Thermal Analysis of Foods 1. Introduction
Thermal Analysis of Foods
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.
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.
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 r = 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 later.
2.2. 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.
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.
3. Experimental Techniques
A variety of different analytical techniques have been developed to monitor changes in the physical properties of food components that occur in response to controlled changes in temperature. A number of the most important of these thermal analysis techniques are described below.
Thermogravimetric techniques continuously measure the mass of a sample as it is heated or cooled at a controlled rate, or is held at a particular temperature for a period of time. Thermogravimetry is useful for monitoring processes that involve a change in the mass of a food or food component, e.g., drying, liberation of gasses, absorption of moisture. To mimic the various types of processing and storage conditions that a food might normally experience, thermogravimetric instruments have been specially designed to allow measurements to be carried out under specific environments, e.g., controlled pressures or atmospheres. Gravimetric instruments typically consist of a sensitive balance situated within a container whose pressure, temperature and gaseous environment can be carefully controlled.
The mass of a sample may either increase or decrease with temperature or time depending on the specific physicochemical processes occurring. Heating often leads to a reduction in mass because of evaporation of volatile components and various chemical reactions that liberate gasses. On the other hand, the mass of a food may increase due to absorption of moisture from the atmosphere. The ability to be able to carefully control the temperature, pressure and composition of the gasses surrounding a sample is extremely valuable for food scientists, because it allows them to model processes such as drying, cooking, and uptake of moisture during storage.
A dilatometer is a device that is used to measure the change in density of a material as a function of time or temperature. Dilatometry measurements are routinely used for monitoring the crystallization and melting of fats in foods. A weighed amount of melted fat is poured into a graduated glass U-tube that is thermostatted in a temperature controlled water bath. The sample is then cooled at a controlled rate and the change in volume of the material is measured as a function of temperature. The density of a solid is usually greater than that of a liquid, thus the volume of a sample decreases when crystallization occurs, and increases when melting occurs. Dilatometry can therefore be used to provide information about the melting and crystallization of fatty foods. For food scientists, the most important information is the temperature at which melting or crystallization begins, the temperature range over which the phase transition occurs, and the value of the solid fat content at any particular temperature.
3.3. Rheological Thermal Analysis
Rheology is the study of the deformation and flow of matter. Rheological techniques used for thermal analysis measure the change in the rheological characteristics of a sample as a function of temperature. A sample is usually contained in a measurement cell whose temperature can be varied in a systematic fashion. A stress is applied to the sample and the resulting strain is measured (or vice versa). The relationship between the stress and strain gives information about the rheological properties of the material being tested. The stress can be applied to a material in a number of different ways (e.g., shear, compression or bending), depending on the type of information required. The stresses used are normally small enough to prevent any changes in the properties of the material during the test. If large stresses were applied to a material they might promote structure breakdown, which would alter the rheological properties of the material during the test.
Rheological thermal analysis techniques are often used to monitor the temperature dependent rheological properties of liquids, gels and solids. For example, they are commonly used to monitor the temperature dependence of the shear modulus of fatty foods, the viscosity of biopolymer solutions, and the shear modulus of biopolymer gels. These techniques provide useful information about the temperature at which thermal transitions occur, the rate at which these changes occur and the final rheological properties of the food. This type of information is used by food scientists to design foods with improved properties, and to optimize processing conditions.
3.4. Differential Thermal Analysis and Differential Scanning Calorimetry
DTA and DSC techniques rely on changes in the heat absorbed or released by a material as its temperature is varied at a controlled rate. These changes occur when components within a food undergo some type of phase transition (e.g. crystallization, melting, evaporation, glass transitions, conformational change) or chemical reaction (e.g., oxidation, hydrolysis).
3.4.1. Differential thermal analysis
DTA is defined as "a technique for recording the difference in temperature between a substance and a reference material against time or temperature as the two specimens are subjected to identical temperature regimes in an environment heated or cooled at a controlled rate". A typical instrument consists of two measurement cells that are located in a temperature-controlled environment, whose temperature can be varied in a controlled fashion. The sample to be tested is placed into the "sample cell", while a reference material of known thermal properties (often distilled water) is placed in the "reference cell". The two cells are then heated or cooled together at a controlled rate. The small difference in temperature between the "sample cell" and "reference cell" (DT = Tsample - Treference) is measured using accurate thermocouples placed below the cells as the temperature of the external environment (Texternal) is varied in a controlled fashion. The output of the instrument is therefore a plot of DT versus Texternal. Information about thermal transitions that occur within a sample can be obtained by analyzing the DT versus Texternal thermogram. If the temperature of the "sample cell" is greater than that of the "reference cell" (DT > 0), then the sample has undergone an exothermic reaction, i.e., it has given out heat. Conversely, if the temperature of the "reference cell" is greater than that of the "sample cell" (DT < 0), then the sample has undergone an endothermic reaction, i.e., it has adsorbed heat. The nature of a peak (exothermic, endothermic, shape) provides information about the type of transition(s) occurring. The position of the peak provides information about the temperature that the transition occurs. The area under a peak depends on the amount of material involved in the transition and the enthalpy change per unit amount of material.
