How important is fermentation?

I believe rum is made in fermentation. As a distiller you can’t really change the science of how a still works – it was invented hundreds of years ago. What changes is what you look for, and what you look for depends on what your market demands or requires. If you have rubbish going into the still, rubbish will come out of the still. You can’t distil quality from a poor-quality wash. That’s why when I train distillers, I don’t train them just in how to operate the still – they must understand the fundamentals of fermentation, because that is where rum is made.

Like a cane eau de vie?

Exactly. How we treat our juice is going to dictate what we get on the other side. So cleanliness is vital, because cane juice is one of the easiest things to be contaminated. If you have cane juice in a dirty tank it will ferment, but what it will give you will be different from having it in a clean tank. We see that in, for one, the percentage of alcohol; if you’re looking for 5-6% alcohol in your fermented juice, in a dirty tank you will not get that.

The instruments used to measure alcohol in the industry is what we call an ebulliometer. It’s a French invention which speaks to the boiling point of alcohol. So you use water as a base; you boil the water and you boil the juice; then and you correlate the boiling point of water with that of the juice and you base the alcohol content of the juice from there, based on the boiling point of water and the boiling point of alcohol. The downside is that if you have a poor fermentation you’ll have compounds in there with a similar boiling point to ethanol. Sometimes you’ll think the wash has 6-7% alcohol but you’ll know theoretically, based on the amount of sugar in it, that it can’t.

Take ethanol and ethanoic acid (vinegar) or acetaldehyde for example – they’re similar boiling points. You’re thinking it’s alcohol and that you have a good wash, but it’s not.

Even from an analytical point of view you have to be careful! I believe in, rather than measuring your fermentation in such a way, measuring your pH. The pH of the juice will give an indication of if there is bacteria. Some juice starts at 5 — and sometimes you will want your juice to start higher; some varieties of cane like ‘Lacalome Red’ give us a pH to start with of 5.24 or 5.16 or whatever — the others are 4 or under. In the fermentation some people add phosphoric acid, which is food grade, to bring the pH up to 5. We don’t of course. For me, once the fermentation goes under 3, that’s where you get some bacterial formation; things like acetic acid — that vinegar smell — which you don’t want.

Notably again, the yeast is what that breaks down the sugars; it acts as an enzyme on the sugars in the juice. Whilst it acts on the sugars it’s breaking down what we call the glycolytic pathway of the yeast. And the glycolytic pathway goes back to what we call in biochemistry The Krebs Cycle. You have over 60-70 different intermediate compounds forming whilst the yeast is acting on those sugars. You have alcohol enzyme A, you have pyruvic acid and all those other compounds breaking down. Now if you don’t have it in a controlled system, some of what will form will be compounds you don’t want in the fermenters because it affects the rum. So that’s why a controlled environment is important.

Now a controlled environment is not that difficult — though it might be more costly. Our horizontal fermenters help in terms of putting less osmotic pressure on the yeast. Because imagine they’re standing up in the air — 40 feet vertically. There’s no way that the yeast at the bottom would be in a comfortable situation with that level of osmotic pressure coming down on them. So those layers of yeasts at the bottom will not be happy, and if they’re not happy they won’t break down properly. And the intermediates will form that you don’t want, and affect the profile of the rum. Whereas if it’s horizontal there’s a larger surface area to volume ratio; there are more yeasts acting on a larger surface area of juice. And that keeps it happy; less stress on the yeasts, so the intermediates that are forming are the ones that you want, to give you the quality of fermented wash that you want for distillation and your final rum.

Walk us through the process after the cane juice has come through the rotary screen.

The clear juice comes through and into the juice tank, which is 25 cubic metres in volume. We don’t allow it to completely fill before sending it on to the fermenters. Whilst it’s in there it could start fermenting. We’re not adding any yeasts at this point, so anything that goes into the juice tank is wild yeast. Our controlled yeasts are introduced as juice goes into the fermentation tanks.

Imagine if we allowed the juice tank to fill for 3 to 5 hours of milling: it would give the wild yeasts a chance to take over the fermentation. Because there’ll be competition — there’ll be all the yeasts that have come in on the cane from the field. Once we reach 5,000 litres in the juice tank I’ll prepare the yeasts and introduce them to that 5,000 litres in the fermentation tank.

