Introduction to Food Science

Introduction
Definition
• Food science is a systematic study of the nature of food materials and
the scientific principles underlying their modification, preservation,
and spoilage.
• To understand food science, the basic concepts of physics, chemistry,
mathematics, and biology and their applications, i.e., biochemistry,
microbiology, and food technology, are necessary to prepare,
package, store, and serve wholesome, high quality products.
Introduction
• Food chemistry is the science that deals with the composition,
structure, and properties of food, and with chemical changes that
take place in food. It forms a major part of food science and is closely
related to food microbiology.
• Micro-organisms have basic growth requirements, namely, food,
moisture, temperature, time, osmotic pressure, pH, and the presence
or absence of oxygen.
• Food chemistry and food microbiology are intimately related to food
processing because the processes to which food needs to be
subjected to improve its taste, texture, flavour, and aroma depend on
its composition and ingredients. The time and temperature for food
processing depend not only on the chemical composition of food but
on its microbial load and the type of packaging to be used.
Measurements
• BASIC SI UNITS OF LENGTH, AREA, VOLUME, AND WEIGHT
Prefixes represent numbers or numerical quantities symbolized by
letters.
mega = M = 1,000,000 = one million
kilo = k = 1,000 = one thousand
deci = d = 1/10 = one tenth
centi = c = 1/100 = one hundredth
milli = m = 1/1,000 = one thousandth
micro = μ = 1/1,000,000 = one millionth
Measurement of Length

• The unit for measuring length is the metre (m).

• Length is measured using a measuring tape or ruler.
• One thousand metres (1,000 m) = one kilometre (km).
• A metre is divided into hundred parts. Each part is called a centimetre
(cm) or
• one metre (m) = 100 centimetres (cm).
• Each centimetre is made up of ten smaller parts called millimetre (mm)
one cenitimetre = 10 millimetres (mm).
• The simplest instrument for measuring length is a scale or ruler
measuring one metre, or a measuring tape.
Measurement of Volume
• Volume and capacity is measured in litres. A litre is made up of 10 decilitres (dl).
Each decilitre is made up of 10 centilitres (cl). A centilitre is made up of 10 millilitres
(ml), which means that a litre is made up of one thousand millilitres (1,000 ml).
• Most measuring cups and jugs are marked in millilitres and litres. The capacity of
cups and spoons is listed below.
The volume of solids that is not greatly affected by water can be
• 1 tablespoon = 15 ml measured by the water displacement method. Solids are
immersed in the displacement can and the volume of water
• 1 teaspoon = 5 ml displaced, equal to the volume of the solid, is noted.
• 1 breakfast cup = 240 ml
• 1 coffee cup = 100-120 ml
• 1 teacup = 150-180 ml
• 1 water glass = 280-300 ml
• The seed method is used to measure the volume of cake and bread. A
large tin box is filled to the brim with seeds and the volume of seeds
required to fill the box is measured in a measuring cylinder. The cake
of which the volume is to be measured is placed in the empty tin and
covered with seeds. The volume of seeds remaining after covering the
cake is equal to the volume of the cake.
Measurement of Weight or Mass
• Weight is the pull experienced on the body by the earth’s force of gravity. Mass
is the amount of matter contained in a known volume of substance. Mass
always remains constant but weight may change in different parts of the world
because the force of gravity varies from place to place.
• Weight is measured on a weighing scale. The kilogram is the unit for measuring
weight and is made up of one thousand smaller parts called grams.
1 kilogram (kg) = 1,000 grams (g)
1 g= 1,000 mg
1 mg = 1,000 ug
From the above we conclude that 1 kg = 1,000,000 mg and a measure of 1 ppm
means 1 mg in 1 kg of a substance.
DENSITY
• Density is the relationship between the weight and volume of a
substance expressed as

• It is expressed in kilograms per cubic metre and is used to compare

the heaviness or lightness of different foods. A fruit cake has a greater
density as compared to a sponge cake. The density of liquids is
measured in g/cm3. Water has a density of 1 g/cm3.
