15) Microbial Spoilage and Public Health Concerns (2)

Question 1

why is milk good for growing bacteria?

Answer 1

due to:

• high water content
• near neutral pH
• available nutrients

Question 2

what does pasteurization do to milk?

Answer 2

pasteurized milk will not support all MOs b/c of low amounts of available AAs

Question 3

milk spoilage bacteria relies on ___ or ____ for energy generation

Answer 3

proteolysis or lipolysis

Question 4

compare lipolysis to proteolysis in milk

Answer 4

lipolysis happens faster at 4C b/c psychotropic bacteria have faster growth rate and produces more lipases

proteolysis happens faster at 7C

Question 5

what is the more common C source for bacterial spoilage of milk? why?

Answer 5

lactose

b/c only select bacteria can degrade lactose

Question 6

after lactose, what is the secondary source of carbon for microbe growth in milk?

what is the third?

Answer 6

2nd: glucose
3rd: fat (only if the globule membrane is physically damaged)

Question 7

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?

Answer 7

if the globule membrane is damaged

otherwise, fat is protected by a protective membrane

Question 8

what is the main source of N for bacterial growth in milk?

Answer 8

casein proteins

Question 9

what are non-protein N sources in milk?

Answer 9

urea
peptides
AAs

Question 10

what are major inhibitors of microbial growth in milk?

what are minor inhibitors?

Answer 10

major:

• lactoferrin
• lactoperoxidase

minor

• lysozyme
• immunoglobulins
• folate and vit B12 binding systems

Question 11

how does lactoferrin act to inhibit microbial growth in milk?

Answer 11

lactoferrin binds iron which makes iron a limiting factor for bacterial growth

Question 12

how is citrate levels different in cow vs human milk?

what effect does this cause?

Answer 12

human milk: low in citrate
cow’s milk: higher in citrate

citrate binds lactoferrin which decreases lactoferrin’s ability to limit bacterial growth

Question 13

describe the lactoperoxidase system in cow’s milk

Answer 13

• 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)

Question 14

what is hypothiocyanite?

Answer 14

• antimicrobial
• highly reactive oxidant
• the final pdt of the lactoperoxidase system

Question 15

what is this limiting substrate in the lactoperoxidase system?

Answer 15

H2O2 (hydrogen peroxide)

Question 16

how can you increase effectiveness of the lactoperoxidase system in milk?

Answer 16

adding H2O2, since it is the limiting substrate

Question 17

what does the lactoperoxidase system limit?

Answer 17

LABs, coliforms, pathogenic bacteria

Question 18

how is shelf life of raw-milk extended in developing countries?

Answer 18

add H2O2, which increases effectiveness of lactoperoxidase system, which decreases LABs, coliforms and pathogenic bacteria

Question 19

preservation of milk relies on ____ and ____

Answer 19

sanitation (pasteurization) and refrigeration

Question 20

what bacteria spoils raw and pasteurized milk?

Answer 20

psychrotropic bacteria that are aerobic gram-neg rods

Question 21

in spoiled milk, what is the main MO found?

Answer 21

Pseudomonas sp: 60-75% of bacteria

Question 22

what MOs will be more commonly found in spoiled refrigerated milk?

Answer 22

obligate aerobes

Question 23

describe the growth of enterobacteriacea and pseudomadaceae during milk storage

what is an exception to this?

Answer 23

enterobacteriacea increases in numbers but is over grown by pseudomadaceae

except when bacterial spores are present

Question 24

which process does psychrotropic milk spoilage primarily consist of? what is a secondary process that may occur?

Answer 24

primarily proteolysis

lipolysis may also occur

Question 25

bacteria most associated w/ milk spoilage Pseudomonas sp. are unable to ______

Answer 25

ferment lactose

Question 26

1) how can raw milk become contaminated by psychrotropic bacteria? 2) how can pasteurized milk be contaminated?

Answer 26

1) by source on the farm | 2) in the milk processing plant equipment and air

Question 27

what causes defects in spoiled milk? what are examples of this?

Answer 27

extracellular enzymes ex - bitter and putrid flavors from proteolysis - rancid and fruit flavors from lipolysis

Question 28

what causes spoilage of milk by LAB?

Answer 28

due to improper storage temperatures when temp is high enough for gram-pos acid producers to outgrow psychrotrophic bacteria

Question 29

compare a) products from desired fermentation b) "wild" LAB produced products

Answer 29

a) lactic acid (tastes good) | b) wild LAB produces acetic acid, propionic acid, unwanted gases (has off-tastes)

Question 30

why do dariy pdts almost always use starter cultures?

