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A collaborative study was conducted to validate a nonenzymatic-gravimetric method for the determination of total dietary fiber (TDF) of samples containing little or no starch such as most fruits, and vegetables and many purified polysaccharides. This simple procedure involves suspension of freeze-dried, ground samples in deionized water and incubation at 37°C for 90 min, followed by pre-cipitation with 4 volumes of 95% ethanol. The weight of the dilute alcohol-insoluble residues after correcting for crude protein and ash corresponds to the TDF content of the sample. Six samples in blind duplicate (apples, apricots, cabbage, carrots, onions, and soy fiber) were sent with Celite to 10 laboratories. The reproducibility relative standard deviation (RSDr) of the TDF values for 9 laboratories ranged from 2.92 to 6.25%. The repeatability standard deviation (RSDr) for the 9 laboratories ranged from 1.50 to 2.70%. The method has been adopted first action by AOAC INTERNATIONAL.

Most gravimetric methods for total dietary fiber (TDF) determination require the complete removal of starch and the partial removal of protein with various combination of enzymes In buffers at different pH values and at temperatures much above ambient condition. A hydrolysis step Is crucial In dietary fiber analysis of samples, such as cereals and legumes, which contain appreciable amounts of starch. However, many vegetables and most fruits contain very little or no starch, and they are often eaten uncooked. It would be unnecessary to use high temperatures and enzymes on these types of samples. Initially, we found that hexane and dilute alcohol extractions of a few selected fruits and vegetables gave residue weights comparable to those after enzymatic treatments. We were able to show later that simply suspending the samples in delonized water for 90 mln at 37°C and then adding 95% ethanol also yields TDF values similar to those obtained from other published methods. Comparison data obtained by using variations of the AOAC/TDF method are presented for 10 fruits and vegetables.

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