Mixtures: Solutions, Colloidal Dispersions, and Suspensions



Food mixtures are classified 

by size of

particles

 distributed throughout the

mixture.



The 

dispersed phase

 refers to particles

that are 

scattered

 throughout a

medium.



The 

medium

 in which particles are

distributed is called the 

continuous

phase

.



A solution is a 

homogenous

 mixture of

two

 or 

more

 substances.



Solutions consist of two parts: the 

solute

which is the 

dispersed

 phase, and the

solven

t, which is the 

continuous

 phase.



The solute 

cannot

 be filtered or

separated out of the solvent.



There are three states of matter, and

solutions are 

possible

 in any combination

of these three states.



Most foods that are solutions exist as

solids

 in liquids, 

liquids

 in liquids, and

gases

 in liquids.



A powdered drink mix when prepared is

an example of a 

solid-in-liquid

 solution.



Most bottled flavorings, such as vanilla

extract, are 

liquid-in-liquid

 solutions of

alcohol in water plus flavoring

compounds.



Club soda is an example of a 

gas-in-

liquid solution

 – it is carbon dioxide gas

dissolved in water.

A number of factors can change how

soluble one substance is in another

:

›

Temperature

›

Particle size

›

Concentration

›

Agitation

›

Vapor pressure



The first factor that affects solubility of

solids is the temperature of the 

solvent

.



As the temperature 

increases

, the

amount of a solid solute the solvent will

hold 

increases

.



The 

type

 of solute will also affect how

much can 

dissolve

 in a solution.



This principle of temperature affecting

solubility is used in the 

candy

 industry.



The base of many candies is 

sugar syrups

– solutions of 

sugar in water

.



As water is heated, it can hold 

more

sugar solute, and the 

concentration

 of

the sugar syrup 

increases

.



The concentration of sugar syrup affects

a candy’s 

sweetness

 and 

texture

.



The temperature of the solvent also

affects the solubility of 

gases

. As the

temperature of a solvent increases, the

amount of gaseous solute it will hold

decreases

. This is why soft drinks lose their

carbonation 

faster

 at 

room

 temperature

than when 

chilled

.



The reason solvent temperature affects

solubility of solids and gases is the

increased number

 of 

molecular

collisions

.



Increased temperature causes

increased molecular 

movement

. The

more

 the molecules of solvent collide

with molecules of a 

solid

 , the 

faster

 the

solid will 

dissolve

.



The 

smaller

 the particles of 

solute

 are,

the 

greater

 the 

exposed

 surface will be.



The greater the surface area is, the 

faster

the 

solute

 will 

dissolve

.



This is because solute molecules must

come in contact

 with the 

solvent

molecules

 to dissolve.



Concentration

 is the 

measure

 of parts of

one substance (the 

solute

) to the known

volume

 of another (the 

solvent

).



A solution holding 

all

 the solute that 

will

dissolve

 in the solvent at a given

temperature is 

saturated

.



Most food solutions are 

mixtures

 of 

polar

compounds

 with 

water

 as the 

solvent

.



When water molecules come close to

other 

polar

 molecules, 

hydrogen bonds

will form.



Once all the water molecules have

bonded to a solute, a solution is at the

saturation point

.



To form a 

supersaturated

 solution, a

solution must be 

heated

 and then 

cooled

.



When solution is heated, it will be able to

hold more solute

.



Then when the solution is cooled, it will

have a 

higher concentration

 than would

normally be possible at that temperature.



Agitation or 

stirring

 will 

speed

 the

dissolving

 process until the saturation

point is reached.



Stirring also 

adds

 a 

small

 amount of

energy

 that 

raises

 the temperature

slightly

.



This is the pressure at which 

gases

escape

 from and 

dissolve

 into a 

liquid

 at

the 

same rate

.



Small

 changes in pressure have 

little

 or

no effect

 on the solubility of 

solids

 and

liquids

 in solution.



The concentration of gas in a liquid is

directly related to the 

pressure

 of the 

gas

over the liquid

. If you 

double

 the pressure

over a gas-in-liquid solution, you can

double

 the 

amount

 of 

gas

 in the 

solution

.



The 

fizz

 in carbonated drinks is caused by

adding gas

 to 

liquid

 under 

pressure

 and

then sealed in containers.



Within the sealed containers, the pressure is

in 

equilibrium

.



This means the pressure of the gas over the

liquid 

equals

 the pressure 

within the liquid

.

The liquid is neither 

gaining

 nor 

losing

 gas

molecules.



