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Food Microbiology > Microbial Spoilage + Public Health Concerns (2) > Flashcards

Microbial Spoilage + Public Health Concerns (2) Flashcards

1
Q

milk is (prone/resistant) to spoilage. why?

A

prone to spoilage

high water content, neutral pH, high in nutrients (sugars, protein, lipids) -> good for bacterial growth

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2
Q

What process is required for all milk in Canada, and what does it achieve?

A

pasteurization (heat treatment)

eliminate most pathogens (to safe levels) & increases shelf life

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3
Q

Why doesn’t milk support growth of some microorganisms?

A

low in available amino acids

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4
Q

True/false: milk at 7C will go rancid faster than milk at 4C

A

False: psychrotrophic bacteria will grow faster and make more lipases -> faster lipolysis

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5
Q

True/false: milk at 7C will undergo proteolysis faster than milk at 4C

A

True

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6
Q

Milk spoilage organisms often rely on ___ and ____ to obtain energy

A

proteolysis; lipolysis

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7
Q

Psychrotrophic milk spoilage organisms will produce ____, ____, and ____ enzymes. What are the products and effects (sensory)?

A

lipolytic -> FFA -> rancid
proteolytic -> peptides, AA -> bitter
esterolytic (esterases) -> ethyl esters -> fruity

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8
Q

What is the effect of Bacillus contamination in milk?

A

produce proteases:

  1. bitter peptides
  2. casein destabilization -> coagulate
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9
Q

What enzyme types do LAB produce in milk, and what is the effect?

A

glycolytic enzymes -> lactic/acetic acid -> sour
oxidase -> 3-methyl-butanal -> malty
polymerase -> exopolysaccharides -> ropey texture

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10
Q

What sugars are present in milk, and how does this affect the bacteria species that grow?

A

some glucose -> can allow for some growth of glucose-utilizing microbes (limited)

main sugar is LACTOSE -> only certain microbes can utilize (milk will be selective for these)

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11
Q

Is milk fat used as a nutrient source for bacterial growth?

A

Yes; but globules must be ruptured for bacteria to have access to fat (protected by membrane)

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12
Q

What provides the N for bacterial growth in milk?

A

Mainly casein proteins; also urea, peptides, AA (NPNs)

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13
Q

The NPNs in milk:

A

urea, peptides, free AA (non protein nitrogen)

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14
Q

What are the major two inhibitory substances for spoilage in milk?

A

lactoferrin

lactoperoxidase

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15
Q

What are some minor inhibitory substances for milk spoilage? (3)

A

lysozyme, immunoglobulins, folate/B12 binding systems

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16
Q

How does lactoferrin inhibit bacterial growth?

A

binds available iron (limiting nutrient for growth)

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17
Q

How does the inhibitory activity of lactoferrin differ in cow vs human milk? Why?

A

Less in cow’s milk:

cow milk higher in CITRATE -> citrate will bind lactoferrin, lowering its iron-sequestering ability

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18
Q

True/false; lactoperoxidase is inactivated by pasteurization

A

False

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19
Q

______ is the most important microbial inhibitory substance in cow milk. How does it function?

A

lactoperoxidase

catalyze oxidation of thiocyanate (with hydrogen peroxide) -> HYPOTHIOCYANITE (highly reactive oxidant; antimicrobial)

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20
Q

Why is H2O2 added to cow milk in developing countries?

A

H peroxide is limiting substrate for lactoperoxidase system; adding will make it more effective as an antimicrobial (in lieu of refrigeration)

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21
Q

Preservation of fresh milk relies on _____ and ______.

A

sanitation/pasteurization

refrigeration

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22
Q

True/False: raw milk does not require refrigeration as it will be subsequently pasteurized to kill off spoilage microbes and pathogens

A

False: bacteria can produce enzymes/toxins that are heat stable and will affect final product
MUST KEEP COLD BEFORE AND AFTER PASTEURIZATION

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23
Q

What is the average shelf life of pasteurized milk kept at a proper temperature? What happens after?

A

16-22 days

psychrotrophic bacteria grow during this period -> results in off flavors, defects

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24
Q

How can the shelf life of pasteurized milk be extended without further heat?

A

microfiltering to remove bacteria in milk

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25
Q

Characterize the microbes responsible for milk spoilage:

A

Aerobic psychrotrophic gram-negative rods
mostly pseudomonas (70%)
Aeromonas, Bacillus, Staphylococcus, Enterococcus

(increase in enterobactericeae but overtaken by pseudomonas)

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26
Q

Most bacterial spores in milk are the genus ______

A

Bacillus

27
Q

True/false The main cause of psychrotrophic spoilage in milk is due to lipolysis

A

False; due to proteolysis

28
Q

True/False: pseudomonas is very good at fermenting lactose

A

False; cannot utilize lactose

29
Q

Where can contamination by spoilage microbes occur for milk?

A

farm (multiple sources), air, processing plant, equipment (especially carton/bag filling process)

30
Q

How does milk at warmer temperatures spoil differently than refrigerated milk?

