Related su Makhshirin 2:12

Planting untithed [seeds] is prohibited, as is covering up untithed [seeds with dirt], as well as making [a partnership] with a Gentile with [respect to planting] untithed [seeds]. A Jew who forgot and planted untithed [seeds], they are exempt before they sprout because [the seeds] have already perished (cf. Ter. 7:7). One who buys [produce] for livestock, for wild animals, and for fowl is exempt from Demai.

We [may] combine fruits [grown] outside the Land of Israel with [exempt, i.e., tithed] fruits [grown] inside the Land of Israel so that they form the majority over [Demai fruits grown] in the Land of Israel in order to exempt them from tithes. We [may] combine fruits [grown] outside the Land of Israel with second-year fruits (i.e., fruits grown in the second year of the agricultural cycle), in order that they form the majority over third-year fruits to exempt them from the second tithe. We [may] combine from the fruits [grown] outside the Land of Israel with third-year fruits in order that they form the majority over fourth-year fruits to exempt them from the poor man's tithe. [?] We [may] combine fruits [grown] immediately before Shevi'it [the seventh year of the agricultural cycle, when fields must lie fallow] in order that they form the majority over fruits [grown during] Shevi'it, to exempt them from Bi'ur ["removal," see Sheviit 7:1]. We [may] combine fruits [grown] outside the Land of Israel with Shevi'it fruits in order that they form the majority over fruits [grown] immediately after Shevi'it in order to exempt them from tithes.

A pan of milk, placed in an oven, underneath a pot of meat - the steam rises, enters into the pot, and renders it forbidden. Rema: If there was actual milk in the pan, then we require sixty times its volume in the meat. All of this is when the pan is open, and the steam rises up from the food itself, to the pan that is above it, and also if the two are so close to each other that the steam that rises is so how that it would make your hand retreat rapidly. If not, then everything is permissible. Therefore, we hang meat (slabs) over dairy pots, and we don't worry about the steam that rises. And if something is covered, then everything is permissible, since it is like two pots that touch each other, which do not make each other forbidden, and all the more so with mere steam. But ideally (lechatchila) one should be strict on all this.