Simple determination of gluten protein types in wheat flour by turbidimetry

A simple method based on turbidimetry has been developed for the quantitati ve determination of total gliadins, glutenin subunits, and high and low mol ecular weight (HMW and LMW) subunits of glutenin. The standard procedure in cludes the subsequent extraction of wheat flour (100 mg) with a salt soluti on, with 50% 2-propanol (gliadins), and with 50% propanol under reducing co nditions and increased temperature (glutenin subunits). Aliquots of the gli adin and the glutenin extracts are mixed with 2-propanol to a final concent ration of 83%, and the turbidity of the precipitates is measured photometri cally at 450 nm and 20 degrees C after 40 min. Another aliquot of the glute nin 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 L MW subunits. Control analyses with reversed-phase HPLC on C-8 silica gel in dicate that the precipitation of the different protein types is quantitativ e and specific, and studies of 16 different wheat flours demonstrate the st rong correlation between quantification by HPLC and turbidimetry. The turbi dimetric measurements are reproducible, linear over a wide absorbance range (0.2-1.7), and sufficiently sensitive to analyze 40 mu g of protein or 20 mg of flour. The absolute amounts of protein types in flour can be determin ed by means of calibration curves with protein standards (gliadins, HMW, an d LMW subunits). Altogether, the developed method is simple, accurate, sens itive, and specific for the different protein types. The total procedure ta kes approximate to 6 hr for the analysis of six flour samples in parallel o r approximate to 4 hr for three samples in overlapping extraction steps. Th e chemicals used are inexpensive, scarcely toxic, and easy to dispose.