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there are flour sieves that are sold with rabbinical approval. You can see one here: http://cdn.instructables.com/FQ7/UKPN/FI8VJOBL/FQ7UKPNFI8VJOBL.LARGE.jpg (archive link)
What are the requirements for a kosher flour sifter? I'd like to buy one that fulfills the kosher requirements, and I don't have access to one with rabbinic approval.
The OU guide to Kashrus in Israel says:
White flour: In the US there are special methods used in processing flour to insure that it remains bug free until use. Unfortunately in Eretz Yisrael flour is not processed in the same way and insect infestation can occur. Therefore it is important always to sift flour before using it. You may sift more flour than you need and freeze the excess. This should be done immediately. Flour that is stored in a freezer immediately after sifting requires no further cleaning.
A 70 mesh sieve is recommended. These are available in most kitchen or housewares stores.
Whole wheatflour: It is nearly impossible to sift whole wheat flour with a 70 mesh sifter. While some poskim are lenient and allow a 50 mesh sieve to sift whole wheat flour, buying either frozen or vacuum packed whole wheat flour with a reliable bug free hashgacha is recommended. When buying frozen flour, check that the flour was kept frozen.
So the criterion is 70-mesh.
