The presence of Bt Rice, a genetically modified rice cultivated in China and including the Cry1Ac gene, was surveyed in thirty imported processed-rice foods in Osaka city, Japan, using published PCR methods. Twenty-nine of the processed-rice foods were negative for Bt rice, although it was difficult to detect rice taxon-specific DNA in one rice paper sample and impossible in one bifun (rice vermicelli) sample. We therefore judged the bifun sample not susceptible to examination and investigated the cause of the failure to detect rice taxon-specific DNA in it. DNA solution extracted from the bifun was diluted to 1, 2.5, 5, and 10ng/μL and used in PCR assay. As no amplification products were obtained in these dilutes, we considered that PCR reaction was not inhibited by impurities. Although the DNA extracted from the bifun was amplified to detect wheat, corn, and common plant DNA, no amplification products were detected. This suggested that DNA in the bifun had been degraded through food processing. Detection of rice taxon-specific DNA using the real-time PCR method was also carried out in the thirty imported processed-rice foods. When PCR reaction was performed at 50 cycles with a threshold of 0.1, the Ct value was 50 in the bifun, 46 in the rice paper, and 26 to 35 in the other processed-rice foods. Under the experimental conditions, there was not enough DNA extracted from the bifun or the rice paper to allow amplification of the rice taxon-specific DNA by PCR. The use of commercial DNA extraction kits was investigated with both the bifun and the rice paper. The detection rate of rice taxon-specific DNA was higher with the Genomic-tip 20/G ion-exchange type kit than with other DNA extraction kits, especially when α-amylase was not used.
