Food Spoilage and Preservation

Varun PathakSchool of Biomedical Sciences,

University of Ulster,Coleraine

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

“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.

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

◦ Hot springs◦ Dead Sea◦ Hydrothermal vents

Where do we find bacteria?

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

Micro-organisms and food Agents of food production

Agents of food spoilage

Micro-organisms and food

Biological Changes

Fermentation: The conversion of carbohydrates to carbon dioxide and alcohol by yeast or bacteria

Food spoilage

Chemical Changes

Enzymes play a significant role in catalyzing these reactions◦ Proteases, also called proteolytic enzymes, split proteins into smaller compounds.

Food spoilage

Physical Changes

The most common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation.

Food spoilage

Food

Vitamins Proteins

Carbohydrate Lipids

Microbial Growth

Energy

Building materials

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

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

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

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 chemicals, drying, radiation

can remain dormant for a long time (years)

endospore - forming bacteria are common in soil

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.

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

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)

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

◦ 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

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 killing all microbes

Perishable foods have a high Aw

drying◦ sun◦ heat◦ freeze - dried

(expensive!)

How to reduce water?

addition of salt or sugar (pickling)

Drying Vacuuming Sublimation

Preservation of food by reducing water content

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)

Preservation of food by preventing microbial

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

Food Preservation by control of bacterial growth

                                           

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

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

Mix depends on food in question

Preservation of food by preventing microbial growth

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...

A good site to visit

◦ http://resources.schoolscience.co.uk/SGM/index.html

A little extra material...

Thank you

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