1. 41
Microbiology of Food
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2. 41.1 Microbial Growth and Food Spoilage
Differentiate between intrinsic and extrinsic factors that influence food spoilage
Explain how the composition of a food item helps determine the kind of microbial growth that might occur
List two physical and two biological features of a food that influence spoilage

3. Microbial Growth and Food Spoilage
Results from growth of microbes in food
alters food visibly and in other ways, rendering it unsuitable for consumption
Involves predictable succession of microbes
Different foods undergo different types of spoilage processes
Toxins are sometimes produced

4. Microbial Growth and Food Spoilage
Microbial growth is controlled by
intrinsic factors
factors related to the food itself
extrinsic factors
environment where food stored

6. Intrinsic Factors Food composition - carbohydrates mold predominates
degrades food by hydrolysis
little odor
ergotism
hallucinogenic alkaloids released by Claviceps purpurea
may cause death

7. Intrinsic Factors Food composition – proteins or fats Putrefaction
bacterial growth predominates
Putrefaction
proteolysis and anaerobic breakdown of proteins; foul smelling amine compounds
Unpasteurized milk spoilage
acid production followed by putrefication
Butter
short-chained fatty acid production; rancid butter

8. Intrinsic Factors pH Presence and availability of water
impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food
e.g., low pH favors yeast and mold
Presence and availability of water
in general, lower water activity inhibits microbial growth

9. Intrinsic Factors Oxidation-reduction potential Physical structure
altered by cooking
lower redox – more bacteria and anaerobes
Physical structure
grinding and mixing distribute microbes; promotes microbial growth
outer skin of vegetables and fruits slows microbial growth

10. Antimicrobial Substances
Coumarins – fruits and vegetables
Lysozyme – cow’s milk and eggs
Aldehydic and phenolic compounds – herbs and spices
Allicin – garlic
Polyphenols – green and black teas

11. Extrinsic Factors Temperature Relative humidity Atmosphere
lower temperatures retard microbial growth
Relative humidity
higher levels promote microbial growth
Atmosphere
oxygen promotes growth
modified atmosphere packaging (MAP)
use of shrink wrap and vacuum technologies to package food in controlled atmospheres

13. 41.2 Controlling Food Spoilage
Review the limitations of refrigeration
Explain why canning is not always a fail-safe means of preserving food
Compare and contrast two pasteurization methods
Describe the principle of water availability and how it can be used to preserve foods
Explain how commonly used chemicals preserve food

14. 41.2 Controlling Food Spoilage
Express an informed opinion on the safety and efficacy of food irradiation
Describe two microbe-based food preservatives
Examine the packaging of food you frequently consume and assess its capacity to prevent spoilage

15. Controlling Food Spoilage
Methods of preservation
goal is to eliminate or reduce the populations of spoilage and disease causing microbes while maintaining food quality

16. Filtration
Water, wine, beer, juices, soft drinks, and other liquids usually by filtration
May better preserve flavor and aroma

18. Low Temperature
Refrigeration at 5°C retards but does not stop microbial growth
microorganisms can still cause spoilage with extended storage
growth at temperatures below 10°C has been observed
fruit juice concentrates
ice cream
some fruits

19. High Temperature
Food heated in special containers (retorts) to 115°C for 25 to 100 minutes
Kills spoilage microbes, but not necessarily all microbes in food
Spoilage of canned foods
spoilage prior to canning
underprocessing
leakage of contaminated
water into cans during
cooling process

20. Pasteurization
Kills pathogens and substantially reduces number of spoilage organisms
Different pasteurization procedures heat for different lengths of time
shorter heating times result in improved flavor

21. Water Availability Dehydration
e.g., lyophilization to produce freeze-dried foods is commonly used to eliminate bacterial growth
food preservation occurs as a result of free- water loss and an increase in solute concentration

22. Chemical-Based Preservation
GRAS
chemical agents “generally recognized as safe”
agents include organic acids, sulfite, ethylene oxide gas, ethyl formate
sodium nitrite – inhibits spore formation in meats, forms nitrosamines
pH of food impacts effectiveness of chemical preservative

24. High Hydrostatic Pressure (HHP)
Alternate way to preserve food
Applies pressures from milliPascals (MPs) without significant changes in temperature
highly detrimental to cell membranes
effective at eliminating eukaryotic microbes
not as effective at elimination of Gram-positive microbes
No industry standards for HHP conditions (yet)

