If one inadvertently spilled milk over a piece of meat cooking in a pot, we need to establish the exact positioning of the meat before we can proceed:
If the meat is submerged within the liquid in which it is cooking (even thought the upper surface of the meat is exposed), we can consider EVERYTHING in the pot to be a single entity to make up the SHISHIM (60) to nullify the effect of the milk. This is because the heat from the cooking process causes the milk to spread evenly throughout all the contents of the pot. The fact that the meat is submerged makes it an intrinsic part of the rest of the pot.
However, if the piece of meat is exposed and is situated above the waterline of the pot (say it is resting on top of another piece of meat that is half in and half out of the cooking liquid), and as soon as the milk spills, one immediately and actively pushes that piece of meat under the waterline (and keeps it submerged for a while) – the same abovementioned law would apply and ALL the contents of the pot would be considered one entity to make up the SHISHIM (60).
Another solution that would achieve a similar result, would be to immediately place the lid onto the pot. This would cause the entire pot to become a single closed system, and thereby spread the milk evenly throughout (even though the meat onto which the milk spilled is still
If one did not think fast enough and immediately push the meat down or close the lid, the spilled milk would remain ‘isolated’ within the exposed piece of meat, and we would not be able to rely on the SHISHIM (60) principle. The reason being that the exposed meat remains ‘detached’, and cannot merge with the rest of the contents of the pot to create SHISHIM (60) to nullify the milk. It therefore becomes a forbidden entity of Basar BeChalav.
The question now remains as what effect this exposed piece of meat has on the rest of the pot.
Here we have divided opinion:
1) VIEW OF THE SHACH:
If the meat in question is lean and non-fatty, then all the adjoining pieces of meat it makes contact with, become forbidden up to KDEI NETILAH (2cm or ¾ inch). This means that we would have to remove 2cm or ¾ inch from the supporting pieces of meat, and the rest of the contents are permissible.
The reason for this is that lean meat cannot spread TA’AM (taste) up to more than KDEI NETILAH (2cm or ¾ inch) into a neighbouring piece of meat.
If the meat in question, however, is fatty, then the only remedy is to find SHISHIM (60) in the rest of the pot. The reason for this is that fatty meat does spread TA’AM (taste) throughout the entire contents of the pot.
2) VIEW OF TAZ (AND MOST OTHER POSKIM):
It makes no difference whether the exposed meat is fatty or lean, it simply has to be removed from the pot, and everything else remaining is permissible.
The reason is a fascinating one: If milk (which itself is considered ‘lean’) falls onto a piece of meat, it remains ‘trapped’ within the meat and cannot be transferred without ROTEV (liquid or gravy). Thus if it remains ‘dry’, as it would be in this exposed piece of meat, it would never leave the meat to affect the rest of the pot.
The only TAAM (taste) that would transfer from this ‘dry’ piece of meat would be the taste of the meat itself (as there still is much heat from the cooking process). But that taste of meat, although emanating from a forbidden entity of BASAR BeCHALAV, is still only considered to be a pure and singular taste of meat alone.
This is an amazing example of tastes ‘splitting up’ and reverting to independent states. All because of the principle that milk (which is considered lean), cannot leave the meat into which it became absorbed, without ROTEV (liquid).
This can be compared to a garment of SHA’ATNEZ (a forbidden mixture of wool and linen), where although it is a forbidden entity (like BASAR BeCHALAV), were the threads of wool and linen to separate, each thread would revert back to its original permissible state. (In a similar vein, the taste of meat, once separated from the milk, reverts back to its original permissible state.)
Of course the SHACH would disagree with this analysis, suggesting a different fundamental understanding of the technical entity of BASAR BeCHALAV: It is not a MIXTURE of milk and meat (that were the two to become separated as in our example, they would revert back to their original permissible states), but rather it is a NEW ENTITY of forbidden substance (and accordingly, the tastes can never be separated back to permissible states).
In any event, were our forbidden piece of exposed meat, to fall back into the pot, all would agree that we would need SHISHIM (60) in the pot to nullify it.
[Daf 37, 38, 39 40, Seif 1-8]
 At first glance, this may not seem to be beneficial, having milk spreading throughout the meat contents. But it actually is, because now we can apply the SHISHIM principle; that milk can be nullified by the meat, provided there is 60 times more meat against the volume of the milk.
 Unless, of course, the exposed meat itself is large enough to be 60 times more the volume of the milk, in which case it would nullify the spilled milk.