• Relative Density
Relative density
• Relative density (R.D.) is the ratio of the mass of a known volume of a
substance to the mass of the same volume of water. It tells us the number of
times the volume of a substance is heavier or lighter than an equal volume
of water. If the R.D. of a volume of lead is 11, it means that it is eleven times
as heavy as an equal volume of water.
• A hydrometer is used to measure the relative density of different liquids. It is
made up of a weighted bulb with a graduated stem calibrated to measure
the relative density of the liquid directly. The liquid to be tested should be at
room temperature and the hydrometer is allowed to float in the liquid. The
depth to which it sinks is read on the graduated stem. Hydrometers are
specifically calibrated to measure the R.D. of different liquids used in the
catering industry.
Devices used for measurements
• Saccharometers are used to determine the concentration of sugar solutions, denoted in
degrees Brix. A 75% sugar solution is called 75 degrees Brix.
• Salinometers are used to determine the R.D. of brine or sodium chloride solutions used for
canning vegetables or pickling ham.
• Lactometers are used for checking the purity of milk. Addition of water or removal of cream
affects the R.D. and is depicted on the graduated scale on the stem. The scale is marked
1.00 to 1.04. ‘W’ denotes R.D. of water, ‘M’ denotes pure milk, and ‘S’ denotes skim milk.
• Alcoholometers are used to test the R.D. of alcoholic beverages. It is used to check the
number of degrees proof or ethanol content of wines, beers, and spirits, and whether it has
been diluted.
• Refractometers are used to measure the sugar or total solids in solution (TSS) while
preparing jam, syrups, etc. They measure the refractive index of light reflected through the
solution.
TEMPERATURE
• Energy is present in many forms. Heat is one form of energy. Solar energy,
electrical energy, and chemical energy are some of the other
• Heat energy is measured in units called joules and the energy present in food
is measured in kilocalories. One kilocalorie is made up of 1,000 calories.
• 1 kilocalorie (kcal) = 4.2 kilojoules (kj) 1 calorie = 4.2 joules
• Temperature refers to the relative hotness or coldness of a substance
compared with melting ice at 0°C and boiling water at 100°C. Thermometers
are used to measure temperature.
• Temperature is measured either in the Celsius or centigrade scale (°C) or in
the Farenheit scale (°F). Each scale has two fixed points: . 1. Melting point of
ice (0°C or 32°F) 2. Boiling point of water (100°C or 212°F).
Types of Thermometers
• Most thermometers are mercury in glass thermometers with different
temperature ranges depending on their purpose. Some common
thermometers are:
• Sugar or confectionery thermometers (40°C to 180°C)
• Dough testing thermometers (19°C to 43°C)
• Meat thermometers with a special spike which can be pierced into
meat and a round dial to record temperature
• Refrigeration thermometers filled with red coloured ethanol (-30°C to
—100 °C).
pH OR POTENTIAL HYDROGEN
• When an acid is diluted with water it dissociates into hydrogen ions and acid radical
ions.

• The term pH (hydrogen ion concentration) is used to express the degree of acidity or
alkalinity of a food.
• It is defined as the negative logarithm to base 10 of the hydrogen ion concentration,
i.e., higher the hydrogen ion concentration, lower will be the pH and vice versa.
• Some foods like fruits contain organic acids and have an acid reaction while others
such as milk are neutral. Bakery products leavened with baking powder, have an
alkaline reaction. Pure water is pH 7 or neutral.
• The pH scale of pH 0 to pH 14, ie., from extremely strong acids to extremely strong
alkali is used to describe the acidity or alkalinity of food.
• A reading between pH 1 to pH 6.5 indicates acidic food while pH 7.5
to pH 14 indicates alkaline food.
• The pH of a solution can be measured electrically using the pH meter
or it may be measured colorimetrically using pH papers which change
colours according to the pH.
Buffers
• They are defined as solutions that can resist a change of pH on addition of
acids or alkalis but within limits.
• These solutions are made up of a weak acid and one of its salts or a weak
base and one of its salts.
• When hydrogen ions (H*) or hydroxide ions (OH’)
are added, they can be absorbed by these systems
without altering the pH of the resulting solution.
• Common buffers are:
1. Acetic acid and sodium acetate mixture
2. Citric acid and sodium citrate mixture.
• A buffer is a special solution that stops massive changes in pH levels.
