Introduction
Honey is a natural sweetener characterized not only by its unique taste, but also by its nutritional and medicinal properties.
All gourmets know that honey may crystallize over time, depending on its composition and the storage conditions. This process affects its texture and appearance as glucose separates from the water phase and forms solid crystals. The stability of honey against CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization is related, amongst other factors, to its thermal properties. In the following, we show how the form of the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition step of honey contains information about its stability.
Experimental
DSC (Differential Scanning Calorimetry) is a typical technique for determining the thermal properties of honey like Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting point and Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature. A DSC measurement was performed on two honey samples of the same brand with different age and appearance. The first one (OLD) was in a crystallized (or partially crystallized) state, the second one (NEW) was clear and in a liquid state.
For the tests, 10.8 mg and 9.6 mg of crystallized and liquid honey were prepared in an aluminum crucible closed with a pierced lid. Each crucible was placed in the DSC cell, cooled down and then subjected to a heating between -80°C and 100°C at a controlled heating rate of 10 K/min-1. During the experiment, the DSC cell was purged with a dynamic flow of nitrogen (40 ml/min).
Measurement Results
Figure 1 depicts the resulting DSC thermogram for both measurements. The endothermal step detected at -44°C (midpoint) in both curves is due to the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition of the honey samples. It was found at the same temperature, but related to different heat capacity step heights.
The step height is connected with the amount of the amorphous phase. The higher the CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization degree of honey, the lower the amount of amorphous phase and the smaller the heat capacity step. This explains the Delta cp of 0.84 J/(g·K) for the crystallized honey in comparison with the higher value of 1.06 J/(g·K) for the liquid one.
An endothermal peak was detected between 30°C and 80°C only in the DSC curve of the crystallized honey. It comes from the transition from the solid to the liquid state and takes place only for honeys that are in the partly or completely crystallized state [1]. The melting peak is superimposed with an endothermal increase in the baseline. This results from the evaporation of the water contained in honey.
The use of closed crucibles allows for the prevention of moisture evaporation, as shown in figure 2. Here, measurements were performed under the same conditions as before, except for one: closed aluminum crucibles instead of crucibles with pierced lid were used. The change in crucible doesn’t influence the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition step. The melting peak can be evaluated more accurately because it is no longer superimposed on evaporation effects. The measurements show that the melting process is connected to an increase in specific heat.
Conclusion
The thermal properties of two honey samples were determined with differential scanning calorimetry. The first one was translucid and liquid, and the second one partly crystallized. The two products differed in their age, the crystallized one being older than the clear one. The Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature of both honeys was detected at -44°C. When honey is stored at a temperature below its Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature, Tg, it will be stable. On the other hand, above the Tg, it is susceptible to CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization. If the product contains a crystallized part, a melting peak is detected in the DSC curve. The higher the Crystallinity / Degree of CrystallinityCrystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.crystallinity, the higher the melting enthalpy.
Thus, DSC analysis can be used to visualize the Crystallinity / Degree of CrystallinityCrystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.crystallinity state of honey and even to predict its stability.