Physical Chemistry of Foods

Pieter Walstra

• Publisher: CRC Press
• Year: 2002
• ISBN:
  9781135565541
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Lebensmittelwissenschaft

Exploring the structure and physical and chemical properties of solutions, dispersions, soft solids, fats, and cellular systems, Physical Chemistry of Foods describes the physiochemical principles of the reactions and conversions that occur during the manufacture, handling, and storage of foods. Coverage progresses from aspects of thermodynamics, b

1
Introduction

1.1 PHYSICAL CHEMISTRY IN FOOD SCIENCE AND TECHNOLOGY

Food science and technology are concerned with a wide variety of problems and questions, and some will be exemplified below. For instance, food scientists want to understand and predict changes occurring in a food during processing, storage, and handling, since such changes affect food quality. Examples are

The rates of chemical reactions in a food can depend on many variables, notably on temperature and water content. However, the relations between reaction rates and the magnitude of these variables vary widely. oreover, the composition of the mixture of reaction products may change significantly with temperature. How is this explained and how can this knowledge be exploited?

How is it possible that of two nonsterilized intermediate-moisture foods of about the same type, of the same water activity, and at the same temperature, one shows bacterial spoilage and the other does not?

Two plastic fats are stored at room temperature. The firmness of the one increases, that of the other decreases during storage. How is this possible?

Bread tends to stale—i.e., obtain a harder and shorter texture—during storage at room temperature. Keeping the bread in a refrigerator enhances staling rate, but storage in a freezer greatly reduces staling. How is this explained?

The physical stability of a certain oil-in-water emulsion is observed to depend greatly on temperature. At 40°C it remains stable, after cooling to 25°C also, but after cooling to 10°C and then warming to 25°C small clumps are formed; stirring greatly enhances clump formation. What are the mechanisms involved and how is the dependence on temperature history explained?

Another emulsion shows undesirable creaming. To reduce creaming rate a small amount of a thickener, i.e., a polysaccharide, is added. However, it increases the creaming rate. How?

Food technologists have to design and improve processes to make foods having specific qualities in an efficient way. Examples of problems are

Many foods can spoil by enzyme action, and the enzymes involved should thus be inactivated, which is generally achieved by heat denaturation. For several enzymes the dependence of the extent of inactivation on heating time and temperature is simple, but for others it is intricate. Understanding of the effects involved is needed to optimize processing: there must be sufficient inactivation of the enzymes without causing undesirable heat damage.

It is often needed to make liquid foods with specific rheological properties, such as a given viscosity or yield stress, for instance to ensure physical stability or a desirable eating quality. This can be achieved in several ways, by adding polysaccharides, or proteins, or small particles. Moreover, processing can greatly affect the result. A detailed understanding of the mechanisms involved and of the influence of process variables is needed to optimize formulation and processing.

Similar remarks can be made about the manufacture of dispersions of given properties, such as particle size and stability. This greatly depends on the type of dispersion (suspension, emulsion, or foam) and on the specific properties desired.

How can denaturation and loss of solubility of proteins during industrial isolation be prevented? This is of great importance for the retention of the protein’s functional properties and for the economy of the process.

How can one manufacture or modify a powdered food, e.g., spraydried milk or dry soup, in such a manner that it is readily dispersable in cold water?

How does one make an oil-in-water emulsion that is stable during storage but that can be whipped into a topping? The first question then is: what happens during a whipping process that results in a suitable topping? Several product and process variables affect the result.

All of these examples have in common that knowledge of physical chemistry is needed to understand what happens and to solve the problem.

Physical chemistry provides quantitative relations for a great number of phenomena encountered in chemistry, based on well-defined and measurable properties. Its theories are for the most part of a physical nature and comprise little true chemistry, since electron transfer is generally not involved. Experience has shown that physicochemical aspects are also of great importance in foods and food processing. This does not mean that all of the phenomena involved are of a physical nature: it is seen from the examples given that food chemistry, engineering, and even microbiology can be involved as well. Numerous other examples are given in this book.

The problems encountered in food science and technology are generally quite complex, and this also holds for physicochemical problems.

In the first place, nearly all foods have a very wide and complex composition; a chemist might call them dirty systems. Anyway, they are far removed from the much purer and dilute systems discussed in elementary textbooks. This means that the food is not in thermodynamic equilibrium and tends to change in composition. Moreover, several changes may occur simultaneously, often influencing each other. Application of physicochemical theory may also be difficult, since many food systems do not comply with the basic assumptions underlying the theory needed.

In the second place, most foods are inhomogeneous systems. Consequently, various components can be in different compartments, greatly enhancing complexity. This means that the system is even farther removed from thermodynamic equilibrium than are most homogeneous systems. Moreover, several new phenomena come into play, especially involving colloidal interactions and surface forces. These occur on a larger than molecular scale. Fortunately, the study of mesoscopic physics—which involves phenomena occurring on a scale that is larger than that of molecules but (far) smaller than can be seen with the naked eye—has made great progress in recent times.

