Question

34. Which of the following is NOT true of water activity? A. It is the amount of water available to organisms B. It is reduced by interaction with solute molecules C. It is reduced by adsorption to surfaces D. Low concentration of salts reduces water activity E. Most microorganisms only grow at water activities around 0.98 or

Answer 1

water activity is defined as the ratio of vapour pressure of water in a particular substance to the vapor pressure of pure water. Therefore, we can also define it as the amount of water available to organisms and most microorganisms grow at water actvities around 0.98 like yeast,bacteria and mould. there are many microorganisms that grow even at lower water activities of 0.60 or 0.75 etc. water activity is decreased when salt or sugar is added so D is not true. water activity is influenced by surface. it is different for hydrophilic and hydrophobic surfaces even. If strong hydrogen bonds are formed by the surface to the interfacial water, then it is expected to have a lower water activity. this proves the fact that water activity is reduced by adsorption to surfaces.so point C is correct. Point B is partially true as various experiments depict that for neutral solutions where there is no solute-solute interaction or the interaction is very weak the water activity was correspondingly lower as compared to any amino-carbonyl reaction where the water activity increased gradually due to the formation of schiff base which led to water liberation at the initial stage. So, point D is not true for water activity. rest of the points are clearly explained.