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5) Food Microbiology > 16) Physical Methods of Food Preservation > Flashcards

16) Physical Methods of Food Preservation Flashcards

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1
Q

why were food preservation methods originally developed?

A

to extend shelf life of food pdts by protecting them from microbio spoilage

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2
Q

define inactivation of bacteria

A

destruction of the organism as judged by its inability o recover on microbio media

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3
Q

what form of food preservation is most common?

A

physical

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4
Q

what are some physical methods of food preservation?

A
  • high temp treatment
  • low temp preservation
  • decreasing water availability
  • ionizing irradiation
  • high pressure processing (HPP)
  • pulsed electric field
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5
Q

describe preservation by high temp

A

includes:
increasing temp above optimal temp of an organism, which slows and stops cell multiplication, resulting in cell death
- damages to cell membranes, proteins and ribosomes

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6
Q

why is heat treatment a good choice for preservation?

what is a drawback to heat treatment?

A

good:

  • can eliminate almost any microbial target
  • customizable to the pdt and organism of concern

bad:
- may damage pdt quality

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7
Q

what are factors that affect heat transfer?

A
  • pdt type
  • container material
  • container shape
  • container size
  • agitation
  • temp of heating medium
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8
Q

what should be considered about pdt type when using heat treatments for preservation?

A

liquid absorbs heat faster than solid foods

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9
Q

what should be considered about container material when using heat treatments for preservation?

A

glass containers heat slower than metal contains

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10
Q

what should be considered about container shape when using heat treatments for preservation?

A

tall and narrow containers heat quicker than other shapes

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11
Q

what should be considered about container size when using heat treatments for preservation?

A

small containers heat quicker

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12
Q

what should be considered about agitation when using heat treatments for preservation?

A

increases heat transfer, especially in viscous pdts

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13
Q

what should be considered about temp of heating medium when using heat treatments for preservation?

A

greater difference in temp between heat transfer medium and pdt = faster rate of heat transfer to pdt

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14
Q

what are major parameters in thermal treatments?

A

temp and time

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15
Q

kinetics of microbial inactivation at a temperature is determined by what?

A

the decline in population survivors during heating time

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16
Q

what are survivor plots?

A

shows the log nature of population inactivation over time

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17
Q

what is the D value?

A
  • decimal reduction time

- the time it takes of a 10 fold reduction in number of survivors at a given temp

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18
Q

greater D value = ___ (higher/lower) resistance of the organism to heat

A

higher resistance

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19
Q

D value ___ (decr/incr) as the processing temp increases

A

decreases

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20
Q

what is the thermal resistance plot?

what shape does it have?

what is it used for?

A

graph of D value vs temp

linear

used to compare difference resistances of a MO at diff temps by calculating the z value

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21
Q

what is the z value?

what does it represent?

how can it be calculated?

A

thermal resistance constant

represents the change in temp needed to change the D value of a MO by 10 fold

can be found by calculating the negative reciprocal of the slope of the thermal resistance plot

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22
Q

larger Z value = ___ (higher/lower) heat resistant organism

A

higher resistance

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23
Q

how can you determine the time required to commercially sterilized food?

A

by taking thermal lethality measurements

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24
Q

what is the F value?

A

time (mins) at a specific temp to achieve a targeted reduction in a homogeneous population with a specific z value

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25
Q

compare F and D values

A

D: time to reduce population by 1 log
F: time to get the population to a specific level (ie. sterile or 10^1)

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26
Q

what is the key pathogen of concern in canned low acid foods?

A

C. botulinum

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27
Q

the canning industry often calculates which value to achieve commercial sterility?

A

F value

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28
Q

how does sterility affect C.bot population?

A

should reduce C. bot by 12 log

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29
Q

define sterilization

A

process of rendering a pdt free of any living organism

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30
Q

what is the problem with sterilization of food pdts?

