Lecture%2017

Lecture%2017 - W ATER A CTIVITY W ATER A CTIVITY All...

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Unformatted text preview: W ATER A CTIVITY W ATER A CTIVITY All organisms need H 2 O Exception: a few osmophiles or halophiles Important in predicting which microbes can grow in a given food Low a W promotes the survival of all microbes W ATER A CTIVITY A. Descriptive Definition a W refers to the H 2 O available to an organism Similar to H 2 O content Food can have high H 2 O content but low available H 2 O For a growing microbe, bound H 2 O is not better than no H 2 O B. M EASUREMENT OF W ATER A CTIVITY a W is understood - a W meat>a W bread 1. Measurement of humidity a W = vapor pressure of the food = Equilibrium Relative Humidity vapor pressure of H 2 O 100 Humidity probe over the food in a sealed container Available (free) H 2 O, but not bound H 2 O, contributes to vapor pressure Quantifying changes in weight = loss/gain of H 2 O food into sealed environment with solution of known osmotic strength 2. Water activity ranges Full (theoretical) scale 0 to 1 (pure H 2 O = 1) Biologically relevant scale: 0.6 to 0.999- Nothing grows at a W <0.6- Physiological saline = 0.996 C ALCULATIONS OF W ATER A CTIVITY Calculation based on solute concentration Solute concentration, major determinant Solutes raise bp and lower a W a W = Molarity of H 2 O = 55.51 Molarity of H 2 O +Molarity of solute 55.51 + solute Molarity a W of 1M solution = 55.51 56.51 = 0.98 L IMITATIONS TO A CCURACY a) Some solutes bind H 2 O more effectively than others Theoretical a W of 1M solution = max a...
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Lecture%2017 - W ATER A CTIVITY W ATER A CTIVITY All...

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