Actually very easy

Invert sugar syrup

The beliefGlucose syrup and ready-made sherbet are assumed to be factory goods, and home syrup certain to crystallise.

The truthSugar, water and a pinch of acid. The acid splits the sucrose in two, and the syrup never crystallises again.

20 minutes

How to make it

What you need

1 kg granulated sugar
600 ml water
The juice of half a lemon, or 1 teaspoon of citric acid

How the engine read this list (3 lines)

1000 g	Granulated sugar	sugar
600 g	Water	water
20 g	The juice of half a lemon	lemon

An empty weight means that line could not be turned into grams — either the amount is open ("to taste") or the ingredient has no record here. Neither enters the sum as zero; they do not enter it at all.

Put the sugar and water in a pan over medium heat. Stir until the sugar has fully dissolved, then stop stirring.
Add the lemon juice once it comes to the boil. Added earlier, inversion starts before the sugar has dissolved and runs unevenly.
Wash any splashes down from the sides with a wet brush. One crystal drying at the rim can seed the whole pan.
Boil for fifteen to twenty minutes: 100–102 °C for a pouring syrup, 105 °C for one that draws threads. Without a thermometer, trust the time.
Take it off, let it come to room temperature and pour it into a clean glass bottle.

Why it works

Table sugar — sucrose — is one glucose molecule joined to one fructose. Heated in acidified water that bond breaks and the two are freed separately; the process is inversion and the product invert sugar. Two things follow. First, crystallisation stops: a sucrose crystal must build a regular lattice, and the loose glucose and fructose molecules do not fit it, so no crystal can grow. That is why invert syrup stays clear for months. Second, it holds moisture: fructose is hygroscopic and pulls water from the air, which is why cakes and lokum made with invert syrup stay soft for longer. Very little acid is needed and it does not change the taste — the juice of half a lemon per kilo of sugar is enough. Boiling time sets the body: at 100 °C the syrup is loose and watery, at 105 °C it draws threads.

The rough sum

1620 grams going in

3876 kcal in total

100% of the mass has figures behind it. This is the sum of what GOES IN rather than of what comes out. Wherever something evaporates, drains or ferments the two differ: a kilo of milk is 150 g of cheese.

Diets it qualifies for

Gluten-free Vegan Vegetarian Lactose-free

Storage & shelf life

How long it keeps

Sealed bottle at room temperature: 6 months
Fridge: 1 year
Flavoured with clove, zest or vanilla: 3 months
If it crystallises: Warm it and add a pinch of acid

What sets an invert syrup’s life is its sugar concentration. Above 60% the water activity is low enough that nothing can grow, so a thick syrup keeps for months while a diluted one lasts weeks. A wet spoon in the bottle, or water added to it, breaks exactly that balance. Flavoured syrups keep less well, because zest and spice bring both water and organisms with them.

When to bin it Bubbles at the surface, cloudiness or a yeasty smell mean fermentation has started and the sugar has been diluted. Do not try to boil it back — bin it.

History

Where it comes from

Sherbet is the backbone of Ottoman confectionery, setting the texture of every pastry from baklava to revani. A master squeezing lemon into the syrup looks like a matter of taste, but the function is chemical: the acid converts part of the sucrose into invert sugar and stops the syrup crystallising — “sugaring” — as it cools. The same logic runs through lokum, which stays soft for as much because of invert sugar as of starch. The hot-and-cold rule is technique too: cold syrup onto hot pastry, or hot syrup onto cold — both hot and the pastry disintegrates instead of drinking it in.

The knack

Never meet syrup and pastry at the same temperature: one hot, one cold — otherwise the pastry falls apart instead of drinking.
Where a recipe asks for glucose syrup this usually stands in for it: both block crystallisation by the same mechanism.
Avoid copper and aluminium pans: the acid attacks the metal and tints the syrup.

Preserves and syrups