When seal on beverage is broken, the

balance

 of pressure 

changes

. The pressure

above the solution 

decreases

.



Mass percent

 is the percentage of the

mass in a solution comes from the 

solute

.



Solutions

 are usually defined or

described in terms of the mass percent

of the solute. A 10% salt solution gets 

10%

of its mass from the 

dissolved salt

.



Water freezes at 

0°C

 or 

32°F

 and boils at

100°C

 or 

212°F

.



When a solute, such as 

salt

 or 

sugar

 is

added

, the freezing point 

drop

s and the

boiling point 

rises

.



The changes in the freezing and boiling

points will be 

greater

 as the mass

percent of the solute 

increases

.



When making ice cream, salt is added to

the ice that surrounds the metal can

containing the ice cream solution. The

combination of salt and ice is

endothermic

.



Endothermic means the mixture absorbs

heat energy

 from its 

surroundings

. As a

result, the ice melts. The salt, ice and

water has a 

lower freezing point

 than

pure water

.



Carbonated soft drinks are solutions.

Water is the solvent. Sweeteners,

flavorings, coloring agents, acids, and

CO

2

 are the 

solutes

.



Artificial

 sweeteners do not give the

solution the same 

body

 as 

sugar

 and

corn syrups

.



Carbohydrate gums

 or 

pectins

 are

added to diet drinks to 

copy

 the 

mouth

feel

 of  regularly sweetened beverages.



CO

2 

 provides fizz to soft drinks as well as

adding to the 

acidity

. Acids act as 

flavor

enhancers

 and 

preservatives

.



Citric acid

 is usually used for fruit flavored

drinks and 

phosphoric acid

 is used more

for non-fruit flavors such as colas and

root beer.



Additionally, 

sodium benzoate

 which

converts to benzoic acid is usually

added as a 

preservative

.



Coffee and tea are considered

nonnutritive beverages and are

consumed for their 

flavors

 and

stimulating effects

.



The 

roasting times

, 

particle sizes

, and

added 

flavoring agents

 affect the

strength and flavor.



When brewed, brewing time and

temperature will also affect 

strength

 and

flavors

.



Colloidal dispersions are 

mixtures

 in

which microscopic 

particles

 of one

substance is 

evenly

 distributed in another

substance

.



The 

nature

 of the 

particles

 is what gives

colloidal dispersions their 

unique

characteristics.



Three types of these mixtures are

emulsions

, 

foams

, and 

gels

.



True colloidal dispersions are made up of

a 

continuous

 phase (medium that holds

the dispersed particles)and a 

dispersed

phase (particles or colloids).



Colloids are 

macromolecules

 or 

clumps

of smaller molecules.



Starch

 and 

protein

 are examples of

colloids that can be distributed in a

colloidal dispersion.



Jelly

, 

mayonnaise

, 

butter

, and 

gelatin

are some foods that are colloidal

dispersions.



The key factor that distinguishes colloids

from solutes is the 

size of the particles

.



Particle size of solutes in solutions is up to

1 nanometer

 (nm) in diameter.



Colloids are usually between 

1 nm

 and 

1

micrometer

. (There are 

1,000

micrometers

 to a millimeter and 

1,000

nm

 to a micrometer.)



Since colloids are 

larger

, they can be

dispersed in another substance, but they

will not

 dissolve.



When light travels through a solution, 

no

change

 in the 

direction

 of the 

light rays

 is

noticeable.



When light passes through a colloidal

dispersion, the light rays 

scatter

 and are

visible

.



This is called the 

Tyndall effect

.



Colloids remain dispersed due to two

factors. The first is the 

motion

 of the

molecules

 of the 

continuous

 phase.



The second factor that keeps colloids

dispersed is their 

electrical charges

. The

colloids are usually 

of the same

substance.



In 

some

 mixtures, the particles 

will not

remain dispersed. This is due to the

density

 of the particles in the 

dispersed

phase compared to the density of the

continuous

 phase.



Examples include 

flour

 in 

water

 and 

fat

 in

fresh milk

 that 

has not

 been

homogenized.



If the particles are 

denser

 than the

continuous

 phase, they will 

sink

 as the

mixture 

sits

. This is why so many sauces must

be 

constantly stirred

 during the 

heating

process

.



Example

: Since 

starch granules

 are denser

than the water in which they are dispersed,

if the mixture is not 

stirred continuously

, it will

become 

lumpy

 and the starch will sink and

burn

 to bottom of the pan.



