Starch in Processed Potatoes -Influence of Tuber Structure, Thermal Treatments and Amylose/Amylopectin Ratio

The potato is one of our most important food crops, being sold in its natural state or processed into a wide range of products. Starch, by far the most abundant component in the tuber, is greatly affected by heat processing. Therefore it is logical to study starch thermal properties within the tuber tissue, although most previous studies have focused on starch-water model systems. One of the main purposes of this work was to analyse thermal events such as gelatinisation, retrogradation and annealing in both tissue samples and starch-water systems, and to relate starch thermal effects to the characteristics of potato tubers and processing conditions.

The main methods used were differential scanning calorimetry (DSC) and microscopy. The DSC method has been widely used to study starch and is also applicable to the analysis of potato tissue samples. Several microscopic techniques were used to study the potato microstructure at different levels of magnification. Information obtained from micrographs was further used to interpret results from DSC analyses.

Several characteristics of the raw material were shown to influence the thermal properties of starch. Potato variety and cultivation year, as well as properties related to dry matter components and their distribution within the tuber, were very important, whereas the tuber size was insignificant. The gelatinisation of starch within tissue is shifted to higher temperatures as compared with starch-water systems, a fact that also influences the effects of annealing. Furthermore, starch from the central parts of the tuber had categorically higher gelatinisation temperatures than starch from other tuber tissue zones. Retrogradation rate, in terms of increased enthalpy of melting recrystallised amylopectin with time, was affected by variety whereas melting temperatures were not. However, the differences in gelatinisation and retrogradation between varieties could not be related to cooking type, i.e. mealiness.

To understand more about the influence of amylose and amylopectin respectively on different starch characteristics, potatoes with large variations in amylose content were analysed. Amylose content roughly ranged from 1 to 78 %. In high-amylose starch, properties such as granule morphology, gelatinisation, and retrogradation were severely altered compared with normal starch. In starch with 99 % amylopectin differences were less extensive. When subjecting some of these starches to different temperature treatments, it was shown that important nutritional features such as resistant starch content and starch hydrolysis rate are mainly affected by amylose content. A synergistic effect between retrograded amylopectin and amylose may have an additional effect on these properties.