Water Activity-Theory and Applications to Food

425 Pages·1987·13.301 MB·\425

by  Rockland (Author)

Water Activity: Theory
and Applications to Food
ifl
Basic Symposium Series
Edited by
INSTITUTE OF FOOD TECHNOLOGISTS
221 N. LaSalle St.
Chicago, Illinois
Foodborne Microorganisms and Their Toxins:
Developing Methodology edited by Mede D.
Pierson and Norman J. Stern
Water Activity: Theory and Applications to
Food edited by Louis B. Rockland and
Larry R. Beuchat
Water Activity: Theory
and Applications to Food
edited by
Louis B. Rockland
Food Science Research Center
Chapman College
Orange, California
Larry R. Beuchat
Department of Food Science and Technology
University of Georgia
Agricultural Experiment Station
Experiment, Georgia
Marcel Dekker, Inc. New York and Basel
Library of Congress Cataloging-in-Publication Data
Water activity.
(IFT basic symposium series)
Proceedings of the tenth basic symposium held in
Dallas, Tex., June 13-14, 1986, sponsored by the
Institute of Food Technologists and the International
Union of Food Science and Technology.
Includes bibliographies and index.
1. Food-Water activity-Congresses. 2. Food-
Moisture-Congresses. I. Rockland, Louis B. II. Beuchat,
Larry R. III. Institute of Food Technologists.
IV. International Union of Food Science and Technology.
V. Series
TX553.W3W36 1987 664 87-8967
ISBN 0-8247-7759-X
COPYRIGHT © 1987 by MARCEL DEKKER, INC.
ALL RIGHTS RESERVED
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without permission in writing from the publisher.
MARCEL DEKKER, INC.
270 Madison Avenue, New York, New York 10016
Current printing (last digit):
10 9 8 7 6 5 4 3
PRINTED IN THE UNITED STATES OF AMERICA
Preface
This book presents the proceedings of the Tenth Basic Symposium sponsored
jointly by the Institute of Food Technologists (IFT) and the International
Union of Food Science and Technology (lUoFST). It was held at the Grenelefe
Hotel, Dallas, Texas, on June 13-14, 1986, prior to the 46th Annual IFT
Meeting.
The symposium was organized for the purpose of disseminating new infor­
mation on relationships between water activity (a^) and the quality and sta­
bility of food products. It featured internationally recognized authorities who
have pioneered this important new technology.
The symposium provided an update on new developments in an expanding
literature on a^ and its influence on the physical and chemical properties of
food products. This new discipline has been hindered by ingrained classical
concepts of the role of water on the quality and stability of food products, and
by the subtle differential influences of tightly bound, partially bound, and free
water. During the past few years it has been shown that the properties of foods
can be related more critically and explicitly to a^. The varied configurations of
moisture sorption isotherms illustrate diagrammatically that a^ is nonlinearly
related to total moisture content, and that the sigmoid shape of isotherms
varies because of the different types and proportions of food product constitu­
ents. Chemical and physical changes that occur during processing and storage
can be related directly to the equilibrium relative humidity, expressed as a^^, of
a food product. As new generations of students are exposed to a^ concepts,
applications of these principles should facilitate significant improvements in
the processing and stabilization of foods. Increased applications\3f a,^ concepts
would also be facilitated by development of rugged, stable, rapid, and precise
instruments for estimating a^ in small containers.
Water activity principles can be applied directly to many common indus­
trial problems, such as dehydration, the development of intermediate moisture
iv PREFACE
foods, Stabilization of flavor, color, and texture, and elimination of micro­
biological problems.
The symposium program v^as approved and supported by the IFT Basic
Symposium Committee, which included Drs. R. V. Josephson (Chairman),
P. G. Crandall, D. T. Gordon,]. A. Maga, M. D. Pierson,]. M. Regenstein, and
R. A. Scanlan. Charles ]. Bates, 1985-86 IFT President and an enthusiastic
supporter, welcomed participants and expressed his gratitude to the speakers
who came from distant places and foreign lands. Calvert L. Wiley, IFT Execu­
tive Director, ]ohn B. Klis, Director of Publications, Anna May Schenck, IFT
Associate Scientific Editor, and other IFT staff members provided the support
