by Jeff Potter
Colloids
One of the more common uses of industrial chemicals in food is to form colloids. A colloid is any mixture of two substances—gas, liquid, or solid—where one is uniformly dispersed in the other, but they are not actually dissolved together. That is, the two compounds in the mixture don’t form chemical bonds, but the overall structure appears uniform to the naked eye.
Common colloids in the kitchen are whole milk and chocolate. In milk, solid particles of fat are dispersed throughout a water-based solution. In chocolate, particles of cocoa solids are dispersed throughout a solid medium of cocoa fat and other ingredients.
The following table shows the different combinations of particles and media, along with examples of foods for each colloid type. The medium of a colloid is called the continuous phase (it’s the watery liquid in milk); the particles are known as the dispersed phase (for milk, the fat droplets).
Gas particles
Liquid particles
Solid particles
Gas medium
(N/A: gas molecules don’t have a collective structure, so gas/gas combinations either mix to create a solution or separate out due to gravity)
Liquid aerosols
Mist sprays
Solid aerosols
Smoke (convertible to a solid-in-liquid colloid via liquid smoke)
Aerosolized chocolate
Liquid medium
Foams
Whipped cream
Emulsions
Milk
Mayonnaise
Sols and suspensions
Commercial salad dressings
Solid medium
Solid foams
Meringue cookies
Soufflés
Gel
Gelatin
Jell-O
Solid sols
Chocolate
Some of these colloid types might remind you of various dishes served at more experimental restaurants.
One of the surprises of this table is the relatively broad swath of techniques that it captures. Foams, spherifications, and gelled foods are all colloids. Even some of the more recent novel dishes are colloids from the gas medium category. Chef Grant Achatz (Alinea, in Chicago) has used solid aerosols by infusing a pillow with smoke and then placing the dish on top of the pillow, forcing the air containing the aerosol to leave the pillow and diffuse into the diner’s environment.
Note
Chef Achatz uses smoke-infused "pillows" to present a pleasant olfactory experience while avoiding the taste sensation for items such as mace and lavender.
Other luxury restaurants have created courses that involve liquid aerosols (by spraying a perfume), and one company (Le Whif) is working on a kitchen gadget that creates solid aerosols from foods such as chocolates.
Some food additives can be used in more than one type of colloid. For example, guar gum can act as an emulsifier (by preventing droplets of oil from coalescing) and as a stabilizer (by preventing solids from settling). Methylcellulose is both a gelling agent and an emulsifier. Don’t think of food additives as directly mapping onto the colloids they create, but it’s a handy framework for thinking about the types of effects you can achieve.
Making Gels: Starches, Carrageenan, Agar, and Sodium Alginate
The food industry uses gels to thicken liquids, to emulsify sauces, to modify texture ("improve mouth-feel," as they say), and to prevent crystal formation in products such as candies (sugar crystals) and ice cream (ice crystals and sugar crystals). Gels are also found in traditional home cooking: both gelatin (see the section on filtration in Chapter 7) and pectin (see the sidebar Make Your Own Pectin in Chapter 4) are used in many dishes to improve mouth-feel, and they also help preserve items such as jams.
From the perspective of modernist cuisine, thickeners and gels are used primarily to create dishes in which foods that are typically liquid are converted into something that is thick enough to hold its shape (this is what pectin does in jam), or even completely solid.
Gels can also be formed "around" liquids to create a gelatinous surface in a technique known as spherification, originally discovered by Unilever in the 1950s and brought to the modernist cuisine movement by Chef Ferran Adrià of elBulli. For our purposes, gels in foods can be classified into two general types: soft gels and brittle gels (true gels).
You can think of a soft gel as a thicker version of the original liquid: it has increased viscosity (it’s "thicker"), but it retains its ability to flow. Soft gels can exhibit a phenomenon termed shear thinning, which is when a substance holds its shape but will flow and change shape when pressure is applied. Substances like ketchup and toothpaste exhibit shear thinning: squeeze the bottle or tube, and it flows easily, but let go, and it holds its shape.
Iota carrageenan (left, 2% concentration) creates a flexible brittle gel, while kappa carrageenan (right, 2% concentration) creates a firm brittle gel. These two samples are resting on top of a narrow bar.
While a soft gel can be described as a "thicker" version of the original liquid, a brittle gel can be thought of as a solid. Brittle gels—foods like cooked egg whites and Jell-O—have a tightly interconnected lattice that prevents them from flowing at all. With sufficient quantities of the gelling agent, this type can form a block or sheet that you can pick up, slice into blocks or strips, and stack as a component in a dish, and it has a "memory" of its cast shape, meaning that it will revert to that shape when no other forces are in play.
In the consumer kitchen, cornstarch is the standard traditional gelling agent. In industrial cooking, carrageenan is commonly used in gelling applications. (Try finding cream cheese that doesn’t have carrageenan in it.) Iota carrageenan is used when a thickening agent is needed, while kappa carrageenan and agar yield firm, brittle gels. While the gelling agents used to create flexible and rigid gels are generally different, you can create a flexible gel with a gelling agent typically used in rigid, brittle applications by carefully controlling the quantity of gelling agent used.
