Sparkling mineral water with freshly squeezed organic lemon juice and lemon zest extract. Nothing else. Serve very cold.
The idea, which is a good one
Water with a slice of lemon in it, made portable. That is genuinely all this is, and the whole point is what is not in it — no sugar, no sweetener, no flavouring, no colour.
The soft drinks aisle offers sweet drinks and it offers plain water, and remarkably little in between. This is the in-between: a drink that tastes of something without tasting of sugar.
Why the zest extract matters
The part worth understanding. Almost all of a lemon's aroma is in the peel, not the juice.
The juice provides acidity — citric acid — and very little smell. The oil glands in the zest hold limonene and citral, which are what your nose registers as "lemon". That is why zesting a lemon perfumes a whole kitchen and squeezing one does not, and why a drink made with juice alone tastes sour rather than lemony.
Using both is what makes this taste like a lemon rather than like acid water.
Serve it properly cold
Not a refinement. Cold does two things here: it suppresses the perception of sourness, and it holds carbon dioxide in solution.
A warm sparkling drink is both flatter and sharper — and with almost no sugar to round it off, this one has nothing to hide behind. Straight from the fridge, or over ice.
Teeth — worth knowing with any citrus drink
Acidic drinks soften enamel temporarily, and citrus plus carbonation is more acidic than most. This is about acid rather than sugar, so a sugar-free drink is not exempt.
Practical version: drink it with a meal rather than sipping it across an afternoon, and do not brush your teeth for about half an hour afterwards — brushing softened enamel is what actually removes it. Rinsing with plain water afterwards helps.
A note on the labelling
The companion grapefruit product carries a footnote reading "*Calorie-free: >4kcal / 100ml". The symbol is the wrong way round — the EU threshold for an energy-free claim is no more than 4 kcal per 100 ml, so it should read "<". A typographical error rather than a misleading claim, and we have flagged it to the producer.