Foaming agents from spent coffee grounds: A mechanistic understanding of the modes of foaming and the role of coffee oil as antifoam

Development of plant-based foaming agents is of relevance to the food industry. The present study hypothesizes that (1) foaming fractions from spent coffee grounds (SCG) may exhibit equivalent or superior foaming properties than synthetic surfactants and that (2) the coffee oil present in SCG could act as antifoam. Foaming fractions were isolated from the defatted and whole SCG (obtained after the hot water brewing of medium-roasted and coarse-ground Arabica and Robusta coffees), after a series of extraction, concentration, and purification steps. The surface activity, foamability, and foam stability of SCG-based foaming fractions were studied in comparison with synthetic surfactants. The foam structure evolution and molecular fingerprint of foaming fractions were elucidated using light microscopy and infrared spectroscopy, respectively. Further, the surface activity of coffee oils was assessed. The high molecular weight foaming fraction obtained from defatted robusta-SCG exhibited higher surface activity, comparable foam stability, and greater foamability than synthetic surfactants. The role of coffee oil as antifoam was ascertained by its ability to retard the foam-forming and stabilizing activities of SCG-based and synthetic foaming agents. Bubble breakage and shrinkage were found to be the antifoaming mechanisms of coffee oil. Infrared spectra of the foaming fractions provided insights into their composition. [Display omitted] •Foaming fraction from defatted robusta spent coffee showed good foaming behavior.•It exhibited higher surface excess concentration than synthetic surfactants.•Coffee oil present in spent coffee grounds acts as an antifoam.•Bubble breakage and shrinkage were the antifoaming mechanisms of coffee oil.•FTIR spectra indicated presence of polysaccharides and Maillard reaction products.