Influence of Water on the Glass Transition Temperature of Honey - METTLER TOLEDO

Influence of Water on the Glass Transition Temperature of Honey

Honey is well-liked as a spread on bread and as a sweetening agent. The ease of spreading of creamed, noncrystalline or slightly crystalline honey depends on the glass transition temperature. The glass transition temperature itself depends on the water content of the honey. In this article we investigate this relationship using a Swiss floral honey.

Drying on Honey by TGA

 

Introduction 

Honey is a natural product that mainly consists of different types of sugar (fructose 27 to 44% fructose, 22 to 41% glucose) and water (15 to 20%). In addition there are small amounts of pollen, proteins, amino acids, vitamins as well as colorants and aromas [1, 2]. The exact composition is determined by the flowering plants from which the bees collect the nectar.

Sugar can bind large amounts of water through hydrogen bonding. The bound water forms a network with the sugar. The ratio of the contents of fructose and glucose in the honey determines its ability to crystallize and hence the consistency of the honey

In general, honey with a high glucose content (for example rape honey) crystallizes more easily. Crystallized honey is granular and cannot be spread very well. Creamed honey is no longer easy to spread if its temperature is below the glass transition temperature.

The glass transition temperature of honey depends on the water content. This article describes how this relationship was investigated using DSC measurements of a creamed Swiss floral honey.

 

Experimental Details

The initial water content of the honey was first determined in a preliminary experiment using a TGA/DSC instrument. The TGA curve is displayed in Figure 1 and shows that the sample mass continues to decrease even after drying for 6 days at 80 °C.

A TGA measurement is therefore not suitable for determining the water content of the honey. This can be done much more quickly and accurately by Karl Fischer titration. The measurement yielded an initial water content 18.5% for the honey sample investigated here.

Following this, two series of measurements were performed. In the first series, samples were prepared in 40-µL aluminum crucibles in which the original water content was systematically reduced through drying for different times at 80 °C.

After determining the loss of water in each case, the sample crucibles were hermetically sealed. The initial sample weight was typically 10 mg. The samples prepared in this way were heated at 10 K/min from –80 to +80 °C, cooled at 10 K/min to –80 °C and then heated again at 10 K/min to 80 °C. In each case the second heating run was evaluated.

In the second series of measurements, different amounts of water were added to the honey. A certain mass of honey (typically 10 mg) was weighed into a 40-µL aluminum crucible. Water was then added to the honey sample using a syringe and the crucible immediately sealed.

The samples were homogenized by heating them at 10 K/min from room temperature to 80 °C. They were then cooled at 10 K/min to –130 °C and heated again at 10 K/min to 80 °C. The cooling run and the second heating run were evaluated.

 

Results and Discussion

First a sample with the original moisture content contained in a hermetically sealed crucible was heated from –80 to auf +80 °C at 10 K/min, cooled at 10 K/min and then heated again from –80 to +80 °C at 10 K/min. The two heating curves are displayed in Figure 2.

In the first heating run, a glass transition was observed at –45.0 °C followed by an endothermic peak between 35 and 80 °C. This peak is sometimes interpreted as melting [2]. Other authors interpret it as the result of gelation of the sugarwater solution [3, 4]. 

Conclusions

In summary, the diagram in Figure 6 shows the relationship between the glass transition temperature and the water content of a creamed Swiss honey. At water contents close to the usual water contents of honey (15–20%), the glass transition temperature depends strongly on the water content. If creamed honey is stored in the open, it is quite possible that it becomes more difficult to spread.

The reason for this is not the crystallization of the honeys but the increased glass transition temperature resulting from the lower water content. The relationship between the glass transition temperature and the water content could in principle be used to estimate the water content in a honey sample. 

 

Influence of Water on the Glass Transition Temperature of Honey | Thermal Analysis Application No. UC 423 | Application published in METTLER TOLEDO Thermal Analysis UserCom 42 

Moisture and Water Content Analysis in Food