A

warm temp -> allows LAB to grow (overtakes psychrotrophs)

31
Q

How does LAB milk spoilage differ from LAB milk fermentation?

A

wild LAB -> produce unwanted byproducts and off flavors (acetic acid, propionic acid)

32
Q

Why would other dairy products (butter, yogurt, cheese, dry milk) spoil differently than milk?

A

different Aw, nutrients added/removed, altered pH

33
Q

Why is it difficult/impractical to generalize spoilage processes in fruit and vegetables?

A

many handling steps/sources of contamination
grown in different climates/countries
commodities vary widely (composition)
methods of cultivation also vary

34
Q

What are the common microbe types and populations on fresh produce?

A

bacteria (aerobic mesophiles): 10^2 - 10^8 CFU/g

yeasts and mold: 10^1 - 10^7 CFU/g

35
Q

How is respiration rate of produce measured (units), and how does it affect shelf life?

A

measured in CO2/kg/hr (at 5C)

higher respiration -> shorter shelf life

36
Q

Plants will produce ___ gas, which will hasten ripening.

A

ethylene

37
Q

Why is ethylene production of concern in produce?

A

Will cause produce to reach full maturity -> begins to age and deteriorate (structure changes)

38
Q

How does respiration lead to damage in produce?

A

aging

moisture loss -> wilting, structure damage

39
Q

Fruit/veg are usually contaminated from:

Spoilage occurs when:

A

the environment they were grown in

microbes penetrate into tissue

40
Q

Fruit/veg is usually protected from microbial invasion by a ________. How is it compromised?

A

cuticle membrane barrier

spoilage microbes produce enzymes; fungi form APPRESSORIUM (at end of germ tube) to penetrate

41
Q

How does bacterial and fungal invasion of fruit/veg differ?

A

bacteria: produce enzymes to attack cuticle
fungi: use mechanical pressure from germ tube to push in and invade

42
Q

What types of microbes can gain immediate access to plant tissue?

A

pathogens (not spoilage)

43
Q

Why might damaged produce spoil faster?

A

injury -> opening to inside tissue -> bacteria/fungi can easily invade

44
Q

What is the main natural mechanism to protect against spoilage in fruit?

A

low pH (as low as 2.0 in some citrus!)

45
Q

What organisms are usually responsible for spoilage of high acid fruit?

A

mold and yeast (more acid tolerant)

46
Q

Give examples of anti-spoilage compounds in fruit:

A

tannins
phenolics (saponine in tomato)
sulfur compounds (onions, garlic)
terpenoids (carrots)

47
Q

Antimicrobial substances in fruit are classified as _____ or ______.

A

phytoalexins: produced in response to attack (detect molecular pattern of pathogens)
phytoanticipins: plant RESISTANCE (anticipate attack)

48
Q

What techniques can be done to prevent spoilage in fruit/veg?

A

Goal is to LOWER RESPIRATION RATE

  1. low temp (0C for most, 10C for tropical fruit)
  2. careful control of humidity
  3. controlled atmosphere
49
Q

True/False: high humidity is recommended for fruit/veg storage

A

True (90% RH for most; except onion and garlic)

50
Q

Why is high RH used for produce storage, and what problems might it have?

A

prevents too much moisture loss (damage)

problem: more water -> more microbe growth

51
Q

General structure of nuts:

A 
outer shell
seed coat (thin or thick)
edible portion (seed, kernel, meat)
52
Q

What is the general preservation method for most nuts? Why?

A

drying to Aw < 0.7

prevent bacteria growth, increase stability

53
Q

True/False: most nuts are not susceptible to microbial spoilage

A

True; shell is very effective, inside usually sterile if undamaged

54
Q

What causes spoilage in nuts?

A

oxidation of fats

55
Q

Most nut microbial spoilage occurs due to:

A

improper drying/storing -> water content increases

56
Q

What is the most commonly consumed nut in North America? What are its spoilage concerns?

A

peanut
groundnut: contamination by mold is unavoidable
Aspergillus flavus will produce MYCOTOXINS (carcinogenic)

57
Q

Can contamination of cereal grains be avoided?

A

No (occurs while growing) - but can minimize growth after harvest by drying & storing properly

58
Q

What are the most common microbes growing on cereals, before and after harvest? What are the effects of each?

A

before: FIELD FUNGI (Alternaria, Cladoporium, Fusarium, Helminthosporium) -> damage crop, losses
after: STORAGE FUNGI (Aspergillus, Rhizopus, Mucor, Wallemia, Penicillium) -> mycotoxins (dangerous!)

59
Q

What are the growth conditions for field fungi and storage fungi in cereals?

A

field fungi: need Aw > 0.9 (encouraged by low temp, high rain)

storage: improperly dried grains

60
Q

The process of baking will destroy _____ and ______

A

mold spores, vegetative bacterial cells

61
Q

The main organism responsible for spoilage of baked goods is:

A

mold

62
Q

A bread with high moisture content is prone to spoilage by: ______. This will manifest as: ______

A

Bacillus subtilis

ropey texture

63
Q

How do commercial baked goods delay mold spoilage?

A

addition of fungistatic agent to product

Food Microbiology (21 decks)

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