A

heating at very high temp can result in unacceptable quality

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31
Q

a food product that undergoes commercial sterilization will be free of… (2)

A
  1. MOs capable of reproducing in the food under normal non-refrigerated storage and distribution temp
  2. viable microbial cells of public health significance
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32
Q

what are mechanisms which cause bacteria to be more naturally resistant than others? (2)

A
  1. protein cross-linking (disulfide bonding)

2. DNA structural changes (higher GC)

33
Q

which is more resistant to heat?

a) stationary phase cells
b) exponential phase cells

A

stationary

34
Q

how does spore formation affect heat resistance

A

more spores = increased heat resistance

35
Q

define aseptic processing

what is a benefit of this?

A

industrial processed where:

  • foods and cans are sterilized separately
  • food is packaged under aseptic conditions

benefit: avoids excess heating of the food which improves food quality and nutrition

36
Q

describe how aseptic processing is done

A
  1. pdt is heated by passing through a set of heat exchangers until the holding time and temp is reached
  2. food is passed through cooling heat exchangers
  3. cooled pdt is placed in pre-sterilized packages and is hermetically sealed
37
Q

what is aseptic processing used for and not used for?

A

used for: fruit juices, dairy pdts, sauces

not used for: low acid or particulate containing foods

38
Q

describe how microwaving works

what are advantages of this?

A

microwaves cause rotation of H2O molecules in food which generates friction and heat, which generates heat throughout the material

this leads to faster heating and shorter processing times

39
Q

why aren’t microwaves used in processing? why are they not good for ensuring safety of a food?

A

they have uneven heat distribution, which leads to uneven lethality of MOs

40
Q

how does freezing affect MOs?

what does it not affect?

A

it stops the metabolic activity of most food-borne MOs

41
Q

what does freezing not affect?

A

does not affect:

  • cryophilic, psychrophilic and psychrotropic MOs
  • toxins

does affect:

  • chemical and biochemical deterioration (eg. lipid oxidation, maillard rxns, enzymatic browning, proteolysis, lipolysis)
  • water activity
42
Q

which survives freezing better?

a) gram-pos
b) gram-neg

A

gram-pos

43
Q

how does freezing affect parasites?

A

they easily destroy them

44
Q

compare rapid freezing go slow freezing

A

rapid freezing:

  • produces smaller ice crystals
  • exposes cells to osmotic stress for shorter time period

slow freezing
- more susceptible to injury and death

45
Q

what are 2 forms in which water can occur in?

A

bound

unbound

46
Q

what can water be bound by?

A
  1. hydroxyl groups
  2. carbonyl and amino acid groups of proteins
  3. salts
47
Q

what two aspects of water influences microbial activity of a food?

A

amount and form

48
Q

what form of water does MOs need for growth and survival?

A

unbound water

49
Q

what form of water can be removed or restricted to decrease microbial activity and extend pdt shelf life?

A

unbound water

50
Q

describe the water activity of a moist food within a package with headspace

what term describes this?

A

water leaves food into the surrounding atmosphere until equilibrium is reached between air and food

water vapor pressure (P)

51
Q

greater P = ____ (greater/lower) moisture content in package air

A

greater

52
Q

more water binding = ___ (higher/lower) P

A

lower

53
Q

what is the equation for Aw?

A

Aw = P / Po

Po = vapor pressure of pure water
P = vapor pressure at the same temp
54
Q

what is ERH? what is the equation?

A

equilibrium relative humidity: air surrounding the food

ERH (%) = Aw x 100

55
Q

describe the relation between Aw and moisture content of food

A

not linear. They are different

Aw: describes chemical and biological activities

moisture content: describes water present in the food

56
Q

what is the Aw for pure water?

what is the Aw for all other foods?

A

pure water: Aw = 1

other foods: Aw = 0-1

57
Q

what are 3 categories of food based on Aw?

for each, describe:

  • Aw
  • example
  • ability of MOs to grow
A
  1. high moisture foods
    - Aw > 0.9
    - eg. milk, meat, fresh fruits
    - foodborne microbes can readily grow
  2. intermediate moisture foods
    - Aw = 0.65-0.9
    - eg. raisins or jam
    - relatively resistant to microbial spoilage
  3. dry foods:
    - Aw < 0.65
    - eg. milk powder or crackers
    - don’t support growth of foodborne MOs; has long shelf life
58
Q

what do living cells rely on water for?

