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4) Food Processing > Lecture 9: Low Temperature Preservation and Freezing > Flashcards

Lecture 9: Low Temperature Preservation and Freezing Flashcards

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1
Q

define low temperature preservation

A

extending shelf life through lowering temperature

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2
Q

what are 2 techniques of low temp preservation? describe each one

A

refrigeration

  • short term solution (days to months)
  • temp: 0-15degC
  • causes moderate physiological changes in tissue structure
  • high free H2O availability

freezing

  • long term storage (months to years)
  • temp: -5 to -40degC
  • causes extensive physical changes
  • no free H2O available
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3
Q

describe the microbial activity of refrigeration compared to freezing

A

refrigeration:

  • microbial, enzymatic, chemical, physiological: moderate
  • respiratory: anaerobic/aerobic

freezing: microbial, enzymatic, chemical, physiological, respiratory: low to none

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4
Q

what temperature is the zone of spoilage w/ no danger to health?

A

4.5 to -10degC

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5
Q

what temp is there RAPID growth of food poisoning organisms? in psychotropic organisms?

A

food poisoning org: above 10degC

psychotropic org: above 4.5degC

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6
Q

what temp is there NO growth of food poisoning organisms? in psychotropic organisms?

A

food poisoning org: below 4.5degC

psychotropic org: below -10degC

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7
Q

compare respiration of phytosystems (plants) to myosystems (animals)

A

phytosystem:

  • aerobic/anaerobic
  • need to maintain
  • softening tissues
  • heat of respiration must be removed

myosystems:

  • aerobic
  • need to suppress
  • rigor mortis
  • tenderization
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8
Q

what is rigor mortis?

A

stiffening of muscle, which needs to be removed by tenderization

occurs after slaughtering

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9
Q

is transpiration desirable in refridgerated storage?

A

no, there should be relative humidity control

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10
Q

what is MAP?

A

modified atmosphere packaging

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11
Q

describe changes muscle tissue characteristics at post-mortem

A
  1. no blood circulation right after slaughtering
  2. anaerobic glycolysis occurs until respiration activity stops after 36 hrs
  3. pH drops (to 5-6) due to lactic acid production
  4. rigor mortis is correlated with loss of glycogen and disappearance of ATP
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12
Q

compare muscle tissue characteristic graphs of colder temp and at warmer temp

A

colder texture tension increases MUCH higher

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13
Q

what are sous-vide products?

what are potential health risks?

A
  • vacuum cooked
  • refridgerated
  • high quality due to minimal heat treatment
  • gourmet type products
  • short distribution time: can be dangerous b/c may cause growth of microorganisms
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14
Q

what is hurdle technology?

A

method of eliminating pathogens in food by lactic acid bacteria, water activity level, antibacterial agents, temperature, salt, MAP (CO2), etc…

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15
Q

define food freezing.

what changes occur?

A
  • lowering the temp below freezing point, which induces crystallization of part of the water and some of the solutes
  • results in change of state and formation of ice crystals
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16
Q

what is the major difference between refrigerated and frozen foods?

A

frozen foods have formation of ice crystals

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17
Q

in the freezing process, how are the phase changes of food different than of water?

A
  • water has very sharp changes

- food changes are slower and does not stay at 0degC or any specific temperature

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18
Q

what are the 3 stages of the freezing process?

A

pre-cooling
phase change
tempering

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19
Q

what is supercooling?

how does it affect food quality?

what is it favoured by?

A
  • occurs before freezing (right before freezing point w/ solidification of crystallization)
  • has almost no effect on food quality
  • favoured by presence of crystals of insoluble salts
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20
Q

what is crystallization?

what two processes characterize it?

A
  • formation of systematically organized solid phase from a solution, melt or vapor

characterized by:

  1. formation of nuclei or nucleation
  2. ripening of ice crystals or crystal growth
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21
Q

what is nucleation?

A

process of combining molecules into an ordered particle of sufficient size to serve as a site for crystal growth

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22
Q

what two factors affect nucleation rates?

A
  1. temperature (lower = better)

2. freezing rate (faster = better)

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23
Q

what are 2 types of nucleation?

when does each one occur?

A
  1. homogenous nucleation: occurs in pure H2O

2. heterogenous nucleation: occurs mostly in foods, when H2O aggregates w/ other solutes

24
Q

what does crystal growth rate depend on?

