Food Dispersion

Food DispersionsNearly all foods contain water

Nutrients + water FoodNutrients are dispersed in waterQuestions1. How are these nutrients dispersed in water2. How does the nutrients interact with the water and

with each other3. How do these nutrients remain in the form in which

they exist and give the characteristics of the food

Components of food

• There are many ways to think of dinner. You could simply divide the meal into the appetizers, main course and dessert.

• Or you could categorize the foods based on their protein, carbohydrate, fat and mineral content. This division is more complex, since the foods may fall into more than one category. (For example, the steak contains protein, minerals and fat, as well as water.)

Components of food

Another method is to divide the food according to how particles are dispersed in it:

In other words, how solids, liquids and gases are suspended, or dissolved, in other solids, liquids, and gases.

For example, cranberry juice is a liquid that contains insoluble solids (fibers) and dissolved solids (sugars).

Jam is also a liquid with a solid (sugar) dissolved in it. However, this liquid is suspended in pectin, which is an insoluble solid.

Dispersions

These dispersions are further classified astrue solutions, colloids and suspensions.

True solutionThe simplest category is a true solution. Dissolving a

solid into a liquid (such as salt into water or sugar into coffee) results in a solution.

Almost every food contains some amount of liquid solvent, usually water. Because of this, all foods with water contain solutions. With water as a solvent, the dispersed phase, or solute, can be a gas, liquid, or solid.

True solution• What do an apple, a steak, and a carbonated

beverage have in common? They all contain solutions . The type of solution varies according to the kind of solute in the food.

• Apple particles may contain dissolved acids, sugars or salts.

• In the steak, dispersed salts and sugars create true solutions.

• And the carbonated beverage contains not only dissolved sugars and salts, but small gas bubbles dispersed in the liquid.

Homogeneous dispersion vs Dispersed particles

• Food scientists generally think of water from two points of view.

• First, the functional properties view defines a "true solution" as a homogeneous dispersion in which the solute is evenly distributed throughout the dispersing liquid and shows no tendency whatsoever to separate.

• Second, the dispersed particles view defines a solution as a dispersion in which the dispersed particles are single molecules, or ions, or both.

Colloids

In a colloidal system, particles are disperse throughout the food without dissolving.

In other words, food ingredients remain separate, but are distributed evenly (more or less) throughout the system

SuspensionsA suspension is the most complex system.

Suspensions contain particles of solid matter which are large enough to settle out. Sometimes you can see the particles and other times you cannot, depending on the dispersing medium and the size and shape of the particles.

How dispersions differ• How can you tell if food is a true solution,

colloid or suspension? These systems differ from each other in at least 20 or 30 ways. Three most critical ways dispersions differ are:

• the size of the particle,• colligative properties, and • ability of the particles to settle out of the

system

Size of particles

True solutions have the smallest dispersed particles. These particles are usually less than 1 nanometer in size.

Ability to settle out

Some particles are unable to dissolve or become evenly dispersed; instead, they visibly "settle out" from food, like fruit settles out from gelatine. The result is a suspension.

Ability to settle out-contd

• In true solutions, particles do not settle out over time. Instead, the solute (in this case, sugar) dissolves completely into the water.


• Adding dry milk solids to water creates a colloid. The lactose and salts in the solids are dissolved into a solution, but the proteins are colloid ally dispersed.


• The particles in colloidal systems may settle out over time; however, most foods would become inedible (or, like milk, would spoil) before the colloids become visible.

Size of particles

Unfortunately, there are exceptions to size for some colloids and suspensions.

• For example, when cornstarch is added to water, it settles out to form a suspension. Yet if the water is heated, the cornstarch granules fill with water, swell, and form a colloid.

• For this reason, food scientists also consider the ability of particles to "settle out" and colligativeproperties when distinguishing between the two systems.

true solutions have the smallest dispersed particles. These particles are usually less than 1 nanometer in size.

The dispersed particles in colloidal systems range from 1 nanometer to 0.2 micrometer

In suspensions, dispersed particles are generally larger than 0.2 micrometer.

