Spoilage is the deterioration of food which makes it go
sour, rotten or moldy.
• Most fresh foods spoil within days when they are stored
without care in warm surroundings.
When is food spoiled?
• Offensive smell
• Mouldy surface
• Strongly sour or sharp flavor
• Different color
• Gas development (e.g. in bulging tins) or shiny surface. 1
When food is spoiled,
the characteristics of
the food are changed
in such a way that it
is no longer
Spoilage of food may be a result of one or more of the
following:(1) Growth and activity of microorganisms in the food,
often a succession of organisms is involved.
(2) Contamination of food by bacteria
(3) Action of enzymes of the plant or animal food product
(4) Worms, bugs, weevils fruit flies and moths may
damage food and renders it unfit for human
The main microorganisms responsible for the
contamination of food are:» Bacteria
Each group has many species which are
responsible for different forms of contamination.
All three groups require a medium in which to grow and
reproduce. food is an ideal medium as it provides nutrient
and adequate moisture.
Microorganism’s contaminate food by producing waste
products or toxins (poisons) or simply makes the food
inedible by their presence.
In some cases, if contaminated food is eaten it may cause
illness due to food poisoning. Microorganism which causes
food poisoning and /or infections are called pathogenic
Note: Not all microorganisms are harmful; some are used in
the food industry to produce foods such as cheese yogurt
and soy sauce.
• Are single celled organisms found in many
places including air, water (including man).
• There are thousands species some of which are
harmful to man and some are beneficial.
They can be classified according to their shape
Rod shaped bacteria
Spiral shaped bacteria
These are called cocci and they form chains called
streptococci which are cause of disease such as
Others forms pairs called diplococci which are the cause of
Clusters: called staphylococci which are the cause of
boils, septic wounds and food poisoning
» Many bacteria of this shape are called bacilli and are the
cause of disease such as diphtheria, tuberculosis typhoid
and food poisoning
Some are called clostridia and cause food poisoning some
have tiny hair projection called cilia to enable them to swim
» These can cause diseases such as cholera, syphilis and
Under suitable conditions of temperature, moisture and
food supply bacteria can multiply or reproduce very
rapidly. They reproduce by binary fission and in a space
of twelve hours under right conditions a single bacteria
can give rise to 16,000,000.
» When a large number of bacteria are present are in
one place, they form a colony which is usually visible
to the naked eye.
» If conditions of division are unfavourable e.g.
moisture is lacking, bacteria are able to form spores
which remain dormant until the right conditions
return, when the spore will germinate. Such spores
are often resistant to heat.
» Many bacteria do not require a source of oxygen to
grow and multiply. Such bacteria are called anaerobic
bacteria and this account for their ability to grow in
food and intestines where oxygen supplies are
limited. Some bacteria do require oxygen and are
called aerobic bacteria
Effect of Temperature and time on
growth of Bacteria
Some bacteria can withstand extreme temperatures, but
most are destroyed at temperature of around 60oC.
Bacteria are able to multiply most rapidly at around 37oC
and this has important implications for the preservation
of food by heat.
At very cold temperatures most bacteria stop multiplying
and become dormant until the temperature conditions
become favourable again, this has important implications
for the freezing and cold storage of foods and their use.
» Many bacteria are affected by high concentration of
salt acid or sugar. These interfere with the normal
metabolic processes of the bacterial cells and either
destroy them or prevent reproduction such
substances are used for food preservation.
» There are some however some bacteria which thrive
in such conditions and may therefore require
different treatment to prevent their growth.
» Bacteria can cause food poisoning symptoms in different ways:
» Physical presence of bacteria in the food if bacteria have had
the opportunity to multiply in large numbers in a food, then
physical present in the intestine may cause irritation and food
poisoning symptoms. This is also referred to as infection
» Production of waster products (Toxins) bacteria like all other
living things has to dispose off waste products which are the
result of metabolic processes. These can cause irritation to the
intestine and food poisoning symptoms even when only a
small when only a small number of bacteria are present. The
toxins are not destroyed by normal cooking temperatures
» Germination of spores: The germination of bacterial spores is
usually accompanied by the productions of highly poisons
substances (exotoxins) only a small amount of which can lead
to severe illness or possibly death. In some cases, just one
spore can produce sufficient toxin to cause food poisoning. 1
˃ Using the same utensils to serve contaminated food
and other foods
˃ Careless attention to personal hygiene while
handling food e.g. not washing hands after visiting
the toilet, touching nose while preparing food.
˃ Leaving skins infection and cuts covered while
˃ Coughing, sneezing or spitting while preparing
˃ Incomplete cleansing of food utensils and serving
˃ Pests e.g. houseflies, cockroaches, beetles and
certain moths, rodent e.g. rats, mice etc
˃ Contaminated water supply
˃ Household pets e.g. dogs, cats, hamsters
˃ Soil and dust.
