Possible Changes By Processing

Dietary fiber (DF) is defined as the material surviving digestion with human enzymes, but the different components, which form the DF complex, can be degraded or modified by other enzymes, e.g. microbial, by acids and bases or by temperature and pressure. While this either reduces the DF content or modifies it, new components, determined as DF, can be increased, too. During production of cereal foods and baked products, different treatments such as grinding, fermentation, cooking, baking, deep-fat frying, extrusion cooking, popping, or steam flaking are possible. This means conditions of temperature, pressure, heat treatment and shear force, electrolytes, and pH can change or be extreme and, therefore, alter DF values.

Heat treatment of starch-containing food may result in chemical modification and fragmentation of the starch. Changes in chemical characteristics may also alter physiological properties of the fiber. In this chapter, alterations in the DF content by physical adjustment or by making a product edible will not be considered. (Milling reduces the particle size, changing the DF content drastically. This reduction is due to the removal of the outer layer of the cereal grain, which contains the highest concentrations of DF. The peeling of fruit and tubers has the same effect.) Most plant food is consumed only after a mechanical and/ or thermal treatment. Most fruits rich in pectins are consumed raw, while DF from cereals contain high amounts of hemicelluloses. Vegetables, on the other hand, are normally eaten after cooking in water. During cooking, hemicelluloses and pectins are leached into the water. These polymers can partly be disintegrated. Pectins are depolymerized by heat and cannot be found in the cooking water.

It is worth mentioning that even the preparation of samples for the analyses can partly enhance the DF content: drying and grinding of bread crumb, made "S

only from wheat starch, was accompanied by an increase in the insoluble DF

(IDF) content from 2.5 to 3.6% d.m. This was found with bread from normal j flour, too. The reason is the building of enzyme-resistant starch (1).

The effect of fiber particle size on functionality has been addressed, and evidence was found that particle size is highly influential on the functionality and nutritional value of dietary fiber containing raw material. For instance, the bile salt-binding capacity decreases with decreased particle size (2). The susceptibility to fermentation in the colon of human subjects is inversely proportional to the size of bran particles (3). The moisture content in feces is significantly higher in humans given a coarse bran diet than in humans given a fine bran diet (4,5). The Englyst definition (6) of DF as non-starch polysaccharidic (NSP) cell-wall material has the benefit of not being effected by processing. So the NSP values

Effect of Processing

for white, brown and wholemeal bread are very similar to the values for the corresponding flours from which they are made; total NSP in the crumb is also similar to NSP values for the breads. Arabinose content in the crust is slightly lower with a higher proportion of xylose residues being measured in the soluble fraction than in the crumb (7). On the other hand, if enzyme-resistant starch (RS) is unfermentable by human enzymes and only is degraded by the colonic microflora in the large intestine, it should be included in the DF value. This part of DF is not accounted for in the Englyst method.

There are different possibilities of changing the DF content by processing (Table 1). These may lead to increased solubility of the fiber, decreased dietary fiber content (if low molecular fragments are formed), and loss of physiological properties of the fiber.

Resistant starch (RS), which is retrograded amylose, is known to develop in starchy food after heating during the cooling period, the amount depending mainly on the content of amylose or the amylose/amylopectin ratio, respectively, and water. However, other important factors are the processing temperature, the physical form, degree of gelatinization, and cooling and storage conditions (1, 8-11). Raw and processed foods contain appreciable amounts of RS, depending on the botanical source of the starch and the type of processing. In the case of waxy maize starch (normally zero amylose content), yields of RS were extremely low at all temperatures in contrast to Hylon VII (nominal 70% amylose), for which yields were high (approximately 34%) (11). By autoclaving and cooling high-amylose corn starch several times, the amount of RS can be increased up to about 60%. DF determined by the AOAC method is known to contain the RS as part of the complex. Today RS is accepted as part of the DF complex (9,10).

