Relationship between pH and Titratable acid

in Cider Apple Juices

There is no direct relationship between titratable acidity and pH in apple juice, although generally the pH goes up as the acid goes down and vice-versa. The exact relationship differs from sample to sample and depends on esoteric concepts like 'buffering capacity' which will vary for a whole host of reasons. Nevertheless some general empirical relationships may be obtained, and some that I've tabulated are shown below. It must be stressed that they only refer to those fruits growing in those locations at that time. They cannot be held to be exact in other circumstances.

In general, titratable acid (TA) relates pretty well to the 'acid taste' of a juice or cider. If the TA doubles, people will tend to perceive it as twice as acidic. The pH relates more to things like microbial stability and susceptibility to mould and bacterial spoilage. In particular, the antimicrobial effectiveness of sulphur dioxide is very pH dependent (see below), and this is the reason that commercial cidermakers measure it (see Jarvis and Lea 2000.) TA is measured by titration, and pH by a pH meter or by narrow-range pH test strips*. A pH meter is tricky to set up and calibrate, and only really worthwhile if it's being used daily in a laboratory environment. The cheap pH 'dipsticks' do not have replaceable electrodes and may only have an effective life of a year or so. For most non-commercial cidermakers, pH test strips are probably a better bet. If you want to measure titratable acidity, you can get kits from home winemaking suppliers or from Vigo.

* [eg Merck indicator strips pH 2.5 - 4.5 (Merck product code 109451; VWR catalogue number 31501). A more readily available alternative may be the 'Vinoferm pH strips 2.8 - 4.6' available in the UK from the Home Brew Shop as http://www.the-home-brew-shop.co.uk/item1860.htm or elsewhere in Europe from Brouwland http://www.brouwland.com as catalogue no 013.073.2. (For distributors elsewhere in the world, check out the links on the Brouwland website)]

Data derived from authentic cider apple juices grown at Little Wittenham (Oxon) and Monnington-on-Wye (Herefs) at various times from 1972 to 2002

Titratable acid is given as % malic. Plots and equations shown are 'best-fit' to the empirical data (about 50 data points, mostly bittersweets) . Both binomial and exponential plots are given.

Exponential equation Y = 64.56*exp(-1.384*X)

where Y is titratable acid in % malic and X is pH.

Binomial plot

Binomial equation Y = 0.4528*X^2 -4.097*X + 9.3632

where Y is titratable acid in % malic and X is pH

So.......how much sulphite should I add for the pH?

The table below gives an approximate idea of how much sulphite needs to be added to a juice at a given pH before fermentation, depending on whether you intend to add a cultured yeast or allow the wild yeasts to do the job. In both cases, addition of sulphite is important to kill bacteria, moulds and adverse wild yeasts, while allowing the beneficial ones to flourish. If a cultured yeast is added, sulphite addition should be made 12- 24 hrs before the addition of the yeast, or the added yeast will be severely inhibited. The dose is given both in parts per million of sulphur dioxide (ppm or milligrams per litre) and the equivalent in Campden tablets per gallon.

It's often easier to use a stock solution of sulphur dioxide. To make a 5% stock solution, dissolve around 10 grams of sodium or potassium metabisulphite in 100 ml of water. (The metabisulphite salts contain around 50 - 60% of available SO₂ depending on how they've been stored). Then 1 ml of this per litre of juice (5 ml per gallon) corresponds to 50 ppm (parts per million) of SO₂.

Note that the correlation between titratable acid (TA) and pH is only approximate (as the graphs above make quite clear!)

pH	Approx TA (% malic)	For total yeast kill (when adding cultured yeast)		For partial yeast kill (for wild yeast fermentation)
pH	Approx TA (% malic)	SO₂(ppm)	Campden tablets per gallon	SO₂(ppm)	Campden tablets per gallon
3.0 – 3.3	1.2 – 0.8	50	1	nil	nil
3.3 – 3.5	0.8 – 0.6	100	2	50	1
3.5 – 3.8	0.6 – 0.3	150	3	100	2
> 3.8	< 0.3	add more acid!	add more acid!	150	3

More information for tecchies!

The information above is derived from long-standing work at the Long Ashton Research Station and other wine research institutes, which started in the 1950's and culminated in the late 1970's. It is based on the empirical fact that the level of molecular SO₂ required to kill adverse yeasts and bacteria but to allow beneficial ones to flourish is around 1 part per million. To get this level of molecular SO₂you actually need a lot more free SO₂ because there is a pH related equilibrium which keeps most of the SO₂ in the inactive sulphite ion form. Hence, in the table above, the amount of SO₂ you need to add depends on the pH.

Unfortunately, that's not all the story. When you add SO₂ to juice or cider, some of it becomes bound to juice components like glucose, galacturonic acid, pyruvate etc. Hence the total SO₂ you need to add must also take account of this binding. It is the total SO₂ which is given in the table above. This is not an exact science because it needs to make certain assumptions about the levels of the binding components, which will differ depending on the nature of the fruit, how many rotten apples got in etc etc! So the figures given in the Table are necessarily approximate. In the table, the column for total yeast kill is based on a target value of 1 ppm molecular SO₂ and for partial yeast kill is based on 0.5 ppm.

If you want to know more, and you have software which can read Excel spreadsheets, then download this file on sulphite binding and addition. It contains three worksheets. The first one gives a table of molecular, free and total SO₂ at different pH values (and is more comprehensive than the table above). The second is just a graphical representation of the free and total SO₂ columns (for a 'typical' apple juice). The third worksheet gives more detail on how the binding calculation is carried out. There are also some literature references given to show where the science comes from (and my publications page also contains a couple of downloads of relevant papers which should give some further background, although they are focussed more on the addition of SO₂to cider after fermentation than to juice beforehand ).

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