Roasted coffee beans darker inside than outside

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
jb-0101
Posts: 175
Joined: 12 years ago

#1: Post by jb-0101 »

Yes that's right. My roasted beans are consistently darker inside than outside. It's very annoying.

What I mean is that when I break the bean along the crack after roasting and look at the bean, you can see that the inside of the bean is darker than the outside. Like the way you would check a steak, but backwards.

There was a thread on here started a while ago that I couldn't reply to where someone asked the same question: Inside of coffee bean is too dark

Here are a couple of pictures:



These are how the beans look from the outside. Look quite evenly roasted, and all to the same roast depth.



Here is a cross section. The darker sections on this photo are not even as dark as they look in real life - the camera hasn't shown the dark sections quite as well.
You can see on the left side close to one end there is what looks like tipping, but it's not right on the end of the bean. Still, I'm pretty sure that's tipping.
If you follow the bean along to the other side you can see that there is a slight crack along that section. This is where it is darker inside than out. Again, it's darker in real life.
Also, if you look at the section above the chaff, it's quite clear that the bean is indeed darker on the inside than the outside.

Any ideas what would cause this?

I'm roasting in a bread machine with a heat gun. The bread pan is insulated and has an insulated lid to try and retain as much heat inside the pan as possible. The fan speed of the heat gun is on low so as to not put heaps of hot air through and try to reduce drying out.

My roasts generally go for about 8 mins to 1C and another 2 min, adding about 10-12 degrees before pulled. Nowhere near second crack.

Any ideas and thoughts would be much appreciated!!

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Contrabass_Bry
Posts: 43
Joined: 10 years ago

#2: Post by Contrabass_Bry »

James,

I saw this quite a bit when I was running by HG/BM setup.

From what I've read, it appears that this is caused by ramping quickly to 1C prior to proper/complete drying. Internal scorching. Basically boiling on the inside prior to 1C.

Willem Boot talks about it in one of his Roast Magazine articles in the context of beans that have an over abundance of moisture (extremely fresh crop or improper drying protocol.) His advice is to extend the drying phase with a bit lower heat.

Best to do that with the HG still on low fan as there might be the propensity to strip too much moisture from the bean in the early stage which is also problematic.

Controling heat level with the heat gun can be accomplished either by increasing physical distance from the bean mass or by regulating voltage (I happened to use a router speed control.)

Best of luck straightening it out!



Kind regards,

Bryan

OldmatefromOZ
Posts: 316
Joined: 10 years ago

#3: Post by OldmatefromOZ replying to Contrabass_Bry »

Completely agree with this post, this was my experience also. You are definitely coming onto first crack to hot.

Try 6 mins to 150C bean temp, then 4 to 5 mins to the first crack.

jb-0101 (original poster)
Posts: 175
Joined: 12 years ago

#4: Post by jb-0101 (original poster) »

Thanks for the replies Bryan and James.

I saw that Willem Boot article - thanks for that.

So is internal scorching caused by the beans drying out too much or not drying out enough?

I got the same results whether I preheated the pan or went from room temp. Is there a preference?

So "drying phase" is generally regarded up to 150*C?

My HG has a variable temp on it so I can control the temp to any level I want which is handy. So 6 min to 150* and 4-5 min to 1C would be 10-11 min to 1C... am I in danger of baking?

And then 4-5 min to 1C, so looking for a ROR of about 10*/min after 150?

I'll give it a crack.

Thanks

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Kabouter
Posts: 57
Joined: 12 years ago

#5: Post by Kabouter »

I had a pro nearby tell me recently that such internal scorching can be caused by too much heat during the (first) exothermic stage, which IIRC takes place just before first crack. She thought that what happens is that the inside of the bean cannot dissipate it's heat energy fast enough due to high environmental temperature and so the inside of the bean progresses much faster than the outside. Disclaimer: I have no idea whether this is correct.

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boar_d_laze
Posts: 2058
Joined: 17 years ago

#6: Post by boar_d_laze »

Kabouter wrote:the (first) exothermic stage, which IIRC takes place just before first crack.
Exothermia begins just before 1stCs, and continues about half way through 1stC, during the "rolling" portion. If you're not pushing Development too hard you'll see the exothermic stage end with an inflection on your plot. Sometimes the inflection is called the "knee."

Rich
Drop a nickel in the pot Joe. Takin' it slow. Waiter, waiter, percolator

dustin360
Posts: 825
Joined: 13 years ago

#7: Post by dustin360 »

How do they taste?

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jb-0101 (original poster)
Posts: 175
Joined: 12 years ago

#8: Post by jb-0101 (original poster) replying to dustin360 »

Ashy, charred, burned, roasty - awful.

I pull the beans out of the roaster about 15* after first crack, before hearing any 2nd crack, and within 3 days there's oil on the outside of the bean.

:(

chang00
Posts: 638
Joined: 16 years ago

#9: Post by chang00 »

Generally, when the heat input exceeds the heat capacity of the coffee bean and heat transfer, charring or uneven roast results.

Looking at your pictures, and the history that you roast with heat gun in a bread machine, it appears there was not enough circulation, ie convection. The heat was concentrated on particular spots, and dispersed along the central crevice, to create the unevenness. It appears some of your beans have reached 2nd crack, and some not. Unless this is a blend, some central chaff is charred and appeared black, and some still yellow.

The heat energy during coffee roasting is commonly thought to be carried by the steam, in addition to the ability of adjacent molecules to transfer the heat to the next. In a drum roaster, the convection can be accomplished by increasing the drum or fan speed. You will have to figure out how to increase convection in a bread machine, possibly by more stirring, increase fan speed, or both.

jb-0101 (original poster)
Posts: 175
Joined: 12 years ago

#10: Post by jb-0101 (original poster) »

Thanks all,

I tried extending out drying time bit by bit, and I ended up with a total roast time of about 20 mins and I can see a reduction of interior scorching, but it's not gone completely. Cupped it 10 mins after roasting and could taste a little bitterness, but i'll cup again later.

Anything fast than that still got interior scorching

The strange thing though is that when I extend the roast times out further, I'm getting less and less audible first cracks. On the 20 min roast I could hardly hear any at all. This makes me think it's too slow and has been baked.

So it seems interior scorching happens due to the beans not drying out enough, but when I dry them out enough to stop this, they're too dry and I get no cracks and baked coffee??

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