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.
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.
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