Gratin: What It Is and the Science of a Golden Crust
A gratin is any dish baked or broiled in a shallow, wide dish until the top forms a golden-brown crust, usually from cheese, breadcrumbs, butter or cream. The word comes from the French gratter, "to scrape," after the tasty browned bits scraped from the pan. Potato gratin, cauliflower gratin, macaroni gratin and French onion soup are all built on the idea.
Why the crust forms
The crust is the Maillard reaction at work. Proteins and sugars in cheese, milk solids and breadcrumbs react in dry heat above about 140 °C / 285 °F, producing hundreds of new flavour compounds and that deep golden colour.
Browning only happens once the surface has dried out. Underneath, the food stays moist, and its temperature can't climb far past 100 °C / 212 °F while water is present. That contrast between a crisp, browned top and a soft, creamy interior is what defines a gratin.
That's also why gratins use a shallow dish, ideally 4–5 cm (1½–2 in) deep. More surface area means more crust per serving, and a thin layer cooks through before the top burns.
Keeping the creamy layer smooth
Milk proteins (mainly casein) can clump and separate under long heating, especially with acid or high heat. Fat protects them. Heavy cream (about 35% fat) holds up through an hour in the oven, while 1% or 2% milk is more likely to curdle into grainy specks.
Starch helps, too. Freshly sliced potatoes release surface starch that thickens the cream as it gelatinizes. Recipes using lower-fat milk usually thicken it first into a béchamel; the flour coats the proteins and keeps the sauce smooth.
Classic potato gratin numbers
- Slice thickness: 3 mm (⅛ in), evenly, ideally with a mandoline.
- Potatoes: Yukon Gold (all-purpose) holds its shape; russets give a softer, starchier result.
- Liquid: about 500 mL (2 cups) cream, or cream and milk mixed, per 1 kg (2.2 lb) of potatoes, coming two-thirds to three-quarters up the layers.
- Bake: 160–180 °C / 325–350 °F for 60–90 minutes, until a knife slides through with no resistance.
- Brown: uncover for the last 20–30 minutes, or broil for 2–4 minutes at the end.
- Rest: 10–15 minutes so the sauce thickens and slices hold together.
Best toppings
Good gratin cheeses melt smoothly and brown well: Gruyère, Comté, Emmental, aged cheddar and parmesan. Very fresh, high-moisture cheeses like fresh mozzarella release water and turn rubbery. Breadcrumbs tossed in melted butter, about 1 tbsp butter per 30 g (½ cup) panko, add crunch.
Common mistakes
- Uneven slices, leaving some potatoes raw and others mushy.
- Oven too hot. The top burns and the cheese turns greasy before the centre cooks.
- Too much liquid, giving a soupy result. Don't fully submerge the top layer.
- Serving straight from the oven. A rest lets the starch set.
Questions
What does au gratin mean?
Au gratin is French for 'with a crust' and describes any dish finished with a browned top, usually from cheese, breadcrumbs, butter or cream. In North America, potatoes au gratin generally means sliced potatoes baked in cream or sauce with cheese.
What is the difference between scalloped potatoes and au gratin?
In North American usage, scalloped potatoes are baked in cream or a milk sauce without cheese, while potatoes au gratin include cheese and often breadcrumbs. The French gratin dauphinois is potatoes baked in cream and garlic, traditionally with no cheese.
Why did my gratin curdle or turn greasy?
Low-fat milk curdles when heated for a long time, especially with acidic ingredients or high oven heat, and cheese turns oily if it overheats. Use cream or a flour-thickened béchamel, keep the oven at about 160 to 180 °C / 325 to 350 °F, and brown the top at the end.
Should you rinse potatoes for gratin?
No. The starch on freshly cut potato slices dissolves into the cream and thickens it as it bakes. Rinsing washes it away and can give a thin, soupy gratin.