Reference for Pesachim 66:1
אמר אמר קרא (דברים יח, ד) ראשית ששיריה ניכרין לישראל יצתה זו שאין שיריה ניכרין
he said, Scripture saith, The firstfruits [of thy corn etc.], [implying] that its residue is distinct [in that i becomes permitted] to the Israelite,<span class="x" onmousemove="('comment',' I.e., by giving the firstfruits, viz., the terumah to the priest, the residue becomes permitted to the Israelite.');"><sup>1</sup></span> [thus] this<span class="x" onmousemove="('comment',' Leaven separated as terumah during Passover.');"><sup>2</sup></span> is excluded, since its residue is not [so] distinct.<span class="x" onmousemove="('comment',' The residue, being leaven, remains forbidden to the Israelite.');"><sup>3</sup></span> R'Ala B'R''Awia sat before R'Joshua and he sat and said in R'Johanan's name: If grapes are defiled, one may tread them out less than an egg in quantity at a time, and their wine is fit for libations.<span class="x" onmousemove="('comment',' On the altar. Unclean food less than an egg in quantity cannot defile other eatables. Hence when he treads out the grapes in such small quantities, there is never enough to defile the exuded juice, and the wine manufactured therefrom is clean, and consequently fit for libations on the altar, for which, of course, only undefiled wine is valid.');"><sup>4</sup></span>