Water activity (a
w) was measured for various two component aqueous solutions at various inter-solute interactions and the observed a
w was compared with the theoretical prediction by Ross equation assuming no inter-solute interactions. For solutions with neutral components including electrolytes, sugars, and neutral amino acid, solute-solute interaction was weak so that no substantial deviation was observed for observed a
w from that predicted by Ross equation. When acid and base were involved, the deviation between the two was large because of the neutralization. For a solution containing macromolecule, bovine serum albumin (BSA) and sucrose mixture showed a strong cooperative effect between solutes to reduce a
w much smaller than the theoretical expectation. In a case with an amino-carbonyl reaction between glycine and ribose kept at 60℃, a drastic increase in optical absorbance at 327 nm was observed with time and a
w increased accordingly because of the water liberation at the initial stage of the Schiff-base formation and the following complex Maillard reaction process to reduce the number of solute molecules in the system.
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