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Cell walls from wheat bran were examined intact by cross-polarization, magic-angle spinning 13 C NMR to determine their polymer composition. The carbohydrate part of the 13 C spectrum was typical of graminaceous cell walls having cellulose and arabinoxylans as their main components. Only a little lignin was observed in the spectrum, but signals from the hydrocarbon chains of cutin were particularly obvious. It is suggested that cutin, rather than lignin, plays an important role in protecting wheat bran from microbial degradation in the rumen and in the human gut and that the hydrophobic properties of the cutin are likely to be responsible for some of its nutritional properties.

Because pectins are released from potatoes and other plants under conditions that cleave ester linkages, it has been suggested that there are other galaturonoyl ester cross-links between pectin chains in addition to the known non-cross-linking methyl esters. A microscale titration method and a copper binding method were developed for the measurement of total polymer carboxyl (essentially pectic) ester content in potato cell walls. Relative to the uronic acid content of the cell walls, the degree of total esterification was 57-58%. Comparison with levels of methanol released on ester hydrolysis allowed nonmethyl uronoyl esters to be estimated to be 14-15% relative to total uronic acid. The possibility of nonmethyl-esterified linkages being formed in potato cell walls by a side-reaction catalyzed by pectin methyl esterase (PME) was investigated, but no increase in nonmethyl-esterified pectin was observed under conditions where pectin was being effectively de-esterified by endogenous PME activity.

Broccoli stems can become tough and stringy owing to excessive development of the vascular ring. Thickened cell walls from the vascular ring were isolated and their composition was determined. They were derived principally from anatomically recognisable xylem vessels, fibres and tracheids but contained an assemblage of polysaccharides typical of primary cell walls. Their pectin content was particularly high and they contained only 6% lignin as estimated by solid state 13 C NMR spectroscopy. They did not differ markedly in composition from parenchyma cell walls within the same stems. Thus, despite their thickness and anatomical appearance, these cell walls resembled the walls of non-woody cells in their polymer composition. INTRODUCTIONUnsatisfactory textural quality of broccoli (Brassica oleracea L var italica) is connected with toughness of 'stringy' tissues within the stems. 1 The most effective approach to solving this problem will depend on the nature of the tissues in question and the reason for their toughness. If existing cell walls become lignified with a consequent increase in their mechanical strength, then preventing lignification will maintain textural quality. If, on the other hand, differentiation of tissues within the stem leads to newly thickened or modified cell walls, then a more appropriate target will be the developmental cues for this differentiation.In the upper region of the flowering stem of certain other Brassica spp the vascular ring contains unusual thickened cell walls, which can be isolated by a sequence of controlled homogenisation and sieving that depends on their resistance to mechanical disintegration. 2,3 These previous studies provide both a method for preparing the problematic cell walls from broccoli and a parallel for their chemical and anatomical nature, which is the subject of this paper.

A group of new methods is described for preparing cell walls from potatoes and processed potato products. Starting from raw domestic potatoes, starch is degraded enzymatically after a very brief 100°C gelatinisation step conducted after homogenisation to minimise the time required for heat transfer. Protein is removed by detergent and phenol extraction. This procedure (method 1) gave cell wall preparations containing <5% starch, with minimal degradation of wall polysaccharides. It did not, however, remove starch ef®ciently from industrial potatoes in which the starch content is much higher. A different procedure, method 2, was used in this case. In method 2 a 20 min starch gelatinisation step was used but the temperature was restricted to 70°C and the pH to 4.0, with the aim of protecting pectins from depolymerisation. Method 2 and method 1A, which is a hybrid procedure involving the starch gelatinisation step from method 2 and other steps from method 1, gave low-starch cell walls from industrial as well as domestic potatoes. These methods are suitable for a range of potato types and potato products and are either more ef®cient or more convenient than previous procedures for cell wall isolation.