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Applied and Environmental Microbiology, April 2002, p. 1864-1871, Vol. 68, No. 4
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.4.1864-1871.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Staphylococcus aureus Growth Boundaries: Moving towards Mechanistic Predictive Models Based on Solute-Specific Effects

Cynthia M. Stewart,^1* Martin B. Cole,² J. David Legan,^1, Louise Slade,¹ Mark H. Vandeven,¹ and Donald W. Schaffner³

Nabisco, Inc., East Hanover, New Jersey 07936,¹ Food Science Australia, North Ryde, New South Wales 1670, Australia,² Department of Food Science, Rutgers University, New Brunswick, New Jersey 08901-8520³

Received 29 August 2001/ Accepted 17 January 2002

The formulation of shelf-stable intermediate-moisture products is a critical food safety issue. Therefore, knowing the precise boundary for the growth-no-growth interface of Staphylococcus aureus is necessary for food safety risk assessment. This study was designed to examine the effects of various humectants and to produce growth boundary models as tools for risk assessment. The molecular mobility and the effects of various physical properties of humectants, such as their glass transition temperatures, their membrane permeability, and their ionic and nonionic properties, on S. aureus growth were investigated. The effects of relative humidity (RH; 84 to 95%, adjusted by sucrose plus fructose, glycerol, or NaCl), initial pH (4.5 to 7.0, adjusted by HCl), and potassium sorbate concentration (0 or 1,000 ppm) on the growth of S. aureus were determined. Growth was monitored by turbidity over a 24-week period. Toxin production was determined by enterotoxin assay. The 1,792 data points generated were analyzed by LIFEREG procedures (SAS Institute, Inc., Cary, N.C.), which showed that all parameters studied significantly affected the growth responses of S. aureus. Differences were observed in the growth-no-growth boundary when different humectants were used to achieve the desired RH values in both the absence and the presence of potassium sorbate. Sucrose plus fructose was most inhibitory at neutral pH values, while NaCl was most inhibitory at low pH values. The addition of potassium sorbate greatly increased the no-growth regions, particularly when pH was <6.0. Published kinetic growth and survival models were compared with boundary models developed in this work. The effects of solutes and differences in modeling approaches are discussed.

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* Corresponding author. Present address: Food Science Australia, P.O. Box 52, North Ryde, NSW 1670, Australia. Phone: 612 9490 8522. Fax: 612 9490 8581. E-mail: cindy.stewart{at}foodscience.afisc.csiro.au.

Present address: Kraft Foods North America, Inc., Glenview, IL 60025.

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Applied and Environmental Microbiology, April 2002, p. 1864-1871, Vol. 68, No. 4
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.4.1864-1871.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

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