What is the thermal shock process?
The thermal shock process is an experimental post-harvest coffee technique in which freshly picked cherries are subjected to rapid alternations between high and low temperatures (typically 60–80 °C then 5–15 °C) before or during fermentation. This thermal shock aims to modify the cellular structure of the mucilage and selectively activate or inhibit specific enzymes and micro-organisms, producing distinct and unusual aromatic profiles.
The thermal shock process is one of the most recent and least documented post-harvest innovations in the specialty coffee community. Developed initially in micro-estates in Costa Rica and Colombia in the early 2010s, this technique draws inspiration from culinary methods (blanching, thermisation) and food fermentation principles.
The basic principle: whole cherries (or the mucilage after depulping) are immersed in hot water (60–80 °C) for a short period (5 to 30 minutes), then immediately transferred to cold or iced water (5–15 °C). This thermal shock provokes several effects: permeabilisation of the mucilage cell membranes, releasing sugars and enzymes otherwise inaccessible; partial or total inactivation of certain wild yeast strains or undesirable bacteria; partial denaturation of enzymatic proteins that modifies the kinetics of subsequent fermentation.
After the thermal shock, cherries are fermented according to various protocols: standard anaerobic fermentation, fermentation with selected yeast inoculation, or direct drying (to produce a profile between natural and thermal shock). Producers report that thermal shock amplifies certain notes — tropical fruits, honey, sugarcane notes — while reducing common fermentation defects (under-fermentation, over-fermentation).
The lack of published scientific research on this technique makes objective assessment difficult. Comparative cuppings by Q-graders on thermal shock vs standard lots from the same origin show measurable differences in perceived acidity and sweetness, but results vary across operators. The community awaits rigorous microbiological and biochemical studies to validate the claimed mechanisms.
Thermal shock process steps
- Selective picking of cherries at optimal ripeness (refractometer, hand selection)
- Immersion in hot water (60–80 °C) for 5–30 minutes depending on protocol
- Immediate transfer to cold or iced water (5–15 °C) — the thermal shock
- Fermentation (anaerobic, inoculated or wild) for 24–96 h depending on objective
- Depulping and washing (if washed) or direct drying (if natural or honey post-thermal shock)
- Drying on raised beds with temperature and humidity control