Every buffer that is made has a certain buffer capacity, and buffer
range. The buffer capacity is the amount of acid or base that can be
added before the pH begins to change significantly. It can be also
defined as the quantity of strong acid or base that must be added to
change the pH of one liter of solution by one pH unit.
• Buffering action is very important in the human body and in food. The
salts of calcium, phosphorus, sodium, and potassium function as
buffers and maintain the pH of milk at a constant level of 6.5.
 Applications of pH
• Preparation of jam—The pectin in jam and marmalade does not form a gel
until the pH is lowered to 3.5. If fruit used for making these preserves does
notcontain sufficient acid, small amounts of citric acid should be added.
• Retaining bright green colour in green vegetables—Green vegetables tend to
get discoloured when cooked. Green colour can be retained by adding a pinch
of sodium bicarbonate to the cooking liquor but B complex vitamins and
vitamin C gets destroyed in an alkaline medium.
• Food digestion—pH of the gastrointestinal juices affects our digestive process.
The pH of gastric juice is strongly acidic, between 1 and 2, and aids in digestion
of food in the stomach while a mildly alkaline pH, between pH 7 and 8 is
needed, to complete digestion in the intestine.
• Texture of cakes—A significant change in texture is observed with a change in pH while
baking cakes. Low pH gives a fine texture and high pH gives a coarse texture to the cake
crumb.
• pH of dough—In bread making, compressed yeast is used for fermentation. During
fermentation, yeasts convert simple sugars to ethyl alcohol and carbon dioxide.
(a) Ethyl alcohol takes up oxygen and forms acetic acid
(b) Carbon dioxide dissolves partially in water to form carbonic acid
(c) Chemical yeast food, i.e., ammonium sulfate and ammonium chloride if used, produce
sulphuric acid and hydrochloric acid respectively.

• All these acids lower the pH of the dough from pH 6.0 to pH 4.5. This change in pH
makes the dough less sticky and more elastic.
 IMPORTANT TERMINOLOGIES
• Boiling Point
• Boiling is the use of heat to change a substance from a liquid to a gas.
• Like the melting point, the boiling point of a pure substance is always constant. It changes if impurities or
dissolved substances are present or by changes in atmospheric pressure. Pure water boils at 100°C.
• Applications of boiling point:
1. Boiling vegetables in salted water increases the boiling point above 100°C.
2. In sugar cookery, the boiling points of sugar solutions is noted at various stages so that fondant, fudge, toffee,
and caramel can be prepared.
• Boiling under pressure
1. When atmospheric pressure is lowered, water boils at a lower temperature of 70°C.
2. When pressure is increased, e.g., below sea level or boiling in a pressure cooker, water boils at higher
temperatures and food cooks faster.
• Applications of boiling under pressure:
1. Food is cooked in pressure cookers to reduce cooking time to one-fourth of ordinary cooking time as water boils
at a higher temperature under pressure.
2. Autoclaves are used for sterilization by moist heat under pressure at 121°C and 15 lb pressure for 20 minutes.
 Evaporation

• Evaporation is a change of state from liquid to gas which takes place continuously from
the surface of a liquid.
• Volatile liquids vaporize easily e.g., petrol and acetone. Non-volatile liquids like oils
evaporate very gradually. Evaporation is faster when there is breeze and low humidity
in the air as well as a large surface area and high temperature.
• Applications of evaporation:
1. Bread and cake if left uncovered, hardens and becomes stale because of loss of
moisture. This can be prevented by storing food in covered tins.
2. Cooking in shallow uncovered pans will cause greater evaporation and is used for
preparing mawa from milk.
3. Milk powder is prepared by dehydration or spray drying in which water from milk is
removed by circulating hot air.
 Melting point
• Melting or fusion is the change of state from a solid to a liquid.
• The temperature at which a solid melts and turns into a liquid is
called its melting point.
• The melting point of fats depends on the percentage of saturated
long chain fatty acids present in it.
• The melting point for any chemical is fixed and is used to measure
the purity of a substance. It is lowered by adding other substances.
