Generating structure in soft solids: heat stable milk chocolate.

Addition of small amounts of glycerol to milk chocolate during the tempering stage can yield a product which exhibits enhanced mechanical strength and retards motion of the continuous phase (cocoa butter, a mixture of fats) of this dense granular suspension above the fat melting point. The mechanisms responsible for this 'heat stability' were probed by an extensive series of experimental investigations. Addition of glycerol did not alter the fat polymorph or melting behaviour. The increase in bulk yield strength, quantified by indentation, was found to be proportional to the square of the volume fraction of glycerol. A quantitative explanation of this behaviour was not identified. The glycerol is not soluble in the fat phase so the possibility of a cage network arising from a capillary suspension mechanism was pursued. The evidence did not support a 'normal' capillary suspension model, and suggested that absorption of glycerol into the lecithin layer coating the particulate phase either resulted in a viscous 'glue' which retards contact motion or rearrangement, or displaces lecithin to allow the formation of sugar-glycerol bridges.