UNIT 7 - EMULSION

EMULSIONS

Theory of emulsification

An emulsion is a system containing two liquid phases, one of which is dispersed as globules in the other.
That liquid which is broken up into globules is termed the dispersed phase, whilst the liquid surrounding the globules is known as the continuous phase or dispersing medium.
The two liquids, which must be immiscible or nearly so, are frequently referred to as the internal and external phases respectively.

Types of emulsions

There are two basic types of emulsions: oil-in-water (O/W) and water-in-oil (W/O). These emulsions are exactly what they sound like, as pictured below.

How to make an emulsion

Mechanical Force - To make an emulsion you first need to apply a mechanical force to break down the dispersed phase into small droplets that become suspended in the continuous phase. For this you can use a whisk, blender or other lab equipments.

Emulsifier - The next problem to solve is to make the emulsion stable. The higher the force you applied when making the emulsion, the smaller the droplets and the more stable the emulsion is. However, no matter how small the droplets are, the ingredients will eventually separate without the presence of an emulsifier that keeps the molecules with different polarity from repelling each other.

Thickener - Finally, adding a thickener to the continuous phase can make the emulsion even more stable as this makes it more difficult for the dispersed droplets to move and combine.
How to make an emulsion
Any mixture of oil and water can be transformed into an emulsion, even without emulsifiers. However, without emulsifiers, the mixture will also quickly separate back into its immiscible (not mixable) parts. You can try a simple experiment with oil and water. Quickly whisk together oil and water, and it will eventually turn into an cloudy mixture. You'll notice plenty of bubbles, however, and those bubbles will quickly grow larger until the water and oil are completely separate.

Properties of an emulsion

Viscosity
When you mix oil and water, the resulting emulsion usually has a higher viscosity than each of the ingredients before the emulsification process. This effect is produced by the interaction of the molecules in the emulsion. For example, mayonnaise is more viscous than the oil and lemon juice it is made of. Most emulsions are shear-thinning fluids which means that the viscosity decreases if you start stirring them strongly.

Color
The transparency and color of the emulsion depend on the size of the droplets of the dispersed ingredient. The smaller they are, the whiter the color of the emulsion. This is due to the way the droplet size affects the light reflection.

Common culinary emulsions

Following are a few of the more common culinary emulsions, both traditional and modernist.

Vinaigrette Emulsion

Vinaigrettes are traditional oil-in-water emulsions made with oil, vinegar, other flavorings, and mustard. The emulsifying ingredient is mustard. Specifically, the network of naturally-occurring mucilage in mustard emulsifies the oil and water. In addition to mustard, a common ingredient in vinaigrettes is honey. While honey is not an emulsifier, its thick consistency helps to stabilize the mixture.

Dairy Emulsion

All dairy products that is, anything made with milk, contains milk proteins that act as emulsifiers and help milk fats stay suspended in water. Milk and cream are O/W emulsions while butter is a W/O emulsion. In addition, any recipe that calls for cream, milk, or butter will benefit from the natural emulsifying properties of these dairy proteins.

Egg Emulsion

Both parts of the egg contain important emulsifiers. The proteins in egg white act as emulsifiers and thickeners. Egg whites or egg white powder can be used to create soft foams . You can also use them to create meringues or add fluffiness to recipes.
Egg yolks contain two important things not found in egg whites: fat and lecithin. Lecithin is a powerful emulsifier. A small addition of lecithin to a vinaigrette or sauce containing oil will help the liquids stay mixed for a longer time. Many modernist chefs use lecithin derived from soy beans instead of egg lecithin because it is cheaper to produce and because recipes using soy lecithin have the added benefit of being completely vegan.
Fat can support emulsions, but can also interfere with them. Fat is what gives vinaigrettes their body, but even a little fat in an egg white foam can cause the foam to destabilize. That's because foams combine nonpolar air with polar liquids. Fat takes the place of air in some cases and makes the foam weaker.

