Microwave baking of potatoes comprised two phases. In the ®rst phase the internal temperature of the potatoes rose to approximately 100°C with little loss of water vapour. The uniformity of internal heating during this phase was assessed by identifying gelatinised starch. Starch gelatinisation began near the outer surface after 1.5 min of cooking and was complete by 2.5 min. During the second cooking phase the thermal energy absorbed was used to evaporate water. When immersion in boiling water was substituted for this second phase of microwave cooking, it took signi®cantly longer for the internal texture to soften adequately for the potatoes to be considered cooked. This suggests that microwave cooking in¯uences texture independently of the thermal pro®le of the cooking process. Damage by escaping steam is suggested as a mechanism.

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.

Potatoes were baked for up to 60 min in a conventional fan-assisted oven. Temperature pro®les within the potato tubers were determined both by direct measurement and by following the inward progress of starch gelatinisation, which occurs at 65°C. Temperature pro®les with time were Sshaped and about 30 min was needed for the centre to reach 100°C. From the nature of the temperature pro®les, long potato tubers will cook faster than round tubers of the same weight, and a varietal`shape factor' was de®ned to quantify this effect. The slow temperature rise, compared to other forms of cooking, was due to evaporative cooling. Moisture loss was linear with time after an initial lag and reached 15±20% of the mass of the potato after 1 h of cooking. Approximately half of the moisture was lost from the outermost layer of the potato under the periderm, leaving behind a dried layer of¯esh that appeared to restrict water transport and, unlike any other part of the potato, could exceed 100°C. This has consequences for the thermal development of¯avour compounds as well as for perceived texture.

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.