Most colloids in foods are 

starches

 or

proteins

.



Once the starch or protein molecules 

open

their molecular structure, intermolecular

bonds

 can form between the molecules.



This creates a 

three-dimensional

 network.



Water gets trapped 

between

 and 

around

molecules.



Once starch molecules have 

swollen

with 

water

, they will stay 

dispersed

, the

mixture will 

thicken

, and a 

colloidal

dispersion

 is formed.



When a 

thickened

 colloidal dispersion

remains 

pourable

, it is a 

sol

.



When 

pH

, 

temperature

, and / or

concentrations

 are altered, the sol can

be transformed into a 

rigid gel

.



Gelatin

 is a 

protein

-based colloidal gel.



Jams

 and 

jellies

 are 

pectin

-based

colloidal gels.



If particles of the dispersed phase are

less dense

 than the continuous phase,

they will 

rise

 to the 

top

 of the 

mixture

.

That is why a 

fat layer

 forms on the top of

gravy – the 

fat colloids

 are 

less dense

than the rest of the gravy.



Milk

 is an example of a colloidal

dispersion. It is also a 

solution

.



Milk is made up of water, 

lactose

,

mineral salts

, 

proteins

, and 

fat

.



The 

lactose

 and 

mineral salts

 are 

solutes

.

They are dissolved in the water forming a

solution.



The 

proteins

 are 

colloids

.



The milk 

fats

 are in 

suspension

.



The colloidal dispersion of the proteins in

milk are 

stable

.



The fat globules in milk 

are not

 a stable

dispersion.



Like fat in the gravy, the 

fat

 particles in

fresh whole

 milk will 

rise

 to the top.



When they rise to the top, they form a

layer of 

cream

.



To keep the fat from separating to the

top, milk is 

homogenized

.



Homogenization is a 

mechanical

 process

that forces milk through 

screens

 or 

small

openings

. This 

ruptures

 the 

membranes

around

 the fat globules and 

breaks

 the

globules into 

smaller

 particles.



Homogenization alters the 

chemical

nature

 of the fat particles keeping them

from 

re-collecting

 and rising to the top.



An 

emulsion

 is a mixture of two

immiscible liquids, where one is dispersed

in 

droplet

 form 

in the other

.



Immiscible

 liquids are liquids that will

separate

 when 

combined

.



Usually one liquid is 

polar

 in nature, like

water-based 

vinegar

, and the other is

nonpolar

, like 

oil

.



Example

: bottle of 

oil

 and 

vinegar

 salad

dressing

. When you 

shake

 the bottle, the

two

 liquids 

combine

. As the bottle 

sets

,

the liquids 

separate

, and the oil 

forms a

layer

 on 

top

 of the vinegar. This type of

dressing is a temporary emulsion.



Temporary

 emulsions are 

unstable

mixtures of two immiscible liquids.



In order for one immiscible liquid to stay

dispersed in another a 

stabilizing factor

must be added. This factor is called an

emulsifier.



An 

emulsifier

 is a 

molecule

 that has a

polar

 end and a 

nonpolar

 end.



The polar end is attracted to and forms

hydrogen bonds

 with other polar

molecules. In the 

same

 way, the

nonpolar end is attracted to nonpolar

molecules

.



Most emulsions require 

agitation

, 

stirring

,

or 

beating

 for the emulsifier to bond to

the molecules and 

stabilize

 the 

mixture

.



Example: 

lecithin

 is an emulsifier found in

egg 

yolks

. During beating, the lecithin in

egg yolks will attach to the molecules of

vinegar and oil to form 

mayonnaise

.



The two most common types of

emulsions are 

oil-in-water

 and 

water-in-

oil

. The 

dispersed

 phase, which is the

liquid

 in 

droplet form

, is listed 

first

. The

continuous

 phase is listed 

second

.



Usually, the 

phase

 present in the 

largest

amount is the 

continuous

 phase.



Examples of 

oil-in-water

 emulsions:

Mayonnaise

, 

salad dressings

, 

ice cream

,

and 

cake batters

.



Examples of 

water-in-oil

 emulsions: 

Butter

and 

margarine

.



A factor affecting the stability of

emulsions is 

temperature

 because fats

and water have 

different thermal

conductivity

 levels.



Thermal conductivity is the 

ability

 to

transfer heat energy

.



Water

 has 

more

 thermal conductivity

than 

fat

.



This means water 

transfers

 heat 

more

readily

 than fat.



