Dietary guidelines for developed countries are consistent in recommending an increased carbohydrate intake, corresponding to at least 55% of total energy. Correspondingly, the carbohydrate content in diets typical for developing countries should be maintained at a high level. The nutritional quality of the carbohydrates and the effects of processing on that quality then becomes a concern, because both the content and the nutritional quality of food carbohydrates can be altered by processing in a number of ways.
Carbohydrate loss through leaching
Low molecular weight carbohydrates
During wet heat treatment, as in blanching, boiling and canning of vegetables and fruits, there is a considerable loss of low molecular weight carbohydrates (i.e. mono- and disaccharides) as well as micronutrients, into the processing water. For example, in the blanching of carrots and swedes (rutabagas) there was a loss of 25% and 30%, respectively of these carbohydrates. With subsequent boiling another 20% was lost. In peas, green beans and Brussels sprouts the loss was less pronounced - about 12% following blanching and another 7-13% at boiling (52).
The loss of glucose and fructose at boiling was higher than that of sucrose (53). The losses of low molecular weight carbohydrates in carrots have also been shown to differ between various cultivars, and also to be different at harvest and in storage. After storage the loss of low molecular weight carbohydrates increases following boiling, most probably due to the higher water content and therefore also a higher diffusitivity (54). The loss of low molecular weight carbohydrates, at least in carrots, seems to be relatively easy to predict by knowing initial concentrations and process conditions of the raw material.
No leaching of dietary fibre into the processing water has been reported with blanching, boiling and canning of carrots, green peas, green beans and Brussels sprouts (52). With swedes, however, there was a 40% loss of dietary fibre (mainly insoluble) with boiling. Also with canning there was a leakage of insoluble fibre into the processing water.
Alterations of low molecular weight carbohydrates
Production of resistant oligosaccharides
The production of resistant oligosaccharides by enzyme technology is an expanding area. More than half of the "functional foods" on the Japanese market contain prebiotic oligosaccharides as active component, with the aim of promoting favourable gut microflora. Fructo-oligosaccharides synthesized from sucrose (55) and galacto-oligosaccharides synthesized from lactose are the most extensively used types of resistant oligosaccharides. Alternatively, fructo-oligosaccharides can be produced by hydrolysis of inulin.
Non-enzymatic browning reactions (Maillard reactions) occur between reducing sugars and ammo groups in foods at processing and in storage. These reactions are temperature dependent and most extensive at intermediate water activities. They are important nutritionally as they may diminish the bioavailability of amino acids, especially lysine, thus diminishing the protein nutritional value. The carbohydrate content and availability is influenced only marginally.
When a non-reducing disaccharide such as sucrose is replaced by, for example, high fructose corn syrup containing glucose and fructose, Maillard reactions occur much more rapidly and extensively. This has to be kept in mind in selecting processing procedures and storage conditions.
Starch - heat-induced effects
Gelatinization refers to the irreversible loss of the crystalline regions in starch granules that occur upon heating in the presence of water. The temperature range during which the crystalline structure of the starch granule is lost is dependent on the water content, and on the type of starch. The gelatinization dramatically increases the availability of starch for digestion by amylolytic enzymes.
Usually, the starch granules are not completely dissolved during food processing, and a food can be regarded as a dispersion in which starch granules and/or granular remnants constitute the dispersed phase. The degree of gelatinization achieved by most commonly used food processes, however, is sufficient to permit the starch to be rapidly digested. Consequently, even food processes which result in a low degree of gelatinization (e.g. steaming and flaking of cereals), produces a postprandial blood glucose and insulin increment similar to that with completely gelatinized foods (56,57).
Gelatinized starch is not in thermodynamic equilibrium. There is, therefore, a progressive re-association of the starch molecules upon ageing (58). This recrystallization is referred to as retro gradation, and may reduce the digestibility of the starch. The retrogradation of the amylopectin component is a long-term phenomena occurring gradually upon storage of starchy foods. Amylose, however, re-associates more quickly. The crystallinity of retrograded amylopectin is lost following re-heating to approximately 70°C, whereas temperatures above 145°C are required to remove crystallinity of retrograded amylose. This is a temperature well above the range used for processing of starchy foods. This implies that retrograded amylose, once formed, will retain its crystallinity following re-heating of the food.
During par-boiling of rice, the kernels are subjected to a pre-treatment involving heating and drying. This process reduces the stickiness of the rice, possibly by allowing leached amylose to retrograde and/or form inclusion complexes with polar lipids on the kernel surface. Parboiling also affects the final cooking properties of the rice.
Starch - texturization
In pasta products, gluten forms a viscoelastic network that surrounds the starch granules, which restricts swelling and leaching during boiling. Pasta extrusion is known to result in products where the starch is slowly digested and absorbed (59,60). Available data on spaghetti also suggest that this product group is a comparatively rich source of resistant starch (61). The slow-release features of starch in pasta probably relates to the continuous glutenous phase. This not only restricts swelling, but possibly also results in a more gradual release of the starch substrate for enzymatic digestion. Pasta is now generally acknowledged as a low glycemic index food suitable in the diabetic diet. However, it should be noted that canning of pasta importantly increases the enzymic availability of starch, and hence the glycemic response (62).
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.
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 (63). 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 (64). 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 (52), whereas baking (conventional and sour-dough baking), steam-flaking and drum-drying had only minor effects on dietary fibre components (65). 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 (65).
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 (66). 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 (67). 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.
Processing of foods affects carbohydrate and micronutrient content and bioavailability in different ways with either desirable or adverse effects on the nutritional value. Losses of water-soluble nutrients at blanching and boiling can be minimized by use of small amounts of water and by adding back the processing water.
The bioavailability of starch is affected dramatically through processing, regarding both rate and extent of small-intestinal digestibility. This permits optimizing the digestion of starch by choice of raw materials and processing conditions.
Processing effects on dietary fibre include solubilization and depolymerization, that can influence physiological effects both in the upper and lower gastrointestinal tract. Formation of resistant starch and use of resistant oligosaccharides as food ingredients provide new opportunities to increase the amount of carbohydrate available for colonic fermentation.