A gummy sweet is a gel cast into a mould made of starch

The mould is made of starch, and that isn’t an eccentricity

Industrial gummy production still runs on a machine whose principle hasn’t changed much in over a century. Trays are filled with dry starch and levelled, a board carrying rows of plaster or plastic shapes is pressed into the surface to leave impressions, and hot gel syrup is deposited into each one. There is no metal mould involved anywhere, which strikes most people as strange until the reasons are laid out.

Starch solves three problems simultaneously. It releases cleanly from a sticky, wet gel where a rigid mould would not. It insulates, so the deposited syrup cools slowly and evenly instead of skinning over. And it is hygroscopic, so it draws moisture out of the sweets over the following days, which is the drying step the process depends on. A mould that also functions as a desiccant is a genuinely elegant piece of engineering.

The syrup has to be gelled, acidified and poured inside a window

The syrup is cooked with sugar and glucose to the target solids, and the gelling agent is then dispersed into it, and from that moment there is a schedule. It must stay hot enough to remain fluid and to deposit accurately into small impressions, and it must not stay hot for long, because heat degrades the gelling agents over time. Gelatine loses strength when held hot, and it loses it faster in the presence of acid.

This is why the acid goes in at the last possible moment, usually dosed into the stream as it is deposited. Acid supplies the sourness and, in pectin systems, it is what triggers the set at all, but acid plus heat plus time hydrolyses the network you are trying to build. A batch that sat in the kettle too long produces sweets that never firm up properly, and no amount of extra drying rescues them.

Setting happens in the trays over days rather than minutes

Once deposited, the trays are stacked and moved into a warm, dry room, and they stay there for something between a day and several days depending on the recipe. Two processes run at once. The gel network forms and strengthens, which for gelatine happens on cooling and continues to develop for a long while afterwards. And water leaves the sweet into the surrounding starch, bringing the solids up to the level at which the finished texture is right and the sweet is stable.

That second process is why the timing is measured in days. Water has to diffuse out of the middle of a small, dense gel, which is not fast, and pulling the trays early gives a sweet that is soft, sticky and short-lived. The starch is dried and recirculated for the next batch, which is the other reason the system persists: the mould material is reusable indefinitely.

The chew is decided by the gelling agent behind it

Gelatine gives the long, elastic chew most people picture, and it is the reason a classic gummy stretches before it yields. Pectin gives a shorter, cleaner bite that breaks rather than stretches, and it suits fruit sweets partly because it is a fruit material. Starch-based jellies are heavier and slower to release flavour, and agar sits at the brittle end of the range. Many products combine two agents to land somewhere between them.

Sugar composition matters alongside the gel. A higher proportion of glucose syrup keeps the sweet chewy and stops sucrose crystallising through it over the weeks it may sit on a shelf, and the ratio also affects how sweet the thing tastes independently of how much sugar it contains. The gel is the skeleton and the sugar phase is the flesh around it, and both are formulated rather than chosen casually.

De-starching, oiling and the finish

Cured sweets are tipped out, the loose starch is brushed and blown off, and what emerges is matt, slightly dusty and unattractive. The finish is applied afterwards. A tiny quantity of oil with a wax such as carnauba or beeswax is tumbled through, which polishes the surface, gives the familiar shine and slows moisture exchange with the air. Sour products are instead tumbled in a mixture of sugar and acid, which is why they feel gritty and why they go sticky faster.

A domestic version diverges at almost every one of these steps, which explains the difference in the result. Home recipes use silicone moulds, which don’t absorb water, so the sweets are set rather than dried and stay softer and wetter. They keep for days rather than months, they are prone to weeping in a humid kitchen, and they never reach the dense chew of a commercial sweet. That is a consequence of skipping the curing room, not of the recipe being wrong.