Properties of Food

1. Presented by
Dr. A.Surendra Babu, Ph.D., PDF
Assistant Professor,
Dept. of Food Science and Technology,
Malla Reddy University, Hyderabad

2. Density
• Density is defined as mass per unit volume, and is used in process
calculations and characterizations of food products.
• Hydrometer is an instrument used for determining density of a
liquid.
• Represented by the greek letter “ρ”
ρ =m/v
• SI unit of density is kilogram per cubic meter

3. • Mass is defined as the amount of matter in a body
• It is important to distinguish between mass and weight.
• Weight is defined as the force acting on an object as a result of
gravity.
• Consequently, the weight of an object changes as the gravitational
force changes, whereas the mass remains constant.
• Density is one of the most important mechanical properties and thus
is widely used in process calculations and product characterization

4. • Different forms of density are defined in the literature: true, material,
particle, apparent, and bulk.
True density
• This is the density of a pure substance. The quotient of mass over the
volume of a sample, without considering pores in the material (true
volume) (ρT).
Material Density
• This is the density measured when a material has been thoroughly
broken into pieces small enough to guarantee that no closed pores
remain (ρm).

5. Particle Density
• This is the density of a particle that includes the volume of all closed
pores, but not the externally connected pores (ρp).
• In this case, the particle is not modified structurally as in material
density.

6. Apparent Density
• This is the density of a substance including all
pores remaining in the material (ρa).
Bulk Density
• This is the density of a material when packed or stacked in bulk (ρB).
• The bulk density of the packed materials depends on the geometry,
size, and surface properties of individual particles
• Common laboratory gravimetric procedures for determining density
are the buoyancy technique, the displacement principle and the
pycnometer method.

7. • The most widely used method is the buoyancy technique which
utilizes Archimedes' principle: a body immersed in fluid indicates an
apparent loss in weight equal to the weight of the fluid it displaces.
• This ancient principle, from around 200 BC, is precisely what is used
nowadays to determine density gravimetrically.
• The solid is weighed in air (A) and then again (B) in the auxiliary liquid
with a known density. The density of the solid ρ can be calculated as
follows:

9. Calculation formula for density in solid sample
ρ = Density of the sample
A = Weight of the sample in air
B = Weight of the sample in the auxiliary liquid
ρ0 = Density of the auxiliary liquid
ρL = Density of air

10. PHASE CHANGE
• Phase change refer to a change in matter from one state to another.
• The most familiar phase changes are seen in the different states of
water, such as freezing liquid water to create ice or boiling liquid
water to create a gas.
• The heat plays major role in changing matter from one phase to
another

12. pH
•pH is defined as the negative logarithimic of H+ ion
concentration.
• pH scale ranges from 0 to 14 in the scale.
• pH has direct role in prolonging shelf life of products.
• pH meter measures acid concentration.
• Yeast- 4-4.5 pH,moulds- 4 pH, bacteria -7 pH.

14. • Classification of foods on the basis of pH:
a) Low acid foods: pH of 5 and higher.
This includes Peas, maize, corn, cauliflower, potato, spinach, beans etc.
b) Medium acid foods: pH of 5 to 4.5.
This class includes meat and vegetable, mixtures, soups, sauces, fish
carrot, okra, cabbage etc.
c) Acid foods: pH of 4.5 to 3.7.
These include tomatoes, pears, pineapple, banana, mango etc.
d) Highly acidic foods: pH is 3.7 and below.
This includes pickles, citrus juices, rhubarb, prune, chutney etc.

15. • The pH above 4.5 required processing at a temperature of 118-121°c
to keep it fit for Consumption and avoids microbial activity

16. Osmosis
• If two solutions of different concentrations are separated by
a semi-permeable membrane which is permeable to the
smaller solvent molecules but not to the larger solute
molecules, then the solvent will tend to diffuse across the
membrane from the less concentrated to the more
concentrated solution.
• This process is called osmosis.
• Examples: osmosis are when plant roots absorb water from
the soil and our kidneys absorb water from our blood

18. Reverse osmosis:
• Reverse osmosis is the process of osmosis in reverse.
• In reverse osmosis there is a need to ‘push’ the liquid through the
reverse osmosis membrane by applying pressure that is greater than
the naturally occurring osmotic pressure whereas osmosis occurs
naturally without energy required.
Example:
• When pressure is applied to the concentrated solution, the water
molecules are forced through the semi-permeable membrane and the
contaminants are not allowed through.

20. Colloidal systems
• The colloidal systems or colloidal dispersions are intermediate between
true solutions and suspensions.
• In other words ,the diameter of the dispersed particles in a colloidal
dispersion is more than that of solute particles in a true solution and
smaller than of suspension.
• In a true solution as sugar or salt in water the salt/sugar particles are
dispersed in the solvent as single molecules or ion.
• Thus the diameter of the dispersed particles ranges from 0.1nm to 1nm.

