Rheology of caramel

Abstract

Caramel is a mixture of sugars, milk proteins, fat and water cooked at high temperatures to initiate Maillard reactions. We study caramels as ‘active emulsion-filled protein gels’, in which fat droplets are chemically-bonded to a background gel matrix of cross-linked proteins in a concentrated aqueous sugar solution. A ‘caramel region’ in composition space is delimited by the varying the four ingredients; sugar, milk protein, fat and water. The boundaries of the composition space define various modes of ‘failure’. Boundaries are determined for transitions to toffee and emulsification failure leaking out of the caramel, and protocol dependent failures, scum formation during the cooking step and subsequent boil over or the formation of a ‘creme Chantilly. Oscillatory rheology within the caramel region reveals that we can superpose the mechanical spectra of all caramels into a single pair of G'(ω),G''(ω) master curves using time-composition superposition (tCS) over 12 decades of frequency, so that all caramels are instances of an underlying ‘universal material’. The master curve is an example of a very lightly cross-linked amorphous polymer, a weak rubber. Utilising the cross-over point of G'(ω) and G''(ω) as a common point to compare caramel master curves the viscous and elastic dependency of caramel is determined and linked to the caramels ingredients. Viscosity is dependent on the aqueous sugar content and the elastic dependency on protein content. This insight constrains the molecular mechanisms for structure formation, and implies that measuring a couple of parameters suffices to predict the rheology of caramels over 12 orders of magnitude in frequency.