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Developing food analysis methods using benchtop NMR spectrometer

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2019
Söyler, Alper
The online monitoring of chemical reactions by using benchtop Nuclear Magnetic Resonance (NMR) spectroscopy has become increasingly attractive for the past few years. The use of quantitative online NMR spectroscopy is a promising alternative to traditional analytical methods with its rapid, quantitative and non-invasive nature that makes it applicable to complex and diverse biochemical mixtures like food systems. In this dissertation, sucrose hydrolysis by invertase and milk lactose hydrolysis were chosen as model reactions for online monitoring. Reactions were performed in a continuous flow mode. Moreover, rather than conventional NMR spectroscopy experiments that mostly rely on the use of deuterated water, the benchtop setting allows working in protonated solvents, and tailored water suppression techniques were used to make quantification more accurate. For the hydrolysis reactions, 10% sucrose solution, 5% lactose solution and a milk sample were hydrolyzed. The kinetic constant was determined by the fractional conversion model. Average rate constants for the hydrolysis reactions of sucrose, lactose solution and milk sample were found as 12.88x10-3 min-1, 1.66x10-2 min-1, 1.52x10-2 min-1, respectively. All the results were comparable to the results obtained in other studies found in the literature. Quantitative online NMR spectroscopy was seen as a promising tool for monitoring food processes in a continuous mode. In addition to online monitoring of enzymatic hydrolysis reactions, authentication of different milk types was achieved using NMR spectroscopy and statistical methods (ANOVA and Discriminant Analysis). Lactose, fat and glycerol contents were obtained from the NMR spectra. Milks from Turkey and France were able to be differentiated with their glycerol content. Lastly, honey adulteration by high fructose corn syrup has been studied by NMR spectroscopy combined with relaxometry techniques. Adulterated samples were discriminated by the relaxation times. Relaxation times of alpha anomer, sugar region, water and bulk sample were calculated using a monoexponential approach. A linear relationship was found between high fructose corn syrup concentration and T2 relaxation times (R2>0.92). Benchtop NMR spectroscopy was seen as a promising, easy to use and affordable tool to analyze food samples either for academic research and for the industry. It has significant potential on developing new analysis methods.