PHYSICOCHEMICAL PROPERTIES

OF FOOD

INTRODUCTION

The important groups of organic compounds present in different foods are:-

1. Carbohydrates

2. Proteins and amino acids

3. Lipids

4. Nucleic acids

5. Enzymes

6. Pigments

7. Organic acids

8. Polyphenols and tannins

9. Flavouring principles

10. Vitamins.

11. Water

CARBOHYDRATES

The different group of carbohydrates are:

1. Sugars 4. cellulose & hemicellulose

2. Starch 5. Pectin, gums and mucilage

3. Glycogen

SUGARS:

the important sugars occurring in foods are glucose,fructose,sucrose,maltose and lactose.

Soluble in water, sweet in taste

Glucose and fructose occur in fruits and honey

Sucrose occurs in sugarcane, beetroot and honey.

Lactose occurs in milk and maltose in malt.

STARCH AND GLYCOGEN:

Complex polysaccharides formed by combination of large number of glucose molecules

Insoluble in cold water and form colloidal dispersion in hot water

Found in cererals,nuts, legumes and oil seeds.

Occurs in small amount in animal foods

CELLULOSE AND HEMICELLULOSE:

1. occur in cell walls, husk and other supporting structure

of plants foods.

2. not digested by human beings.

PECTINS,GUMS AND MUCILAGES:

1. It occurs in many fruits and vegetables.

2. Used in preparation of jams and jellies.

3. Plant gums are used as thickeners in certain food

products.

AMINO ACIDS AND PROTEINS

They occur in free state in all foods.

Proteins are complex compounds formed by combination of a large

number of amino acids.

Animal foods,nuts and legumes are rich in proteins while cereals

are moderate sources, vegetables are fair sources and fruits are poor

sources.

Form colloidal dispersion in water.

E.g.. Egg white, curd from milk and gelatine.

LIPIDS

The main compounds are fats and oils.

They are compounds of glycerol and fatty acids.

Several complex lipids such as phospholipids occur in many foods.

Fats occur in the form of emulsion in milk and egg.

Phospholipids help in the formation of emulsions.

OTHER COMPONENTS

NUCELIC ACIDS: present in varying amount in different foods

Enzymes: organic catalyst occurring both in animal and plant foods. Essential in many biochemical reactions that takes place in plant and animal tissues.

PIGMENTS: plant food contain pigments such as chlorophyll, xanthophyll, anthocyanin's, carotenoids etc. Meat contains myoglobin while egg yolk contains xanthophyll and carotenoid pigments.

ORGANIC ACIDS: citric acid, malic acids occur in fruits. Also found in vegetables and animal tissues.

POLYPHENOLS AND TANNINS: occur in plant foods

FLAVOURING PRINCIPLES: each food has a characteristic flavour for e.g. condiments and spices have distinct flavour because of essential oils and flavour principles.

VITAMINS: occur in varying amounts in different foods. A

part of them is lost due to processing etc. they can be

reduced or made up by fortifying the foods.

WATER: present in large amount in fresh fruits,

vegetables,meats,fish,egg and milk (70-90%). Cereal ,

pulses, and dries nuts contains only small amount of water

(8-12%).

It is present in two forms

1. Free water

2. Bound water

When water is removed from vegetables it turns from soft to

a different texture due to dehydration. The perishable nature

is due to the presence of high water content.

ORGANIC COMPOUNDS PRESENT IN FOODS

Organic compounds present in foods are:

1) Colloids i:e, compounds having large molecular weights

and forming only dispersion in water. E.g.,

protein,starch,glycogen and agar-agar.

2) Crystalloids i;e, compounds having small molecular

weights and can form true solutions e.g., sugar and

amino acids.

Lipids exist in free form in oilseeds, nuts, meat and fish

while they exist in form of emulsion in milk and egg yolk.

COLLOIDS

Colloidal solution contains substances whose molecular

aggregates possess a diameter greater the 1mµ and less than

100mµ.

CLASSIFICATION:

Divided into:

1. Lyophobic colloids( no affinity for water)

2. Lyophilic colloids(greater affinity for water e.g., proteins,

starch)

PROPERTIES:

1) large molecular weight

2) possess electrical charge.

3) can be separated by crystalloids by dialysis while mixture

such as protein can be separated by ultracentrifuge.

OSMOTIC PRESSURE

The process by which water is drawn into a solution

through a semi permeable membrane is called osmosis.

The pressure that must be exerted on the solution to

prevent the flow of water into the porcelain vessel is equal

to the osmotic pressure exerted by the solution.

OSMOTIC PRESSURE OF NON ELECTROLYTES:

1g molecular wt. of different non electrolytes when dissolved

in 1lt will exert an osmotic pressure of 22.4 atmospheres.

Molecular wt. of glucose and cane sugar are 180 and 342

while that of serum albumin is 73,000. hence serum albumin

will exert the same osmotic pressure as 342g of cane sugar

or 180g of glucose when dissolved in the same volume of

water.

