Physicochemical Properties of Squash (Cucurbita Maxima) Cell Walls
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Plant cell walls are thought to make a major contribution to the texture of vegetables. This thesis illustrates the relationship between texture and cell-wall composition of four squash (Cucubita maxima) cultivars. Texture profile analysis and rupture tests were carried out on raw and cooked tissues of 'Delica', 'CF 2', 'CF 4' and 'Red Warren' squash during the storage. The raw tissues of 'CF 2' was the firmest i.e., high tissue strength (high failure force) combined with less rigidity i,e., less modulus of deformability (MOD) and high compressibility (high stain and deformation at failure). Whereas, 'Red Warren' had the least firm tissues (low failure force) combine with high rigidity (high MOD) and low compressibility. Firmness of the 'Delica' and 'CF 4' squash was relatively moderate. The texture parameters such as MOD, failure force, hardness, gumminess and chewiness measured for raw and cooked tissues for four squash cultivars showed no significant different up to two months of storage then decreased (P < 0.05) between two to three months of storage. The size of the cells and the thickness of the parenchyma cell walls among 'Delica', 'CF 2' and 'CF 4' were not different to each other. The increased size of the 'Red Warren' parenchyma cells and larger inter-cellular spaces together with their thinner cell walls account for the lower tissue strength compared to other cultivars. The changes in cell-wall polysaccharides (CWP) of the tissues of four squash cultivars during storage and after cooking were investigated. A procedure for isolating cell walls in tissues containing high levels of starch has been developed. An initial extraction of the tissues with HEPES buffer-containing dithiothreitol (DTT) was followed by grinding the residue in a cold ring grinder and a second HEPES buffer extraction. Any remaining starch was solubilised using dimethyl sulphoxide (DMSO) followed by degradation using ?-amylase. The starch free CWP was sequentially fractionated using CDTA, dilute Na2CO3 and 4MKOH. Cellulose made up 40% of total CWP each for 'Delica' and 'CF 4' squash at harvest and the cellulose contents for other cultivars were 35% and 42% of the total CWP for 'Red warren' and 'CF 2', respectively' The ratio of UA: (Ara + Gal) of the CDTA-' Na2CO3-and residue wash-soluble fractions for 'Red Warren' was higher than that of the ratio from equivalent fractions for other cultivars suggest that 'Delica', 'CF 2' and 'CF 4' walls consists of more highly branched pectic polysaccharides than that from Red Warren' squash. Consequently, 'Red Warren' squash cell walls consist of more smooth regions of pectic polysaccharides and/or less highly branched pectic polysaccharides than other cultivars. The lower highly branched pectic polysaccharide content of 'Red Warren' squash correlates with its lower firmness. Steaming resulted in an increase in the water- and DMSO-soluble pectins and a decrease in the pectins associated with cellulose whereas the total squash cell-wall polysaccharide at harvest showed no change on cooking. The total CWP content of raw and cooked tissues of all four squash cultivars remained unchanged up to two months of storage then markedly decreased between two to three months of storage. The amount of sugars including galactose, mannose and xylose in all squash cultivars remained relatively constant from harvest up to two months of storage and then decreased markedly during two to three months of storage. The major changes occurred for the galactose. It is speculated that solubilisation of the pectic polysaccharides, probably as a result of the loss of galactan, may have caused deterioration of the cell-wall architecture, accounting for the loss of firmness of squash tissues. However, the insoluble residue, primarily cellulose, of all squash cultivars did not change appreciably as storage increased. The degree of esterification (DE) of galacturonic acid in 'Red Warren' and 'Delica' squash as determined by linkage analysis, were 7l and 6l% respectively which are described as high methoxy pectins. A broad signal at l7l ppm in subspectra B than the signal at 174 ppm in subspectra A of the proton spin relaxation editing (PSRE) experiments from the solid state 13C nuclear magnetic resonance (NMR) spectroscopy confirmed the high levels of DE of galacturonic acid in 'Red Warren' squash' Higher ratio of galacturonan to arabinogalactan (Type I) in the cell walls of raw and cooked 'Red Warren' squash (2.0) compared to that from 'Delica' squash (1.2) suggest that 'Red Warren' consisted of more smooth regions of pectic polysaccharides. The amount' of arabinan and arabinogalactan (Type I) present in 'Red Warren' did not change on cooking. The low content of xyloglucan (3-4 mol% of the total CWP) in the cell walls of 'Red Warren' and 'Delica' did not change on cooking. Shorter T1?(H) values for raw 'Red Warren' cell walls than those for cooked tissues, computed from PSRE l3C NMR experiments suggest that the greater interactions between the crystalline domain with non-cellulosic matrix of raw tissues of 'Red Warren' squash compared to cooked tissues of 'Red Warren' squash. The higher ratio of crystallite interior to crystallite surface for cooked 'Red Warren' squash cell walls compared to that for raw tissues suggests that cooking may remove a chain/chains from the surface of the cellulose crystallites. The lateral fibril dimensions of the cellulose crystallites for raw 'Red Warren' squash tissues in a 5 x 4 array was 2.68 nm.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/2000