Microbial Spoilage + Public Health Concerns (2) Flashcards
milk is (prone/resistant) to spoilage. why?
prone to spoilage
high water content, neutral pH, high in nutrients (sugars, protein, lipids) -> good for bacterial growth
What process is required for all milk in Canada, and what does it achieve?
pasteurization (heat treatment)
eliminate most pathogens (to safe levels) & increases shelf life
Why doesn’t milk support growth of some microorganisms?
low in available amino acids
True/false: milk at 7C will go rancid faster than milk at 4C
False: psychrotrophic bacteria will grow faster and make more lipases -> faster lipolysis
True/false: milk at 7C will undergo proteolysis faster than milk at 4C
True
Milk spoilage organisms often rely on ___ and ____ to obtain energy
proteolysis; lipolysis
Psychrotrophic milk spoilage organisms will produce ____, ____, and ____ enzymes. What are the products and effects (sensory)?
lipolytic -> FFA -> rancid
proteolytic -> peptides, AA -> bitter
esterolytic (esterases) -> ethyl esters -> fruity
What is the effect of Bacillus contamination in milk?
produce proteases:
- bitter peptides
- casein destabilization -> coagulate
What enzyme types do LAB produce in milk, and what is the effect?
glycolytic enzymes -> lactic/acetic acid -> sour
oxidase -> 3-methyl-butanal -> malty
polymerase -> exopolysaccharides -> ropey texture
What sugars are present in milk, and how does this affect the bacteria species that grow?
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)
Is milk fat used as a nutrient source for bacterial growth?
Yes; but globules must be ruptured for bacteria to have access to fat (protected by membrane)
What provides the N for bacterial growth in milk?
Mainly casein proteins; also urea, peptides, AA (NPNs)
The NPNs in milk:
urea, peptides, free AA (non protein nitrogen)
What are the major two inhibitory substances for spoilage in milk?
lactoferrin
lactoperoxidase
What are some minor inhibitory substances for milk spoilage? (3)
lysozyme, immunoglobulins, folate/B12 binding systems
How does lactoferrin inhibit bacterial growth?
binds available iron (limiting nutrient for growth)
How does the inhibitory activity of lactoferrin differ in cow vs human milk? Why?
Less in cow’s milk:
cow milk higher in CITRATE -> citrate will bind lactoferrin, lowering its iron-sequestering ability
True/false; lactoperoxidase is inactivated by pasteurization
False
______ is the most important microbial inhibitory substance in cow milk. How does it function?
lactoperoxidase
catalyze oxidation of thiocyanate (with hydrogen peroxide) -> HYPOTHIOCYANITE (highly reactive oxidant; antimicrobial)
Why is H2O2 added to cow milk in developing countries?
H peroxide is limiting substrate for lactoperoxidase system; adding will make it more effective as an antimicrobial (in lieu of refrigeration)
Preservation of fresh milk relies on _____ and ______.
sanitation/pasteurization
refrigeration
True/False: raw milk does not require refrigeration as it will be subsequently pasteurized to kill off spoilage microbes and pathogens
False: bacteria can produce enzymes/toxins that are heat stable and will affect final product
MUST KEEP COLD BEFORE AND AFTER PASTEURIZATION
What is the average shelf life of pasteurized milk kept at a proper temperature? What happens after?
16-22 days
psychrotrophic bacteria grow during this period -> results in off flavors, defects
How can the shelf life of pasteurized milk be extended without further heat?
microfiltering to remove bacteria in milk
Characterize the microbes responsible for milk spoilage:
Aerobic psychrotrophic gram-negative rods
mostly pseudomonas (70%)
Aeromonas, Bacillus, Staphylococcus, Enterococcus
(increase in enterobactericeae but overtaken by pseudomonas)
Most bacterial spores in milk are the genus ______
Bacillus
True/false The main cause of psychrotrophic spoilage in milk is due to lipolysis
False; due to proteolysis
True/False: pseudomonas is very good at fermenting lactose
False; cannot utilize lactose
Where can contamination by spoilage microbes occur for milk?
farm (multiple sources), air, processing plant, equipment (especially carton/bag filling process)
How does milk at warmer temperatures spoil differently than refrigerated milk?
warm temp -> allows LAB to grow (overtakes psychrotrophs)
How does LAB milk spoilage differ from LAB milk fermentation?
