ADSC of Amorphous Sugar

Sample

Sugar (Saccharose)

Conditions

Measuring cell: DSC821^e with IntraCooler

Pan: Aluminum standard 40 µl, with pierced lid

Sample preparation: The sugar is weighed in the pan and heated in the DSC cell at 5 K/min. Immediately after the endothermic fusion peak the pan is removed and shock-cooled to ambient temperature on an aluminum plate in order to obtain the amorphous phase.

DSC measurement: Heating from 50 °C to 170 °C at a mean heating rate of 2 K/min. Modulation amplitude 1 °C, period one minute. The empty cell and an aluminum sample are measured under the same conditions for calibration purposes

Atmosphere: Nitrogen, 50 cm³/min

Interpretation

In the upper coordinate system the DSC curve obtained using the alternating temperature program is displayed. It shows a number of changes.

Fast Fourier Analysis allows the recording of the mean signal (= total heat flow signal, corresponding to a classical DSC curve measured at 2 K/min), the amplitude curve (reversing curve) and the so-called non-reversing curve.

The changes of the specific heat capacity due to the glass transition, crystallization and other phenomena can be observed in the reversing curve. The reversing curve allows the calculation of the temperature function of the specific heat capacity (lower diagram) again showing the cp changes but in this case directly in J/gK.

The non-reversing effects include for example chemical reactions, cold crystallization on heating amorphous samples, relaxation effects during the glass transition and the evaporation of volatiles (drying)

.There is a broad range of substances that are usually crystalline but, when shock-cooled from the molten phase, form a glassy or amorphous state of aggregation. Examples of such substances are sulfur, most organic compounds, some semicrystalline polymers, certain alloys and quartz. On heating they undergo a glass transition in which they change from a solid and brittle state and become soft and rubbery. Due to the mobility of the molecules, they are then able to form crystals (devitrification). This crystallization process leads to an exothermic peak on the DSC curve (total heat flow and non-reversing curve).

Evaluation

The glass transition midpoint of 75.6 °C is typical for anhydrous sugar; moisture would act as a plasticizer and lower the temperature range of glass transition. The heat of crystallization is obtained by integrating the crystallization peak on the non-reversing curve. The value of 94.6 J/g does not correspond to complete crystallization since the heat of fusion of completely crystalline saccharose is approximately 130 J/g. The corresponding change in c_p is 0.31 J/gK.

Conclusion

In food technology the glass transition temperature is becoming increasingly important in connection with stability considerations of sweets, with other amorphous dried foods and with deep-freeze processes. The most attractive feature of ADSC is its ability to separate c_p changes from certain enthalpy changes as shown in the case of crystallization

_{ADSC of Amorphous Sugar | Thermal Analysis Application No. HB 1000 | Application published in METTLER TOLEDO TA Application Handbook Food}