|Measuring Whey Acidity by using Titration|
|Written by Pav|
|Monday, 21 June 2010 02:18|
The premise behind acidity is that some types of molecules can release hydrogen
ions (H+) into a solution of water. The ability of the molecules to
release H+ influences how strong an acid is. A very strong acid,
such as HCl will disassociate and release all or nearly all of its H+ into solution. In a weaker acid, such as lactic acid or acetic acid, only some
of the molecules disassociate and give up H+, and the rest of the
acid molecules stay in solution, but may give up their H+ later.
Two Ways to Measure Acidity
The two common ways to measure acidity are pH and titratable acidity (TA). pH measures the concentration of disassociated hydrogen ions in the solution, and titratable acidity measures the concentration of both disassociated hydrogen ions and un-disassociated hydrogen ions. In other words, TA measures all the acid in solution, both molecules that have given up their H+ and ones that have not.
Suitability of pH and TA for Cheesemaking
Because lactic acid is a weak acid, and all of it does not disassociate in water, pH does not measure the exact quantity of lactic acid present, just the dissolved H+ ions. However, this does not make TA inherently better, just different. Both indicators work for cheesemaking. Pick whichever one is easier for you. Even TA doesn’t measure the exact amount of lactic acid because milk and whey contain other acids and buffers.
Principle Underlying TA Measurement
The fundamental idea behind TA measurement is that if you add a base to an acid, the base will neutralize the acid, and if you do this slowly, you can mathematically calculate the quantity of acid. Bases also dissociate in water, but instead of releasing H+ ions, they release hydroxyl ions (OH-). Together H+ and OH- form HOH, aka H2O, aka water. An OH- ion will first combine with all the available H+ ions, and then strip the H+ ions from weak acid molecules that have not disassociated and neutralize them, too.
Requirements for TA Measurement
Practically, to measure TA, you need the following:
Figure 1. Basic TA setup
Making a Standard Solution
As already mentioned, it is vital that you use a solution of known normality. You can either buy this or make it yourself. Making it yourself is not difficult, but if you’re not up for it, you can buy it. The normality available commercially may vary with your location. This is because different parts of the world use different standard methods and units to measure dairy products. You can choose from the following values commonly used with TA, and prepare the normality accordingly:
It is critical that if you are trying to follow a cheese recipe that you know which unit is used. Table 1 shows the basic relationship between the units.
Table 1. Common acidity measurement units
More About Normality
If you don’t want to read about science-y stuff, feel free to skip this section. You don’t need to understand this and can just follow the procedure.
Normality, as already mentioned is the measure of the concentration of something in a solution. More specifically, it is 1 gram equivalent weight (gEW) per liter of solution. The equivalent weight is the molecular weight divided by the valence. Another way to think about it is that the equivalent weight is the volume of solute you need to have to equal one mole of ions (either OH- or H+). This gives us the following formula: Normality = Grams/(Equivalent Weight X Volume)
Putting the above details into practice results in the following quantities shown in Table 2 that are necessary to create the various normalities. The quantities assume a pure assay of 100% NaOH, which gives it a molecular weight of almost exactly 40g per mole.
Table 2. Measurements for making common normalities
The process is relatively straightforward, as follows:
Method to Measure TA
Once you acquire a titration kit you will have most of the things you need to measure the titratable acidity in your milk or whey. The NaOH solution should be in one of the three normalities mentioned above. The acidity in milk is measured by titrating milk that has 3-5 drops phenolphthalein added to it with the NaOH solution. Phenolphthalein changes color at a pH value of 8.2. Adding a drop at a time of NaOH slowly brings up the pH level, and then it is possible to calculate the TA by measuring the volume of NaOH solution used.
Figure 2. Correct color goal
Figure 3 shows the color when you have added too much NaOH.
Figure 2. Color when too much NaOH is added
Easier Procedure for .1 N NaOH
The previous procedure uses a general 25 g sample to account for the larger sample size necessary when using a more concentrated NaOH solution (like the .25 N). If you have a standard .1 N NaOH solution, you can save yourself some math calculations if you use a 9 gram sample size. With a 9 g sample and using .1 N NaOH, the % lactic acid is equal to the volume of NaOH divided by 10. So if you use 1.8 ml NaOH that is .18 % lactic acid. You can purchase a special dairy burrette that has the ml/10 marking on it directly so you can quickly read the % lactic acid. The process is as follows for .1 N NaOH:
Measuring TA by Using a pH Meter
It is possible to measure the TA by using a pH meter instead of phenophthalein. The procedure is similar to the ones above, except instead of stopping when there is a color change, you stop when the pH meter reads 8.2.
If you follow the first procedure using the 25 gram sample size, you need to calculate the TA. This is also relatively straightforward. The theory here is that you first calculate the molecular weight of the acid, and then plug it into a formula. Lactic acid is an organic acid with one carboxylic acid, CH3-CHOH-COOH, having a molecular weight of 90. The formula to calculate the % titratable acidity (this is in the industry the same as the % lactic acid) is as follows:
After you have the %TA, use the equations in table 3 to get the desired measurement unit. For more details about the relationships among the units, see Table 1.
Table3. Converting among the various units
|Last Updated on Saturday, 26 June 2010 03:47|