Determination of acidity by titration with lyes. This test allows us to find more than just presence, but also the amount of acid in the milk. It is about this, that some phenolphthalein is added to the milk as an indicator and gradually as much of the standard solution of sodium or potassium lye, until all the acid has neutralized and a little excess lye is formed. This moment is indicated by phenolphthalein, which turns pink with a slight excess of lye.
I ignore the Soxhlet and Henkel methods as impractical in cheese making, for he measures in too great a step, requires rarely used ¼ normal lye and does not give direct amounts of acid in percentages or numbers easily convertible to percentages of lactic acid. It is not known, why it has been preserved so far in our dairy industry, when 3/4 the globe uses the Marshall's method, which we will see in the future.
If you want to convert the degrees S — H to the percentages of lactic acid you have to multiply them by 0,0225, e.g. 7° S — H corresponds to 7 • 0,0225 = 0.1575% acid.
Dornic used for titration 1/9 normal lye. In this way, degrees of lactic acid are obtained that are easy to convert into percentages.
Dornic's method changed by Marshall. Due to this, that lye 1/9 normal is rarely used, Marschall changed Dornic's method this way, that he only tried 9 cm³ of milk, 5 drops 2 % alcoholic phenolphthalein solution and lye 0,1 normal, easy to buy everywhere. Using this revised method, the same grades are obtained as with the Dornic method, because 1/9 :10 = 1/10 :9.
The execution of the test is as follows: The test tube is measured with a suction cup 9 cm³ of milk and add it with a dropper 5 drops of phenolphthalein. The test tube is held with the left hand, on the right, the lye is measured with a suction cup or a burette divided by co 0,1 cm3, the alkali titration should start from zero. Then lye is added dropwise until a weak one appears, but clear and persistent pink color. After each addition of lye, the contents of the test tube are shaken. The number of tenths of a cm3 of lye used represents the number of Marshall degrees of acidity.
Therefore 0,1 cm³ of used lye = 1 ° M = 0,01% lactic acid
1,0 cm³ of used lye = 10 ° M = 0.10% lactic acid
10,0 cm³ of used lye = 100 ° M = 1.00% lactic acid
The acidity of milk converted to lactic acid:
normal fresh milk 0,12 do 0,19%
alkaline below 0,12 „
not withstanding boiling 0,26 do 0,28 „
sour, curdled 0,60 do 1,00 „
Initial acidity of fresh, by bacteria not yet disturbed milk depends on the acidic nature of the casein, acidic mineral salts and gas. The next increase in acidity is due to the activity of the bacteria.
It is significant, that the initial acidity in milk with fat content 3 do 3,5% is approx 0,12 do 0,15%, and containing milk 4 do 6% fat rises to 0,16 -f- 0,19%. In the Bażanowice cowshed near Cieszyn (cattle of the algau breed) Witold Licznerski stated it; he also found significant individual variations depending on the individual milkings, especially in winter.
The initial acidity drops after 1 do 2 hours from milking by what 0,01 up to 0.02o / ° due to gas volatilization (carbon dioxide).
The stock of standard lye must be protected from air. For this purpose, the bottle with the reserve is tightly closed with a rubber stopper, and for everyday use it pours small amounts.
The phenolphthalein solution is 2% in 96% alcohol.