|MadSci Network: Biochemistry|
Salt performs several functions in preserving food: 1. Salt shifts the growth conditions to favor Gram-positive instead of Gram-negative bacteria. Most human pathogens are Gram-negative. These include Salmonella, Escherichia coli and Clostridia. There are only a few Gram-positive pathogens, such as Listeria and Staphylococcus. 2. Salt removes available water from the food by changing the osmotic pressure. This makes it harder for bacteria to obtain water to grow. The availability of water in food is called "water activity", and every species of bacteria has a minimum water activity below which it won't grow. Most pathogens will not grow below a water activity of 0.92 (pure water is 1.0). 3. Salt in itself eventually becomes poisonous to the microbes by creating an electrolyte imbalance within the cell. 4. A brine (salt / (salt + moisture)) of 3.5% will normally prevent serious pathogens (e.g., Clostridium botulinum) from growing. Brines of more than 10% will normally prevent all pathogenic bacteria from growing.
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