If tahini lasts for ever, why do sesame seeds go rancid so quickly?
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This is answered easily: in solid form the oil exposed to air and air causes the high fat content in the seeds to go rancid, in tahini form the oils are blended and, more importantly, the tahini is stored in the refrigerator, which is a cold environment, keeping the fats from oxidizing as quickly. If you keep your sesame seeds in the freezer then they will last as long as tahini.