Effects of Processing on Dietary Fibre in Vegetables
Abstract: Several beneficial effects have been connected with dietary fibre. Insoluble fibre has a good faecal bulking capacity, whereas soluble and viscous dietary fibre has been shown to have beneficial effects on carbohydrate and lipid metabolism. Fruit and vegetables provide about one third of the dietary fibre intake in Northern Europe and more than half in Mediterranean countries. These food items are also the most important source of soluble dietary fibre. Most vegetables are, however, processed in one way or another, in industry or in the home, before consumption. Processing may affect the physicochemical properties of dietary fibre, which in turn may lead to changes in the physiological effects. The present work was performed to study how physicochemical properties of nutritional significance, such as content and solubility of dietary fibre as well as molecular weight distribution and the viscosity of water-soluble polysaccharides (WSP) in vegetables (green beans, Brussels sprouts, green peas and carrots) may be affected by processing. In addition, the physicochemical properties of WSP at their actual site of action in the gastrointestinal tract were evaluated in rats. The effects of processing on dietary fibre in vegetables were shown to be very complex. Considerable differences were found in raw material, both between vegetables, and also between different cultivars of the same vegetable. Moreover, post-harvest changes during storage were found to influence the properties of dietary fibre. Heat processing of vegetables generally degraded the dietary fibre polysaccharides. The degradation was dependent on the severity of the heat treatment. Boiling of carrots and severe microwave treatment of green beans reduced the content of total dietary fibre. In some cases there was also a changed distribution between soluble and insoluble dietary fibre. Following heat processing there was a shift towards lower molecular weight of the WSP in all vegetables. This degradation was strongly dependent on the severity of the heat treatment, with more extensive degradation resulting from more intense heat treatment. In accordance with the molecular weight measurements the viscosity of the WSP decreased following more intense heat treatment. Furthermore, the WSP isolated from carrots and green beans exhibited a higher viscosity than those from Brussels sprouts and green peas. Freezing and souring of carrots had only minor influence on the molecular weight distribution and viscosity of the isolated WSP. The WSP in green beans were degraded during passage through the gastrointestinal tract in rats. The differences in molecular weight seen between blanched and microwaved green beans had been lost after passage through the gastrointestinal tract of antibiotic-treated rats. The in vivo fermentability of the bean fibre in rats, as well as the bulking capacity, was not affected by mild or by severe microwave treatment of the beans.
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