In terms of the juice — thinking about drinks like wine where we might talk about things like acidity or tannins and certain phenols. What are the key flavour compounds in sugar cane juice?

The same! When you ferment sugar cane juice you get 56 different flavour compounds. You get flavours from the phenols of course, the aromatic chains that will give you some fruity notes. You’ll have some esters in the sugar cane, depending on how long you ferment it for and what you add to it. You have aldehydes, you have some lighter alcohols like butanol and you have some heavier alcohols — oils and so on. And then you have a lot of volatile acids.

And those volatile acids must be why you need to be quick to the mill and controlled thereafter — so that the juice doesn’t turn vinegary?

Yes, because of the sugar. It’s very reactive if it’s left in an oxidative environment. We also have some traces of phopherol because of the cane; the cane itself gives that particular compound, phopherol. And in the crushing of the bagasse you take that from the fibre of the cane. So there’s a wide range of flavour compounds in sugar cane juice and in rum — it isn’t just about esters!

Talk me through fermentation times and the ‘slow cook’ and why that matters.

Cane juice ferments very fast. After the first 10 hours or so you could have had a lot of activity happening in sugar cane juice. And it will begin fermentation by itself if you’re not careful. So as soon as the cane arrives at the distillery we clean it and then we mill it three times. The first milling is just the cane, the second is what’s left of the cane, rehydrated with sugar cane juice from the first milling, and the third is the final remnants of the sugar cane rehydrated with warm water — because it’s critical that we’ve removed all traces of sugar before the milled bagasse goes through to the biomass boiler.

So the bagasse goes to the boiler and the juice from the three millings is collected. We’ll then screen the juice to clean it and then send it to the juice tank. So there are critical control points here. That rotary screen is very important; we want clean juice coming across; juice free of any bits of the bagasse.

To make sure you’re fermenting a clear wash?

Exactly. So that screen is critical. So critical that during the process it is constantly being cleaned by a tank of water. It’s programmed so that every half an hour or so you will have it flushed for about 5-10 seconds. Constantly cleaned. Because this is one of the critical control points. Because as it comes through from the mill you will find some debris; some bits of the bagasse. You might have a bit of soil, especially if you’re harvesting in the wet season. So for me, to get the proper juice, that screen is critical. And it’s maintained by flushing that hot water. After every single run, even if you don’t wash any thing else, you wash the rotary screen. Because that’s where you can have a lot of hidden ‘dead legs’ as we call them. And then they just mess up everything. It’s a filter with a very fine mesh — less than one micron — and you can rotate it, so that it screens out all the debris.