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food processing > Refrigeration/Freezing > Flashcards

Refrigeration/Freezing Flashcards

1
Q

2 techniques for low temp. preservation:

A
  • refrigeration

- freezing

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2
Q

What are some deteriorative activities occuring in food? (5)

A 
enzymatic breakdown
microbe growth
physiological activity
chemical rxn
respiration
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3
Q

How does the rates of deteriorative activity compare in frozen vs refrigerated vs fresh?

A

fresh: full activity
refrigerated: moderate activity (can have full physiological activity/respiration)
frozen: none or very low levels

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4
Q

What are the temp ranges for refrigerated and frozen?

A

refrigerated: 0-15C
frozen: -5 to -40C

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5
Q

What factors differ between frozen and refrigerated storage? (6)

A
  • time of storage
  • temperature
  • atmosphere
  • availability of free water
  • amount of deteriorative activity
  • effect on tissue structure
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6
Q

How do atmospheric conditions compare for frozen vs refrigerated storage?

A

frozen: no need to control atmosphere (just RA)
refrigerated: CA/MA/RA - need to control environment to limit deterioration

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7
Q

What are some pros and cons of freezing rather than refrigerating?

A

pros: longer storage, slower deterioration, less microbe growth, no need for modified atmosphere
cons: more tissue damage, water loss

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8
Q

What type of changes in tissue structure are caused by frozen vs refrigerated storage?

A

refrigerated: moderate, physiological
frozen: extensive, physical

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9
Q

True/False: free water content in food is reduced by refrigerating.

A

False; free water activity is full

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10
Q

Why is controlling the atmosphere not necessary in frozen storage?

A

less water activity and very low respiratory rates will already slow deterioration significantly.

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11
Q

True/False: in frozen food, the water activity is nearly 0.

A

True, free water converted to ice

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12
Q

Can bacteria be killed at low temperatures? Can this be used to make food safe?

A

Yes, but slowly. No, too slow and is rarely completely killed.

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13
Q

When holding produce in refrigerated storage, ____ respiration should be maintained.

A

aerobic

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14
Q

The temp range -10 to 10C is known as: _____.

Why?

A

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

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15
Q

What happens above/below the “zone of spoilage without danger to health?”

A

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

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16
Q

_____ is moisture loss, and is prevented by:

A

transpiration; controlling RH

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17
Q

What are the modified atmospheric conditions used for myosystems vs phytosystems in refrigeration? Why?

A

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.

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18
Q

In refrigerated storage, what 2 factors need to be considered, and what is done to control them? (5)

A

respiration; transpiration - air and gas exchange

modifying atmosphere, temperature, packaging, waxing, humidity

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19
Q

How does respiration in plant and animal systems in refrigeration differ?

A

plant: AEROBIC, leads to softening
animal: ANAEROBIC, leads to toughening (rigor mortis), pH decline

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20
Q

Plant systems are known as ___, while animal systems are ____.

A

phytosystems; myosystems

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21
Q

Define rigor mortis. When does it happen, and what is the cause?

A

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

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22
Q

What can be applied to produce in refrigeration to reduce transpiration?

A

edible wax coating

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23
Q

What are “minimally processed refrigerated foods?” Why might they be desirable, and what is the major concern?

A

mild thermal treatment (pasteurize, blanch) -> vacuum pack -> refrigerate

good: short distribution time, high quality (‘gourmet’)

major concern: anaerobic growth of C. botulinum

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24
Q

Give some examples of pathogen growth prevention that can be used in addition to heat. (7)

A

low pH, low Aw, low temp storage, salt, antibacterial agents (bacteriocin), modified atmosphere, lactic acid bacteria

25
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.
26
True/False; freezing predates canning
True, used since ancient times. BUT canning predates COMMERCIAL FREEZING
27
The major difference between refrigerated and frozen storage is _______.
frozen is change in state; formation of ICE CRYSTALS
28
Freezing involves reducing the temperature to ____, which will induce _______ of ______.
-18C (0F) | crystallization of part of the water, and some solutes
29
The 3 phases of the freezing process:
precooling phase change tempering
30
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
31
Define "nucleation," and the 2 types.
molecules organized into a particle that serves as the site for further crystal growth (ripening) 1. homogenous - in pure water (no impurities) 2. heterogenous - water molecules aggregate on nucleating agents (other solutes) - in food systems
32
How are crystal size and crystal number related?
inverse relationship more crystals = smaller less crystals = bigger size
33
What is crystallization, and what are its 2 processes?
formation into an organized solid phase 1. nucleation - formation of crystal nuclei 2. ripening - crystals increase in size
34
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
35
What factors affect the nucleation process? How can we maximize the amount of nucleation?
temperature, freezing rate LOWER temperature, FASTER freezing rate
36
What factors promote large ice crystal formation?
higher temp (close to zero), slow freezing
37
What affects the LOCATION of ice crystal formation?
freezing rate, temp, nature of cells
38
A rate slower than ____ is considered slow freezing.
lowering <1C/minute
39
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
40
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
41
Why would the thermophysical properties of food be different from water?
presence of solutes elevates BP and depress FP
42
Ionic solutes have a much (higher/lower) Kf. What does this imply?
ionic solutes will depress FP much more
43
How is freeze point depression determined?
FP depression = Kf * m Kf = molar depression constant m = molarity
44
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
45
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
46
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 ```
47
What property changes are caused by freezing?
increased volume -> decreased density decreased heat capacity increased thermal conductivity increased thermal diffusivity
48
list in order of increasing density: water 20C, water 5C, ice -10C, ice -40C
ice -10C < ice -40C < water 20C < water 5C
49
Density (increases/decreases) with lower temperature, for both water and ice. (water/ice) state will have a lower density.
increases ice
50
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
51
Heat capacity is ___ in unfrozen state, but will (increase/decrease) with decreasing temperature in the frozen state.
constant decrease
52
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)
53
What is thermal conductivity?
ability of material to conduct heat
54
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
55
What does thermal diffusivity describe?
ability of material to respond to and undergo temperature change
56
Removal or adding heat to ice is ____ times (faster/slower) than for water.
9x | faster (higher thermal diffusivity)
57
Multiplying the density by heat capacity will give what?
describes ability of material to absorb heat
58
How would changes in density, Cp, or conductivity affect the thermal diffusivity?
density decrease -> higher TD Cp decrease -> higher TD conductivity increase -> higher TD
59
Which is faster: freezing or thawing? Why?
Freezing - makes ice layer, diffuse heat better | thawing will have outer water layer with poorer heat diffusion

food processing (17 decks)

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