Microbial Spoilage + Public Health Concerns (2) Flashcards
(63 cards)
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
