When milk falls into a pot that contains meat, it can create the forbidden substance known as basar b’chalav[1] if there is enough milk to give a taste to the meaty food. The halachic assumption is that if the volume of milk is at least 1/60th of the volume of the meaty food, the milk’s taste will be noticeable in the mixture and make it forbidden.[2] This method of measuring works well when the milk spreads uniformly throughout the contents of the pot, which happens when the food is soupy. However, if the milk fell onto a solid piece of meat, then we have to ascertain how far into the food the milk penetrated, as we shall see. (If the milk fell into a pot containing solid pieces of meat protruding from gravy, the situation is much more complicated and beyond our present scope.[3])
Several factors affect if and how far the taste of one food is absorbed by the solid food on which it falls. The most basic factor is the heat of the food. In a case where the bottom food is cooking, there is significant absorption even if that which falls upon it is cold (Shulchan Aruch,[4] based on the rule, tata’ah gavar[5]). However, even if the drop of milk is absorbed, will it spread throughout the roast?
We work under the assumption that the milk can spread at least up to kedei netilla,[6] which is a little less than an inch around the place where the milk fell, in depth as well as on the top surface. However, when the food(s) is fatty, it is likely that the taste will spread throughout the piece. To make a very long story short,[7] we consider the possibility that the milk taste can spread throughout the roast.
The supposition that the milk taste will spread might appear to create more issur, but, in theory, it can be a cause for leniency. If, as likely, the roast is at least sixty times the volume of the milk, the milk taste should be diluted to the point of bitul, where it loses its impact on the meat. Thus, a big roast and/or a small spill will allow the roast to remain kosher. However, since it is probable that all or a large portion of the milk will collect near the area where it fell and affect the taste there, a kedei netilla amount around that area must be removed anyway.[8]
If the whole roast does become forbidden, or if some milk rolls onto the pan in which the roast is cooking in a manner that there isn’t enough gravy for bitul, then the pan needs to be kashered.