Since six acids have just been named, a simple question: how much of it is there? Six published sources answer, and the comparison is instructive for anyone who wants to read scientific literature without being fooled.

For citric acid, the main one: Van der Stegen and Van Duijin give, in 1987, 5.0 to 14.9 g per kilo. Balzer, in 2001, 0.5 to 1.49% on a wet basis. Steiman, in 2003, 0.7 to 1.8%. Ferreira and coauthors, in 2008, 9 to 13.5 mg per gram. Borém, in 2014, 0.5 to 1.58%.

Four different units for six sources. That is the first obstacle, and it is not trivial: these figures cannot be compared without converting them, and most of the articles that cite them do not.

Let us do it. Five grams per kilo make 0.5%. Nine milligrams per gram make 0.9%. Once everything is brought down to a percentage, the first five sources overlap neatly, somewhere between 0.5 and 1.8% of the mass. Forty years of independent measurements that agree: this is reassuring, and it was not apparent.

The sixth stands out. Evangelista and coauthors, in 2013, give 0.02 to 0.59 g per kilo, that is, a hundred times less. A gap of this order is not explained by a slightly different method: it means that the two are not measuring the same thing. Without the full article in front of us, we cannot say which one, and it is better to flag this than to average incomparable numbers.

Keep the reflex more than the figures: faced with a published range, look first at the unit, then at the material measured, and only then at the number.

Comparison drawn up from a summary document reproducing Van der Stegen and Van Duijin (1987), Balzer (2001), Steiman (2003), Ferreira et al. (2008), Evangelista et al. (2013) and Borém (2014); unit conversions made here