Physical changes of foods

Change in state of matter from solid, to liquid, to gas requires energy input.

A change from gas, to liquid, to solid requires energy removal. Such changes occur frequently in the preparation of foods..

• Such a change also alters the way water influences the properties of food. For example, ice (which is a solid) has a restricted and well-defined structure.

Physical changes of foods

• If energy is applied -- in this case, heat energy -- the structure becomes fluid and the orientation of molecules is less definitive.

• Finally, if the water changes to a gas (steam), the structure becomes a single molecule of H2O

Latent Heat

A change in the state of matter requires energy (which is usually measured in calories) but no temperature change. Each gram of pure water requires 1 calorie to raise its temperature by 1 degree C.

• Latent heat requirements have many implications in both the processing and preparation of foods.

• For example, in the preparation of frozen food, at least 80 calories must be removed for every gram of water which is changed from liquid to ice. Ice cream and frozen desserts also require significant energy removal to form ice crystals.

Latent Heat

• On the other hand, dehydrating and drying fruits and vegetables requires input of energy to convert liquid water into water vapor.

Since fruits and vegetables may consist of at least 90 percent water, the cost of processing is significant

• For example, this diagram shows the amount of latent heat required to bring 1 gram of ice to its melting point, changing it from a solid state to a liquid state without a rise in temperature. How many calories are required to change 1 gram of ice at 0 degrees to water?

Colligative propertiesColligative properties in foods include:

boiling point, freezing point, osmosis and

water activity.

Colligative properties• The colligative properties in true solutions

are strongly affected by the dispersed particles.

• These properties are slightly affected by the dispersed phase of colloids.

• The dispersed particles do not affect colligative properties of suspensions

Boiling and freezing points

• The number of particles dissolved in a solution significantly affects both boiling and freezing points

• Every gram of solute particles added to water lowers the freezing point by a precise amount


The boiling point of water is 100 degrees Celsius (C), which is equal to 212 degrees Fahrenheit (F). The freezing point of water is 0 degrees Celsius, or 32 degrees Fahrenheit.

Boiling and freezing points-contd

Adding 1 gram molecular weight of sugar to 1 liter of water increases boiling temperature by 0.52 degrees C, while the same amount lowers freezing temperature to -1.86 degrees C.

For example, in household measurement terms, it would take 6.1 cups of sugar to change the boiling temperature of 1 liter of water by 1.86 degrees C, compared to 1.71 cups of sugar to change freezing point by the same amount.

Boiling and freezing pointsCompared to a nonionizing solute, an ionizing solute (like

salt) has a far greater impact on the freezing temperature of water. The amount that the freezing temperature lowers depends on the number of particles into which the ionizing solute dissolves.

• 1 gram molecular weight ionizing solute per liter = -1.86 degrees C decrease in freezing point per particle


For example, using salt (which divides into two particles) lowers the freezing temperature of water twice as much as a nonionizing solute (like sugar) -- by -3.72 degrees C per gram molecular weight of the solute per liter of water.

An ionizing solute that divides into three particles would lower the freezing temperature of water three times lower than a nonionizing solute, and so on.

Saturated solutionsAll true solutions consist of dispersed

particles (the solute) in a liquid solvent; in foods, the liquid is usually water.

• These true solutions may have different characteristics, depending on the concentration of the solute.

• This concentration is defined as Saturation.

Supersaturated solutionsMost foods contain unsaturated and saturated

solutions.Supersaturated solutions. critical to preparing

candies and table salt.Supersaturated solutions are relatively

unstable, and solute tends to precipitate out of the mixture to form crystals, resulting in a saturated solution.

Saturated Solution• To understand saturation, think of adding sugar to iced tea. Initially, this

combination forms an unsaturated solution because the solution (the tea) dissolves all the solute (the sugar), yet has the ability to dissolve even more.

