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food processing > Dehydration fundamentals + drying process > Flashcards

Dehydration fundamentals + drying process Flashcards

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1
Q

drying is the process of ______, through ____.

A

water removal; VAPORIZATION

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2
Q

What are the 3 main methods of drying?

A

sun (uncontrolled)
dehydration (highly controlled conditions)
solar (controlled, using heat from sun)

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3
Q

How is drying different from concentration or cooking/baking?

A

concentration can also be accomplished by ADDING solute; not sufficient moisture removal
cooking/baking will have other changes involved; not sufficient moisture removal

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4
Q

What does the process of drying involve, in terms of mass/energy balance?

A

latent heat supplied from external source to cause vaporization
water loss = mass lost

uses both heat + mass transfer

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5
Q

the two types of drying process:

A

conventional: evaporate water from liquid/semiliquid/solid/paste by applying heat

freeze-dry: sublimate water from frozen matrix

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6
Q

Why is drying necessary? (4)

A
  1. lower Aw
  2. convert to more stable form
  3. reduce bulk
  4. make convenience food
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7
Q

What are other processes that involve water removal, and how do they differ from dehydration? (3)

A

concentration: evaporate water to increase solute concentration - not enough moisture removal to count as drying, but less energy needed

physical removal (filter, centrifuge, sedimentation) - not considered drying (not enough) and less energy needed

osmotic drying: only partial drying - remove liquid by diffusion (more economical)

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8
Q

The Aw represents ______, and can be expressed as:

A

free water available in food,

ERH/100

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9
Q

Water acts both as a ___ and a ___, making it necessary for what undesirable outcomes?

A

solvent; reactant

deterioration reactions, bacterial growth

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10
Q

Aw of pure water is:

Aw of dried foods should be:

A

1

<0.7

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11
Q

What happens for high lipid foods as Aw decreases further and further?

A

oxidation will increase! Some water is needed to protect lipid from O2 exposure

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12
Q

What are the different stages of drying?

A 
Transition
Constant Drying Rate
Falling Rate
Second Falling rate
Equilibrium (end)
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13
Q

The solids % can be described on a ____ basis or ____ basis.

A

wet; dry

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14
Q

What happens to the moisture lost? Where does the energy lost from the hot air go?

A

moisture picked up by air

heat from air transferred to food

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15
Q

Compare the inlet air and outlet air temperatures and the food temperature during the phases of drying.

A

Beginning: Ti>To, Tf=To (saturated active drying)
Middle: Ti>To>Tf (not saturated, restricted drying)
End: Ti=To=Tf (equilibrium reached;no drying)

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16
Q

When does the fastest rate of drying occur?

A

Constant drying rate (linear)

17
Q

What type of drying happens during the constant rate, and why?

A

saturated: product gives all the moisture the air can carry
rate of moisture from product to surface is GREATER than rate of moisture loss from surface to air
product is non-blocking porous structure (no resistance)

18
Q

True/False: no drying happens during the transition period.

A

False; some drying, not well defined. Adjusting to environment conditions.

19
Q

length of the constant rate drying depends on ______, and is usually ____.

A

product type; small

20
Q

constant rate drying is also known as:_____. How do you determine if this is occurring?

A

active drying

wet bulb temp of food = wet bulb temp of leaving air

21
Q

the rate of drying during the constant period depends on:

A

air conditions (RH, temp, flow rate)

22
Q

Is the outlet air during the falling period saturated?

A

No. product not supplying sufficient moisture anymore.

23
Q

true/false: the falling rate has less drying occurring because of less evaporation occuring on product surface.

A

False. surface moisture picked up quickly, but DIFFUSION from inside product is limited
(product cannot move moisture for air to pick up)

24
Q

what happens to rate of migration of moisture in product vs rate of evaporation from surface during the falling period?

A

migration in product < evaporation from surface

25
Q

What decreases during the falling period?

A

saturation of leaving air
moisture loss
diffusion of moisture from inside -> surface

26
Q

how does the temperature of product vs outlet air compare during falling period?

A

wet bulb outlet air > wet bulb product

27
Q

When has the drying process reached the final phase?

A

When Me = ERH

28
Q

When the process has reached equilibrium, moisture in product is: _____, and moisture in air is:____.

A

moisture in product = Me (equilibrium moisture)

moisture in air = ERH (eq. relative humidity)

29
Q

drying follows a _____ MSI curve, while rehydration follows a ____ MSI curve.

A

desorption

adsorption

30
Q

Me vs ERH relationships are called:

A

moisture sorption isotherms

31
Q

What factors affect the equilibrium moisture content?

A
  1. temp (higher temp -> lower Me)

2. RH of air (higher RH -> higher Me)

32
Q

The greater the temperature, the (higher/lower) the moisture content at the same RH

A

lower

33
Q

True/False: the same MSI can be used for drying and wetting a product

A

False

34
Q

The difference between adsorption/desorption curves is known as:

A

Hysteresis

35
Q

How does the ERH for adsorption/desorption curves compare? Why?

A

ERH always greater for adsorption curves

in drying: tissue will collapse and block, so moisture confined (lower Aw)
In wetting: added moisture will be mostly free (high Aw)

36
Q

How is the MSI obtained for a product?

A

desorption: place in salt chamber w/ specific Aw, allow to equilibrate -> find moisture content (plot for several different Aw)
adsorption: condition at different humidity/moisture conditions to obtain certain moisture content -> use meter to determine Aw (plot several different points)

37
Q

As temperature increases, water activity will ____.

A

decrease

38
Q

What are some dessicants that can be used for MSI chambers?

A

NaCl, KCl, KI, MgCl2, KCO3, CH3COOK, LiCl

food processing (17 decks)

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