Rav Menashe Klein
R' Menashe Klien, published in Nezer HaTorah Kislev 5772
The Mishna [Chullin 122] records that that skins of various beasts are deemed to be flesh and carry ritual impurity but they lose that status with tanning. The expression used, “Battel MiToras Bassar – it is no longer classified as meat” means that it is no longer meat in any respect and it may now be eaten as a Kosher product. [See RaMBaM Avos HaTumos 1:13: meat that is deteriorated to the point that a dog would not eat it, is no longer meat]
Radvaz [979 or 545] rules similarly, that the flesh of a mummy is not food and there is not even a Rabbinic restriction.
Regarding gelatine manufactured from hides the principle is the same; it is Kosher. [Heter Gamur]
One does not transgress the prohibition of eating non-Kosher food, when it is either combined with foul tasting additives, or is eaten in an abnormal fashion. Transgression of the Torah command requires that one eat non-Kosher food. In the cases described above, food has been consumed but food has not been "eaten". The foul component can be removed, it is not an intrinsic part of the food. The foul taste merely makes the act of consuming it, irregular, which disqualifies it from being deemed "eating".
The Chavas DaAs [YD 103] explains that this is completely different to a case where the food itself becomes spoiled, in which case it permanently loses its identity as a food. These two situations are supported by different Pesukim from the Torah.
Furthermore, food that has lost its identity as a food can never be restored to its earlier identity, even if we make it edible through various processes.
Although the Beis SheOrim [OCh 206] appears to disagree, suggesting that any spoiled food can be restored and will be re-identified with its original non-Kosher status, the source of this Halacha [a discussion in the Gemara Bechoros 13] proves otherwise. It records a dispute in this matter, however, the dispute is restricted to flesh that has been rendered unsuitable for human consumption but is still food suited for a dog; meaning it has not lost its identity completely. However, where the food has become utterly spoiled and no longer suited even for a dog, there is consensus that it is permanently re-identified as a new entity.
This is supported by the words of the ReAh [Bedek haBayis 4:1 and again in 4:4, p35b] and the Rashba [MishMeres HaBayis 4:1] who both agree that once fully spoiled a food cannot be restored to its previous non-Kosher identity. In fact the Rashba maintains that just being disqualified from human consumption renders it permanently disqualified from being a food.
The Avnei Nezer [YD 2:242] proves from the RaMBaM that food that is dehydrated to the point that it requires more than 24 hours of soaking in warm water to be rejuvenated, is permanently disqualified from being considered food.
Furthermore, the NoDa BiYehudah 1:26 YD, explains that any stringencies in this matter apply only apply to a product that is eaten in its own right but not where it is used as an adjunct to another food.
The Rosh [Berachos 43] quoting Rabbenu Yona [who dismisses the concerns of Raz HaLevi], rules that musk is Kosher although it was believed to originate from a blood fluid, since it is now an altered product. The Rosh also rules this in his Teshuvos [24:66] Although the Magen Avraham disagrees, arguing that the Gemara [Bechoros 6] understands that milk of a Kosher beast ought to be non-Kosher since it is derived from a blood fluid. Now following the logic of Rabbenu Yona, it is as much an altered product as is musk and should accordingly be recognised as a Kosher product. Why does the Gemara find it necessary to invoke a special permit? But this is easily deflected – the expression Dam NaAcher VeNaAse ChaLav does not mean completely altered. NeAcher means clouded or muddied. MaYim AchuRim, are the waters that have had the settled dust kicked up and are now murky. So we would not argue that milk is an altered product. According to this opinion milk is still blood but it has become murky. Obviously it would be prohibited if not for a disclosure from the Torah that it is permitted.
A more complicated answer suggests that the Gemara really means to say that if one considers milk to be an unaltered product then we can still rule that it is Kosher because of a special Derasha.
See also RaMBaN Shemos 30:23, that we can use Mor Derror [possibly musk] for service in the Mikdash even though it originates in a non-Kosher beast, since it is now altered from its origin.
HaRav Klein also explains that extracting collagen from the skins is a process of destruction and is permitted therefore even for a Jew to do; and is not to be confused with combining small amounts of non-Kosher with Kosher which a Jew is not permitted to do.
HaRav Klein notes that he has studied the response of HaRav Feinstein and sees that he agrees to rule leniently other than regarding use of soft edible skins. Although HaRav Klein does not accept HaRav Feinsein’s concerns about soft skins, he nevertheless is reluctant to openly disagree with him and therefore regarding gelatine from soft skins reserves his ruling.