See discussion of pH and acidity in Section 3.5.
Apparatus and Reagents
- An acidimeter equipped with a burette graduated in tenths of a ml up to 10 ml, and some means of filling the same without undue exposure of the solution to the carbon dioxide of the atmosphere.
- N/10 sodium hydroxide solution.
- A dropping bottle containing a 1% alcoholic phenolphthalein solution.
- White cup, glass stirring rod, 17.6 ml pipette (or 8.8 or 9.0 ml pipette)
- For cream, Torsion balance and 9 g weight.
- Mix sample thoroughly by pouring it from one container to another. The temperature of the sample should be near 20C.
- Pipette 17.6 ml of milk or cream into a white cup. Note: 8.8 ml pipettes may also be used but are no longer as readily available as 17.6 ml pipettes. Readily available 9 ml pipettes are also used but require application of a correction factor to the final result.
- Add six drops of phenolphthalein indicator solution to milk, 10 drops if the product is cream.
- Titrate the sample with the N/10 sodium hydroxide solution (0.1 Normal NaOH) while stirring the sample with the glass rod. Look for the appearance of a faint pink colour which signals the endpoint. Add another drop or half a drop of NaOH if the pink colour does not persist for 30 s.
- Record the number of ml of NaOH used to reach the endpoint. This value is called the 'titre'. Titratable acidity reported as percent lactic acid is dependent on the volume of sample.
For the 8.8 ml pipette, % Lactic acid = titre
For the 17.6 ml pipette, % Lactic acid = 0.5 x titre
For the 9.0 ml pipette, % Lactic acid = 0.98 x titre.
Note that there is practically no lactic acid in fresh milk, but it is a North American convention to report TA in terms of % lactic acid.
As described in the next section, both titratable acidity (TA) and pH are measures of acidity. However, for most process control purposes, pH is a more useful measurement. Many cheese makers, however, still use TA to monitor initial acid development (that is to check for culture activity) during the first hour after adding the culture. For this purpose, TA is a more reliable indicator because relative to pH measurement, it is more sensitive to small changes in milk acidity.
When using TA to monitor initial culture activity note that:
- You are looking for a measurable increase in TA to confirm that the culture is active. For example, if the initial TA taken immediately after the culture was added is 0.183% lactic acid, and the TA after one hour of ripening is 0.194 % lactic acid, the change in TA is 0.194 - 0.183 which is 0.011%.
- Different people will interpret the coloured endpoint differently, so it is important that the same person takes both the initial and final TA measurements.
- Carefully performed, it is possible to reliably measure a change in TA of 0.05% lactic acid, so if the TA increase is greater than 0.05% you can conclude that the culture is active. In most cases TA increases in the range of 0.05% to 0.10% are obtained after about 30 minutes of ripening (that is, 30 minutes after adding the culture).
- It is critical to take the initial TA reading after the culture is added, because the culture (especially the bulk culture) is acidic.