Aeration and rheology of buttercream icings

Abstract

Buttercream icings – 4 phase materials comprising sugar, butter (an emulsion) and air – are widely used for coating and decorating sweet goods. The aeration of 2:1 w/w sugar-butter mixtures was investigated in 2 benchtop planetary mixers and the rheology of the resultant viscoplastic material was quantified using lubricated compressive testing (squeeze flow and upsetting) over extensional shear rates of 10−3–100 s−1. The aeration dynamics exhibited asymptotic behaviour which fitted a simple phenomenological model well. Optical microscopy and X-ray microtomography indicated that the air was incorporated as small bubbles of similar size to the sugar particles or as voids generated by the mixer wires. The measured yield stress was strongly dependent on the air volume fraction, φ. The flow behaviour for one batch could be reasonably described as a Herschel-Bulkley fluid whereas a second batch gave overall better agreement with a modified power-law description: in both cases the viscous friction term decreased strongly with φ.