“Question: Is there any problem with doing hagoloh to a (metal) grater?
Introduction: Hagoloh is one of the methods of kashering by utilising boiling water to purge forbidden bliyos from the utensil being kashered. Hagoloh is discussed at length in Hilchos Pesach Siman 451 and 452, and can be used to kasher wood, stone and metal utensils (Shulchan Aruch Siman 451 se’if 8). Prior to performing hagoloh, one must ensure that the utensil being kashered is completely clean of any food residue or rust (that may in fact contain within it particles of issur – see Shulchan Aruch ibid se’if 3 and Mishna Berura s.k. 22).
A dovor chorif (discussed at length in Yoreh De’ah Siman 96) is a sharp or spicy food item. We find various severities regarding a dovor chorif; including that it has the ability to rejuvenate eino ben yomo bliyos, it has the ability to transfer bliyos when cut even while cold and it can also absorb into itself a large proportion of bliyos present in a utensil in which it is cooked (or with which it is cut) and is thus not subject to standard nat bar nat principles. Consequently, although eino ben yomo bliyos generally do not pose a problem ex post facto, if the food item they are absorbed into is a dovor chorif then the dovor chorif may in fact rejuvenate the pogum bliyos and become forbidden to be eaten. Additionally, even though permitted heter bliyos often do not pose an issue in a nat bar nat scenario, if the permitted food item in question if a dovor chorif, there will often be no nat bar nat leniencies.
Discussion: The Remo (Orach Chayim Siman 451 se’if 18) writes that hagoloh is ineffective for a grater.
The Magen Avraham (ibid, s.k. 35) explains that this is because it is usual to use the grater for bread and/or horseradish that has been previously cut with a chometz knife and it is inevitable that some chometz residue will be left on the grater [even after it has been cleaned, and if this is the case then obviously hagoloh will be ineffective because there is actual chometz substance on the utensil, which can subsequently be absorbed into food on Pesach].
The reason that horseradish cut with a chometz knife can pose a problem (even though the knife, horseradish and grater were all cold) is because horseradish is a dovor chorif and consequently when cut it has the ability to transfer bliyos to and from the implement with which it is cut. Therefore, if the chometz knife used to cut the horseradish (prior to grating) had upon it even a small amount of bread (or other chometz products) then the horseradish will absorb bliyos from these chometz items, and since some of the horseradish inevitably remains on the grater it will prevent hagoloh from being effective.
Consequently, the Magen Avraham (ibid) writes, even if hagoloh was mistakenly performed on the grater and the grater was subsequently used on Pesach, the food grated becomes forbidden to be eaten. [And even though the permitted food may contain sixty times the volume of the miniscule chometz crumbs; on Pesach this is inconsequential, because on Pesach even a minute amount of chometz will prohibit an entire mixture of permitted food (see Shulchan Aruch Siman 447 se’if 1).]
It should be mentioned that with the types of sophisticated graters (and cleaning agents) available today, it may be actually be possible to thoroughly clean a grater and then to kasher it with hagoloh. This will have to be determined on a case-by-case basis, by a competent Halachic authority.
Conclusion: We do find in the Remo that hagoloh is indeed ineffective for a grater. The reason given for this are because it is inevitable that will be a miniscule particle of chometz (or of a sharp food cut with a chometz knife) that remains in the grater and thus hagoloh will have no effect. If one is certain that they have cleaned a grater until there is no minute amount of food left on it, a Rav should be consulted about whether this grater can be kashered with hagoloh.