Food Spoilage and Preservation
Professor James Dooley
School of Biomedical Sciences,
University of Ulster,
Coleraine
Food Spoilage and Preservation
Essential element of modern society

Not appreciated by most individuals
A changing environment requiring constant
innovation
Will always be a problem for humans
Hunter-Gatherer society
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supported low numbers/ self-sufficient
unreliable food supply
limited specialisation of individuals
Industrial and Agricultural society
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supports high numbers/ produce excess
supports specialisation
generally predictable food supply
Vitamins
Proteins
Food
Energy
Carbohydrate
Building materials
Lipids
Microbial
Growth
Human Growth
What are microbes?
“Organisms that are too small to be seen
with the naked eye”
Bacteria
Viruses
Fungi
Protozoa
What are bacteria?
Unicellular organisms
Very small!!!!!!!

1-10 microns
Enormous diversity

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Shape
Habitat
Nutrition
Many bacteria require similar growth and nutrition
conditions to humans

very many do not but we do not deal with them when considering
food spoilage and preservation.
Light Microscope
x 1,000
Where do we find bacteria?
Everywhere!
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Soil
Plant roots
Water
Bodies of animals, fish, birds etc,
Hot springs
Dead Sea
Hydrothermal vents
What are microbes?
“Organisms that are too small to be seen
with the naked eye”
Bacteria
Viruses
Fungi
Protozoa
General features of Fungi
unicellular (yeasts) and multicellular (moulds)
Non-photosynthetic,plant-like organisms
Multicellular, filamentous organisms
Normally inhabitants of the soil, rhizosphere and
water
Can tolerate acidic and dry conditions
Fungi in Nature
Metabolic by-products form the raw material for
many industries:
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ethanol
antibiotics
enzymes (washing powders etc.)
solvents
food flavours
Cholesterol-lowering drugs – mevacor
Fungi are the main organisms involved in the
decay of organic material and the recycling of
essential elements (C, N, etc.)
Yeast are good model organisms for genetic
manipulation.
Micro-organisms and food
Agents of food production
Micro-organisms and food
Agents of disease
Micro-organisms and food
Agents of food spoilage
Food Spoilage and How to
Prevent it
We need to know about how spoilage
organisms live
We need to understand their biology
We need Microbiologists!
Laboratory study of bacteria
Bacterial growth
Doublings
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Number of cells
1
2
4
8
16
32
64
128
256
512
1,024
2,048
4,096
8,192
16,384
32,768
65,536
1,310,752
2,621,504
5,243,008
10,486,016
20,972,032
41,944,064
83,888,128
Doubling is a Big Deal
Some bacteria can double
every 30 min. and a few
can double in 20 minutes!!
• Escherichia coli
• 20 minutes
•Mycobacterium tuberculosis
• 15 hours
What do bacteria need to grow?
Source of nutrients
amino acids, sugars, lipids, vitamins
 released by action of enzymes operating outside
the cell
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starch digested by amylase
Correct temperature
Bacteria grow within temperature ranges
 mesophiles (10-45oC)
 psycrophiles (0-20oC)

What do bacteria need to grow?
pH
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6-7.5
Absence of toxic chemicals
Correct atmosphere (O2)
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Aerobic
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Anaerobic
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Bacillus
Clostridium
facultative anaerobes
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Salmonella
Super Tough Bacteria!
some bacteria produce endospores
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response to stress
very resistant to heat
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121oC
very resistant to harsh chemicals,
drying, radiation
can remain dormant for a long time
(years)
endospore - forming bacteria are
common in soil
What happens when bacteria grow?
Nutrients
Suitable
Environment
Time
More Bacteria!
What happens when bacteria
grow in food?
Food Components: Starch, protein etc.
Sugars, amino acids etc.
Digestive enzymes
Waste products:
Altered
Food
CO2
Alcohol
Lactic acid etc.
Altered
Environment
Microbial Food Spoilage
Microbial growth introduces unwanted alterations
in food
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appearance
smell
Taste
Nutritional content
Changes not necessarily harmful!
Each food unique microbial environment
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unique spoilage agents
Three groups of foods:
based upon rate of spoilage
highly perishable
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meat
fruit
milk
vegetables
eggs
WET
semi perishable
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potatoes
nuts
stable
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rice
flour
dry beans
Dry
What defines each group?
Amount of water
Food Spoilage
Each food has it’s own unique microbial
population
Uncontrolled growth of the microbes results
in food spoilage
We can predict (and therefore control) food
spoilage
Milk spoilage (unpasteurised)
Bacterial growth on milk sugars

(Lactobacillus spp., Lactococcus spp.)
pH reduction
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lactic acid build up (bitter taste!)
Change in bacterial population
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further pH reductions and much more
lactic acid, continues until all sugars
depleted
Yeasts and moulds dominate
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use lactic acid for growth.
pH rise
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allowing further bacterial growth
Bacteria use proteins as major nutrient
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(Primary amines produced- Smelly!!!!!)
Food spoilage has
major economic
impacts
Microbial food spoilage

