1. Chapter 41 Microbiology of Food
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Chapter 41
Microbiology of Food

2. Microorganism growth in foods
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Microorganism growth in foods

3. Intrinsic factors Composition pH Presence and availability of water
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Intrinsic factors
Composition pH
Presence and availability of water
Oxidation-reduction potential
altered by cooking
Physical structure
Presence of antimicrobial substances

4. Composition and pH Putrefaction
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Composition and pH
Putrefaction
proteolysis and anaerobic breakdown of proteins, yielding foul-smelling amine compounds
pH impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food

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Water availability
In general, lower water activity inhibits microbial growth
Water activity lowered by:
drying
addition of salt or sugar
Osmophilic microorganisms
prefer high osmotic pressure
Xerophilic microorganisms
prefer low water activity

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Physical structure
Grinding and mixing increase surface area and distribute microbes
promotes microbial growth
Outer skin of vegetables and fruits slows microbial growth

8. Antimicrobial substances
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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

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

10. Microbial growth and food spoilage
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Microbial growth and food spoilage
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
algal toxins may contaminate shellfish and finfish
fungal toxins contaminate grains

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Left: fresh milk
Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey.

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13. Toxins Ergotism Aflatoxins Fumonisins
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Toxins
Ergotism
toxic condition caused by growth of a fungus, Claviceps purpura, in grains
(← production of hallucinogenic alkaloids)
Aflatoxins
carcinogens produced in fungus-infected grains and nut products (Aspergillus flavus)
Fumonisins
carcinogens produced in fungus-infected corn
(Fusarium moniliforme)

14. Intercalate into DNA, aflatoxins causing frameshift mutations
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Intercalate
into DNA,
causing
frameshift
mutations
aflatoxins
The letter designations refer to the color of the compounds under ultraviolet light after extraction from the grain and separation by chromatography. The B1 and B2 compounds fluoresce with a blue color and the G1 and G2 appear green. The two type of M aflatoxins are found in the milk of lactating animals that have ingested type B aflatoxins.

15. Fumonisins Disrupt synthesis and metabolism of sphingolipids
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Fumonisins
FB1: R = OH
FB2: R = H
Disrupt synthesis and metabolism of sphingolipids

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Algal toxins

17. Controlling food spoilage
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Controlling food spoilage

18. Removal of microorganisms
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Removal of microorganisms
Usually achieved by filtration
Commonly used for water, beer, wine, juices, soft drinks, and other liquids

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Low temperature
Refrigeration at 5°C retards but does not stop microbial growth
microorganisms can still cause spoilage with extended spoilage
growth at temperatures below -10°C has been observed

20. High temperature Canning Pasteurization
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High temperature
Canning
Pasteurization

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Canning
Food heated in special containers to 115° C for 25 to 100 minutes
Kills spoilage microbes, but not necessarily all microbes in food

22. Spoilage of canned goods
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Spoilage of canned goods
Spoilage prior to canning
Underprocessing
Leakage of contaminated water into cans during cooling process

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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

24. Chemical-based preservation
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Chemical-based preservation
GRAS
chemical agents “generally recognized as safe”
pH of food impacts effectiveness of chemical preservative

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26. Radiation Ultraviolet (UV) radiation Radappertization
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Radiation
Ultraviolet (UV) radiation
used for surfaces of food-handling equipment
does not penetrate foods
Radappertization
use of ionizing radiation (e.g. gamma ray) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables
kills microbes in moist foods by producing peroxides from water
peroxides oxidize cellular constituents

27. Microbial product-based inhibition
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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
* e.g., nisin
- produced by some strains of Streptococcus lactis
- used in low-acid foods to inactivate Clostridium botulinum during canning process

28. Food-borne diseases Two primary types food-borne infections
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Food-borne diseases
Two primary types
food-borne infections
food intoxications

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

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31. Nucleic acid can be detected even when plaque-forming ability is lost
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Nucleic acid can be detected
even when plaque-forming
ability is lost

32. Food-borne intoxications
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Food-borne intoxications
Ingestion of toxins in foods in which microbes have grown
Include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning

