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3. The Rabbinical prohibition of Separating:
Using a Knon and plate: If one separated the food from the waste with a Knon or a plate then he is Biblically exempt [from liability] although it is Rabbinically forbidden to be done due to a decree [that one may come to use] a sieve and sifter. [The same applies for any irregular Borer utensil, that it is only Rabbinically forbidden to be used, while a regular Borer utensil is Biblically forbidden.]
Separating an egg yolk from its white: See Halacha 6
May one use a garlic tube peeler?
 Admur 319:1
 This refers to a wooden tube which is wide on top and narrow on bottom. One places the legumes in from the wide part and then shakes off its peels and the legumes fallout from the narrow bottom while the peels remain inside. Nevertheless, this item is only Rabbinically forbidden as it is not the commonly used item for Borer, and is hence considered a Shinuiy, which makes the Melacha only Rabbinical. [Rashi in Shabbos 74a; M”B 319:2] Vetzaruch Iyun as to why a Knon is only Rabbinical, is it because it is considered 1) a shinuiy, or 2) is not a manufacturers choice vessel, or 3) it has other usages and is not designated for separating per say.
 Mishneh Berurah 2