Food Spoilage and Preservation

1. Food Spoilage and Preservation Dr James Dooley School of Biomedical Sciences, University of Ulster, Coleraine

2. Food Spoilage and Preservation • An essential element of modern society • Not appreciated by most individuals • A changing environment requiring constant innovation • Will always be a problem for humans

3. Hunter-Gatherer society • supported low numbers/ self-sufficient • limited specialisation • unreliable

4. Industrial and Agricultural society • supports high numbers/ produce excess • supports specialisation • generally predictable

5. Vitamins Proteins Food Energy Building materials Lipids Carbohydrate Microbial Growth Human Growth

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

7. 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!)

8. Light Microscope x 1,000

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

10. spores

11. Endospore formation • 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

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

13. General features of 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

14. Fungi in Nature • Metabolic 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.

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

18. Micro-organisms and food • Agents of disease

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

21. 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!

22. Laboratory study of bacteria

23. Bacterial growth

24. Doubling is a Big Deal Some bacteria can double every 30 min

25. Bacterial growth • Binary fission • Divide in two • Growth rate varies • Escherichia coli • 20 minutes • Mycobacterium tuberculosis • 15 hours

26. What do bacteria need to grow? • Source of nutrients • amino acids, sugars, lipids, vitamins • released by action of enzymes operating outside the cell • Correct temperature • Bacteria grow within temperature ranges • mesophiles (10-45oC) • psycrophiles (0-20oC)

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

28. Nutrients Suitable Environment Time More Bacteria! Consequences of bacterial growth

29. Waste products • CO2 • Alcohol • Lactic acid etc. Altered Environment Altered Food Consequences of bacterial growth in food Nutrients: Starch, protein etc.

30. Microbial Food Spoilage • Microbial growth introduces alterations in food • appearance • smell • Taste • Nutritional content • Changes not necessarily harmful! • Each food unique microbial environment • unique spoilage agents

31. 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

32. 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

33. 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!!!!!)

34. Food spoilage has major economic impacts

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

36. Food Spoilage Shapes History

37. 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. • In 1810, Appert published the first known book on canning • 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!!!!

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

39. 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

40. 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

41. How to reduce water? • drying • sun • heat • freeze - dried (expensive!)

42. How to reduce water? • addition of salt or sugar • water needed to keep salt and sugar in solution

43. 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

44. 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

45. 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)

46. 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

47. 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

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

49. 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

50. Food Spoilage Tests- do not take these literally THE GAG TEST - Anything that makes you gag is spoiled (except for leftovers from what you cooked for yourself last night). Ditto for things that make you violently ill. 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. MAYONNAISE - If it makes you violently ill after you eat it, the mayonnaise is 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. EXPIRATION DATES - This is NOT a marketing ploy to encourage you to throw away perfectly good food so that you'll spend more on groceries. Perhaps you'd benefit by having a calender in your kitchen. MEAT - If opening the refrigerator door causes stray animals from a three- block radius to congregate outside your house, the meat is spoiled. (or the smell alone can make you violently ill and/or unconscious) 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.