The hydrolysis and safety assessment of food flavouring esters

To facilitate the safety assessment process, a predictive model of flavouring ester hydrolysis has been constructed through an evaluation of the hydrolysis of 44 esters of monoterpene alcohols (citronellol, geraniol, nerol, linalool, and α-terpineol), cinnamyl alcohol, cinnamic acid and furfuryl alcohol. This was achieved by investigating the rates of hydrolysis in artificial gastrointestinal fluids, rat and human tissue homogenates and following single dose oral and intraperitoneal administration to rats. Substrate specificity was found to be similar in each system investigated and was due to defined structural criteria (which are numbered in order of decreasing importance): 1. Steric hindrance of nucleophilic attack by esterases on the ester bond, by hydrocarbon groups present on carbon atoms directly adjacent to the ester bond; 2. The presence of bulky and rigid groups; 3. Substrate specificity as a result of the linear length of the alcohol and carboxylic acid carbon chains.The serine esterases of rat liver which hydrolysed selected flavouring esters were found to consist of 2 isoenzymes: one has low affinity but high stereoselectivity for (S)-linalyl esters; the other has high affinity but is not stereoselective for linalyl esters. The low affinity but not the high affinity isoenzymes was found to be highly sensitive to inhibition by phenylmethylsulfonyl fluoride.In vivo studies with rats demonstrated the rapid sequestration of lipophilic esters into adipose tissue and confirmed the glucuronic acid conjugation of citronellol, linalool and α-terpineol following the hydrolysis of their esters.Overall these studies add increased confidence to the metabolic predictions necessary during the safety assessment of food flavouring esters.