Ginger beer plant – about a tablespoonful. Remember that you cannot make this from ordinary yeast – you have to find one. They are sometimes available online.
2 litres water (should be chlorine free – a large pinch of ascorbic acid (vitamin C), or the juice of a lemon will remove the chlorine if you are unsure)
½ teaspoon cream of tartar (this stabilises the acid level but is not essential)
Juice of 1 lemon
2 or 3 inches of ginger, peeled and grated (dried ginger powder is an alternative)
The GBP is a pretty resilient creature but it is safest to make sure everything is clean in the kitchen. Tie the grated ginger in a small piece of muslin. Place it (or dried ginger) with the sugar, lemon juice, cream of tartar and water in a large jug. Stir until the sugar has dissolved. Add the GBP. Cover with a cloth fixed in place with an elastic band, or just a lid if there is one.
Leave for about five days (or until it tastes just a little sweeter than you would like the finished product to be) then carefully pour into plastic pop bottles or swing top bottles using a fine sieve and a funnel. (You will find a larger GBP in the sieve than you started with. Remove the muslin then rinse the GBP in fresh water. Use it to make another batch). It is ready to drink within two or three days, though a week is fizzier. The low activity of the yeast and small amount of sugar used will ensure that explosive levels of CO2 are not reached, but it may be worth checking a test bottle every now and then by loosening the lid just to make sure. Chilling in the fridge will stop any further fermentation if you are happy with the level of fizz and sweetness.