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Food Microbio > Physical methods of food preservation > Flashcards

Physical methods of food preservation Flashcards

1
Q

originally, what were food preservation methods developed for?

A

to extend the shelf life of food products by protecting them from microbiological spoilage

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2
Q

what are modern preservation methods designed for?

A

extend shelf life, but also to ensure its safety by inactivating pathogens, or in some cases jus preventing their growth

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3
Q

what is the definition of bacterial inactivation?

A

destruction of the organism as judged by its inability to recover on microbiological media (destroy to the point if you plate it on media, it does not grow)

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4
Q

what are the most common food preservation methods?

A

physical methods

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5
Q

what are examples of physical preservation methods?

A
  • high-temperature treatment (cooking)
  • low-temperature preservation (freezing)
  • Decreasing Water Availability (i.e. drying, salting)
  • Ionizing Irradiation
  • High-Pressure Processing (HPP)
  • Pulsed Electric Field
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6
Q

what happens when the temperature exceeds the optimal temperature for an organism?

A

cell multiplication slows and eventually stops.
Additional temperature may result in cell death.
protein denaturation, collapse of the cytoplasmic membrane, thermal lysis

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7
Q

what happens when the temperature is very low?

A

organisms don’t die, their growth slows down (transport processes so slow) (membrane gelling, enzymatic reactions are slow)

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8
Q

freezing does not kill …. but kills ….

A

bacterial pathogens

but kills parasites

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9
Q

thermal processing kills ….

A

bacterial pathogens

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10
Q

what is a drawback of thermal processing? give example.

A

over heating may damage the product quality so process development and validation is important
e.g. chocolate gets burnt before you kill salmonella

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11
Q

what are 6 factors that affect heat transfer?

A
  1. product type
  2. container material
  3. container shape
  4. container size
  5. agitation
  6. temperature of heating medium
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12
Q

… absorb heat faster than … foods

A

liquids absorb heat faster than solid foods

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13
Q

… containers heat more slowly than … containers

A

glass containers heat more slowly than metal containers

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14
Q

.. and … containers heat more quickly than other shapes (heat o centre faster)

A

tall and narrow

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15
Q

the centre of … containers get reached more quickly Han the centre of … containers

A

small ; large

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16
Q

… and … help to increase heat transfer especially in viscous or semi-viscous products

A

agitation and mixing

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17
Q

the greater the difference in … between the heat transfer medium and the product, the more rapid the rate of heat transfer to the product (oil vs. air) (temperature of heating medium)

A

temperature

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18
Q

what are the major parameters that affect thermal treatments?

A

time and temperature

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19
Q

what are survivor plots?

A

plots depict the logarithmic nature of population inactivation over time

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20
Q

what is the decimal reduction time (D value)

A

the time it takes for a 10-fold reduction in the number of survivors at a given temperature
(it takes 6min [D value] to reduce the bacterial population from 1000 to 100 a 120C)

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21
Q

what does a big D value at a given temperature mean about an organism?

A

the greater the D value, the more resistant that organism is to heat, at a given temperature.

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22
Q

heat doesn’t kill all organisms, it is just a …..

A

log reduction

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23
Q

the D value of a microorganisms … as the processing temperature …

A

decreases

increases

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24
Q

what is a thermal resistant plot?

A

when D value is plotted against temperature. it is generally linear and is useful or comparing the different resistances of a microorganism at different temperatures

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25
Q

what is calculated from the thermal resistant plot? what is it?

A 
z value (thermal resistance constant), mathematically equal to the negative reciprocal of the slope 
=> represents the change in temperature required to change the D value of a microorganism by an order of magnitude (10-fold)
(at 90C it takes 20min, at 100C it takes 3min, at 110C it take 0.3min => z value=10C
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26
Q

what does a large z-value indicate?

A

a more resistant organism

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27
Q

why are thermal lethality measurements taken?

A

to determine the amount of time required to commercially sterilize food at a given temperature

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28
Q

what is the F value?

A

the time, in minutes, at a specified temperature, required to achieve a targeted reduction in a homogenous population having a specific z value

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29
Q

what is the difference between the F value and the D value?

A

D value defines the time it takes to reduce the population by 1 log, F value represents the time it takes to get the population to a specific level (i.e. 10^1 or sterile)
F-value is how much time did it take to go from this many to this many

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30
Q

It is common for the canning industry to calculate … with the time at a specific temperature to achieve commercial sterility

A

F value

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31
Q

what does the canning industry assume?

A

It is assumed that this process can reduce a C. botulinum population by 12 log

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32
Q

…. is the key pathogen of concern for canned low acid foods

A

Clostridium botulinum

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33
Q

what is the D value of C. botulinum at 121C? and z value?

A

D121C value of 0.21 minutes, and a Z value of 10C.

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34
Q

what does it mean for C botulinum to have a D121C value of 0.21 minutes and a z value of 10C?

A

to achieve commercial sterility at F121C you multiply 0.21 minutes by 12 which equals 2.52 minutes

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35
Q

what is sterilization?

A

Sterilization is the process of rendering a product free of any living organisms
(autoclaving)
Application of sterilization to food would require exposing products to excessive heating and result in products with unacceptable quality

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36
Q

what is commercial sterilization?

