Detection of cashew nuts in food by real-time polymerase chain reaction

A real-time PCR-based method for the detection of cashew (Anacardium occidentale L.) nuts in confectionery products is described. The method consists of DNA isolation by chaotropic solid phase extraction and the subsequent PCR with cashew-specific primers and a TaqMan fluorescent probe. The method was positive for 10 samples of cashew nuts available on the market and negative for all other tested plant materials used in food industry including almonds, brazil nuts, chestnuts, hazelnuts, peanuts, pistachio nuts and walnuts. The intrinsic detection limit of the method was 1.25 pg DNA, which corresponds to approx. 2.5 genome equivalents (1C). Using a series of model pastry samples with defined cashew nut contents, a practical detection limit of 0.01% (w/w) was determined. Practical applicability of the PCR method was tested by the analysis of 22 confectionery products. For all but one of the food samples, results obtained conformed to the labelling. The one incorrectly labelled food product was Macadamia nougat containing approx. 0.040% (w/w) of cashew nuts, which suggests that it had been contaminated rather than adulterated. The presented real-time PCR method is useful for sensitive and selective detection of cashew nuts in food samples.