Grinding home roasted beans

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Grinding home roasted beans

Postby voice_of_reason » Sun Jan 04, 2009 2:01 pm

I tried grinding some supermarket Starbucks beans (just to experiment, you understand!) on the one but finest setting on my new Dualit burr grinder, and the grind looked very fine, and worked very well in the Aeropress - very little drip through to begin with, but not hard to press, and everything seemed OK.

I then tried roasting some beans (my first attempt!) in a Prima popcorn popper - some Brazil Camocim Iapar from HasBean - did a medium roast, as suggested, left to de-gas, ground on exactly the same setting (one up from finest), but in the Aeropress I found it practically impossible to press, like the grind was way too fine, and I also ended up with about half the amount of liquid in the cup compared to the 'cheap bean' grind, as though the coffee grounds had retained a lot more of the water. The coffee still tasted great (lovely nutty flavour) but was perhaps a little weak, probably due to the difficulty pressing it and 'losing' some of the water in the coffee grounds.

So why would this happen when I ground on exactly the same setting on both? I'm new to this, so maybe I'm missing some basic coffee princple! Could it be caused by under/over roasting?
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RE: Grinding home roasted beans

Postby lukas » Sun Jan 04, 2009 6:23 pm

Welcome to the madhouse (if I forgot that before :))! The reason for this is that you compare probably very old and stale coffee that needs a very fine grind to be extracted well (if at all possible) to fresh, homeroastet coffee, that tends to need quite a coarser grind than the old supermarket stuff. Try a coarser grind, and you might find you need to tighten up the grind a little over the next days as the coffee ages. Welcome to the world of fresh specialty coffee :)
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Re: RE: Grinding home roasted beans

Postby voice_of_reason » Sun Jan 04, 2009 6:30 pm

Thanks for the welcome - and thanks for the tips! I assumed that no matter what I ground, if I did it to the same fineness on the same setting it would behave the same in the AeroPress - because it would be the same fineness each time, whether it was old stale coffee or nice freshly roasted beans.

Thanks again - it's all a learning curve!
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Postby JulieJayne » Wed Jan 07, 2009 10:48 am

Furthermore, some coffees need a finer grind than others, even if everything else is the same.

From my experience, good Brazil, Java, Ethiopia all need a coarser grind than normal, to produce the best results.
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Postby voice_of_reason » Wed Jan 07, 2009 11:37 am

JulieJayne wrote:Furthermore, some coffees need a finer grind than others, even if everything else is the same.

From my experience, good Brazil, Java, Ethiopia all need a coarser grind than normal, to produce the best results.


I can definitely understand getting better results depending on how finely you grind different types of beans from a taste point of view. I just assumed that no matter what I ground, if I always did it to the same fineness it would behave the same in the AeroPress, i.e. it may not taste its best, but it shouldn't be any easier or harder to press.

From my first experience though, that's definitely not the case!
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Postby JulieJayne » Wed Jan 07, 2009 12:17 pm

Taste is only a part of it. I have customers who own, good expensive brand name machines, who always complain that what we grind in the shop is not fine enough. (Yes they should have grinders at home!)

But if I can once get them to try the right coffee (Java, Eth. Limu) then they come back sheepishly asking if we can grind a little coarser, as it choked their expensive espresso machine!
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Postby Tristan » Wed Jan 07, 2009 1:27 pm

The way that I understood it (which might be wrong) is that lighter roasted coffees usually need a coarser grind because they contain more water and are less porous.

And as already said, fresher ones have gas issues too.
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Postby voice_of_reason » Wed Jan 07, 2009 4:45 pm

Tristan wrote:The way that I understood it (which might be wrong) is that lighter roasted coffees usually need a coarser grind because they contain more water and are less porous.

