Submitted to: Hawaii Macadamia Nut Association
Publication Type: Other
Publication Acceptance Date: June 23, 2006
Publication Date: August 29, 2006
Citation: Nishijima, K.A., Wall, M.M., Siderhurst, M.S. 2006. Update of gray kernel disease of macadamia. Proc. Hawaii Macadamia Nut Assoc. 46th Annual Conf. pp. 16-25. Technical Abstract: Gray kernel is an important disease of macadamia that affects the quality of kernels with gray discoloration and a permeating, foul odor that can render entire batches of nuts unmarketable. We report on the successful production of gray kernel in raw macadamia kernels artificially inoculated with strains of the bacterium Enterobacter cloacae. Our studies prove that Enterobacter cloacae is the causal agent of this disease. We identified some of the conditions that are conducive to the production of gray kernel in artificially inoculated kernels as: incubation period of 2 months at 30ºC (= 86ºF), low oxygen (anaerobic) atmospheres, presence of free water, and 1-2 d of air exposure at 30ºC after kernels are removed from shells. In addition, headspace analysis was conducted and major volatiles were identified from control nuts (ethanol and acetic acid) and infected, gray nuts (ethanol, acetic acid, and additional compounds: 3-hydroxy-2-butanone, 2, 3-butanediol, phenol and 2-methoxyphenol). The information on the volatiles involved in gray kernel is a first-time report. The establishment of gray kernel as a bacterial disease caused by E. cloacae acknowledges that gray kernel is a food quality as well as a food safety concern for the macadamia industry.