Transcript Food Spoilage and Spoilage

Food Spoilage and Preservation
Dr James Dooley
School of Biomedical Sciences,
University of Ulster,
Coleraine
Food Spoilage and Preservation
An 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
limited specialisation
unreliable
Industrial and Agricultural society
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supports high numbers/ produce excess
supports specialisation
generally predictable
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!!!!!!!
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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!)
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
spores
Endospore formation
• some bacteria produce endospores
•response to stress
• very resistant to heat
•121oC
• very resistant to harsh chemicals, drying,
radiation
• can remain dormant for a long time (years)
• endospore - forming bacteria are common
in soil
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
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8
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128
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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
Bacterial growth
Binary fission
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Divide in two
Growth rate varies
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Escherichia coli
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20 minutes
Mycobacterium
tuberculosis
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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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Correct temperature
Bacteria grow within temperature ranges
 mesophiles (10-45oC)
 psycrophiles (0-20oC)
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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
Consequences of bacterial growth
Nutrients
Suitable
Environment
Time
More Bacteria!
Consequences of bacterial growth
in food
Nutrients: Starch, protein etc.
Waste products
Altered
Food
CO2
Alcohol
Lactic acid etc.
Altered
Environment
Microbial Food Spoilage
Microbial growth introduces 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
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(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
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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.
In 1809, Appert received 12,000 francs for his method
of enclosing food in airtight jars which were then
heated.
In 1810, Appert published the first known book on
canning
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boiling products in jars for four to six hours and then pouring
molten wax over the jars.
By this method, food could be preserved indefinitely.
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
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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
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–taste
–texture
–nutritional content
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
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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
Food Spoilage Tests- do not take these literally
THE GAG TEST - Anything that makes you gag is spoiled (except for leftovers from what
you cooked for yourself last night). Ditto for things that make you violently ill.
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.
MAYONNAISE - If it makes you violently ill after you eat it, the mayonnaise is 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.
EXPIRATION DATES - This is NOT a marketing ploy to encourage you to throw away
perfectly good food so that you'll spend more on groceries. Perhaps you'd benefit by having
a calender in your kitchen.
MEAT - If opening the refrigerator door causes stray animals from a three- block radius to
congregate outside your house, the meat is spoiled. (or the smell alone can make you
violently ill and/or unconscious)
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.
CEREAL - It is generally a good rule of thumb that cereal should be discarded when it
is two years or longer beyond the expiration date.
LETTUCE - Bibb lettuce is spoiled when you can't get it off the bottom of the
vegetable crisper without Comet. Romaine lettuce is spoiled when it turns liquid.
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.
EMPTY CONTAINERS - Putting empty containers back into the refrigerator is an old
trick, but it only works if you live with someone or have a maid.
UNMARKED ITEMS - You know it is well beyond prime when you're tempted to
discard the Tupperware along with the food. Generally speaking, Tupperware
containers should not burp when you open them.
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 refrigerator to gauge this.