• If you continue to add sugar to the tea, eventually you'll reach a point when the sugar will no longer dissolve. That's because the solution dissolves all the solute it can hold at a given temperature. At this point, the solution is Saturated

Supersaturated solution

• Finally, let's say you want to dissolve more sugar into the tea after it's reached saturation point at a given temperature. You may be able to do this by heating the tea, dissolving all the sugar possible at this high temperature, then cooling the tea without precipitating the sugar. A solution that contains more solute than normal at a specified temperature is called Supersaturated

Water ActivityWater activity, or Aw, is the ability of water to take

part in biological or chemical reactions. As solute is added to water, the water forms a structure

around it. At that point, the water surrounding the solute is no longer available.

Water Activity• Pure water has an Aw of 1.0. A high level of water

activity, in the range of .75 to .95, means that the food permits growth of bacteria, yeast and molds. Aw decreases as solutes (such as salt and sugar) are dissolved in food.

• A low Aw level means that the food is less likely to allow the growth of molds, yeasts and other detrimental biological and chemical reactions.

Water Activity• Foods are processed and produced for shelf

stability based on water activity. As you'll see on the graph, foods with a low Aw level have fewer reactions; therefore, they have a longer shelf life.

Suspension• In some foods, the three states of matter

may appear to dominate as separate entities in the mixture. Foods consisting of a liquid with particles visibly settling out are called suspension.

• The particles may be solids, gases or even other liquids. The system is further complicated because each of the three states may exist as a solution and/or colloid.

Suspension-contd• Jello with fruit is an example of a

suspension. The chef prepares a liquid gelatin sol to the consistency of egg white. When fruit pieces are added they settle out, forming a suspension. If the preparation is done properly, the liquid sol will change to a gel, trapping the fruit. .

Suspension-contd• An example of another suspension is

cornstarch (a solid) added to water. If left standing, the cornstarch will settle out.

• A liquid that visibly settles out from a liquid is also a suspension. For example, if non-homogenized milk is left in a container, large globules of fat will rise to the surface. This suspension -- cream -- is then easily removed

Colloidal systemA colloidal system is made up of large

molecules of a substance dispersed in water.

The molecules form the disperse phase. The disperse phase may be solid, liquid or gaseous and the properties of colloidal systems are dependent on the large surface area of the disperse phase.

Behaviour of colloids

• During the processing and cooking of food, colloids behave in different ways, depending on a number of conditions. These include:

• The particle size and extent of dispersion may be affected by:

• heat • beating • pH: acid or alkaline conditions affect food

colloids in different ways.

Behavior of colloids-contd

For example, in breadmaking, acidic conditions during fermentation may increase gluten dispersion but in biscuit and cake manufacture, alkaline conditions have this effect, the difference being due to the relative proportions of water and fat.


• Acidic conditions will cause milk protein(casein) to curdle and become non-colloidal. A further reduction in pH will reverse this process. Control of pH is therefore very important in food manufacture.

• enzyme activity: amylase and proteinase can increase or decrease particle size and dispersion in starch and protein colloids, respectively.


• Acidic conditions will cause milk protein(casein) to curdle and become non-colloidal. A further reduction in pH will reverse this process. Control of pH is therefore very important in food manufacture.

• enzyme activity: amylase and proteinase can increase or decrease particle size and dispersion in starch and protein colloids respectively.

Examples of colloidal systems

a. For example, before cooking, the egg consisted of a protein (solid) in a liquid as a sol. Cooking the egg converted it into a firm gel with the solid now forming a matrix, trapping the liquid.

b. The butter on the toast is an emulsion, formed by dispersion of a liquid (water) into a liquid (oil) with crystalline fat and other ingredients.

Colloids

Remember:Colloids consists of 2 phases:Continuous phase (dispersing phase)Discontinuous phase (dispersed phase)For example, margarine, oil is the Continuous

phase (dispersing phase) while water is the Discontinuous phase (dispersed phase)

Colloidal systems

Colloidal systems in foods are:emulsions

solsgels

foams

Colloidal systems• Liquid dispersed into liquid forms an

emulsion, such as mayonnaise.• A solid dispersed into a liquid forms a Sol.

Here, cooked starch is colloid ally dispersed in a liquid to make gravy.

• A liquid trapped in a solid creates a gel, like gelatin dessert.

• Air dispersed into a liquid forms a foam, such as whipping cream.

