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.

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….

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.

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 = 4.186 J/g T(K)=T(°C)+273.16 T(°C)=[T(°F)-32]5/9 Heat flow is represented in either calories/s or watts 1 watt = 1 J/s = 0.2388 cal/s

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.

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. 156.64 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.

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).

Thermal Transitions

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

TGA Applications