A
  • nutrient acquisition
  • waste removal
  • medium for metabolic rxns
59
Q

what is the range of Aw for:

a) spoilage and pathogenic bacteria
b) spore forming bacteria
c) S. aureus
d) yeasts and molds

A

a) Aw > 0.95 (very high)
b) Aw = 0.91-0.95
c) Aw >0.86
d) lowest Aw reqs than bacteria

60
Q

describe the growth of molds in terms of Aw

A
  • xerotolerant (grows in “dry” conditions)
  • grows at lower moisture contents than other spoilage MOs
  • can produce mycotoxins under xerophilic conditions
61
Q

what can be used to lower the Aw of a food?

A

humectants

62
Q

Does lowering Aw kill MOs in food?

what happens when Aw is increased again?

what is an example of this?

A
  • lowering Aw may inhibit microbial growth, but may not kill MOs in food
  • MOs may be dormant until food is rehydrated, then resume metabolic activity and multiply
  • eg. salmonella in chocolate bars
63
Q

describe drying and how it decreases water availability

A
  • the process of mobilizing h2o present in a food matrix to its surface, then removing it from the surface by evaporation
  • involves heat and mass transfer
64
Q

describe freeze-drying and how it decreases water availability

A
  • pdt is frozen first, then moisture is removed via sublimination
  • minimal impact on structure and flavor
  • high cost (used as a value added technique)
65
Q

describe osmotic dehydration and how it decreases water availability

A
  • involves removing h2o from fresh food w/ a hypertonic solution (sucrose, salts, glycerol)
  • food cells permit water out of food w/o surrounding solutes entering cells
  • final result: food is mostly dehydrated
  • can be used as a pre-treatment to other methods of dehydration
66
Q

what are 3 methods of preservation by decreasing water availability?

A
  1. drying
  2. freeze drying
  3. osmotic dehydation
67
Q

describe preservation by ionizing radiation

A
  • uses electromagnetic radiation with shorter wavelengths (such as gamma radiation), which have lethal effects on MOs
  • primary target is the cell’s DNA (smaller genome = greater resistance to ionizing radiation) by using reactive oxygen species (ROS) generated during radiation
68
Q

what is an example of short and long wavelengths?

which is more effective at killing MOs?

A

short: gamma radiation
long: microwaves

longer wavelengths just generate heat = less effective

69
Q

smaller genome in cell = ____ (greater/lower) resistance to ionizing radiation

A

greater

70
Q

what is ROS?

how do they function in ionizing radiation?

A

reactive oxygen species

they are generated during radiation and causes DNA damage to MOs

71
Q

how are x rays produced?

A

when an electron beam strikes a metal

72
Q

what are X rays effective at treating? why?

A
  • fresh fruit and veg for microbial lethality

- b/c X rays for food are restricted to 5MeV or less

73
Q

what is a drawback of X rays?

A
  • requires high power

- requires cooling treatment

74
Q

how is gamma radiation emitted?

A

emitted by an excited nucleus of a radioisotope, causing the nucleus to go to its ground state (lowest energy)

75
Q

what are radiation sources for gamma radiation?

how are they stored?

A

cobalt 60
cesium 137

stored: under water
during treatment: in concrete and steal

76
Q

what is an advantage of gamma radiation?(3)

A
  • high penetration power
  • constant energy throughout treatment
  • continuous source that doesn’t need to be turned off
77
Q

what is:

a) redappertization
b) radicidation
c) radurization

A

a) redappertization: high dose; produced commercial sterility
b) radicidation: medium dose; produces thermal pasteruization; effective against vegetative bacteria but not spores
c) radurization: low dose; controls insects in grains or sprouting of potatoes

78
Q

what is HPP? describe it.

A

high pressure processing

  • treats food with pressure
  • inactivates most bacteria, yeasts, molds, viruses
  • does not affect spores
  • may cause deformation in food materials with air pockets due to compression
79
Q

what is PEF processing? describe it

A

pulsed electric field

  • treats pumpable food
  • involves short burst of high voltage to a pdt between two electrodes
  • eliminates vegetative bacterial cells
  • need to know food comp, pH and electrical conductivity for successful treatment
  • should not have air bubbles in food

5) Food Microbiology (21 decks)

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