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Food Microbiology > Physical Methods of Food Preservation > Flashcards

Physical Methods of Food Preservation Flashcards

1
Q

What are the purposes of modern food preservation methods?

A
  1. increase shelf life

2. improve safety (inactivate/prevent pathogens)

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2
Q

What is “inactivation” of microbes in food?

A

damaging/reducing population to point where it will not recover

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3
Q

True/False; Physical methods of food preservation are rarely used today

A

false: still most common food preservation method

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4
Q

List the physical methods of food preservation:

A
  1. High temp
  2. Low temp
  3. Decrease Aw
  4. Ionizing irradiation
  5. High pressure
  6. Pulsed electric field
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5
Q

Describe the effect on bacteria as heat is increased:

A

enzyme activity and growth will increase -> reach optimum temp, peaks -> even higher temp will cause rapid decline/stop (cell death)

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6
Q

High heat will cause ____ in most bacteria. Why?

A 
cell death
damages membranes (collapse -> lysis), ribosomes, enzymes and proteins (denatured)
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7
Q

Describe the effects of high heat and very low temp on bacterial membranes:

A

low temp: membrane “gels” -> cannot transfer efficiently -> no growth

high temp: membrane lysis -> cell death

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8
Q

What is an issue with heat treatment in food?

A

overheating will damage quality of food

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9
Q

What factors affect heat transfer in food?

A
  1. product type
  2. container material (conductance)
  3. container shape (surface area)
  4. container size
  5. agitation
  6. heating medium temperature
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10
Q

What is the best container shape for fast heating of food?

A

small, tall, narrow

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11
Q

Liquids will heat (faster/slower) than solids

A

faster

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12
Q

What can be done to improve heating rate of viscous fluids?

A

agitation/mixing -> help better distribute heat

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13
Q

The (greater/less) the difference in temperature between product and heating medium, the greater the heat transfer

A

greater

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14
Q

The major parameters of heating processes for food:

A

time

temp

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15
Q

The microbial population will decline following a _____ pattern during heat treatment

A

logarithmic

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16
Q

What is a survivor plot?

A

depicts the inactivation of the bacteria population (logarithmic curve) over time

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17
Q

What is D value?

A

Decimal reduction time: time needed at given temperature to make a 1 log reduction (kill 90% of current population)

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18
Q

An organism with a high D value is (resistant/not resistant) to heat

A

resistant (take longer treatment)

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19
Q

Why might heat treatment times differ for different products?

A
  1. different bacteria -> different D values

2. different initial population (higher population need longer time)

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20
Q

As temperature increases, D value will (increase/decrease).

A

decrease

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21
Q

D value plotted vs. temperature gives a ____ plot. This follows a ___ pattern.

A

thermal resistance; linear

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22
Q

What is the z value?

A

thermal resistance constant
change in temp needed to decrease D value by 90%
negative reciprocal of slope on thermal resistant plot

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23
Q

The higher the Z value, the more _______ the organism

A

thermally resistant

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24
Q

________ measurements are taken to determine amount of time needed to sterilize food at a given temp
Lethality is represented by: ______

A 
thermal lethality
F value (minutes)
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25
Q

What is the difference between D value and F value?

A

D value: time for 1 log reduction

F value: time to reduce population to certain level (10^1 or 0)

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26
Q

F value is very important to consider in the ____ industry

A

canning

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27
Q

What standards are the canning heat treatments based on (especiallly low acid food)?

A

reducing C. botulinum population by 12 log

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28
Q

What is the difference between commercially sterile and sterile?

A

sterile: done by autoclaving; totally kills all microbes (extreme high heat, long time)

commercially sterile: free from microbes that can reproduce under common storage conditions; no viable cells/spores that could harm health
(less heat treatment to avoid totally destroying food)

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29
Q

What are the most heat resistant microbes in food?

A 
Geobacillus stearothermophilus (spoilage)
C. botulinum (A and B) - (toxin)
C. sporogenes - (spoilage)
B. coagulans - (spoilage)
B. licheniformis - (spoilage)
30
Q

What mechanisms allow for greater heat resistance in bacteria?

A
  1. Disulfide bonding (protein cross-linking, prevent total denaturation)
  2. DNA structural changes (more G-C: 3 bonds so stronger linking)
  3. form spores/stationary cells (more resistance when dormant, especially spores)
31
Q

How can we lower the heat resistance ability of microbes?

A

add in another stressing factor: low pH, salt, reduce water, antimicrobials, etc

32
Q

What is aseptic processing? What are its benefits, and what is it used for?

A

sterilize container & product separately, then package
avoids excess heating (better quality)

used for liquids (juice/dairy/sauce); NOT for low acid/chunky products

33
Q

Describe the general aseptic process:

A

liquid pass through heat exchangers until reach target temp/time -> cooled (heat exchangers) -> packed in pre-sterilized packages & hermetically sealed

34
Q

How do microwaves heat food, and why isn’t this used for heat treatment?

A

causes rotation of water molecules -> friction, heat generation (generates heat within food)

causes uneven heating - uneven lethality

35
Q

Microwave heating is (faster/slower) than conventional heating. Why?

A

faster

generates heat within/throughout product, rather than penetrating from outside

36
Q

How does freezing preserve food (microbiologically)?

A

Stops metabolic activity of microbes (except psychrophiles, psychrotrophs, cryophiles)

No water activity (frozen, not available)

freeze/thaw will damage cells of microbes

37
Q

What are the impacts of freezing on food quality?

A

preserves:
stops chemical/biochemical deterioration (enzymes halted) - prevent lipid ox, Maillard rxn, enzymatic browning, lipolysis, proteolysis

but will rupture some cells (ice crystals) -> altered structured

38
Q

True/False: freezing will destroy toxins

A

False; toxins not affected

39
Q

microbes growing at subzero temperatures must be both cold-tolerant and ___tolerant. These microbes are mostly ____ and _____.

