INVESTIGATION OF VITRIFICATION BY NUCLEAR-MAGNETIC-RESONANCE AND DIFFERENTIAL SCANNING CALORIMETRY IN HONEY - A MODEL CARBOHYDRATE SYSTEM

The vitrification of honey, a model system, was studied by utilizing nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). Various dilutions of honey, 100-0%, were analyzed, and heat capacity (Cp) of vitrification was calculated by using DSC. Complementary information was obtained by using NMR for the determination of the degree of proton mobility and magnetic relaxation of aqueous solution protons during DSC-observed events. DSC transitions and thermal events, Tg, Tc, Td, and Tm, detected heat flow and plots of the first derivative of heat capacity correlated to peak amplitude of the free induction decay (FID) signal observed by NMR. The first derivative of heat capacity, when plotted against temperature, yielded a definitive separation of thermal events. It was observed, on cooling at -5 °C min-1, that 0% honey (100% ddH20) crystallized (Tc) at -13 °C and as the concentration of honey increased above 35% the Tc decreased as did the exponential decay of the FID. At concentrations >50 % honey there was a loss of the FID signal due to the increase in viscosity which decreased relaxation times. At 50% dilution the FID showed two changes in slope, at -42 and -60 °C, which correlate to two DSC-observed Tg's at the same temperatures following a bulk Tc. It was determined that the second glass was not dependent upon the first as there was no T¿ observed in DSC-monitored heat flow or decrease in FID signal between those two temperatures. At concentrations >80% there was a Tg at -77 °C and no ice formation on cooling, but a Td at a warming rate of +5 °C min-1 or less. In undiluted honey (100%) the Tg was -40 °C and the corresponding change in Cp was 0.81 J °C g'1. An isothermal hold at -62.5 °C for 90 min followed by warming through the Tg showed an overshoot of Cp. This demonstrates the relaxation of the vitreous sample and concomitant loss of entropy. The carbohydrate content of the undiluted sample (referred to as 100% honey) analyzed by high-performance liquid chromatography was 47.2% fructose, 43.2% glucose, 1.6% sucrose, and 8% undetermined component(s), possibly maltose. © 1990, American Chemical Society. All rights reserved.