Chapter 7 :- Emulsions

Introduction:- An emulsion is a colloidal dispersion of tiny droplets of one liquid suspended in another. In this colloidal system, liquids form the dispersed as well as continuous phase. One liquid is dispersed as droplets in another liquid. For an emulsion to form, agitation or shaking the two liquids is necessary till they are well mixed. Emulsions form only when the two liquids are immiscible in each other e.g oil and water. The liquid liquid with higher surface tension forms small droplets or the dispersed phase. When an emulsion is formed the dispersed liquid has a much larger surface area as compared to the two liquids as separate layers. 

Natural and synthetic emulsifiers:-

Food emulsifiers act as an interface between the conflicting components of food like water and oil. While preparing the food, often conflicting natural components of food have to be combined into a consistent and pleasing blend. Each component of food (carbohydrate, protein, oil and fat, water, air, etc.) has its own properties which are sometimes conflicting to one another just like oil and water. To make the two components compatible, emulsifiers are used.

Proteins, polysaccharides, phospholipids, and saponins can be used as natural emulsifiers.

Synthetic emulsifiers comes from synthetic sources. Examples of synthetic emulsifiers are carboxymethylcellulose (CMC) or polysorbate 80 (P80).

 (A.) Theory of emulsification:-

1.) During the process of emulsification, the main step is to break down the bulk liquid into small droplets and then stabilize the emulsion.

2.) In a stable emulsion, the droplets remain dispersed. But due to interfacial tension, there is a tendency for droplets to coalesce and separate out.The interfacial tension is lowered down by the addition of emulsifiers.EMULSIFIERS or emulsifying agents are surface active agents which lower the interfacial tension, i.e. the tension at the interface of two immiscible liquids.

3.) The dispersed droplets which are of colloidal dimensions tend to form spherical structures in the continuous phase.

4.) To prepare a stable emulsion, it is necessary to reduce the size of the droplets, prevent their coalescing, and increase their surface area.

5.) Mechanical aids such as beaters, stirrers, homogenizers and colloid mills help to reduce the size of the dispersed droplets, thereby increasing surface area.Energy is required to work against the interfacial tension and allow the continuous phase to sketch out and cover the dispersed droplets.

6.) Emulsifiers are used to reduce interfacial tension. They get absorbed at the interface.

7.) In an O/W emulsion, example mayonnaise, the non-polar group of the emulsifiers is oriented towards the oil droplet and is absorbed in the outermost layer of the droplet.

8.) The polar group of the emulsifiers is oriented towards the continuous phase of water (vinegar, lime juice, water from egg) surrounding the oil droplet.

9.) The molecules of emulsifier surround the droplet completely forming a monomolecular layer of emulsifier (lecithin) around the droplet.

10.) The oil droplet is thus protected by a film consisting of 3 layers:

      a.) The outermost layer of oil molecule

      b.) A layer of emulsifier

       c.) The innermost layer of water 

This protective film/layer prevents two oil droplets from coalescing when they collide.

11.) Emulsions are further stabilized by electrical charges.

12.) The ingredients used should not be chilled. Warm ingredients emulsify sooner as they are more fluid and spread or split into droplets faster.

13.) The consistency of an emulsion ranges from liquid to a plastic solid. 

(B.) Types of emulsions:-

Food emulsions are of two types:

1.) Oil in water emulsion or O/W emulsion in which the droplets of oil are dispersed in water, e.g.,mayonnaise and milk.

2.) Water in oil emulsion or W/O emulsion in which droplets of water are dispersed in oil, e.g., margarine and butter.

Emulsions may also be classified on the basis of stability as follows:-

1.) Temporary emulsions, e.g., French dressing 
2.) Semi-permanent emulsions, e.g., milk
3.) Permanent emulsions, e.g., mayonnaise, homogenised milk 

(C.) Emulsifying agents:-

Emulsifiers are also known as emulsifying agents or emulgent. The substance which is used in the emulsion to stabilize it by increasing its kinetic energy is called an emulsifying agent. These have more or less solubility either in oil or water as emulsifiers are those compounds which have polar or hydrophilic means water soluble part  and a non-polar or hydrophobic (or lipophilic part) means water insoluble part. In oil-in-water emulsion hydrophilic emulsifiers are used while water-in-oil emulsion, hydrophobic or lipophilic emulsifiers are used.

Examples of emulsifying agents:- 

Lecithin, soy lecithin, sodium phosphates, monoglycerides, diglycerides, sodium stearoyl lactylate,etc. These are generally used as food emulsifiers. 

Instability of Emulsions:- 

Stability of emulsion refers to ability of emulsion to retain its properties. Following four types of instabilities occur in emulsions-

• Flocculation - It is caused by aggregation of particles of dispersed phase. It is generally found in oil-in-water emulsions.
• Coalescence- It is caused by increasing the average particle size of the discontinuous phase. It can be seen in fog.
• Creaming- It is caused by the influence of buoyancy or centripetal force. It can be found in dairy products when they are left for a long time. 
• Ostwald Ripening- It is caused by In this,  change of inhomogeneous structure over time takes place. It is generally found in water-in-oil emulsions.

Uses of Emulsions:-

• Oil in water emulsions are used in food industries. For example- vinaigrettes are made by suspending oil in vinegar or something acidic. 
• Cutting fluid is a type of coolant or lubricant which is also an emulsion and used in metalworking processes.
• Mayonnaise, various sauces,etc. are oil in water emulsions which are stabilized by lecithin. 
• Margarine is also an emulsion which is used in the flavouring, baking and cooking. 
• Creams, ointments,balms, etc. pharmaceutical products are emulsions only. Emulsions are frequently used in pharmaceutics.
• Many hair creams and gels are emulsions. 
• Emulsions are used in personal hygiene also.
• Emulsions are used in vaccines.
• Nano-emulsions are used as disinfectants for surfaces. 

Characteristic of Emulsion –

• It should not damage the fiber.
• The emulsion should be colorless.
• It should be stable for a certain period of time.
• Its softening quality should be high.
• The emulsion droplets should be very small.

(D.) Role of emulsifying agents in food emulsions:-

In mayonnaise, the emulsifier is the phospholipids present in egg yolks – they are such successful emulsifiers that as much as 80% oil can be dispersed in the aqueous phase.

Ice cream is another food that would not exist were it not for emulsifiers. It is both a foam and an emulsion, and its texture results from the ice crystals and unfrozen water it contains. But it’s not just creamy products where emulsifiers are crucial – bread and other baked products, where solid particles are dispersed in an airy foam, are enhanced by emulsifiers.

The emulsifiers that are used commercially come from both natural and synthetic sources. They include:-

Lecithins (E322) are mixtures of phospholipids such as phosphatidylcholine and phosphatidylethanolamine and are usually extracted from sources such as egg yolk and soybeans. The precise composition of the phospholipids depends on the source. Uses include salad dressings, baked goods and chocolate.

Esters of monoglycerides of fatty acids (E472a-f) are made from natural fatty acids, glycerol and an organic acid such as acetic, citric, lactic or tartaric. The fatty acids are usually from a vegetable source, though animal fats can be used. Products that use them include ice cream, cakes and crisps.

Mono- and diglycerides of fatty acids (E471) are semi-synthetic emulsifiers made from glycerol and natural fatty acids, which can be from either plant or animal sources. They are used in products like bread, cakes and margarine.