This second edition of Water Activity in Foods furnishes those working within food manufacturing, quality control, and safety with a newly revised guide to water activity and its role in the preservation and processing of food items. With clear, instructional prose and illustrations, the book's international team of contributors break down the essential principles of water activity and water-food interactions, delineating water's crucial impact upon attributes such as flavor, appearance, texture, and shelf life.
The updated and expanded second edition continues to offer an authoritative overview of the subject, while also broadening its scope to include six newly written chapters covering the latest developments in water activity research. Exploring topics ranging from deliquescence to crispness, these insightful new inclusions complement existing content that has been refreshed and reconfigured to support the food industry of today.
GUSTAVO V. BARBOSA-CÁNOVAS is Professor of Food Engineering and Director of the Center for Nonthermal Processing of Food at Washington State University, Pullman, WA, USA.
ANTHONY J. FONTANA, JR. is Technical Services Manager, ALS Global USA, Corp., Irvine, CA.
SHELLY J. SCHMIDT is Professor of Food Chemistry in the Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL, USA.
THEODORE P. LABUZA is Morse Alumni Distinguished Professor of Food Science and Engineering in the Department of Food Science and Nutrition at the University of Minnesota, Saint Paul, MN, USA.
Introduction: Historical Highlights of Water Activity Research
Jorge Chirife1and Anthony J. Fontana Jr.2
1Catholic University, Buenos Aires, Argentina
2ALS - Truesdail, Irvine, CA, USA
The concept of water activity (aw) is more than 50?years old. William James Scott showed in 1953 that microorganisms have a limiting aw level for growth. It is now generally accepted that aw is more closely related to the microbial, chemical, and physical properties of foods and other natural products than is total moisture content. Specific changes in color, aroma, flavor, texture, stability, and acceptability of raw and processed food products have been associated with relatively narrow aw ranges (Rockland and Nishi 1980). Next to temperature, aw is considered one of the most important parameters in food preservation and processing (van den Berg 1986). This chapter is not a review of the literature on aw but rather a highlight of some early key aw research as it relates to microbial growth, moisture sorption isotherms, prediction and measurement of aw in foods, and, to a lesser extent, the influence of aw on the physical and chemical stability of foods.
Australian-born microbiologist Scott (1912-1993) received his bachelor's degree from the University of Melbourne (1933) and a doctorate of science degree from the Council for Scientific and Industrial Research (CSIR) Meat Research Laboratory (1933). He then took a position as senior bacteriologist at the CSIR Division of Food Preservation and Transport from 1940 to 1960. In 1960, he moved to the Meat Research Laboratory, where he served as assistant chief of division until 1964 and officer-in-charge until 1972. In 1979, he became a fellow of the Australian Academy of Technological Sciences and Engineering.
Scott's early work was concerned with handling, cooling, and transport conditions that would enable chilled beef to be successfully exported to Britain. During World War II, he was concerned with the microbiology of foods supplied by Australia to the Allied Forces. After the war, he pioneered studies on the water relations of microorganisms. In 1953, Scott related the relative vapor pressure of food to the thermodynamic activity of water, using the definition aw = p/po, where aw is the water activity derived from the laws of equilibrium thermodynamics, p is the vapor pressure of the sample, and po is the vapor pressure of pure water at the same temperature and external pressure. He showed a clear correlation between the aw of the growth medium and the rate of Staphylococcus aureus growth. The summary of his paper stated:
Table 1.1 Papers by Scott and Christian.
Author Year Title of Paper
Scott, W.J. 1953
Water relations of Staphylococcus aureus
at 30°C Christian, J.H.B. and Scott, WJ. 1953
Water relations of Salmonella
at 30°C Christian, J.H.B. 1955a
The influence of nutrition on the water relations of Salmonella oranienburg
Christian, J.H.B. 1955b
The water relations of growth and respiration of Salmonella oranienburg
at 30°C Scott, W.J. 1957
Water relations of food spoilage microorganisms
Fourteen food-poisoning strains of Staphylococcus aureus have been grown in various media of known aw at 30°C. Aerobic growth was observed at water activities between 0.999 and 0.86. The rate of growth and the yield of cells were both reduced substantially when the aw was less than 0.94. The lower limits for growth in dried meat, dried milk, and dried soup were similar to those in liquid media. Aerobic growth proceeded at slightly lower water activities than anaerobic growth. All cells were capable of forming colonies on agar media with water activities as low as 0.92. The 14 strains proved to be homogeneous with similar water requirements.
