13 SIEVES MEANS UNITY OR ONENESS

The Mishnah Menachot 10:1-5 describes the ceremony.

If the barley was ripe it was taken from the vicinity of Jerusalem; otherwise, it could be brought from anywhere in Israel …The grain was then brought to the Temple where it was threshed, parched, spread on the courtyard floor to be dried by the wind, milled and ground into fine flour.

It was then sifted through 13 sieves …