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
Graphical abstract
Introduction
The structurally simple spiroketal conophthorin (7-methyl-1,6-dioxaspiro[4.5]decane, 1 in Fig. 1) has recently been reported as an active component in a blend of host plant volatiles that attract both male and female navel orangeworm (Amyelois transitella) moths (Beck et al., 2012a). A. transitella is a major insect pest that inflicts significant economic loss to California almonds, pistachios, and walnuts (Campbell et al., 2003). Moreover, A. transitella larvae are purported to vector toxigenic aspergilli to almonds (Palumbo et al., 2008). Yet, conophthorin and the isomeric chalcogran (2-ethyl-1,6-dioxaspiro[4.4]nonane, 2 in Fig. 1) have long histories as semiochemicals of scolytid beetles with insect and plant origins (Francke and Kitching, 2001). Compared to the history of these spiroketals and scolytid beetles, reports of conophthorin (1) and chalcogran (2) from almonds are relatively new. As an example, recent investigations reported compound 1 from almonds at hull split (Beck et al., 2012a) and both 1 and 2 from mechanically damaged almonds (Beck et al., 2008). More recent was a report of 1 and 2 from various fungal spores on fatty acids common to almond and pistachio (Beck et al., 2012b).
What was interesting regarding the recent detection of 1 and 2 from almonds (all components present – hull, shell, and kernel) was their lack of detection in a number of investigations that reported on the volatile emissions of almonds or pistachios under varying conditions. For instance, spiroketal 1 or 2 was not detected from the following: the vacuum steam volatiles of almond hulls (Buttery et al., 1980); in situ intact and undamaged almonds (Beck et al., 2009); ambient almond orchard volatiles (Beck et al., 2011a); intact ex situ Pistacia spp. (Roitman et al., 2011); and, almond kernels naturally contaminated with orchard fungi (Beck et al., 2011b).
The present investigation was initiated to help delineate the true origin of conophthorin from almonds, and to determine if pistachios were a possible source. This is important for a number of reasons: the inconsistent detection of spiroketals in the aforementioned studies; the rich history of spiroketals and scolytid beetles; the recent assessment that conophthorin (1) is a semiochemical of A. transitella; and, the recent study that demonstrated spores produce both spiroketals (Beck et al., 2012b). Despite the mounting evidence, the question remains – is conophthorin produced by fungi, the host plant, or both? The fungal spore study demonstrated that spores were capable of producing both conophthorin (1) and chalcogran (2), but did not decisively eliminate the plant as an active participant – not just a carbon source for the spores.
Anecdotal evidence from previous volatile emission studies and preliminary exploratory experiments suggested we turn our attention to the hulls of almonds and pistachios as a possible source of compounds 1 and/or 2. Thus, the objectives of this study were to: (1) monitor the volatiles emitted from ground almond and pistachio components, primarily the hulls, to determine a condition for consistent spiroketal production; (2) determine the corresponding fatty acid profiles of the hulls at progressive stages of hull development; and, (3) determine the water content and water activity of the hulls at the varying stages of hull development.
Section snippets
Results and discussion
The objectives of determining the relative fatty acid composition, water content, and water activity of the hulls were successfully met. Furthermore, both spiroketals were observed from ground almond hulls and shells at varying times and amounts (Table 1), yet no spiroketals were observed from the ground almond kernel treatments or any of the pistachio material. The objective of determining specific conditions for consistent spiroketal production was more elusive; however, based on our results
Almond and pistachio collections
In the 2011 growing season, 10 undamaged Nonpareil almonds from three different trees were removed and placed in a 1-quart wide-mouth Mason jar (Ace Hardware, El Cerrito, CA) with a modified lid containing a 1 cm hole and lined with Teflon. Jars were shipped overnight to the laboratory. For pistachios, ca. 40 undamaged Kerman pistachios were removed from three different trees and placed in similar jars. Both almonds and pistachios were collected from commercial orchards located in the southern
Acknowledgements
The authors thank Nausheena Baig, Divya Donthi (USDA-ARS), and Johnny Magana (Paramount Farming) for their valuable contributions. Research was conducted under USDA-ARS CRIS Project 5325-42000-037-00D, TFCA 5325-42000-037-05 with the Almond Board of California, TFCA 5325-42000-037-07 with the California Pistachio Research Board, and portions of RCA 5325-42000-037-13 with the California Department of Food and Agriculture.
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