A little bit later than promised, but here is a bit about our acidity meter together with a little picture of it sitting on the shelf in our dairy. To the right of it is a thermometer that shows the time, ambient temperature and humidity. We always make a note of this when at the start of cheese making on our make sheet because it adds to the story of the day’s make. To the left of the acidity meter is a small dropper bottle of phenolphthalein. Behind, on the wall is a signed copy of our Slow Vat Procedure, our Sharps and Metals check sheet and also our Dairy Cleaning Schedule.
We take an acidity reading at the start of the make, before we put the starter culture in. I might take one after we have cut the curd (it will always drop on cutting because of a rapid release of whey causes a dilution effect and a temporary fall in acidity) but really start taking readings at the beginning of and throughout the pitching stage to know when to let the whey off and then during the cutting, blocking and shovelling stage to know when to salt.
So how does the acidity meter work? What we are testing is the increase in acidity to give us an indication of the increase in bacterial activity. I add three tiny drops of phenol into a measuring cup containing 10ml of whey. At pitching stage I squeeze whey out of the curd as this whey will be more representative of bacterial activity than say the whey the curd is sitting in. I use medicine cups, the sort that are given to patients in hospital containing prescription tablets. They are graduated from 5ml to 30ml in little notches up the side. It is easy enough to measure off 10ml and avoids having to use (and clean!) glass pipettes. The only glass in my dairy is the burette in the acidity meter.
So, you have the three drops of phenol in 10 ml of whey, then, ensuring that your acidity meter is completely full and measuring zer0, gently release enough of the sodium hydroxide solution, swirling the cup around to enable an even distribution, until the whey turns pink. Stop and measure off at the bottom of the meniscus and this is the percentage of lactic acid, (although not all acid titrated in milk is lactic,) but you know what I mean.
I always say that only one person should do TA tests throughout a make, as one person’s idea of pink can differ to another’s. But when I do it, I take it to pale pink, not crimson. If there is any doubt and you have a ‘rogue’ reading, then yes, have two of you do it and compare.
What Can Go Wrong Testing Titratable Acidity (TA)?
- Forgetting to add the drops of phenol will not give you any reading as the solution will not turn pink. Don’t titter – we’ve all done it!
- Remember to always squeeze your acidity meter back to zero immediately after testing. Otherwise you will have double the reading. This happened to me one day when I had to nip out for 10 minutes and I asked Lydia to do a TA whilst I was gone. I got back and she was a little panicky because the acidity had gone wild. What had really happened is that I didn’t return the burette back to zero so we had a double reading!
- Have you released the control screw that drips Sodium Hydroxide (NaOH) into the whey? The reason I ask is that if the little screw like thing is still screwed tight, it culd be dripping small drops of NaOH on the table, so you are not starting from a zero base.
- Phenol and Sodium Hydroxide can ‘go off’ – they do have a best before date. This is usually 3-5 years. As a point of interest, I asked whether they should be stored in the dark/cold but was told by a manufacturer that they keep stocks at ambient on shelves in an open warehouse.
- Have dirty measuring equipment may give false readings. We have to make sure that the little measuring cups we use (and re-use) are squeaky clean. It also helps if they are dry.
- Rogue readings – they happen. It could be because an extra drop of phenol fell in when you were maybe distracted, or that you have slightly more or slightly less whey than 10mls. It happens, do another reading and if it still happens, ask someone else to do one to compare.
- Ambient temperature can sometimes speed up or slow down acidity development for us, for example, when it is very cold or very hot in our little dairy. In the summer, if it is actually hot in the more usually cold and grey North Yorkshire, we know to take the whey off our big vat at between 1.6 – 1.7 because the ambient temp can increase the rate of acidity quite quickly during the time it takes to take the whey off. In the winter, I would leave this to about 2.0. Our little vat is different altogether and I take whey off later. It is like knowing your oven when baking a cake.
- Have you cut your cheese a little smaller or a little larger than usual? A smaller cut will usually result in dry slow acid cheese and a larger cut a more moist, acid, quick maturing cheese.
- Have you rinsed all detergent/degreaser and santiser/steriliser out of your vat or cheese making vessel? Any residue can slow down starter activity.
- Have you scalded your curd higher than normal? This varies from starter to starter, but some starter lactic acid development is reduced as you take the temp beyond around 37 degrees C. After 44 degrees C, some starter is destroyed.
- Phage can slow down the rate of acidity development and this can often be confused with a slow vat – see below. If you think you may have phage, rotate your starter culture and clean your vat down with hypo and rinse thoroughly.
- A slow vat is one where the desired acidity levels are not achieved in the usual timescales, most likely because there is some other bacteria in the milk preventing the starter culture from working properly, whose purpose is to convert lactose into lactic acid. It could be antibiotics or something far more serious. All cheese makers have a slow vat procedure, it is usually a CCP (critical control point) in our HACCP plans – after a set time, if the curd has not developed to the desired level, make it, i.e. get it out of the vat, and quarantine it and have the cheese tested by the lab. But keep it separate.
If any one would like me to add/change anything, give me a shout and will do. Hope this has been useful.