A

Commercial sterility of food products is the application of heat that renders the food free of:
1. Microorganisms capable of reproducing in the food under normal non-refrigerated storage and distribution temperatures
2. Viable microbial cells or spores of public health significance (any pathogen is destroyed)
Commercially sterile foods are heat treated just enough to produce a safe and shelf-stable product (not microbiologically sterile)

37
Q

what makes a bacteria more naturally resistant than others to heat?

A
  • protein cross-linking (disulphide bonding - more heat resistant if there are many)
  • DNA structural changes (higher amounts of G and C [3 bonds] - holds DNA better together)
38
Q

… phase cells are more resistant to heat than … phase cells

A

stationary

exponential

39
Q

… formation dramatically increases heat resistance

A

spore

40
Q

state of … is an important factor in heat resistance

A

dormancy

41
Q

what food characteristics influence heat resistance?

A
  • water activity
  • pH
  • antimicrobial constituents
    (low pH, low water activity requires less heat cause bacteria is already stressed-out)
42
Q

what is aseptic processing?

+ uses

A

used industrially, where foods and cans are sterilized separately and then foods are packaged under aseptic/sterile conditions (hermetically sealed cans)
generally used for fruit juices, dairy products and sauces NOT for low-acid, or particulate-containing foods

43
Q

what are the advantages of aseptic processing?

A

avoids having to heat the food to excess to ensure that the centre of the can reaches temperature, and improves food quality and nutrition

44
Q

explain process of aseptic processing

A
  • product is heated by passing through a set of heat exchangers until the holding time and temperature have been reached, then the food is immediately passed through a set of cooling heat exchangers
  • The cooled product is placed in pre-sterilized packages and hermetically sealed
45
Q

why aren’t microwaves used in processing, and why aren’t they a good idea for foods that require cooking for safety?

A
  • Microwaves cause rotation of water molecules in foods, and these rotations generate friction and therefore heat. Unlike convectional heat, which penetrates from the outside inward, microwaves generate heat through out the material leading to faster heating and shorter processing times
  • Distribution of the heat is very uneven, leading to uneven lethality of microorganisms (don’t microwave your chicken fingers) => cold spots allow MO to survive
  • This has hindered the use of the microwave in commercial food processing.
46
Q

how does freezing act on food-borne pathogens

A

sops the metabolic activity of most food-borne pathogens

47
Q

which food microbiota will still alter foods at sub-zero?

A
  • cryophilic
  • psychrophilic
  • psychrotropic
48
Q

… are generally not affected by freezing

A

toxins

49
Q

freezing retards chemical and biochemical deterioration such as: (5)

A
  • lipid oxidation,
  • maillard reactions
  • enzymatic browning
  • proteolysis
  • lipolysis
50
Q

freezing decreases the …. .

what impact does that have?

A

water activity

only cold-tolerant and xerotolerant microorganisms can grow, but some yeast and fungi have these characteristics

51
Q

Gram-… bacteria survive freezing better than gram-… bacteria

A

positive ; negative

52
Q

……. (pork worm- parasite) is eliminated if the raw product is held briefly at -23C

A

Trichinella spiralis

53
Q

which is more safe: home freezing or commercial freezing? why?

A

Rapid freezing (commercial) produces smaller ice crystals (cells don’t get punctured), and exposes cells to osmotic stress for shorter time periods, therefore, microbial cells are more susceptible to injury and death during slow freezing (like in your home freezer)

54
Q

what are the states of water found in food?

A

bound (by Hydroxyl groups, Carbonyl and amino acid groups of proteins, or Salts) or unbound

55
Q

… water is essential for microbial growth and survival

A

unbound

56
Q

removing or restricting …. water can preserve the food. how?

A

unbound

by suppressing microbial activity and extending the product’s shelf life

57
Q

moisture content vs. water availability

A

High moisture content but low water availability (due to high content of bound water)

58
Q

what is the food’s vapour pressure P?

A

If a moist food is contained within a package with headspace the water molecules will eventually leave the food and enter the surrounding atmosphere until a state of equilibrium is obtained with the air and surrounding food

59
Q

the greater the P, the … its moisture content

A

greater

60
Q

formula of water activity

A

aw=P/P0

P0: equilibrium achieved with Pure water

61
Q

values of water activity aw

A

pure water: aw= 1

other foods aw= 0 to 1

62
Q

aw is directly related to the ……. in the surrounding air. give the formula

A

equilibrium relative humidity

ERH(%) = aw x 100

63
Q

chemical and biological activities are better described by which parameter?

A

water activity

64
Q

what are the three categories of foods based on their water activity?

A
  1. high-moisture foods (foods with a water activity > 0.9 like milk, meat, fresh fruit/veg -> foodborne microbes can readily grow in these foods)
  2. Intermediate-moisture Foods (Foods with an aw 0.65-0.90, such as raisins or jam, because of the moderately low aw these foods are relatively resistant to microbial spoilage)
  3. Dry Foods (Foods with an aw < 0.65, such as milk powder or crackers -> these foods do not support the growth of foodborne microorganisms and have a very long shelf life)
65
Q

what is the water activity of properly prepared maple syrup?
what is the Brix (sugar concentration) of properly prepared maple syrup?

