An alternative approach to control the oxygen permeation across single-dose coffee capsules

The objective of this study was to investigate the potential of a hybrid coating made of pullulan and a silicon-dioxide precursor to act as an oxygen barrier on polypropylene (PP) capsules used for single-dose ground coffee. PP capsules were first treated by an oxidizing flame to allow better adhesion of the water-based coating solution. The barrier performance of the capsules (uncoated and coated) was assessed by a non-invasive approach based on fluorescence quenching. The deposition of the coating (approximately 7.0μm thick) led to a dramatic decrease in the oxygen permeation across empty capsules at 40°C and 25% RH (uncoated capsules: 496.91mL [package 24hatm]-1; coated capsules: 14.01mL [package 24hatm]-1). In the presence of coffee inside the capsules, the barrier performance was even improved (pack permeability 1.87mL [package 24hatm]-1), which was ascribed to the residual oxygen inside the capsules at the beginning of the analysis. At 66% RH, the hybrid coating, while still acting as a good barrier, partially lost its performance due to the simultaneous swelling of the polymer matrix and the presence of fractures on the coating surface. The approach presented in this work can present opportunities for an alternative design of the packaging intended for coffee capsules, with a potential advantage also from an environmental perspective represented by the upstream reduction in the use of plastics.