Note we're only covering bottled tonic water; there's also tonic syrups on the market to which you add carbonated water yourself. We'll cover those separately in a future series.

Carbonation - effervescence

Those bubbles that tickle your nose when you bring your tonic water up to your lips? That’s the result of carbon dioxide being released when you pop the top off the bottle. In simple terms, the process of carbonation involves adding carbon dioxide in water. Most tonic brands will do this by using pressure - the carbonation for alcoholic beverages like champagne is created via the process of fermentation.

When the bottle is open, the pressure is released, which results in the bubbles coming up to the surface. Those bubbles will also carry some of the aromatic molecules from the bottom as it rises to the top, which influences what you get on the nose. Look out for how big the bubbles are, how aggressively they fizz as you pour into the glass, how long those bubbles stay in the drink (thereby keeping the fizziness going), and ultimately how it dances on your tongue, i.e. is it an aggressive fizz or is it soft on your tongue.

Doing things differently however, is Fentimans, who practice what they call botanical brewing. Traditionally brewers of ginger beer, the process involved simmering crushed and milled ginger root in spring water, filtering the result then adding sugar, herbs, fresh water and brewer's yeast. The mixture is left to ferment, decanted and then carbonated with fresh water. Today, their tonic water is still made using the same multi level process, key to which is the fermentation process. 

Sugar - sweetness

If you’ve ever gone down the sugar aisle in your local supermarket and seen the sheer array of sugar now available, you’ll understand why sugar matters. Cane sugar, demerara sugar, caster sugar – each will contribute to the flavour profile of the tonic. In addition, cheaper tonics will likely use synthetic sweeteners such as saccharine or controversial sweeteners such as high corn fructose syrup. Most of the boutique producers tout their use of natural sugar such as cane sugar. Apart from the influencing the flavour, Tim Warrilow, co-founder of Fever-Tree, suggests that the use of artificial sweeteners contribute to larger, more aggressive bubbles that dissipate faster, thereby resulting in a flat tonic.

Different producers will also use different levels of sugar in flavouring the tonic - most tonics marked as "light" or "dry" will generally have lower levels of sugar in them.

Quinine - bitterness

As many species of cinchona trees that exists, so there are different strands of quinine that can be extracted from the bark. East Imperial for example, sources its quinine from Java grown cinchona bark, Fever-Tree goes for what it deems to be the best cinchona bark from the Rwandan jungle whilst Kiwi based Quina Fina looks to Ecuador. Different tonic brands can also dial up or dial down the amount of quinine (and therefore bitterness) they use in the tonic. Note that whilst cinchona bark is available for the average home experimenter, pure quinine extract is only available commercially, and is what maintains the clear appearance of the tonic - using raw cinchona bark in the production process of tonic water itself would lend a brown-ish colour to the final product, which explains the colour for some brands of tonic water.

Other flavourings

Aside from the three base ingredients, many tonic water brands also include flavourings to impart a bit of citrus and tartness; for many brands, this takes the form of citric acid. Depending on the brand also, some may also add other natural botanicals. For example, Fever-Tree also uses marigold extract and bitter orange in its Indian tonic, Fentimans’ Indian tonic also has kaffir lime and lemongrass in it, Strangelove adds a touch of juniper into its No 8 tonic and Sydney based PS Bush Tonic also uses native botanicals such as lemon myrtle and native lemongrass. Naturally, there's also more obvious "flavoured" tonics such as Fever-Tree elderflower tonic, East Imperial's pink grapefruit tonic.

Getting thirsty and want to get stuck in testing out some tonics instead? Try our Tonic Tasting Set here and put your knowledge to the test here.