Evaluation of surface disinfection procedures for enumerating fungi in foods: a collaborative study

An effective technique for assessing fungal infections of paniculate foods such as cereals, nuts, fruits and vegetables is surface disinfection with a sodium hypochlorite solution before plating directly onto agar media. The surface disinfection procedure is used to inactivate the surface mycoflora then permitting assessment of the percentage of panicles invaded by specific fungi. A previously recommended procedure treating samples with 0.4% chlorine for 2 min has been shown to be ineffective for barley highly contaminated with conidia of either A. flavus or A. parasiticus. An international collaborative study has demonstrated that modifying the procedure by increasing the chlorine concentration to 0.8%, or increasing the contact time to 8 min, or increasing the volume of chlorine solution is not the answer, especially with surface contamination loads of approximately 106 conidia/g. The inclusion of a 70% ethanol pre-rinse prior to a 0.8% chlorine treatment for 2 min made a significant improvement to the inactivation of surface conidia of A. flavus on barley. However, pretreatment with ethanol inactivated some of the internal Alternaria infections especially when the ethanol was not rinsed off. Further studies including an ethanol pre-rinse are recommended to optimise the surface disinfection procedure for seeds and nuts which have a high concentration of fungal surface contaminants.