Submitted to: Journal of Association of Official Analytical Chemists International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/1998
Publication Date: N/A
Interpretive Summary: Methods for dietary fiber analysis generally fall into two major classes. They are enzymatic-gravimetric and enzymatic-chemical methods. The main difference between these is in the chemical characterization of the fiber residues that are isolated after enzyme treatments. Such compositional analysis of dietary fiber polysaccharides usually requires acid hydrolysis to release their constituent neutral sugars, which can then be quantitatively determined chromatographically. The acid hydrolysis is traditionally carried out in two stages, involving different concentrations of sulfuric acid and reaction time from 2 to 3 hours in water-bath or convection oven. The loss of sugars during the second stage of hydrolysis is well documented. It was found that microwave energy reduced hydrolysis time to less than 6 minutes, yet the neutral sugar compositions were found to be very comparable to those obtained using convection oven. From these results, other analysts may consider using microwave energy for similar hydrolysis reaction to improve yield and shorten time.
Technical Abstract: Hydrolysis of dietary fiber polysaccharides (DFP) is an integral part of any enzymatic-chemical method for fiber analysis. Residues obtained after enzyme treatments of fiber-containing foods are usually suspended in 12 M sulfuric acid and kept at ambient temperature or slight above for at least 1 h, then the mixtures are diluted with deionized water to a final concentration of 1 or 2M acid followed by heating at 100 degree C in a water bath or convection oven for 1 or 2 h. Under these hydrolytic conditions, some degradation of the released monosaccharides generally take place over the duration of hydrolysis. In an attempt to reduce the hydrolysis time and degradation of the sugars, we investigated the feasibility of using microwave energy as a heat source. Preliminary tests were done on a well characterized soy polysaccharide, FibrimR. With a microwave digestion system equipped with temperature and pressure devices, optimum settings of power, time, temperature, or pressure were determined. Results obtained with less than 6 minutes of heating were in good agreement between the microwave and convection oven hydrolysis of DFP for 5 frequently consumed foods.