3.4.2. Differential scanning calorimetry
DSC is a technique for recording the energy required to keep a zero temperature difference between a sample cell and a reference cell which are either heated or cooled at a controlled rate. The thermocouples constantly measure the temperature of each cell and heaters supply heat to one or other of the cells so that they both have exactly the same temperature. If a sample were to undergo a phase transition it would either absorb or release heat. To keep the temperature of the two samples the same an equivalent amount of energy must be supplied to either the test or reference cells. Special electrical circuitry is used to determine the amount of energy needed to keep the two measurement cells at the same temperature. DSC data is therefore reported as the rate of energy absorption (Q) by the sample relative to the reference material as a function of the external temperature. Information about thermal transitions that occur within a sample are obtained by analyzing the Q versus Texternal thermogram. It should be noted that it is also possible to measure the change in the heat released by a material as a function of time under isothermal (constant temperature) conditions.
3.4.3. Isothermal titration calorimetry
ITC is used to measure enthalpy changes that occur as the result of interactions between different types of molecules. An ITC instrument consists of a reference cell, a sample cell and an injector. A reference material (e.g., distilled water), that does not undergo any enthalpy changes during the experiment is placed in the reference cell. A solution of one type of molecule is placed in the sample cell ("sample solution"), and a solution of another type of molecule is placed in the injector ("injection solution"). Small aliquots of the injection solution are then injected periodically into the sample solution contained within the sample cell (e.g., 10 mL every 300 seconds), and the energy required to keep the sample and reference cells at the same temperature is measured as a function of time. The resulting thermogram consists of a plot of Q versus time, which consists of a series of enthalpy peaks corresponding to the series of injections. By analyzing the nature (exothermic, endothermic), magnitude (area under the curve) and shape of the peaks it is possible to obtain valuable information about interactions between molecules in the injector and in the sample cell (see below).
Specific Heat Capacity. The specific heat capacity is an important quantity in the food industry because it determines the amount of energy that must be supplied or withdrawn from a material in order to increase or decrease its temperature by a given amount. Knowledge of the specific heat capacity of a material is therefore important in the design of processes such as chilling, freezing, warming, sterilization and cooking. DSC and DTA can be used to measure the specific heat capacities of food materials. A known mass of material is placed in a sample cell, which is then heated or cooled at a controlled rate. For DSC, the specific heat capacity is determined from the equation: Q = m´ CP´ dT/dt, where Q is the heat flow per unit time, m is the sample mass, CP is the specific heat capacity of the material, and dT/dt is the rate of change of the external temperature.
Phase transitions. DSC and DTA are routinely used in the food industry to characterize phase transitions in foods, e.g. crystallization, melting, glass transitions and conformational changes. They can be used to provide information about the temperature at which transitions occur (Ttr), the enthalpy change associated with a transition (DHtr), the type of transition involved (exothermic or endothermic), and the quantify of material that undergoes a transition. As an example, we will consider the use of DSC to study the melting and crystallization of food components. When a material changes its physical state from solid-to-liquid (melting) or from liquid-to-solid (crystallization) it absorbs or gives out heat, respectively. A process that absorbs heat is an endothermic process, whereas a process that evolves heat is an exothermic process. Pure substances usually have very sharp melting or crystallization points and therefore all the heat is absorbed or evolved over a narrow range of temperatures, leading to a sharp DSC or DTA peak. Many food components are chemically complex materials and therefore the phase transitions occur over a wide range of temperatures, e.g. edible oils contain a wide variety of different triacylglycerols each with its own melting point. Peaks from food oils may also be complicated by the fact that triacylglycerols can crystallize in more than one different crystalline structure, i.e., they are polymorphic.
Molecular interactions. ITC can be used to provide valuable information about interactions between different types of molecules, e.g., binding interactions or conformational changes. As an example, we will consider the use of ITC for quantifying the binding of a ligand molecule (L) to a protein molecule (P): P + L ® PL. A solution containing the ligand is placed into the injector, while a solution containing the protein is placed into the sample cell. Small aliquots of the ligand solution are then injected into the sample solution at regular intervals (e.g., 10 mL every 300 seconds). The interval between each injection should be long enough to allow any reactions to go to completion. The instrument records the enthalpy change that occurs after each injection as a result of the interaction between the ligand and protein molecules. By measuring the change in the enthalpy with ligand concentration in the sample cell it is possible to obtain information about the number of binding sites on the protein, the strength of the binding interaction and the thermodynamics of the binding interaction.