Unique colloidal systemsColloids, true solutions and suspensions may all be found

in the same food. For example, fresh milk from a dairy farm contains milk

protein solids, which are colloid ally dispersed as a sol.Some of the fat may be naturally emulsified as a liquid in

liquid emulsion.Sugars, salts and water-soluble vitamins are present in

solution. And if the milk is not homogenized, the unemulsified,

suspended fat will "cream" or settle on top of the milk

Properties of colloidal dispersions

The general properties of colloidal dispersions

• surface activity - colloids can, in some circumstances act as surface active agents as in the formation of foams.

• viscosity - colloids can vary in consistency from sols, which appear as solutions, to a high degree of plasticity - a gel.

Starch colloids• Starch granules form a suspension in cold

water but when heated to above 60°C, the granules rupture releasing polymeric starch (amylose and amylopectin) to form a colloid dispersion.

• With increasing concentrations of starch, the colloid becomes more viscous because connections are formed between the molecules.

• Eventually the colloid becomes a gel (i.e. semi-solid) but this may be reversed by stirring or heating to break the connections

Starch colloids

A wide range of products made from colloidal starch consist of foams stabilized by coagulated protein (bread, cakes, biscuits, many snack foods and breakfast cereals). The viscous properties are widely employed in sauces and soups.

.

The essence of food manufacture is to produce a consistent product consistently - an objective that may be thwarted by the small but definitive differences of behavior of natural starches. This has resulted in the development and widespread use of a range of modified starches.

Starch colloids-contd

Protein colloidsIn nature, protein usually occurs as a colloidal

dispersion, sometimes in a highly organized state as in muscle.

Protein colloids exhibit all the properties of colloid dispersions and all the susceptibilities to changes induced by pH, but have the added property that when heated the protein denatures (i.e. coagulates) so that the colloidal is 'set' or stabilized.

Most baked products utilize this phenomenon as does the cooking of meat and fish.

Emulsion• An emulsion is a mixture of two components

which normally do not mix, such as oil and water, in which one component is distributed as droplets in the other.

• They are described as being immiscible. The droplet component is known as the 'dispersed' phase and the other as the 'continuous' phase.

• In food, most emulsions are oil in water or water in oil so that the commonly used emulsifiers exhibit solubility in fat and water. Emulsifiers make the emulsion permanent.

Emulsions

Liquid/liquid systems of 2 immiscible substances are called emulsion. Substances

or particle size = 10-100 microns.Examples: butter (w/o), margarine (w/o),

mayonnaise (o/w), salad dressing (o/w), milk (o/w), cream (o/w)

CHEMICAL STRUCTURE OF SOME COMMON EMULSIFIERS (SURFACTANTS)

Stabilizers

• Stabilizers are other substances in common use to make emulsions permanent.

• starches and modified starches are examples, as are proteins, which have the added attribute(property) that when heated they may coagulate leading to rigid or semi rigid structures as in meringues, bread and cakes.

Foam• A foam is similar to an emulsion but the

dispersed element is gaseous in nature, usually air or carbon dioxide (produced by fermentation).

• Commonly a foam is formed in an oil in water emulsion (e.g. ice cream). Usually the gaseous phase is enclosed in protein membrane derived from egg white.

Foam-contd

• The droplet size in both emulsions and foams is dependent on the amount of energy used to create the dispersion (e.g. mechanical beating), the effectiveness of the emulsifier and of the stabilising substance.

Some desirable characteristics of food emulsifers

1. Ability to reduce interfacial tension below 10 dynes/cm

2. Ability to be rapidly absorbed at the interface3. Ability to function effectively at low

concentrations4. Resistance to chemical change5. Lack of odor, color, and toxicity6. Economical

FOAM

• Gas is dispersed in liquid or semi-liquid.• Dispersed-phase: gas• Continuous-phase: liquid• It requires a 3rd component possessing

protective or stabilizing properties to maintain the dispersion.

