Monitoring of lactose hydrolysis in milk by single-bounce attenuated total reflectance Fourier transform infrared spectroscopy

A Fourier transform infrared (FTIR) method for monitoring the hydrolysis of lactose in milk has been developed based on the use of a single-bounce attenuated total reflectance (SB-ATR) accessory. Preliminary work indicated that the SB-ATR accessory was suitable for the analysis of sugars in milk and that the trace by-products produced by transgalactosylation were unlikely to interfere with the analysis. Calibration standards were formulated by mixing milk and Lacteeze(R) (99% lactose-free milk) gravimetrically with lactose, glucose, galactose, and distilled water to introduce compositional variability. The spectra of the formulated calibration standards were recorded and partial-least-squares (PLS) regression was used to derive a calibration relating the spectral information to the sugar concentrations of the standards, calculated from gravimetric and HPLC data. Lactose in milk was hydrolyzed at 4degreesC using beta-galactosidase obtained from Kluyveromyces lactis, with samples taken for FTIR and HPLC analysis at selected times over 48 h. The concentrations of lactose, glucose, and galactose were predicted for the FTIR-analyzed samples using the PLS calibration and compared to HPLC results, yielding standard errors of prediction of 0.20, 0.13, and 0.10% (w/v), respectively. The results indicate that the SB-ATR/FTIR method is highly accurate and could serve as a convenient means by which to monitor lactose hydrolysis in the manufacture of lactose-reduced milk.