Abstract
This study is the first to provide a comprehensive characterization of the liquid and volatile fractions of goat whole milk powder. For this, robust nuclear magnetic resonance (NMR)- and gas chromatography-mass spectrometry (GC-MS)-based fingerprinting methods were implemented. The untargeted 1H-NMR analysis resolved 44 metabolites in the liquid fractions of goat whole milk powder. The NMR fingerprinting technique effectively identified metabolites coming from the aliphatic, sugar, and aromatic regions that can be important in defining the technological properties and quality of the milk sample. The untargeted headspace-GC-MS fingerprinting was able to detect a total of 50 volatiles including alkanes, ketones, alcohols, aromatics, alkenes, aldehydes, esters, acid, and sulphur-compounds. Goat milk is a premium product and vulnerable to fraudulent activities such as adulteration or counterfeit. Therefore, proper characterization/identitation is an important first step to verify its authenticity and quality.
Following characterization, this work provided an insight regarding the metabolite and volatile changes in goat whole milk powder during its shelf-life. For this, the chemical fingerprinting was integrated with advanced chemometrics and feature selection methods. For the metabolite composition in the water-soluble liquid fraction, the optimized untargeted 1H-NMR fingerprinting method was able to detect complex metabolite changes during storage of the milk powder. Results showed a decreasing metabolite content in the stored goat whole milk powder over time. Particularly, metabolites in the carbohydrates and amino acids group showed significant decrease in concentration due to possible Maillard reaction-associated degradations. The present study identified adenosine triphosphate (ATP) as an important metabolite which has the highest correlation with storage time. This component may cause changes in the nutrients during milk powder shelf-life, and thus, may affect the quality of milk. Other metabolites selected as potential markers were galactose, glycerophosphocholine, and methionine.
Finally, an integrated GC-MS-based fingerprinting and chemometrics was used to study volatile changes during the storage of the goat whole milk powder. A total of 14, 21, and 39 volatile compounds were found to have significantly changed when stored at 20 ℃, 30 ℃, and 40 ℃ respectively for up to 44 weeks. These volatiles showed an increase in concentration over time and can be attributed to lipid oxidation and Maillard reactions. In addition, the suitability of these volatiles as potential shelf-life markers was further investigated. There were 11 candidate volatile shelf-life markers selected including pentanal, heptanal, hexanal, (E)-2-octenal, octanal, 1-octanol, 1-pentanol, 1-hexanol, 1-octen-3-ol, 3-octen-2-one, and 2-heptanone. These compounds have key roles in the odour of dairy-based products. To quantify the effects of storage time and temperature on these compounds, a kinetic study using a one-step logistic model was done. Shorter lag phases of markers and their higher rates of formation at increased storage temperature show its clear effect on the sample. Moreover, their increased formation during storage indicates possible flavour changes and can be useful to monitor product quality during the shelf life of goat whole milk powder.