Measurement of casein in milk by Kjeldahl and sodium dodecyl sulfate–polyacrylamide gel electrophoresis

Our objectives were to determine if milk casein as a percentage of true protein (CN%TP) estimated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) is equivalent CN%TP Kjeldahl, and the proportion (CN), proteolysis products (CNPP), serum (SP) from (TP) that goes into Kjeldahl noncasein nitrogen (NCN) filtrate stays in NCN precipitate using SDS-PAGE. Raw samples collected 16 mid-lactation Holstein cows twice week for 2 wk. These milks analyzed total nitrogen, nonprotein content duplicate, The determined was compared with SDS-PAGE calculated ways: only intact caseins divided TP both CNPP TP. Three varying fat, lactose, TP, CN, SP formulated. duplicate content, each relative quantity (%) CNPP, SP. We found estimate higher than CN all bands. However, no difference detected when included calculation results. Based on results, we majority (89%) (approximately 10.13 out 11.41% TP) retained precipitate. Thus, measured underestimates amount proteolytic damage has been done milk. It important dairy industry correctly rapidly measure extent value product quality yield point view. A rapid quantitative needed.