A

xerotolerant (lack of water)

yeast, fungi

40
Q

Are Gram positive or Gram negative bacteria more susceptible to freezing? Why?

A

Gram negative (no cell wall for protection)

41
Q

What process is done to eliminate parasites from pork?

A

Freeze briefly at -23C (parasites inactivated by freezing) - inactivates Trichenella spiralis (pork worm)

42
Q

How does slow freezing compare to fast freezing in terms of quality and microbial death?

A

fast freezing (industrial): smaller ice crystals, shorter period of osmotic stress -> better quality, but less damage to microbial cells too

slow freezing (home): large ice crystals -> damaged food quality, but also more effective against microbes

43
Q

Water in food is either ____ or _____ by ___, ____ groups, or ____ groups.

A

free

bound: salts, hydroxyl, carbonyl/amino (on proteins)

44
Q

___ or ____ the ___ water in food will restrict microbial growth.

A

removing; restricting; free

45
Q

How is the available water expressed?

A

Aw (water activity)

ratio of water vapor pressure (P) with water vapor pressure of pure water (Po)

46
Q

How is water vapor pressure determined?

A

seal food in container -> water will escape from food into headspace until reach equilibrium; pressure at this point is P (water vap pressure)

47
Q

Aw will have values between ___ and ____. The lower the value, the lower the _____ and the longer the ____

A

0 and 1
lower the water activity/microbial growth
longer shelf life

48
Q

what is ERH?

A

equilibrium relative humidity %

Aw x 100%

49
Q

How is moisture content and Aw different?

A

moisture content: total water in food (bound and free)

Aw: only free water (better indicator of biochem & chem activity)

50
Q

Division of food based on Aw:

A

High moisture food: Aw>0.9, prone to spoilage (milk, meat, veg, etc)
Intermediate moisture food: Aw 0.65-0.9, relatively resistant (jam, raisins)
Low moisture food: Aw<0.65, long shelf life (dry milk, crackers)

51
Q

Maple syrup has Aw of ______, making it a _____ moisture food.

A

0.88

intermediate

52
Q

What is the maple syrup making process?

A

boil maple sap to 104C, Brix should be 66

53
Q

Why is water necessary for microbial growth?

A

nutrient acquisition
waste removal
medium for metabolic rxn

54
Q

How do water requirement differ among microbes?

A

Pathogens/spoilage bacteria: high water req (need 0.95 to grow)
Spore-forming bacteria: can grow at 0.91-0.95
Yeast: tolerate lower moisture than bacteria
Mold: most xerotolerant

55
Q

What bacteria requires the least moisture to grow?

A

Staphylococcus aureus; can grow at as low as 0.86 (but cannot make toxins)

56
Q

What are the usual spoilage microbes in intermediate moisture foods?

A

Yeast; bacteria inhibited by lower moisture but yeast are more tolerant

57
Q

Low moisture may not be able to inhibit mold growth, but it will prevent: ____

A

yeast growth, bacteria growth, toxin production (prevent mycotoxins)

58
Q

Agents used to lower Aw are called ____. Give examples:

How do their effects differ?

A

humectants
glycerol, salt, sugar
glycerol can permeate bacterial membrane

59
Q

Are microbes killed by low Aw?

A

No; only dormant. Can be reactivated

60
Q

How is water removal from food achieved?

A

drying: apply heat, mobilize water to surface and evaporate

osmotic dehydration: immerse in hypertonic solution; water drawn out by osmosis (less effective, usually used as pre-treatment)

freeze drying: frozen, then sublimated; high cost but preserves structure/flavor

61
Q

what types of electromagnetic waves are effective in killing microbes? What is the mechanism?

A

short wavelengths: gamma and x-rays (ionizing radiation)

creates ROS that will attack DNA

62
Q

What affects a microbe’s susceptibility to radiation?

A

size of genome; smaller genome will be more resistant

63
Q

How are X-rays produced, and what foods is it used for What are the limitations?

A

produced when electron beam strikes metal (tungsten, stainless steel)
Used for fresh fruit/veg (<5MeV)

High power requirement
Lot of heat generation; need cooling system as well

64
Q

How is gamma radiation produced? What are advantages and disadvantages?

A

emitted from radioisotope (excited) - Ce137 or Co60

high penetration power, constant energy during treatment, continuous source

safety concerns (need large facility, store source under water), cannot turn off

65
Q

What are the dosage levels for irradiation in food?

A

Redappertization: high dose (10-75kGy) -> sterile
Radicidation: med dose (1-10kGy) -> similar to pasteurization (spores unaffected)
Radurization; low (0.05-1kGy) -> inhibit sprouting, kill insects

66
Q

The main advantage of HPP and PEF processing is:

A

maintains food in “fresh” state, and microbiologically safe

67
Q

What are the parameters and considerations for HPP?

A

High pressure processing:
treat with 100-600MPa -> inactivate most bacteria/yeast/mold/virus (spores survive)

need to consider package type, food composition (air pockets will cause deformity due to difference in compressibility)

68
Q

PEF is _____. It is designed to treat _____ food.

A

pulsed electric field

pumpable (liquid)

69
Q

The process for PEF involves: _________ to _______.

The success will depend on: _______

A

apply short (micro/milliseconds) burst of high voltage, to eliminate vegetative bacterial cells

food composition, conductivity, pH

70
Q

Air pockets are problematic in PEF and HPP. Why?

A

non-uniformity:
PEF: air is poor conductor
HPP: air has different compressibility -> leads to deformation

Food Microbiology (21 decks)

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