Means to Optimize the Nutritional Properties of Starch in Potato Products - Impact on glycaemia, satiety and resistant starch content

Sammanfattning: Despite decreased consumption potatoes still constitute an important staple in the Swedish diet. An objection to potatoes and potato products from a nutritional point of view is the ease by which the starch is digested and absorbed in the gastrointestinal tract. This results in unfavourably high glycaemic and insulinaemic responses (glycaemic index, GI and insulinaemic index, II, respectively). Sustained hyperglycaemia may result in insulin resistance and associated diseases such as diabetes and cardiovascular disease included in the metabolic syndrome. The relatively high content of resistant starch (RS) in boiled potatoes is noteworthy and since RS affects the colonic microflora similarly to dietary fibre, it may have positive effects on colonic and metabolic functions. The purpose of the present thesis was to characterize the nutritional properties of starch in potato products and to evaluate raw material and optimize processing conditions to improve these properties with particular focus on postprandial glycaemia, satiety and RS content. Potatoes and potato products were characterized by high GI values (GI = 77-169). No differences were found in predicted GI irrespectively of potato variety, tuber weight or storage time after lifting among varieties with normal amylose contents. Neither were differences found between winter- and new potato varieties. The GI of french fries was 41 % lower compared with boiled potatoes added with the same type and amount of fat. A possible cause may be the particular processing conditions used for manufacturing and/or reheating of the french fries, rather than the fat content per se. RS content was comparatively high in boiled potatoes (4.5 %, starch basis) and could be increased further in potato tubers by cold storage (7 %) and/or temperature treatment (10 %). Cold storage of boiled potatoes resulted in lower glycaemic and insulinaemic responses (- 25 %). Addition of an oil/vinegar mixture to the cold potatoes reduced GI to 96 (-43 %). Based on current potato consumption; exchanging freshly boiled potatoes for such a pickled potato product would lower dietary GI by approximately 30 %. Tubers from potato genotypes with increased amylose contents (up to 78 % amylose) contained less starch and had higher content of low molecular weight carbohydrates than tubers with low or normal amylose contents. The over-all predicted GIs for the high-amylose tubers (81 and 83) where somewhat lower than for tubers from their mother lines (118 and 95). The lowest values obtained after processing of common potato varieties were 96 as obtained for boiled, cold stored and pickled potatoes and 77 for french fries. Thus, from a nutritional point of view, there is a potential for genetic modification of potatoes. At present, transgenic potatoes are not approved for human consumption. Isolated high-amylose starches were characterized by elevated RS contents, (18-34 % RS, total starch basis) compared with starches with normal or low amylose contents (<1 % RS). The results within the thesis altogether suggest retrograded and/or ungelatinized amylose as the main constituent of the RS fraction in time/temperature treated tubers or starches. However, retrograded amylopectin, as obtained after cold storage, or cold storage and subsequent reheating to 30 °C appeared to decrease the rate of starch hydrolysis thus reducing predicted GIs in both tubers and isolated starches with normal or low amylose contents. Satiating properties of potatoes and potato products were evaluated in meal studies in healthy subjects. Boiled potatoes were considerably more satiating than french fries on an energy equivalent basis, whereas no differences were observed between boiled or mashed potatoes neither in glycaemia nor satiety. The early high postprandial glycaemia for boiled and mashed potatoes coincided with increased satiety; a feature that may have effects on meal termination. In the long-term, highly satiating foods may have positive implications on weight regulation. Compliance to an energy restricted diet is most likely facilitated by the inclusion of highly satiating foods, such as boiled potatoes.