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

16) Physical Methods of Food Preservation Flashcards

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
compare F and D values
D: time to reduce population by 1 log F: time to get the population to a specific level (ie. sterile or 10^1)
26
what is the key pathogen of concern in canned low acid foods?
C. botulinum
27
the canning industry often calculates which value to achieve commercial sterility?
F value
28
how does sterility affect C.bot population?
should reduce C. bot by 12 log
29
define sterilization
process of rendering a pdt free of any living organism
30
what is the problem with sterilization of food pdts?
heating at very high temp can result in unacceptable quality
31
a food product that undergoes commercial sterilization will be free of... (2)
1. MOs capable of reproducing in the food under normal non-refrigerated storage and distribution temp 2. viable microbial cells of public health significance
32
what are mechanisms which cause bacteria to be more naturally resistant than others? (2)
1. protein cross-linking (disulfide bonding) | 2. DNA structural changes (higher GC)
33
which is more resistant to heat? a) stationary phase cells b) exponential phase cells
stationary
34
how does spore formation affect heat resistance
more spores = increased heat resistance
35
# define aseptic processing what is a benefit of this?
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
describe how aseptic processing is done
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
what is aseptic processing used for and not used for?
used for: fruit juices, dairy pdts, sauces not used for: low acid or particulate containing foods
38
describe how microwaving works what are advantages of this?
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
why aren't microwaves used in processing? why are they not good for ensuring safety of a food?
they have uneven heat distribution, which leads to uneven lethality of MOs
40
how does freezing affect MOs? what does it not affect?
it stops the metabolic activity of most food-borne MOs
41
what does freezing not affect?
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
which survives freezing better? a) gram-pos b) gram-neg
gram-pos
43
how does freezing affect parasites?
they easily destroy them
44
compare rapid freezing go slow freezing
rapid freezing: - produces smaller ice crystals - exposes cells to osmotic stress for shorter time period slow freezing - more susceptible to injury and death
45
what are 2 forms in which water can occur in?
bound unbound
46
what can water be bound by?
1. hydroxyl groups 2. carbonyl and amino acid groups of proteins 3. salts
47
what two aspects of water influences microbial activity of a food?
amount and form
48
what form of water does MOs need for growth and survival?
unbound water
49
what form of water can be removed or restricted to decrease microbial activity and extend pdt shelf life?
unbound water
50
describe the water activity of a moist food within a package with headspace what term describes this?
water leaves food into the surrounding atmosphere until equilibrium is reached between air and food water vapor pressure (P)
51
greater P = ____ (greater/lower) moisture content in package air
greater
52
more water binding = ___ (higher/lower) P
lower
53
what is the equation for Aw?
Aw = P / Po ``` Po = vapor pressure of pure water P = vapor pressure at the same temp ```
54
what is ERH? what is the equation?
equilibrium relative humidity: air surrounding the food ERH (%) = Aw x 100
55
describe the relation between Aw and moisture content of food
not linear. They are different Aw: describes chemical and biological activities moisture content: describes water present in the food
56
what is the Aw for pure water? what is the Aw for all other foods?
pure water: Aw = 1 other foods: Aw = 0-1
57
what are 3 categories of food based on Aw? for each, describe: - Aw - example - ability of MOs to grow
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
what do living cells rely on water for?
- nutrient acquisition - waste removal - medium for metabolic rxns
59
what is the range of Aw for: a) spoilage and pathogenic bacteria b) spore forming bacteria c) S. aureus d) yeasts and molds
a) Aw > 0.95 (very high) b) Aw = 0.91-0.95 c) Aw >0.86 d) lowest Aw reqs than bacteria
60
describe the growth of molds in terms of Aw
- xerotolerant (grows in "dry" conditions) - grows at lower moisture contents than other spoilage MOs - can produce mycotoxins under xerophilic conditions
61
what can be used to lower the Aw of a food?
humectants
62
Does lowering Aw kill MOs in food? what happens when Aw is increased again? what is an example of this?
- 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
describe drying and how it decreases water availability
- 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
describe freeze-drying and how it decreases water availability
- 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
describe osmotic dehydration and how it decreases water availability
- 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
what are 3 methods of preservation by decreasing water availability?
1. drying 2. freeze drying 3. osmotic dehydation
67
describe preservation by ionizing radiation
- 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
what is an example of short and long wavelengths? which is more effective at killing MOs?
short: gamma radiation long: microwaves longer wavelengths just generate heat = less effective
69
smaller genome in cell = ____ (greater/lower) resistance to ionizing radiation
greater
70
what is ROS? how do they function in ionizing radiation?
reactive oxygen species they are generated during radiation and causes DNA damage to MOs
71
how are x rays produced?
when an electron beam strikes a metal
72
what are X rays effective at treating? why?
- fresh fruit and veg for microbial lethality | - b/c X rays for food are restricted to 5MeV or less
73
what is a drawback of X rays?
- requires high power | - requires cooling treatment
74
how is gamma radiation emitted?
emitted by an excited nucleus of a radioisotope, causing the nucleus to go to its ground state (lowest energy)
75
what are radiation sources for gamma radiation? how are they stored?
cobalt 60 cesium 137 stored: under water during treatment: in concrete and steal
76
what is an advantage of gamma radiation?(3)
- high penetration power - constant energy throughout treatment - continuous source that doesn't need to be turned off
77
what is: a) redappertization b) radicidation c) radurization
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
what is HPP? describe it.
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
what is PEF processing? describe it
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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