Gelatinization of Starches

Starches

Starch gelatinization is the process where starch and water 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 to provide 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 cooking pasta in boiling water, the starch in the pasta swells as it absorbs water, and 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 cannot 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 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.

Hydrocolloids

A hydrocolloid is a substance that forms a gel in contact with water. There are two main categories:

1. Thermo-reversible gel: A gel that melts upon reheating and sets upon cooling. Examples are gelatin and agar-agar.
2. 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 hydration is associated with swelling, which easily causes lumping. It is therefore necessary to disperse hydrocolloids in water. Classically, as done with cornstarch, a portion of the liquid from the recipe is mixed to form a “slurry” before adding to the cooking liquid using 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.

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 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 Panna Cotta; 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 also used in hard and soft gel capsules for the pharmaceutical industry.

Agar-agar

Is an extract from red algae often used to stabilize emulsions or foams and to thicken or gel liquids. It is thermo-reversible and heat resistant. It is 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 185ºF (85ºC).

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 varieties of pectin that react differently according to the ingredients used. Low-methoxyl pectin (activated with the use of calcium for gelling) and high-methoxyl pectin, which requires sugar for thickening, are the two most common types used in cooking. High-methoxyl pectin 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.

Crystallization

Many factors can influence crystallization in food. Controlling the crystallization process can affect whether a particular product is spreadable, or whether it will feel gritty or smooth in the mouth. In some cases, crystals are something you try to develop; in others, they are something you try to avoid. It is important to know the characteristics and quality of the crystals in different food. Butter, margarine, ice cream, sugar, and chocolate all contain different types of crystals, although they all contain fat crystals. For example, ice cream has fat crystals, ice crystals, and sometimes lactose crystals. The fact that sugar solidifies into crystals is extremely important in candy making. There are two categories of candies: crystalline (candies that contain crystals in their finished form, such as fudge and fondant); and non-crystalline (candies that do not contain crystals, such as lollipops, taffy, and caramels). Recipe ingredients and procedures for non-crystalline candies are specifically designed to prevent the formation of sugar crystals because they give the resulting candy a grainy texture. One way to prevent the crystallization of sucrose in candy is to make sure that there are other types of sugar—usually fructose and glucose—to get in the way and slow down or inhibit the process. Acidic items will have a similar effect. When boiling sugar for any application, the formation of crystals not desired.

The following lead to crystal growth:

• Pot and utensils that are not clean
• Sugar with impurities in it (a scoop used in the flour bin, and then used for sugar, may have enough particles on it to promote crystallization)
• Water with a high mineral content (“hard water”)
• Too much stirring (agitation) during the boiling phase

Crystallization can be prevented by adding an interferent, such as acid (lemon, vinegar, tartaric, etc.) or glucose or corn syrup, during the boiling procedure.

As mentioned above, ice cream can have ice and fat crystals that co-exist along with other structural elements (emulsion, air cells, and hydrocolloid stabilizers such as locust bean gum) that make up the “body” of the ice cream. Some of these components crystallize either partially or completely. The bottom line is that the nature of the crystalline phase in the food will determine the quality, appearance, texture, feel in the mouth, and stability of the product. The texture of ice cream is derived, in part, from the large number of small ice crystals. These small ice crystals provide a smooth texture with excellent meltdown and cooling properties. When these ice crystals grow larger during storage (recrystallization), the product becomes coarse and less enjoyable. Similar concerns apply to sugar crystals in fondant and frostings, and to fat crystals in chocolate, butter, and margarine. Control of crystallization in fats is important in many food products, including chocolate, margarine, butter, and shortening. In these products, the aim is to produce the appropriate number, size, and distribution of crystals in the correct shape because the crystalline phase plays such a large role in appearance, texture, spreadability, and flavor release. Thus, understanding the processes that control crystallization is critical to controlling quality in these products.

Certain factors to control crystallization in foods:

• Number and size of crystals
• Crystal distribution
• Proper polymorph (crystal shape)

Crystallization is important in working with chocolate. The tempering process, sometimes called precrystallization, is an important step that is used for decorative and molding purposes, and is a major contributor to the mouth feel and enjoyment of chocolate. Tempering is a process that encourages the cocoa butter in the chocolate to harden into a specific crystalline pattern, which maintains the sheen and texture for a long time.

When chocolate is not tempered properly, it can have a number of problems. For example, it may not ever set up hard at room temperature; it may become hard, but look dull and blotchy; the internal texture may be spongy rather than crisp; and it can be susceptible to fat bloom, meaning the fats will migrate to the surface and make white streaks.