Tosefta do Terumot 3:2

From what time may they take terumah from olives? From the time that [the olive press beam] bears down on them (משיטעינו), and Rabbi Shimon says, from the time that they grind them up. Rabbi Yosei son of Rabbi Yehudah says, from the time that they bring them [to the press] in a basket and place [the basket] under the press-beam/crushing-stone (ממל), or from when [the workers] tread on them vertically and horizontally (as with grapes, see Y. Ter. III.2.6). They said to him, "Grapes and olives are not alike. Grapes are soft and their juice (lit, "wine") is [easily] removed [from the fruit]. Olives are hard and their oil is not [easily] removed."

The terumah of tithes of demai (doubtfully tithed produced) that fell back into their [former] place creates a forbidden mixture [of tithed and untithed produce], [but if it fell into] a different place, it does not create a forbidden mixture, the words of Rabbi Eliezer. And the Sages say, whether in its [former] place or in a different place it creates a forbidden mixture. Rabbi Shimon says, whether in its [former] place or in a different place, it does not create a forbidden mixture.