Food Spoilage and Preservation

1
Food Spoilage and Preservation

• Dr James Dooley
• School of Biomedical Sciences,
• University of Ulster,
• Coleraine

2
Food Spoilage and Preservation

• A cornerstone of modern society
• Not appreciated by most individuals
• A changing environment requiring constant
  innovation
• Will always be a problem for humans

3

• Hunter-Gatherer society
• supported low numbers/ self-sufficient
• limited specialisation
• unreliable

4

• Industrial and Agricultural society
• supports high numbers/ produce excess
• supports specialisation
• generally predictable

5
Microbial Growth
Human Growth
6
What are microbes?

• Organisms that are too small to be seen with the
  naked eye

Bacteria Viruses Fungi Protozoa
7
What are bacteria?

• Unicellular organisms
• Very small!!!!!!!
• 1-10 microns
• Enormous diversity
• Shape
• Habitat
• Nutrition
• Many bacteria require similar growth and
  nutrition conditions to humans (very many do not!)

8
Light Microscope x 1,000
9
Where do we find bacteria?

• Everywhere!
• Soil
• Plant roots
• Water
• Bodies of animals, fish, birds etc,
• Hot springs
• Dead Sea
• Hydrothermal vents

10
(No Transcript)
11
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

12
Life cycle of an endospore -forming bacterium
vegetative cell
sporulating cell
spore
spore germination
outgrowth
vegetative cell
13
What are microbes?

• Organisms that are too small to be seen with the
  naked eye

Bacteria Viruses Fungi Protozoa
14
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

15
Fungi in Nature

• Metabolic by-products form the raw material for
  many industries
• 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.

16
(No Transcript)
17
(No Transcript)
18
Micro-organisms and food

• Agents of food production

19
Micro-organisms and food

• Agents of disease

20
Micro-organisms and food

• Agents of food spoilage

21
(No Transcript)
22
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!

23
Laboratory study of bacteria
24
Bacterial growth
25
Doubling is a Big Deal Some bacteria can
double every 30 min
26
Bacterial growth

• Binary fission
• Divide in two
• Growth rate varies
• Escherichia coli
• 20 minutes
• Mycobacterium tuberculosis
• 15 hours

27
What do bacteria need to grow?

• Correct atmosphere (O2)
• aerobic
• anaerobic
• facultative anaerobes
• Correct temperature
• mesophiles (10-45oC)
• psycrophiles (0-20oC)

28
What do bacteria need to grow?

• pH
• 6-7.5
• Absence of toxic chemicals
• Source of nutrients
• amino acids, sugars, lipids, vitamins
• released by action of enzymes operating outside
  the cell

29
Consequences of bacterial growth
Nutrients
Suitable Environment
Time
30
Consequences of bacterial growth in food
Nutrients Starch, protein etc.
31
Microbial Food Spoilage

• Microbial growth introduces alterations in food
• appearance
• smell
• Taste
• Nutritional content
• Changes not necessarily harmful!
• Each food unique microbial environment
• unique spoilage agents for each

32
Three groups of foodsbased upon rate of spoilage

• highly perishable
• meat
• fruit
• milk
• vegetables
• eggs
• semi perishable
• potatoes
• nuts
• stable
• rice
• flour
• dry beans

• What defines each group?
• Amount of water

WET
Dry
33
Food Spoilage

• Each food has its own unique microbial
  population
• Uncontrolled growth of the microbes results in
  food spoilage
• We can predict (and therefore control) food
  spoilage

34
Milk spoilage (unpasteurised)

• Bacterial growth on milk sugars
• (Lactobacillus spp., Lactococcus spp.)
• pH reduction
• lactic acid build up (bitter taste!)
• Change in bacterial population
• further pH reductions and much more lactic acid,
  continues until all sugars depleted
• Yeasts and moulds dominate
• use lactic acid for growth.
• pH rise
• allowing further bacterial growth
• Bacteria use proteins as major nutrient
• (Primary amines produced- Smelly!!!!!)

35

• Food spoilage has major economic impacts

36
Microbial food spoilage

• Foods are characteristically spoiled by known
  organisms

37
Food Spoilage Shapes History
38
Nicholas Appert

• 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
• 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!!!!

39
Food Preservation by control of bacterial growth

• Temperature
• canning
• sterilization by heat
• 121oC for 15 minutes
• all bacteria and endospores killed

40
Preservation of Food

• 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

41
Preservation of Food

• 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

42
How to reduce water

• drying
• sun
• heat
• freeze - dried (expensive!)

43
How to reduce water

• addition of salt or sugar
• water needed to keep salt and sugar in solution

44
Food Preservation by control of bacterial growth

• pH
• 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
• lactic acid bacteria

45
Food Preservation by control of bacterial growth

• Temperature
• storage at 4oC degrees
• rate of spoilage decreased
• storage at -20oC degrees
• rate of spoilage extremely slow
• need -70oC to eliminate spoilage

46
Food Preservation by control of bacterial growth

• Temperature
• Pasteurization
• mild heat treatment
• overall microbial population is reduced
• pathogens are eliminated since these tend to be
  more heat sensitive than other organisms.
• 63C for 30 min.
• 72C for 15 sec. (flash pasteurization)

47
Food Preservation by control of bacterial growth

• Radiation
• use of gamma rays from Co
• microbes killed by free radicals
• Food can be packaged!
• No recontamination possible
• Pasteurization of meat, poultry, cheese
• No alteration of food
• controversial claim

48
Food Preservation by use of Modified Atmosphere
Packaging

• Oxygen
• Nitrogen
• Carbon Dioxide
• Mix depends on food in question

49
A little extra material...

• BBC Radio 4 Science
• On the shelf
• http//www.bbc.co.uk/radio4/science/connect_200210
  30.shtml
• Food Safety Through the Ages
• Dr. Bill Grierson
• http//www.acsh.org/healthissues/newsID.767/health
  issue_detail.asp
• Food Preservation site
• Good links to related material
• http//www.bookrags.com/sciences/biology/food-pres
  ervation-wmi.html

50
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.