Foods are characteristically spoiled by known
organisms
Food
Organism
Type of Spoilage
Chicken
Pseudomonas spp. Sliminess
Green colour
Milk (pasteurised)
Lactobacillus
thermophilus
Sour
Bread
Rhizopus nigrans
Bread mould
Food Spoilage
Shapes
History
Nicholas Appert
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a Frenchman who invented a method to preserve perishable
organic materials.
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In 1809, Appert received 12,000 francs for his method of
enclosing food in airtight jars which were then heated.
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boiling products in jars for four to six hours and then pouring
molten wax over the jars.
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By this method, food could be preserved indefinitely.
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Unfortunately, the glass jars often broke on their trip to the
army!!!!
Preservation of food by killing all
microbes
Temperature
canning
 sterilization by heat
 121oC for 15 minutes
 all bacteria and
endospores killed

Preservation of food by killing all
microbes
Removal or killing of all microbes from a
food will prevent spoilage!
Removal or killing of all microbes from a
food will drastically alter the food
taste
 texture
 nutritional content
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Preservation of food by
preventing microbial growth
A number of parameters can be manipulated
to slow down microbial growth
Moisture content {water activity (Aw)}
Perishable foods have a high Aw

preserve by lowering Aw
How to reduce water?
drying
sun
 heat
 freeze - dried
(expensive!)
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How to reduce water?
addition of salt or
sugar
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water needed to
keep salt and
sugar in solution
Preservation of food by
preventing microbial growth
pH
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very few bacteria grow below
pH 5.0
How to make food acidic?
Add acid e.g. acetic acid
 Allow bacteria to make acid
from natural food components
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lactic acid bacteria
Preservation of food by
preventing microbial growth
Temperature
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storage at 4oC degrees
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storage at -20oC degrees
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rate of spoilage decreased
rate of spoilage extremely
slow
need -70oC to eliminate
spoilage
Preservation of food by
preventing microbial growth
Temperature
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Pasteurization
mild heat treatment
overall microbial population is reduced
pathogens are eliminated since these tend to be more
heat sensitive than other organisms.
63°C for 30 min. (batch pasteurization)
72°C for 15 sec. (flash pasteurization)
Food Preservation by control of
bacterial growth
Radiation
use of gamma rays from Co60
 microbes killed by free radicals
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Food can be packaged!
No recontamination possible
Pasteurization of meat, poultry, cheese
No alteration of food
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controversial claim
Irradiation is controversial
Irradiation of various
foods accepted in US
and many other
countries
UK only allows for
irradiation of herbs,
spices or vegetable
seasonings
Preservation of food by
preventing microbial growth
Modified Atmosphere
Packaging
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Oxygen
Nitrogen
Carbon Dioxide
Argon
Mix depends on food in
question
A little extra material...
BBC Radio 4 Science
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“On the shelf”
http://www.bbc.co.uk/radio4/science/connect_2002103
0.shtml
Food Safety Through the Ages
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Dr. Bill Grierson
http://www.acsh.org/healthissues/newsID.767/healthiss
ue_detail.asp
Food Preservation site
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Good links to related material
http://www.bookrags.com/sciences/biology/foodpreservation-wmi.html
Food Standards Agency
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www.food.gov.uk/
Good site for general information
A little extra material...
A good site to visit
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http://resources.schoolscience.co.uk/SGM/index.html
Food preservation challenges
Tests for food spoilage
EGGS - When something starts pecking its way out of the shell, the
egg is probably past its prime. Especially if the something is NOT a
chicken.
DAIRY PRODUCTS - Milk is spoiled when it starts to look like
yogurt. Yogurt is spoiled when it starts to look like cottage cheese.
Cottage cheese is spoiled when it starts to look like regular cheese.
Regular cheese is nothing but spoiled milk anyway and can't get any
more spoiled than it is already. Cheddar cheese is spoiled when you
think it is blue cheese but you realize you've never purchased that kind.
Blue cheese, by definition, is never spoiled.
FROZEN FOODS - Frozen foods that have become an integral part
of the defrosting problem in your freezer compartment will probably
be spoiled - (or wrecked anyway) by the time you pry them out with a
kitchen knife.
Tests for food spoilage
MEAT - If opening the fridge door causes stray animals to congregate
outside your house, the meat is spoiled.
BREAD - Sesame seeds and Poppy seeds are the only officially
acceptable "spots" that should be seen on the surface of any loaf of
bread. Fuzzy and hairy looking white or green growth areas are a good
indication that your bread has turned into a pharmaceutical laboratory
experiment.
FLOUR - Flour is spoiled when it wiggles.
SALT - It never spoils.
LETTUCE - lettuce is spoiled when you can't get it off the bottom of
the fridge without Mr Muscle.
Tests for food spoilage
CANNED GOODS - Any canned goods that have become the size or
shape of a softball should be disposed of. Carefully.
CARROTS - A carrot that you can tie in a clove hitch in is not fresh.
RAISINS - Raisins should not be harder than your teeth.
POTATOES - Fresh potatoes do not have roots, branches, or dense,
leafy undergrowth.
CHIP DIP - If you can take it out of its container and bounce it on the
floor, it has gone bad.
GENERAL RULE OF THUMB - Most food cannot be kept longer
than the average life span of a hamster. Keep a hamster in or nearby
your fridge to gauge this.