The following diketopiperazines have been identified in roasted cocoa: Cyclo(‐Pro‐Leu‐), Cyclo(‐Val‐Phe‐), Cyclo(‐Pro‐Phe‐), Cyclo(‐Pro‐Gly‐), Cyclo(‐Ala‐Val‐), Cyclo(‐Ala‐Gly‐), Cyclo(‐Ala‐Phe‐), Cyclo(‐Phe‐Gly‐), Cyclo(‐Pro‐Asn‐), Cyclo(‐Asn‐Phe‐). The typical bitterness of cocoa is due to an interaction of the purine theobromine (2) and diketopiperazines, which are formed during the roasting of the coca beans.
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Pickenhagen et al. (1975) studied this question.
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