Abstract
Next to prostitution, counterfeiting of food products is considered to be the oldest profession of mankind. Currently costing the global food industry US$40 billion a year, this risk to food safety prevails due to globalization and trade liberalization. The bovine milk market is one of the primary victims of this fraudulent practice, thus demands methods to determine product authenticity and provenance.
Therefore, this project aimed to evaluate the potential of using multi-element stable isotope ratio analyses for differentiating milk samples according to their geographical origin. This study provides new insights into non-targeted milk provenance applications where compound specificity was prioritized in which milk casein was used as the primary component.
This study began by investigating the relationship between surface adhered water and acidic H in the sample matrix under different drying temperatures using a series of organic reference materials with different hygroscopic properties. This was important to explore as isotopic exchange reactions which take place between residual water vapor, and acidic H of the sample matrix could hugely affect the accuracy and reproducibility when measuring the stable isotopic composition of hydrogen (Delta2H) and oxygen (Delta18O) in complex organic matrices. Choosing the appropriate sample pre-treatment (e.g. drying temperature) for different sample matrices was shown to be crucial in producing accurate and reproducible Delta2H and Delta18O measurements. These findings were carried forward in the later stages of the study.
Milk samples collected from the dairy farms across New Zealand were utilized in a pilot study where Principal component analysis (PCA) based on stable isotopic parameters (Delta2H, Delta18O, Delta15N and Delta13C) was employed to verify caseins ability in discriminating between milk samples of different origin. Delta15N was the strongest predictor of provenance, demonstrating a strong relationship with climatic variability, while Delta2H and Delta18O by themselves were inadequate for identifying milk regional authenticity.
Subsequent exploration of the isotopic link between casein nitrogen and climatic variability revealed that rumen metabolic reactions' attributions play a significant role in the strong correlation observed between casein Delta15N and climatic parameters. Results from PCA analysis and multiple linear regression (MLR) showed the efficacy of casein Delta15N to be used as a tool for understanding the bio-physical conditions of the cattle milking environment, thus its potential for identifying the climatic origin of milk.
A preliminary assessment was also carried out to investigate the potential of 87Sr/86Sr ratios to determine the geographical origin of milk. At the same time, this study presents the optimization and adaptation of prep-FAST MC to allow for a sufficient and rapid Sr/matrix separation of Ca-rich milk matrix. Overall, this was an initiative towards establishing an authentic reference database for future provenance applications.
In summary, the knowledge that was obtained in this study demonstrates the necessity of using integrated isotopic systems (Delta13C, Delta15N, Delta18O, Delta2H and 87Sr/86Sr) for verifying the provenance of milk products. Each specific approach could be further developed as suggested in this study and adopted for milk authentication purposes.