Chapter 7 · What heat does to food
7.4 Starches swell: gelatinization
Hydrocolloids
A hydrocolloid is anything that forms a gel when it meets water. Hydrocolloids come in two groups:
- Thermo-reversible gels melt when you heat them again and set again as they cool. Gelatin and agar agar behave this way.
- Thermo-irreversible gels stay set when reheated. Cornstarch and pectin behave this way. Too much heat can still dry the gel out and make it shrink.
A hydrocolloid does not take up water all at once. As it takes water in, it swells, and swelling grains easily stick together in lumps. So it must be spread through the liquid first. The classic way is a slurry: stir the starch into a little of the recipe's liquid, then add that to the pot. A stick blender or a regular blender also works, and so does mixing the powder first with sugar, oil or alcohol.
Starch gelatinization
When starch and water are heated together, the starch grains (granules) swell and soak up the water. This cannot be undone. The liquid turns thick and see-through, and the result is a gel. Sauces, puddings and creams all rely on it for their texture.
You see gelatinization in three places:
- Sauces. Starch goes into a liquid, usually as it heats. It soaks up liquid, swells and thickens the sauce. The starch you pick sets the finish: some stay cloudy, others turn clear.
- Pasta. Pasta is made mostly from semolina, a durum wheat flour rich in starch. In boiling water the starch takes in water and swells, and the pasta goes soft.
- Baked goods. Starch makes up most of a baked good. It cannot hold up the shape alone, but it gives the structure its bulk. Baked starch is softer than baked protein. The soft inside (crumb) of bread comes mostly from starch, and the more protein structure a bread has, the chewier it is.
Thickeners and gelling agents compared
Some starches are simple plant extracts, such as cornstarch, tapioca and arrowroot. Others are modified starches or pre-gelatinized starches, made for special jobs.
| Thickener | Amount for 1 L of liquid | How to add it | What it does |
|---|---|---|---|
| Cornstarch | 20–40 g | Make a slurry and whisk it into simmering liquid, or mix with sugar and add cold liquid; simmer until it no longer tastes of starch | Clear and glossy; good for dessert sauces and Asian-inspired dishes; thickens more on cooling into a gel you can slice; goes thin if kept hot too long |
| Waxy maize, waxy rice | 20–40 g | Dissolve in cold water, then whisk into hot liquid | Clear; does not thicken more as it cools and does not gel when cold, so it suits cold sauces; stands up to heat and freezing |
| Modified starches | 20–40 g | Dissolve in cold water, then whisk into hot liquid | Changed to cope better with heat and freezing; common in processed and convenience foods; translucent; thickens more on cooling |
| Pre-gelatinized starches | 20–40 g | Dissolve in cold liquid; add to liquid of any temperature | Thickens with no cooking, so it suits liquids that would lose color or flavor on the heat; shiny and fairly clear; no gel when cold |
| Arrowroot | 20–40 g | Dissolve in cold liquid, then whisk into hot liquid | Very clear and shiny, with a gooey texture; a very light gel when cold; used in Asian cuisines |
| Gelatin | 15–30 g | Soften powder or sheets in cold water, then add to cold or simmering liquid; heat activates it and cold sets it | Clear and firm; melts again when reheated; for aspic, glazes, desserts and cold sauces |
| Agar agar | 15–30 g | Dissolve powder in cold water, then add to cold or simmering liquid; heat activates it and cold sets it | Clear and firm; does not thin on reheating; used in vegetarian dishes in place of gelatin, and in Asian desserts |
These amounts are for thickeners and gelling agents used in the bakeshop.
Gelatin, agar and pectin
▶ LectureGelatin is a protein taken from animal tissue, from the collagen in bones and meat. It dissolves in water and does several jobs:
- it gels
- it thickens
- it emulsifies
- it works as a whipping agent
- it stabilizes
- it gives food a smooth feel in the mouth
You can buy it as powder or as sheets, also called leaves. It firms up whipped cream and mousse, gummy sweets and marshmallows, and panna cotta, and it turns up in low-fat spreads, yogurt and ice cream.
Agar agar comes from red algae. It thickens or gels liquids and keeps foams and emulsions stable. It takes up water in boiling liquid and copes with a wide range of acidity. It starts to set once it cools to about 40°C (100°F), and it does not melt again until 85°C (185°F).
Pectin comes from citrus and from tree fruits such as apples and pears. It is found in jams, jellies, sweets, fruit juices and milk drinks. Cooks mostly use two kinds:
- High-methoxyl pectin needs sugar to thicken. It is the traditional pectin for jams and jellies.
- Low-methoxyl pectin needs calcium to gel. Modern kitchens like it because it reacts well to calcium and forms a gel that heat does not undo.
Sources
This chapter is written from these open textbooks:
- Culinary Foundations — Cheramie & Thibodeaux, Chef John Folse Culinary Institute, Nicholls StateCC BY-NC-SA 4.0
- Modern Pastry and Plated Dessert Techniques — BC Cook Articulation Committee, BCcampusCC BY 4.0
- Basic Scientific Food Preparation Lab Manual — Dept. of Food Science and Human Nutrition, Iowa State UniversityCC BY-NC-SA 4.0
- The Chemistry of Cooking — Sorangel Rodriguez-Velazquez, LibreTextsCC BY-NC-SA 4.0