Professor James Dooley School of Biomedical Sciences, University of Ulster, Coleraine

Food Spoilage and Preservation

Professor James DooleySchool of Biomedical Sciences,

University of Ulster,Coleraine

We will talk about………

Space

Exploration

We will talk about………

WAR

Main points of today’s lecture

◦ Essential element of modern society Not appreciated by most individuals

◦ A changing environment requiring constant innovation Will always be a problem for humans

Food Spoilage and Preservation

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/Repair

Energy

Building materials

Microbial Growth

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

What are microbes?

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

Everywhere!◦ Soil◦ Plant roots◦ Water◦ Bodies of animals, fish,

birds etc,

◦ E. coli◦ C. difficile (C diff)◦ Superbugs

MRSA

Where do we find bacteria?

Laboratory study of bacteria

Bacterial growth

Doublings Number of cells 1 1 2 2 4 3 8 4 16 5 32 6 64 7 128 8 256 90 512 10 1,024 11 2,048 12 4,096 13 8,192 14 16,384 15 32,768 16 65,536 17 1,310,752 18 2,621,504 19 5,243,008 20 10,486,016 21 20,972,032 22 41,944,064 23 83,888,128 24 167,776,256

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

Source of nutrients◦ amino acids, sugars, lipids, vitamins

released by action of enzymes operating outside the cell

E.g starch digested by amylase releases glucose Correct temperature

◦ Bacteria grow within temperature ranges◦ mesophiles (10-45oC)◦ psycrophiles (0-20oC)

What do bacteria need to grow?

Suitable pH◦ 6-7.5

Absence of toxic chemicals

Correct atmosphere (N/CO2/O2)◦ Aerobic

Bacillus◦ Anaerobic

Clostridium◦ facultative anaerobes

Salmonella

What do bacteria need to grow?

Super Tough Bacteria!

some bacteria produce endospores◦ response to stress

very resistant to heat◦ 121oC

very resistant to harsh conditions◦ chemicals, drying, radiation

can remain dormant for a long time (years)

endospore - forming bacteria◦ universal in soil

◦ Bacillus anthracis: anthrax

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

What are microbes?

BacteriaVirusesFungiProtozoa

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

General features of Fungi

Fungi are the main organisms involved in the decay of organic material

◦ recycling of essential elements

◦ C, N, H, O, S etc.

Fungi in Nature

Agents of food production

Micro-organisms and food

Agents of disease

Micro-organisms and food

Agents of food spoilage

Micro-organisms and food

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

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 growth introduces unwanted alterations in food◦ appearance◦ smell◦ Taste◦ Nutritional content

Changes not necessarily harmful!◦ Hákarl

Each food unique microbial environment◦ unique spoilage agents

Microbial Food Spoilage = unwanted microbial growth

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

Food Spoilage: when we understand it we can control it

Bacterial growth on milk sugars◦ Lactose ◦ Lactobacillus spp., Lactococcus spp.

Bacterial waste products◦ lactic acid build up (bitter taste!)◦ pH reduction

Only lactic acid bacteria can grow and all others inhibited◦ continues until all sugars used up

Yeasts and moulds tolerate acidity◦ use lactic acid for growth.◦ pH rise◦ allowing different bacterial species to

grow New bacteria use proteins as

major nutrient◦ (Primary amines produced-

Smelly!!!!!)

Milk spoilage (unpasteurised)

Microbial food spoilage

Food Organism

Type of Spoilage

Chicken

Pseudomonas spp.

Sliminess Green colour

Milk (pasteurised)

Lactobacillus thermophilus

Sour

Bread

Rhizopus nigrans

Bread mould

Foods are characteristically spoiled by known organisms

Food spoilage has major economic impacts

Now we will talk about………

WAR

Food Spoilage Shapes History,

Honestly it does…

“An army travels on its stomach”

◦ 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 journeys with the army!!!!

Nicholas Appert

Microbes killed

Microbial GROWTH causes food spoilage

◦ Kill microbes: no growth = no spoilage

◦ Stop microbes growing: no growth = no spoilage

Microbes and Food Spoilage

Temperature◦ canning◦ sterilization by heat◦ 121oC for 15 minutes◦ all bacteria and

endospores killed

Preservation of food by killing ALL microbes

Killing of all microbes from a food will prevent spoilage!

Killing of all microbes from a food will drastically alter the food◦ taste◦ texture◦ nutritional content

Preservation of food by killing ALL microbes

Temperature◦ Less extreme than canning

◦ Pasteurization

◦ mild heat treatment◦ overall microbial population is reduced◦ disease causing microbes 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 killing SOME microbes

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

Preservation of food by killing SOME microbes

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

Irradiation of food fuels space exploration

NASA

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

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

Preservation of food by preventing microbial growth 1

drying◦ sun◦ heat◦ freeze - dried

(expensive!)

How to reduce water in foods?

addition of salt or sugar◦ water needed to

keep salt and sugar in solution

How to reduce water in foods?

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 2

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 3

Modified Atmosphere Packaging◦ Oxygen◦ Nitrogen◦ Carbon Dioxide◦ Argon

Mix depends on food in question◦ Vacuum packing also used

Preservation of food by preventing microbial growth 4

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.76

7/healthissue_detail.asp

Food Preservation site◦ Good links to related material◦ http://www.bookrags.com/sciences/biology/fo

od-preservation-wmi.html

Food 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

A little extra material...

Food preservation challenges

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.

Tests for food spoilage