Scott's classic demonstration that it is not the water content but the aw of a food system that governs microbial growth and toxin production was a major contribution to food microbiology. Many scientists, most notably his Australian colleague, J.H.B. Christian, expanded Scott's work. Key papers published in the 1950s by both Scott and Christian are listed in Table 1.1. These papers laid the foundation for future research into the survival and growth of microorganisms in foods at low aw.
In the field of food science, the general acceptance and application of the concept of a minimum aw for microbial growth began with the review by Scott published in 1957, Water Relations of Food Spoilage Microorganisms. Taken from the table of contents in Scott's classic review, the following are some of the aspects discussed:
- III. Methods for controlling aw:
- Equilibration with controlling solutes
- Determination of the water sorption isotherms
- Addition of solutes
- IV. Water requirements for growth
- General relationships
- V. Factors affecting water requirements
- Nutrition, temperature, oxygen, inhibitors, adaptation
- VI. Special groups
- Halophilic bacteria
- Osmophilic yeasts
- Xerophilic molds
- VII. Some applications in food preservation
- Fresh foods, dried foods, concentrated foods, frozen foods, canned foods
Table 1.2 Selected early work on the minimal water activity for growth of pathogenic and spoilage microorganisms.
Author Year Title of Paper
Baird-Parker, A.C. and Freame, B. 1967
Combined effect of water activity, pH, and temperature on the growth of Clostridium botulinum
from spore and vegetative cell inocula Ohye, D.F. and Christian, J.H.B. 1967
Combined effects of temperature, pH, and water activity on growth and toxin production by Clostridium botulinum
types A, B, and E Pitt, J.I. and Christian, J.H.B. 1968
Water relations of xerophilic fungi isolated from prunes Anand, J.C. and Brown, A.D. 1968
Growth rate patterns of the so-called osmophilic and non-osmophilic yeasts in solutions of polyethylene glycol Ayerst, G. 1969
The effects of moisture and temperature on growth and spore germination in some fungi Emodi, A.S. and Lechowich, R.V 1969
Low temperature growth of type E Clostridium botulinum
spores. II. Effects of solutes and incubation temperature Kang, C.K., Woodburn, M., Pagenkopf, A., and Cheney, R. 1969
Growth, sporulation, and germination of Clostridium perfringens
in media of controlled water activity Horner, K.J. and Anagnostopoulos, G.D. 1973
Combined effects of water activity, pH, and temperature on the growth and spoilage potential of fungi Troller, J.A. 1972
Effect of water activity on enterotoxin A production and growth of Staphylococcus aureus
Beuchat, L.R. 1974
Combined effects of water activity, solute, and temperature on the growth of Vibrio parahaemolyticus
Northolt, M.D., van Egmond, H.P., and Paulsch, W.W. 1977
Effect of water activity and temperature on aflatoxin production by Aspergillus parasiticus
Pitt, J.I. and Hocking, A.D. 1977
Influence of solute and hydrogen ion concentration on the water relations of some xerophilic fungi Lotter, L.P. and Leistner, L. 1978
Minimal water activity for enterotoxin A production and growth of Staphylococcus aureus
Hocking, A.D. and Pitt, J.I. 1979
Water relations of some Penicillium
species at 25°C Briozzo, J., de Lagarde, E.A., Chirife, J., and Parada, J.L. 1986
Effect of water activity and pH on growth and toxin production by Clostridium botulinum
type G Tapia de Daza, M.S., Villegas, Y., and Martinez,...