25. Radiation Radappertization
use of ionizing radiation (gamma radiation) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables
excellent penetrating power – food not rendered radioactive
kills microbes in moist foods by producing peroxides from water
peroxides oxidize cellular constituents

26. Microbial Product-Based Inhibition
Bacteriocins
bactericidal proteins active against related species
some dissipate proton motive force of susceptible bacteria
some form pores in plasma membranes
some inhibit protein or RNA synthesis

27. Microbial Product-Based Inhibition
e.g., nisin from Lactococcus lactis
used in low-acid foods to inactivate Clostridium botulinum during canning process
e.g., bacteriophages that kill Listeria monocytogenes
sprayed onto ready-to-eat meats prior to packaging

28. Packaging Modified atmosphere packaging (MAP)
gases in stored food affect microbial growth
shrink wrap materials and vacuum technology control atmosphere
impermeable to gasses
high CO2 content packaging can be used to prevent fungal growth
high O2 content packaging produces superoxide radicals that inhibit microbial growth

29. 41.3 Food-Borne Disease Outbreaks
Summarize a major food-borne infection that had substantial health and economic consequences
Describe the origin and consequences of a major food intoxication event in the United States
Explain why these cases of food-borne disease outbreaks serve as cautionary tales for government regulatory agencies, food producers, and consumers
Contrast and compare the aflatoxins with the fumonisins

30. Types of Food-Borne Disease
About 48 million cases/yr in U.S.
approximately 128,000 hospitalizations
at least 3,000 deaths/yr in U.S.
only 14 million attributed to known pathogens
Pathogens
Noroviruses, Campylobacter jejuni, Salmonella are major causes
E. coli and Listeria are also important pathogens

31. Types of Food-Borne Disease
Two primary types
food-borne infections
food intoxications
Transmission
breakdown in hygiene
fecal-oral route key
fomites also important

32. Food-Borne Infection
Ingestion of pathogen, followed by growth, tissue invasion, and/or release of toxins
Raw foods (e.g., sprouts, raspberries, and seafood) are important sources

34. Food-Borne Infections
Listeriosis
pregnant women, the young and old, and immunocompromised individuals most vulnerable
responsible for the largest meat recall in U.S.
at-risk people should not eat soft cheeses, refrigerated smoked meats, deli meats, and undercooked hot dogs

35. Food-Borne Intoxications
Ingestion of toxins in foods in which microbes have grown
Produce symptoms shortly after the food is consumed because growth of the disease- causing microorganism is not required
Include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning

36. Other Food Disease Enterohemorrhagic E. coli (O157:H7)
normal inhabitant of intestines of mammals including cattle, swine
contamination of meat during slaughter
in salad vegetables contaminated in field
shiga toxin from horizontal gene transfer from Shigella
may result in life-threatening hemolytic uremic syndrome, most common in children and elderly

37. Other Food Disease Fungus-derived toxins Algal toxins aflatoxins
carcinogens produced in fungus- infected grains and nut products
fumonisins
carcinogens produced in fungus- infected corn
Algal toxins
contaminate fish and shellfish

38. 41.4 Detection of Food-Borne Pathogens
List the principles that guide the development of food- testing strategies
Describe how a food-borne pathogen consumed by an individual might be traced back to the food source

39. Key U.S. Food Safety Legislation
Driven in part by Upton Sinclair’s 1905 novel The Jungle, the Federal Meat Inspection Act was passed
Other food safety legislation followed

40. Detection of Food-Borne Pathogens
Must be rapid, sensitive and simple and for prevention
Methods include:
culture techniques – may be too slow but are most likely to detect food pathogens
immunological techniques - very sensitive
molecular techniques
probes used to detect specific DNA or RNA
sensitive and specific

41. Detection of Food-Borne Pathogens
PulseNet
established by Centers for Disease Control
uses pulsed-field gel electrophoresis to determine distinctive DNA pattern of each bacterial pathogen
enables public health officials to link pathogens associated with disease outbreaks in different parts of the world to a specific food source

42. Detection of Food-Borne Pathogens
In addition to PFGE, PCR and RFLP are increasingly developed and used
amplify small quantities
species specific
microarray techniques directly from food or water

