Concepts of Acidity and pH
All aqueous systems (including the water in you and in cheese) obey the following relationship (Equation 3) between the concentration of hydrogen ions (H+) and hydroxyl ions (OH-). Note, the square brackets indicate concentration in moles per litre. A mole is 6 x 1023 molecules, that is, the numeral six with 23 zeros after it.
[H+] x [OH-] = 10-14
Because the actual concentrations in moles per litre are small, it is customary to express the values as exponents. For example, if we know that the concentration of hydrogen ions [H+] in a sample of milk is 0.000001 moles/l which is equivalent to 10-6 moles/l, we can calculate the concentration of hydroxyl ions as 10-14/10-6 = 10-8 moles/l which is the same as 0.00000001 moles/l.
* If [H+] = [OH-] the solution is neutral with respect to acidity.
* If [H+] > [OH-] the solution is acidic.
* If [H+] < [OH-] the solution is basic or alkaline.
* Chemicals which contribute H+ or absorb OH- are acids, while bases contribute OH- or absorb H+.
The concept of pH evolved as a short hand method to express acidity. We have already seen that a hydrogen ion concentration of 0.000001 moles/l can be expressed as [10-6], an expression which defines both the unit of measurement and the numerical value. The concept of pH is a further abbreviation which expresses the concentration of hydrogen ions as the negative log of the hydrogen ion concentration in units of moles/l. This sounds complex but is quite easy to apply. For example, the log10 of hydrogen ion concentration of [10-6] is equal to -6. The final step is to take the negative of the log, that is -1 x -6 which is 6. So, 0.0000001 moles/l = [10-6] = pH 6. From the relationship expressed in Equation 3, if the concentration of one of OH- and H+ is known, it is always possible to calculate the concentration of the other. So, if the pH of a solution is 6, the pOH is 14 - 6 = 8. Because this relationship is understood, the convention is to only report pH. Note, that because the negative sign was dropped by convention, decreasing pH values mean increasing acidity, that is, increasing concentration of H+ ions. So, although both TA and pH are measures of acidity, pH decreases with increasing acidity.
All of this can be summarized by a description of the pH scale. The pH scale for most practical purposes is from 1 to 14, although a pH of less than one is theoretically and practically possible.
pH 7.0 is neutral acidity [H+] = [OH-]
pH < 7.0 = acid condition [H+] > [OH-]
pH > 7.0 = alkaline condition [H+] < [OH-]
pH Versus Titratable Acidity
TA and pH are both measures of acidity but, for most purposes, pH is a better process control tool, because the pH probe measures only those H+ which are free in solution and undissociated with salts or proteins. This is important because it is free H+ which modifies protein functionality and contributes sour taste. It is also the pH rather than titratable acidity which is the best indicator of the preservation and safety effects of acidity. It must be emphasized, that the most important factor available to the cheese maker to control spoilage and pathogenic organisms is pH control. The pH history during and after cheese manufacture is the most important trouble shooting information. Cheese moisture, mineral content, texture and flavour are all influenced directly by the activity of free hydrogen ions (i.e. pH).
Titratable acidity (TA) measures all titratable H+ ions up to the phenolphthalein end point (pH 8.5) and, therefore, varies with changes in milk composition and properties. During cheese manufacture, the pH gives a true indication of acid development during the entire process so that the optimum pH at each step is independent of other variables such as milk protein content. However, the optimum TA at each step in cheese making will vary with initial milk composition and the type of standardization procedure used.