HYPOTONIC,HYPERTONIC AND ISOTONIC

SOLUTIONS:

Water moves readily across cell membranes through

special protein-lined channels, and if the total concentration

of all dissolved solutes is not equal on both sides, there will

be net movement of water molecules into or out of the cell.

Whether there is net movement of water into or out of the

cell and which direction it moves depends on whether the

cell’s environment is isotonic, hypotonic, or hypertonic.

APPLICATIONS OF OSMOSIS TO FOOD

PROCESSING:

1. Osmosis plays an important role in food

preservation and processing.

2. It maintains the original size and shape of pickled

whole fruit.

3. Mangoes are usually pickled with salt, when salt is

added it draws out water from mangoes and it

shrinks .this is due to osmotic pressure exerted by

strong salt solution.

FOOD DISPERSIONS

It may be divided into five groups:

a) Solids in liquids (e.g., gelatine dissolved in water)

b) one liquid in another insoluble liquid ( water in oil

emulsions)

c) Gas in liquid (foam)

d) Gas in solid (solid foam (foam candy))

e) Solid in gas (solid aerosol – smoking of meat and fish)

FOOD SOLS AND GELS:

Colloidal dispersions of polysaccharides can be

divided into:

1) Sols – free flowing liquids at room temp.

2) Gels- relatively firm, do not flow.

FOOD SOLS: have a diameter of 1mµ to 100mµ.

E.g., solution of egg albumin in water and dilute

solution of gelatine in water. Important properties of

sols are

A) Electrical double layer

B) Rheology

C) Optical property

ELECTRICAL DOUBLE LAYER:

1) In sols possessing continuous aqueous phase, the

particles have electrically charged surface.

2) The ionized groups are the source of these

charges

3) In dissolved electrolyte, and electrical double layer

may exist around each particle.

RHEOLOGY:

1) The viscosities of hydrophilic sols are much greater

than water. The increase in the concentration of the

colloids increases the viscosity of sols.

FOOD GELS:

1) Consist of continuous phase of interconnected

macromolecules intermingled with a continuous

liquid phase such as water.

2) Common food gels include- jelly,gelatin gels and

starch gels.

3) Posses varying degree of rigidity and elasticity,

depending on the type of gelling agent.

4) A sol can be transformed into gel under certain

conditions:

a) Lowering of temp.

b) Chemical alteration of the gelling agent

c) Adjustment of pH or addition of salt

d) Addition of water competitive compound e.g.,

sugar.

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), and

chip-dip (o/w).

An emulsifying agent is made up of two

parts.

One is hydrophilic (water loving) and the

other is hydrophobic (water hating).

The emulsifier holds the disperse phase

within the continuous phase. This results

in the emulsion becoming stable.

A) Mayonnaise is an example of a stable emulsion of

oil and vinegar, when egg yolk (lecithin) may be used

as an emulsifying agent.

B) Stabilisers are often added to emulsions to increase

the viscosity of the product. These help improve the

stability of the emulsion, as over time the emulsion

may separate. Stabilisers also increase shelf life,

E461 methylcellulose, used in low fat spreads.

PROPERTIES OF EMULSIONS:

1) Droplet size distribution:

2) Optical properties

3) Rheology of emulsions

Droplet size distribution:

Emulsions change their size distributions over time withthe average droplet size shifting to larger valuesA sharply defined distribution containing a the maximumfraction of small-diameter droplets is usually more stable

Rheology of emulsions:

Continuous Phase: O/W emulsion can be partially controlled by clays and gums W/O emulsion by the addition of high-melting waxes and polyvalent metal soapsInternal Phase: No impact to final emulsion viscosityDroplet Size & Dist: The viscosity of emulsions having similar size distributions about a mean diameter is inversely proportional to the mean diameter

MECHANISM OF ACTION OF EMULSIFIER

Emulsifiers ma be grouped into three heads depending on

their mechanism of action in forming stable emulsions:

a) Emulsifiers obtained at the oil water interface

b) Finely divided particle absorbed at the interface

c) Water dispersible hydro colloids which increase viscosity

of the continuous phase.

when all the three classes of emulsifiers are used in

combination

optimal emulsion stability may be achieved.

STABILITY OF EMULSIONS

stability changes in food emulsions can occur through

the process of

a) Creaming

b) Flocculation

c) Coalescence

1) The creaming phenomena involves the flotation or

sedimentation of dispersed emulsified droplets into

emulsions layer one richer and other other poor in fat.

2) Flocculation is the agglomeration of droplets to form

loose and irregular clusters.it is the first step before

creaming takes place.

3) Coalescence is the irreversible union of small

droplets to form large droplets .

4) Emulsion can be stabilised against creaming,

flocculation and coalescence by introducing a strong

interfacial film around each droplet, by adding electric

charges to the droplet surfaces and increasing the

viscosity fo continuous phase.

FOAM

It is a dispersion of gas bubbles in a liquid or

semisolid phase.

Bubbles are separated from each other by liquid or

semisolid wall (lamellae) that are elastic in stable

foams.

Depending on thickness of bubble size, foam can be

as dense as continuous liquid phase or almost as

light as the dispersed gaseous phase.

