Phase Transitions in Foods

Yrjo H Roos, Stephan Drusch

• Publisher: Academic Press
• Year: 2015
• ISBN:
  9780124079229
• Edition: 2
• Topic: Technik & Maschinenbau
• Subtopic: Lebensmittelwissenschaft

Phase Transitions in Foods, Second Edition, assembles the most recent research and theories on the topic, describing the phase and state transitions that affect technological properties of biological materials occurring in food processing and storage. It covers the role of water as a plasticizer, the effect of transitions on mechanical and chemical changes, and the application of modeling in predicting stability rates of change.

The volume presents methods for detecting changes in the physical state and various techniques used to analyze phase behavior of biopolymers and food components. It should become a valuable resource for anyone involved with food engineering, processing, storage, and quality, as well as those working on related properties of pharmaceuticals and other biopolymers.

• Contains descriptions of non-fat food solids as "biopolymers" which exhibit physical properties that are highly dependent on temperature, time, and water content
• Details the effects of water on the state and stability of foods
• Includes information on changes occurring in state and physicochemical properties during processing and storage
• The only book on phase and state transitions written specifically for the applications in food industry, product development, and research

Chapter 1

Introduction to phase transitions

Abstract

Phase transitions are changes in the physical state of materials. Thermodynamic properties of materials are required for the analysis of phase and state transitions. Gibbs energy is a useful parameter for the understanding of equilibrium and nonequilibrium states in foods. Classification of phase transitions may follow that of Ehrenfest. First-order transitions show discontinuity in heat capacity while second-order transitions have a step-change in heat capacity at the transition temperature. Raoult’s law provides an important relationship for composition dependence of equilibrium properties of food materials.

Keywords

Thermodynamics; Gibbs energy; Phase Transitions; First order transitions; second order transitions; Raoults law; phase diagrams

1.1 Introduction

Phase transitions are changes in the physical state of materials, which have significant effects on their physical properties. Chemically pure compounds such as water or many organic and inorganic compounds in foods have exact phase transition temperatures. There are three basic physical states, which are the solid, the liquid, and the gaseous states. The term transition refers to the change in the physical state that is caused by a change in temperature or pressure. Heating of solid foods is often used to observe temperatures at which changes in thermal or physical properties, for example, in heat capacity, viscosity, or textural characteristics, occur.

Water is one of the most important compounds in nature and also in foods. It may exist in all of the three basic states during food processing, storage, and consumption. The effect of water on the phase behavior of food solids is of utmost importance in determining processability, stability, and quality. Well-known examples include transformation of liquid water into ice (freezing) or water into vapor (evaporation). These transitions in phase are the main physical phenomena that govern food preservation by freezing and dehydration. Engineering and sensory characteristics of food materials are often defined by the complicated combination of the physical state of component compounds. The main constituents of food solids are carbohydrates, proteins, water, and fat. These materials may exist in the liquid state and the solid crystalline or amorphous noncrystalline state. Many of the component compounds, for example, sugars, fats, and water, when they are chemically pure, crystallize below their equilibrium melting temperature.

Stability is an important criterion in food preservation. Materials in thermodynamic equilibrium are stable, that is, they exist in the physical state that is determined by the pressure and temperature of the surroundings. However, most biologic materials are composed of a number of compounds, and they often exist in a thermodynamically nonequilibrium, amorphous state. Such materials exhibit many time-dependent changes that are not typical of pure compounds, and they may significantly affect the shelf life of foods. The physical state of food solids is often extremely sensitive to water content, temperature, and time. This chapter introduces the basic terminology of thermodynamics and phase transitions and describes the common thermodynamic principles that govern the physical state of foods.

1.2 Thermodynamics

Thermodynamics describes the physical state of materials in terms of basic state variables and defines the basis for the description and understanding of the physical state at equilibrium and various transitions, which may occur due to changes in the quantities of the state variables that define the equilibrium. General principles of thermodynamics, which are reported here according to Atkins and de Paula (2009), can be found in most textbooks of physical chemistry.

1.2.1 Basic Terminology

Thermodynamics describes relationships between various systems that are in equilibrium, that is, no changes in the physical state of the systems are observed as a function of time. A system is a defined fixed quantity of mass, that is, the specific object under investigation. Such systems are often pure compounds at a given temperature and pressure. Thermodynamic systems are separated from their surrounding by boundaries. Boundaries may allow the occurrence of heat transfer between the system and its surroundings, for example, a food product may gain heat from its surroundings. However, boundaries of an isolated system prevent all interactions of the system with its surroundings and there can be no transfer of energy or matter between the system and its surroundings (Figure 1.1). Foods are seldom isolated systems, although the principle can be applied when insulators are used to avoid rapid cooling of hot foods or warming of cold drinks. A system may also be open or closed. An open system may have transfer of both energy and matter with the surroundings. A closed system may only have transfer of energy with surroundings. In food processing food systems may be open systems, for example, they can be heated by steam infusion, which includes transfer of matter and energy from the surroundings to the food. Foods may also be closed systems, which is true when they are hermetically sealed in containers.

Figure 1.1 Open, closed, and isolated systems.

Systems that are uniform in all properties are called homogeneous. These properties comprise, for example, optical properties, the composition, or the physical state of the constituents. Heterogeneous systems usually consist of different phases separated b...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• About the Authors
• Preface
• Chapter 1. Introduction to phase transitions
• Chapter 2. Physical state and molecular mobility
• Chapter 3. Methodology
• Chapter 4. Water and phase transitions
• Chapter 5. Food components and polymers
• Chapter 6. Prediction of the physical state
• Chapter 7. Time-dependent phenomena
• Chapter 8. Reaction kinetics
• Chapter 9. Food processing and storage
• Index