High gravity brewing and dilution

What high gravity brewing is

In high gravity brewing you brew and ferment a wort stronger than the beer you sell, then dilute it with treated water to the sales strength, usually after fermentation and before or after filtration. A brewery that brews at 16 °P and sells an 11 °P beer gets close to 50% more saleable beer out of every brew and every fermenter turn.

Large lager breweries have done this for decades, including in India, where many mainstream lagers are made this way. For a growing craft brewery that has run out of tank space, it is one of the cheapest capacity increases available, if you can control it.

Doing the dilution maths properly

Degrees Plato is a percentage by weight, so the honest calculation goes through kilograms of extract, not litres.

Take 10 hL of wort at 16 °P. Its specific gravity is about 1.065, so it weighs about 1,065 kg and holds 1,065 x 0.16 = about 170 kg of extract. At a target of 11 °P (SG about 1.044), that extract can make 170 / 0.11 = about 1,549 kg of wort, which is 1,549 / 1.044 = about 1,484 litres.

The shortcut, 10 hL x 16 / 11, gives about 1,455 litres. It is close, but it undershoots by around 2%. That 2% of every tank adds up across a year. Our guide to original and final gravity covers the conversions.

When you dilute finished beer, you are blending to an alcohol and original extract target, so use a lab measurement of the strong beer rather than the brew sheet. Confirm the blend after mixing.

The water is part of the beer

Dilution water can be a quarter or more of the finished beer, so it has to meet beer standards.

• Deaerated. Oxygen in dilution water stales beer as surely as oxygen at packaging. Breweries strip it with vacuum or CO2 down to very low levels and check it with a meter, as covered in our guide to measuring dissolved oxygen.
• Microbiologically clean. It goes in after the boil, so it must be sterile filtered or otherwise treated.
• Chemically matched. Borewell water with high alkalinity or chlorine will show in a pale lager. Most breweries start from RO water and add back minerals to match the beer.
• Carbonated and chilled to suit the point of addition.

What it does to flavour and yeast

Strong worts change fermentation. Yeast faces more osmotic stress and higher alcohol, so it tends to make more esters, especially ethyl acetate and isoamyl acetate. A beer brewed at 18 °P and diluted rarely tastes identical to the same recipe brewed at sales gravity.

To keep the yeast healthy, pitch more cells in proportion to the gravity, oxygenate well, dose zinc and keep the temperature under tight control. Many breweries cap high gravity at around 15 to 17 °P because yeast health and flavour drift get hard to manage above that.

Hop utilisation also falls in stronger worts, so bitterness has to be recalculated, not scaled. Foam can suffer as well, because foam-positive proteins are diluted along with everything else.

Indian adjuncts and excise

Rice, maize grits and sugar syrups make it easier to reach high gravity without a bigger mash tun, which is one reason they go hand in hand with high gravity brewing. Keep an eye on free amino nitrogen, because adjunct-heavy strong worts are the ones most likely to run short.

Dilution also touches excise. Where in the process dilution is allowed, how strength is declared and how volumes are recorded are matters for your state excise rules, which vary and change. Check the current notification or speak to a licensing consultant before you change the process.

Where to start

Start small. Brew your core lager 2 to 3 °P stronger than usual, dilute to strength and run a blind triangle test against the normal version. If the panel cannot tell, move up another step. If it can, you have found your limit for that recipe and that yeast.

The arithmetic is the easy part. Getting a diluted beer that matches the original every time, across water, oxygen, yeast and bitterness, is where a brewery's process knowledge is really tested.