Arabica Coffee Cherry Cross Section

A “sciency” look at the fruit of the coffee shrub, and the endosperm AKA “coffee bean” we so love to roast.

I created these images from macro photos I took. It is a cross section cut of a coffee cherry, with information gleaned from the article Coffee Bean Physiology that we have in our academic coffee article archives.

I find it really informative to look at the coffee plant, the fruit and the seed from this perspective. We are producing this imagery as a postcard, one we ship out with orders for the coming month.

The “coffee bean” we roast is a food supply for the nascent embryo of the coffee plant. It’s not about us and our love of aroma, flavor and caffeine! Caffeine functions for the plant to ward off insects and fungus, and the seed is just a “food reserve” to support the embryonic plant since the embryo itself cannot store it’s own.

You can click on these to see the larger image size:

The “bean” of the arabica coffee cherry is a seed from this flowering tree, but given that we call a peanut a nut, and a tomato a vegetable, I suppose it’s par for the course. In fact, the coffee fruit was most often called a berry in early literature, and now it is most commonly called a cherry, which it is also not.

The wikipedia states “the coffee fruit is also a so-called stone fruit,” which does not sync with my understanding of a stone fruit as having a single “pit” / stone/ seed. Other sites define it as a drupe, which essentially means the same thing: stone fruit.

“In botany, a drupe (or stone fruit) is an indehiscent fruit in which an outer fleshy part (exocarp, or skin, and mesocarp, or flesh) surrounds a single shell (the pit, stone, or pyrene) of hardened endocarp with a seed (kernel) inside.

Coffee doesn’t seem to conform entirely to this definition as it has almost no flesh and has 2 seeds that are not encapsulated in a single shell,nor is that shell “hardened”. I would be interested in other opinions on this (comment below please). I am no botanist … just very interested in coffee. -Thompson

6 Responses

  1. The classification system of fruits by botanists is something imposed on plants by humans; not all plants fit nicely into the categories we came up with. As I understand it, C. arabica can occasionally produce some fruits with 3 seeds (Ukers (1922) – and, of course 1-seeded peaberries. I found it interesting that Ukers says that peaberries come from flowers at the ends of branches. I had thought that it was due to insufficient pollination. Have you hear this before?
    Yes, the endocarp is suppose to be hard and stony, but parchment is much less so. Yet, it is certainly harder than the endocarp of a tomato, which is as fleshy as the mesocarp.
    In one of the labelled photos, pectin is given, as well as endocarp. I take these to be synonyms – correct?

    1. Very welcome feedback and comments. Thanks Tom!
      Last thing first …I’m pulling from a lot of different sources on this, so I can’t track down the source on pectin layer being separate from the skin/exocarp and mesocarp/pulp/mucilage. But its something I have seen in text descriptions and can see in the images too.There’s a slightly clearer, almost gelatinous fruit layer closer to the endocarp/parchment. It really seems to have less fibrous content, and the text I read stated it was a concentration of pectin. As far as the seeds … definitely the peaberry single seed cherry and triple seed are cases of a failure to develop the endosperm and in the case of 3 beans, polyspermy. (In the PB cherry you will find a failed remnant of the endocarp/parchment, a flat sliver that would have held the seed that failed to develop. Anyway, I want to be clear I am just curious and like to read this stuff but don’t feel super confident , nor have any real depth of knowledge here! And as far as pectin layer verus mesocarp this certainly could be a false distinction!

  2. Thanks Tom,
    I suspect the pectin layer is a portion of the mesocarp that is a bit different in composition than the rest (and is somewhat unique to coffee fruits). Apparently it has more pectin, a polysaccharide that breaks down as fruits ripen and over-ripen, causing them to soften (pectin is found in the cell wall and normally contributes to rigidity but it breaks down to simple sugars as fruits ripen – or are fermenting in a dry process).

    Thanks for the comments on peaberries. Another thing I’ve always wondered about (having no on the ground experience with coffee production) is why peaberries tend to be offered from only certain countries, like Tanzania and Kenya. Are they just not sorted out in other countries or are they not found when picking?

    1. I realized Tom that the cherry in that photo is one I traveled home to California with from Indonesia, and likely was degrading a bit. Your comment reminded me that there might be some decomposition in this particular fruit making that pectin layer more defined visually. I’m not sure really… Yes the pebearry question, basically one of these seed ovules failing to develop, is interesting too! I was on a farm in Indonesia and someone pointed out a “peaberry tree”. I laughed because I never heard of this but they were right in a way… a very high percentage of PB. But I also noticed the tree was on the embankment edge of the road and its root system was likely restricted because of it. I know it is the same in places like Tanzania ad Kenya. Poor root structure, and varieties with poor tap roots, produce more peaberry, and so called Elephant beans. And yes its true that I have seen more peaberry at the end of branches. So I think these issues with poor roots and uptake from soils, or just poor soils, and poor delivery in the plant to extremities might be all contributing. I’ll see if I can find any real support for these observations!

  3. Tom,

    great post and interesting to me as a photographer and as a roaster. It raised questions in my mind (as all good posts do) about the anatomy of coffee, roasting, and flavor. My questions are: I know in some recipes I’ve seen, like those involving lotus seed, that the sprout form the seeds have to be removed or the bitterness they add ruins the recipe; does the presence of the embryonic plant greatly effect the flavor of the roasted coffee (is this s source of bitterness)? On the practical side, I wonder if there is really any way to know if a coffee bean has “sprouted” or if it has a developing embryo before it’s roasted or even after, as it seems such a test would be destructive.

    Similarly, do we have any information or understanding of which flavors the other layers or structures add to the mix?

    1. There are a few really interesting articles I put in our library of research papers about the coffee seed and changes in the development of the embryo that might have an impact on quality. Dirk Selmar is the author on these. There is one titled “Biochemical Insights into Coffee Processing”. But it’s not really about what you are focused on because the coffee embryo never is triggered to develop in either wet processing or dry processing. I think it’s like this: coffee is a fruit that propagates seedlings around a mother plant when the fruit falls, and reaches a stage of decomposition of the outer layers that then triggers the embryo to activate. But when picked and processed, it never has time to reach that point before it is dried down below the level of being a viable seed. When they produce seed from coffee cherry, they do actually pulp off the skin, and sometimes wash it too, just like the wet process. But they never put it out in the sun – they dry it very slowly in shade to something like 22-25% moisture. But for consumption they put it in the sun after something like 2-5 days, and within a few more days it can be down to 15% moisture, ultimately reaching 10-12% or so. At that level the embryo won’t swell and grow. BUT … it is still intact in there. In fact, take 20 green beans from us and put them in water. in a day or so you will see the start of an emerging plantlet coming from a few! But 99.9% chances are it wont grow. I have tried many times! It would be interesting to then re-dry that coffee, roast it and see what it tastes like. Really not sure … that is something I have never done, since I think the re-wetting and redrying itself would damage flavor too much to tell what else has happened… but I could be wrong! Anyway, really interesting question Lee! Thanks!

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