Disclosed are extracts containing 2,5-piperazinedione,3,6-bis(phenylmethyl)-,(3S,6S)-, also known as cyclo(L-phe-L-phe) and (3S,6S)-3,6-Dibenzyl-2,5-piperazinedione, which have a pH less than 5. Also disclosed is a process for preparing the acidic extracts by treating meat in heated water to remove water-soluble proteins, replacing the water and heating again, and subsequently adding acid then filtering to obtain the liquid extract. The use of these extracts in formulating acidic beverages is also disclosed, such as for improving learning motivation.
