Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2007
Publication Date: 7/28/2007
Citation: Kozukue, N., Park, M., Choi, S., Lee, S., Levin, C.E., Friedman, M. 2007. Kinetics of Light-Induced cis-trans Isomerization of Four Piperines and their Levels in Ground Black Peppers as Determined by HPLC and LC/MS. Journal of Agricultural and Food Chemistry. 55:7131-7139. Interpretive Summary: As part of an effort designed to explore activities of biologically active pungent pepper compounds against foodborne pathogens and bacterial toxins, we participated in a collaborative study carried out in Korea on the effect of light on piperine. Piperine is a secondary metabolite present in the outer part of the fruits and in the seeds of black, white, and green peppers (Piper nigrum). Interest in piperine arises from the fact that, in addition to its sensory properties that impact the taste of food, the compound is reported to possess several non-sensory beneficial pharmacological/insecticidal/antimicrobial properties that may also benefit agriculture and human health. Reported activities against pathogenic microbes and protozoa include inhibition of Clostridium botulinum and Entamoeba histolytica. One of the objectives of the present study was to define conditions for the preparation of four light-induced piperine isomers. This objective was achieved by subjecting commercial piperine to fluorescent light and isolating the formed isomers by chromatography. In the course of this study we also developed an HPLC method to determine the distribution of the piperine isomers in four commercial ground black peppers, two from the United States and the other two from Korea. Because light may influence the nature, amounts, and biological effects of piperines present, labeling of peppers and pepper-containing foods for piperine content would undoubtedly benefit consumers.
Technical Abstract: The pungent compound piperine, a secondary metabolite present in black, white, and green pepper fruit, undergoes light-induced isomerizations. To facilitate studies in this area, an HPLC method has been developed for analysis and isolation of the following four possible piperine photo-induced isomers: piperine, isopiperine, chavicine, and isochavicine. The limits of detection estimated from calibration plots were ~15-30 ng for each isomer. Recoveries of spiked samples were 95.9 ± 9.5%, n = 3. To determine the kinetics of these isomerizations, fluorescent light, sunlight, and UV radiation at 254 nm was used to induce cis-trans geometric isomerization of each isomer as a function of light intensities and time of exposure determined with the aid of HPLC and LC-MS. HPLC was also used to determine the distribution of the isomers in four commercial ground black pepper products used as spices in culinary practice. Isomerization increased with light intensities and time of exposure and leveled off at the so-called photostationary phases. The piperine levels of the four products were quite similar, ranging (in wt. %) from 10.17 to 11.68, a 13% difference from highest to lowest level. The levels of the other three isomers were low, ranging (in wt. %) from 0.01 to 0.07 of the total for chavicine; from 0.15 to 0.23 for isopiperine; and from 0.37 to 0.42 for isochavicine. The results establish the utility of the HPLC method for simultaneous analysis of the four isomers both in pure form and in black pepper extracts. Possible mechanisms of light-induced isomerizations of piperines and other food ingredients containing conjugated double bonds and the possible dietary significance of the results are discussed.