Cheese Emulsion

While cheese is technically a dairy product, it is worth mentioning separately here due to it unique properties. Most cheeses are solids at room temperature, but many become creamy emulsions when heated. But if you've ever accidentally overheated a brie in the oven or tried making your own cheese sauce in the microwave, you know that some cheeses will split into unattractive masses of oil and cheese matter. The problem lies in emulsifiers. Once again, it is the milk proteins in cheese that act as emulsifiers. The particular ratio of fats to other ingredients in cheese determine how it will behave when melted.

Emulsion stabilizers and emulsifiers

Traditional emulsifiers such as dairy and eggs continue to be widely used in modernist cuisine. Where modernist technique innovation comes in is with the addition of emulsion stabilizers and modern surfactant emulsifiers.

Emulsion Stabilizers

Modernist thickeners and gelling ingredients can make emulsions more stable. Besides giving emulsions more texture, thickeners also help to slow down the rate at which emulsions separate. As the liquid is more viscous, the suspended droplets can't move around so easily to eventually combine in a specific area. This is important for everything from a sauce, which needs to stay emulsified for as long as it would take a dinner to eat it, to ice cream, which needs to stay emulsified to prevent the growth or ice crystals.
Agar, carrageenan,  sodium alginate , gellan,  xanthan gum , gelatin, guar gum, can all be used as emulsion stabilizers.

Emulsion techniques

The appearance, texture, and stability of an emulsion depend both on its ingredients and on how you prepare it. There are several tools you can use to make an emulsion but the higher the force, the smaller the droplets in the emulsion and the more stable it will be.

Handheld whisk

Using a handheld whisk will produce the weakest emulsions but it may be more than enough if you prepare it right before serving. Some emulsions, such as mayonnaise, can be mixed by hand and will stay stable in the refrigerator for days, even weeks. But one problem with mayonnaise is that it can curdle or invert. That is, the ratio of oil added to eggs and other ingredients can cause clumps of butter-like water-in-oil emulsion bits to form. This makes the finished product less than ideal.

Immersion blender

Use an immersion blender for small quantities or when you need to make the emulsion while heating. To combat the previously mentioned problem when making mayonnaise, try using an immersion blender.

Blender

If you're making larger batches of emulsion, nothing beats a powerful blender. Use traditional blender for larger quantities or for more difficult emulsions that need more power. Commercial blenders have more power than household blenders and produce better emulsions with smaller droplets.
The quality of a blender-made emulsion depends on two factors: shear stress and carafe shape. Shear has to do with the speed at which the blender's blades turn through a liquid. The faster the shear, the smaller the particles in an emulsion. The smaller the particles, the creamier the result, until the particles are so small that your tongue can no longer distinguish them, resulting in a milk-like texture. In addition to shear, the size and shape of a blender's carafe determine the blade's ability to fully interact with all of the carafe's contents. If only a small portion of an emulsion benefits from maximum shear, the end result will still be poor.

Rotor-stator homogenizer

Not a traditional kitchen tool and expensive, rotor-stator homogenizers are used to make fine emulsions. It generates much higher shear forces than a regular blender thanks to the two blades that move very close to each other to produce fine emulsions. One of the blades is stationary and the other one rotates. (shown in left picture)

Ultrasonic homogenizer

Another expensive lab tool usually used to make an existing emulsion even finer through mechanical vibrations that create microscopic bubbles.

High-pressure homogenizer

The ultimate lab tool to make the finest emulsions ever works by applying pressure to the liquid.

Emulsion tips

In addition to picking the right emulsifier for the desired application and using the right tool as described above, there are a few other things to consider when making an emulsion.

Viscosity

An emulsion can be made more stable by increasing the viscosity (thickening) of the continuous phase in the emulsion as this makes it more difficult for the dispersed droplets to move and combine. Remember that in an O/W emulsion the continuous phase is the water and in an W/O emulsion the continuous phase is the oil. So adding a thickener to the continuous phase can help you make the emulsion more stable.  This is a widely used technique and that's why you'll frequently see the use of thickeners and emulsifiers combined to make effective stable emulsions.

