Food Spoilage and Preservation
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Transcript of Food Spoilage and Preservation
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Food Spoilage and Preservation
Varun PathakSchool 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”
What are microbes?
BacteriaVirusesFungiProtozoa
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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 but we do not deal with them when
considering food spoilage and preservation.
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Everywhere!◦ Soil◦ Plant roots◦ Water◦ Bodies of animals, fish, birds etc,
◦ Hot springs◦ Dead Sea◦ Hydrothermal vents
Where do we find bacteria?
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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
Fungi
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Micro-organisms and food Agents of food production
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Agents of food spoilage
Micro-organisms and food
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Biological Changes
Fermentation: The conversion of carbohydrates to carbon dioxide and alcohol by yeast or bacteria
Food spoilage
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Chemical Changes
Enzymes play a significant role in catalyzing these reactions◦ Proteases, also called proteolytic enzymes, split proteins into smaller compounds.
Food spoilage
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Physical Changes
The most common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation.
Food spoilage
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Food
Vitamins Proteins
Carbohydrate Lipids
Microbial Growth
Energy
Building materials
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We need to know about how spoilage organisms live
We need to understand their biology
We need Microbiologists!
Food Spoilage and How to Prevent it
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Laboratory study of bacteria
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Bacterial growth
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Doublings Number of cells 1 1 2 2 3 4 4 8 5 16 6 32 7 64 8 128 9 256 10 512 11 1,024 12 2,048 13 4,096 14 8,192 15 16,384 16 32,768 17 65,536 18 1,310,752 19 2,621,504 20 5,243,008 21 10,486,016 22 20,972,032 23 41,944,064 24 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
Correct temperature◦ Bacteria grow within temperature ranges◦ mesophiles (10-45oC)◦ psycrophiles (0-20oC)
What do bacteria need to grow?
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pH◦ 6-7.5
Absence of toxic chemicals
Correct atmosphere (O2)◦ Aerobic
Bacillus◦ Anaerobic
Clostridium◦ facultative anaerobes
Salmonella
What do bacteria need to grow?
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Super Tough Bacteria!
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 happens when bacteria grow in food?
Food Components: Starch, protein etc.
Waste products:
CO2
Alcohol
Lactic acid etc.
Altered Environment
Altered Food
Digestive enzymesSugars, amino acids etc.
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highly perishable◦ meat◦ fruit◦ milk◦ vegetables◦ eggs
semi perishable◦ potatoes◦ nuts
stable◦ rice◦ flour◦ dry beans
Three groups of foods:based upon rate of spoilage
What defines each group?
Amount of waterWET
Dry
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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!!!!!)
Milk spoilage (unpasteurised)
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Microbial food spoilage
Food Organism Type of Spoilage
Chicken Pseudomonas spp. SliminessGreen colour
Milk (pasteurised) Lactobacillusthermophilus
Sour
Bread Rhizopus nigrans Bread mould
Foods are characteristically spoiled by known organisms
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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. 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!!!!
Nicholas Appert
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Temperature◦ canning◦ sterilization by heat◦ 121oC for 15 minutes◦ all bacteria and
endospores killed
Preservation of food by killing all microbes
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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
Preservation of food by killing all microbes
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Perishable foods have a high Aw
drying◦ sun◦ heat◦ freeze - dried
(expensive!)
How to reduce water?
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addition of salt or sugar (pickling)
Drying Vacuuming Sublimation
Preservation of food by reducing water content
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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
Preservation of food by preventing microbial growth
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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
Preservation of food by preventing microbial growth
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Temperature
◦ 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)
Preservation of food by preventing microbial
growth
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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
Food Preservation by control of bacterial growth
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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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Modified Atmosphere Packaging◦ Oxygen◦ Nitrogen◦ Carbon Dioxide◦ Argon
Mix depends on food in question
Preservation of food by preventing microbial growth
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BBC Radio 4 Science◦ “On the shelf”◦ http://www.bbc.co.uk/radio4/science/connect_20021030.shtml
Food Safety Through the Ages◦ Dr. Bill Grierson◦ http://www.acsh.org/healthissues/newsID.767/healthissue_detail.asp
Food Preservation site◦ Good links to related material◦ http://www.bookrags.com/sciences/biology/food-preservation-wmi.html
Food Standards Agency◦ www.food.gov.uk/◦ Good site for general information
A little extra material...
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A good site to visit
◦ http://resources.schoolscience.co.uk/SGM/index.html
A little extra material...
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Food preservation challenges
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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.
Tests for food spoilage
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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.
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
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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.
Tests for food spoilage