This bit is a bit technical…so if you don’t want to read it I’ll understand.
Last time I talked about how gravity is a measure of sugar concentration and how that is related to alcohol. This time I’ll explain where the sugar comes from and how I’m going to control it to fit in with my beer design.
For the purposes of designing my beer I’m going to start with the final gravity. I’ve decided that it should be 1012 – 1014. Let’s say 1012 for the sake of argument. This should provide a reasonable mouthfeel but not make it too sickly sweet. There are a number of ways to control the final gravity.
The first is controlling the types of sugars that you convert during mashing by managing the mash temperature. (Mashing is basically the process by which starch in the malt is converted into sugars and is done by mixing milled malt (grist) with water…more on that later). As a general rule, temperatures over 65-66⁰C result in a sugar profile with more dextrinous sugars. The yeast cannot ferment these and so they remain in the beer.
The second is the choice of yeast. Some yeasts are high attenuating and some low attenuating. To explain it a different way some yeasts are well hard and will eat everything ignoring the rising alcohol levels (which, by the way, is one of their waste products). Others are a bit more sensitive and once they have eaten their fill they object to sitting in their own waste products and stop playing – basically they go to sleep. I have a particular yeast that tends to attenuate down to around 1012 on a good day – not the hardest yeast on the market but nor is it the softest.
The final way is to control the temperature at the end of fermentation. By cooling the beer quickly the yeast will stop fermenting. The drawback to this method is that unless you remove the yeast from the beer completely by filtering then when the beer warms up the yeast is likely to start fermenting again. Seeing as some of my beer is destined for cask and is unfiltered then this is probably not the best method.
Cool….that’s final gravity sorted. Now for original gravity(OG). First I’ll decide on the ABV (alcohol by volume). It’s certainly not going to be at the 140/- level of almost 9% mentioned in my last blog. Just to re-iterate, I want my beer to refreshing and flavourful and more-ish. Anything too strong in the alcohol department may mean that although people would love to have more they’re incapable of asking for it or unable to get up and walk to the fridge. Nope, it’s going to be a “small” beer – relative to the good old 19th century days of course. I reckon I aim for 4.4% ABV. So, I know I want a final gravity (FG) of 1012 and an alcohol of 4.4%. I plug that into the calculation (OG-FG) x 0.129 = ABV, do a bit of jiggling about and hey presto! I need an OG of 1046.
So far so good, but where do I get all the sugar to get this OG of 1046. Well, the sugars come from the malt. Rather than me boring you to death about malt and mashing here is a link to an excellent website that explains it all rather more entertainingly than I can.
Read it? Ok, you’ll be a malt expert by now. What it doesn’t explain is how brewers calculate how much malt they need to get the desired gravity. This may be a bit boring but bear with me if you really want to understand beer design. Otherwise feel free to go back to reading Hello magazine.
Each malt has a theoretical extract – that is the theoretical amount of sugar that you can extract from the malt. This varies according to the type of malt and the malting processes used. However, as an example, pale malt may have a theoretical extract of 300 l⁰/kg (litre degrees per kilogram). That is, 1 kg of malt will provide 300⁰ (degrees) of extract (sugar) in 1 litre of water. So if I want to brew 20 litres of beer starting with an original gravity of 1046 I need 20 litres x 46⁰ (for some reason you subtract 1000 from the OG) = 920 l⁰ (litre degrees). Therefore the amount of malt I would need is 920 l⁰/300 l⁰ per Kg = 3.06 Kg of pale malt. Simples!
Oh if only it was so! Actually I need to take into account a few other factors. Brewing equipment isn’t perfect and often the actual extract doesn’t match the theoretical extract. This is often determined by the type of kit and the process used and will vary from brewery to brewery. The efficiency of the brewery in extracting sugar is determined by trial. Some, more modern breweries, may have efficiencies over 100% but most small brewers are around the 80% mark. So, say the efficiency of the brewing kit is 80%, then I need to adjust my calcs. Instead of 920 l⁰ I need 920 x (1/0.8) = 1150. The amount of malt is then 1150/300 = 3.83 Kg.
The second factor I need to consider is if I am going to use more than one malt. For example I may decide for reasons of colour and/or taste that I want to add a small amount of crystal malt, say 4%. Crystal malt has an extract value around 260 l⁰/Kg. Therefore the amount of crystal malt required is (4% x 1150)/260 = 0.18 kg. The amount of pale malt will be (96% x 1150)/300 = 3.68 Kg.
Sooooo…just to summarise...I'm aiming for a 4.4% beer with an original gravity of 1046 and a final gravity of 1012. I guess the next decision is the type of malts to use……I’ll save that for the next blog.