Density

Mixing oil and water is not only hard because of the polar differences but also because of the difference in density. The oil molecules are lighter than the water molecules but if you make the oil heavier, the emulsion will be more stable. This is not usually necessary in most culinary applications.
Speed of incorporating ingredients
When making an emulsion, it is usually better to disperse the emulsifier first in the continuous phase and then slowly incorporate the dispersed phase while you are mixing with the selected tool.

Role of emulsifying agents in food emulsions

The use of food emulsifier began with adding monoglyceride and lecithin to margarine. Originally, these substances were only known as emulsifiers. However, as studies progressed, more functions were found and they began to be used in various fields, such as bread, ice cream and cake. Nowadays, emulsifiers are applied to Japanese foods like tofu and minced fish products. Why do bread and tofu need emulsifiers? Generally speaking, an emulsifier is well known for its emulsifying effects, however, actually it has various functions and followings are just some examples: Modifies oil crystal and prevents water spattering in cooking;Destroys emulsion to stabilize foam and to make smooth texture in ice cream, and keeps its shape; Reacts with proteins to make a smooth easy-rising dough in bread;Acts on starch to make bread soft.
Emulsifiers have various effects on the production process of food and improve its quality. They are used in various types of food. Functions of emulsifiers are listed below.


Emulsifiers have various effects on the production process of food and improve its quality. They are used in various types of food. Functions of emulsifiers are listed below.
Bread and sweet rolls sold at super-markets and convenience stores are usually mass-produced. Mass production and mass distribution require time and speed from start to finish. Emulsifiers are used to maintain the softness as long as possible and to make bread dough suitable for machine production.
Emulsifiers are not only used for emulsification, but also for dough modification, that is, dough gets tolerable against mechanical force by modulating the proteins in wheat flour.


Emulsifiers make a rigid complex with starch to protect starch granules and improve the quality of starchy foods.

Noodles

Effects of emulsifiers on macaroni, spaghetti of low water content, fresh noodles containing high water content and dried instant noodles are different, however, the basic effects on starch are similar.
For macaroni and spaghetti, emulsifiers provide elasticity and smooth uniform surface which prevents sticking after boiling.
In fresh noodles, emulsifiers make easy-to-handle dough and increase the water absorption rate by 1-2%. The surface of noodles becomes smooth, uniform, and less sticky, which improve and streamline the production process.

In instant noodles, emulsifiers improve absorption and decrease cooking time.

Cakes are classified on the basis of the compounding ratio of fat to flour as shown in the following table.

Oils and Fats Content Ranging
0-10% Sponge Cake
10-50% Butter Sponge Cake
30-100% Butter Cake
Sponge cake is made by utilizing the foaming ability of eggs.
Butter sponge cake is made by further additions of butter and margarine.
On the other hand butter cake is produced from a batter foamed after the addition of large amount of butter or margarine.

Sponge and butter sponge cake are basically made from sugar batter method. Sugar is added to whole eggs after foaming and combined with flour.
The use of emulsifier makes it possible to produce an all in one-mixture method, in which all materials are mixed at the same time and foamed.

ICE CREAM
Concerning the manufacturing process of ice cream, a liquid form mixture of raw materials is prepared and homogenized by a high pressure homogenizer. After short period of high-temperature sterilization, the homogenate is stabilized by letting it stand sit for several hours at a temperature below 5℃.
Soft cream is produced by foaming in a freezer at -2 to -9°C, and pouring into a small container followed by rapid freezing.
Oils and fats, ones of the major components of ice cream, form fat balls by emulsification, and their surfaces are covered with emulsifiers and are coated with milk-like protein like casein. By combining with an emulsifier, the fat balls become fine and stable.