1.2: Gelatinization
Hydrocolloids
A hydrocolloid is a substance that forms a gel in contact with water. There are two main categories:
- Thermo-reversible gel: A gel that melts upon reheating and sets upon cooling. Examples are gelatin and agar agar.
- Thermo-irreversible gel: A gel that does not melt upon reheating. Examples are cornstarch and pectin.
Excessive heating, however, may cause evaporation of the water and shrinkage of the gel. Hydrocolloids do not hydrate (or dissolve) instantly, and that hydration is associated with swelling, which easily causes lumping. It is therefore necessary to disperse hydrocolloids in water. Classically, this has always been done with cornstarch, where a portion of the liquid from the recipe is mixed to form a “slurry” before being added to the cooking liquid. This can also be done with an immersion blender or a conventional blender, or by mixing the hydrocolloid with a helping agent such as sugar, oil, or alcohol prior to dispersion in water.
Starches
Starch gelatinization is the process where starch and water are subjected to heat, causing the starch granules to swell. As a result, the water is gradually absorbed in an irreversible manner. This gives the system a viscous and transparent texture. The result of the reaction is a gel, which is used in sauces, puddings, creams, and other food products, providing a pleasing texture. Starch-based gels are thermo-irreversible, meaning that they do not melt upon heating (unlike gelatin, which we will discuss later). Excessive heating, however, may cause evaporation of the water and shrinkage of the gel.
The most common examples of starch gelatinization are found in sauce and pasta preparations and baked goods.
- In sauces, starches are added to liquids, usually while heating. The starch will absorb liquid and swell, resulting in the liquid becoming thicker. The type of starch determines the final product. Some starches will remain cloudy when cooked; others will remain clear.
- Pasta is made mostly of semolina wheat (durum wheat flour), which contains high amounts of starch. When pasta is cooked in boiling water, the starch in the pasta swells as it absorbs water, and as a result the texture of the pasta softens.
Starch molecules make up the majority of most baked goods, so starch is an important part of the structure. Although starches by themselves generally can’t support the shape of the baked items, they do give bulk to the structure. Starches develop a softer structure when baked than proteins do. The softness of the crumb of baked bread is due largely to the starch. The more protein structure there is, the chewier the bread.
Starches can be fairly straightforward extracts of plants, such as cornstarch, tapioca, or arrowroot, but there are also modified starches and pre-gelatinized starches available that have specific uses. See Table 1 for a list of different thickening and binding agents and their characteristics.
| Starch or Gel | Ratio | Preparation | Characteristics and Uses |
|---|---|---|---|
| Cornstarch | 20-40 g starch thickens 1 L liquid |
A slurry (mixture of cornstarch and water) is mixed and added to a simmering liquid while whisking until it dissolves and the liquid thickens;
or
Cornstarch mixed with sugar, and cold liquid added Thickened mixture simmered until no starch taste remains |
Used to thicken sauces when a clear glossy texture is desired, such as dessert sauces and in Asian-inspired dishes
Translucent, thickens further as it cools; forms a “sliceable” gel Sensitive to extended heat exposure, so products become thin if held at heat for long periods of time |
| Waxy maize, waxy rice |
Dissolved in cold water
20-40 g starch thickens 1 L liquid |
Added to hot liquid while whisking until it dissolves and the liquid thickens |
Used in desserts and dessert sauces
Clear, does not thicken further as it cools Does not gel at cool temperatures, good for cold sauces Quite stable at extreme temperatures (heat and freezing) |
| Modified starches |
Dissolved in cold water
20-40 g starch thickens 1 L liquid |
Added to hot liquid while whisking until it dissolves and the liquid thickens |
Modified starches are often used in commercially processed foods and convenience products
Modified to improve specific characteristics (e.g., stability or texture under extreme conditions; heat and freezing) Translucent, thickens further as it cools |
| Pre-gelatinized starches |
Powder, dissolved in cold liquid
20-40 g starch thickens 1 L liquid |
Added to liquid at any temperature |
Used when thickening liquids that might lose colour or flavour during cooking
Become viscous without the need for additional cooking Translucent, fairly clear, shiny, does NOT gel when cold |
| Arrowroot |
Powder, dissolved in cold liquid
20-40 g starch thickens 1 L liquid |
Added to hot liquid while whisking until it dissolves and the liquid thickens |
Derived from cassava root
Used in Asian cuisines Very clear; possesses a gooey texture Translucent, shiny, very light gel when cold |
| Gelatin | 15-30 g gelatin sets 1 L liquid |
Powder or sheets (leaves) dissolved in cold water
Added to cold or simmering liquid Activates with heat, sets when cold |
Derived from collagens in bones and meats of animals
Used in aspic, glazes, cold sauces, and desserts Clear, firm texture Dissolves when reheated, thickens when cold |
| Agar agar | 15-30g agar agar sets 1 L liquid |
Powder dissolved in cold water
Added to cold or simmering liquid Activates with heat, sets when cold |
Extracted from seaweed
Used in Asian desserts and molecular gastronomy cooking Used in place of gelatin in vegetarian dishes Clear firm texture Does not thin when reheated, thickens more when cold |
Gelling agents
Gelatin is a water-soluble protein extracted from animal tissue and used as a gelling agent, a thickener, an emulsifier, a whipping agent, a stabilizer, and a substance that imparts a smooth mouth feel to foods. It is thermo-reversible, meaning the setting properties or action can be reversed by heating. Gelatin is available in two forms: powder and sheet (leaf).
Gelatin is often used to stabilize whipped cream and mousses; confectionery, such as gummy bears and marshmallows; desserts including pannacotta; commercial products like Jell-O; “lite” or low-fat versions of foods including some margarines; and dairy products such as yogurt and ice cream. Gelatin is also used in hard and soft gel capsules for the pharmaceutical industry.
Agar agar is an extract from red algae and is often used to stabilize emulsions or foams and to thicken or gel liquids. It is thermo-reversible and heat resistant.
It is typically hydrated in boiling liquids and is stable across a wide range of acidity levels. It begins to gel once it cools to around 40ºC (100ºF) and will not melt until it reaches 85ºC (185ºF).
Pectin
Pectin is taken from citrus and other tree fruits (apples, pears, etc.). Pectin is found in many different foods such as jam, milk-based beverages, jellies, sweets, and fruit juices. Pectin is also used in molecular gastronomy mainly as a gelling agent, thickener, and stabilizer.
There are a variety of types of pectin that react differently according to the ingredients used. Low-methoxyl pectin (which is activated with the use of calcium for gelling) and high-methoxyl pectin that requires sugar for thickening are the two most common types used in cooking. High-methoxyl pectin is what is traditionally used to make jams and jellies. Low-methoxyl pectin is often used in modern cuisine due to the thermo-irreversible gel that it forms and its good reaction to calcium. Its natural capability to emulsify and gel creates stable preparations.
Increasingly, cooks, bakers, and pastry chefs are turning to many different gels, chemicals, and other substances used in commercial food processing as new ingredients to modify liquids or other foods. These will be outlined in detail in the section on molecular gastronomy.