Rare but in most
foods cases, exotoxin
especially meat produced
12-36hrs fatal Double vision
Salmonella typhi 1-3 weeks 1-2 Headache
Sewage, water Can fatal slow
flies cream, cake recovery
Severe vomiting Nose, skin cuts Poisoning due to
toxins heat does
Headache fever Faeces, sliced
abdominal pain poultry,
pies Poisoning due to
bacteria in gut
» Moulds are tiny microorganisms which are just visible
to the naked eye. They grow on many types of food,
especially cheese, bread and fruit. They require warm
most conditions to grow, but are able to grow at a
slower rate in cool places. Moulds reproduce by means
of sporulation. Spores are released into the
atmosphere and carried in the air. If they land on a
suitable food, the spores germinate and a new mould
appears. There are many types of mould, but among
the more common types are
» Food that is contaminated with mould often appears to
be safe to eat as only the outer part is affected by mould
growth. However, recent research has shown that
metabolites (toxins) produced by mould which migrate
into the food could be harmful to many organs of the
» These toxins are called mycotoxins it is therefore
advisable to discard mould food completely rather just
remove the mouldy part.
» Mould growth is prevented by cool dry storage, heating
to destroy moulds and acidic conditions. However, not
all moulds are harmful. Specially produced moulds are
added to certain cheeses e.g. stilton, Danish blue, to
develop characteristic flavours.
» Yeasts are microscopic single celled microorganisms
which are found in the air and soil and on the surface of
fruits. Some are able to tolerate fairly high acidic, salt
and sugar concentrations and can grow without the
» Yeasts reproduce by budding in presence of water,
warmth and food.
» The cells remain dormant in very cold
conditions and are killed at temperature
» Growth is inhibited in the presence of high salt
concentrations. Yeast can spoil foods such as
jam and fruits by fermenting the sugar, to
produce alcohol and carbon dioxide gas.
» The time that this takes will depend on the
concentration of sugar in the food and length
of time it is stored. Fruit flavoured yogurts may
also be affected in this way.
» Enzymes are found in all fresh food and although their
actions is important to the ripening of certain foods
(such as fruits and vegetables) the continuance of this
action after the peak of maturity brings about
undesirable changes in the food tissue these changes
include darkness of cut tissue or surfaces the formation
of soft sports and the development of off flavours.
The bruises and cuts caused by these insects serve as
pathways by which microorganisms reach the inner 2
There are various sources of food spoilage
microorganisms such as
vi. During handling and processing
Sources of microorganisms from animals include the surface
flora, the flora of the respiratory tract and the flora of the
The natural surface flora of meat usually is not as important
as the contaminating microorganisms from their intestinal
or respiratory tract. Howerer, hides, hoofs and hair
contain not only large numbers of microorganisms from
soil, manure, feed, and water but also important kind of
Feathers and feet of poultry carry heavy contamination from
similar sources, the skin of animals may contain
micrococci and staphylococci may find their way into the
carcass and then to the final raw products. The faeces and
fecal contaminated products of animals can contain many
enteric organisms including salmonella
» The natural surface flora of plants varies within plants
but usually include, Pseudomonas Micrococcus and
Coliforms and lactic acid bacteria.
» Exposed surfaces of plants become contaminated from
soil, water sewage, air and animals some fruits have
been found to contain viable microorganism in their
» Normal healthy tomatoes have been found to contain
Pseudomas and Coliforms. Yeasts have been found inside
When untreated domestic sewage is used to fertilize crops,
there is likelihood that the raw plant foods will be
contaminated with human pathogens especially those
causing gastrointestinal diseases.
The use of night soil as a fertilizer still persists in some
countries such as China.
» The soil contains the greatest variety of
microorganisms than any source of contamination
not only numerous kinds of microorganism but also
large total numbers are present in fertile soil ready to
contaminate surfaces of plants growing on or in them
and the surfaces of animals coming over the land.
» Modern methods of foods handling usually involve
washing the surfaces, of foods and hence the removal
of much of the soil from those surfaces.
» Natural water contains not only their natural flora but also
microorganisms from soil and possibly from animals or
» Surface water from animals or pools and stored water in
lakes and dams vary considerably in their microbial
content. Ground water from springs and or wells have
passed through layers of rocks and soil to a definite level
and hence most of the bacteria as well as the greater part
of suspended material, have been removed by subsequent
» The kinds of bacteria in natural water are chiefly species of
Pseudomonas, Proteus Micrococcus Bacillus and
» Contamination of foods from the air may be important
for sanitary as well as economic reasons.
» Disease organisms especially those causing respiratory
infection may be spread among employees by air or to
» Microorganisms get into the air by means of dust, solid
suspended materials and moisture droplets from
coughing sneezing or talking.
Contamination of foods from the natural sources as
discussed above take place before the food is harvested
or gathered or during handling and processing of the
Additional contamination may come from equipment
coming in contact with foods, from packaging materials
and from the personnel.
The processor attempts to clean and sanitize equipment
to reduce such contamination and to employ packaging
materials that will minimize contamination.
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