Brown sugar is cane sugar (sucrose) – similar to white ‘table’ sugar, but less refined (some flavourful ‘impurities’ are purposefully left over from the refinement process).
It is readily fermentable (nearly 100 %) but we typically advise caution when using sucrose sugars for beer fermentation.
This is because, the use of these type of sugars typically results in higher levels of acetaldehyde in the beer, which can cause a cidery ‘green apple’ flavour/taint. This can be beneficial when brewing cider, or even wine, but in beer it is commonly regarded as an undesirable off-flavour.
We wouldn’t go as far to say it’s never acceptable to use sucrose sugars in beer, but we would generally advise against it – that said, the Belgians commonly use Candi sugar when brewing their beers, and this is a form of sucrose (albeit from sugar beet rather than sugar cane).
So if you’re going to use sucrose sugars in brewing beer, it’s important to be aware of the potential for a detectable ‘green apple’ note – and if you’re happy to take the risk and give it a try, the advice would be to moderate the proportion used.
In terms of proportion, using 1 kg (2.2 lb) of this type of sugar with a 1.7 kg (3.75 lb) liquid malt extract kit will mean >40% of the sugar is coming from the sucrose, so it it advised to try at most 0.5 kg (1.1 lb) sucrose with 0.5 kg (1.1 lb) dextrose, thus keeping the percentage of sugar from sucrose <25%.
If this produces good results it may be worth increasing to 1 kg (2.2 lb) next time, but on the understanding that such a high percentage may have a more negative impact on the made beer quality.