1 Introduction

Cinnamic acids are a series of trans phenyl-3-propenoic acids, differing in their ring substitution. These compounds are widely distributed as conjugates in plant material including many foods and beverages. The cinnamates are rarely found as free acids in unprocessed plant material, but they may be analysed after deconjugation and the contents be reported as free acids rather than as the original conjugates. The commonest is caffeic acid (3,4-dihydroxycinnamic acid): ferulic (3-methoxy-4-hydroxy), sinapic (3,5-dimethoxy-4-hydroxy) and p-coumaric (4-hydroxy) also are widespread and a few others occur much less frequently, for example 2-hydroxycinnamic and 3,4-dimethoxycinnamic which are unusual also in that they occur naturally as free acids in commercial green coffee beans.1

The best known conjugate is 5-caffeoylquinic acid (IUPAC numbering2), commonly referred to as chlorogenic acid (CGA), a term which should now be used only to refer to the family of related quinic acid conjugates. There have been several substantial reviews covering the chemistry, complex trivial nomenclature and quantification of CGA,3 ^,4 and their significance for the consumer5 as well as the dietary burden of cinnamates in general. ^{6 8} This review focuses on analytical and physical aspects.

2 The Cinnamate Conjugates

The cinnamate conjugates may be divided in to two major categories.