What does each component of the homebrew actually do?

JumpShivers

What does each component of the homebrew actually do? And what does varying them achieve?

I think Malt extract gives flavour and body and some sugars for fermentation.

Yeast is what breeds and causes the ethanol as a bi-product of it's (an)aerobic?? respiration.

Sugar is what gives the yeast "food" to respire and carbonation.

My OH and I usually add one batch of malt extract, some brown and caster sugar, some golden syrup and some yeast into 25 litres of bottled water.

We are thinking of adding double yeast and double malt extract.

Is this what brewers mean when they say double and triple?

Will adding more malt extract and yeast increase our alcohol content? Will we need to add more sugar to allow for the extra fermentation?

How much priming sugar is normal for 25 litres?

BeerNut

In general it's not a good idea to add sugar in quantity. You get a better beer from an all-malt brew, so for a typical 1.7kg kit I would add 1kg of spraymalt, and then just a small amount of sugar for priming at bottling time.

Adding more fermentable material, eg malt or sugar, will give you a stronger beer. You can use brewing software to work out in advance how much of an effect it will have. It's not infinitely flexible though -- yeast, especially dry yeast, can struggle if there's too much malt/sugar. It's also best not to mess about with kits too much: they're almost-finished beers and you can throw the whole thing out of balance if you try to do too much. Extract and all-grain brewing does give you more freedom to make beer to your own specifications.

Adding more yeast won't make much of a difference, though if you are making something very strong then maybe two packets might be a good idea.

How much priming sugar you add depends on how fizzy you like your beer. There are carbonation calculators on the web for working it all out. 2.4 vols of CO2 should be fine for most ales, so 25L at 18C would need about 140g of sugar to hit that.

And the double and triple in beer names doesn't really mean anything measureable, just that they're stronger relative to each other.

dukeellington

Don't know if I agree with BeerNut that you shouldn't add sugar in quantity, very dependent on the beer style you're going for, lots of Belgian recipes would call for high sugar addition, any German style, obviously, wouldn't have any sugar addition. The real trick is understanding why you're adding or not adding anything to your recipe, if you don't know why you're putting it in, don't.

As for the original question, that is not what brewers mean by double and triple, as BeerNut saying they're just indicators of relative strength, doubles being in the range of about 6-8% ABV and triples probably around 7-9%. A lot of doubles are dark and triples golden, but that's not gospel.

Malt extract is perfectly capable of giving all the sugar required to feed the yeast and create the alcohol, if you're adding sugar a lot of the time its to increase the alcohol content without creating a fuller, maltier mouthfeel without imparting flavour. Often for a triple this would be required. More complex sugars from darker sugar, syrups or fruit can be added with the intention of adding flavour. I wouldn't recommend doubling up on your sugars, as noted there's a limit to the amount alcohol the yeast can create, complex malt sugars left over in beers with high final gravity's can leave nice flavour, but simple sugars would presumably leave you with sugary water flavour.

There is also no need to double your yeast, I've been up and down with making yeast starters, and I don't really know how much impact a higher yeast pitching rate has, a big starter will definitely get the fermentation going quicker, but you run the risk of raising the temperature with a fast reaction and creating off-flavours.

I aim for highly carbonated beer and use about 150-200g DME to prime before bottling usually, you could always take a litre of your wort before you pitch your yeast, hold it in the fridge and then use that to prime if you like too.

Carawaystick

I'd be with Beernut, unless you're brewing a specific style of beer which needs sugar, then sugar is only for priming.

The vast majority of beers shouldn't have sugar added.
If you're brewing a belgian style ale which needs it, you'll have the belgian yeast to pitch first.