textural qualities of the Selva strawberry. Two major Oregon-grown commercial cultivars, Totem and Benton, were also included to facilitate comparisons between pectic substances and textural profiles of strawberry fruits. Fruit firmness was measured by the resistance of berry halves to shear and the amount of juice released after thawing. Very firm characteristic of Selva fruit was documented in that the shear value of thawed Selva was equal to fresh Totems and even greater than that of fresh Bentons. Juice-release after thawing was lowest in Selva samples. The quantity of acetone-insoluble solids obtained from freeze-dried powders of strawberry fruits was high for Selva and Benton, and low for Totem. Selva contained higher amounts of total soluble polyuronides and lower quantities of hemicellulose fraction as compared to Totem and Benton. Elution profiles of water-soluble polyuronides and chelator-soluble polyuronides from DEAE ion-exchange chromatography showed that Selva samples had a longer retention time and wider peak distribution, suggesting that its pectin species were of higher molecular weight and/or charge than Totem or Benton samples. The elution profile of ripe Selva was similar to that of underripe Totem and Benton. Ratios of uronic acid to rhamnose, indicating the extent of kinking and branching, were high for Selva and Totem in the water-soluble fraction and for Selva in the chelator-soluble fraction. Arabinose and galactose levels were relatively high in Selva base soluble polysaccharides, indicating that an arabinogalactan may be present in the hemicellulose fraction. This arabinogalactan may play a role in connecting the rhamnogalactan to the glycoprotein in the cell wall.
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