15) Microbial Spoilage and Public Health Concerns (2) Flashcards
why is milk good for growing bacteria?
due to:
- high water content
- near neutral pH
- available nutrients
what does pasteurization do to milk?
pasteurized milk will not support all MOs b/c of low amounts of available AAs
milk spoilage bacteria relies on ___ or ____ for energy generation
proteolysis or lipolysis
compare lipolysis to proteolysis in milk
lipolysis happens faster at 4C b/c psychotropic bacteria have faster growth rate and produces more lipases
proteolysis happens faster at 7C
what is the more common C source for bacterial spoilage of milk? why?
lactose
b/c only select bacteria can degrade lactose
after lactose, what is the secondary source of carbon for microbe growth in milk?
what is the third?
2nd: glucose
3rd: fat (only if the globule membrane is physically damaged)
when is fat used as a C source for spoilage microbe growth in milk?
what is fat protected by when not used as a carbon source?
if the globule membrane is damaged
otherwise, fat is protected by a protective membrane
what is the main source of N for bacterial growth in milk?
casein proteins
what are non-protein N sources in milk?
urea
peptides
AAs
what are major inhibitors of microbial growth in milk?
what are minor inhibitors?
major:
- lactoferrin
- lactoperoxidase
minor
- lysozyme
- immunoglobulins
- folate and vit B12 binding systems
how does lactoferrin act to inhibit microbial growth in milk?
lactoferrin binds iron which makes iron a limiting factor for bacterial growth
how is citrate levels different in cow vs human milk?
what effect does this cause?
human milk: low in citrate
cow’s milk: higher in citrate
citrate binds lactoferrin which decreases lactoferrin’s ability to limit bacterial growth
describe the lactoperoxidase system in cow’s milk
- an effective microbial inhibitor in cow’s milk
- lactoperoxidase survives pasteurization
- lactoperoxidase catalyzes oxidation of thiocyanate (w/ H2O2 presence) which creates hypothiocyanite (an antimicrobial and highly reactive oxidant)
what is hypothiocyanite?
- antimicrobial
- highly reactive oxidant
- the final pdt of the lactoperoxidase system
what is this limiting substrate in the lactoperoxidase system?
H2O2 (hydrogen peroxide)
how can you increase effectiveness of the lactoperoxidase system in milk?
adding H2O2, since it is the limiting substrate
what does the lactoperoxidase system limit?
LABs, coliforms, pathogenic bacteria
how is shelf life of raw-milk extended in developing countries?
add H2O2, which increases effectiveness of lactoperoxidase system, which decreases LABs, coliforms and pathogenic bacteria
preservation of milk relies on ____ and ____
sanitation (pasteurization) and refrigeration
what bacteria spoils raw and pasteurized milk?
psychrotropic bacteria that are aerobic gram-neg rods
in spoiled milk, what is the main MO found?
Pseudomonas sp: 60-75% of bacteria
what MOs will be more commonly found in spoiled refrigerated milk?
obligate aerobes
describe the growth of enterobacteriacea and pseudomadaceae during milk storage
what is an exception to this?
enterobacteriacea increases in numbers but is over grown by pseudomadaceae
except when bacterial spores are present
which process does psychrotropic milk spoilage primarily consist of? what is a secondary process that may occur?
primarily proteolysis
lipolysis may also occur
bacteria most associated w/ milk spoilage Pseudomonas sp. are unable to ______
ferment lactose
1) how can raw milk become contaminated by psychrotropic bacteria?
2) how can pasteurized milk be contaminated?
1) by source on the farm
2) in the milk processing plant equipment and air
what causes defects in spoiled milk?
what are examples of this?
extracellular enzymes
ex
- bitter and putrid flavors from proteolysis
- rancid and fruit flavors from lipolysis
what causes spoilage of milk by LAB?
due to improper storage temperatures
when temp is high enough for gram-pos acid producers to outgrow psychrotrophic bacteria
compare
a) products from desired fermentation
b) “wild” LAB produced products
a) lactic acid (tastes good)
b) wild LAB produces acetic acid, propionic acid, unwanted gases (has off-tastes)
why do dariy pdts almost always use starter cultures?
to avoid production of unwanted flavors and gases due to “wild” LAB
why do dairy pdts spoil differently than milk pdts?
the environment they provide is different:
- nutrients have been removed/concentrated
- pH altered
- diff water activity
why is it hard to make generalizations about spoilage of fruits and veg?
b/c a produce item can be grown under various conditions and climates
each variable affects bacterial load and spoilage
what is respiration measured in?
