Transcript Food: Battleground of the Species!

Varun Pathak
School of Biomedical Sciences,
University of Ulster,
Coleraine
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Hunter-Gatherer society
◦ supported low numbers/ self-sufficient
◦ unreliable food supply
◦ limited specialisation of individuals
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Industrial and Agricultural society
◦ supports high numbers/ produce excess
◦ supports specialisation
◦ generally predictable food supply
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“Organisms that are too small to be seen
with the naked eye”
Bacteria
Viruses
Fungi
Protozoa
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Unicellular organisms
Very small!!!!!!!
◦ 1-10 microns
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Enormous diversity
◦ Shape
◦ Habitat
◦ Nutrition
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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.
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Everywhere!
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Soil
Plant roots
Water
Bodies of animals, fish, birds etc,
◦ Hot springs
◦ Dead Sea
◦ Hydrothermal vents
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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
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Agents of food production
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Agents of food spoilage
Biological Changes
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Fermentation: The conversion of
carbohydrates to carbon dioxide and alcohol
by yeast or bacteria
Chemical Changes
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Enzymes play a significant role in catalyzing
these reactions
◦
Proteases, also called proteolytic
enzymes, split proteins into smaller
compounds.
Physical Changes
The most common physical changes occurring in
foods as they spoil are evaporation, drip loss, and
separation.
Vitamins
Proteins
Food
Energy
Carbohydrate
Building materials
Lipids
Microbial Growth
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We need to know about how spoilage
organisms live
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We need to understand their biology
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We need Microbiologists!
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
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Source of nutrients
◦ amino acids, sugars, lipids, vitamins
◦ released by action of enzymes operating outside
the cell
 starch digested by amylase
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Correct temperature
◦ Bacteria grow within temperature ranges
◦ mesophiles (10-45oC)
◦ psycrophiles (0-20oC)
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pH
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Absence of toxic chemicals
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Correct atmosphere (O2)
◦ 6-7.5
◦ Aerobic
 Bacillus
◦ Anaerobic
 Clostridium
◦ facultative anaerobes
 Salmonella
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some bacteria produce
endospores
◦ response to stress
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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
Food Components: Starch, protein etc.
Sugars, amino acids etc.
Digestive enzymes
Waste products:
Altered
Food
CO2
Alcohol
Lactic acid etc.
Altered
Environment
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highly perishable
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meat
fruit
milk
vegetables
eggs
WET
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semi perishable
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stable
◦ potatoes
◦ nuts
◦ rice
◦ flour
◦ dry beans
Dry
What defines each group?
Amount of water
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Bacterial growth on milk sugars
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pH reduction
◦ (Lactobacillus spp., Lactococcus spp.)
◦ lactic acid build up (bitter taste!)
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Change in bacterial population
◦ further pH reductions and much more
lactic acid, continues until all sugars
depleted
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Yeasts and moulds dominate
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pH rise
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Bacteria use proteins as major
nutrient
◦ use lactic acid for growth.
◦ allowing further bacterial growth
◦ (Primary amines produced- Smelly!!!!!)

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
◦ 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.
 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!!!!
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Temperature
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canning
sterilization by heat
121oC for 15 minutes
all bacteria and
endospores killed
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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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Perishable foods
have a high Aw
drying
◦ sun
◦ heat
◦ freeze - dried
(expensive!)
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addition of salt or
sugar (pickling)
Drying
Vacuuming
Sublimation
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pH
◦ very few bacteria grow below pH
5.0
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How to make food acidic?
◦ Add acid e.g. acetic acid
◦ Allow bacteria to make acid from
natural food components
 lactic acid bacteria
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Temperature
◦ storage at 4oC degrees
 rate of spoilage decreased
◦ storage at -20oC degrees
 rate of spoilage extremely
slow
◦ need -70oC to eliminate
spoilage
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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)
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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
◦ controversial claim
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Irradiation of
various foods
accepted in US and
many other
countries
UK only allows for
irradiation of herbs,
spices or vegetable
seasonings
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Modified Atmosphere
Packaging
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Oxygen
Nitrogen
Carbon Dioxide
Argon
Mix depends on food in
question
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BBC Radio 4 Science
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Food Safety Through the Ages
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Food Preservation site
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Food Standards Agency
◦ “On the shelf”
◦ http://www.bbc.co.uk/radio4/science/connect
_20021030.shtml
◦ Dr. Bill Grierson
◦ http://www.acsh.org/healthissues/newsID.76
7/healthissue_detail.asp
◦ Good links to related material
◦ http://www.bookrags.com/sciences/biology/f
ood-preservation-wmi.html
◦ www.food.gov.uk/
◦ Good site for general information
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A good site to visit
◦ http://resources.schoolscience.co.uk/SGM/index.html
Thank you
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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.
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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.
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FLOUR - Flour is spoiled when it wiggles.
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SALT - It never spoils.
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LETTUCE - lettuce is spoiled when you can't get it off the
bottom of the fridge without Mr Muscle.
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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.