Cooking crystalline candies and the ductile to brittle transition in concentrated suspensions

The existence and origin of the ductile to brittle transition in non-Brownian suspensions and pastes is underexplored despite the ubiquity of such materials in practical applications. We demonstrate the phenomenon in candies of sugar crystals in a water-protein-fat matrix prepared by boiling a sugar-cream-butter mixture (known as ‘fudge’ in some countries). As cooking time or final cooking temperature increases, we observe a transition from a fluid to a ductile solid, then to a brittle solid that abruptly fractures in compression. We propose that this is driven by rising solid sugar crystal volume fraction, and indeed find the same sequence of behaviour in a suspension of non-Brownian calcite particles as the solid fraction moves from frictional jamming to random close packing. Particle-based simulations reveal the sensitivity of the observed phenomenon to boundary conditions.