Annals of Botany

The secret of good coffee – keep it under wraps

I am sitting at my computer taking sips from a cup of delicious dark-roast black coffee and it seems appropriate that I am writing about green coffee (i.e. dried, unroasted beans), the subject of a paper by Selmar et al. (Braunschweig, Germany, pp. 31–38). The first stage in coffee manufacture is to extract the beans from the flesh of the coffee ‘cherry’; this is done by different methods that result in different flavours in the final product. In wet treatments, the first stage is a mechanical de-pulping followed by immersion in water (‘fermentation’) to remove the remaining pulp. The beans are then dried and the endocarp (‘parchment’) is removed. In semi-dry processing, the fermentation step is omitted; residual pulp and endocarp are removed mechanically after drying. In dry processing, whole cherries are dried and the pulp and endocarp are removed mechanically. In the authors’ research, green coffee was treated by these three methods under strictly controlled conditions. For the wet treatment, only half the beans had the endocarp removed. The green coffee beans were stored at 22 °C and 63 % relative humidity for up to 2 years. Viability and germination tests gave very clear results: beans with endocarp retained viability, in some instances for up to 2 years; bare beans lost viability rapidly over the first 4 months. The reason for this was unclear since it seems unlikely that the dried endocarp (parchment) could affect significantly the metabolism of the bean. Distinct biochemical changes such as slow loss of sugars and significant loss of glutamine occurred in all beans and again these changes gave no clue about changes in viability. Sensory tests showed that storage of green coffee caused a distinct loss of aroma within 6 months. Intriguingly, this loss of aroma was slightly less in the beans with retained endocarp; maintenance of viability is thus correlated with an improved retention of quality.