titratable acidity

Splitting Curds and Curd Drainage

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The mystery of the splits in the curd continues. After a period in late August and September with minimal or no splits at ladling, it came back again in October and November. As it happens this also coincided with a period in which I joined the Baron Bigod team who were in the middle of making Christmas cheese, battling staff illness and had a couple of people off on compassionate leave. Earlier in the year when we mentioned the splits to Jonny he had sounded surprised having not heard from his team about any issue but it soon became clear that although the Baron Bigod curd splits were smaller than those in the St Jude curd, they still existed.

One theory, especially because the splits tend to appear and be exacerbated at the end of the set was that the milk protein might have been made up of the wrong sorts. Julie sent a sample of milk to a lab in Cornwall for them to analyse its composition and they returned a result that suggested the milk while high in protein overall was perhaps a little low in casein which is responsible for the set and that therefore the total protein was made up of too many other types of protein like albumins which are whey proteins. Having had that result returned, she sent further samples off that had been frozen earlier in the year at a time when the set was particularly bad – not only split but also quite fragile. Unfortunately the results this time were not conclusive. So while there may be mileage in this avenue of research we can’t be sure yet.

Another theory had been that the starters were not active enough and to be honest here we’re a little hazy on the details. Thierry had wondered if the cheeses were prone to phage but Julie didn’t feel the make was getting that much longer. A week ago we also had a visit from Martial Reynard, a technologist from Coquard who explained to us that the titratable acidity readings we were recording would show us the lactic acid development and by comparison to the pH we can gather information, not only on how active the starters are but it can perhaps help us put together a picture of how the chemical composition of the milk might be inhibiting our starters. Julie measures the TA of the fresh milk, at renneting and at ladling and hopes to find a reading of around 60 Dornic indicating that lactic acid is developing nicely. 60 degrees Dornic would indicate 6g lactic acid per litre. Recently we are getting readings closer to 40 and in fact the other week on the day of Martial’s visit our reading was 37. However, I began measuring TA on the Baron Bigod make this week and using the faster of their starters, there seems to be no starter inhibition on that make. Lactic acid developed to 90 – 109 Dornic by the end of the make, meaning they had between 9 & just under 11 grammes of lactic acid per litre. 6 to 7 grammes per litre would indicate that the milk has reached the acidity at which it would naturally coagulate or its isometric point. So it appears that in the St Jude curd for some reason we aren’t reaching the isometric point at ladling and the Baron Bigod whose set is more rennet based anyway is exceeding it by the end of the day and the final turn. Both cheeses do use the acid development to aid drainage. Brie style cheeses drain as a mixture of acid development shrinking the curd and squeezing out moisture as well as the rennet also carrying out that action. Lactic cheeses of course rely on the acid development more than that of the rennet as the rennet is basically there to support the acid rather than act as an equal draining agent.

Strangely although the curd is splitting, and the isometric point may not be being reached when we want it, the cheeses seem to be draining quite well by the first turn. This evening, when Julie measured the TA of the whey coming off the draining table at the first turn of the cheeses (a short wait after the end of ladling) it showed that the lactic acid was developing still, having finally made it to 60 Dornic or 6g lactic acid per litre and therefore the isometric point. So as this is achieved in the end, it should explain why the cheeses are draining despite the splits. However we also noticed this evening that the more split vats tended to have the lower TA at ladling despite having hit the right pH. This is something to monitor as so far this year just when we think we’ve identified a pattern eg different rennet amount or different temperature, the curd bucks the trend and confuses us all over again.

However the splits but also drainage theme seems to be echoed in the Baron Bigod makes this week too. The Bigods have had split curd before cutting but drained nicely. Last week however there were splits and the cheese had a softer consistency and stuck to the drainage mats (it makes the morning clean down much longer and more difficult). Last week’s St Jude also held in more moisture and Julie sent an instruction to Jacob at Neal’s Yard Dairy to dry a couple of the batches on arrival to help the rind set.

