Author: Dave Meadows is a chemical engineer and is the Technology Director at Tongaat Hulett. Dave is the author of technical papers, contributing author to two books on sugar technology and engineering, and is a recipient of the international Sugar Industry Technologists organisation’s Crystal Award for his contribution to sugar technology.
Article appeared in: South African Sugar Journal. November 2015.
Sugar isn’t manufactured. It really isn’t. As someone who has spent his career designing, building, operating and optimising sugar factories, that feels a little shocking to say – almost heretical.
But it isn’t manufactured. It’s grown. Sugar is made by plants – in fruit, vegetables and stalks – out of carbon dioxide and water, using the energy from the sun. All of the expertise that goes into making sugar comes from Mother Nature. Plants use sugar to store energy, especially plants such as sugarcane and sugar beet, which produce more of it than other plants. Nature gives us sugar ready-made. Which is a little humbling if, like me, you’re an engineer?
So, before I precipitate wholesale engineering job-losses, if sugar factories don’t actually make the sugar, what do they do?
Well, they get the sugar out and clean it off. I know, that doesn’t sound terribly impressive. Perhaps this will be better: remember the famous story about the sculptor who said he sculpted an elephant from a block of stone by simply removing everything that wasn’t elephant? That’s what sugar factories do. They simply remove anything that isn’t sugar.
So how do they do that? Let’s start with the sugarcane that arrives on the back of a truck at a sugar mill. About 12 to 15% of each stalk of sugarcane is sugar (or sucrose, to be a little more technical). The rest? About 15% is fibre – the solid, cellulosic material that gives the plant its physical structure – and about 70% is water. The remaining few percent are the rest of the soluble salts, sugars and so on that make up plants.
Smash, Soak and Squeeze
The first part of the sugar milling process is rated PG13 for violence because to get the sugar out of the sugarcane, we use rotating knives and shredders to smash the cane stalks to little pieces of fibre. Then to get the sugar out of those little pieces, most sugar mills in South Africa use a process called diffusion. This involves dragging the shredded sugarcane through an enormous “bath” in which it is soaked in hot water. The hot water washes the sugar out of the fibre, in much the same way that hot water gets tea out of tea-leaves.
Once we’ve washed out as much of the sugar as we can, we squeeze the remaining water out of the fibre by pressing it between heavy rollers (called a mill, which is where sugar factories get their name).
The dry fibre that comes out of the mill is a very useful product. Some factories use it as an ingredient in animal feeds, some use it as a feedstock for paper-making, but all sugar mills use it as a fuel. A lot of heating goes on in a sugar factory, and for that we need a source of energy. The sugarcane fibre is an excellent fuel, so sugar mills burn it in boilers to produce steam.
Before the steam gets used in the factory, it first drives turbines to produce electricity – our sugar factories are self-sufficient in both steam and electricity – all using a fully renewable fuel, sugarcane. Some sugar factories even export some of their “green” electricity into the national grid.
Clarify and Crystallise
But let’s go back to the diffuser – after all, the question was, how is sugar made? Having soaked the sugar out of the fibre, we now have a lot of hot water with sugar dissolved in it. But it isn’t very clean. It’s full of particles of sand, clay and plant material from the sugarcane. So, the next step is to add a flocculant which “collects” all the solid particles and sticks them together so, that they all settle to the bottom of a large vessel called a clarifier, while the clear liquid (free from flocculated particles) is drawn off the top.
But the sugar is still dissolved in the water. And by the way, it’s not the only stuff in there – there are those other salts and things that came with the cane – how do we get rid of them and get the sugar out? Well, Mother Nature provides the solution to both problems at once, with an amazing process called crystallisation.
You remember those school experiments, where you grew a crystal in a beaker on the end of a string? The amazing thing about crystallisation, is that it excludes everything else from the crystal – only sugar crystallises onto a sugar crystal. So by crystallising the sugar, we not only get it out of the solution, but we leave behind the other stuff.
Evaporate, Spin and Dry
Getting back to our sugar factory process, what we do is use steam to heat the sugar solution, evaporate the water and concentrate up the sugar so that it forms crystals. We then spin this mixture at high speed in a perforated drum, and the centrifugal force pushes the liquid out of the perforations, while the sugar crystals are trapped inside. All that remains then is to dry the crystals with warm air, and we’ve made sugar.
What about the liquid left behind? That still contains quite a lot of sugar, plus all of the other materials from the sugarcane plant – and we call that molasses. It has a number of uses, such as feedstock for distilleries, or use in animal feeds. See what I mean about the sculptor and his elephant? All we do is take away the things that aren’t sugar – first the fibre, then the solids, then the water and the other plant materials.
I should probably be clear here – the process that I’ve just described is how we make “raw” or brown sugar. “Aha!” I hear you say “but what about refined sugar? Surely refining isn’t just taking stuff away?” Well, actually, it is.
You see raw sugar is brown because it contains tiny amounts of “colour molecules”. In a sugar refinery, the raw sugar is dissolved, and then various processes are used to “trap” those molecules and remove them from the sugar solution. This is usually done by getting them to stick to solid particles such as carbon granules or resin beads. We then filter the sugar solution to make sure no solid particles get through. And lastly, we again use the magic of crystallisation to make sugar crystals but with the colour molecules left behind, the crystals are now white. So, whether brown or white, that’s how sugar is made. Mother Nature delivers it – we just have to remove the wrapping. Not a bad job, being a sugar engineer, come to think of it.