When I buy homemade ginger beer (in South Africa), there are often small black raisins floating in it, about 1cm in diameter. Why did the makers decide to put them in?
I'm not sure if this is true in ginger-beer making, but in mead-making, raisins are sometimes added to supply nutrients for the yeast. The all honey and water mixture which is the mead starting point cannot host a very healthy yeast colony without the trace minerals supplied by the raisins.
Maybe it is the same for ginger beer...
BTW. If you can get your South African supplier to share, please post their ginger beer recipe (as part of your question, or as a comment on your question, or even as a comment on my answer). I am sure that I am not the only person here who would be interested in seeing how things are done there.
The skin of sultanas or raisins have natural occurring yeast needed for the fermentation process.
Older recipes typically used a 'ginger bug' derived from the yeast on sultana skin in stead of brewers yeast. It is not coincidental for the sultanas to float as this indicative of the fermentation process active also within the swollen sultanas.
I'm South African and my grandmother used to make ginger beer all the time when we were little and it always had raisins in. Unfortunately she is no longer around for me to ask her why but whatever the scientific reason is, I think it's the traditional way to make ginger beer in South Africa. I loved the raisins and will be adding some to my recipe!