The literature concerning cooked-potato texture is reviewed and discussed in the light of three new hypotheses. First, that "mealiness" is the subjective perception of the flow characteristics ("viscosity") of cooked tissue and that this aspect of texture is controlled primarily by the solids content of the tuber. Second, that the reduction in tissue strength which occurs on cooking is due to water uptake by the polysaccharides of the cell wall. This increases the thickness of the cell wall and reduces the viscosity of the cell wall matrix: both effects combine to reduce the stress required to separate cells. Third, that, when the intrinsic properties of the cell wall material lead to excessive hydration, the adhesion between cells may be sufficiently reduced to allow spontaneous separation or "sloughing".
New methods are described for the objective assessment of mouthfeel (mealiness) using a Kramer Shear Press and for the amount of breakdown by sedimentation of sloughed tissue. Their application to several hundred samples of canned potatoes is reported. These objective methods correlate significantly (P < 0.001) with their subjective counterparts and demonstrate a clearer separation of mouthfeel and breakdown than do subjective methods. Examination of partial correlation coefficients indicates that mouthfeel is more significantly related to the specific gravity of the raw material than is breakdown. In separate experiments, late planting was shown to affect mouthfeel but not breakdown and the addition of calcium chloride to affect breakdown whilst leaving mouthfeel unaltered. Evidence is presented that mouthfeel is a rheological property controlled primarily by solids content; in contrast breakdown is interpreted as a failure of intercellular adhesion.
Total esterified polyuronide in plant tissue can be assayed by titrating the uronic acid liberated from the alcohol-insoluble fractions of the tissue by alkaline hydrolysis. The total polyuronide can be determined in a similar manner if the carboxyl groups are initially esterified with methanolic hydrochloric acid. Comparison of the two results gives the degree of esterification of the total polyuronide. The divalent metals present in the same alcohol-insoluble fractions are assumed to be bound to the uronide groups and their assay gives a measure of the extent of interchain cross-linking by metal salt formation.Application of these procedures to samples from different parts of potato tubers showed that polyuronide generally had a low degree of esterification (about 40 %) and was present in higher concentrations (about 4.6% a.i.s.) in the cortex and periderm than in the interior tissues (about 2.7% a.i.s.). The degree of crosslinking due to calcium and magnesium ions is lowest in the outer layers of the potato tuber and may be the reason that sloughing occurs in this area.
The effects of variety, harvest date and plant population on breakdown are considered in the light of two current theories relating chemical composition to the texture of cooked potato tubers. Differences between cell surface area could not account for the differences in breakdown found. The results also show the inadequacy of the theory ascribing a primary role in texture control to the "swelling pressure" generated by starch on gelatinisation. It seems likely that breakdown of cooked potato tubers is due to excessive hydration of cell wall material. On this theory high levels of polyuronides and phytate and low levels of polyvalent metals in the cell wall material favour breakdown. Correlation analysis indicates that other chemical factors which have yet to be identified may also play a role in the control of cooked potato texture.
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