• Melting point of fats: Vanaspati 39°C Butter 37- 36 Lard 44°C Tallow
48°C Coconut oil 26°C
 Applications of melting point
1. Ice has a melting point of 0°C. If adequate sodium chloride is
added to ice, the melting point falls to -18°C. This lowering of
melting point is made use of in the setting of ice cream.
2. Fat is removed from adipose tissue of animals by a process
called rendering which is based on the melting point. Boiling
water or dry heat is used to liberate the oil from the fat cells.
 Smoke point
• When fats and oils are heated strongly above frying temperature, they
decompose and a stage is reached at which visible thin bluish smoke is
given off. This temperature is called the smoke point.
• The temperature varies with different fats and ranges between 160 and
260°C. The bluish vapour is because of formation of acrolein from
overheated glycerol. Acrolein has an acrid odour and is irritating to the
eyes.
• The smoking point is lowered by the following factors: 1. Presence of
large quantities of free fatty acids 2. Exposure of large surface area
while heating 3. Presence of suspended food particles.
• Flash point This is the temperature at which the
decomposition products of fats and oils can be ignited, but will
not support combustion. The flash point varies with different
fats and ranges between 290 and 330°C.
• Fire point This is the temperature at which the decomposition
products of fats and oils support combustion. It ranges
between 340 and 360°C for different fats. The oil or fat may
catch fire and burn. The smoke point, flash point, and fire point
are lowered by the presence of free fatty acids.
• Normal frying temperature for most oils is 180-195°C. The
smoke point is 25-40°C above normal frying temperature. The
application of smoke point is in frying foods. Fats and oils used
for deep fat frying should have a high smoke point. Moist foods
should be coated well before frying as moisture present in food
tends to hydrolyse the fat and increase the free fatty acids
present.
 Surface Tension
• Surface tension is a force experienced on the surface of a liquid. It is caused by cohesion,
i.e., a force that causes the molecules of a substance to be attracted to one another.
• Surface tension is also defined as the force of attraction which exists between liquid and
solid surfaces.
• The molecules of a liquid that are below the surface are pulled by cohesive forces from
all directions. But the molecules at the surface behave differently because they are only
pulled downwards or sideways. This downward or sideways attraction causes a constant
pull on the surface molecules which makes the liquid behave as if it is covered by a thin
elastic film. For example, the surface of water can support needles if they are placed
carefully.
• Because of surface tension, drops of liquid take a spherical shape, which has the smallest
possible surface area, e.g., dew drops.
 Osmosis
• Osmosis is the passage of water from a weak solution to a stronger solution through a
semipermeable membrane.
• When raisins are soaked in a cup of water for sometime, the raisins swell because water
from the cup enters the raisins. Similarly, if raisins are placed in a concentrated sugar
solution, they shrivel up after sometime because water from the raisins passes into the
sugar solution because of osmosis.
• Plant and animal cell membranes act as semipermeable membranes and selectively
permit water and electrolytes to enter or leave the cell
• Applications of osmosis: 1. Osmosis plays an important role in food processing and
preservation to retain the original shape and size of canned fruits in syrup and of
vegetables in pickles. 2. The freshness of fruits and vegetables depends on the osmotic
pressure in the cells. Salads lose their crisp crunchy texture and become limp if salt and
sugar is sprinkled much in advance. Lettuce leaves can be revived by immersing then in
chilled water.
 Humidity
• Humidity refers to the presence of water vapour in the air.
• Water vapour is produced by respiration of plants and animals,
evaporation from food during cooking and from water bodies, from
rain during the monsoons, etc.
• The humidity of the air is measured with the help of a hygrometer.
This instrument depicts the percentage of water vapour in the air. It is
a ratio between the amount of water vapour which air could hold and
what it actually holds at the same temperature. Humidity of 60-70%
is considered normal and does not cause discomfort or undue
spoilage of food.
 Applications of humidity
1. Spoilage organisms multiply and spores germinate at high
moisture levels in the atmosphere.
2. Humidity needs to be controlled in air-conditioned rooms
along with ventilation and heating which is done by humidifier
water sprays which maintain 60-70% humidity.
3. Processed foods are prevented from drying up by adding
substances with hygroscopic properties called humectants.
Glycerine and sorbitol are used as humectants in jam.
