Food Spoilage and Preservation

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Food Spoilage and Preservation

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

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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

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

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

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Human Growth
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Microbial Growth
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What are microbes?

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

Bacteria Viruses Fungi Protozoa
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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!)

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

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

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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

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What are microbes?

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

Bacteria Viruses Fungi Protozoa
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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

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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.

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Fungi in Nature
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Micro-organisms and food

• Agents of food production

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Micro-organisms and food

• Agents of disease

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Micro-organisms and food

• Agents of food spoilage

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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!

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

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

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What do bacteria need to grow?

• Source of nutrients
• amino acids, sugars, lipids, vitamins
• released by action of enzymes operating outside
  the cell
• Correct temperature
• Bacteria grow within temperature ranges
• mesophiles (10-45oC)
• psycrophiles (0-20oC)

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What do bacteria need to grow?

• pH
• 6-7.5
• Absence of toxic chemicals
• Correct atmosphere (O2)
• Aerobic
• Bacillus
• Anaerobic
• Clostridium
• facultative anaerobes
• Salmonella

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Consequences of bacterial growth
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Consequences of bacterial growth in food
Nutrients Starch, protein etc.
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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

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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
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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

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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!!!!!)

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• Food spoilage has major economic impacts

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Microbial food spoilage

• Foods are characteristically spoiled by known
  organisms

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Food Spoilage Shapes History
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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!!!!

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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
• 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

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How to reduce water?

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

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How to reduce water?

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

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Preservation of food by preventing microbial
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

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Preservation of food by preventing microbial
growth

• 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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Preservation of food by preventing microbial
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. (batch pasteurization)
• 72C for 15 sec. (flash pasteurization)

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Food Preservation by control of bacterial growth

• Radiation
• use of gamma rays from Co60
• microbes killed by free radicals
• Food can be packaged!
• No recontamination possible
• Pasteurization of meat, poultry, cheese
• No alteration of food
• controversial claim

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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

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Preservation of food by preventing microbial
growth

• Modified Atmosphere Packaging
• Oxygen
• Nitrogen
• Carbon Dioxide
• Argon
• Mix depends on food in question

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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
• Food Standards Agency
• www.food.gov.uk/
• Good site for general information

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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.
• 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 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.

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Food Spoilage Tests- do not take these literally
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 Flash. Romaine lettuce is spoiled when it
turns liquid. CANNED GOODS - Any canned goods
that have become the size or shape of a baseball
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.