The acidity of milk can be expressed in various ways. There are three main scales:
- Soxhlet Henkel degree: Number of ml of 0.25 N $\ce{NaOH}$ required to neutralize 100 ml of milk
- Thörner degree: Number of ml of 0.1 N $\ce{NaOH}$ required to neutralize 100 ml of milk
- Dornic degree: Number of ml of 0.11 N $\ce{NaOH}$ required to neutralize 100 ml
of milk
One ml of 0.11 N $\ce{NaOH}$ = 0.01 g lactic acid i.e 1 Dornic degree corresponds to 0.01g of lactic acid per 100 ml of milk or 0.01%. Therefore, number of Dornic degrees divide by 100 gives the content of lactic acid in percent. The measurement is done through titration with phenolphthalein as indicator. During analysis, the milk is diluted to several degrees which can be used to make comparison of various degrees of acidity. Table of comparison is as follows:

These scales are very important during cheese making. Cheese makers wants to reduce the acidity of milk from 0.3% (30° Dornic) down to 0.16% (16° Dornic). The calculation below determines the total amount of acid contained in 100 litres of milk:
$\mathrm{0.3-0.16 = 0.14~kg~of~acid}$
$\mathrm{(30 ^\circ D - 16 ^\circ D) \times 100~kg = 1400^\circ D = 140~g~of~acid}$
Neutralization will be done with the fact that 40 g of $\ce{NaOH}$ will nautralize 90 g of lactic acid so the quantity of $\ce{NaOH}$ needed to neutralize the above amount of acid would be:
$\mathrm{\frac{40 \times 140}{90} = 62.2~g~of~NaOH}$
Reference
- Robinson: Modern Dairy Technology: Volume 1 Advances in Milk Processing by R. Robinson, Springer Science & Business Media, 2012
- The Oxford Companion to Cheese by Catherine W. Donnelly, Oxford University Press, 2016
- The Technology of Making Cheese from Camel Milk (Camelus Dromedarius) by J.P. Ramet, Food and Agriculture Organization of the United Nations
Food & Agriculture Org., 2001