I discovered that a local spice shop, Oaktown Spice, has tonic water kits: a set of herbs, spices, and citric acid to make a concentrate.
I got one and prepared it today. You simmer the kit’s contents with orange, lemon, and lime zest and juice. Then you add citric acid and strain it through a sieve and then cheesecloth (I used a paper towel). The recipe calls for a lot of sugar, and the main reason I was interested was that I don’t want all the sugar in commercial tonic waters (more than in Coca Cola, and sugar free tonics are sweetened with saccharine). So I used Splenda.
It’s delicious! More complex spice notes than the one-dimensional Schweppes. The tint is from the juices and zest. I made half the kit, which came to about a cup and a half of concentrate. Two tablespoons go into the drink, mixed with sparkling water.
