Gelatin, Derived From Meat but not Meaty, But it’s Not Kosher
Gelatin is derived from collagen, a substance found in the skins and bones of animals. There is no vegetarian source for collagen; “vegetable gelatin” is agar agar, a seaweed derivative and is called gelatine in recognition of the supremacy of true gelatine.
Gelatin is Kosher in spite of its derivation from non-kosher sources [either non-Kosher beasts or Kosher beasts that have not been ritually slaughtered or have been ritually slaughtered but are found to be not Kosher due to internal blemishes] since in extracting the collagen, it forms a product that is no longer considered food and is therefore seen as having a new identity.
Other rabbis disagree, with many Kosher agencies adopting this harsher line. The intensity of their position can be determined by their determination to require Kosherization of plant and machinery that has used gelatine.
Nevertheless, all Rabbis agree that gelatin when derived from Kosher sources, is Pareve. It is not meaty and may be combined with dairy ingredients. Why is this so? Although Kosher laws regarding segregation of milk and meat, are very strict and prohibit all benefit from such combinations, the processing renders them Pareve since in extracting the collagen, it forms a product that is no longer considered food and is therefore seen as having a new identity. [Why this logic works to make gelatine Pareve but does not work to make it Kosher is a great mystery.]
In recent times with the growth of those seeking non-animal gelatine [Muslim and Hindu communities] fish gelatin has become a commercially viable product. Collagen of a reasonable quality can be extracted from skin and bone of Kosher fish. This is a value adding process, providing a better return than dumping or pulverising for sale as animal feed supplement. From the Kosher perspective, it is very easy since all the waste skin and bone from a tuna canning factory for example, can be accepted as Kosher. Fish do not require special ritual slaughter, nor are they disqualified from Kosher because of blemishes, as are beasts.