To make ginger beer you must, of course, add water, the juice of ginger, and sugar. Then after a period of fermentation the liquid is filtered and bottled and the process starts again. A spoon of sugar is added to each bottle for the secondary fermentation but after 2 days they must be refrigerated to block the fermentation and avoid exploding bottles.
You could always take a leaf out of the book of those good folk in Porto and stop the fermentation with a dose of brandy.
As to Kefir, although I've never tried it, as an Archers listener, I'm fully up to speed.
