A rapid method to quantify casein in fluid milk by front-face fluorescence spectroscopy combined with chemometrics

Abstract

Casein in fluid milk determines cheese yield and affects cheese quality. Traditional methods of measuring casein in milk involve lengthy sample preparations with labor-intensive nitrogen-based protein quantifications. The objective of this study was to quantify casein in fluid milk with different casein-to-crude-protein ratios using front-face fluorescence spectroscopy (FFFS) and chemometrics. We constructed calibration samples by mixing microfiltration and ultrafiltration retentate and permeate in different ratios to obtain different casein concentrations and casein-to-crude-protein ratios. We developed partial least squares regression and elastic net regression models for casein prediction in fluid milk using FFFS tryptophan emission spectra and reference casein contents. We used a set of 20 validation samples (including raw, skim, and ultrafiltered milk) to optimize and validate model performance. We externally tested another independent set of 20 test samples (including raw, skim, and ultrafiltered milk) by root mean square error of prediction (RMSEP), residual prediction deviation (RPD), and relative prediction error (RPE). The RMSEP for casein content quantification in raw, skim, and ultrafiltered milk ranged from 0.12 to 0.13%, and the RPD ranged from 3.2 to 3.4. The externally validated error of prediction was comparable to the existing rapid method and showed practical model performance for quality-control purposes. This FFFS-based method can be implemented as a routine quality-control tool in the dairy industry, providing rapid quantification of casein content in fluid milk intended for cheese manufacturing.

Spectroscopic technologies and data fusion: Applications for the dairy industry

“…In the study, casein was precipitated using acetic acid prior to fluorescence analysis. PLS and elastic net regression models both performed well in predicting casein% with R 2 value of 0.91 for both PLS and elastic net and cross validated RMSE of 0.12% ( 85 ). Heat-induced coagulation was also measured using fluorescence ( 86 ).…”

“…Acidification of casein can also be measured using FFFS as, when casein is in an acidic environment, it undergoes structural changes that increase the fluorescence intensity of tryptophan. This is associated with structural and conformational changes in the casein micelle as colloidal casein phosphate (CCP) dissociates from the micelle at low pH, exposing more tryptophan residues ( 85 ). In the study, casein was precipitated using acetic acid prior to fluorescence analysis.…”