Making gels: Starches
Starches are used as thickeners in everything from simple roux to pie filling. They’re easy, plentiful, and exist in almost all of the world’s cuisines: cornstarch, wheat flour, tapioca starch, and potato "flour" (not actually a flour) being the most common. While there are differences among these starches—size of the starch granules, length of the molecular structure, and variations on the crystalline structure—they all act essentially the same. Expose to water, heat up, then cool down, and they thicken up.
Gelatinization temperature of common starches.
Starch is composed of repeating units of amylopectin and amylose that form crystalline structures. The gelatinization temperature—the temperature at which these crystalline structures melt and then absorb water and swell—can vary, depending upon the ratio of amylopectin and amylose groups. We’ll examine cornstarch here, but as you play with the others, keep in mind that the gelatinization temperature can vary.
Instructions for use. To use cornstarch (called "corn flour" in the UK) to make a gel, mix it with a small amount of cold liquid such as water to create a slurry. Adding cornstarch directly to a hot liquid will result in clumps. Add the slurry to the desired dish and bring to a simmer.
Uses. Cornstarch is used as a thickener and has about twice the thickening ability of flour. When a recipe calls for a teaspoon of flour, use half a teaspoon of cornstarch. Cornstarch is gluten-free, making it a good thickening substitute for those with gluten allergies.
(Flour isn’t as good a thickener because it contains other stuff in addition to starch, such as gluten, fat, fiber, and minerals.)
Origin and chemistry. Derived from corn (shocker, I know). Like other starches used in cooking (e.g., potato, tapioca, wheat), cornstarch is a carbohydrate composed of repeating units of amylopectin and amylose that form crystalline structures. On heating, these structures swell up and break down. Upon cooling, the leached amylose molecules can link together to create a 3D mesh, trapping other mo
lecules into the network. For more on the chemistry of starches, see http://www1.lsbu.ac.uk/water/hysta.html.
Technical notes
Gelatinization temperature
203°F / 95°C; maximum thickness at 212°F / 100°C.
Gel type
Thixotropic. (This means it becomes less viscous when pressure is applied. Think ketchup: it holds its shape, but flows under pressure.)
Syneresis ("weeping")
Extensive if frozen and then thawed.
Thermoreversible
No—after gelatinizing, the amylose is leached out from the original starch molecules.
Lemon Meringue Pie
Like many savory foods in which multiple discrete components are combined to create the dish, lemon meringue pie is the combination of three separate components: pie dough, a meringue, and a custard-like filling. We’ve already covered pie dough (Simple Pie Dough in Chapter 5) and meringues (French and Italian Meringue in Chapter 5), so the only thing left for making a lemon meringue pie is the filling itself. Flip to those recipes for instructions on how to make the pie dough and meringue topping.
To make the lemon custard, place in a saucepan off heat and whisk together:
2 ½ cups (500g) sugar
¾ cup (100g) cornstarch
½ teaspoon (5g) salt
Add 3 cups (700g) of water, whisk together, and place over medium heat. Stir until boiling and the cornstarch has set. Remove from heat.
In a separate bowl, whisk together:
6 egg yolks
Save the whites for making the meringue. Make sure not to get any egg yolk in the whites! The fats in the yolk (nonpolar) will prevent the whites from being able to form a foam when whisked.
Slowly add about a quarter of the cornstarch mixture to the egg yolks while whisking continuously. This will mix the yolks into the cornstarch mixture without cooking the egg yolks (tempering). Transfer the entire egg mixture back into the saucepan, whisk in the following ingredients, and return to medium heat and cook until the eggs are set, about a minute:
1 cup (240g) lemon juice (juice of about 4 lemons)
Zest from the lemons (optional; skip if using bottled lemon juice)
Transfer the filling to a prebaked pie shell. Cover with Italian meringue made using the six egg whites (double the recipe in French and Italian Meringue in Chapter 5, which is for three whites), and bake in a preheated oven at 375°F / 190°C for 10 to 15 minutes, until the meringue begins to turn brown on top. Remove and let cool for at least four hours—unless you want to serve it in soup bowls with spoons—so that the cornstarch has time to gel.
To create decorative peaks on the meringue, use the back of a spoon: touch the surface of the unbaked meringue and pull upward. The meringue will stick to the back of the spoon and form peaks.
Note
Gelling agents typically come as a powdered substance that is added to water or whatever other liquid you are working with. Upon mixing with the liquid, and typically after heating, the gelling agent rehydrates and as it cools forms a three-dimensional lattice that "traps" the rest of the liquid in suspension. By default, add your gelling agent to a cold liquid and heat that up. Adding gelling agents to hot liquid usually results in clumps because the outer layer of the powder will gel up around the rest of the powder.
