You probably know this ... Table two in Joseph's acid section shows roughly 75 ppm phsophoric acid in coffee, while the test is only good up to 10 ppm. If you do the correct dilutions, it might work. But there's a ton of reagents in coffee, including organo-phosphates, which may stop it from working.

You may be better off trying to sweet talk a Chem lab into running GC/MS on the sample. The biggest sex appeal is in anything potentially publishable

The health police has currently declared war on phosphoric acid, so you may be better off playing down this aspect of your namesake bean

In his SCAA acids course, Joseph Rivera talks about a project the CQI did where they identified high phosphoric acid as being the main difference between high quality SL28 and low quality RU11 plantings. Reference 2 in the paper you cite: Rivera, J. Organic Acid Analysis of Kenya SL 28 and other Cultivars. SCI Technical Papers, 1997, 5-9 is probably the source.

The paper makes for an interesting puzzle since Joseph's story jibes with the food uses of phosphoric acid, while the paper's does not. Phosphoric acid is a soft drink additive, since it lends the acidic "sparkle" without the sourness of organic acids; so phosphoric acid is the one which doesn't need to be sweetened; malic, tartaric, acetic and citric do.

Here's a suggestion: The paper says phosphoric acid rises as the coffee gets less acidic, gets older, or gets roasted darker -- so, in effect, they've discovered a staleness marker. In other words, the level of phosphoric acid isn't rising in these coffees, all the other solubles are dropping as their molecules either oxidize into insoluble rust or polymerize into brown goo. Phosphates just happen to be the most shelf stable soluble in coffee, so as it stales, the proportion of phosphate solubles rises.

But it could still be that RU11 has a problem that shows up by its inability to fix soil phosphates.

In other words, I think both the papers are correct in the chemistry. Joseph's work certainly slipped into the post hoc fallacy, confusing the symptom or correlate of higher phosphoric acid levels in SL28 for the cause of its better taste. The paper you cite also flirts with post hoc when it implies that differing phosphoric acid levels are a basic characteristic of the different coffees, rather than their terroirs (e.g. fertilizer use, perhaps lower in forest coffees, higher in plantation coffees) and processing. No fault lies in either author -- mistaking correlates for causes is a necessary occupational hazard for scientists, and the main reason results have to be replicated using other methods.

Thanks for the reference.