EFFECTS OF FOOD PROCESSING ON DIETARY FIBER

Effects of Food Processing on Dietary Fiber

 Milling and peeling

During milling of cereal grains to refined flours the outer fibre-rich layers are removed, resulting in a lower content of total dietary fibre. This reduction is due mainly to a decrease of insoluble fibre. The dietary fibre composition in both whole-grain and refined flours is different. Refined flours of oats, barley, rice and sorghum contain mainly glucans, while arabinoxylans dominate in refined flours of wheat, rye and maize. Whole-grain flours all contain considerable amounts of cellulose. The husk which surrounds barley, rice and oats also contains considerable amounts of xylans. This fraction is generally removed before consumption, but oat and rice husks are used for fibre preparation to enrich foods.

 Heat-treatment

Processes involving heat-treatment may affect the dietary fibre in different ways. An increased temperature leads to a breakage of weak bonds between polysaccharide chains. Also glycosidic linkages in the dietary fibre polysaccharides may be broken. These changes are important from analytical, functional and nutritional points of view.

A decreased association between fibre molecules, and/or a depolymerization of the fibre, results in a solubilization. If the depolymerization is extensive, alcohol soluble fragments can be formed, resulting in a decreased content of dietary fibre with many of the currently used fibre methods. Moderate depolymerization and/or decreased association between fibre molecules, may have only minor influence on the dietary fibre content, but functional (e.g. viscosity and hydration) and physiological properties of the fibre will be changed. Other reactions during processing that may affect the dietary fibre content and its properties are leakage into the processing water, formation of Maillard reaction products thus adding to the lignin content, and formation of resistant starch fractions. Also structural alterations in the cell wall architecture are important to follow during processing as these are highly correlated to sensory and nutritional characteristics.

The architecture of the fibre matrix in the cell wall differs between various types of plant material. The cross-linking of constituent polysaccharides and phenolics within the cell wall is important in determining the properties of the fibre matrix, as the solubility of the fibre is highly dependent on the type and amount of cross-links present. During heat-treatment the cell-wall matrix is modified and the structural alterations that occur may be important not only for the nutritional properties of the product but also for its palatability.

With extrusion-cooking of wheat-flour, even at mild conditions, the solubility of the dietary fibre increases . The solubilization seems to be dependent on the water content used in the process, and the lower the content of water, the higher the solubilization of the fibre, at least for whole-grain wheat flour and wheat bran . The screw speed and the temperature had minor effects in those experiments. An increased solubility of the fibre has also been obtained with 'severe' popping of wheat, whereas baking (conventional and sour-dough baking), steam-flaking and drum-drying had only minor effects on dietary fibre components 

One reason why popping caused an increased solubility of the fibre was that the outer fibrous layers were removed and the content of insoluble fibre decreased. Considerable amounts of Maillard reaction products were also formed during this process. A loss of insoluble dietary fibre has also been reported with autoclaving of wheat flour, which was attributed to degradation of the arabinoxylans .

Hydration properties (swelling, water-holding and water-binding capacity)

Most raw materials containing cereal fibres are ground for better acceptance of the final product and this process can affect hydration properties. Swelling and water-binding capacity of pea hull fibres are decreased by grinding, whereas the water-holding capacity was slightly increased . The kinetics of water-uptake was also different, and the ground product hydrated instantaneously in contrast to the unground product, which reached equilibrium only after 30 minutes. This was related to the differences in surface area.

Heat-treatment can also change hydration properties. For example, boiling increased the water-binding capacity slightly in wheat bran and apple fibre products, whereas autoclaving, steam-cooking and roasting had no significant effects . The kinetics of water uptake, however, was different for steam-cooking and roasting. Thus, both products exposed to steam-cooking had a very rapid water-uptake, whereas the roasted sample had a slow uptake. Extrusion-cooking of pea-hulls, sugar-beet fibres, wheat bran and lemon fibres had only slight effects on the water-binding capacity