and coordination required to facilitate a nearly flawless symposium.
Primary credit for the success of this symposium belongs to the speakers.
They performed professionally, meeting all the necessary deadlines for sub­
mission of abstracts and manuscripts to result in the rapid and timely publica­
tion of this volume.
Louis B. Rockland
Larry R. Beuchat
Introduction
Critical interest in the influence of water activity (a.^) on food product, quality,
and stability began unceremoniously about 30 years ago. It was promoted by
empirical observations inconsistent with a direct relationship between total
moisture and product stability.
Microbiologists were among the first to recognize that a^, rather than total
moisture content, governed the growth, survival rate, death, sporulation and
toxin production by diverse microorganisms. Scott (1936, 1953, 1957) and
Mossel (1949, 1951, 1955), who conducted the classical work in this area, did
not consider applications of their observations to dry products having low a^^.
As early as 1949, Karon and Hillery noted that at any given a^, the moisture
content of various tissues of peanuts were very different, and that despite these
differences, all of the tissues were at moisture equilibrium. In classical papers.
Lea et al. (1943) and Bryce and Pearce (1946) reported an optimum moisture
content for dried milk powder. Rockland (1957) reported an optimum mois­
ture content for shelled walnuts and related this to an optimum a^^ range.
These observations were in contrast to the generally accepted belief that sta­
bility increased consistently with decreasing moisture content. It was also
suggested that the optimum a^^ related to the manner in which the mois­
ture was bound by individual chemical moieties (Pauling, 1945) and that this
could be delineated using a mathematical treatment of moisture sorption data.
This provided the basis for the Local Isotherm Concept and the utilization of
moisture sorption data for derivation of Stability Isotherms (Rockland, 1957,
1969). On the basis of the Local Isotherm Concept, which delineated, at least
qualitatively, moisture binding properties and hence stability, it was also pro­
posed that changes in temperature would displace moisture sorption iso­
therms and result in predictable changes in quality and stability. Previously
Bull (1944) and subsequently others have subscribed to the premise that the
Brunauer-Emmet-Teller (BET) equation (1938) might be useful in providing a
Vi INTRODUCTION
theoretical basis for differentiating moisture binding characteristics. Of classi­
cal importance was the thermodynamic treatment of moisture sorption phe­
nomena by McLaren and Rowen (1951). During the past 20 years, there has
been an avalanche of equations proposed to describe moisture sorption iso­
therms and to provide theoretical justification for moisture sorption phe­
nomena (Van den Berg and Bruin, 1981).
During recent years, emphasis has been placed upon the relationships be­
tween moisture sorption characteristics, i.e., physical and chemical changes, as
well as microbiological influences upon the stability and quality of food and
other natural products (see figure, facing page). The primary objective of the
1986 Basic Symposium was to disseminate new knowledge and to explore
some basic principles relating to the influences of a^^ on food quality.
Louis B. Rockland
REFERENCES
Brunauer, S., Emmett, P. H., and Teller, E. 1938. J. Am. Chem. Soc. 60:309.
Bryce, W. A. and Pearce, J. A. 1946. Can.J. Res. 24:61.
Bull, H. B. 1944. J. Am. Chem. Soc. 66:1499.
Karon, M. L. and Hillery, R. E. 1949. J. Am. Oil Chem. Soc. 26:16.
Lea, C. H., Moran, T., and Smith, J. A. B. 1943. J. Dairy Sci. 13:164.
McLaren, A. D. and Rowen, J. W. 1951. J. Polymer Sci. 7:289.
Mossel, D. A. A. 1951. Antonie von Leeuwenhoek 17:146.
Mossel, D. A. A. and Kuijk, H. J. L. 1955. Food Res. 20:415.
Mossel, D. A. A. and Westerdijk, J. 1949. Antonie von Leewenhoek 15:190.
Pauling, L. 1945. J. Am. Chem. Soc. 67:555.
Rockland, L. B. 1957. Food Res. 22:604.
Rockland, L. B. 1969. Food Technol. 23:11.
Scott, W. J. 1936. J. Council Sci. Ind. Res. 9:177.
Scott, W. J. 1953. Aust.J. Biol. Sci. 6:549.
Scott, W. J. 1957. Adv. Food Res. 7:83.
Van den Berg, C. and Bruin, S. 1981. In Water Activity: Influences on Food
Quality (L. B. Rockland and G. F. Stewart, eds.), p. 1. Academic Press, New
York.
WATER ACTIVITY - STABILITY DIAGRAM