To be honest, we still have not got to the bottom of this yet but we have learned a lot about drainage and milk composition in the process so far. We’re now awaiting a technical paper from Martial to help us understand what information the TA might unlock.

Acidity and Cheesey Chemistry

Before any of my long standing friends wonder who I am and what I’ve done with Anne, I should point out that the chemical knowledge in this post is courtesy of the collective brains of Dr Jemima Cordle, Dr Katie Jewell and M. Ivan Larcher who have very kindly either taught me the info or have answered my questions on the subject.  I hope I have interpreted it correctly.  Any chemists out there, please pick me up on any mistakes so I can correct them.

The difference between titration and pH is that you use titration to calculate the concentration of Hydrogen ions by means of measuring the amount of alkali ions you have added to cause a certain colour change.
A pH measurement is the log (base10) of the concentration of Hydrogen ions.
 
pH meter at Neal’s Yard Creamery recording acidity development of Finn curd
Titratable Acidity kit set up at Old Hall Farm
 
Hydrogen Ions, Hydroxyl Ions and Acidity
In a basic solution there will be Hydrogen ions (H+) and Hydroxyl ions (OH-).  The H+ ions determine the acidity of a solution and the OH- ones are in predominance in a basic solution (when you stick both ions together you get H2O which is water)
Although pure water is present largely in its molecular form rather than atomic (which means that there are bonds linking the H atoms to the O atom to form the water molecule) a portion of it is ionised and present as H+ and OH-.
Because these two ions are present in equal quantities in a pure solution of water it remains neutral with a pH of 7. (The actual pH number is derived from the concentration of H+ present). 
When substances are dissolved in water they often disturb the H+ OH- balance. 
When the concentration of H+ is higher than the concentration of OH- the solution is said to be acidic and the pH is less than 7. 
When the concentration of H+ is less than the concentration of OH- the solution is said to be basic and the pH is more than 7. 
Acids and bases can vary hugely in strength. Hydrochloric acid for instance has a lot of H+ ions present and has a pH of 1. Citric acid on the other hand has less H+ ions present in it and has a pH of around 2.5. 
How the pH meter measures
A pH meter is able to directly measure the concentration of H+ ions present in a solution into which the probe is stuck. It then converts this concentration to the pH reading. 
How the TA measures
Titratable acidity measurement on the other hand is an indirect technique to measure the H+ concentration. There are certain ‘indicator’ solutions available that change colour when the solution they are in gets to a certain pH.
In cheesemaking we use phenolphthalein which turns pink when the solution gets to pH8.3. So we add a basic solution (NaOH or sodium hydroxide) to the milk or whey until enough OH- have been added to cause the colour change.
The amount that has been needed will tell you how many H+ ions were present in the original solution.
To milk you need to add very little NaOH because milk contains a very small amount of H+ but to whey from cheese the morning after making, you need to add a lot more NaOH because the lactic acid bacteria have produced a lot of acid and therefore a lot of H+ ions are present.
Buffering
Having said this there is a complication which means that the pH/titratable acidity measurements are not 100% correlated. The complication is an interesting acid/base phenomenon known as buffering.
Buffering happens when an acid or base is added to a solution, but the pH does not change. This happens in cheesemaking because there is a lot of protein present.
The protein is able to capture H+ ions as they are formed and therefore prevents them from ‘being seen’ in the solution. They do not add to the H+ concentration that determines the acidity.
This can also happen with OH- ions – the protein can capture them and stop them from causing a pH change.  
The latter is relevant during titratable acidity measurements because sometimes even though OH- ions are being added they are not being used to neutralise the H+ present and raise the pH to 8.3.  Instead the protein is capturing them.  This distorts the relationship between pH and TA in a non-uniform way.
It will depend on the specific composition of the milk and the stage of acidity that has been reached. 
Comparisons of pH and TA therefore on a daily basis can help you extrapolate some physical properties of the milk you are working with – see the chart that follows.