Mid-infrared spectrometry of milk for dairy metabolomics: A comparison of two sampling techniques and effect of homogenization

Milk production is a dominant factor in the metabolism of dairy cows involving a very intensive interaction with the blood circulation. As a result, the extracted milk contains valuable information on the metabolic status of the cow. On-line measurement of milk components during milking two or more times a day would promote early detection of systemic and local alterations, thus providing a great input for strategic and management decisions. The objective of this study was to investigate the potential of mid-infrared (mid-IR) spectroscopy to measure the milk composition using two different measurement modes: micro attenuated total reflection (μATR) and high throughput transmission (HTT). Partial least squares (PLS) regression was used for prediction of fat, crude protein, lactose and urea after preprocessing IR data and selecting the most informative wavenumber variables. The prediction accuracies were determined separately for raw and homogenized copies of a wide range of milk samples in order to estimate the possibility for on-line analysis of the milk. In case of fat content both measurement modes resulted in an excellent prediction for homogenized samples (R2 > 0.92) but in poor results for raw samples (R2 < 0.70). Homogenization was however not mandatory to achieve good predictions for crude protein and lactose with both μATR and HTT, and urea with μATR spectroscopy. Excellent results were obtained for prediction of crude protein, lactose and urea content (R2 > 0.99, 0.98 and 0.86 respectively) in raw and homogenized milk using μATR IR spectroscopy. These results were significantly better than those obtained by HTT IR spectroscopy. However, the prediction performance of HTT was still good for crude protein and lactose content (R2 > 0.86 and 0.78 respectively) in raw and homogenized samples. However, the detection of urea in milk with HTT spectroscopy was significantly better (R2 = 0.69 versus 0.16) after homogenization of the milk samples. Based on these observations it can be concluded that μATR approach is most suitable for rapid at line or even on-line milk composition measurement, although homogenization is crucial to achieve good prediction of the fat content.

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► The potential of two sampling techniques for FT-IR spectroscopy, as well as the effect of homogenization as sample pre-treatment, was examined in order to measure the composition of raw milk.
► μATR IR spectroscopy resulted in excellent predictions for the crude protein, lactose and urea content of raw milk, while homogenization was necessary to obtain good measurements for milk fat.
► Although the results for HTT IR spectroscopy were not as good, they were still acceptable. For HTT, homogenization was essential for good fat and urea measurements.