Pectic Substances in the Plant Cell Wall and Their Role in Potato Processing

Plant sciences

Jardine, William Gordon

University:University of Glasgow

关键词: Plant sciences;   

Others  :  http://theses.gla.ac.uk/74687/1/10992309.pdf

来源: University of Glasgow

【 摘 要 】

The objective of this thesis was to characterise the chemistry of the pectin in the primary cell wall of two potato varieties, cv. Record and cv. Saturna, used in the crisp industry. The varieties chosen produce crisps with different textures. The original aim was to relate chemical characteristics of the pectin in the raw cell wall to textural differences in the crisp. A new procedure for the isolation of the cell wall, free from starch was developed. Methodologies to measure the pectic galacturonan, total ester and the extent of beta-elimination in the isolated cell wall were developed. Cell walls were isolated from raw, cold wash, hot wash, cold wash crisp and hot wash crisp samples from the 1995 and 1996 harvest. No significant differences in the pectin content or structure were found between the cell walls isolated from cv. Record and cv. Saturna. Nevertheless, the cell walls isolated following the crisping process had a reduced pectic galacturonan and methyl ester content. The neutral sugar content of the cell wall remained unchanged; however, the galactan and arabinan side chains had a greater mobility. As no significant differences between the varieties were found, the objectives were changed to monitor the pectin changes during processing. For comparison with crisping, slices of cv. Maris Piper were boiled for 2.5 minutes. Boiling resulted in pectin solubilisation (8%) with no reduction in the percentage galacturonan methyl esterification. Saponification followed by boiling caused 44% pectic galacturonan solubilisation. The pectin solubilised during cooking following saponification had a greater proportion of neutral sugars to galacturonan and had a smaller molecular-size distribution than the fraction solubilised by boiling alone. The maximum extent of beta-elimination in isolated cell walls of boiled cv. Saturna was estimated at 1%. Non-methyl esters were found at values of 14-15% in cv. Marfona and cv. Fianna. Chelation of divalent cations from the middle lamella resulted in the preferential solubilisation of a predominantly linear, non- methylated fraction of pectin. Saponification resulted in cleavage of ester crosslinks between pectic chains. Pectic chains whose anchorage in the cell wall relied on ester links were now held in the cell wall by divalent bridges. Boiling of the saponified tissue would result in the cleavage and loss of the cations into the cooking water, thus resulting in pectic solubilisation. Acid hydrolysis may have been an additional mechanism of cleavage of the pectic chain. Slices of the 1996 cv. Saturna harvest were crisped after saponification. The cffect of saponification on the quantities of pectin solubilised prior to the cooking process was reversed from boiling. Saponification prior to frying reduced the quantities of pectin solubilised due to the crisping process, 8% being solubilised from the saponified crisps and 40 % solubilised from the non-saponified crisps. Both saponification experiments resulted in solubilisation of the rhamnogalacturonan I fraction suggesting that the non-methyl esters occur within or close to the rhamnogalacturonan I fraction. The galacturonan and rhamnose units within the rhamnogalacturonan I do not provide sufficient sites for nonmethyl esterification. Ester linkages between the ester group of the homogalacturonan and the galactan side chains may provide alternative sites for ester linkages. A taste panel experiment was run on crisps following different pre-treatments. The pre-treatments that reduced the softening upon heating also conferred strength to the crisp. The crisping process involves cleavage of the methylated pectic chain due to beta-elimination in the cell wall resulting in the preferential solubilisation of a methylated homogalacturonan fraction. This mechanism of cleavage is prevented in the saponified crisp due to removal of the methyl ester groups. Anchorage of pectic chains whose ester cross links have been broken by saponification is maintained by divalent bridging. The saponified galacturonan has a greater potential for the formation of bridging due to the removal of the ester groups. There is a clear role for pectins in controlling the texture of the crisp, however, varietal differences can not yet be explained by differences in the raw cell wall.