Responsa for Shabbat 288:15

מתיב רבינא טמא מת שסחט זיתים וענבים

Rab Judah said in Samuel's name: One may squeeze out a cluster of grapes into a pot,<span class="x" onmousemove="('comment',' Of food, for obviously the juice will not be drunk separately but is meant to season the food; as such it remains a food, i.e., a solid, itself. ');">²⁸</span> but not into a plate.<span class="x" onmousemove="('comment',' As it may then be drunk separately, notwithstanding that one does not generally drink from a plate. ');">²⁹</span> R. Hisda observed: From our master's words we may learn [that] one may milk a goat into a pot [of food], but not into a plate. This proves that he holds: a liquid that unites with<span class="x" onmousemove="('comment',' Lit., 'comes into'. ');">³⁰</span> a [solid] foodstuff is [accounted] a foodstuff. Rami b. Hama objected: If a <i>zab</i> milks a goat, the milk is unclean.<span class="x" onmousemove="('comment',' A zab defiles everything through hesset (v. p. 395, n. 1); here too he exercises hesset on the milk. ');">³¹</span> But if you say, A liquid that unites with a [solid] foodstuff is a foodstuff, whereby did it become susceptible?<span class="x" onmousemove="('comment',' To defilement, for no foodstuff can be unclean unless a liquid has previously fallen upon it (v. p. 45, n. 1). — The law is stated generally- which implies that it is so even if he milks it into a pot of food. ');">³²</span> — As R. Johanan said [elsewhere], By the drop [of milk] smeared on the nipple: so here too by the drop smeared on the nipple.<span class="x" onmousemove="('comment',' The milker smears the first drop around the nipple, to facilitate the flow. This drop of course counts as a liquid, and all the subsequent milk is touched thereby. ');">³³</span> Rabina objected: If a person unclean through a corpse squeezes out olives or grapes

Maharach Or Zarua Responsa