Simple determination of gluten protein types in wheat flour by turbidimetry

A simple method based on turbidimetry has been developed for the quantitative determination of total gliadins, glutenin subunits, and high and low molecular weight (HMW and LMW) subunits of glutenin. The standard procedure includes the subsequent extraction of wheat flour (100 mg) with a salt solution, with 50% 2-propanol (gliadins), and with 50% propanol under reducing conditions and increased temperature (glutenin subunits). Aliquots of the gliadin and the glutenin extracts are mixed with 2-propanol to a final concentration of 83%, and the turbidity of the precipitates is measured photometrically at 450 nm and 20 degrees C after 40 min. Another aliquot of the glutenin extract is mixed with acetone to a final concentration of 40% acetone, and precipitated HMW subunits are determined turbidimetrically after 30 min. The sample is then filtered, and an aliquot of the filtrate is mixed with 2-propanol to a final concentration of 77% to determine the precipitated LMW subunits. Control analyses with reversed-phase HPLC on C8 silica gel indicate that the precipitation of the different protein types is quantitative and specific, and studies of 16 different wheat flours demonstrate the strong correlation between quantification by HPLC and turbidimetry. The turbidimetric measurements are reproducible, linear over a wide absorbance range (0.2-1.7), aud sufficiently sensitive to analyze 40 microgram of protein or 20 mg of flour. The absolute amounts of protein types in flour can be determined by means of calibration curves with protein standards (gliadins, HMW, and LMW subunits). Altogether, the developed method is simple, accurate, sensitive, and specific for the different protein types. The total procedure takes approximately equal to 6 hr for the analysis of six flour samples in parallel or approximately equal to 4 hr for three samples in overlapping extraction steps. The chemicals used are inexpensive, scarcely toxic, and easy to dispose.