Cinchona bark is sold “cut” or in powder form. I like to use cut bark because it has a clean flavor and it is easier to strain out of the mixture than the powdered form. You’ll find that the bark pieces are sold in various sizes, so it is best to weigh out the bark instead of using a volume measurement. I like to use one ounce, which (using my cinchona bark) measures out to around 1/4 cup. Add this to the jar as well.

If you have a favorite flavor of gin that you’re planning on drinking with the tonic, you can consider adding spices to pair with its botanicals. For example, if you like a juniper-forward gin such as Aviation, you could add a few juniper berries to the jar. Or perhaps up the citrus flavor of the tonic by adding coriander seeds. Allspice and cardamom are also common additions. Personally, I like to keep the tonic a blank slate in order to use it with a wide variety of gins, so I don’t add anything else in the way of flavorings.

Add water to the mix of fruit and aromatics and let the mixture steep for three days. Photo: Kate Williams

Once you’ve got all the flavor elements in the jar, cover the mix with 2 1/2 cups of water. Close the jar, give it a shake, and place it in the fridge. Forget about it for three days.

At some point over those 72 hours, make concentrated simple syrup. This extra-thick syrup will be used to sweeten and thicken the tonic concentrate. Heat 3 cups natural cane sugar (I like its richer flavor, but feel free to use regular granulated instead) and 1 1/2 cups water in a medium saucepan over medium-high heat. Once the mixture comes to a simmer, reduce the heat and stir gently until all of the sugar dissolves. If you’re using natural sugar, the syrup will be brown. That’s okay.

Citric acid acts as a preservative and flavor enhancer. Photo: Kate Williams

Remove the syrup from the heat and whisk in 1/4 cup citric acid. This tart powder is used as both a preservative and a flavoring agent for the tonic. It adds a bit of pucker to the final concentrate. Also add a big pinch of kosher salt to balance the flavor. The citric acid and salt should dissolve easily in the syrup with a few good whisks. Transfer the syrup to another 1-quart glass jar, let it cool to room temperature, and then refrigerate until the cinchona mixture is ready.

At the end of the steeping period, taste a small spoonful of the cinchona mixture. It should be bitter, of course, but should also taste of citrus and lemongrass. If you think the flavor is weak, you can let it steep a little longer in the fridge. Be careful though -- you don’t want it to be too bitter. If you like the flavor, it’s time to strain.

Use coffee filters and a fine-mesh strainer to remove even the smallest bits of cinchona residue from the steeped mixture. Photo: Kate Williams

I like to use a fine-mesh strainer lined with coffee filters to ensure that I get out every last bit of cinchona residue from the tonic. Help the straining along by wetting the coffee filters before adding the cinchona mixture. You’ll probably need to add the mixture in batches, unless you have an extra-large strainer. It will take several minutes for all of the mixture to seep through the filters. Be patient.

Once everything is strained, discard the solids. Pull out the citric acid-filled simple syrup and add it to the cinchona mixture in stages. Since we’re adding the syrup last, you can control how sweet you’d like the final tonic to be. Add all of the syrup for an extra sweet tonic, or hang back a little for a dryer result. Taste as you go, keeping in mind that you will be diluting the concentrate with seltzer water for serving. I added about 2 cups of simple syrup, bringing the total amount of concentrate to 4 cups. Give everything a good whisk to ensure that the sugar is well distributed in the cinchona mixture.

Whisk the strained cinchona mixture with the citric acid-sugar syrup to taste. Photo: Kate Williams

At this point, the tonic concentrate is ready to go. You can serve it right away or transfer it back to a (clean) 1-quart glass jar and refrigerate for up to two months. To serve, you’ll want to add seltzer water. I have a home soda maker, but you can also use store-bought seltzer or club soda to dilute. I like to use equal parts tonic concentrate and seltzer, but if you’d like a more subtle drink, you can dilute further to a 2:1 ratio of water to tonic. If you’re making just one drink, you can mix it right in a highball glass. (It’s pretty.) Otherwise, mix the tonic and soda up in a pitcher before serving. To make a gin and tonic, start with 1 to 2 ounces of gin in a highball glass filled with ice. Top with tonic water and a lime wedge.

Recipe: DIY Tonic Water

Makes 4 to 5 cups, enough for 8 to 16 cups tonic water
Note: Cinchona bark and citric acid are available online at Mountain Rose Herbs or at Lhasa Karnak Herb Company in Berkeley.

Ingredients:

1 orange
1 lemon
1 lime
4 cups water
1 stalk lemongrass, chopped coarse
1/4 cup (1 ounce) cut cinchona bark
1–2 tablespoons additional herbs and spices, such as allspice, cardamom, or juniper (optional)
3 cups natural cane sugar
1/4 cup citric acid
1/8 teaspoon kosher salt

Equipment:

Vegetable peeler
Cooking scale
2 (1-quart) glass jars with tight fitting lids
Medium saucepan
Fine-mesh strainer
Coffee filters
Whisk

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