Lecture 9: Low Temperature Preservation and Freezing Flashcards
define low temperature preservation
extending shelf life through lowering temperature
what are 2 techniques of low temp preservation? describe each one
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
describe the microbial activity of refrigeration compared to freezing
refrigeration:
- microbial, enzymatic, chemical, physiological: moderate
- respiratory: anaerobic/aerobic
freezing: microbial, enzymatic, chemical, physiological, respiratory: low to none
what temperature is the zone of spoilage w/ no danger to health?
4.5 to -10degC
what temp is there RAPID growth of food poisoning organisms? in psychotropic organisms?
food poisoning org: above 10degC
psychotropic org: above 4.5degC
what temp is there NO growth of food poisoning organisms? in psychotropic organisms?
food poisoning org: below 4.5degC
psychotropic org: below -10degC
compare respiration of phytosystems (plants) to myosystems (animals)
phytosystem:
- aerobic/anaerobic
- need to maintain
- softening tissues
- heat of respiration must be removed
myosystems:
- aerobic
- need to suppress
- rigor mortis
- tenderization
what is rigor mortis?
stiffening of muscle, which needs to be removed by tenderization
occurs after slaughtering
is transpiration desirable in refridgerated storage?
no, there should be relative humidity control
what is MAP?
modified atmosphere packaging
describe changes muscle tissue characteristics at post-mortem
- no blood circulation right after slaughtering
- anaerobic glycolysis occurs until respiration activity stops after 36 hrs
- pH drops (to 5-6) due to lactic acid production
- rigor mortis is correlated with loss of glycogen and disappearance of ATP
compare muscle tissue characteristic graphs of colder temp and at warmer temp
colder texture tension increases MUCH higher
what are sous-vide products?
what are potential health risks?
- 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
what is hurdle technology?
method of eliminating pathogens in food by lactic acid bacteria, water activity level, antibacterial agents, temperature, salt, MAP (CO2), etc…
define food freezing.
what changes occur?
- 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
what is the major difference between refrigerated and frozen foods?
frozen foods have formation of ice crystals
in the freezing process, how are the phase changes of food different than of water?
- water has very sharp changes
- food changes are slower and does not stay at 0degC or any specific temperature
what are the 3 stages of the freezing process?
pre-cooling
phase change
tempering
what is supercooling?
how does it affect food quality?
what is it favoured by?
- 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
what is crystallization?
what two processes characterize it?
- formation of systematically organized solid phase from a solution, melt or vapor
characterized by:
- formation of nuclei or nucleation
- ripening of ice crystals or crystal growth
what is nucleation?
process of combining molecules into an ordered particle of sufficient size to serve as a site for crystal growth
what two factors affect nucleation rates?
- temperature (lower = better)
2. freezing rate (faster = better)
what are 2 types of nucleation?
when does each one occur?
- homogenous nucleation: occurs in pure H2O
2. heterogenous nucleation: occurs mostly in foods, when H2O aggregates w/ other solutes
what does crystal growth rate depend on?
freezing rate
faster crystal growth occurs at slower freezing rates and near freezing temp
lower temp and faster freezing rates favour ____ number and ____ size crystals
larger
smaller
temp near the FP and slower freezing rates favour ____ numbers and ____ size crystals
smaller
larger
crystal size varies _____ w/ the number of nuclei formed
inversely
what 3 factors does ice crystal location depend on?
- freezing rate
- temp
- nature of cells
in slow freezing, where do ice crystals form?
what does this cause (in terms of quality damage)?
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
in rapid freezing, where do ice crystals form?
how does this affect quality damage compared to slow freezing?
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
which freezing condition is least disruptive to cell structure?
rapid freezing
- causes firmer texture in thawed product
- favoured by formation of a large number of well distributed smaller size ice crystals
in rapid freezing, how is crystal growth affected by crystal formation?
what is the result of this?
growth is suppressed by formation
result: large umber of small crystals
how is the freezing point of pure water different than FP in presence of solutes?
pure h2O: 0degC
H2O with solutes: higher BP and lower FP
what is the equation of FP depression?
delta Tf = Kf * m
where Kf is molar depression constant
m is molarity of solute
ionic solutes depress FP ____ (more/less) than non-ionic solutes
more
what is the initial FP of most foods?
-1 to -2 degC
foods don’t have a sharp FP, but freezes over _____
what is this called?
a temperature range (-1 to -5C)
caled the zone of max ice crystal formation
as H2O starts to freeze, the conc of dissolved solids __ (inc/dec)
this causes the FP to ____ (inc/dec)
increase
decrease
why can’t all H2O in food be frozen?
since some are bound to solutes even at low temp, which remain in liquid state as bound H2O
how does freezing affect the density?
- freezing results in volume expansion, since ice occupies more volume than water
thus, density of ice < density of water
which has a higher density?
A) water
B) ice
water
what is the heat capacity equation?
Q= m * Cp * deltaT
define heat capacity
heat required to raise T of a unit mass by one degree
how does Cp of ice compare to Cp of water and Cp of food?
water: 4.186 KJ/Kg deg C
ice: 2.1 KJ/Kg deg C
food: depends on composition
what are the four models that are used to estimate Cp in food based on composition?
siebel
dickerson
charm
heldman and singh
which two models are used to estimate Cp in foods for high moisture?
which two are more versatile?
siebel and dickerso: simpler and accurate for high moisture
charm and helman/singh: more versatile
what is the unit of heat capacity?
Kg/Kg deg C
what is the unit for enthalpy and latent heat?
KJ/Kg
what are units for thermal conductivity?
W/ (m*degC)
what is the equation of thermal conductivity?
Q = (k * A * delta T) / x
what is thermal conductivity?
what does density x Cp represent?
thermal conductivity: ability to conduct heat
density x Cp = ability to absorb heat
what is the equation of thermal diffusivity?
alpha = k / (p * Cp)
k = thermal conductivity p = density Cp = heat capacity
what is the significance of thermal diffusivity?
can be used to determine the ability to respond to temp changes and ability to undergo temp change
if thermal diffusivity is high, the food responds ____ to temp change
fast
if thermal diffusivity is low, the food responds ____ to temp change
slow
how does the thermal diffusivity of ice compare to that of water?
what does this represent?
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
