Lecture 9: Low Temperature Preservation and Freezing

Question 1

define low temperature preservation

Answer 1

extending shelf life through lowering temperature

Question 2

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

Answer 2

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

Question 3

describe the microbial activity of refrigeration compared to freezing

Answer 3

refrigeration:

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

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

Question 4

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

Answer 4

4.5 to -10degC

Question 5

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

Answer 5

food poisoning org: above 10degC

psychotropic org: above 4.5degC

Question 6

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

Answer 6

food poisoning org: below 4.5degC

psychotropic org: below -10degC

Question 7

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

Answer 7

phytosystem:

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

myosystems:

• aerobic
• need to suppress
• rigor mortis
• tenderization

Question 8

what is rigor mortis?

Answer 8

stiffening of muscle, which needs to be removed by tenderization

occurs after slaughtering

Question 9

is transpiration desirable in refridgerated storage?

Answer 9

no, there should be relative humidity control

Question 10

what is MAP?

Answer 10

modified atmosphere packaging

Question 11

describe changes muscle tissue characteristics at post-mortem

Answer 11

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

Question 12

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

Answer 12

colder texture tension increases MUCH higher

Question 13

what are sous-vide products?

what are potential health risks?

Answer 13

• 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

Question 14

what is hurdle technology?

Answer 14

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

Question 15

define food freezing.

what changes occur?

Answer 15

• 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

Question 16

what is the major difference between refrigerated and frozen foods?

Answer 16

frozen foods have formation of ice crystals

Question 17

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

Answer 17

• water has very sharp changes

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

Question 18

what are the 3 stages of the freezing process?

Answer 18

pre-cooling
phase change
tempering

Question 19

what is supercooling?

how does it affect food quality?

what is it favoured by?

Answer 19

• 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

Question 20

what is crystallization?

what two processes characterize it?

Answer 20

• 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

Question 21

what is nucleation?

Answer 21

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

Question 22

what two factors affect nucleation rates?

Answer 22

1. temperature (lower = better)

2. freezing rate (faster = better)

Question 23

what are 2 types of nucleation?

when does each one occur?

Answer 23

1. homogenous nucleation: occurs in pure H2O

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

Question 24

what does crystal growth rate depend on?

Answer 24

freezing rate

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

Question 25

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

Answer 25

larger smaller

Question 26

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

Answer 26

smaller larger

Question 27

crystal size varies _____ w/ the number of nuclei formed

Answer 27

inversely

Question 28

what 3 factors does ice crystal location depend on?

Answer 28

1. freezing rate 2. temp 3. nature of cells

Question 29

in slow freezing, where do ice crystals form? what does this cause (in terms of quality damage)?

Answer 29

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

Question 30

in rapid freezing, where do ice crystals form? how does this affect quality damage compared to slow freezing?

Answer 30

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

Question 31

which freezing condition is least disruptive to cell structure?

Answer 31

rapid freezing - causes firmer texture in thawed product - favoured by formation of a large number of well distributed smaller size ice crystals

Question 32

in rapid freezing, how is crystal growth affected by crystal formation? what is the result of this?

Answer 32

growth is suppressed by formation result: large umber of small crystals

Question 33

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

Answer 33

pure h2O: 0degC H2O with solutes: higher BP and lower FP

Question 34

what is the equation of FP depression?

Answer 34

delta Tf = Kf * m where Kf is molar depression constant m is molarity of solute

Question 35

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

Answer 35

more

Question 36

what is the initial FP of most foods?

Answer 36

-1 to -2 degC

Question 37

foods don't have a sharp FP, but freezes over _____ what is this called?

Answer 37

a temperature range (-1 to -5C) caled the zone of max ice crystal formation

Question 38

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

Answer 38

increase decrease

Question 39

why can't all H2O in food be frozen?

Answer 39

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

Question 40

how does freezing affect the density?

Answer 40

- freezing results in volume expansion, since ice occupies more volume than water thus, density of ice < density of water

Question 41

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

Answer 41

water

Question 42

what is the heat capacity equation?

Answer 42

Q= m * Cp * deltaT

Question 43

define heat capacity

Answer 43

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

Question 44

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

Answer 44

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

Question 45

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

Answer 45

siebel dickerson charm heldman and singh

Question 46

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

Answer 46

siebel and dickerso: simpler and accurate for high moisture charm and helman/singh: more versatile

Question 47

what is the unit of heat capacity?

Answer 47

Kg/Kg deg C

Question 48

what is the unit for enthalpy and latent heat?

Answer 48

KJ/Kg

Question 49

what are units for thermal conductivity?

Answer 49

W/ (m*degC)

Question 50

what is the equation of thermal conductivity?

Answer 50

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

Question 51

what is thermal conductivity? what does density x Cp represent?

Answer 51

thermal conductivity: ability to conduct heat density x Cp = ability to absorb heat

Question 52

what is the equation of thermal diffusivity?

Answer 52

alpha = k / (p * Cp) ``` k = thermal conductivity p = density Cp = heat capacity ```

Question 53

what is the significance of thermal diffusivity?

Answer 53

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

Question 54

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

Answer 54

fast

Question 55

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

Answer 55

slow

Question 56

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

Answer 56

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