Recently there's been an interesting post by Ronald Pattison (xtreme beer-history buff) on use of "sugar" in brewing. In the comments following it's become apparent that some people hold very strong views on the subject. It's looking like one of those religious issues. It also seems that some people are quite confused about the whole thing.
Nowadays we use sucrose or invert sugar in brewing not so much for economy, but rather to allow us to control the fermentability of our worts. To understand how this works, it's important to be aware of the difference between what's commonly called sugar (sucrose) and sugars generally. It might be better to talk about carbohydrates really, but I'm comfortable with sugars v. sugar , so try to keep up. Wort carbohydrates include the monosaccharides fructose & glucose, the disaccharides sucrose & maltose, maltotriose (a trisaccharide), and more complex sugars (often called dextrins). Our yeast can only eat fairly simple sugars - the 'mono' and 'di' saccharides and some others.
A typical British Pale Ale wort with an original gravity (OG) of 1040 might contain a little more than 9 grams of sugar per 100 millilitres, of which less than 7g will be fermentable (including 0.5g sucrose - table sugar). This leaves something like 2g / 100ml of the higher sugars to contribute to taste and body in the finished beer and final gravity (FG) of around 1008.
Broadly, we can say that a beer with high levels of unfermented sugars will be sweeter, with more body than a beer with relatively low levels of these dextrins. But of course, we don't always want sweet beers with lots of body. Indeed, Belgian brewers speak of the digestibility of their beers, by which I guess they mean the relatively light body (considering the alcoholic strength) of many of their offerings. Digestible beers don't fill you up. A relatively light body, with dryness are also characteristics of some British beers, it's part of what makes our session beers so sessionable - so how do we go about achieving this?
To a certain extent we can control the fermentability of the wort by controlling mash temperature. The various enzymes in the malt have different optimum temperatures for their actions. So a little bit warmer (say 67C) might give us a less fermentable wort (lower proportion of fermentable sugars), while a bit cooler (64C) might give us a more fermentable wort leading to a drier (thinner) finished beer.
We can also make a difference by our choice of yeast. Yeast tends to consume sugars in sequence. It's like a child eating the chips first, then the sausage, followed by the beans. So the simpler sugars are utilised first, the more complex ones (which require the mobilisation of more yeast cell machinery) only when the easy stuff is used up. A yeast that's highly flocculent, i.e. falls out of suspension quickly, will tend to leave a higher proportion of the more complex sugars unfermented, even though it has the right machinery. A less flocculent yeast might produce a drier beer, by hanging around for long enough to have a good go at all the fermentables. Some yeasts are constitutionally unable to use maltotriose - they just don't have the tools for the job.
We can control the fermentable / unfermentable balance of the wort by directly adding fermentable sugars while keeping the OG the same (i.e. reducing the amount of malt somewhat). We can chose to add mainly simple sugars, a useful technique allowing us to use more flocculent yeasts (good for producing clear beers in a short time with simple equipment), without leaving too much unfermented at this stage.
This is the basis for brewers adding invert sugar to their wort. Invert sugar is sucrose (table sugar) which has been treated to break it down into glucose and fructose. This is often done by heating an acidified solution of sucrose. It's Quite Interesting to note that as the hydrolysis proceeds, the optical properties of the solution change, with more fully hydrolysed solutions rotating polarised light in the opposite direction to a sucrose solution - which is why we call it invert sugar...
The fermentability of commercially available invert sugars seems to be in the range 94% - 99%, although some of the really dark ones, which will have more unfermentable stuff in them, will be less fermentable (90% or so). Other, starch derived products can be got with more or less whatever fermentability you want, and are commonly used as brew-length extenders (which is a good way of saying profit enhancers).
Anyway, throwing good old sucrose into the boil works well (although you'd be advised to make a solution / syrup up first unless you particularly want to be chipping burnt toffee of your elements all the next day). In the acid conditions of the boiling wort a proportion of the sucrose is hydrolysed for you. I should think.
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