• Example: whipped topping

FOAM

The important foam stability factors are:1. Surface tension2. Concentration of separate phase3. Presence of foaming agent to lower surface

tension4. Viscosity of liquid - the higher the

viscosity, the more stable the foam.5. Presence and thickness of adsorption layer

(a 3rd stabilizing material

GEL

• semi-solid state with 2 continuous phases.• Continuous phase of interconnected

particles and/or macro-molecules intermingled with

• a continuous phase of liquid phase such as water.

Example jam and jello

Stability of colloids• The stability of colloids is of great importance

to food technology and is influenced by: • surface charge - this may produce

aggregation of particles and change the colloid to a suspension. The reverse process may also occur.

• hydration - many food substances have an affinity for water and can remain as a colloidal dispersion when little water is present because they 'hold' the water molecules around the particles.

• (Refer to Food colloids for further slides)

CHEMICAL STRUCTURE OF SOME COMMON EMULSIFIERS (SURFACTANTS)

STABILITY OF A FOOD DISPERSION

1. Dispersed particle size2. Viscosity of continuous phase3. Dispersed phase concentration4. Density difference between 2 phases

Stabilizers• Many colloids require a stabilizer to keep

the dispersed particles from separating out (precipitating) or blending together (coalescing). This stabilizer is frequently called a surfactant.

• All colloidal systems may be stabilized in part by any or all of three characteristics: absorbed solvent layer, electrical charge, and Brownian Movement.

Mechanisms for stabilizing colloidal systems

Electrical charge• Dispersed colloidal particles may have an

electrical charge either because ions are adsorbed, or groups of the colloid are ions.

• It's similar to how magnets work: "like" polarities repel each other, keeping the solute from coalescing with unlike charges and possibly precipitating

Example of electrical charge stabilizing a colloid

• For example, in milk, the large macro-molecules of calcium caseinate have "like" charges, causing the milk protein to exist as a sol. When milk sours, the positive acid ions (hydrogen) change the electrical charge of the caseinate, allowing the caseins to bond intramolecularly and intermolecularly. This, in turn, forms a milk gel.

• In addition to the inherent properties of the colloid, a surfactant may contribute to the charge.

Stability of colloids• The stability of colloids is of great importance

to food technology and is influenced by: • surface charge - this may produce

aggregation of particles and change the colloid to a suspension. The reverse process may also occur.

• hydration - many food substances have an affinity for water and can remain as a colloidal dispersion when little water is present because they 'hold' the water molecules around the particles.

• (Refer to Food colloids for further slides)

Destabilization of a dispersion

no influenceslight influenceappreciable influence

Water Activity

no influenceslight influenceappreciable influence

Osmosis

no influenceslight influenceapprecableinfluence

Boiling Point

no influence of dispersed particles

slight influence by dispersed particles

appreciable influence by dispersed particles

Freezing Point

SuspensionsColloidsTrue SolutionsColligativeProperty

WaterWater FunctionsImportant component of food.1. Universal solvent (salt, vitamins, sugar, gases,

pigment)2. Capable of ionizing (H3O+, OH-)3. Affects the texture4. Chemical reactions (hydrolysis of protein = amino

acids)5. Stabilizing the colloids (by hydration)6. Necessary for micro-organisms grow

Hydrogen Bond

The bond is formed due to the affinity of electro-positive hydrogen atoms for electro-negative atoms such as O. Binding energy of hydrogen bond is about 10% of covalent bond. H-bond strength = 10 Kcal/mol.

Water is a good dissolving solvent - Why?

1. Hydration of the solute by a chemical complex such as the "hydrogen bond"

2. Physical action of dispersion of solute molecule due to the high-activity of water molecules at the surface of the solute.

3. The high di-electric constant of water (80x that of vacuum) diminishes the effectiveness of attractive forces that tend to hold the solute molecules together

Dissolving of ions by high di-electric constants of water

Good food ...

.......tastes, looks and smells delicious, is fresh, wholesome and pure , has nutritional value and will do no harm if eaten in moderation, is produced by methods, traditional or modern, the producer would be proud to show the public, has had minimal processing,has been humane in the keeping and slaughter of animals, has minimal chemical content, contains no unnecessary additives.

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Food Dispersion