The verse speaks of two terumot; one is standard teruma and one is teruma of the tithe. Since they are mentioned together, they can be compared: Just as standard teruma is taken by estimate, as there is no requirement for the amount separated to be measured precisely, and it can be taken by thought, as one is not required to physically separate it before consuming the remaining produce, so too, teruma of the tithe may be taken by estimate and by thought. And this comparison also teaches that just as the homeowner has permission, i.e., the right, to separate standard teruma, so too, the homeowner has permission to separate teruma of the tithe.

MISHNA: With regard to one who sets aside produce with the understanding that he will separate terumot and tithes with it, so that when he has untithed produce he can render it fit by declaring that the teruma and tithes that must be separated will be from the produce that he had set aside for this purpose, or one who sets aside money with the understanding that he will separate and redeem second tithe with it, then he may later separate the teruma or tithe with them, based on the presumption that the produce or the money are extant. He need not be concerned that perhaps the produce or money was lost in the meantime. If he discovers that they were lost, then he must be concerned that the produce or money that he set aside was lost, from the time until the same time, as will be explained in the Gemara, and he must separate teruma and tithes from the produce a second time; this is the statement of Rabbi Elazar.

Rabbi Yehuda says: One checks the wine that is set aside to be used for separating terumot and tithes for other wine to see if it has turned to vinegar, which would render it unfit for this purpose, at three times during the year: When the east wind blows at the conclusion of the festival of Sukkot, and when the blossoms fall and the grape buds emerge and appear as small clusters, and at the time when water enters and fills the unripe grape. Since there is a change in the weather at these times, one should check to ensure that the wine has not turned to vinegar.