The effects of food processing on plant cell walls with special reference to extensin

Abstract

Changes in plant cell wall composition caused by a variety of cooking and manufacturing processes have been investigated. These chemical changes have been compared with corresponding structural modifications which were assessed by light and electron microscopy. The different processing treatments have been applied to a single plant tissue, namely mung been seedlings, thus enabling comparisons to be made between the processes. Amino acid analysis was employed to assess changes in cell wall composition. The predominant amino acids in extensin, and isodityrosine, the critical cross-linking unit in this glycoprotein, were measured. Changes resulting from separate chemical extraction of pectin and glycoprotein wall components were also examined. An automated amino acid analysis method for isodityrosine has been established and a novel system for synthesising this dimer developed. The latter involves simple incubation of isolated cell walls with tyrosine, an arrangement which has achieved a six-fold increase in isodityrosine concentration. A reduction in the wall content of hydroxyproline and isodityrosine was observed in samples which had been stored in sulphite solution. It is suggested that this process may degrade pectin and glycoprotein. This idea is discussed with reference to the wall structure seen by microscopical examinations. The soluble extract resulting from boiling cell walls in dilute bicarbonate solution and the water-soluble pectin fraction were both found to contain significant quantities of isodityrosine. It is proposed that these two treatments may extract extensin glycopeptides or even cross-linked extensin oligomers from the wall. It is concluded that most of the processes damaged cell wall pectin to some extent while some also affected extensin. Results from the chemical fractionation experiments demonstrated that these two wall components tend to be co-extracted. This observation is discussed in relation to current models of cell wall structure.