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.
A good practical illustration of the difference between TA and pH is the effect of cutting. Up to the time of cutting, TA of the milk increases with the development of acidity by the culture. After cutting the TA of the whey is much lower. This does not mean that acid development stopped. It simply means that titratable H+ ions associated with the milk proteins are no longer present in the whey. This leads to the concept of buffer capacity, which is an important principle in cheese making. The effect of protein removal on the TA of whey, is related to the ability of protein to 'buffer' the milk against changes in pH. That same buffer property is the reason it helps to take acidic medication, like aspirin, with milk.
Buffer capacity can be described as the ability of an aqueous system, such as milk, to resist changes in pH with addition of acids (added H+) or bases (added OH-). Specifically, buffer capacity is the amount of acid or base required to induce a unit change in pH. For example, a small addition of acid to distilled water will cause a large reduction in pH. The same amount of acid would have a small effect on the pH of milk because milk proteins and salts neutralize the acidity.
The two most important buffer components of milk are caseins (buffer maximum near pH 4.6) and phosphate (buffer maxima near pH 7.0). The buffer maximum near pH 5.0 is extremely important to cheese manufacture because the optimum pH for most cheese is in the range of 5.0 - 5.2. As the pH of cheese is reduced towards pH 5.0 by lactic acid fermentation, the buffer capacity is increasing (i.e., each incremental decrease in pH requires more lactic acid). The effect is to give the cheese maker considerable room for variation in the rate and amount of acid production. Without milk's built in buffers it would be impossible to produce cheese in the optimum pH range.
Another way to illustrate the difference between TA and pH is to consider typical ranges of pH and TA for normal milk. TA is a measure of the total buffer capacity of milk for the pH range between the pH of milk and the phenolphthalein end point (about pH 8.3). The pH of milk at 25C, normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acidity of herd milks is 0.12 to 0.18% lactic acid In other words, pH is a good indicator of initial milk quality, while the traditional measurement of TA to indicate bacterial growth in milk is less precise.
The pH of cheese milk, whey and soft cheese can be measured directly. Firm and hard cheese must be fragmented before analysis. Always measure cheese pH in duplicate and use extreme care in handling the electrode. Place the fragmented cheese in a 30 ml vial or small beaker and gently push the electrode into the cheese ... too much haste is likely to break the electrode on the bottom of the beaker. To ensure good contact, press the cheese around the electrode with your fingers. There is no need to rinse the electrode between cheese samples. However, if the electrode is stored in buffer it should be rinsed with distilled water before measuring cheese pH. Always store the electrode in pH 4 buffer or as directed by the manufacturer. Do not rub the electrode. The electrode should be washed with detergent and rinsed with acetone occasionally to remove fat and protein deposits.