 Food Rheology
• It is the science of measuring forces which are needed to
deform food materials or to study the flow properties of liquid
foods. It deals with the viscous behaviour of a system.
• The texture is determined when we chew food and it is
described as crisp, tough, chewy, creamy, sticky, spongy, etc.
• Viscosity is defined as the resistance of a liquid to flow. It is
measured by an instrument called a viscometer. This property
of a liquid is seen in batters, sauces, syrups, etc.
• Compression It is the pressure needed to squash foam or spongy foods to find out
their freshness or tenderness. The compressimeter or tenderometer is used to
measure the lightness of a product.
• Shearing It is force needed to cut or slice through meat, vegetables, fruits, etc., and
indicates the toughness of a food the. Penetrometers measure the force needed to
penetrate a food, such as jelly, cooking fat, canned and fresh fruits, and vegetables.
• Elastic substances These substances do not flow, but flow when force is applied.
However, when the force is removed it regains its original shape, e.g., sponge cake.
• Plastic substances These substances resist flow to a certain point, but beyond that
point they flow, i.e., they become plastic in nature. Plasticity is an important
property of margarine. A plastic fat is one which can be creamed as well as forms a
thin sheet or layer in dough when the dough is rolled out, e.g., flaky pastry.
 Types of food dispersions
• Foods are mixtures or dispersions of two or more types of
substances. These substances are present as particles of
various sizes. Depending on the particle size or size of the
molecule in the mixture, these substances may be classified as
a true solution, a colloidal dispersion, or a coarse suspension.
True Solution
In a true solution, ions or molecules smaller than one millimicron
are dissolved in a liquid.
• They have the smallest particle size of the three types of
dispersions. A solution is homogenous, i.e., alike in all parts,
e.g., sugar syrup and brine.
Suspension
• Suspensions are dispersions of coarse particles in a liquid. The
particles are large and require continuous agitation to keep
them dispersed. When agitation ceases, these coarse
suspended particles settle down because of force of gravity.
When the mixture is stirred, the suspension is formed again. In
a suspension the particle size is larger than one micrometre or
micron, e.g., starch and cold water paste.
Colloidal Systems
• Between the particle sizes of the solutions and those of suspensions, lies the
area of colloidal systems.
• Colloidal systems deal with dispersions of definite size, since it is the size of the
particles in the colloidal range that impart the specific and characteristic
properties to the system.
• Colloidal dispersions are characterized by particles ranging between one
millimicron (0.001 µm) and 100 millimicrons (0.1 µm) with maximum size of up
to one micrometre (µm) in diameter.
• All colloidal dispersions or colloidal systems have two phases: a continuous
phase and a discontinuous or dispersed phase. The continuous phase extends
throughout the system and surrounds the dispersed phase completely.
• Proteins, carbohydrates, and fats exist in foods as particles of colloidal
dimensions.
• The system is a colloidal system as long as the particle size of the dispersed
• Colloidal systems may be a combination of solid, liquid, or gas as the
continuous or dispersed phase. In food, the following colloidal
systems are of importance.
• Sol — Colloidal dispersion of a solid dispersed in a liquid.
• Gel — Colloidal dispersion of a liquid dispersed in a solid.
• Emulsion — Colloidal dispersion of a liquid dispersed in a liquid.
• Foam — Colloidal dispersion of a gas dispersed in a liquid.
• Solid foam or suspensoid — Colloidal dispersion of a gas dispersed
in a solid.
• Dispersions may be simple or complex. In a simple dispersion
a colloid may consist of a solid dispersed in a liquid, e.g., when
gelatin is dissolved in warm water, a simple dispersion called a
sol is formed. Mayonnaise is an example of a complex
dispersion since it is an emulsion, a sol, and foam combined in
one. Milk is another example of a complex dispersion, i.e.,
more than one phase is dispersed in a liquid. Milk is a solution
of lactose in water, an emulsion of fat in water, and a sol as
milk protein is dispersed in water.
• The rigidity, elasticity, and brittleness of the gel depends on the type
and concentration of the solid or gelling agent, the pH, salt content, and
temperature, e.g., pectin does not form a gel unless the pH is acidic.