Making gels: Carrageenan
Carrageenan has been used in food as far back as the 15th century for thickening dairy products. Commercial mass production of carrageenan gums became feasible after World War II, and now it shows up in everything from cream cheese to dog food, where it acts as a thickener. Modernist cuisine dishes use it for the same reason, although typically to thicken liquids into gels in ways that we might not think of at first glance (beer gel, anyone?).
Instructions for use. Mix 0.5% to 1.5% carrageenan into room-temperature liquid. Gently stir liquid to avoid trapping air bubbles into the gel; lumps are okay at this stage. (They’re hard to get out unless you have a vacuum system.) Allow to rest for an hour or so; carrageenan takes a while to rehydrate. To set carrageenan, bring to a simmer either on a stovetop or in an oven. If you are working with a liquid that can’t be heated, create a thicker concentration using just water, heat that, and then mix it into your dish.
Uses. Carrageenan is used to thicken foods and to control crystal growth (e.g., in ice cream, keeping ice crystals small prevents a gritty texture). Carrageenan is commonly used in dairy (check the ingredients on your container of heavy whipping cream!) and water-based products, such as fast-food shakes (keeps ingredients in suspension and enhances mouth-feel) and ice creams (prevents aggregation of ice crystals and syneresis, the expulsion of liquid from a gel).
Origin and chemistry. Derived from seaweed (such as Chondrus crispus—common name Irish moss), carrageenan refers to a family of molecules that all share a common shape (a linear polymer that alternates between two types of sugars). The seaweed is sundried, treated with lye, washed, and refined into a powder. Variations in the molecular structure of carrageenan cause different levels of gelification, so different effects can be achieved by using different types of carrageenan (which, helpfully, grow in different varieties of red seaweed). Kappa carrageenan (k-carrageenan) forms a stronger brittle gel, and iota carrageenan (i-carrageenan) forms a softer brittle gel.
On the molecular level, carrageenan, when heated, untangles and loses its helical structure (left); when cooled, it reforms helices that wrap around each other and form small clusters (right). The small clusters can then form a giant three-dimensional net that traps other molecules.
Technical notes
i-carrageenan
k-carrageenan
Gelling temperature
95–149°F / 35–65°C
95–149°F / 35–65°C
Melting temperature
131–185°F / 55–85°C
131–185°F / 55–85°C
Gel type
Soft gel: gels in the presence of calcium ions
Firm gel: gels in the presence of potassium ions
Syneresis
No
Yes
Working concentrations
0.3% to 2%
0.3% to 2%
Notes
Poor solubility in sugary solutions
Interacts well with starches
Insoluble in salty solutions
Interacts well with nongelling polysaccharides (e.g., gums like locust bean gum)
Thermoreversible
Yes
Yes
Gelled Milk with Iota and Kappa Carrageenan
This isn’t, in and of itself, a tasty recipe (add some chocolate, though, and you’ve got something close to commercial prepackaged food). Still, it will give you a good sense of what adding a gelling agent does to a liquid and provides a good comparison between soft and brittle gels.
Flexible brittle version
In a saucepan, whisk to combine and then bring to a boil:
1 teaspoon (1.5g) iota carrageenan
3.5 oz (100 ml) milk
Pour into a glass, ice cube tray, or mold and chill in the fridge until set (about 10 minutes).
Firm brittle version
Again in a saucepan, whisk to combine and then bring to a boil:
1 teaspoon (1.5g) kappa carrageenan
3.5 oz (100 ml) milk
Pour into a second glass, ice cube tray, or mold and chill in the fridge until set.
Notes
Try modifying the recipe by adding 1 teaspoon (4g) of sugar, substituting some cream for a portion of the milk, popping the mixture into a microwave for a minute to set it, and pouring it into a ramekin that has a thin layer of jam or jelly and toasted sliced almonds on the bottom. Once gelled, invert the set gel onto a plate for something roughly approximating a flan-style custard.
Since the carrageenan is thermoreversible (once gelled, it can still be melted), you can take a block of food gelled with kappa carrageenan, slice it into cubes, and do silly things like serve it with coffee or tea (
one lump or two?).
You can take a firm brittle gel and break up the structure using a whisk to create things like thick chocolate pudding.
Making gels: Agar
Agar—sometimes called agar-agar—is perhaps the oldest of all the food additives commonly used in industry, but has only recently become known in western cuisines, mostly as a vegetarian substitute for gelatin. First used by the Japanese in the firm, jelly-type desserts that they’re known for, such as mizuyokan, agar has a history stretching back many centuries.
When it comes to playing with food additives, agar is one of the simplest to work with. You can add it to just about any liquid to create a firm gel—a 2% concentration in, say, a cup of Earl Grey tea will make it firmer than Jell-O—and it sets quickly at room temperature. It comes in two general varieties: flakes or powder. The powdered form is easier to work with (just add to liquid and heat). When working with the flake variety, presoak it for at least five minutes and make sure to cook long enough so that it breaks down fully.