We prepare our yeasts in the conditions we want. The yeast that we use is a specially-designed yeast for Rum Agricole. It’s dry yeast — dehydrated so that it lasts longer, so we have to rehydrate it to get the enzymes active again. But you can’t hydrate it with juice — you should rehydrate it with water at 34-36 degrees celsius. We’re so particular about it that if the water exceeds 36 degrees, we add cooler water. We don’t take a chance. If it’s less than 34 we’re so particular that we get a kettle and incrementally add to get to the temperature we want. It’s 150 to 160 litres of water, together with the yeasts. So the ratio of yeast to juice will be 400-650 parts per million — 400-650 milligrams of yeast for every litre of juice.

We’ll calculate how much juice we’ll ultimately want in each washback and we’ll hydrate the corresponding amount of yeast accordingly. Now as I say, the hydration is with water, because the water will go under the cells in the yeasts and start releasing the active ingredients in there. The sugar cane juice doesn’t have that ability; sugar cane juice won’t rehydrate the yeasts properly, because it’s a different pH and there are other compounds in the juice that the yeast isn’t ready for yet. So that’s why you have to rehydrate with water. If you use sugar cane juice to rehydrate the yeast it’ll shock the yeast. There’s something called denaturing the enzymes in the yeast; enzymes are living things and they want a particular pH and a particular temperature. So we rehydrate with water because to wake up those enzymes we need a pH between 6 and 7. And juice is less than that, so it would be too acidic.

When we have 5,000 litres in the juice tank, and the yeasts are rehydrated and become active we put it into the mixer and then into the fermenter. The yeasts are multiplying now; the yeast cells start acting on the sugars and multiplying. And at the same time we continue milling and the rest of the juice is entering our juice tank. And every time we reach around 3,000 litres we send it into the fermenter, so it never has a chance to start fermenting by itself in the juice tank. So that’s why we talk about ‘slow cooking’ – letting it take its time. You could leave it in the juice tank to fill up totally – but there’d be too much wild yeast and bacterial stuff happening.

A bit like leaving food out on the side?

Exactly. So we continue until we’ve filled up a particular fermenter. It’ll take about 2 hours to get the first 5,000 litres, milling about 10 tonnes of cane per hour. Each fermenter can hold 40,000 litres, but we try to work them at 30,000. Because if we fill our fermenters and then realise we’ve got just a couple of tonnes of cane left from the particular field we’re working on, there’s no point putting it in a new fermenter. Our temperature controls kick in only when we’re at 9,000 litres, so if you only put a tiny bit into one of them, the recirculation and the temperature control won’t take place. So we make sure there’s space in the fermenters to add any final little bits below that amount. And the reason we programmed it that way is that the minimum batch for the pot still is 8000 litres.

Firstly, how does that incremental addition benefit the yeasts, and secondly what are the conditions of Renegade Rum’s fermentation?

The advantage of having that incremental policy is that when we add that first 5,000 litres the yeast cells start multiplying. You want the yeast cells to multiply to give you more than 150 million cells for when the rest comes in. The yeasts start feeding on the sugars and multiplying and in the process converting the sugar to alcohol. The sugar is the substrate, the yeasts are the enzyme. The yeast nourishes itself so it multiplies, and what you get from the sugar is alcohol. Just as when we digest we get energy. The benefit of the policy is propagation. We want to create a larger number of yeasts so that when more juice comes through with more sugars there are more yeasts to act on those sugars. You’re not giving the initial number of yeasts too much food all at once.

Yeasts by nature work best between 32-38 degrees celsius but we try to balance it between 32-36. So this is where temperature control comes in. When the yeasts are acting upon the sugars it’s exothermic; it gives off a lot of heat. So I set the fermenters so that when the temperature reaches 32, the temperature control kicks in. If it goes higher than 36 the coolant starts circulating to bring it down to 32.

And what would happen if the temperature went too high — in terms of fermentation and final flavours?