In the third place, a student of the physical chemistry of foods has to become acquainted with theories derived from a range of disciplines, as a look at the table of contents will show. Moreover, knowledge of the system studied is essential: although basic theory should have universal validity, the particulars of the system determine the boundary conditions for application of a theory and thereby the final result.

All of this might lead to the opinion that many of the problems encountered in food science and technology are so intricate that application of sound physical chemistry would hardly be possible and that quantitative prediction of results would often be impossible. Nevertheless, making use of the basic science involved can be quite fruitful, as has been shown for a wide variety of problems. Reasons for this are

Understanding of basic principles may in itself be useful. A fortunate characteristic of human nature is the desire to explain phenomena observed and to create a framework that appears to fit the observations. However, if such theorizing is not based on sound principles it will often lead to wrong conclusions, which readily lead to further problems when proceeding on the conceived ideas with research or process development. Basic knowledge is a great help in (a) identifying and explaining mechanisms involved in a process and (b) establishing (semi— )quantitative relations.

Even semiquantitative answers, such as giving the order of magnitude, can be very helpful. Mere qualitative reasoning can be quite misleading.For instance, a certain reaction proceeds much faster at a higher temperature and it is assumed that this is because the viscosity is lower at a higher temperature. This may be true, but only if (a) the reaction rate is diffusion controlled, and (b) the relative increase of rate is about equal to the relative decrease in viscosity. When the rate increases by a factor of 50 and the viscosity decreases by a factor of 2, the assumption is clearly wrong.

Foods are intricate systems and also have to meet a great number of widely different specifications. This means that process and product development will always involve trial and error. However, basic understanding and semiquantitative relations may greatly reduce the number of trials that will lead to error.

The possibilities for establishing quantitative relations are rapidly increasing. This is due to further development of theory and especially to the greatly increased power of computer systems used for mathematical modeling of various kinds. In other words, several processes occurring in such complex systems as foods—or in model systems that contain all the essential elements—can now be modeled or simulated.

Altogether, in the author’s opinion, application of physical chemistry and mesoscopic physics in the realm of food science and technology is often needed—besides food chemistry, food process engineering, and food microbiology—to solve problems and to predict changes that will occur during manufacture, storage, and use of foods.

1.2 ABOUT THIS BOOK

1.2.1 What Is Treated

The book is aimed at providing understanding, hence it primarily gives principles and theory. Moreover, facts and practical aspects are included, because knowledge of the system considered is needed to apply theory usefully, and also because the text would otherwise be as dry as dust. Basic theory is given insofar as it is relevant in food science and technology. This implies that several physicochemical theories are left out or are only summarily discussed. It also means that many aspects will be treated that are not covered in standard texts on physical chemistry, which generally restrict the discussion to relatively simple systems. Since most foods are complicated systems and show nonideal behavior, treatment of the ensuing complexities cannot be avoided if the aim is to understand the phenomena and processes involved.

As mentioned, molecular and mesoscopic approaches will be needed. The first part of the book mainly considers molecules. We start with some basic thermodynamics, interaction forces, and chemical kinetics (Chapters 2–4). The next chapter is also concerned with kinetic aspects: it covers various transport phenomena (which means that a few mesoscopic aspects are involved) and includes some basic fluid rheology. Chapters 6 and 7 treat macromolecules: Chapter 6 gives general aspects of polymers and discusses food polysaccharides in particular, with a largish section on starch; Chapter 7 separately discusses proteins, highly intricate food polymers with several specific properties. Chapter 8 treats the interactions between water and food components and the consequences for food properties and processes.

Then mesoscopic aspects are treated. Chapter 9 gives a general introduction on disperse or particulate systems. It concerns properties that originate from the division of a material over different compartments, and from the presence of a large phase surface. Two chapters give basic theory. Chapter 10 is on surface phenomena, where the forces involved primarily act in the direction of the surface. Chapter 12 treats colloidal interactions, which primarily act in a direction perpendicular to the surface. Two chapters are concerned with application of these basic aspects in disperse systems: Chapter 11 with emulsion and foam formation, Chapter 13 with the various instabilities encountered in the various dispersions: foams, emulsions, and suspensions.

Next we come to phase transitions. Chapter 14 mentions the various phase transitions that may occur, such as crystallization, gas bubble formation, or separation of a polymer solution in two layers; it then treats the nucleation phenomena that often initiate phase transitions. Chapter 15 discusses crystallization, a complicated phase transition of great importance in foods. It includes sections on crystallization of water, sugars, and triacylglycerols. Chapter 16 introduces glass transitions and the various changes that can occur upon freezing of aqueous systems.

Finally, Chapter 17 is about soft solids, a term that applies to the majority of foods. It gives an introduction into solids rheology and fracture mechanics, but otherwise it makes use of many of the theories treated in earlier chapters to explain properties of the various types of soft solids encountered in foods.

1.2.2 What Is Not Treated

Some aspects are not covered. This includes analytical and other experimental techniques. A discussion of these is to be found in specialized books. Basic principles of some methods will be given, s...