A

0.87-0.88 making it an intermediate-moisture food

Brix= 66

66
Q

how is maple syrup prepared?

A

boiling the sap from sugar maple trees until the boiling temperature is 4.1C higher than that of pure water (which will vary based on air pressure but is around 100C) = 104.1C

67
Q

… and … tend to need a very high water activity for growth (>0.95)

A

pathogenic and spoilage bacteria

68
Q

…. can grow in the range of 0.91-0.95

A

Spore forming bacteria, L. monocytogenes, and V. parahaemolyticus

69
Q

… has the lowest water requirements and can grow with an aw of >0.86. However, it cannot produce toxins at this level

A

S. aureus

70
Q

… have a water activity requirements that are lower than those of …

A

yeasts ; bacteria

71
Q

… are xerotolerant and grow at Lower moisture contents than the other types of spoilage microorganism

A

molds

72
Q

mycotoxin production is restricted under …. conditions

A

xerophilic (low water activity)

73
Q

Growth of microorganisms is restricted not only by the aw but also based on which … was used to lower the aw

A

humectant (binds water)

glycerol vs sodium chloride or sucrose

74
Q

does lowering water activity kill the microorganisms in the food?

A

No, These microbes may remain dormant until the food is rehydrated and then resume their metabolic activity and multiply (i.e. Salmonella in chocolate bars)

75
Q

what is drying?

A

the process of mobilizing water present in a food matrix to its surface and then removing it from the surface by evaporation. It often involves simultaneous heat and mass transfer. Most drying operations involve converting liquid water in the material to vapor and then removing it by passing hot air over the product (i.e. beef jerky).

76
Q

what is freeze-drying?

A

During freeze-drying the product is frozen first and the moisture is removed via sublimation. Freeze-drying has a minimal impact on the structure and flavor of the food compared with other drying techniques, but has a relatively high cost, and is therefore used as a value added technique (i.e. camp dinners, instant coffee, infant formulas).

77
Q

what is osmotic dehydration?

A

a procedure that involves removing water from fresh food using a hypertonic solutions (sucrose, salts, glycerol). It relies on the ability of the food cells to selectively permit the passage of water out of the food without allowing surrounding solutes to enter these cells. As a result food is dehydrated. It cannot remove all the moisture from the food, so it is mainly used as a pretreatment to other methods of dehydration. Primarily used for dehydrating fruits and vegetables (apples, blueberries, pineapples, mangoes etc.).

78
Q

electromagnetic radiation with ….. has lethal effects on MO

A

shorter wavelengths (gamma radiation)

79
Q

examples of ionizing radiations

A

X rays and gamma radiation

80
Q

what is the primary target of ionizing radiation? which MO are more resistant

A

the cell’s DNA

MO with smaller genomes are more resistant to these radiations

81
Q

how is DNA damage caused by ionizing radiation?

A

caused by the reaction of reactive oxygen species (ROS) that are generated during radiation

82
Q

what is the drawback of using X-rays?

A

have a high-power requirement, and have a high requirement for cooling treatment (most of the energy is lost as heat)

83
Q

what is a gamma radiation and what is its advantage?

A

Gamma radiation is emitted by an excited nucleus of a radioisotope, which permits the nucleus to go to its lowest energy or ground state
The advantage is radiation has a high penetration power, and the energy is constant over the duration of the treatment, it is a continuous source that doesn’t need to be turned off

84
Q

what is the radiation source for gamma radiation?

A

cobalt-60, and cesium-137

85
Q

what are the 3 different dosage levels used to produce different levels of food preservation. how are they selected?

A
  • Redappertization. Involves a high dose (10 to 75 kGy) to produce commercial sterility
  • Radicidation. Involves medium dose (1 to 10 kGy) to produce an equivalent effect to thermal pasteurization – effective against vegetative bacteria but spores are not inactivated
  • Radurization. Involves low dose irradiation (0.05 to 1 kGy) to control the presence of insects in grains or stop the sprouting of potatoes, or delay fruit ripening
    => selected based in the end objective for the food
86
Q

what are high-pressure processing and pulsed electric field preservation methods?

A

identifying minimal preservation technologies to produce food that are microbiologically safe, but have the desirable “fresh” quality

87
Q

what is High-Pressure Processing? what do we need to look out for?

A
  • involves treating food with pressures in the range of 100 to 600 MPa, which will inactivate most bacteria, yeasts, molds, and viruses (spores survive)
  • Food materials containing air pockets may be deformed due to the differences in compressibility between the air and food material + Appropriate packaging must also be chosen (flexible plastic not glass jar)
88
Q

how does pulsed electric preservation method work? what influences the success of this treatment?

A
  • designed to treat pumpable food and involves the application of a short burst of high voltage to a product between two electrodes
    This treatment will eliminate vegetative bacterial cells
  • relies on food composition, pH, and electrical conductivity
    Air bubbles are also poor conductors and can create process non-uniformity (i.e. pumpable food)

Food Microbio (11 decks)

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