• The gelling agent may be a polysaccharide like cornflour, a protein like
albumin in caramel custard or complex colloidal particles like calcium
caseinate in curds. Gums, pectins, and gelatin can form gels even at low
concentrations.
• The liquid which was entrapped in the three-dimensional meshwork of
the gel is expelled from the interstitial spaces and the gel shrinks. This
condition is called syneresis or weeping gel.
 Emulsions
• 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 with the 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.
• Food emulsions are of two types: (a) Oil in water emulsion or O/W emulsion
in which the droplets of oil are dispersed in water, for example, mayonnaise
and milk. (b) Water in oil emulsion or W/O emulsion in which droplets of
water are dispersed in oil, for example, margarine and butter.
• During the process of emulsification, the main step is to break down the
bulk liquid into small droplets and then stabilize the emulsion. 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 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.
• Mechanical aids such as beaters, stirrers, homogenizers, and colloid mills
help to reduce the size of the dispersed droplets, thereby increasing
surface area.
 Stability of emulsion
• The stability of an emulsion depends on the following factors:
1. The presence and type of emulsifying agent present
2. The amount or concentration of the emulsifying agent
3. The size of the droplets in dispersed phase
4. The ratio of oil and water used gel
5. The viscosity of the continuous phase.

Substances that increase the viscosity of a colloidal system are called stabilizers.
They do not orient themselves at the interface as an emulsifier, but reduce the
speed with which the dispersed droplets move. As viscosity increases the
collision between droplets decreases and the droplets remain dispersed for a
fairly long time. Examples of stabilizer are starch, sugar, gelatin, gums, finely
powdered spices, carboxy methyl cellulose, sodium alginate, pectin, etc.
Formation of stable emulsions is of utmost
importance in the food industry. A broken
emulsion loses its viscosity, cannot be
spread, and gives the product an
unappetizing curdled appearance. Broken
emulsions affect the texture and
consistency of the final product.
• Egg yolk contains a phospholipid lecithin which is a good emulsifying agent and
forms O/W emulsions. Lecithin contains fatty acids as the hydrophobic group,
and phosphate and choline as the hydrophilic group. Caseinogen a protein
found in milk acts as a natural emulsifying agent. Glycerly monostearate (GMS)
is added as an emulsifier in ice creams. Mayonnaise is a stable emulsion
because of lecithin in the egg yolk. The most widely used natural emulsifiers
are lecithins present in egg yolk and extracted from soya beans which is more
economical.
• Synthetic food emulsifiers most commonly used are mono- and diglycerides.
• Some of the other emulsifying agents are stearyl tartarate, lactic acid
monoglyceride, polyoxyethylene monostearate, etc.
 Common Food Emulsions
• Milk and cream - O/W emulsion stabilized by phospholipids and protein caseinogen
• Butter and Margarine - W/O emulsion containing approximately 80% fat. Butter is
stabilized by caseinogen and margarine is stabilized by GMS
• Egg yolk - O/W emulsion. It is a good emulsifier as it contains lecithin
• Salad dressings(a) Mayonnaise - O/W emulsion stabilized by egg yolk. Not less than
65% oil by weight. Synthetic emulsifiers may be added like mono- and diglycerides
of fatty acids, e.g., GMS
• Gravies, sauces, cream soups - O/W emulsions contain high percentage of water
stabilized with refined flour
• Icecream — O/W emulsion stabilized by caseinogen, GMS, alginates, gums/ gelatin.
 Foams
• Foams are of two types: 1. Gas in liquid 2. Gas in solid
• Beaten egg white and sugar foam is a gas in liquid dispersion. When it is baked it
becomes a gas in solid dispersion, e.g., meringue.
• To form a foam, energy is required to overcome the surface tension of the liquid and
stretch it into thin films, which surround bubbles of gas. Liquids which form foams
easily have a low vapour pressure and low surface tension.
• The presence of solid matter increases the stability of a foam. When egg white,
cream, or gelatin is whipped into a foam, the protein which collects at the air—water
interface gets denatured or coagulated by the energy used for whipping and helps in
making the foam stable.
• Foams used in cookery include egg white, egg yolk, gelatin, and cream. They
contribute towards lightness, volume, and texture of the product.