CO2/kg/hr at 5C
describe the relationship with resp rate and shelf life one fruit&veg is detached from the plant
negative relationship (shelf life decreases with higher resp rate)
pdts with low resp rate has a ____ shelf life
pdts with a high resp rate has a ____ shelf life
low resp rate = long shelf life
high resp rate = short shelf life
what is ethylene?
produced by plants
increases produce maturity
respiration results in what?
- moisture loss
- structural damage
- wilting (in greens)
spoilage in F&V is mainly caused by what?
describe this process
- penetration of microbial cells into plant tissue
- cuticle barrier is compromised by spoilage bacteria by enzymatic processes
- spoilage fungi forms an appressorium at end of their germ tubes, which attached and penetrates plant surfaces by mechanical pressure to initiate fungal spoilage
describe natural and accidental openings in F&V
what do openings allow for?
natural: blossom end, stem scars
accidental: wounds
openings allow for spoilage MOs to penetrate into tissues
why are F&V susceptible for microbial growth?
- high water content
- high nutrients available
what is the main limitation preventing growth of bacteria in F&V?
pH
can be very acidic in some citrus fruits
what is spoilage caused by in low-pH fruits? why?
molds and yeasts since they are more tolerant to high acidity
what antimicrobial compounds do F&V have? what is an example of each one?
- tannins and phenolic compounds (saponine in tomatoes)
- sulfur derived antimicrobials (onions or garlic)
- terpenoids (carrots)
- phytoanticipins (for plant resistance)
- phytoalexins (produced in response to PAMPs on bacteria)
how can you prolong shelf life of F&V?
using optimal storage conditions (control temp, RH, composition of gas atmosphere, modified atmosphere) = reduces resp activity = delays ripening = prolongs shelf life
what temp is recommended for storage for F&V?
what is an exception to this?
close to 0C
except for produce that can suffer from chilling injuries
what relative humidity is recommended for F&V?
RH of 90%
what is the edible portion of the nut called?
seed, kernel or meat
seed coats on nuts can be ____ or ___
thin (peanut)
or thick and hard (coconut)
all nuts have ______
a rigid outer casing or shell
how are nuts generally preserved?
low water activity (below 0.7) which prevents all bacteria and most fungi.molds from growing and increasing shelf stability
in nuts, what acts as a barrier to microbes?
the thick shells
as a result, the internal surface of a nut is almost sterile
what causes or encourages bacterial colonization in nuts?
damage to the outer shell
how is nut spoilage affected by microbiology and oxidation of nut fats?
microbiology: not related
oxidation of nut fats: major cause of spoilage
when does microbiological spoilage occur in nuts?
when moisture is not controlled properly (must be below 0.7)
what kind of nut is the peanut?
ground nut, not a tree nut
where does the microbiota of peanuts originate from?
what is the result of this?
soil, thus mold contamination of the pods are inevitable
how is spoilage molds reduced in peanuts?
rotating peanut fields with other crops
when does microbiological contamination of cereal grains occur?
when do they increase?
while grains are growing
contaminats increase while grains are growing, during and after harvest
how can you limit growth of microbes in cereal grains
post harvest interventions (drying grains + good storage practices)
what are the most common contaminants of grains in the field ?
molds
some are toxigenic
what are field fungi?
what are storage fungi?
describe each
field:
- molds that infect the grain before harvest
- requires water activity above 0.9
- encouraged by low temp and high precipitation
- can damage the grains to the point of total crop loss
storage:
- infects grains after harvest
- growth occurs due to improperly dried grains
- can also result in high mycotoxin levels
how can you kill mold spores and MOs in cereal pdts
baking
what is the predominant cause of baked pdt spoilage?
how does this occur?
how can this be avoided
mold
occurs b/c surface of baked pdts can be re-contaminated w/ airborne mold spores during pdt cooling and packaging
can avoid by using fungistatic agents to retard mold growth
what are bakery pdts w/ a moist interior more prone to?
why?
rope spoilage
due to extracellular material by Bacillus subtilis
