Relationship between pH and Titratable acid

in Cider Apple Juices

 
The pH is a logarithmic measure of the concentration of free hydrogen ions in a chemical or biological system. It's a very important concept in cell biochemistry. Titratable acid, on the other hand, is a simple measure of the (related) amount of acid 'anions' in a juice. 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.  A more detailed exploration, with additional data, is given at the end of the page.

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 the sulphite page), 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, I used to recommend pH test strips as a better bet.  But now after doing a formal comparison in 2011 I'm not so sure. Read what I found here at pH measurement comparisons. If you want to measure titratable acidity, you can get kits from home winemaking suppliers or from Vigo. Or you can 'do it yourself' from my titration page here.

* [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 see  http://www.the-home-brew-shop.co.uk/acatalog/Acid_Testing_Equipment.html 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)]  

Note that the Vinoferm strips do not read accurately in most artificial light, only by daylight. The reference dyes show a 'metachromic shift' and will not then correspond to the colour of the test portion. But if you cannot read the strips by day, I'm advised by Rose Grant that a small halogen desk lamp (though not a regular fluorescent  tube) works just as well. That's a useful tip!

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.  

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

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

Some much better data!

In fact, because pH is a logarithmic scale, you actually get a more sensible looking 'straight-line' relationship if you plot  pH vs TA on a 'semilog' plot.  This is what has been done for the data below  which was put together by Claude Jolicoeur in Québec from a number of sources.  The straight blue line is the 'best fit' to the data, but the important thing is the two red lines which enclose the '95% confidence limits'.  What this shows is that there is considerable natural variation in the relationship - for instance a juice with an acidity of 5 g/L (0.5%)  could have a pH below 3.3 or above 3.9. Plainly this is not  much use if you want to be within 0.1 / 0.2 pH units for SO₂ dosage, for instance. Conversely, pH is little use as a measure of titratable acidity - for instance, a juice with a pH of  3.6 could have an acidity as low as 2.5 g/L or as high as 10 g/L. Hence pH and titratable acidity do not measure the same thing and you cannot get one to stand in for the other by using a fixed conversion factor.

If you want to read an excellent essay on the pH / acidity relationship and for a further discussion of that data, you should download it from Claude Jolicoeur's site http://cjoliprsf.ca/Documents/Acidity-pH.pdf

Updated  31st October 2011