Control by Reduced Water Activity and Drying ◾ 471
and 0.887 at 25°C). The minimum A W for microbial growth can vary with the type of solutes in
a food. Clo. botulinum Type E failed to grow below an A W of 0.97 when NaCl was used as a solute, but grew up to an A W of 0.94 when glycerol was the solute. Pseudomonas fluorescens similarly
showed a minimum A W of 0.957 for growth with NaCl but 0.94 with glycerol. This is because
glycerol enters freely inside the cell and thus does not cause osmotic stress as do nonpermeable
NaCl, sucrose, and similar solutes. Germination of spores in glycerol occurs at lower A W because
Clo. botulinum E spores, with glycerol, germinate at an A W of 0.89 but do not grow below an A W
of 0.94. The growth rate of microorganisms also decreases as the A W value is lowered. The growth
rate of Sta. aureus reduces to about the 10% level at an A W of 0.90 of its optimum growth rate
at an A W of 0.99.
Among microbial groups, halophiles, osmophiles, and xerophiles grow better or grow preferentially at lower A W values. Halophiles, such as some vibrios, need NaCl in varying amounts for
growth. Osmophiles and xerophiles, yeasts, and molds grow at an A W less than 0.85 as they do
not have competition from bacteria.
Optimum growth of most microorganisms in foods occurs at an A W ≥ 0.98. At A W ≥ 0.98,
gram-negatives, having a faster growth rate, predominate if other needs for optimum growth are
fulfilled. As the A W drops to 0.97, gram-positive bacteria, such as Bacilli, Lactobacilli, Micrococci,
and Clostridia, predominate. Below an A W of 0.93, gram-positive bacteria, such as Micrococci,
Staphylococci, Enterococci, and Pediococci as well as yeasts and molds grow preferentially. As the
A W drops below 0.86, osmophilic yeasts and xerophilic molds predominate. They can grow in
foods with an A W up to 0.6; as A W drops below 0.6, microbial growth stops. 3,5,6
Methods
The water activity of foods can be reduced by using one or more of these three basic principles:
(1) removing water by dehydration, (2) removing water by crystallization, or (3) adding solutes to
bind water. Some of these methods and their effects on microorganisms are briefly described. 4,7
Natural Dehydration
Natural dehydration is a low-cost method in which water is removed by the heat of the sun. It
is used to dry grains as well as to dry some fruits (raisins), vegetables, fish, meat, milk, and curd
(from milk), especially in warmer countries. The process is slow; depending on the conditions
used, spoilage and pathogenic bacteria as well as yeasts and molds (including toxigenic types) can
grow during drying.
Mechanical Drying
Mechanical drying is a controlled process, and drying is achieved in a few seconds to a few hours.
Some of the methods used are tunnel drying (in which a food travels through a tunnel against
a flow of hot air that removes the water), roller drying (in which a liquid is dried by applying a
thin layer on the surface of a roller drum heated from inside), and spray drying (in which a liquid is sprayed in small droplets, which then come in contact with hot air that dries the droplets
instantly). Vegetables, fruits, fruit juices, milk, coffee, tea, and meat jerky are some foods that are
dried by mechanical means. Liquids may be partially concentrated before drying by evaporation,
reverse osmosis, freeze-concentration, and addition of solutes.
and 0.887 at 25°C). The minimum A W for microbial growth can vary with the type of solutes in
a food. Clo. botulinum Type E failed to grow below an A W of 0.97 when NaCl was used as a solute, but grew up to an A W of 0.94 when glycerol was the solute. Pseudomonas fluorescens similarly
showed a minimum A W of 0.957 for growth with NaCl but 0.94 with glycerol. This is because
glycerol enters freely inside the cell and thus does not cause osmotic stress as do nonpermeable
NaCl, sucrose, and similar solutes. Germination of spores in glycerol occurs at lower A W because
Clo. botulinum E spores, with glycerol, germinate at an A W of 0.89 but do not grow below an A W
of 0.94. The growth rate of microorganisms also decreases as the A W value is lowered. The growth
rate of Sta. aureus reduces to about the 10% level at an A W of 0.90 of its optimum growth rate
at an A W of 0.99.
Among microbial groups, halophiles, osmophiles, and xerophiles grow better or grow preferentially at lower A W values. Halophiles, such as some vibrios, need NaCl in varying amounts for
growth. Osmophiles and xerophiles, yeasts, and molds grow at an A W less than 0.85 as they do
not have competition from bacteria.
Optimum growth of most microorganisms in foods occurs at an A W ≥ 0.98. At A W ≥ 0.98,
gram-negatives, having a faster growth rate, predominate if other needs for optimum growth are
fulfilled. As the A W drops to 0.97, gram-positive bacteria, such as Bacilli, Lactobacilli, Micrococci,
and Clostridia, predominate. Below an A W of 0.93, gram-positive bacteria, such as Micrococci,
Staphylococci, Enterococci, and Pediococci as well as yeasts and molds grow preferentially. As the
A W drops below 0.86, osmophilic yeasts and xerophilic molds predominate. They can grow in
foods with an A W up to 0.6; as A W drops below 0.6, microbial growth stops. 3,5,6
Methods
The water activity of foods can be reduced by using one or more of these three basic principles:
(1) removing water by dehydration, (2) removing water by crystallization, or (3) adding solutes to
bind water. Some of these methods and their effects on microorganisms are briefly described. 4,7
Natural Dehydration
Natural dehydration is a low-cost method in which water is removed by the heat of the sun. It
is used to dry grains as well as to dry some fruits (raisins), vegetables, fish, meat, milk, and curd
(from milk), especially in warmer countries. The process is slow; depending on the conditions
used, spoilage and pathogenic bacteria as well as yeasts and molds (including toxigenic types) can
grow during drying.
Mechanical Drying
Mechanical drying is a controlled process, and drying is achieved in a few seconds to a few hours.
Some of the methods used are tunnel drying (in which a food travels through a tunnel against
a flow of hot air that removes the water), roller drying (in which a liquid is dried by applying a
thin layer on the surface of a roller drum heated from inside), and spray drying (in which a liquid is sprayed in small droplets, which then come in contact with hot air that dries the droplets
instantly). Vegetables, fruits, fruit juices, milk, coffee, tea, and meat jerky are some foods that are
dried by mechanical means. Liquids may be partially concentrated before drying by evaporation,
reverse osmosis, freeze-concentration, and addition of solutes.