I have seen some varieties of unroasted coffee beans listed as a dry process. What exactly is this referring to?
There are generally speaking two categories of processing methods and some hybrids of the two. You will often see either washed coffee or natural coffee.
1. Washed coffee
Washing is the newer method, usually applied in humid countries as dry processing is prone to cause mold there and has been considered the gold standard in terms of flavor since the industrialization of coffee production. For traditional small scale farmers however it's not been viable due to the high investment and maintenance costs and the need for electricity (change of organization to cooperative forms has mitigated this somewhat). In this processing method the coffee cherry is completely depulped (mechanically and through up to 36h fermentation in water) before the bean is dried. The pulp, mucilage and parchment skin is removed. Only some of the silver skin stays on the bean in this way of processing and falls off during roasting as chaff. Usually there are some chaff leftovers in the crack after roasting, which is a way to tell apart washed and natural processed coffees. Washed coffee is very true to the taste of the variety/cultivar. Only the actual coffee bean gives flavor. Washed coffee is all else equal generally more acidic and has brighter fruit flavors than natural coffee.
2. Dry processed / natural coffee
Dry processing is the "natural", traditional method to process coffee and still dominant in Ethiopia (although washed becomes more popular) and especially in Brazil, where basically all coffee is dry processed. The cherries are dried in the sun for about 4 to 6 weeks on flat beds leaving the skin, pulp and mucilage on the bean. Only after the drying is complete the cherry being hulled (that means the pulp, mucilage and parchment skin removed). During the drying the cherry is fermenting and some of the sugars and flavors of the mucilage, the fruit, are transferring to the bean. So you tend to get different flavors than with washed coffee. The coffee usually is fuller bodied, has more red fruit notes like strawberry, cherry and is higher in sugar content because some of the sugars from the mucilage transfer to the bean during the drying.
3. Hybrid methods / Semi-Washed / Honey processing
Recently some producers started combining washed and dry processes. Here the cherry is partly depulped, leaving the mucilage on. There are different stages that are differentiated by the amount of mucilage left on:
- Red honey: all the mucilage is left on for the drying process, the bean turns red, thus the name.
- Yellow honey: only a small part of the mucilage is left on for drying and the beans turn yellow.
- White honey: almost all the mucilage is removed, it's closest to washed coffee and there's no discoloring of the beans.
There are other variations due to the rise in specialty coffee and as producers get more creative and experimental. Read more here. A very informative interview about different processing methods you can find here. Read this wikipedia article for a good overview.
With the dry process, the fruit is left intact for an initial fermentation stage. Then the beans are hulled, washed, dried again, sorted and graded (as with other processes).
It's more expensive than other processing methods because of the real-estate and time and labor invested in the initial fermentation. But it seems to get more fruit into the bean. Dry-processed coffees tend to have very complex fruit and wine notes that are not found (or rarely found) in washed and semi-washed process coffees.