Proton transfer reaction-mass spectrometry (PTR-MS) for rapid detection of oxidised odour in milk during storage

Milk flavour varies greatly depending on its oxidation state, which can be measured by several methods like HPLC or GC-MS. However, due to constraints associated with methodological complexity, time of analysis and costs, implementation of these methods in food industry is often limited. Recently, proton transfer reaction-mass spectrometry (PTR- MS) has been proposed as an alternative analytical technique, especially for food quality control. PTR-MS offers advantages over some traditional methods including the involvement of a mass spectrometer based on soft ionisation. This means that the resulting mass signals are mainly composed of protonated parent ions derived from volatile organic compounds (allowing their straightforward interpretation). Moreover, PTR-MS allows direct and continuous measurement and suitable for rapid on-line monitoring of foods during processing or storage. Therefore, PTR-MS was used here for rapid detection of milk oxidation. Oxidation state was determined by measuring hexanal, a well-known marker of fat oxidation in milk samples. Hexanal was monitored by tracking the changes in the abundance of the mass fragment at m/z 83. Different oxidation states in milk samples were induced by spiking different concentrations of Cu++ (i.e. increasing from 0 to 32 ppm) during storage conditions (4°C). Results showed that the intensity of the mass fragment at m/z 83 evolved following a sigmoidal trend with a maximum rate proportional to the Cu++ content in milk. The proposed analysis procedure is fast, sensitive (8.8 ±ppbv/µM), precise (RSD < 6%; low limit of detection: 0.5 µM), and non-invasive. Therefore, this PTR-MS procedure is suitable for rapid detection of hexanal to monitor the oxidative changes in milk. Keywords: milk oxidation, copper, hexanal, rapid method.