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Food Spoilage and Preservation
Professor James Dooley
School of Biomedical Sciences,
University of Ulster,
Coleraine
Food Spoilage and Preservation
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 supported low numbers/ self-sufficient unreliable food supply limited specialisation of individuals
Industrial and Agricultural society supports high numbers/ produce excess supports specialisation generally predictable food supply
Food
Vitamins Proteins
Carbohydrate Lipids
Human Growth
Energy
Building materials
Microbial Growth
What are microbes?
“Organisms that are too small to be seen with the naked eye”
BacteriaVirusesFungiProtozoa
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.
Light Microscope
x 1,000
Where do we find bacteria?
Everywhere! Soil Plant roots Water Bodies of animals, fish, birds etc,
Hot springs Dead Sea Hydrothermal vents
What are microbes?
“Organisms that are too small to be seen with the naked eye”
BacteriaVirusesFungiProtozoa
General features of Fungiunicellular (yeasts) and multicellular (moulds)Non-photosynthetic,plant-like organismsMulticellular, filamentous organismsNormally inhabitants of the soil, rhizosphere and waterCan tolerate acidic and dry conditions
Fungi in NatureMetabolic 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.
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
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
What do bacteria need to grow?
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?
pH 6-7.5
Absence of toxic chemicals
Correct atmosphere (O2) Aerobic
Bacillus Anaerobic
Clostridium facultative anaerobes
Salmonella
Super Tough Bacteria!
some bacteria produce endospores response to stress
very resistant to heat 121oC
very resistant to harsh chemicals, drying, radiationcan remain dormant for a long time (years)endospore - forming bacteria are common in soil
What happens when bacteria grow?
More Bacteria!
NutrientsSuitable Environment
Time
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.
Microbial Food Spoilage
Microbial growth introduces unwanted alterations in food appearance smell Taste Nutritional content
Changes not necessarily harmful!
Each food unique microbial environment unique spoilage agents
Three groups of foods:based 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 waterWET
Dry
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 (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!!!!!)
Food spoilage has major economic impacts
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
Food Spoilage Shapes History
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.
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
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
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
How to reduce water?
drying sun heat freeze - dried
(expensive!)
How to reduce water?
addition of salt or sugar water needed to
keep salt and sugar in solution
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
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
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.
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
Food can be packaged!No recontamination possiblePasteurization of meat, poultry, cheeseNo alteration of food 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 Oxygen Nitrogen Carbon Dioxide Argon
Mix depends on food in question
A little extra material...BBC Radio 4 Science “On the shelf” http://www.bbc.co.uk/radio4/science/
connect_20021030.shtmlFood Safety Through the Ages Dr. Bill Grierson http://www.acsh.org/healthissues/newsID.767/healthiss
ue_detail.aspFood Preservation site Good links to related material http://www.bookrags.com/sciences/biology/food-preser
vation-wmi.htmlFood Standards Agency www.food.gov.uk/ Good site for general information
A little extra material...
A good site to visit
http://resources.schoolscience.co.uk/SGM/index.html
Food preservation challenges
Tests for food spoilage
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
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
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.
