uite a few
attempts have been made to determine the composition and structure of
this compound and to assign it a formula. Among them may be noted those
of Allen,[123] who gives it the empirical formula C_14_H_16_O_7;
Hlasiwetz,[124] who represents it as C_15_H_18_O_8; Richter, as
C_30_H_18_O_16; Griebel,[125] as C_18_H_24_O_10, and Cazeneuve
and Haddon,[126] as C_21_H_28_O_14. It is variously supposed to
exist in coffee as the potassium, calcium, or magnesium salt. In regard
to the physical appearance of the isolated substance there is also some
doubt, Thorpe[127] describing it as an amorphous powder, and Howard[128]
as a brownish, syrup-like mass, having a slight acid and astringent
taste.

The chemical reactions of "caffetannic acid" are generally agreed upon.
A dark green coloration is given with ferric chloride; and upon boiling
it with alkalies or dilute acids, caffeic acid and glucose are formed.
Fusion with alkali produces protocatechuic acid.

K. Gorter[129] has made an extensive and accurate investigation into the
matter, and in reporting upon the same has made some very pertinent
observations. His claim is that the name "caffetannic acid" is a
misnomer and should be abandoned. The so-called "caffetannic acid" is
really a mixture which has among its constituents chlorogenic acid
(C_32_H_38_O_19), which is not a tannic acid, and coffalic acid.
Tatlock and Thompson[130] have expressed the opinion that roasted coffee
contains no tannin, and that the lead precipitate contains mostly
coloring matter. They found only 4.5 percent of tannin (precipitable by
gelatin or alkaloids) in raw coffee.

Hanausek[131] demonstrated the presence of oxalic acid in unripe beans,
and citric acid has been isolated from Liberian coffee. It also has been
claimed that viridic acid, C_14_H_20_O_11, is present in coffee. In
addition to these, the fat of coffee contains a certain percentage of
free fatty acids.

It is thus apparent that even in green coffee there is no definite
compound "caffetannic acid," and there is even less likelihood of its
being present in roasted coffee. The conditions, high heat and
oxidation, to which coffee is subjected in roasting would suffice to
decompose this hypothetical acid if it were present.

In the method of analysis for caffetannic acid (No. 24) given at the end
of this chapter, there are many chances of error, although this
procedure is the best yet devised. Lead acetate forms three different