An oil-in-water emulsions will freeze at a

higher

 temperature as well as 

faster

 than

water-in-oil emulsion because heat

begins transferring out of the continuous

phase  

before

 transferring out of the

dispersed phase.



High

 temperatures as well as 

low

temperatures can affect the 

stability

 of

emulsions. This is especially true when the

emulsifier is a 

protein

.



High

 temperatures cause 

proteins

 to

coagulate

 which 

prevents

 the proteins

from being 

evenly

 distributed through

the mixture. The emulsion 

separates

because the protein can no longer act

as an emulsifier.



Hollandaise sauce is an example of an

emulsion that can 

easily

 become

unstable

 and 

curdle

 due to high

temperatures.



Hollandaise sauce is made from a

mixture of 

egg yolk

, 

butter

, 

lemon juice

,

water

, 

salt

, and 

cayenne pepper

. The

egg 

yolk

 serves as the emulsifier that

keeps the 

butter dispersed

.



A second factor that can affect the

stability of emulsions is 

electrical charges

.



Emulsions work because the water-

based liquid is 

electrically attracted

 to

the polar end of the emulsifier.



Running electrical currents through an

emulsion can 

disrupt

 the electrical fields

in the emulsion which can cause the

emulsion to 

destabilize

.



This principle can be used to 

separate

parts

 of 

emulsions

.



Ice cream is an example of an emulsion

that must 

remain stable during freezing

.



Ice cream is a water-based sugar

solution that forms an emulsion with 

fat-

based cream

.



The mixing or 

agitation

 during freezing

adds air

 to this emulsion.



As the water freezes, the 

concentration

of the unfrozen sugar solution 

increases

.



Once the 

solution

 reaches the 

saturation

point

 for sugar, 

lactose

 and other 

sugars

will begin to 

settle out

 as 

crystals

.



The crystals can give a 

gritty

, 

sandy

 feel

to the ice cream.



Emulsifiers give ice cream a 

smooth

,

creamy

 texture because the emulsifier

stabilizes

 the ice cream solution so the

cream

 and 

sugar solution

does not

separate

 during freezing.



Emulsifiers also help keep the 

sugar

, 

fat

,

and 

water

 particles dispersed so ice and

sugar 

crystals

 remain small.



A sugar molecule 

cannot

 bond to another

sugar molecule if there is a 

fat

 molecule

between

 them.



The emulsifiers used in commercially made

ice cream are 

gums

, 

pectins

, and 

lecithin

.



They work by 

thickening

 the mixture. The

more viscous

 the mixture is, the 

harder

 it is

for fat droplets to 

collect

 and 

separate

.



Foams are colloidal dispersions of 

gas

 or

air bubbles

 dispersed in a 

liquid

.



The 

froth

 on top of root beer is an

example of an 

unstable

 foam. These

foams will 

quickly collapse

.



Whipped cream

 and 

meringue

 are

examples of 

stable

 food foams that will

last for extended periods.



For a foam to remain stable , four

conditions are necessary:

1.

The liquid needs to be 

viscous

 enough

to 

hold

 the 

air

.

2.

The dispersion needs to contain a

stabilizer

 that will 

stretch

 to form a 

thin

film

 around 

air bubbles

.



For a foam to remain stable , four conditions are necessary

(cont.):

1.

The liquid needs to be viscous enough to hold the air.

2.

The dispersion needs to contain a stabilizer that will stretch

to form a thin film around air bubbles.

3.

The 

surface tension

 of the film should

be 

less than

 that of 

water

.

4.

The liquid in the foam needs 

low

vapor pressure.



Temperature

 is one factor affecting

foam stability.



In most cases, the 

viscosity

 of a liquid is

decreased

 as the temperature 

rises

.



However, egg whites reach the 

fullest

volume

 when allowed to stand at 

room

temperature for about 

30

 minutes 

before

beating.



Moderate

 temperatures make is 

easier

for the 

proteins

 to 

denature

 and 

form

 the

films

 necessary to create the 

air bubbles

.



Whipping cream forms the best foam

when the 

cream

, 

mixing bowl

, and

beaters

 have been 

chilled first

.



The reason for this is because the

viscosity

 of the 

fat

 is 

greater

 when

cooled

. The foam is formed by the

nonfat protein portion

 of the cream.



Heavy whipping cream with a 

40%

 fat

content produces the 

highest

 foam

volume.



The 

addition

 of 

solutes

 can 

alter

 foam

stability.



Sugar

 is a common solute in food foams.