If it goes too high the yeast cells will die. There are two things that affect the enzymes of the yeasts — temperature and pH. If it exceeds 38 degrees it will be too hot and it will denature the yeasts. The yeasts will be in there, but they won’t be able to act upon the sugars. If you imagine the sugar as a particular shape, the yeast will mould itself in a way that it can feed directly inside that shape, so it can act upon the sugars. Above 38 degrees, if it’s become denatured; it’s as if it’s not able to make that shape; to fit directly into the sugar, so it’s not eating efficiently. So if the yeast becomes denatured, by temperature or pH, it can’t work properly. There’s an optimum pH and optimum temperature. Also, going back to the ‘slow cook’, you don’t want too much alcohol to form too fast. Because if there’s too much alcohol that can also denature the yeasts. You want alcohol to form from the sugars, but if you have very high alcohol in the wash it will denature the yeasts before the yeasts have acted on any remaining sugars. Even if there’s remaining sugar in the wash.

That’s why we do what we call a viability test. After a certain number of hours we’ll take a sample and look under the microscope to count the number of yeast cells we have and the number of active cells. Once it’s acted on the sugars the yeast will die, so in the sample we’ll see dead yeasts and active yeasts. And we call the viability high if we have plenty of active yeasts to act on the sugars. If the viability is low then we know that the fermentation is coming to an end. So that’s one way that we gauge the fermentation; measuring the viability of the yeast cells.

So what you’re trying to do is make sure there are enough active yeasts cells for long enough to fully interact with all the sugars to create the most flavours? It must vary depending on the terroir and the variety, but what kind of times are we talking about?

You can have fermentation between 24-36 hours. You’ll have had some fermentation before that, but we’re talking about a complete fermentation. You can have a rapid fermentation in 24 hours but we don’t want rapid fermentation, so we go 36 and beyond. We try and control our yeast to give us a minimum of 36 hours and up to 96 hours.

It’s also worth noting that whilst we can denature the yeasts at excessive temperatures, we can make the yeasts inactive at lower temperatures. If it’s too cold the yeasts will shut down. So if we’re away for the weekend, or for two days or three days, or if we find a particular fermenter is going too fast, we lower the temperature, which we can just do on the computer by monitoring. And if I want the yeasts to be inactive I can just reduce the temperature to 30 degrees, shut the yeasts down, then come back and activate them again. That’s how we try and control the fermenters.

So juice pH is dropping through fermentation and if it’s below 3 you want it to stop — but what is the pH range of cane generally?

It starts at 5. Lacalome Red and Purple Tallboy have higher pH and brix to start. But there can be a lot of terroir variation too. After every run they’ll send me a pH and brix report. So you can see two examples from the same farm here:

Farm: Dunfermline
Field: George
Variety: Lacalome Red
Tons milled: 40.56

Washback 41
Start brix: 13.3
Start pH: 4.18
Volume: 38,613L

Farm: Dunfermline
Field: De Gale
Variety: Lacalome Red
Tons milled: 29.30

Washback 42
Start brix: 14.6
Start pH: 5.04
Volume: 34,395L.

We’ve talked a lot about processes — let’s talk about what’s happening flavour-wise. What are you looking to see, what are you looking to avoid?

Obviously we’re looking to see that we’re getting the right flavours. To monitor that we have what we call the life cycle of the fermenter. Our process operators are working 24/7 in shifts, and every four hours they will take a sample. And we’ll monitor the brix, pH, temperature, basic nosing and so on. And from that we’ll see if we’re getting the flavours that we want. So if Ifwe want more aromatic flavours we’ll leave the fermentation a bit longer to get the phenols, the higher-chain alcohols. We don’t do high-ester rum; our ester content is less than 80-100ppm. But if we wanted a little more, to get a bit of that vanilla or banana, we might leave it a little longer. Once you form alcohol, the alcohol starts oxidising into acids. A little of that is good, and the molecule chains can combine to give you the ester — the banana and vanilla notes — but if you go too far you can get those vinegar smells.

We also have the aldehydes, some ketones, butanol, long-chain alcohols, then branch-chains that give you the heavier flavours. Then there are the volatile acids. This is the one that you want to get out — it’s part of the glycolytic pathway of the yeasts. It’s easier to get out during distillation, because these volatile notes go up the column or the pot faster. And so some of the notes are converted to the grassy or lemony kind of aromas. I think because we’re working with the agriculture, it’s all coming directly from the field, you get that grassy kind of a nose. But you can look at the wash itself to see that you have this nice green, clear wash with the right coenzymes A and B forming. We test the pH every 4 hours because if you have bacterial fermentation happening, the sort that we want to avoid, you can see it, you can smell it and the pH will give you a clear indication.