Abstract
We have confirmed that the determination of DF (dietary fiber) in foods by the AOAC enzymaticgravimetric method yields very low recovery of pectin which is stable in weak acidic solution at pH 3 to 5, but degrades on heating of the solution at near neutral pH 5 to 7. This degradation might be due to splitting of high-molecular-weight pectin into low-molecular-weight fragments. Therefore, enzymatic digestion for determination of DF should be done in weakly acidic conditions. Heat-stable α-amylase has considerable activity under weakly acidic conditions at low temperature. Foods were dissolved in hot dimethyl sulfoxide, and digested by heat-stable α-amylase at pH 4.7 and 70°C for 2h in dialysis tubes. The tubes were then dialyzed against running deionized water for 7h at room temperature, and the inner fluids were evaporated to dryness in a rotary evaporator flask. The residues in the flasks gave almost no iodine coloration of starch, except for residues from heated food homogenates. For the separation of lipids, residues were refluxed twice with ethanol and once with acetone, and weighed. Then the ash and protein contents were estimated. This procedure consisted of only α-amylase treatment in a dialysis tube and successive separation of the digestion products by dialysis. The estimation of DF in foods by this new simplified method yields somewhat less variable results.
