Think again about full

batchsize
harvesting green coffee

Why you should use 100% of your roaster’s capacity

First of all, thank you Philipp Schallberger. I am a big fan of my friends from Basel, Switzerland: Kaffeemacher:innen. They always share nice input and it’s not always just about coffee.

His recommendation was to read the book: The Carbon Almanac.

Immediately it reminded me to one urban myth that I have been hearing all over the world: “You should roast at 50 % – 75 % of your roaster’s capacity.” When and why I personally don’t agree and encourage roasters to think about it (again).

What does it mean to roast full batches?

Your roasting machine is usually designed to roast (up to) a specific amount of green coffee beans. As always you could argue: it depends. That’s always pretty right when we talk about coffee. Some would talk about the volume/ density of green coffee, others would argue that roasting full batches would burn your coffee, others would say: “But I read that I need specific drum speed for a specific batch size, but my RPM is fixed so I always use 7.3 kg of green coffee”, etc.

What if I would tell you, what roasting a full batch also means: save time, save money, save the environment.

Let’s say you’re also using a 12 kg roaster like we do and you have to roast 5 different coffees in a roasting day:

  • Coffee A: A traditional 83 points coffee, 48 kg
  • Coffee B: An Espresso blend, 50% – 50%, 48 kg
  • Coffee C: A Typica Kombucha processed from Colombia 88.5 points, 8 kg
  • Coffee D: A Geisha from Panamá 91 points, 6 kg
  • Coffee E: Another specialty coffee at 87 points, 8 kg

 

Save time, money and gas

You usually roast 50% of your roaster’s capacity – 17 batches at 6 kg and you dare to roast 2 batches with 8 kg. You roast let’s say 12 minutes per batch, your between-batch-protocol is 8 minutes, including your destoner, that means you have 6:20 hours of pure roasting time – no break, no lunch.

If you would have roasted with your full batch capacity (at least roasting coffee A & B), you would have roasted only 11 batches. Your total roast time would have been around 4 hours (full batches take usually slightly longer than 50% batches). After that you would have had lunch, cupped the coffees from the day before, tried them in Espresso and Filter and would have saved almost 33% of the gas.

What does batch-size mean for coffee roasting?

If you have ever cooked rice for one person and suddenly for let’s say 10 people, it usually fails at the first trial. But should you stop trying to cook bigger amounts of food at a time? Probably not.

The bigger the batch size, the slightly longer the roast time, but shorter between-batch-protocols.

The smaller the batch size, the faster you can roast, the longer the between-batch-protocol and more often you have to roast.

On a 12 kg roaster you can roast a 5 kg batch as slow as a 12 kg batch. But usually you will touch the limit of your roaster, when you try to roast a 12 kg batch as fast as a 5 kg batch.

Apart from the actual roast time, batch size can also be important for the rotation speed of your coffee: if you sell a lot from one specific coffee, it’s better to roast big batches to save time and money. If e.g. more expensive competition lots don’t sell so fast and you want to guarantee freshness, it might be better to roast smaller batches.

At Desarrolladores de Café we are using a Diedrich IR-12 using basically three batch sizes: 5 kg for coffees that we sell small amounts, 8 kg for coffees with medium sales, 12 kg for our most commercial classical Colombian coffee: the Sancocho Project.

Is it always good to roast half batch/ full batch?

If you’re roasting a 90+ points high end super competition specialty coffee without any experience at roasting full batches, it’s probably a really bad idea to start roasting full batches.

Sometimes, machines might be limiting the roast speed and it can be a good idea to use smaller batch sizes, if you wish to roast extra fast and you touch the limit of energy transmission via conduction/ convection/ radiation or experience other roasting defects.

Sometimes, your coffee has undergone such an intense processing and it resulted in a super fragile bean and when you roast a full batch you end up with burned, smoky flavors, or other roasting defects.

But most of the roasteries I know sell most of the time something between 80-86 points coffees, which are really good, decent, nice, but not super complex and delicate coffees. This is where I recommend to think about roasting full batch size. This also means to understand your roasting machine, energy principles, and all the variables of your green coffee.

How to roast full batches

My personal approach to coffee roasting is pretty technical: measure your green bean variables, understand the coffee processing it has undergone and know your roasting machine. But here are some facts:

  • A bigger batch size needs more energy (conduction, convection, radiation)
  • A bigger batch size needs more time to cool down
  • A bigger batch size needs more control to not cause roasting defects
  • A bigger batch size saves time, money and most of the time gas

What we do different when roasting full batches

  1. We charge at higher temperatures (up to 15 °C more than 8 kg batches, depending on the coffee variables)
  2. When repeating the curve we double check our bean temperature probe, air in and air out temperatures (thank you Cropster)
  3. We roast around 11:30 – 13:00 minutes instead of 09:40-11:00
  4. Our development time is slightly longer
  5. End temperature stays pretty much the same
  6. Cooling takes slightly longer

Our Cup profiles compared to 8 kg Sancocho roasts

When we started roasting full batches (sometimes up to 18 kg, if we anyway need a super slow roast, e.g. for aged coffees from clients), we always compared via blind cupping to not sacrifice cup quality.

For our standard Colombian coffee (83+ points) it has been a really good development:

Slower roasts helped us to improve the coffee’s body and sweetness, with slightly less acidity and slightly less complex aromatic compounds.

Please, roast full batches for most of your coffees

I encourage you to start using your roasting machine’s capacity especially for the environmental impact. It also can improve your flavor profile, lower some astringency, improve body and sweetness and might hit your customer’s gusto for coffee. Feel free to share your experiences while using full batch sizes.

If you’re worried about quality

We’re happy to help you out. Change is many times scary and needs courage and adjustments in your processes. Check out our next roasting course dates and join the environmentally friendly roaster crew!

Our next course dates

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