Thermal behaviour of foods

Calorimetric techniques have become essential tools for the characterization of materials such as ceramics, polymers, metals, drugs, foodstuffs as a function of their thermal properties. The knowledge of these thermal properties (melting temperature, crystallization, glass transition, denaturation or decomposition temperature) plays a role of great importance in the selection of the material and its manufacturing conditions. Despite the importance of thermal characterization there are no simple and accessible alternatives to commercial instruments, whose main limitations are their high costs and the fragility of their constitutive parts during operation. In this work we present the design and construction of a portable, robust and low cost differential thermal analysis instrument which allows for the simultaneous analysis of three samples and it is suitable when corrosive substances are intended to be thermally characterized. This device represents an alternative to commercial devices, with applications in research and development of materials in various types of industries and apt for academic training.

The melting behavior of six fat samples hydrogenated on Pd catalysts (uniform 2% Pd/C and eggshell 0.5% Pd/Al₂O₃) in supercritical dimethylether (DME) or propane as reaction solvents was investigated. Hydrogenation was carried out at 484 K, with pressures between 20 and 28 MPa and a molar hydrogen content ranging from 4 to 9%, and using a continuous internal-recycle reactor. The melting and crystallization behavior of the hydrogenated products was characterized by differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR), which allowed the determination of solid fat content (SFC). DSC values were generally higher than NMR values. Melting behavior is one of the most important characteristics of edible fats and margarines as it has an impact on their organoleptic properties. These are strongly affected by supercritical-fluid (SCF) processing, so readjustment is necessary. The results obtained show that supercritical hydrogenation on the eggshell 0.5% Pd/Al₂O₃ catalyst in supercritical DME leads to an improved melting profile of the hydrogenated products compared with uniform 2% Pd/C catalyst in propane. In addition, supercritical conditions have a greater influence on melting behavior than conventional hydrogenation.

Kleiber's definitions of what constitutes direct and indirect calorimetry are accepted as the beginning of a commentary on the advantages and disadvantages of direct and indirect calorimetry in which calorimetry is divided into a number of categories based on the kind of calorimetric measurement. For non-reaction calorimetry such as entropy determinations and differential scanning calorimetry, the only means of measurement is by direct calorimetry. For reaction calorimetry, a preference of direct over indirect calorimetry depends on the accuracy needed and the ability of the experimenter to define the system. The data necessary to correct the observed heat loss in direct calorimetry are often all that are needed to make an indirect calculation of the true heat loss. In general, because they are convenient and inexpensive to use, indirect calorimetric methods are preferable to direct methods. However, when possible, one method can be used to verify the results of the other.

This chapter focuses on “food polymer science” approach and illustrates various recent examples of the way it has been used to define structure–function relationships in food products and processes and to assess and understand the effects of glass transitions and water plasticization on food quality, safety, and stability. The chapter reviews some theoretical principles from the field of synthetic polymer science that have been shown to be applicable to the studies of the glassy state phenomenon in foods and illustrates those applications by means of a broad overview focusing primarily on the most recent experimental studies by a number of groups that have been especially active in this rapidly growing field of research. The results of such studies have demonstrated the major opportunity offered by the food polymer science approach to expand not only the quantitative knowledge but also, of broader practical value in the context of industrial applications, the qualitative understanding of food products and processes with respect to (1) structure–functional property and water relationships and (2) the influences of nonequilibrium glassy and rubbery states on time-dependent mechanical and rheological properties related to quality and storage stability.

What is a glass? What is a glass transition? Why is the temperature at which a glass transition occurs (T_g) so important to the processing and storage stability of so many different types of foods ? Why is the effect of water as a plasticizer on T_g of such widespread relevance to food products and processes? Why are considerations of non-equilibrium glassy solid and rubbery liquid states in foods, rather than equilibrium phases, more germane to issues of food quality and safety? Why are the kinetics of heat/ moisture processes for foods and of deteriorative changes in food systems during storage more often appropriately interpreted in terms of the Williams-Landel-Ferry (WLF), rather than the Arrhenius, equation? What is the ‘food polymer science’ approach, with its central concepts of ‘glass dynamics’ and ‘water dynamics’, and why has this research approach proved so useful to the study of glasses and glass transitions in foods? And why has there been, since the early 1980s, such interest in these questions, and such increasing research activity, especially in the last few years, in this area of food science and technology? These are the questions addressed in this paper.

The answers to these questions are illustrated by a review that emphasizes, in the first part, (a) theoretical principles from the field of synthetic polymer science that are applicable to studies of glasses and glass transitions in aqueous food systems, and (b) a broad compilation focusing primarily on the most recent experimental studies by a number of groups that have been especially active in this growing field of research. In the second part of this paper, a general discussion of the effects on T_g of (a) structure and molecular weight of water-compatible solutes, and (b) plasticization by water, is highlighted. This is followed by specific illustrations of the application of the food polymer science approach as an interpretive and experimental framework for studies of (a) structure-function relationships of polymeric flour components, and (b) flour functionality in cookie baking. The paper describes how results of these studies have demonstrated the major opportunity offered by the food polymer science approach to expand not only quantitative knowledge but also, of broader practical value in the context of industrial applications, qualitative understanding of (a) the functionality of flours and flour components, and (b) the importance of glassy and rubbery states, in commercial cookie products and processes.