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food processing > Refrigeration/Freezing > Flashcards

Refrigeration/Freezing Flashcards

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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
Q

What is the “hurdle concept?”

A

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
Q

True/False; freezing predates canning

A

True, used since ancient times. BUT canning predates COMMERCIAL FREEZING

27
Q

The major difference between refrigerated and frozen storage is _______.

A

frozen is change in state; formation of ICE CRYSTALS

28
Q

Freezing involves reducing the temperature to ____, which will induce _______ of ______.

A

-18C (0F)

crystallization of part of the water, and some solutes

29
Q

The 3 phases of the freezing process:

A

precooling
phase change
tempering

30
Q

What is “supercooling?” When does it happen, and what are the effects on the product?

A

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
Q

Define “nucleation,” and the 2 types.

A

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
Q

How are crystal size and crystal number related?

A

inverse relationship
more crystals = smaller
less crystals = bigger size

33
Q

What is crystallization, and what are its 2 processes?

A

formation into an organized solid phase

  1. nucleation - formation of crystal nuclei
  2. ripening - crystals increase in size
34
Q

What is crystal growth? How does it affect quality?

A

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
Q

What factors affect the nucleation process? How can we maximize the amount of nucleation?

A

temperature, freezing rate

LOWER temperature, FASTER freezing rate

36
Q

What factors promote large ice crystal formation?

A

higher temp (close to zero), slow freezing

37
Q

What affects the LOCATION of ice crystal formation?

A

freezing rate, temp, nature of cells

38
Q

A rate slower than ____ is considered slow freezing.

A

lowering <1C/minute

39
Q

How does the location of ice crystals differ for slow vs fast freezing? How does this affect quality?

A

slow: EXTRACELLULAR spaces - more tissue damage
fast: INTRA and EXTRACELLULAR space, smaller size - less tissue damage, higher quality

40
Q

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.

A

nucleation; crystal growth
intra and extracellular
less disruptive
firmer texture

41
Q

Why would the thermophysical properties of food be different from water?

A

presence of solutes elevates BP and depress FP

42
Q

Ionic solutes have a much (higher/lower) Kf. What does this imply?

A

ionic solutes will depress FP much more

43
Q

How is freeze point depression determined?

A

FP depression = Kf * m
Kf = molar depression constant
m = molarity

44
Q

How does the freezing of foods compare to pure water? What happens as the freezing process continues?

A

lower freeze point
no sharp FP

freezing -> solids increasingly concentrated -> FP depressed even further

45
Q

What is the initial freezing point of foods? In what temp range does most freezing occur?

A

-1 or -2C

“zone of maximum ice crystal formation:” -1 to -5C

46
Q

Can all water in food be frozen? Why or why not?

A 
No (theoretically impossible)
Bound water (strongly bound to solutes) will not be frozen even at very low temp (-40)  - remain liquid, unfreezable
47
Q

What property changes are caused by freezing?

A

increased volume -> decreased density
decreased heat capacity
increased thermal conductivity
increased thermal diffusivity

48
Q

list in order of increasing density: water 20C, water 5C, ice -10C, ice -40C

A

ice -10C < ice -40C < water 20C < water 5C

49
Q

Density (increases/decreases) with lower temperature, for both water and ice. (water/ice) state will have a lower density.

A

increases

ice

50
Q

Define heat capacity:

What factors affect it, and what are the models used?

A

energy required to raise temperature of unit mass by 1 degree
affected by COMPOSITION: moisture, fat, solids
models: Siebel, Dickerson, Charm, Heldman & Singh

51
Q

Heat capacity is ___ in unfrozen state, but will (increase/decrease) with decreasing temperature in the frozen state.

A

constant

decrease

52
Q

Which of the heat capacity models is most specific? What is an advantage of the Siebel method?

A

Heldman & Singh - account for lipid, carb, protein, ash, moisture

Siebel: has calculations for both above and below FP (others only above)

53
Q

What is thermal conductivity?

A

ability of material to conduct heat

54
Q

What are the models for thermal conductivity? What do they apply for?

A

Sweat: high moisture formula, meats formula, and when stored below -40
Earle: others

55
Q

What does thermal diffusivity describe?

A

ability of material to respond to and undergo temperature change

56
Q

Removal or adding heat to ice is ____ times (faster/slower) than for water.

A

9x

faster (higher thermal diffusivity)

57
Q

Multiplying the density by heat capacity will give what?

A

describes ability of material to absorb heat

58
Q

How would changes in density, Cp, or conductivity affect the thermal diffusivity?

A

density decrease -> higher TD
Cp decrease -> higher TD
conductivity increase -> higher TD

59
Q

Which is faster: freezing or thawing? Why?

A

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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