Shelf-life prediction of processed milk by solid-phase microextraction, mass spectrometry, and multivariate analysis

A technique based on solid-phase microextraction, mass spectrometry, and multivariate analysis (SPME-MS-MVA) was used to predict the shelf life of pasteurized and homogenized reduced-fat milk and whole-fat chocolate milk sampled over a 7 month period. Using SPME-MS-MVA, which is essentially a mass spectrometry-based electronic-nose instrument, volatile bacterial metabolites were extracted from milk with SPME (Carboxen-PDMS) and injected into a GC capillary column at elevated temperature. Mass fragmentation profiles from the unresolved milk volatile components were normalized to the intensity of a chlorobenzene internal standard mass peak (m/z 112) and subjected to MVA. Prediction models based on partial least-squares regression of mass intensity lists were able to predict the shelf life of samples to approximately +/-1 day, with correlation coefficients greater than 0.98 for the two types of milk samples. Using principal component analysis techniques, the procedure was also useful for classifying samples that were rendered unpalatable by nonmicrobial sources (contamination by copper and sanitizer) as well as by bacteria.