Food Spoilage and Preservation

1. Food Spoilage and Preservation VarunPathak School of Biomedical Sciences, University of Ulster, Coleraine

2. Hunter-Gatherer society • supported low numbers/ self-sufficient • unreliable food supply • limited specialisation of individuals

3. Industrial and Agricultural society • supports high numbers/ produce excess • supports specialisation • generally predictable food supply

4. What are microbes? • “Organisms that are too small to be seen with the naked eye” • Bacteria • Viruses • Fungi • Protozoa

5. 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.

6. Where do we find bacteria? • Everywhere! • Soil • Plant roots • Water • Bodies of animals, fish, birds etc, • Hot springs • Dead Sea • Hydrothermal vents

7. 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

8. Micro-organisms and food • Agents of food production

9. Micro-organisms and food • Agents of food spoilage

10. Food spoilage Biological Changes • Fermentation:The conversion of carbohydrates to carbon dioxide and alcohol by yeast or bacteria

11. Food spoilage Chemical Changes • Enzymes play a significant role in catalyzing these reactions • Proteases, also called proteolytic enzymes, split proteins into smaller compounds.

12. Food spoilage Physical Changes The most common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation.

13. Vitamins Proteins Food Energy Building materials Lipids Carbohydrate Microbial Growth

14. 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!

15. Laboratory study of bacteria

16. Bacterial growth

17. 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

18. 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)

19. What do bacteria need to grow? • pH • 6-7.5 • Absence of toxic chemicals • Correct atmosphere (O2) • Aerobic • Bacillus • Anaerobic • Clostridium • facultative anaerobes • Salmonella

20. 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

21. Sugars, amino acids etc. Digestive enzymes Altered Environment Altered Food What happens when bacteria grow in food? Food Components: Starch, protein etc. • Waste products: • CO2 • Alcohol • Lactic acid etc.

22. 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 water WET Dry

23. 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!!!!!)

24. Microbial food spoilage • Foods are characteristically spoiled by known organisms

25. 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!!!!

26. Preservation of food by killing all microbes • Temperature • canning • sterilization by heat • 121oC for 15 minutes • all bacteria and endospores killed

27. 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

28. How to reduce water? • Perishable foods have a high Aw • drying • sun • heat • freeze - dried (expensive!)

29. Preservation of food by reducing water content • addition of salt or sugar (pickling) • Drying • Vacuuming • Sublimation

30. 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

31. 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

32. 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)

33. 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

34. 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

35. Preservation of food by preventing microbial growth • Modified Atmosphere Packaging • Oxygen • Nitrogen • Carbon Dioxide • Argon • Mix depends on food in question

37. A little extra material... • 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

38. A little extra material... • A good site to visit • http://resources.schoolscience.co.uk/SGM/index.html

39. Thank you

40. Food preservation challenges

41. 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.

42. 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.

43. 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.