What is Dornic acidity and how to measure it?

Question

Context: I work with dairy product (cheese making) and in order to improve the quality of my process, I have documentation (in French) which refers to Dornic acidity over time.

This graph represents the optimal acidity range over time for the production of cheese and reduce the risk of certain bacterial infections. The Y axis is the Dornic acidity and the X axis is the time in hours from coagulation of milk with rennet.

Problem:

1. What exactly is Dornic acidity?
2. How do I measure it? (At low cost preferably)
3. I am familiar with pH and I am already equipped with pH meters. Is there a pH-to-Dornic correspondence? (And if not then why?).

Answer 1

1. Interesting term...never heard of it nor it is common in English chemical analysis literature. It turns out that it is an ancient method from >100 years. The unit is named after a person. The original definition from a 1900 document is ""D" indicates acidity in degrees Dornic, the number of cubic centimeters of a NaOH solution containing 4.445 g NaOH per liter required to neutralize the acidity of 100 cc of milk." Journal of Dairy Science, 1925, Volume 8, pg 172.

2. It is a simple acid-base titration, phenolphthalein is used as an indicator.

3. "To be considered fresh, a milk must have an acidity less than or equal to 18 °D. Dornic acidity is the result of the natural acidity of milk (linked to its richness in proteins and minerals). It is an indicator of the degree of milk preservation. Naturally the lactose contained in the milk gradually degrades to lactic acid by the bacteria. The less milk is fresh, the more it contains lactic acid." from Adv. Biores., Vol 9 (2) March 2018: 166-177.

Answer 2

The acidity of milk can be expressed in various ways. There are three main scales:

1. Soxhlet Henkel degree: Number of ml of 0.25 N $\ce{NaOH}$ required to neutralize 100 ml of milk
2. Thörner degree: Number of ml of 0.1 N $\ce{NaOH}$ required to neutralize 100 ml of milk
3. 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

1. Robinson: Modern Dairy Technology: Volume 1 Advances in Milk Processing by R. Robinson, Springer Science & Business Media, 2012
2. The Oxford Companion to Cheese by Catherine W. Donnelly, Oxford University Press, 2016
3. 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