Posted 09 January 2011 - 06:45 AM
"Milk chocolate contains a significant amount of butterfat and, because butterfat creates a strong eutectic with cocoa butter, the resultant product is much softer than plain chocolate. Milk chocolate rarely if ever undergoes Form VI bloom. The reason for this is the composition of butterfat contains a large range of TAG structures and molecular weights. This is due to the presence of significant amounts of fatty acids from C4 to C14. These serve to block the move from Form V to Form VI because, as they co-crystallise with cocoa butter TAG, the packing density of the crystals does not permit the thermodynamic change. The butterfat TAG also cause a change in the solid solution and thus a softer product. Both these effects reduce the incidence of bloom. This action of butterfat can (and is) harnessed to reduce the occurrence of bloom in plain chocolate. Butterfat can be added at a low level where softening of the chocolate is not significant but the effect on crystal form change is very significant."
Quoted from -
Eugene Hammond and Susan Gedney, United Biscuits (UK) Ltd, Group Technical, Lane End Road, Sands, High Wycombe, Bucks, HP12 4JX, UK
The way I've always pictured it is that dark chocolate in temper has all it's little chair shaped crystals lined up in military fashion allowing lines and planes that will allow other fats to creep through - when you add the more amorphous milk fat to it you fill in those lines between the crystalline planes and make it more impermeable to fat creeping.