33. Detection of food-borne pathogens
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Detection of food-borne pathogens
Must be rapid and sensitive
Methods include:
culture techniques – may be too slow
immunological techniques - very sensitive
molecular techniques
probes used to detect specific DNA or RNA
sensitive and specific

34. Comparison of PCR sensitivity and growth for detection of Salmonella
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Comparison of PCR sensitivity and growth for detection of Salmonella

35. Surveillance for food-borne disease
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Surveillance for food-borne disease
PulseNet
established by Centers for Disease Control
uses pulsed-field gel electrophoresis under carefully controlled and duplicated conditions 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

36. Surveillance… FoodNet
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Surveillance…
FoodNet
active surveillance network used to follow nine major food-borne diseases
enables public health officials to rapidly trace the course and cause of infection in days rather than weeks

37. Microbiology of fermented foods
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Microbiology of fermented foods
Major fermentations used are lactic, propionic, and ethanolic fermentations

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39. Fermented milks Mesophilic – Lactobacillus and Lactococcus
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Fermented milks
Mesophilic – Lactobacillus and Lactococcus
Thermophilic – Lactobacillus and Streptococcus
Therapeutic – Lactobacillus and Bifidobacterium
Yeast lactic – yeasts, lactic acid bacteria, and acetic acid bacteria
Mold lactic – filamentous fungi and lactic acid bacteria

40. Cheese production Milk Curd Cheese lactic acid bacteria and rennin
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Cheese production
Milk
lactic acid bacteria and rennin
Curd
removal of whey 
ripening by microbial action 
Cheese

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42. Meat and fish Sausages Hams Bologna Salami
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Meat and fish
Sausages
Hams
Bologna
Salami
Izushi (fermentation of fish, rice, and vegetables by Lactobacillus spp.)
Katsuobushi (fermentation of tuna by Aspergillus glaucus)

43. Production of alcoholic beverages
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Production of alcoholic beverages
Begins with formation of liquid containing carbohydrates in readily fermentable form
must
juice from crushed grapes
mashing
hydrolysis of complex carbohydrates in cereals by addition of water and heating mixture
yields wort – clear liquid containing fermentable carbohydrates

44. racking – removes sediments
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racking – removes sediments or
Microbial oxidation of ethanol to acetic acid yields wine vinegar

45. Beers and ales Malt Mash Bottom yeasts Top yeasts
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Beers and ales
Malt
germinated barley grains having activated enzymes
Mash
the malt after being mixed with water in order to hydrolyze starch to usable carbohydrates
Bottom yeasts
used in production of beers
Top yeasts
used in production of ales

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pitched
Beer production requires malting, and the use of hops and boiling for clarification, flavor development, and inactivation of malting enzymes, to produce the wort. Only after completion of these steps can the actual fermentation be carried out.

47. Distilled spirits Similar to beer-making process
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Distilled spirits
Similar to beer-making process
usually begins with ‘sour mash’
mash inoculated with homolactic bacterium: lower the mash pH
following fermentation, is distilled to concentrate alcohol

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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 causes ropiness

49. Other fermented foods Silages
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Other fermented foods
Silages
fermented grass, corn, and other fresh animal feeds

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Making sauerkraut

51. Microorganisms as foods and food amendments
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Microorganisms as foods and food amendments
Variety of bacteria, yeasts, and other fungi are used as animal and human food sources
- mushrooms, Spirulina, etc.
Probiotics
microbial dietary adjuvants
microbes added to diet in order to provide health benefits beyond basic nutritive value

52. Benefits of probiotics
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Benefits of probiotics
Immunodilation
Control of diarrhea
Anticancer effects
Possible modulation of Crohn’s Disease
In beef cattle,
Lactobacillus acidophilus decrease E. coli
In poultry,
Bacillus subtilis limits Salmonella colonization of gut by the process of competitive exclusion

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Prebiotics
Oligosaccharide polymers that are not processed until they enter large intestine
Synbiotic system
combination of prebiotics and probiotics
results in an increase in production of butyric acid and propionic acids that may be responsible for possible beneficial effects of probiotics