Slow drinks: Lacto-fermented rhubarb cordial, by Phillip Aldridge

Millie Milliken

Millie Milliken

21 May 2020

With more time than ever to experiment at home, we've launched a special series on slow drinks – recipes for those DIY tinctures, tipples and toddys that take time to create. This week it's the turn of Manchester bartender, Phillip Aldridge

Why I'm fermenting

Fermentation is a great process for a couple of reasons. Firstly, it allows you to expand your guest's mindset about what flavour they might expect from a certain ingredient. Secondly, it's never a perfect science, ferments can come out a little bit different every time you do it which, if you're anything like me, just encourages you to keep trying new combinations and methods. If you stay curious, you can't go wrong.

What I'm fermenting

Rhubarb! For this method we'll be using a process known as 'facto-fermentation'. It's great because you don't need a lot other than good quality organic produce and non-iodised salt to get it started. What we do is create an ideal salty environment to allow bacteria known as 'lactobacillus genus' to flourish. These are naturally occurring bacteria that break down the sugars naturally found in fruit into lactic acid and carbon dioxide – it's this lactic acid that gives the signature 'tang'. 

What my ferment can be used in

You can take something as simple as fermented rhubarb juice and apply it to many different products. You could use it as a substitute for fresh citrus, you could add acids/dilution and make a soda, or you could add sugar and acid and stabilise the end product into a cordial or syrup.

A drink I like to make uses a lacto-fermented rhubarb cordial, stabilising the fermented rhubarb juice with sugar and both citric/ascorbic acid. This halts the fermentation process and allows for increased shelf life. With this cordial you can make a simple, but unbelievably versatile, highball using 40ml-50ml Japanese whisky (I like Yamazaki DR), 15-20ml lacto-fermented rhubarb cordial and 100-125ml strained coconut water. 


  • A good-quality centrifugal juicer, a brand like Sage is ideal (this is essential as it improves yield massively)
  • A kilner jar or similar non-reactive container with a tight fitting lid. (If you have access to a vacuum packer, this is best)
  • A bowl or mixing bowl
  • Scales that measure to 0.1 decimal place


Organic rhubarb stalks

Non-iodised salt (brands like Maldon are good)

Super-fine caster sugar

Citric acid

Method: Run your rhubarb through the juicer on the appropriate setting to deal with hard/fibrous veg. Weigh your rhubarb juice on a scale to find out its total volume, then add 2% total volume in non-iodised salt and mix with a sterilised utensil to ensure salt is evenly incorporated.

If you have access to a vacuum packer, then vac the liquid into a bag, if not you can use a container like a Kilner jar with a sealed lid (sterilise your container before use). You can buy powder for sterilising from homebrew/hobby shops or you could use boiling water/steam.

Once your liquid is in your chosen container, seal it tightly – you don't want any oxygen to get to the product while its fermenting. Over time, it will release CO2 which (if you aren't using a vacuum sealed bag) will create its own buffer against any oxygen that was present in the vessel when you sealed it. 

Find a suitable place with a consistent temperature range, somewhere away from sudden changes in temperature throughout the day (ie windows, near the oven, doorways etc) and especially make sure it's kept out of direct sunlight. You're looking for a temperature in the mid 20°Cs, with 24°C-26°C being ideal, but nothing higher than 28°C. The hotter your chosen area is, the faster the fermentation process will occur.

Leave your rhubarb to ferment for between two to three days, starting to keep a close eye on it as it enters the second day. If you're using a vacuum-sealed bag then you'll notice the bag begin to inflate – this will be a great indicator of how far along it is and you'll need to catch it before the bag pops. If you're using a container then by all means pop the lid off and have a taste every six hours and see how it's getting along (remember to sterilise anything you're going to put into the juice). You're looking for a flavour profile that's very much a complex acidic rhubarb juice with no hints of 'off' flavouring, if it starts to taste off, then you can stop the fermentation process by putting it in the fridge.

Once you have your fermented rhubarb juice, it's up to you where you want to go from here and how you want the end product to turn out. If you want to retain that acidic flavour of the rhubarb then you can add less sugar, if you want a more balanced flavour then add more etc. 

NOTE: If you plan on using the rhubarb juice in something that's going to be carbonated you may wish to freeze-filter your rhubarb juice (pour it into a shallow container then freeze solid, once frozen you can put a clean/damp dishcloth over a sieve and allow the juice to defrost in the fridge, the water will melt first leaving behind a lot of the solids). This will help clarify your lacto-rhubarb juice for carbonation, however be aware this method is slightly inconsistent and you will lose some flavour (not a significant amount).

Weigh your rhubarb juice. I like to add to it 75% total volume in caster sugar as well as 2.5% total volume (before you add the sugar) in citric acid, and mix to incorporate. This may take some time at room temperature so feel free to do it sporadically. Both the sugar and the acid help stabilise the ferment but it's still important that the resulting cordial is kept in the fridge as much as possible to ensure shelf life.

And you have a cordial to do as you wish with! If kept properly, it should last for up to six weeks/two months in the fridge. You can even freeze batches of the lacto-fermented rhubarb juice to have on hand if you've made a bit too much.

In other parts of this series, learn how to make lacto-fermented tomato water, kvass, sima and watermelon whey soda.

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