After the last hop addition, the wort is finished and ready to ferment with the tasty yeast. We transfer it all to the fermenting vessel, leaving behind the squashy hops. It's dead important that the sugar content (specific gravity, or just gravity) of this wort is spot on, because it's the sugar which yeast turns to alcohol and we need the beer to be the correct ABV (we'll have worked this out before hand to use the right amount of malt etc). Boiling wort serves several main purposes, including developing hop bitterness, getting it clear, caramelising the sugars, and reducing it to enhance all the flavours.
After doing this, the gravity can often be to high because so much water has been evaporated. To correct this, we'll need to dilute it before fermentation. This dilution is called a 'hop sparge' because we sparge (spray on hot water, or liquor) through the hops and then also drain this liquor into the fermenter. Hopefully by doing it this way, we have collected up all the remaining wort and imparted lots of lovely hop aromas into the liquor- much better than just shoving in straight liquor. You can aim for a high gravity,meaning you'll have to do a hop sparge which extracts maximum hop flavours for a really aromatic brew.
So that's the hops.
Next is the dilution calculation step which is in theory a way of working out how much liquor to use in a hop sparge to give finished wort at the correct gravity. Although it is an actual calculation, we use a fiddle factor. The fiddle factor (which itself is frequently fiddled) tries to account for the amount of wort left in the pipe work and the hops, as this contains sugar and will contribute to the gravity.
In short, it varies so much between brews that it rarely works first time and often takes a couple of small hop sparges to get the gravity right. If we're really brave we might just cross our fingers and do one big hop sparge after working it out using a big fiddle factor. This is a bit risky though as over sparging means the wort gravity will be lower than intended.
So why is over sparging bad? Well the sugar is needed for fermentation, firstly in the fermenting vessel, and secondly during cask conditioning. To get beer at the right percentage, its essential that enough sugar is fermented during this first stage. We always ferment the beer to the right alcohol content in the fermentation vessel (or as close as we can), but there wont be as much conditioning in the cask because of the lack of remaining sugar. The reduced conditioning doesn't really make a significant difference to the alcohol content, but without this little extra fermentation the beer might be a bit flat. This is because fermentation gives you alcohol and carbon dioxide. The beer will be a little less sweet too, although these are two elements you might not really notice a difference in.
So a majorly over hop sparged beer might give you a flat, less sweet pint. I over hop sparged my special, I hope that's not too flat but I'm going to try it tomorrow and will let you know the verdict.