Gravity

The Thing in question is a hydrometer. It looks a bit like a thermometer; a blown glass tube with a printed paper scale trapped inside and a blob on the bottom. Inside the blob, instead of mercury, is a cluster of lead beads to weight the base and keep it upright. It’s read by floating it in a tall jar of booze and looking to see where the meniscus aligns with the scale inside. Like the Dead Sea, a must with a lot of stuff dissolved in it is super buoyant - counteracting Earth’s gravity - and the hydrometer sits higher in the jar. The less dissolved stuff there is, the lower it sits. The dissolved stuff we’re interested in here, though, is sugar - not salt.

Get the sugaring wrong and you won’t get the drink you were anticipating. So that’s why a hydrometer is essential equipment in the home winemaker’s armoury. It’s your guide to the invisible - and while you can make a wine without metering the buoyancy, officially known as the ‘gravity’, what you end up with will inevitably be a bit of a stab in the dark.

But! You can also use the hydrometer for prediction, and there are charts you can use to determine the potential alcohol in a brew simply by looking up the reading. For example, a reading of 1.010 represents a sugar content of about 27 grams per litre. A reading of 1.100 represents a sugar content of 243 grams per litre. That one decimal place means the difference between a potential ABV of 0.4% - just about enough alcohol to preserve a cordial or fancy lemonade - and a brisk and eminently drinkable 13.4% Sauvignon Blanc. 

There are quite a few different gravities, gravity fans! So here are the gravities you need to know:

‘Specific gravity’ - all gravity measurements are ‘specific’, because it’s how we express the gravity in terms of its ratio to, specifically, the gravity of water. This is how the scale inside the little glass tube is calibrated: it’s a ratio, with the measured liquid’s density as ‘n’ and water the ‘1’. So the gravity ratio for that 1.010 lemonade with 27g sugar per litre is 1.01:1. The 13.4% Sauvignon Blanc you’ll be having tonight reads 1.100, so is 1.1:1 denser than water, and ’n’ here is 1.1. Pure alcohol is much less dense, so less buoyant, than water, so a bucket of meths would read something like 0.791:1 and the hydrometer would sink right down. Being a physical gadget, the glass hydrometer makes it all a lot easier to get your head around: it will visibly sit several centimetres deeper in Sauvignon Blanc than it does in lemonade. Just like you at bedtime.

’Original gravity’ is pretty self-explanatory: it’s the starting point; the specific gravity of the must before you start fermentation. It’s an important measurement because it’s the key to all future measuring and working out. 

‘Final gravity’, equally obviously, is the density of the finished brew - the fermentation’s end point. Or, at least, the point where you realise it’s got further to go and start it up again…!

Let’s learn how we measure and adjust the gravity - and use the readings to predict the eventual ABV – by making a new batch of nettle wine. It’s a good time of year for it! Nettle wine is a good place to get to grips with Gravity, because its liquor starts with zero sugar, and we add sugar until the yeast is provided with the right amount of fuel. 

Nettle wine starts off with stewing fresh young nettle tips, lemon peel and ginger until you have a rich, aromatic tea. It smells amazing, and you could actually drink it at this stage - especially if you have arthritis, or any other inflammatory ailment, which it’s meant to help. But we’re making alcohol, so the next stage is to add a bit of lemon juice - and quite a lot of sugar. For a fruit wine, you’d account for the natural sugars and take a hydrometer reading before you sugared as necessary to a good fermenting level. For sweet grapes, or a dessert apple cider, you’d probably not add any at all; for a beer, you’d make hop tea and add a little bit of a low sucrose sugar - malt, for example.

I added the sugar in two parts, partly so I could show you the difference it makes to the gravity and eventual alcohol content, but mainly because the only yeast I had handy was a low-alcohol one for making Belgian beer. So initially, I added only 1kg sugar to the 4.5 litres of nettle tea so the beer yeast could make a start while the tough stuff came in the post. This first gravity was 1.085, which would give potential alcohol content of 10-13%. When the new yeast arrived, I added the remaining 0.75kg and the gravity went up to 1.110, giving a potential ABV of 14-17%.

How did I work all that out though? Well, by the magic of looking things up in published charts. There are a few things (not least ambient temperature, yeast type, sugar type, water quality, hardness, and so on) that will affect readings, and how thoroughly the sugars are converted into alcohol and how fast. So I like to check more than one chart and work to a range.

For a home winemaker, of course, this stuff is ‘nice to know’. It helps you learn, and it helps you understand what that murky liquid is likely to actually do to you. But for a commercial operation, not only is it essential to ensure that your product is consistent in both taste and quality, but it’s also a legal requirement before you can offer anything for sale. At the Three Choirs Vineyard in Gloucestershire, Winemaking Director Martin Fowke uses a £2,250 portable density meter from Anton Paar to make multiple tests throughout the process. Every batch of grapes that comes into the winery is tested for sugar level and acidity at pressing; post fermentation testing establishes ABV, and then wines are also tested before and after bottling.  

“Each wine”, says Martin, “must legally have a declared alcohol that is within 0.5 of the actual alcohol of the wine in the bottle. So a wine labelled with an 11.5% label must have an actual alcohol of between 11 and 12%. We are only permitted to state alcohol in amounts of 0.5% increments, so if a wine is 11.2% alcohol then it can legally have an 11% or an 11.5% label declaration”. Martin tells me they do still use glass hydrometers here and there, but with all that testing and reporting to be done, you can see why he prefers something that’s “quick and easy, adjusts for temperature, and is calibrated to give accurate results every time”.

A handheld digital meter that instantly reads specific gravity, density, sugar and alcohol contents, automatically adjusts its readings to account for temperature, and even sucks up its own samples through an inbuilt tube? I want one. Now. Oh, and I forgot to say: one other thing about getting an accurate gravity reading is that even a tiny difference in temperature affects it, and then more maths must be applied to get an accurate result. But don’t worry: if you can’t afford two and a bit grand for a handheld digital miracle, there are charts for looking that up too.

Nettle wine recipe

Ingredients

• 2 litres (about 125g) lightly packed fresh young nettle tips

• Peel and juice of 2 unwaxed lemons

• 5g fresh ginger, sliced

• 7g dried root ginger, smashed

• 4.5 litres water, preferably filtered

• 1.75kg sugar

Equipment

• Large saucepan with lid

• 6 litre fermenting bucket

• Stirring spoon

• Sterilising solution

Method

Thinly peel the lemons and discard any white pith, because that would make the wine bitter. Juice them and put the juice to one side.

Put peel, ginger and washed nettles in a pan with 2 litres water. Bring to the boil, cover, then simmer gently for 45 minutes.

Strain into sterilised fermenting bin or jar, stir in the sugar until completely dissolved, then add the remaining 2.5 litres water.

Allow to cool until your hand can stay in comfortably, then add the lemon juice, yeast and nutrient. 

Cover and leave for a few days until the fermentation calms down a little, then transfer to a sterilised demijohn, fit airlock, and stash somewhere warm until bubbling stops.