“It’s all about sight and sound,” said Kai Cairney, chief roaster at Gordon Street Coffee.

He has been roasting coffee beans at the coffee shop for 18 months, and since December has been solely in charge of Deirdre – a large, red, roasting machine which stands in an upstairs corner of the Glasgow Central Station cafe.

“It’s all a lot more sciencey and complex than I first thought,” Kai said.

“The way the beans react to the heat, the drum itself, the paddles.

“It can be really high-tech, some people are using laptops and everything now to roast their beans.”

As Deirdre hums away, rotating a batch of beans Kai put into the machine earlier, he explains how the Gordon Street Coffee roasting process works – without laptops.

“Our green beans go up here, in the top, they get dropped into the drum,” he said.

The “green beans” are unroasted coffee beans, pale in colour, and in varying shapes depending on which region they are from.

Gordon Street have two blends; a Glasgow roast and a house roast which use different combinations of beans.

“We’ve got all our different beans together, our Guatemalan, Brazilian, Indian red cherry, and Kenyan beans, we’ll mix them all together.

“I won’t tell you in what quantity though, that’s our trade secret!” Kai said.

Before Kai can pour the beans in the top, Deirdre needs to be heated to around 150C by two infrared burners either side of its drum.

Once the beans are in the drum Kai has to keep a watchful eye on the temperature and alter the airflow, which affects the taste of the coffee.

“We’ll change the air flow to change the chemical reaction process, remove the unwanted chaff and CO2 from it,” Kai said.

The chaff is the coating which separates from each bean as it’s heated, it can be used as compost in gardens. Kai said the cafe has been discussing options to donate the chaff from their beans so it is no longer just a waste product.

When the drum reaches around 190C Kai changes the air flow again so it blows through the drum.

“That will slow down our chemical reaction and will really burn off our amino acids and get our reaction of complex sugars and everything else,” Kai said.

Once the beans start to make cracking and popping sounds Kai knows they are nearly ready to come out, but timing is key.

If they are roasted too lightly – for too short a time – the coffee may taste acidic, but roasted for too long it can taste burnt.

Gordon Street’s most favoured Glasgow roast is left in for a few seconds longer than the house blend, “it has a different taste, it’s stronger,” Kai said.

When the beans are ready, Kai pulls open the hatch and the chocolatey-brown beans flow down into the secondary drum below.

The airflow is switched to this second drum and large paddles push the beans round in a mesmerising clockwise motion to cool them down.

“We have to cool them as quickly as possible,” said Kai. “Else they just keep roasting.”

Deirdre, with the help of Kai, is able to roast around 100 kilograms of coffee beans a day. The freshly roasted beans are ground to make coffee in store for customers, but also sold in bags on their shelves and online.

“We sell around 4,000 cups of coffee a week and 350 bags of coffee,” Kai said.