Friday, 22 July 2016

The best coffee?

Like many people, I enjoy drinking coffee. My preference is for lightly roasted beans and I don't mind whether the coffee is made using a French press, a filter, or a commercial coffee machine. 

World-wide demand is such that many countries now produce coffee beans and the resultant crops vary in quality, depending on whether they are grown at altitude, as an understory in forests, in intensive plantations, or by other farming methods. Coffea arabica, the most widespread coffee plant, comes originally from north-east Africa and its global spread has brought problems, with much effort being given to controlling coffee borer beetles and other pests. Spraying of the crop is sometimes needed as there is often no natural biological control of the pests, and monoculture provides optimal conditions for pest transmission.

Ripe "cherries" are red, those that are over-ripe are dull brown-red and those that are yet to ripen are green (see above), so hand picking is often the best method of collection. The bean that we roast is contained within the cherry and, in the picture below, a cherry has been halved and the two beans it contains partly removed. The pulp and mucilage that surrounds each are obvious, as is the white parchment-like coating of the bean itself.

Coffee beans only develop their flavour upon roasting so they must first be extracted from the cherry.  As in many fruits, the soft pulp evolved to protect the bean (seed) from drying, or abrasion, and the mucilage layer provides an extra barrier that also allows easy penetration of the shoot when the seed germinates. Mucilage has many functions in Nature [1] and the layer is important in allowing the production of the famous Kopi Luwak from Indonesia [2]:

Although Kopi Luwak.. ..comes from the Indonesian islands of Java, Sumatra and Sulawesi, it is not its exotic location of origins but rather its unusual and quite unexpected method of production which contribute to its mystique and price. The desire to consume unique food products is a characteristic of passionate coffee drinkers.

So, what is the method of production of Kopi Luwak? The skin and pulp of ripe coffee cherries are naturally sweet and are thus attractive as foods for a number of vertebrates. Among these, the African civet feeds on coffee cherries in north-east Africa, the original home of the plant, utilising the pulp and then depositing the beans, protected largely by the mucilaginous coat, in their faeces. This passage is the basis of the production of Kopi Luwak, and begins with the collection of faeces of palm civets (below, upper) that climb trees and eat the beans in situ. They would originally have fed upon the many other forms of fruit available, but the arrival of coffee plantations provided a new, and concentrated, source of food. Coffee beans are retrieved from the palm civet faeces (below, lower) and are then washed and prepared for export. The collection of faeces from the wild has now been superseded by a battery-farming approach [3], with caged civets being fed coffee cherries, but the beans that have passed through civets are still a very tiny part of the total market for coffee beans. The same is true of coffee beans that pass through Jacu birds [4], that rival the fame of Kopi Luwak.

Ingestion of fruits results in the dispersal of seeds, but there are many other methods for this essential process that are more familiar to us:

- Coconuts falling into the sea and being carried large distances to other islands
- The winged seeds of sycamore that we used as "helicopters" when we were children
- The "clocks" of dandelions, with each seed having a parachute
- The explosive pods of gorse that fling seeds large distances
- Seeds of avens with hooks that attach to animal fur

These are just a few examples of a large number of dispersal mechanisms that have evolved and all readers will know of others. However, the passage of seeds through animal guts is less familiar to us, yet we eat many berries and pay little attention to what happens to the seeds they contain. The purple droppings that result from birds eating blackberries, elderberries and other fruits are more familiar, especially to car owners in late summer, but we are less aware that the droppings often contain seeds and that these are deposited in fertiliser, far from the parent plant.

Anna Traveset has reviewed the effect of frugivores (birds, non-flying mammals, bats, reptiles, fishes) on the germination of seeds that pass through the gut of animals and concludes [5]:

In addition to moving seeds from the parent plant to sites that can be suitable for recruitment and seedling growth, frugivore seed dispersers have the capacity to modify the germination patterns of many plants by varying the potential germinability of seeds, the rate of germination, or both.

The effect on germination is complex and depends on many factors, varying also from individual to individual. Each plant is likely to produce many seeds, so the main beneficial effect of having fleshy fruits is dispersal, just as it is for the other mechanisms listed above. All worth thinking about when eating strawberries, or when savouring a cup of Kopi Luwak, This unusual coffee is yet another example of the extraordinary power of evolution and of the ingenuity of humans in taking advantage of natural processes. Is it the best coffee though?

[2] Massimo F. Marcone (2004) Composition and properties of Indonesian palm civet coffee (Kopi Luwak) and Ethiopian civet coffee. Food Research International 37: 901-912.

[5] Anna Traveset (1998) Effect of seed passage through vertebrate frugivores' guts on germination: a review. Perspectives in Plant Ecology, Evolution and Systematics 1/2: 151-190.

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