Discrepancies in the declaration of the RS content of foods occur because RS was initially defined as that starch which was determined after solubilization

Table 1 Possible changes in DF content by processing

Leaching of SDF, like pectins or fragments of hemicelluloses, into the process water

Decomposition of SDF or IDF into fragments, not determined as DF Decomposition of insoluble to soluble DF (accompanied with a relatively constant DF content) Decrease of degradable products like starch or protein, which means an indirect DF enrichment Formation of enzyme-resistant starch

Formation of acid-insoluble material Maillard products ("lignins") Increase of enzyme-resistant compounds such as Maillard products ("lignins") and RS

Rabe with 2 M KOH or DMSO. RS was the starch solubilized in the DF residue or the difference in starch content with and without added DMSO. Different methods for starch determination were used, too. In products with a high starch to fiber ratio, formation of enzyme-resistant starch during processing may contribute significantly to the DF content of the product. RS has been identified in cooked and baked foods such as bread, boiled potatoes, pasta, legumes, porridge, and corn flakes at levels of 1-4 g/100 g d.m. Cooked, dried, and ground legume seeds, such as beans, which are rich in amylose, showed an RS content of 3-6%, which is considerably higher than that formed during baking of bread. As the raw beans showed 0.8% RS it may be that these products have a high content of RS2 (12,13), which is resistant starch like raw potato starch.

Although Englyst (14) proposed a method for the determination of different kinds of RS, which was accepted initially, there is still the need for a quick and reliable method, mainly for the determination of RS3, the retrograded amylose. The true RS content of foods may be higher than that recorded for RS3 in fiber residues since only a fraction of retrograded starch remains in the residues. The continuing development of new methods (15) shows there still is demand for it. Although interesting from a scientific point of view, the physically inaccessible kinds of RS are not of great importance, as most food is consumed after heating. To take this to the extreme, it means RS2 from potatoes should not be considered, because when they are consumed, there is only RS3. On the other hand, in eating raw cereals like muesli, there really is a greater consumption of RS2, physically trapped starch, but only a small part of the population consumes it.

The repeated heating of starch-rich foods (Table 2), measured by the IDF fraction, shows the possibility of accumulation of RS. Potatoes (variety ''Hansa'') were cooked in water, drained, cooled to room temperature, and stored in a refrigerator for 18 h (overnight). Heating of the cooked potatoes, the cooked polished rice, and the pasta (100% durum wheat) in the microwave oven was done without additional water.

It can be seen from Table 2 that the content of IDF, or of RS, respectively, "S

is greater in products heated with excess water in a normal cooking pot than in products heated without water, e.g. in a microwave oven, as expected. On the j other hand, in heating in the microwave oven, there was an increase, too. The highest amount of RS, compared to the original but low value for the uncooked food, was formed in rice heated in a normal cooking pot or a microwave oven. In pasta, too, there was a great increase, but only about half that in rice. The reason was that these foods are rich in amylose and underwent complete starch gelatinization during cooking. The least amount was formed in potatoes, independent of the method of heating. The decrease in RS in rice during heating in the normal pot results from the fact that in this case, there was a great amount of insoluble material poured away with the cooking water, which was opaque from it (16).

Effect of Processing 399

Table 2 RS formation in reheated foods as seen by IDF (% d.m.)

Food

Normal Cooking pot

Microwave oven

Pressure cooker

Potatoes, uncooked:

5.0%

5.8%

Cookng time: 15 min

7.2%

6.2% (6 min)

+ 2 min

8.3%

6.1%

8.6%

+ 2 min

8.9%

8.1%

9.8%

Rice, uncooked

1.0%

Cooking time: 10 min

4.6%

+ 2 min

7.9%

5.4%

+ 2 min

5.1%

7.2%

Pasta, uncooked

1.8%

Cooking time: 12 min

4.0%

+ 3 min

5.0%

4.4%

+ 3 min

5.8%

4.8%

As can be seen from these results, the reheating of already cooked pasta, (salad-) potatoes, and rice is a way to increase the RS or DF content, respectively, in normal starchy food without adding other material. In order to reach a higher intake of RS, consumers can be told to prepare a greater amount than is actually needed, refrigerate the surplus, and reheat as needed. This heating is best done with additional water and in a normal cooking pot or in a pressure cooker. The increase in the absolute amounts is relatively small after repeated heating/cooling cycles compared to the first heating (16).

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