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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.

enter image description here

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?).
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  • $\begingroup$ I hope my compound questions are acceptable as they really are closely related. $\endgroup$ May 27 at 12:52
  • $\begingroup$ From ncbi.nlm.nih.gov/pmc/articles/PMC3568963: "Dornic acidity is a way of expressing the acidity degree of milk by measurement of the titrable acidity." $\endgroup$
    – Buck Thorn
    May 27 at 14:15
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    $\begingroup$ 1 Dornic degree (1°D) is equal to 0.1g of lactic acid per liter. chal. Interesting is Dornic acidity seems to have FR and DE but not EN Wikipedia pages. $\endgroup$
    – Poutnik
    May 27 at 14:36
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    $\begingroup$ Different regions have different measures of acidity: dairyscience.info/newcalculators/acidity $\endgroup$ May 27 at 18:36
  • $\begingroup$ I would think a regular pH meter would do poorly with milk and even worse with cheese production because of the liquid fats would interfere. $\endgroup$ May 27 at 22:32

2 Answers 2

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  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.

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  • $\begingroup$ In a complement to AChem's informations, the titration should be carried out until the measured $p$H value is equal to $8.30$, even if there is no visible $p$H leap at this $p$H value. . $\endgroup$
    – Maurice
    May 27 at 15:57
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    $\begingroup$ That is not answer to part 3. It is however a reasonable response to the main question. $\endgroup$ May 27 at 22:31
  • $\begingroup$ @user2617804 There is no general relation between pH and acid content. There would have been such a relation if milk had been nothing but a lactic acid solution. For milk, one can construct an empirical chart pH versus Dornic acidity. $\endgroup$
    – Poutnik
    May 28 at 4:33
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    $\begingroup$ " It turns out that it is an ancient method from >100 years. The unit is named after a person" I tried searching but couldn't find exactly when the scale was first introduced nor who invented it. Is it really a person who invented it? Who is Dornic? $\endgroup$ May 28 at 9:17
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    $\begingroup$ @NilayGhosh, Yes, it is indeed a French person. See German Wikipedia de.wikipedia.org/wiki/Grad_Dornic and machine translate it. $\endgroup$
    – AChem
    May 28 at 9:53
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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:

enter image description here

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
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  • $\begingroup$ How would titration be effected once the milk solidified in to curds? The document from which I got the graph suggests a measurement of acidity over 24 hours after adding rennet to the milk, by which time the cheese would already be in solid form and have drained. It implies a measurement should be possible during that stage too. But then is it possible to do titration with a solid? $\endgroup$ May 28 at 15:43
  • $\begingroup$ I am sure, they would do filtering. Impossible to titrate a curd directly. $\endgroup$
    – AChem
    May 28 at 16:18
  • $\begingroup$ I was given an answer to the measurements of acidity post-coagulation of curds, and the answer to that is the measurement is made from the remaining whey. I was under the impression one could not assume the whey and the cheese would share the same acidity but apparently that is the practice. Source: private discussion with a French microbiologist who studies fungal and bacterial growths on cheese. $\endgroup$ May 29 at 16:30

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