An important criterion of the total dietary fiber (TDF) value determined by any method is that it should not be influenced by processing in order to be used for food labelling. If not, processing conditions favoring high values may be preferred, but this may not correspond to beneficial physiological effects.
The complete removal of starch from the dietary fiber residue has been a challenge for all dietary fiber methods (12,42-44). When present, residual starch is measured mostly as part of the insoluble fiber in gravimetric analyses and as glucose in GC analyses. In boiled legumes, the amount of starch in the residue using certain official methods can outweigh that of dietary fiber (12,43). This situation is partly due to the choice of enzymes and to the sequence of enzyme treatments. Non-mammalian (e.g., bacterial) sources of enzymes need to be puri- "S
fied to prevent the degradation of dietary fiber by contaminating enzymes. How- g ever, purified enzymes may have more limited activities due to loss of cofactors.
A less pure mammalian enzyme has more diverse bond-breaking capacities, and t?
multiple attacks by amylase and protease are critical (12,43). Two sequential ^
short treatments are better than a very long one. Ideally, the fiber residue should contain only the cell wall structural protein and no other protein. Larger amounts |
indicate the presence of residual digestible protein and possibly an incomplete starch digestion. Starch-protein interactions are present in certain food products and the absence of a protease treatment may reduce the rate of in vitro starch hydrolysis (45). It is therefore important that proteolytic enzymes are included in the in vitro assay (28) either as a separate treatment or included with an unpurified mammalian alpha-amylase preparation (43).
Mongeau et al.
Structural protein (46-47) and lignin are non-carbohydrate components of dietary fiber. One method (Englyst) overlooks lignin and only one method (Mongeau) measures the small amount of undigestible protein. Permanganate lignin and Klason lignin values are not always in agreement (43-44). Klason lignin is generally higher and more variable than permanganate lignin. Lignin is difficult to separate from some hemicelluloses and can react with protein during food processing (48). The rapid Mongeau method includes treatments that represent an appropriate preparation for measuring permanganate lignin in foods, and values are little influenced by food processing. According to Selvendran (7), the acetyl bromide method needs only a 5-10 mg sample but comparison of values has not been made. Lignin is regarded as a minor component of most plant foods but the total polyphenolic material (including lignin) represents 5-20% of the dry cell wall (7).
Food processing can solubilize some of the insoluble fiber (49). The resulting higher soluble: insoluble fiber ratio should not be interpreted as having oat-gum like physiological effects. The measurement of soluble dietary fiber increases the time and cost of analysis with some methods, and the soluble fiber values are highly method-dependent. Only the total dietary fiber values can be compared among methods, but still with certain limitations.
The Mongeau (AOAC #992.16) TDF method (42) was developed over 11 years by testing various combinations of treatments taking into account rapidity, precision and accuracy. The method uses neutral detergent but the total dietary fiber values are different from neutral detergent values. The neutral detergent method is not appropriate for measuring insoluble fiber in human foods unless it incorporates a specific preparation of alpha-amylase. The combination of neutral detergent and unpurified pancreatic amylase has a unique efficiency for digesting protein and it ensures extensive and reproducible digestion of starch. The best results with the above-mentioned TDF method are obtained when the soluble and insoluble fractions are determined sequentially in the same sample. The method provides a soluble: insoluble ratio among the highest. The solubility of dietary "S
fiber in vivo is not well documented but it is thought to be high. g
In various food samples collected throughout Canada over several years j and prepared as they would have been at home, the rapid Mongeau method (y) t?
was shown to correlate well with the integral Prosky method (x) for total dietary fiber (y = 1.02x — 0.04, r = 0.993, n = 38). These methods gave higher values ^
than Englyst NSP (non-starch polysaccharides) values but the difference was I
partly due to the fact that lignin is not measured by the Englyst method. In a method comparison including 65 foods, the TDF values by the Mongeau method were about 1.3 times higher than Englyst NSP values. The factor remained the ©
same when processed foods and legumes were included or excluded since both methods are not affected by processing. By contrast, when the Prosky TDF and |
Englyst NSP methods were compared, the factor was 1.3 when processed food u
Definition and Analysis of Dietary Fiber
and cooked dried legumes were excluded, but it reached 1.6 when they were included in the comparison (NSP X 1.6 = Prosky) (44). Thus, in spite of the close correlation mentioned above, discrepancies among dietary fiber methods can be substantial for some foods. TDF differences even between the Prosky and Lee-modified version were noticed for cooked dried legumes, but these discrepancies were attenuated when an unpurified pancreatic alpha-amylase was used in both versions of the Prosky method (12,43). On the other hand, dietary fiber values for 45 foods (fruits, vegetables, cereals and canned legumes) analyzed by the Mongeau method (y) correlate well with those published by Marlett (50) for the corresponding foods analyzed by a modified Theander method (x): y = 0.99x + 0.06, r = 0.96. Overall, the comparison of the dietary fiber value for various (>60) foods suggests a general agreement among several dietary fiber methods, and a slight modification of the Prosky and Lee methods would permit an even better agreement.
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