This document discusses using thermal analysis techniques to monitor changes in physical properties of foods as temperature varies. It explains that temperature changes can cause alterations like evaporation, melting, crystallization or gelation that influence properties like taste, texture and stability. Thermal analysis techniques measure changes in mass, density, heat capacity and other physical properties during phase transitions or gelation processes as these are important for food manufacturers to optimize processing.

1. Thermal analysis of food

2. Temperature Dependent Properties of
Foods
• Initially, it is useful to highlight some of the physical changes that occur in food
components when the temperature is varied.
• 2.1. Density
• The density of pure materials, which do not undergo phase transitions (e.g.,
melting, crystallization or evaporation), usually decrease as the temperature is
increased. This is because the atoms in the material move around more vigorously
when they gain thermal energy, and so the space between the molecules
increases. The mass of a material is independent of temperature (provided
evaporation or condensation do not occur), and so an increase in volume with
temperature leads to a decrease in density (since = m/V). Knowledge of the
temperature-dependence of the density of a food material is often used by
engineers to design processing operations, e.g., containers for storing materials or
pipes through which materials flow. In materials that do undergo phase transitions
the variation of the density with temperature is more
• dramatic. A solid usually has a higher density than a liquid, and so when a solid
melts or a liquid crystallizes there is a significant change in density superimposed
on the normal variation of density with temperature. The use of density
measurements to monitor melting and crystallization of materials will be discussed
late

3. 1. Introduction
• Most foods are subjected to variations in their temperature during production,
transport, storage, preparation and consumption, e.g., pasteurization, sterilization,
evaporation, cooking, freezing, chilling etc. Temperature changes cause alterations
in the physical and chemical properties of food components which influence the
overall properties of the final product, e.g., taste, appearance, texture and
stability. Chemical reactions such as hydrolysis, oxidation or reduction may be
promoted, or physical changes, such as evaporation, melting, crystallization,
aggregation or gelation may occur. A better understanding of the influence of
temperature on the properties of foods enables food manufacturers to optimize
processing conditions and improve product quality. It is therefore important for
food scientists to have analytical techniques to monitor the changes that occur in
foods when their temperature varies. These techniques are often grouped under
the general heading of thermal analysis. In principle, most analytical techniques
can be used, or easily adapted, to monitor the temperature-dependent properties
of foods, e.g., spectroscopic (NMR, UV-visible, IR spectroscopy, fluorescence),
scattering (light, X-rays, neutrons), physical (mass, density, rheology, heat capacity)
etc. Nevertheless, at present the term thermal analysis is usually reserved for a
narrow range of techniques that measure changes in the physical properties of
foods with temperature, e.g., mass, density, rheology, heat capacity. For this
reason, only these techniques will be considered in this lecture.

4. 1. Introduction
• Most foods are subjected to variations in their temperature during production,
transport, storage, preparation and consumption, e.g., pasteurization, sterilization,
evaporation, cooking, freezing, chilling etc. Temperature changes cause alterations
in the physical and chemical properties of food components which influence the
overall properties of the final product, e.g., taste, appearance, texture and
stability. Chemical reactions such as hydrolysis, oxidation or reduction may be
promoted, or physical changes, such as evaporation, melting, crystallization,
aggregation or gelation may occur. A better understanding of the influence of
temperature on the properties of foods enables food manufacturers to optimize
processing conditions and improve product quality. It is therefore important for
food scientists to have analytical techniques to monitor the changes that occur in
foods when their temperature varies. These techniques are often grouped under
the general heading of thermal analysis. In principle, most analytical techniques
can be used, or easily adapted, to monitor the temperature-dependent properties
of foods, e.g., spectroscopic (NMR, UV-visible, IR spectroscopy, fluorescence),
scattering (light, X-rays, neutrons), physical (mass, density, rheology, heat capacity)
etc. Nevertheless, at present the term thermal analysis is usually reserved for a
narrow range of techniques that measure changes in the physical properties of
foods with temperature, e.g., mass, density, rheology, heat capacity. For this
reason, only these techniques will be considered in this lecture.

5. Phase Transitions
• The term phase transition refers to the process whereby a material is
converted from one physical state to another. The most commonly
occurring phase transitions in foods are melting (solid-to-liquid),
crystallization (liquid-to-solid), evaporation (liquid-to-gas), condensation
(gas-to-liquid), sublimation (solid-to-gas) and glass transitions (glassy-to-
rubbery). When a material changes from one physical state to another it
either absorbs or gives out heat. A process that absorbs heat is an
endothermic process, whereas a process that evolves heat is an
exothermic process. The overall properties of foods may be drastically
altered when key components undergo phase transitions, and so it is
important to have analytical techniques for monitoring these processes.
These techniques utilize measurements of physical properties of a
material that change when a material undergoes a phase transition, e.g.,
molecular structure, molecular mobility, density, rheology, heat capacity.
• 2

6. Gelation
•
• Many foods contain components that are capable of forming a gel when the food is heated or
cooled under appropriate conditions. Most food gels are three-dimensional networks of aggregated
or entangled biopolymers or colloidal particles that entrap a large volume of water, to give the
whole structure "solid-like" characteristics. The physical properties of gels, such as appearance
(transparent or opaque), water holding capacity, rheology and stability, depend ultimately on the
type, structure and interactions of the molecules or particles that they contain. Common examples
of foods in which gelation makes an important contribution to their overall properties are eggs,
starches, jellies, yogurts and meat products. In some foods a gel is formed on heating (heat-setting
gels), whilst in others it is formed on cooling (cold-setting gels). Gels may also be either thermo-
reversible or thermo-irreverisble, depending on whether gelation is reversible or not. Gelatin is an
example of a cold-setting thermo- reversible gel: when a solution of gelatin molecules is cooled
below a certain temperature a gel is formed, but when it is reheated the gel melts. Egg-white is an
example of a heat-setting thermo-irreverisble gel. When an egg is heated above a temperature
where gelation occurs a characteristic white gel is formed, however, when the egg is cooled back to
room temperature the gel remains white, i.e., it doesn't revert back into the liquid from which it
was formed. For ingredients that gel it is important to know the temperature at which gelation
occurs, the gelation rate, and the
• nature of the gel formed. Thus thermal analytical techniques are needed by food scientist to
measure these properties.