Water Activity

Theory and Applications to Food

Rockland

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351405102
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencia de los alimentos

This book presents the proceedings of the Tenth Basic Symposium sponsored by the Institute of Food Technologists and the International Union of Food Science and Technology. The key aim of the Symposium was to explore some basic principles relating to the influences of water activity on food quality.

1

Mechanics and Influence of Water Binding on Water Activity

Marc Le Maguer

The University of Alberta Edmonton, Alberta, Canada

INTRODUCTION

Water, as the main component of food and biological materials, plays a predominant role in determining their shape, structure, and physical and chemical properties. It also is a major control component in mass transfer, chemical reactions, and the activity of microorganisms. The food technologist involved in product development, process design, or production is confronted daily with the effect, desirable or undesirable, of water on the food material. In either case, a better understanding of the water relations with the food would certainly be of help. Most of the unit operations used in food processing have as a goal, in one way or another, either the removal of water to stabilize the material, as in drying and concentration, its transformation into a nonactive component in freezing, and its immobilization in gels, structured foods, and low and intermediate moisture foods. The main and essential way in which the immobilization of the water is measured is through the consideration of a_w and its relationship to moisture content. Based on the thermodynamic concept of water chemical potential in solutions, a_w has served and still is serving as an index of how successful we are at controlling water behavior in food systems. It is also the parameter that controls the driving force in water removal operations and is therefore essential for design purposes. Its effect on many of the other properties important to food manufacturing and preservation will be reviewed in the subsequent chapters in this volume. What all this indicates, however, is that we need to be able to deal with a_w quantitatively if we want to effectively control our food processing.

This review is concerned with understanding water and a_w as they relate to the other components of the food material, and will subsequently describe an approach to the modeling of the complex material that food represents.

BASIC STRUCTURE OF THE FOOD MATERIAL

In all of the operations involved in food processing, it is rare that we deal with a pure, well-characterized component (with the possible exception of sucrose). For the sake of simplification and clarity the following categories are proposed.

Cellular Material of Plant or Animal Origin

The process used in the transformation of plant and animal tissue will preserve to a great degree the initial cellular structure of the material. Blanching, conditioning in syrup or brine, freezing, and drying of fruits and vegetables and some meat products would fall in this category. The material can be considered made of a matrix, usually water insoluble, cell membranes, intact, partially or totally destroyed, and a complex aqueous solution of sugars, amino acids, proteins, salts, aromatic compounds, and lipids. The cell membranes may or may not play a role in the water relation depending on whether or not they have been damaged during processing. This last consideration is important in the removal of water from high-moisture fresh fruits or vegetables.

Mixture Water-Soluble and Water-Insoluble Food Components

No structure of the initial cellular material remains. Usually a mixing step of the ingredients with water is involved and subsequent processing may or may not induce structure. Texturization, filtration, wet mechanical separation, membrane separation, and concentration are typical processing operations inv...