wild LAB -> produce unwanted byproducts and off flavors (acetic acid, propionic acid)
Why would other dairy products (butter, yogurt, cheese, dry milk) spoil differently than milk?
different Aw, nutrients added/removed, altered pH
Why is it difficult/impractical to generalize spoilage processes in fruit and vegetables?
many handling steps/sources of contamination
grown in different climates/countries
commodities vary widely (composition)
methods of cultivation also vary
What are the common microbe types and populations on fresh produce?
bacteria (aerobic mesophiles): 10^2 - 10^8 CFU/g
yeasts and mold: 10^1 - 10^7 CFU/g
How is respiration rate of produce measured (units), and how does it affect shelf life?
measured in CO2/kg/hr (at 5C)
higher respiration -> shorter shelf life
Plants will produce ___ gas, which will hasten ripening.
ethylene
Why is ethylene production of concern in produce?
Will cause produce to reach full maturity -> begins to age and deteriorate (structure changes)
How does respiration lead to damage in produce?
aging
moisture loss -> wilting, structure damage
Fruit/veg are usually contaminated from:
Spoilage occurs when:
the environment they were grown in
microbes penetrate into tissue
Fruit/veg is usually protected from microbial invasion by a ________. How is it compromised?
cuticle membrane barrier
spoilage microbes produce enzymes; fungi form APPRESSORIUM (at end of germ tube) to penetrate
How does bacterial and fungal invasion of fruit/veg differ?
bacteria: produce enzymes to attack cuticle
fungi: use mechanical pressure from germ tube to push in and invade
What types of microbes can gain immediate access to plant tissue?
pathogens (not spoilage)
Why might damaged produce spoil faster?
injury -> opening to inside tissue -> bacteria/fungi can easily invade
What is the main natural mechanism to protect against spoilage in fruit?
low pH (as low as 2.0 in some citrus!)
What organisms are usually responsible for spoilage of high acid fruit?
mold and yeast (more acid tolerant)
Give examples of anti-spoilage compounds in fruit:
tannins
phenolics (saponine in tomato)
sulfur compounds (onions, garlic)
terpenoids (carrots)
Antimicrobial substances in fruit are classified as _____ or ______.
phytoalexins: produced in response to attack (detect molecular pattern of pathogens)
phytoanticipins: plant RESISTANCE (anticipate attack)
What techniques can be done to prevent spoilage in fruit/veg?
Goal is to LOWER RESPIRATION RATE
- low temp (0C for most, 10C for tropical fruit)
- careful control of humidity
- controlled atmosphere
True/False: high humidity is recommended for fruit/veg storage
True (90% RH for most; except onion and garlic)
Why is high RH used for produce storage, and what problems might it have?
prevents too much moisture loss (damage)
problem: more water -> more microbe growth
General structure of nuts:
outer shell seed coat (thin or thick) edible portion (seed, kernel, meat)
What is the general preservation method for most nuts? Why?
drying to Aw < 0.7
prevent bacteria growth, increase stability
True/False: most nuts are not susceptible to microbial spoilage
True; shell is very effective, inside usually sterile if undamaged
What causes spoilage in nuts?
oxidation of fats
Most nut microbial spoilage occurs due to:
improper drying/storing -> water content increases
What is the most commonly consumed nut in North America? What are its spoilage concerns?
peanut
groundnut: contamination by mold is unavoidable
Aspergillus flavus will produce MYCOTOXINS (carcinogenic)
Can contamination of cereal grains be avoided?
No (occurs while growing) - but can minimize growth after harvest by drying & storing properly
What are the most common microbes growing on cereals, before and after harvest? What are the effects of each?
before: FIELD FUNGI (Alternaria, Cladoporium, Fusarium, Helminthosporium) -> damage crop, losses
after: STORAGE FUNGI (Aspergillus, Rhizopus, Mucor, Wallemia, Penicillium) -> mycotoxins (dangerous!)
What are the growth conditions for field fungi and storage fungi in cereals?
field fungi: need Aw > 0.9 (encouraged by low temp, high rain)
storage: improperly dried grains
The process of baking will destroy _____ and ______
mold spores, vegetative bacterial cells
The main organism responsible for spoilage of baked goods is:
mold
A bread with high moisture content is prone to spoilage by: ______. This will manifest as: ______
Bacillus subtilis
ropey texture
How do commercial baked goods delay mold spoilage?
addition of fungistatic agent to product
