Refrigeration/Freezing Flashcards
2 techniques for low temp. preservation:
- refrigeration
- freezing
What are some deteriorative activities occuring in food? (5)
enzymatic breakdown microbe growth physiological activity chemical rxn respiration
How does the rates of deteriorative activity compare in frozen vs refrigerated vs fresh?
fresh: full activity
refrigerated: moderate activity (can have full physiological activity/respiration)
frozen: none or very low levels
What are the temp ranges for refrigerated and frozen?
refrigerated: 0-15C
frozen: -5 to -40C
What factors differ between frozen and refrigerated storage? (6)
- time of storage
- temperature
- atmosphere
- availability of free water
- amount of deteriorative activity
- effect on tissue structure
How do atmospheric conditions compare for frozen vs refrigerated storage?
frozen: no need to control atmosphere (just RA)
refrigerated: CA/MA/RA - need to control environment to limit deterioration
What are some pros and cons of freezing rather than refrigerating?
pros: longer storage, slower deterioration, less microbe growth, no need for modified atmosphere
cons: more tissue damage, water loss
What type of changes in tissue structure are caused by frozen vs refrigerated storage?
refrigerated: moderate, physiological
frozen: extensive, physical
True/False: free water content in food is reduced by refrigerating.
False; free water activity is full
Why is controlling the atmosphere not necessary in frozen storage?
less water activity and very low respiratory rates will already slow deterioration significantly.
True/False: in frozen food, the water activity is nearly 0.
True, free water converted to ice
Can bacteria be killed at low temperatures? Can this be used to make food safe?
Yes, but slowly. No, too slow and is rarely completely killed.
When holding produce in refrigerated storage, ____ respiration should be maintained.
aerobic
The temp range -10 to 10C is known as: _____.
Why?
zone of spoilage without danger to health
pathogen growth is limited/none, so is safe for health; but psychrophiles may still grow and cause deterioration
What happens above/below the “zone of spoilage without danger to health?”
above: rapid growth of pathogens, food quickly spoils and is also not safe
below: almost no growth of pathogens or spoilage microbes - safe and long term preservation
_____ is moisture loss, and is prevented by:
transpiration; controlling RH
What are the modified atmospheric conditions used for myosystems vs phytosystems in refrigeration? Why?
phytosystem: increase CO2:O2 ratio to suppress rates of aerobic respiration (but want to maintain some!) - to prevent over softening and generating too much heat
myosystem: use CO2 or NO2: Oxygen not needed since ANAEROBIC respiration, so use inert gas to preserve/prevent spoilage.
In refrigerated storage, what 2 factors need to be considered, and what is done to control them? (5)
respiration; transpiration - air and gas exchange
modifying atmosphere, temperature, packaging, waxing, humidity
How does respiration in plant and animal systems in refrigeration differ?
plant: AEROBIC, leads to softening
animal: ANAEROBIC, leads to toughening (rigor mortis), pH decline
Plant systems are known as ___, while animal systems are ____.
phytosystems; myosystems
Define rigor mortis. When does it happen, and what is the cause?
stiffening of muscle after death
around 36 hours after death
no blood flow -> anaerobic glycolysis -> lactic acid -> pH decline to around 5.5 -> glycolysis ceases (enzymes inactivated) -> No ATP -> muscle stiffens due to Ca
What can be applied to produce in refrigeration to reduce transpiration?
edible wax coating
What are “minimally processed refrigerated foods?” Why might they be desirable, and what is the major concern?
mild thermal treatment (pasteurize, blanch) -> vacuum pack -> refrigerate
good: short distribution time, high quality (‘gourmet’)
major concern: anaerobic growth of C. botulinum
Give some examples of pathogen growth prevention that can be used in addition to heat. (7)
low pH, low Aw, low temp storage, salt, antibacterial agents (bacteriocin), modified atmosphere, lactic acid bacteria
What is the “hurdle concept?”
Rely on multiple factors to prevent bacteria growth, not just heat.
Using multiple prevention measures allows for lower processing temp/time and higher quality.
True/False; freezing predates canning
True, used since ancient times. BUT canning predates COMMERCIAL FREEZING
The major difference between refrigerated and frozen storage is _______.
frozen is change in state; formation of ICE CRYSTALS
Freezing involves reducing the temperature to ____, which will induce _______ of ______.
