Determination of honey crystallization and adulteration by using time domain NMR relaxometry

Berk, Berkay
Honey is an important source of sugar and widely used as a natural sweetener. The demand for honey has increased significantly due to increase in the consumer tendency to use natural sugars as an alternative to starch based sweeteners (SBS). Thus, honey as being expensive compared to SBS has become very susceptible to adulteration and started to suffer from significant quality problems. In addition to adulteration, crystallization is another problem that could affect the quality of the honey. Crystallization is a natural process that could occur in honey due to several reasons and precipitation of glucose is one of them. In this study, a non-conventional TD-NMR pulse sequence: Magic Sandwich Echo (MSE) was used to monitor the crystallization of honey at 14℃. Honey samples were seeded with glucose to induce crystallization and samples were monitored for 96 hours. T1 and T2 relaxation times were measured during the process to follow changes in the state of water. Mono-exponential and bi-exponential relations were used to explain T1 and T2 relaxation times, respectively. As a complementary method to NMR, crystal melting enthalpy was also measured by differential scanning calorimetry (DSC). High correlations were detected between NMR and DSC results (r=0.87). During crystallization, T1 relaxation times did not change, whereas both components of T2 decreased with increasing time. Besides crystallization, MSE sequence was used to detect adulteration of honey by glucose (GS) and high fructose corn syrups (HFCS) accompanied with T1 and T2 relaxation time measurements. Mono-exponential and bi-exponential relations were also used to explain T1 and T2, respectively in this part of the study. Higher maltose in GS (r=0.96) and changing glucose to water ratio of HFCS (r=0.91) gave high correlations with the MSE output. In both HFCS and GS adulteration, two different proton pools of T2 were detected. Addition of GS resulted in decrease in fast-relaxing component of T2 time due to decreased mobility. All components of T2 time increased with HFCS addition due to higher water content of HFCS resulting in higher mobility. In addition to crystallization and adulteration, the melting of crystallized honey was also performed at 50℃ inside the NMR system. Crystal melting was monitored in real-time, and the rate of crystal removal was fitted to a mono-exponential decay model. Results obtained from NMR experiments showed that TD-NMR is a powerful technique to investigate crystallization, detect adulteration and monitor melting of honey samples.


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Citation Formats
B. Berk, “Determination of honey crystallization and adulteration by using time domain NMR relaxometry,” M.S. - Master of Science, Middle East Technical University, 2020.