Characteristics of foam:

1) Large amount of entrapped gas.

2) Extensive surface area

3) Rigid, semi rigid and elastic walls

FORMATION OF FOAM

The formation of foam depends on the presence of

a foaming agent in the continuous phase prior to

dispersion of gas.

The foaming agent is absorbed to reduce surface

tension and resist the coalescence of bubbles.

Typical foams include whipped cream, ice-cream,

cake mix, meringue, and the froth on beer.

Low density and a thin walled turbid structure are

essential to yield a fluffy product.

Foaming agent may be classified as:

1) Surface0active lipids (phospho-lipids)

2) Glucosides and saponins

3) Cellulose derivatives

4) Proteins (albumins)

Foam can be formed either by dispersion or

condensation.

In dispersion, gas is injected into the solution

through orifice.

In condensation, gas under pressure is passed into

the solution that is to be formed.

FOAM STABILITY

Foam stability is related to the resistance of the foam

wall to bursting stresses.

Loss of liquids in wall is caused by :

a) Gravitational force

b) A suction effect at the periphery of the wall dur to

high curvature.

c) Deformation forces brought about by gas movement.

Foam stability can be increased by increasing the

viscosity of the solution and by producing

particulate matter.

FOAM DESTRUCTION

In products like fruit juices, coffee extract, excessive foaming may lead to waste of material.

to avoid this wastage use of antifoam agents can be done.

Water insoluble dimethyl poly-siloxanes(silicone oils) are used in food industry.

The use of antifoam agents

causes an immediate collapse

of foam.

HYDROGEN ION CONCENTRATION(PH)

The acidity and alkalinity is of great importance in foodprocessing.

Fruits contains organic acid and have an acid reactionwhile foods such as milk and eggs have neutralreaction.

The term hydrogen ion concentration is used toexpress the degree of acidity or alkalinity of a foodor a given solution.

STRONG ACIDS AND WEAK ACIDS

A strong acid is one which is virtually 100% ionised

in solution. The strength of the acid depends on its

hydrogen ion concentration.

The larger the degree of dissociation, stronger is

the acid.

In strong acid all the hydrogen ion exist as free ion.

A weak acid is one which doesn't ionise fully when

it is dissolved in water.

In week acids only a small part exist as free ion

while the rest exist as bound ion.

Titrable acidity and free H ion:

It refers to the total H ions present in the acid.

STRONG BASES AND WEAK BASES:

A strong base like NaOH or KOH dissociates

completely in dilute solutions while a weak base like

ammonium hydroxide dissociates only to a small

degree. The degree of alkalinity of a base depends

on the concentration of OH ion.

IONSIATION OF WATER

Water is neutral, distilled water contains small

amount of hydrogen ions and hydroxyl ions.

Since the concentration of H ion is equal to that of

the OH ion in water is constant i;e, irrespective of

weather acids or alkalis are added to water.

PH AND PK SCALES

pK is a measure of acid strength. It depends on the identity and chemical properties of the acid. pKa is defined as the negative logarithm to the base 10 of the Kain g ions/ L or as logarithm to the base 10 of the reciprocal of Ka.

The pH of a solution is defined as the negative logarithm to the base 10 of the hydrogen ion concentration. It is a measure of [H+] in a solution.

pKa = - log Ka

pH = - log H+

The relationship between pH and pK is given by Henderson - Hassel Bach equation.

pH = pKa + log [A−]/[HA]

pH scale

The pH scale measures how acidic or basic a

substance is. The pH scale ranges from 0 to 14. A pH

of 7 is neutral. A pH less than 7 is acidic. A pH greater

than 7 is basic.

BUFFER SOLUTIONS

A buffer solution is an aqueous solution consisting of

a mixture of a weak acid and its conjugate base or a

weak base and its conjugate acid.

It has the property that the pH of the solution

changes very little when a small amount of strong

acid or base is added to it.

Buffer solutions are used as a means of keeping pH

at a nearly constant value in a wide variety of

chemical applications.

Many life forms thrive only in a relatively small pH

range; an example of a buffer solution is blood.

acidic buffer solutions

An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt.

example would be a mixture of ethanoic acid and sodium ethanoate in solution. In this case, if the solution contained equal molar concentrations of both the acid and the salt, it would have a pH of 4.76.

You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts.

Alkaline buffer solutions

An alkaline buffer solution has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts.

example is a mixture of ammonia solution and ammonium chloride solution. If these were mixed in equal molar proportions, the solution would have a pH of 9.25.

BOUND WATER IN FOODS

Bound water is usually defined in terms of the ways it

is measured; different methods of measurement give

different values for bound water in a particular food.

Some characteristics of bound water include:

1. It is not free to act as a solvent for salts and sugars

2. It can be frozen only at very low temperatures (below

the freezing point of water).

3. It exhibits essentially no vapor pressure

4. Its density is greater than that of free water

Bound water has more structural bonding than liquid or

free water thus it is unable to act as a solvent.

Eg. cacti or pine tree needles