3,478. How to Determine the Origine of Produce

Hilchos Maaser 13:12

The Sages said that if there’s a doubt as to whether produce came from Israel or other lands, the determination is not made based on its appearance, flavor or smell, but on where most of the produce came from. If most of the produce from other lands, it is exempt from demai; if from Israel, it is obligated. Therefore, if most of the produce in a city is from other lands but this is not the case in its outskirts, or vice versa, or if most of the produce in the mountains is from other lands but this is not the case in the valley, or vice versa, or if most of the produce of shopkeepers comes from other lands but not that of homeowners, or vice versa – in all of these cases, for those for whom produce from other lands is the majority, the rules of demai don’t apply. For those for whom such produce is not in the majority, the rules of demai do apply.

Hilchos Maaser 13:13

If the produce in the market started out with a majority from other lands but it decreases until it becomes the minority, the market reverts to its previous status, with the result that one who buys there is obligated in demai.