At 8 a.m. on Saturday and Sunday, the thermometer in Brussels read almost the same: 14.3°C (57.7°F) on Saturday, 13.8°C (56.8°F) on Sunday. Going by that number alone, the first batch of coffee roasted outdoors started from the same place on both days. The air said something else. Here is what it said, and what you can take from it for the grinder in your kitchen.
Why the first batch of coffee roasted outdoors tells you the most
In the morning, Golden's metal is cold when the flame goes on. The first batch is the only one that starts in a cold machine, after the preheat we described in our piece on the three weather variables. In cold weather the gas bottle matters too, which was Friday's subject in the gas roaster in cold weather. Today we stay with the air itself: the first batch is where the morning weather shows most clearly.
A guide Perfect Daily Grind published on 17 September asks roasters to preheat the same way every day and to watch the start of the roasting day closely. The same guide notes that cold mornings and humid afternoons change how heat moves through the roaster. That advice is written for indoor roasteries. Outside, the 8 a.m. air changes from one day to the next.
The first batch of coffee roasted outdoors on 3 and 4 October
Figures for Brussels at 8 a.m. (Open-Meteo):
- Saturday 3 October: 14.3°C, 71% humidity, almost no wind (2 km/h, about 1 mph), fully overcast.
- Sunday 4 October: 13.8°C, 81% humidity (85% at 7 a.m.), northeast wind around 9 km/h (about 6 mph), clear sky right after sunrise.
- Both days: dry, high pressure, around 1030 hPa at sea level.
The difference between the two days sat in humidity and wind. Sunday meant roasting coffee on a humid morning: the air was ten points wetter, and it was moving. The northeast wind, light as it was, also pulls at the flame.
Laurent reads all four numbers before lighting up and adjusts the fire and the time on Golden for the same flavour target. The target stays fixed, and the road to it changes from one morning to the next. That is where the small differences between batches from the same weekend come from.
The weekend's lesson for anyone who checks the forecast: temperature is one number out of four. When you open your weather app, look at humidity and wind as well.
The water you can't see: from the first batch of coffee roasted outdoors to your grinder
The same invisible water is at work in your kitchen, this time inside the bean. As you grind, the beans rub against the burrs and pick up an electric charge. Grounds cling to the grinder's spout and the walls of the catch cup, and some fly onto the counter. That is coffee grinder static, and it is why ground coffee sticks to the grinder.
Researchers at the University of Oregon measured it in a study published in the journal Matter in January 2024 (Méndez Harper, Hendon and colleagues, DOI 10.1016/j.matt.2023.11.005). What they found: the water left inside the bean changes how strongly the grounds charge. Darker, drier roasts charge more. A little water added to the beans before grinding cuts both the charge and the clumping.
A bean taken a little further in the roast keeps less water. If a new bag throws more dust than the last one, the cause usually lies with the bean, and your grinder is fine.
The drop test: after the first batch of coffee roasted outdoors, your grinder's turn
Baristas call it RDT (Ross Droplet Technique). The RDT coffee method takes two minutes:
- Weigh 15 g of beans and grind them as usual. Tap the catch cup and look at how much stays stuck to the walls and the spout.
- For the next dose, run the handle of a teaspoon under the tap, shake it off and stir the beans with it for a few seconds. One drop of water on the beans before grinding is enough, roughly 0.1 to 0.2 ml for 15 to 20 g.
- Grind and compare. If the grounds fall cleanly into the cup, you have found the cause.
Three rules so the test leaves your coffee intact:
- Wet only the dose you are grinding right now. The beans in the hopper stay dry.
- For espresso, check your time again after the drop: 18 g in the basket, 36 g in the cup, 25 to 30 seconds. If you fall outside that window, move the grinder one step finer or coarser as needed.
- If the dust shows up with one particular bag, note the roast date printed on it and tell us at the stall.
The coming weekend: the first batch of coffee roasted outdoors in cooler air
Today's forecast (RMI and Open-Meteo) shows a cooler end to the week: highs of 14 to 15°C (57 to 59°F) on Saturday 10 and Sunday 11 October, patchy drizzle or light rain on Saturday, a drier Sunday, and more wind than last weekend. The first batch will start in cooler air than a week ago.
If you come by Marché de l'Abattoir on Saturday morning (exit of the Manufakture car park, Anderlecht), ask Laurent which numbers were read before the first batch. And if you try the drop test tomorrow, tell us what changed in your grinder. We are there on Saturday and Sunday with coffee roasted outdoors in Anderlecht, and this week's batches are also in the shop.
Brut. Sauvage. Imprévisible.
