Exploring the Seasonal and Geographical Effect on the Chemical Signature of Raw Milk

Abstract

Nowadays raw milk plays an important role in dairy industry, particularly from chemical composition and microbiological point of view. The objective of this research was to compare the raw milk samples in two different regions (Auckland and Waikato) and two different seasons (summer and winter) by means of Nuclear Magnetic Resonance (NMR) and Infrared (IR) spectroscopy. As there exists fat globules in raw milk and it will have influence on milk analysis, a good way to homogenize milk was also explored. It was found that homogenizing by ultrasound was the optimal way to homogenize raw milk rather than mixing the sample on a stirrer hot plate or stirring by vortex alone. For IR analysis, two different measurement modes ─ attenuated total reflection (ATR) and transmission mode were used to measure milk metabolomics. It was observed that transmission mode of FTIR was the best method for raw milk investigation. In terms of NMR measurements, the water suppression method ─ presaturation was applied to NMR to observe the signals of milk components properly. Two methods for multivariate statistical analysis ─ Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) in ‘ChemoSpec’ by R were applied to investigate whether there are differences among raw milk samples and whether they can be separated according to region, season, or season and region. The results showed that no any obvious separation can be observed between different regions and seasons, but there is clear clustering of samples according to individual farm, which indicates that milk from each farm is unique and its spectrum can serve to classify the farm of origin.