A

freezing rate

faster crystal growth occurs at slower freezing rates and near freezing temp

25
Q

lower temp and faster freezing rates favour ____ number and ____ size crystals

A

larger

smaller

26
Q

temp near the FP and slower freezing rates favour ____ numbers and ____ size crystals

A

smaller

larger

27
Q

crystal size varies _____ w/ the number of nuclei formed

A

inversely

28
Q

what 3 factors does ice crystal location depend on?

A
  1. freezing rate
  2. temp
  3. nature of cells
29
Q

in slow freezing, where do ice crystals form?

what does this cause (in terms of quality damage)?

A

in extracellular locations only

causes more damage to food quality

  • causes max dislocation of H2O
  • shrunken appearance
  • less desirable food quality
  • large drip loss when thawing
  • high local solute concentration
30
Q

in rapid freezing, where do ice crystals form?

how does this affect quality damage compared to slow freezing?

A

there is uniform distribution of ice crystals (intra and extra cellular)

causes less damage to food texture than slow freezing

  • little water dislocation
  • tissues are less damaged
  • muscle fibers (sarcolemma) are not damaged
  • lower drip loss when thawing
31
Q

which freezing condition is least disruptive to cell structure?

A

rapid freezing

  • causes firmer texture in thawed product
  • favoured by formation of a large number of well distributed smaller size ice crystals
32
Q

in rapid freezing, how is crystal growth affected by crystal formation?

what is the result of this?

A

growth is suppressed by formation

result: large umber of small crystals

33
Q

how is the freezing point of pure water different than FP in presence of solutes?

A

pure h2O: 0degC

H2O with solutes: higher BP and lower FP

34
Q

what is the equation of FP depression?

A

delta Tf = Kf * m

where Kf is molar depression constant
m is molarity of solute

35
Q

ionic solutes depress FP ____ (more/less) than non-ionic solutes

A

more

36
Q

what is the initial FP of most foods?

A

-1 to -2 degC

37
Q

foods don’t have a sharp FP, but freezes over _____

what is this called?

A

a temperature range (-1 to -5C)

caled the zone of max ice crystal formation

38
Q

as H2O starts to freeze, the conc of dissolved solids __ (inc/dec)

this causes the FP to ____ (inc/dec)

A

increase

decrease

39
Q

why can’t all H2O in food be frozen?

A

since some are bound to solutes even at low temp, which remain in liquid state as bound H2O

40
Q

how does freezing affect the density?

A
  • freezing results in volume expansion, since ice occupies more volume than water

thus, density of ice < density of water

41
Q

which has a higher density?
A) water
B) ice

A

water

42
Q

what is the heat capacity equation?

A

Q= m * Cp * deltaT

43
Q

define heat capacity

A

heat required to raise T of a unit mass by one degree

44
Q

how does Cp of ice compare to Cp of water and Cp of food?

A

water: 4.186 KJ/Kg deg C
ice: 2.1 KJ/Kg deg C
food: depends on composition

45
Q

what are the four models that are used to estimate Cp in food based on composition?

A

siebel

dickerson

charm

heldman and singh

46
Q

which two models are used to estimate Cp in foods for high moisture?

which two are more versatile?

A

siebel and dickerso: simpler and accurate for high moisture

charm and helman/singh: more versatile

47
Q

what is the unit of heat capacity?

A

Kg/Kg deg C

48
Q

what is the unit for enthalpy and latent heat?

A

KJ/Kg

49
Q

what are units for thermal conductivity?

A

W/ (m*degC)

50
Q

what is the equation of thermal conductivity?

A

Q = (k * A * delta T) / x

51
Q

what is thermal conductivity?

what does density x Cp represent?

A

thermal conductivity: ability to conduct heat

density x Cp = ability to absorb heat

52
Q

what is the equation of thermal diffusivity?

A

alpha = k / (p * Cp)

k = thermal conductivity
p = density
Cp = heat capacity
53
Q

what is the significance of thermal diffusivity?

A

can be used to determine the ability to respond to temp changes and ability to undergo temp change

54
Q

if thermal diffusivity is high, the food responds ____ to temp change

A

fast

55
Q

if thermal diffusivity is low, the food responds ____ to temp change

A

slow

56
Q

how does the thermal diffusivity of ice compare to that of water?

what does this represent?

A

thermal diffusivity is 9 times higher

thus, ice responds to temp changes 9 times faster than water

removal and addition of heat through ice is 9 times faster than through water

4) Food Processing (18 decks)

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