Changes in chlorophylls, chlorophyll degradation products and lutein in pistachio kernels (Pistacia vera L.) during roasting

Abstract

Bright green pistachio kernels are desirable in the food industry and degradation of color is often associated with a loss in quality. Chlorophylls (a and b) and lutein are the major pigments in raw pistachios. The heat treatment of chlorophyll-containing foods results in color alteration, due to the conversion of chlorophylls (bright green color) to pheophytins and pyropheophytins (yellow-brown olive color). In this study, changes in the concentration of chlorophylls, chlorophyll degradation products and lutein in pistachios were monitored during oven roasting. An efficient, high performance liquid chromatography method with photodiode-array detection (HPLC-PDA) was developed to separate and quantitate these pigments in pistachios. Extractable chlorophylls a and b were higher in pistachio kernels roasted for 5 and 10min than in the raw kernels. The most drastic losses were observed with pheophytins a and b, which both decreased by approximately 85% after 60min of roasting. Pyropheophytins a and b increased significantly during roasting and were 10–12 fold higher in the pistachios roasted for 60min than in the raw pistachios. Extractable lutein concentration increased by 37% after 5min, but did not change significantly with longer roasting times. Initial increases in chlorophylls a and b and lutein were likely due to enhanced extractability with roasting. Color measurements of the pistachios corresponded with changes in concentration of chlorophyll pigments observed at the later roasting times. Interestingly, pyrochlorophylls a and b were detected in pistachios as confirmed by mass spectrometry. There are no prior reports of these compounds being detected in pistachios. We hypothesize that the conditions of pistachio roasting favor the formation of pyrochlorophylls. Color changes observed after oven roasting significantly affect quality and value perceptions of pistachios.