(Water)
Part FDSC500 2: Water in Foods
1. The structure of water and solutions
a)
Bulk Properties of Solutions
Probably the main bulk property of a solution we can readily vary in foods is pH (=-log10[H+]). The pH of most foods is in the range 3-7. The pH is important as it alters the structure of molecules present, by either protonating or deprotonating the ionizable groups. The main protonation/deprotonation reactions we are interested in involve carboxylic acid and amine groups: R-COOH R-NH3+ R-COOR-NH2 + + H+ H+
As the reaction proceeds, the charge on the carboxylic acid will change from zero to -1 and the charge on the amine from +1 to zero. Qualitatively we can see that as the pH drops ([H+] increases) the reactions will be forced to the left hand side. However different functional groups have different affinity for protons that can be expressed quantitatively in terms of the equilibrium constant of the reaction. So for a general protonation reaction: AH K= A[A-][H+]/[AH] = + H+
[A-]/[AH] * [H+]
Taking negative logs of both sides we get the Henderson Hasselbach equation: pH=pKa+ log [A-]/[HA] The Henderson Hasselbach equation can tell us the ratio of protonated to deprotonated groups (i.e., -COOH and COO- for a carboxylic acid or NH3+ and NH2 for an amine) as a function of pH and the pK of that particular functional group. Plotting the concentration of both molecules as a function of pH we get a characteristic sigmoidal shape with [protonated]=[deprotonated] at pH=pK. The concentration ratio changes rapidly around the pK, but more than 2 pH units above and below changes in pH will not affect the ratio.
7
100% Percentage of component Protonated form Deprotonated form 50%
0% 0 2 4 6 pH 8 10 12
Figure 1: Characteristic acid-base dissociation for a pK=5 salt calculated using the Henderson-Hasselbach equation. Example: Vinegar is acetic acid solution. It deprotonates according to the following reaction with a pK of 4.74 CH3-COOH = CH3-COO- + H+ If we made a meat sauce containing acetic acid and ended up with a 1 M solution at pH 5, w...
FDSC500 Water