The efficacy of grapefruit seed extracts as postharvest agents

Organic bananas for export need to be protected against crown rot, for which purpose formulations based on grapefruit seed extracts (GSE) are commonly used. The production process and chemical composition of these GSEs are not precisely known, but various formulations have been declared to be compatible with organic agriculture standards as "plant preparations" by organic certifying bodies. Mid-2012, widespread contamination of bananas with quaternary ammonium compounds (QACs), notably benzalkonium chloride (BAC) and didecyl dimethyl ammonium chloride (DDAC), was detected. Some GSE-based postharvest agents were found to be contaminated with QACs. Several GSE based postharvest agents, a fresh grapefruit seed extract and diluted solutions of BAC were mixed with an isolate of fungi causing crown rot in bananas and grown on a growing medium. A strong correlation was found between the presence of QACs and efficacy in suppressing the fungi. One agent free of QAC showed strong efficacy, but needs to be tested further to elucidate its mechanism for action and rule out any other synthetic chemical antimicrobials. The means of production and chemical composition of GSEs are not clear, nor is their mechanism of antimicrobial action. No credible chemical pathway for natural formation of these QACs during extraction from grapefruit seeds has been proposed. Hence, QACs found in GSEs are probably chemical contaminants. Certifying bodies in organic agriculture need to be more critical when evaluating the compatibility of 'natural products' for use in organic agriculture. To approve organic plant protection agents, certifying bodies need to require information about the exact nature, origin, production process and chemical composition, as well as sound scientific evidence of efficacy to avoid opportunities for fraud.