Peter scratches an itch to know what tallow is: he learns some scary details about its nutritional qualities, but doesn’t mention what it actually is.
tallow : Online Dictionary at Datasegment.com:
Tallow Tal”low, n. [OE. taluh, talugh; akin to OD. talgh, D. talk, G., Dan. and Sw. talg, Icel. t[=o]lgr, t[=o]lg, t[=o]lk; and perhaps to Goth. tulgus firm.] 1. The suet or fat of animals of the sheep and ox kinds, separated from membranous and fibrous matter by melting. [1913 Webster]
It’s the melted down fat that you get when you boil offal (the bones and otherwise inedible parts of animals). The grease that comes out of a burger or that’s left in the pan in which a roast has been cooked — the dripping, as we called it growing up — is tallow, if impure. And the addition of that is what makes Mickey D’s fries taste so good.