Aflatoxin contamination is not uniform and will vary in amount from kernel to kernel. While you can’t see aflatoxin, removing obviously damaged, shriveled or fungus-infected nuts removes some of the most contaminated nuts or kernels.
Still, a few kernels or nuts may be highly contaminated, but show no visible signs. When these are mixed with the other less contaminated kernels, the entire lot is above the regulatory limit.
Since most test methods destroy the kernels, the entire lot cannot be tested, so a sample of kernels must be taken that represents the entire lot.
The solution is to take a sample from the lot. But it’s important to make sure you sample well enough to get those rare, contaminated kernels.