Effect of heat treatment on the dietary fibre contents of potato and tomato

The effects of autoclaving at 100, 120 and 130°C, baking in an oven, and microwave heating on the dietary fibre (DF) content and composition of potato and tomato were studied at normal pH and after acidification. The fibres were fractionated by sequential hydrolysis, and the sugar compositions were determined by gas chromatography (a modification of Englyst's method). All heat treatments appeared to increase the total DF content of the potato; the fraction analysing as 'cellulose' showed the major change, which was probably due to the retrogradation of starch. Autoclaving, especially at 130°C, reduced the total DF of tomato, while baking increased it. Excessive autoclaving may have hydrolysed some of the water soluble non-cellulosic polysaccharides (w.s. NCP), and solubilized a part of the water insoluble (w.i.s.) NCP. Baking increased the w.i.s. NCP and cellulose fractions, in particular. Slight acidification caused only minor additional changes in the fibre fractions.