43. 41.5 Microbiology of Fermented Foods
Compare and contrast mesophilic and thermophilic fermentations
Differentiate between a yeast-lactic and a mold-lactic fermentation
Outline the process by which cheese is made
List some fermented meats
Provide an overview of the production of wine, champagne, beer, and distilled spirits
Describe a fermented food item not commonly eaten in the United States

44. Microbiology of Fermented Foods
Chemical changes in food brought about microbial action
Major fermentations used are lactic, propionic, and alcoholic fermentations

45. Fermented Milks
Majority of fermented milk products rely on lactic acid bacteria (LAB) in the genera Lactobacillus, Lactococcus, Leuconostoc, and Streptococcus
Gram-positives that tolerate acidic conditions, non- spore forming, aerotolerant, strictly fermentative

46. Fermented Milks Mesophilic (app. 20-30° C): Buttermilk/sour cream
Lactobacillus spp. and Lactococcus lactis
Thermophilic (app. 45° C): Yogurt
Lactobacillus spp. and Streptococcus thermophilus
Yeast lactic: Kefir (ferm. milk with 2% ethanol)
yeasts, lactic acid bacteria, and acetic acid bacteria
Mold lactic: Viili (Finnish fermented milk)
filamentous fungi and lactic acid bacteria

47. Cheese Production
Approximately 2,000 distinct varieties representing 20 general types
Classified based on
texture, hardness (soft, semi-soft, hard, very hard)
All from lactic acid fermentation
molds may further enhance flavor

48. Cheese Production milk curd cheese lactic acid bacteria and rennin
removal of whey 
ripening by microbial action 
cheese

50. Meat and Fish Sausages Hams Bologna Salami
Izushi – fish, rice, and vegetables
Katsuobushi – tuna

51. Wines and Champagnes Enology (wine production) crushed grapes
separation and storage of liquid (must) before fermentation
fresh must
treated with sulfur dioxide fumigant
Saccharomyces cerevisiae or S. liposideus added for consistent results
fermented for 3–5 days at 20–28oC

52. Wines and Champagnes Dry or sweet wines Racking
controlled by regulating initial must sugar content
Racking
removes sediment produced during fermentation

53. Beers and Ales Cereal grains used for fermentation malt mash
germinated barley grains having activated enzymes
mash
the malt after being mixed with water in order to hydrolyze starch to usable carbohydrates
mash heated with hops
hops provide flavor and assist in clarification of wort
heating inactivates hydrolytic enzymes

54. Beers and Ales Wort inoculated (pitched) with desired yeast
bottom yeasts
used in production of beers
top yeasts
used in production of ales
Freshly fermented beers are aged or lagered
CO2 usually added at bottling
Beer can be pasteurized or sterilized by filtration

55. Distilled Spirits Similar to beer-making process begins with sour mash
mash inoculated with homolactic bacterium
following fermentation, is distilled to concentrate alcohol

56. Production of Breads
Involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions
maximizes CO2 production, which leavens bread
Other microbes used to make special breads (e.g., sourdough bread)
Can be spoiled by Bacillus species that produce ropiness

57. Other Fermented Foods Sufu – from fermentation of tofu
Sauerkraut (sour cabbage) – from wilted, shredded cabbage
Pickles – from cucumbers
Silage – from grass, chopped corn, and other fresh animal feeds

59. 41.6 Probiotics
Describe how a food is classified as a probiotic and identify two microorganisms used in probiotic foods

60. Probiotics and Standardization
live microorganisms, which when administered in adequate amounts, confer a health benefit to the host
specific requirements should be met
Microorganisms
Lactobacillus, Bifidobacterium

61. Possible Benefits of Probiotics
Immunomodulation
Control of diarrhea
Possible modulation of Crohn’s Disease
Lactobacillus acidophilus and Bifidobacterium
help minimize lactose intolerance
improve general intestinal health and balance
may lower serum cholesterol
may have anti-tumor activity

62. Probiotic Microbes in Farm Animals
Probiotics
Lactobacillus acidophilus in beef cattle
decrease E. coli O157:H7
Bacillus strain in poultry
limit colonization of gut by the process of competitive exclusion
reduces Salmonella and Campylobacter