[Detection of wheat as an allergenic substance in models of processed foods by a nested PCR methods]

A polymerase chain reaction (PCR) method for verifying the allergen labeling of foods (i.e., the presence of wheat, buckwheat, or peanut) was adopted as the official Japanese identification test by the Ministry of Health, Labour and Welfare of Japan in 2002. We have verified the wheat labeling of several commercial food items by using the adopted PCR method. The study has revealed that some foods with positive results in the screening test yielded negative results in the identification test. When the result of the screening test disagrees with that of the identification test, the validation of food labeling is remarkably difficult. Therefore, we developed a nested PCR method with high sensitivity and specificity and employed this method in our routine testing as necessary. In this study, we examined 11 types of models of processed foods containing 10 microg/g wheat protein by using the adopted PCR and nested PCR methods; these samples were prepared by various processes and had varying physical properties. The adopted and nested PCR methods enabled the detection of wheat in 8 and 10 types of food models, respectively. The reasons for the failure in detecting the food allergens include DNA fragmentation due to the processing of food and the presence of DNA from other sources in the extracted DNA. In both PCR methods, an increase in the amount of template DNA in the PCR mixture enabled the detection of wheat DNA in all the food samples, but an excessive increase in the amount of template DNA hindered PCR amplification. These results indicate that an increase in the amount of template DNA increases the efficiency of the detection of allergens in processed foods by conventional PCR. Further investigation is needed to remove factors that inhibit PCR amplification of the extracted DNA.