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I bought a bag of Guatemala/Colombia/Ethiopia blend after having single origin Peru beans (all Arabica), and was struck by the difference in bean size: Peruvian beans were almost twice larger than the beans in the blend.

Now I'm wondering whether the difference is conditioned by some environmental factors, or regional differences of the coffee plant, or the timing of harvesting. Of course, it can also be that any coffee plant anywhere produces beans of (significantly) different size, and then they are sorted into size-consistent batches (as per this guide, for instance).

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The single biggest factor in the size of a coffee bean is the botanical variety of the plant. There is no amount of nutrition or growing conditions that can overcome the effect of genetics.

In this picture from our garden at Coffea Diversa you can see two examples at the extreme.

  • On one hand you have mokka, this variety has the smallest bean of all coffee varieties.

  • On the other hand you have Maragogype, this variety has the largest bean of all coffee varieties.

There is no amount of fertilizer that you can give to a mokka grain to make it even come close to the size of a ripe Maragogype bean (and viceversa).

The difference in sizes between your blend and peruvian coffees are probably due to the botanical varieties of the beans.

Mokka vs Maragogype

enter image description here

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    Seem like what I was looking for, thanks! What a nice illustration.
    – Ivan Kapitonov
    Oct 12, 2015 at 22:06
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    @IvanKapitonov I would like to add a chart that I have encountered at another site, just for reference. Please also see that one, it visualizes a nice set of differences among beans.
    – MTSan
    Jun 10, 2016 at 9:10
  • @MTSan That's a great chart! I'll add it to my answer.
    – PabTorre
    Jun 15, 2016 at 17:09
  • This isn't entirely true. Different varietals will prefer different conditions. While it's true that there are no conditions under which a mokka plant will produce maragogype sized beans, there are poor conditions under which a maragogype plant may produce undersized beans much closer to mokka beans. There is also the fact that plants can be selectively bred to produce vast differences over time. While no plant can escape it's genetics, the expression of it's genetics is reliant entirely on the growing conditions it is given.
    – Suspended User
    Feb 25, 2020 at 16:52
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There are a large variety of factors that can affect bean size. Obviously growing conditions can vary, and there is often variation of size within a single crop. Keep in mind coffee is an agricultural product. No one blinks when a crop of apples or oranges has a great variety in sizes, but many people seem to prefer their coffee beans a more uniform size. Most places accomplish this with sorting but it is artificial to some extent.

On the other hand, roast level is a controllable process that greatly affects bean size. Beans expand and become less dense as they are roasted and lose moisture. It is entirely possible that the same crop of sorted beans (all relatively uniform in size) will have different sizes due to variation in roast level.

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  • Thanks! As for apples (and maybe oranges?), there are different sorts of them at least. But +1 for the roasting, it didn't occur to me that they become larger as they roast. My bigger beans were a bit darker indeed!
    – Ivan Kapitonov
    Oct 9, 2015 at 0:16
  • Sorry, Chris, had to withdraw the acceptance :-( See PabTorre's answer.
    – Ivan Kapitonov
    Oct 12, 2015 at 22:08
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Just to add one more variation, which is a little unusual. A Monsooned Malabar is also a very large bean. However, this is because the green beans, already bagged, are left to "air" in the moonsoon winds and they absorb moisture and expand to about twice their original size and also change to an ivory colour.

I haven't had Monsooned Malabar for a long time but I don't recall them expanding a whole lot more in the roasting. (The airing process results in them roasting quickly and they move very rapidly from first crack to second crack.)

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  • Very interesting! Will be keeping an eye out on it.
    – Ivan Kapitonov
    Jun 16, 2016 at 10:31

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