Answer 30

to avoid production of unwanted flavors and gases due to "wild" LAB

Question 31

why do dairy pdts spoil differently than milk pdts?

Answer 31

the environment they provide is different: - nutrients have been removed/concentrated - pH altered - diff water activity

Question 32

why is it hard to make generalizations about spoilage of fruits and veg?

Answer 32

b/c a produce item can be grown under various conditions and climates each variable affects bacterial load and spoilage

Question 33

what is respiration measured in?

Answer 33

CO2/kg/hr at 5C

Question 34

describe the relationship with resp rate and shelf life one fruit&veg is detached from the plant

Answer 34

negative relationship (shelf life decreases with higher resp rate)

Question 35

pdts with low resp rate has a ____ shelf life pdts with a high resp rate has a ____ shelf life

Answer 35

low resp rate = long shelf life high resp rate = short shelf life

Question 36

what is ethylene?

Answer 36

produced by plants increases produce maturity

Question 37

respiration results in what?

Answer 37

- moisture loss - structural damage - wilting (in greens)

Question 38

spoilage in F&V is mainly caused by what? describe this process

Answer 38

1. penetration of microbial cells into plant tissue 2. cuticle barrier is compromised by spoilage bacteria by enzymatic processes 3. spoilage fungi forms an appressorium at end of their germ tubes, which attached and penetrates plant surfaces by mechanical pressure to initiate fungal spoilage

Question 39

describe natural and accidental openings in F&V what do openings allow for?

Answer 39

natural: blossom end, stem scars accidental: wounds openings allow for spoilage MOs to penetrate into tissues

Question 40

why are F&V susceptible for microbial growth?

Answer 40

- high water content | - high nutrients available

Question 41

what is the main limitation preventing growth of bacteria in F&V?

Answer 41

pH | can be very acidic in some citrus fruits

Question 42

what is spoilage caused by in low-pH fruits? why?

Answer 42

molds and yeasts since they are more tolerant to high acidity

Question 43

what antimicrobial compounds do F&V have? what is an example of each one?

Answer 43

- 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)

Question 44

how can you prolong shelf life of F&V?

Answer 44

using optimal storage conditions (control temp, RH, composition of gas atmosphere, modified atmosphere) = reduces resp activity = delays ripening = prolongs shelf life

Question 45

what temp is recommended for storage for F&V? what is an exception to this?

Answer 45

close to 0C except for produce that can suffer from chilling injuries

Question 46

what relative humidity is recommended for F&V?

Answer 46

RH of 90%

Question 47

what is the edible portion of the nut called?

Answer 47

seed, kernel or meat

Question 48

seed coats on nuts can be ____ or ___

Answer 48

thin (peanut) or thick and hard (coconut)

Question 49

all nuts have ______

Answer 49

a rigid outer casing or shell

Question 50

how are nuts generally preserved?

Answer 50

low water activity (below 0.7) which prevents all bacteria and most fungi.molds from growing and increasing shelf stability

Question 51

in nuts, what acts as a barrier to microbes?

Answer 51

the thick shells as a result, the internal surface of a nut is almost sterile

Question 52

what causes or encourages bacterial colonization in nuts?

Answer 52

damage to the outer shell

Question 53

how is nut spoilage affected by microbiology and oxidation of nut fats?

Answer 53

microbiology: not related oxidation of nut fats: major cause of spoilage

Question 54

when does microbiological spoilage occur in nuts?

Answer 54

when moisture is not controlled properly (must be below 0.7)

Question 55

what kind of nut is the peanut?

Answer 55

ground nut, not a tree nut

Question 56

where does the microbiota of peanuts originate from? what is the result of this?

Answer 56

soil, thus mold contamination of the pods are inevitable

Question 57

how is spoilage molds reduced in peanuts?

Answer 57

rotating peanut fields with other crops

Question 58

when does microbiological contamination of cereal grains occur? when do they increase?

Answer 58

while grains are growing contaminats increase while grains are growing, during and after harvest

Question 59

how can you limit growth of microbes in cereal grains

Answer 59

post harvest interventions (drying grains + good storage practices)

Question 60

what are the most common contaminants of grains in the field ?

Answer 60

molds some are toxigenic

Question 61

what are field fungi? what are storage fungi? describe each

Answer 61

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

Question 62

how can you kill mold spores and MOs in cereal pdts

Answer 62

baking

Question 63

what is the predominant cause of baked pdt spoilage? how does this occur? how can this be avoided

Answer 63

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

Question 64

what are bakery pdts w/ a moist interior more prone to? why?

Answer 64

rope spoilage due to extracellular material by Bacillus subtilis