1
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.

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:

1. III. Methods for controlling aw:
   • Equilibration with controlling solutes
   • Determination of the water sorption isotherms
   • Addition of solutes
2. IV. Water requirements for growth
   • Molds
   • Yeasts
   • Bacteria
   • General relationships
3. V. Factors affecting water requirements
   • Nutrition, temperature, oxygen, inhibitors, adaptation
4. VI. Special groups
   • Halophilic bacteria
   • Osmophilic yeasts
   • Xerophilic molds
5. 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.