It helps 

stabilize

 the 

water

 in the foam.



Sugar must be 

added

 at the 

right point

in foam formation to produce the 

most

stable

 foam.



Adding sugar 

too early

 in the 

beatin

g process

will delay the 

denaturation

 of 

protein

 and

also cause 

part

 of the 

protein

 to 

bond

 to the

sugar

 molecules.



This will 

reduce

 the 

ability

 of the mixture to

foam

, resulting in a foam that 

lacks

 height

and stiffness.



Adding

 sugar 

after

 a foam is completely

formed

 will cause the air bubbles to be

coarse

 in texture.



The best time to add sugar is 

after

 a

foam 

begins

 to 

form

 but 

before

 it

reaches the 

soft peak stage

. This results

in a 

fine-textured

 egg white foam with

maximum

 height.



The foam needs to be 

beaten until

 the

sugar dissolves

 because when

granulated

 sugar is 

heated

, it tends to

drop through

 a foam.



Weeping

, or 

syneresis

 of a 

meringue

, is

the result of a foam 

destabilizing

 and

partially

 returning to a 

liquid

 state.



Acidity

 is another factor affecting the

stability of foams.



Whipping cream works best when 

fresh

because as the 

cream ages

, bacteria

change

 the 

lactose

 in cream into 

lactic

acid

. This causes a 

decrease in pH

,

which 

disrupts

 the ability of the 

casein

 in

cream 

to form

 an emulsion.



Cream of tartar

, or tartaric acid, is used

to 

lower

 the 

pH

 of eggs.



The pH range of fresh egg whites is

between 

7.0

 and 

8.0

.



The 

fresher

 the egg the 

lower

 the 

pH

.



During storage, 

CO

2 

escapes through the

porous shell

 which causes an 

increase

 in pH

to over 

9.0

.



The 

increase

 in pH causes the 

protein

 to

break down

, 

reducing

 the egg’s ability to

foam.



Most stable foams from 

protein albumin

 are

formed when the pH is between 

4.6 and 4.8



Care must be taken when 

separating eggs

to 

prevent

 any 

egg yolk

 getting into the

egg whites to be used to make  a foam.



The 

fat

 in the yolk will 

react

 with the 

protein

and 

prevent filming

 action.



It is 

easier

 to separate eggs when the eggs

are 

cold

. Then let set for 

30

 minutes to

reach 

best foaming

 temperature.



Plastic

 bowls are 

porous

 and 

may retain

egg yolk particles that can reduce foam

volume – use 

glass

 or 

metal

 bowls.



A 

suspension

 is a mixture of 

undissolved

particles in a liquid.



Suspensions may be 

very unstable

 due

to the 

size

 of the 

dispersed particles

. The

mass

 of the particles will cause them to

sink

 to the 

bottom

 of the mixture.



If the particles are 

lighter

 than water,

they will 

rise

 to the 

top

 of a mixture.



Fresh milk

 contains a 

suspension

 of 

fat

globules

.



When milk is 

homogenized

, the fat

globules 

decrease

 in size and a 

stable

colloidal dispersion

 is created.



Hot chocolate is a 

suspension

 of 

cocoa

in 

milk

.



Stabilized Italian dressings are a

suspension

 of 

spices

 and 

herbs

 in an

emulsion

.



Pulp

 in fruit juice, 

fruit

 in gelatin, and

crushed berries

 in ice cream are also

examples of suspensions.



Many baked goods are made from

batters and doughs that have chocolate

chips, 

raisins

, chopped 

nuts

, or coconut

suspended throughout

. Batters and

doughs are the 

continuous

 phase of

these complex mixtures.



Batters are 

pourable

.



The particles most often suspended in

batters are 

air bubbles

.



These bubbles provide 

leavening

. When

they are 

evenly

 distributed, the result is

light, airy

 baked goods.



Too much mixing

 can cause the 

air

 to

rise out

 a batter suspension.



Allowing a batter 

to sit

before baking

can also have this effect.



When batter loses air, the baked product

will be 

denser

 and lack 

height

.



Baking

 coagulates the 

proteins

 in batters

and 

creates

 a 

stable

 suspension.



Stability

 of a batter can be 

increased

 by

decreasing particle size

 or 

increasing

 the

viscosity

 of the batter.



The 

proportion

 of 

flour

 to liquid is 

higher

 in

doughs

 than it is in batters. This makes the

doughs 

too thick

 to 

pour

. 

Because doughs

are thick

, it is easy to keep large particles

suspended evenly.