MEASUREMENT OF CASEIN CONTENT IN MILK AND MILK-BASED PRODUCTS USING KJELDAHL, SODIUM DODECYL SULFATE POLYACRYLAMIDE GEL ELECTROPHORESIS, AND MID INFRARED SPECTROSCOPY

Abstract

The objective of the first part of the study was to develop and validate partial least squares (PLS) models using MIR spectra to predict true protein (TP), casein (CN), serum protein (SP), and casein as a percentage of TP (CN%TP) content of micellar casein concentrates (MCC) and unflavored milk-based beverages. The PLS models, developed using 625 samples of formulated milk with a range of CN%TP and orthogonal matrix of fat, TP, and lactose were used for model development based on Kjeldahl reference chemistry and mid-infrared spectra collected for each sample. The ratio of performance to deviation (RPD) values were 62.7, 31.4, 14.1, and 3.8 for the prediction of TP, CN, SP, and CN%TP, respectively. External validation of the PLS models using 18 formulated MCC and 28 formulated unflavored milk-based beverage produced good validation performance, which may be attributed to the high RPD values achieved by orthogonal design of the modeling population used for the PLS model development.The objectives of the second part of the study were to determine if milk CN%TP estimated by sodium dodecyl sulfate polyacrylamide gel (SDS-PAGE) is equivalent to CN%TP estimated by Kjeldahl, and to determine the proportion of CN, casein proteolysis products (CNPP), and SP from milk TP that goes into the Kjeldahl non-casein nitrogen (NCN) filtrate and the proportion that stays in the NCN precipitate using SDS-PAGE. The estimate of CN%TP by Kjeldahl was higher than the estimate of CN%TP by SDS-PAGE that was calculated as only intact CN divided by the total of all protein bands. However, no difference was detected in the estimate of CN%TP by Kjeldahl compared to CN%TP by SDS-PAGE when CNPP were included as CN in the calculation of SDS-PAGE results. SDS-PAGE analysis of the Kjeldahl NCN filtrate and NCN precipitate of three formulated milks with varying fat, lactose, TP, CN, and SP content showed that a majority (89%) of the CNPP from the milk (approximately 10.13% out of 11.41% TP) were retained in the Kjeldahl NCN precipitate. In conclusion, the CN%TP measured by Kjeldahl underestimates the amount of proteolytic damage that has been done to casein in milk.