Precipitation of inulins and oligoglucoses by ethanol and other solvents

We investigated the ethanol precipitation step of the gravimetric method of dietary fiber analysis (AOAC Official Method 985.29). Four different solvents: ethanol, propanol, acetone and acetonitrile at four ratios: 1:1, 2:1, 3:1 and 4:1 (solvent : supernatant, v/v) were studied. Using inulins that contain components with a full range of degree of polymerization values (DP), we found that the percents of precipitation by these solvents were proportional to the average DP of the products. Use of ethanol and propanol produced similar results. In general, acetonitrile and acetone precipitated more of the inulins than did ethanol. Supernatant solutions were analyzed by high performance anion exchange chromatography in order to determine the size of the components that were precipitated. Our data showed that components of inulin with DP 1-10 remain in solution after the addition of ethanol at a ratio of 4:1. However, significant amounts of molecules of DP 11 and 12 and smaller amounts of molecules of DP 14-18 also remain in solution. The precipitation patterns of oligoglucoses with DP 1-DP 7 were also investigated. Our data suggest that the precipitation behavior of oligoglucoses follows a pattern similar to that of inulins.