The microorganisms associated with dry processing are much more variable and complex than those found in wet processing (Silva et al., 2008). The processes through which mucilage is degraded in natural processing are far more complex — and less well understood by science — than the simple acidification process that occurs with wet processing (see Lesson 3.02). With dry processing, the flora are far more actively involved in mucilage removal.
At the start of the natural process, the Gram-positive and Gram-negative bacteria are dominant. Later in the process, after the cherries have begun to dry, the water activity (Aw) drops. (To learn all about water activity in coffee, read our post here.) Once the level has dropped to below 0.8 Aw, bacteria can no longer reproduce. The initial population of bacteria is then supplanted by yeasts and fungi (R. F. Schwan and G. H. Fleet [eds.], 2014).
Note: Water activity is a relatively new measurement in speciality coffee, though it is well established in other parts of the food industry. Rather than measuring the total amount of moisture in a sample, it measures how tightly bound the water is in the sample. In a sample with a high water activity, the water is less tightly bound and can therefore take place in chemical reactions or other processes more easily. In other industries, it is water activity rather than moisture content that has been shown to affect, for example, how quickly food goes mouldy or stale.
Another factor that supports the rise of yeasts and fungi is the work done by the pectinolytic bacteria. These species break down the pectins into simple sugars through fermentation. This provides the food the yeast population needs to get the upper hand (P. Poltronieri and F. Rossi, 2016).
It wasn’t until 2008 that the microorganisms in natural processing were isolated from arabica coffee for the first time (C.