Ex situ volatile survey of ground almond and pistachio hulls for emission of spiroketals: Analysis of hull fatty acid composition, water content, and water activity

Authors: Mahoney, Noreen; Gee, Wai; HIGBEE, BRADLEY - Paramount Farming Company, Inc; Beck, John

Submitted to: Phytochemistry Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2013
Publication Date: 3/29/2013
Publication URL: http://handle.nal.usda.gov/10113/58706
Citation: Mahoney, N.E., Gee, W.S., Higbee, B.S., Beck, J.J. 2013. Ex situ volatile survey of ground almond and pistachio hulls for emission of spiroketals: Analysis of hull fatty acid composition, water content, and water activity. Phytochemistry Letters. 7:225-230.

Interpretive Summary: The volatile compound conophthorin has recently been implicated as an important form of chemical communication (semiochemical) of the navel orangeworm moth (Amyelois transitella), which is a major insect pest to California tree nuts. Conophthorin is considered to be a spiroketal, which is a compound with two rings connected to the same carbon via oxygens. Additionally, new evidence demonstrates conophthorin is produced by fungal spores when placed in the presence of linoleic acid – a common fatty acid component of almond and pistachio kernels. Though numerous investigations have analyzed the volatile emissions of almonds and pistachios under varying conditions there are few reports of conophthorin as a volatile component. Previous studies by our laboratories have suggested almond hulls may be a source of conophthorin production. Accordingly, the volatile emissions of ex situ almond and pistachio (ones that have been removed from the tree and studies in a laboratory setting) ground hulls were surveyed at several developmental stages during the 2011 growing season. Each ground sample was analyzed at various intervals for one week to determine if conophthorin was produced. The almond and pistachio samples were presumed to have a natural fungal bouquet present. Additionally, the fatty acid content, water activity, and water content were analyzed for each sample. Conophthorin, along with the structurally similar and isomeric compound chalcogran, were detected at intermittent times from almond hulls, but not detected from any of the pistachio samples. The relative percentage of four fatty acid components, palmitic, oleic, linoleic, and linolenic, were monitored. The fatty acid composition of almond hulls varied greatly throughout the growing season, whereas the composition of pistachio hulls stayed relatively constant. Both the water activity and content stayed constant in the early stages of almond growth and dropped significantly in the later stages of hull split. Spiroketal as well as green leaf volatiles and associated volatile emissions are discussed.

Technical Abstract: The spiroketal conophthorin has recently been implicated as an important semiochemical of the navel orangeworm moth (Amyelois transitella), which is a major insect pest to California tree nuts. Additionally, new evidence demonstrates conophthorin is produced by fungal spores when placed in the presence of linoleic acid – a common fatty acid component of almond and pistachio kernels. Though numerous investigations have analyzed the volatile emissions of almonds and pistachios under varying conditions there are few reports of conophthorin as a volatile component. Previous studies by our laboratories have suggested almond hulls may be a source of conophthorin production. Accordingly, the volatile emissions of ex situ almond and pistachio ground hulls were surveyed at several developmental stages during the 2011 growing season. Each ground sample was analyzed at various intervals for one week to determine if conophthorin was produced. The almond and pistachio samples were presumed to have a natural fungal bouquet present. Additionally, the fatty acid content, water activity, and water content were analyzed for each sample. Conophthorin, along with the structurally similar and isomeric compound chalcogran, were detected at intermittent times from almond hulls, but not detected from any of the pistachio samples. The relative percentage of four fatty acid components, palmitic, oleic, linoleic, and linolenic, were monitored. The fatty acid composition of almond hulls varied greatly throughout the growing season, whereas the composition of pistachio hulls stayed relatively constant. Both the water activity and content stayed constant in the early stages of almond growth and dropped significantly in the later stages of hull split. Spiroketal as well as green leaf volatiles and associated volatile emissions are discussed.