Pesachim 11A

שלא ברצון חכמים (הן עושין), דברי רבי מאיר. רבי יהודה אומר: ברצון חכמים היו עושין. ולא קא גזר רבי יהודה דילמא אתי למיכל מיניה? אמר רבא: שאני חדש, מתוך שלא התרת לו אלא על ידי קטוף, הוא זכור.

However, with regard to those people who harvest the crop before the omer is sacrificed, they act contrary to the will of the rabbis. This is the statement of R' Meir. The concern is that while working with the grain they might come to eat from it, despite the fact that it is still prohibited. R' Yehuda says: They act in accordance with the will of the rabbis. And in that case, R' Yehuda did not issue a decree lest one eat from it. Why, then, does he issue a decree with regard to leaven? Rava said that the prohibition of new grain is different: Since before the omer you permitted one to harvest the crop only by picking it by hand and may not harvest it in the typical manner, he will remember the prohibition and refrain from eating it. That is not the case with regard to leaven.

אמר ליה אביי: תינח בשעת קטיפה, טחינה והרקדה מאי איכא למימר? הא לא קשיא: טחינה ברחיא דיד, הרקדה על גבי נפה.

Abaye said to him: This works out well in explaining R' Yehuda’s opinion with regard to the time when one is picking the grain; however, with regard to the time of grinding and sifting, what can be said? Apparently, it is permitted to perform these acts in a typical manner. Why, then, is there no concern lest one eat the grain at that stage? The Talmud responds: This is not difficult, as one also performs grinding in an atypical manner. One must grind the grain before the sacrificing of the omer with a hand mill, not with a mill powered by an animal or by water. Likewise, sifting is performed atypically, not in the interior of the sifter. Instead, it is performed on top of the sifter. Since all of these actions are performed in an atypical manner, there is no concern lest he come to eat the grain.