Grandma's ginger beer recipe:

This traditional recipe uses two stages: making the "plant", then fermentation in sealed bottles. 

The quantities below make 9 x 750ml bottles. 
The product should be fermented for about two weeks in warm weather.

Making the plant (GBP)
One teaspoonful brewer's yeast
Juice of two (medium sized) lemons
One teaspoonful lemon pulp
Two teaspoonful of ground ginger
Four teaspoonsful raw sugar
Two cups of cold water

Notes:  baker's yeast can be used if brewer's yeast is not available. If any yeast is not available, use 10 sultanas (utilising the natural yeast on the surface of the dry fruit - if you do this start the feeding after three or four days, to allow the yeast to build up). Ordinary sugar can be used if raw sugar is not available. Water should be free of chlorine (or let the water stand in an open container for 24 hours before use). The alcohol content of the final product is said to be 0.5%.

Method:  Add all ingredients to a large clean glass jar with a lid, leaving at least 50 mm air space at the top of the jar. Leave in a warm dark place to ferment for one week. During this time, feed the plant each day with two teaspoonsful of ground ginger, and four teaspoonsful of sugar. Do not miss feeding each day or the plant might die. The lid should not be airtight. At the end of the week the plant is ready. See "Making the ginger beer" below.

Note: the jar should NOT be sealed airtight, so if using a jar as above, do not close the lid tightly.

Conversions:  One teaspoon = 5 ml. One cup = 250 ml.

Making the ginger beer.
Three cups of raw sugar
Four cups of boiling water
Juice of four lemons (medium sized)
Five litres of cold water
- optional - one teaspoon of cream of tartar

Mix the boiling water and the sugar in a bowl, and stir until the sugar has dissolved, then add the lemon juice, and the cream of tartar if desired. Place the cold water in a separate container. Line a large strainer with muslin or cheesecloth. Strain the ginger beer plant.  Add the resulting liquid to the cold water. Throw 3/4 of the GBP residue away - the rest can be used to start a new ginger beer plant, after adding two cups of cold water. Now add the contents of the bowl to the cold water, and mix well. Poor this mixture into clean screwtop plastic brewing bottles, leaving 30 - 50 mm air gap at the top (if you overfill, the bottles may explode). Screw on the lids and leave in a cool dark place for about two weeks. Select a place where damage from leakage or explosion will be limited. If a bitter taste is desired, use less sugar.


Notes about commercial ginger beers
Making commercial ginger beer at an industrial scale cannot, in practice, follow the traditional approach, so if you want a 'real' ginger beer you must make it yourself. However some commercial products come close in taste. Bundaberg GB, for example, replaces fresh lemon with flavours and acids, and only uses a token brewing process, so that artificial carbonation is necessary to get the right fizz; nevertheless the taste is not far off that of the 'real' product made by the traditional recipe.