Ability to re-foam frothed milk at different solid concentrations and their foam structure

Left-over frothed milk in most coffeehouses is typically discarded, leading to wastage of a lot of milk due to a common belief that frothed milk is unable to re-foam. We investigated the foaming and physical properties of frothed reconstituted skim milk (1.5–15%, w/w) foamed up to four times. The results showed that at all investigated solid concentrations and foaming times, milk samples retained their high foamability and foam stability, and uniform micro-foam structure. Similarly, properties of milk samples including viscosity (1.9–3.3 Pa.s), absolute zeta-potential (25.5–27.2 mV) and surface tension (54.8–59.7 mN/m) remained unchanged even after re-foaming multiple times.