Thanks for that, that's a good, scientific reason - I can relate to those! To me, that would explain why it's harder to press it even though it's ground to the same fineness.
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Postby bruceb » Wed Jan 07, 2009 6:43 pm

As good as "scientific reasons" may appear, they may also be dead wrong (as v_o_r said). The difference in moisture content of lighter roasts vs. darker is minimal. The effect is one of caramelisation of the sugars and liberation of oil, among others. It is very difficult to generalise about which beans need to be ground finer or coarser. In the end it always comes down to an empirical science.
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Postby Tristan » Thu Jan 08, 2009 8:51 am

bruceb wrote:As good as "scientific reasons" may appear, they may also be dead wrong (as v_o_r said). The difference in moisture content of lighter roasts vs. darker is minimal. The effect is one of caramelisation of the sugars and liberation of oil, among others. It is very difficult to generalise about which beans need to be ground finer or coarser. In the end it always comes down to an empirical science.


Ok, so I probably ought to check out the science... referring to Espresso Coffee: The Science of Quality.

Assuming that you're sticking with the same grinder and dial-in it looks like the other main variables are:

Variability of the coffee - Varietal, processing, origin, all contribute to the hardness and density of the bean.

Roasting degree - Overall expansion of the bean, tenacity and elasticity, water vaporisation, dry mass loss. As a rule of thumb (IIRC) African coffee expands more than S.American?

Gas - Effects the stability of the grind.

Moisture of the bean - Either in relation to degree of roast, or environmental moisture (storage).

The water content of a green is usually somewhere around 12%, that drops to 2-3% for a medium to dark roast, but I wonder what the water content is (on average) at first and second crack? Because the numbers are so small it could be as much as double h2o% between the two, which would imagine does play a role in the grinding?
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Postby bruceb » Thu Jan 08, 2009 10:21 am

Some years ago I did some experiments using NMR to determine the differences in moisture levels of roasted coffee. Thermogravimetric methods (moisture balance, scales, etc.), used in measuring moisture of greens, are inadequate for these measurements as the moisture levels are so low.

I quickly came to the conclusion that the moisture levels in roasted beans depend almost wholly on the ambient moisture levels, ie. the atmospheric humidity. Since I could not roast beans in the lab where the nuclear magnetic resonance equipment is located it took too long to get the ground roasted beans into the machine and consequently the moisture was always the same, regardless of whether the beans were roasted to 1st crack, just to 2nd crack or well into becoming charcoal. Roasted beans are very hygroscopic and pick up moisture instantly after roasting. To my knowledge the best studies on this topic are being made at the ETH in Zürich, but I don't have access to the data or the time or interest to follow them up closely. We use "rehydrated" beans to make our coffee in any case.

The roast dependent weight difference that is easily determined by weighing the beans before and after roasting appears not to be so much varying loss of moisture as it is conversion of solids into gaseous (carbon dioxide, etc.) products.
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Postby Tristan » Thu Jan 08, 2009 5:16 pm

Very interesting, I suppose it makes sense that the lower the moisture level of the bean gets, the more it is affected by environmental humidity. I used a moisture reader when I was processing the Eden Project coffee, I wish i'd played around with it a bit more now.

Agreed: Weight can't be directly correlated to moisture.
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Postby bruceb » Thu Jan 08, 2009 6:19 pm

In fact, moisture plays a huge role in the price of coffee, but only in regard to greens. When buying tons of unroasted beans a couple of percent difference in moisture means $$$. That's why all of the moisture balances are aimed at green values.

When I was young I worked for a company that made products from maize. Starches and dextrins were the main products I worked with and measuring moisture was very important. We used special balances with infrared heaters above them, put a certain amount of the product in a pan and pressed a button. The machine measured original weight, then turned on the heater to dry the product and after a given time provided a reading of %H2O. If you turned off the heat and let the dried product sit on the balance you could watch the moisture being reabsorbed. That happened in a very short time and the process with coffee will be the same.
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I decided I needed a bit of a change so I roasted some Monsooned Malabar. That was a change!
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