-18C (0F)
crystallization of part of the water, and some solutes
The 3 phases of the freezing process:
precooling
phase change
tempering
What is “supercooling?” When does it happen, and what are the effects on the product?
temperature is lowered below freezing point, but no crystallization
- happens under highly controlled freezing conditions, favored by insoluble salt crystals in solution
no effects on final product
Define “nucleation,” and the 2 types.
molecules organized into a particle that serves as the site for further crystal growth (ripening)
- homogenous - in pure water (no impurities)
- heterogenous - water molecules aggregate on nucleating agents (other solutes) - in food systems
How are crystal size and crystal number related?
inverse relationship
more crystals = smaller
less crystals = bigger size
What is crystallization, and what are its 2 processes?
formation into an organized solid phase
- nucleation - formation of crystal nuclei
- ripening - crystals increase in size
What is crystal growth? How does it affect quality?
increase in size of crystal nuclei
large ice crystals in EXTRACELLULAR spaces
results in cell wall rupture -> drip loss when thawed
water loss, shrunken appearance, quality loss
What factors affect the nucleation process? How can we maximize the amount of nucleation?
temperature, freezing rate
LOWER temperature, FASTER freezing rate
What factors promote large ice crystal formation?
higher temp (close to zero), slow freezing
What affects the LOCATION of ice crystal formation?
freezing rate, temp, nature of cells
A rate slower than ____ is considered slow freezing.
lowering <1C/minute
How does the location of ice crystals differ for slow vs fast freezing? How does this affect quality?
slow: EXTRACELLULAR spaces - more tissue damage
fast: INTRA and EXTRACELLULAR space, smaller size - less tissue damage, higher quality
fast freezing will increase _____, while suppressing _____. Crystals form in both ____ and ____ spaces. This is (less/more) disruptive to tissue structure , and leads to a ____ texture.
nucleation; crystal growth
intra and extracellular
less disruptive
firmer texture
Why would the thermophysical properties of food be different from water?
presence of solutes elevates BP and depress FP
Ionic solutes have a much (higher/lower) Kf. What does this imply?
ionic solutes will depress FP much more
How is freeze point depression determined?
FP depression = Kf * m
Kf = molar depression constant
m = molarity
How does the freezing of foods compare to pure water? What happens as the freezing process continues?
lower freeze point
no sharp FP
freezing -> solids increasingly concentrated -> FP depressed even further
What is the initial freezing point of foods? In what temp range does most freezing occur?
-1 or -2C
“zone of maximum ice crystal formation:” -1 to -5C
Can all water in food be frozen? Why or why not?
No (theoretically impossible) Bound water (strongly bound to solutes) will not be frozen even at very low temp (-40) - remain liquid, unfreezable
What property changes are caused by freezing?
increased volume -> decreased density
decreased heat capacity
increased thermal conductivity
increased thermal diffusivity
list in order of increasing density: water 20C, water 5C, ice -10C, ice -40C
ice -10C < ice -40C < water 20C < water 5C
Density (increases/decreases) with lower temperature, for both water and ice. (water/ice) state will have a lower density.
increases
ice
Define heat capacity:
What factors affect it, and what are the models used?
energy required to raise temperature of unit mass by 1 degree
affected by COMPOSITION: moisture, fat, solids
models: Siebel, Dickerson, Charm, Heldman & Singh
Heat capacity is ___ in unfrozen state, but will (increase/decrease) with decreasing temperature in the frozen state.
constant
decrease
Which of the heat capacity models is most specific? What is an advantage of the Siebel method?
Heldman & Singh - account for lipid, carb, protein, ash, moisture
Siebel: has calculations for both above and below FP (others only above)
What is thermal conductivity?
ability of material to conduct heat
What are the models for thermal conductivity? What do they apply for?
Sweat: high moisture formula, meats formula, and when stored below -40
Earle: others
What does thermal diffusivity describe?
ability of material to respond to and undergo temperature change
Removal or adding heat to ice is ____ times (faster/slower) than for water.
9x
faster (higher thermal diffusivity)
Multiplying the density by heat capacity will give what?
describes ability of material to absorb heat
How would changes in density, Cp, or conductivity affect the thermal diffusivity?
density decrease -> higher TD
Cp decrease -> higher TD
conductivity increase -> higher TD
Which is faster: freezing or thawing? Why?
Freezing - makes ice layer, diffuse heat better
thawing will have outer water layer with poorer heat diffusion
