1. Department of Food Science
Food Analysis
Lecture 15 (3/11/2005)
Thermal Analysis
Qingrong Huang
Department of Food Science
Reading materials: Chapter 31, Food Analysis.

2. Thermal Processes Temperatures of transition; Heat capacity changes;
Weight losses or gains;
Energies of transition or enthalpic changes (H);
Viscoelastic or mechanical property changes during phase
changes or chemical reactions, etc….

3. Thermal Analysis Techniques
Differential Scanning Calorimetry (DSC): Measure the differential
temperature or heat flow to or from a sample versus a reference
material, and this is displayed as a function of temperature or time.
Two type of thermal events:
Endothermic: taking up heat; Exothermic: Giving off heat.
Thermogravimetric analysis (TGA): Measure changes in the weight
of a sample a function of temperature; both losses and gains can occur.

4. Principle of Calorimetry
Calorimetry involves the measurement of T or heat, or the determination of the
T and/or the quantity of heat absorbed or given off when a definite amount of
material undergoes a specific chemical or physicochemical change.
Thermal transitions:
- Endothermic: crystalline melting (heat of fusion), dehydration
- Exothermic: crystallization (ordering or freezing), polymerization, etc..
Units of thermal energy:
1 cal/g=0.001 Kcal/g = J/g
T(K)=T(°C)
T(°C)=[T(°F)-32]5/9
Heat flow is represented in either calories/s or watts
1 watt = 1 J/s = cal/s

5. DSC
“defined as a technique for recording the energy necessary to establish a zero
temperature difference between a substance and a reference material against
either time or temperature, as the two specimens are subjected to identical
temperature regimes in an environment heated or cooled at a controlled rate.
A test sample and an inert reference
material are used. Both test and
reference samples are heated or
cooled at the same time under
identical conditions.
To maintain the same temperature
between sample and reference, the
resistance of the temperature sensor
must be changed to influence the rate
by which heat is supplied to the sample
and reference, respectively.

6. DSC Experiment
Instrument output: (1)Sample pan size, material; (2) Heating rate; (3)
Placement of sample pan inside sample holder; (4) Furnace atmosphere;
(5) Size and shape of the furnace.
Sample Properties affecting instrument output:
(1) Weight; (2) Particle size; (3) Thermal conductivity; (4) Packing; etc…
Calibration with standard like indium (H=28.4 J/g; m.p C)
Calibration constant: KR= Hindium Windium/Aindium
Then the enthalpy of sample is:
Hsample = (KR )(Asample)/ Wsample
Where Asample is the area of the transition peak for the sample at scan
rate R; Wsample is the weight of the sample.

7. DSC Data Interpretation
A: Curve with no change in
heat capacity;
B: A broad transition;
C: A transition with a concomitant
change in heat capacity;
D: Heat capacity change during
a glass transition;
E: The effect of an increase in
sample size (dashed line) on
the transition;
F: Comparison of a primary
thermogram and its first
derivative (dashed line).

8. Thermal Transitions

9. DSC Applications The process of potato starch gelation
with different volume fractions of
Water.

10. TGA Applications