21. • A colloidal system contains two separate phases: a dispersed phase
and a continuous phase.
• The part which is dispersed is known as the dispersed phase and is
suspended in the continuous phase.
• Colloidal systems in food can be classified into different groups based on the
states of matter constituting the two phases.
• Food colloids are sols, gels, emulsion, and foam. For example, egg white foam
is a simple colloid system.
• Air bubbles (disperse phase) are trapped in the egg white (continuous phase)
resulting in a foam.

23. • When the diameter of the particles of a substance dispersed in a
solvent ranges from about 1 nm to 200 nm.
• The system is termed a colloidal solution, colloidal dispersion or
simply a colloid.
FOAM:
• A colloidal system with a liquid or solid continuous phase and a
gaseous dispersed phase.
SOLS:
Sols are colloidal systems in which a solid is dispersed in a liquid.
these are sud divided into two classes.

24. A) Lyophilic sols (solvent loving):
These are those in which the dispersed phase exhibits a definite affinity of
the medium or solvent.
Ex: Dispersions of starch, gum and protein in water.
B) Lyophobic sols (solvent hating):
These are those in which the dispersed phase has no attraction for the
medium or the solvent
Ex: Sulphur in water, metal hydroxides sols
EMULSIFICATION:
• Creation of an emulsion by the dispersion of one immiscible liquid in the
form of small droplets in a second immiscible liquid.
• There are two types of emulsions : oil-in-water and water-in-oil

25. SURFACE TENSION
• The term surface tension refers to the cohesive state that occurs at a
liquid-gas interface or liquid-liquid interface.
• Surface tension may be regarded as the resistance offered by liquid
water to forces attempting to deform or break through the surface
film of water.
• The interface between two immiscible fluids behaves like a stretched
membrane, and surface tension comes from intermolecular
attraction, or cohesion.

26. • At an interface between two fluids or between a fluid and a solid
container, the fluid molecules try to pull the interface inward.
• The surface tension is then defined as the tensile force per unit
length of the line at the interface.
• It is an interesting property and, for water, the surface tension,
measured in force per unit length.
• Surface tension is measured in SI units of N/m (newton per meter),
although the more common unit is the cgs unit dyn/cm (dyne per
centimeter).

28. SPECIFIC GRAVITY
• It is the ratio of density of a substance to the density of reference material
at a specific temperature and pressure. Also called relative density.
• SG =density of gas/density of air (for gas)
• SG =density of material/density of water (for solids)
• It is expressed as the dimensionless quantity
• An object with a specific gravity less than 1 will float in water
• While a specific gravity greater than 1 means it will sink

30. • The usual standard of comparison for solids and liquids is water at
4°C, which has a density of 1.0 kg per litre.
• Gases are commonly compared with dry air, which has a density of
1.29 grams per litre under standard conditions (0 °C and a pressure of
1 standard atmosphere).
• Specific gravity of normal milk is 1.028 to 1.032. Specific gravity of
water is one.
• Addition of water to milk tends to decrease the specific gravity of
milk.

31. • The specific gravity of potato tubers is an important quality criterion
for processing potatoes.
• It is used as an estimate of the solids or dry matter content of tubers
— the higher the dry matter content the lower the water content and
the higher the specific gravity.

32. Physical attributes of foods
• Physical attributes of foods consist of size,
shape, volume, density, and porosity,
surface area etc.
SIZE:
• Size is an important physical attribute of
foods used in screening solids to separate
foreign materials, grading of fruits and
vegetables, and evaluating the quality of
food materials.
• Size can determine the loss of products
during processing and the final product
yields.

33. • Size features include weight,
volume, diameter, area, surface
area, perimeter, length, skeleton
length and width.
• Particle sizes are expressed in
different units depending on the size
range involved.
• Coarse particles are measured in
millimeters, fine particles in terms of
screen size, and very fine particles in
micrometers or nanometers.

34. SHAPE:
• Shape is generally referred to the physical structure of objects
geometrically.
• Shape is also important in heat and mass transfer calculations,
screening solids to separate foreign materials, grading of fruits and
vegetables, and evaluating the quality of food materials.
• Shape affects the grade given to fresh fruit.
• To make the highest grade a fruit or vegetable must have the
commonly recognized expected shape of that particular
fruit/vegetable.

35. • Misshapen fruit and vegetables will be down-graded and may sell at
a lower price in high volume markets.
• The shape of a food material is usually expressed in terms of its
sphericity and aspect ratio.

36. VOLUME:
• Volume is defined as the amount of three-dimensional space
occupied by an object
• Usually expressed in units that are the cubes of length units, such as
cubic inches and cubic centimeters, or in units of liquid measure,
such as gallons and liters.
• In the SI system, the unit of volume is m3.
• Volume is an important quality attribute in the food industry.
• It appeals to the eye, and is related to other quality parameters.