Moisture uptake during storage of coffee packed into compostable capsules decreases the quality of coffee brew

Compostable bioplastics for coffee capsule production should satisfy compostability requirements while providing a moisture barrier able to guarantee the espresso coffee quality. The present study aimed at exploring the time required for coffee packed in biobased PBS capsules and stored under different temperature and relative humidity to reach critical moisture levels triggering quality decay. Samples were stored in plastic boxes containing supersaturated solutions of Mg(NO₃)₂, NaNO₂ or NaCl guaranteeing 54, 65 or 75 % RH, placed at 20, 30 and 45 °C in thermostatic incubators. During storage, the coffee powder was analysed for moisture uptake and water activity, and the coffee brew was extracted to measure the pH, selected as the quality indicator. Over 18 months, moisture uptake rapidly increased, reaching critical levels within 3 weeks in the worst-case scenario (i.e., 45 °C and 75 % RH). The evolution of pH presented an initial lag phase and a subsequent linear decay, which were respectively shorter (< 15 days) and faster (pH < 5.1 within 1 month) in the worst-case scenario. The findings highlight the role of T and RH in affecting coffee quality decay and emphasize the potential drawbacks of adopting biopolymer-based packaging. These outcomes could help food manufacturers in scouting new packaging materials for coffee capsule applications, evidencing the potential drawbacks of replacing conventional packaging materials with biobased ones. In this regard, it is recommended that a thorough cost-benefit analysis is carried out before transitioning from conventional to compostable packaging to ensure sustainability goals are effectively met while maintaining product quality.