Re: What causes milk to 'clabber' when milk or meat tenderizer is added?

It all has to do with proteins! 
Milk contains apart from water and Calcium about 5% proteins. A large 
portion of this are the so-called caseins. These proteins are present in 
the milk as large aggregates named casein micelles, because of their 
‘large’ size [200-500 nm] milk has that white appearance. These micelles 
remain in solution because ONE of the 4 different caseins (k-casein) has 
the particular characteristic to keep these aggregates in solution. What 
happens to milk when meat tenderizer is added is that proteins with a 
particular function [named proteolytic enzymes*] interact with that 
protecting k-casein and as a result it loses its protective function. 
Apart from that the acid value [pH] of the milk, which is usually around 6.7 
will drop to lower values. Because of the combined action of the lowering 
in pH and the loss of protective action of  k-casein, all proteins will 
precipitate - come out of solution - which is noted as the separation in 
curd and a liquid phase named whey. The last one is the one containing the so 
called whey proteins that are not sensitive to the the drop in pH.

* Proteolytic enzymes are proteins that have the ability to hydrolyse - 
breakdown - other proteins. For this effect only very small amounts of 
enzyme are required, some bacteria as the ones used for yogurt and cheese 
production produce these enzymes in a natural way resulting in curd 
formation in the same way as described above.