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food processing > Drying curves > Flashcards

Drying curves Flashcards

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1
Q

The point at which constant rate ends and falling rate begins is known as:

A

critical point (tc, Mc)

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2
Q

What does the residual moisture ratio describe?

A

the remaining moisture in food that can be removed by drying

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3
Q

During the constant period, moisture levels vary from ___ to ____.
During the falling period, moisture levels vary from ___ to ___.

A

Mi to Mc

Mc to Me

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4
Q

Graphically speaking, M = Mc at t = ?
M = Me at t = ?
What are these points known as?

A 
M = Mc at t=tc (critical point)
M = Me at t=infinity (equilibrium; not reachable since log curve)
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5
Q

What is “MR?” What are the values at beginning vs end of drying (theroretically)?

A

Residual Moisture ratio (M-Me)/(Mi-Me)

beginning: 1
end: 0

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6
Q

How is MR determination different for constant vs falling rate periods?

A

constant: (M-Me)/(Mi-Mc)

falling: (M-Me)/(Mc-Me
ratio = 1 when M=Mc)

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7
Q

What is the relationship between MR and time for the constant vs falling rate drying?

A

constant: linear
falling: semi-logarithmically

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8
Q

What processing factors affect drying? (5)

A 
state/shape of food
operation: batch vs continuous
operating conditions/type
heating mode
energy source
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9
Q

What types of drying operations can be used, and what factors are considered for each?

A 
atmospheric - (temp, RH, flow rate)
vacuum - (temp, pressure)
freeze dry - (temp, pressure)
IR - (IR source, temp)
superheated steam dry - (temp, flow rate)
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10
Q

What factor is important for all types of drying?

A

temperature

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11
Q

Pressure is an important factor for what types of drying? (2)

A

vacuum, freeze dry

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12
Q

What are the different possible heating modes? What is most common for drying?

A

convection
conduction
radiation

most common: convection

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13
Q

using dry steam instead of hot air is known as ____ drying.

A

superheated steam drying

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14
Q

In what type of drying is RH an important consideration?

A

atmospheric

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15
Q

solar dehydration is a form of ____ heat transfer.

Give examples of the other 2 types.

A

radiation

Conduction: drum drying
convection: Hot air/hot dry steam

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16
Q

What is the difference between normal and superheated steam?

A

normal: around 100C - moist
superheated: around 150C - dry steam, able to pick up moisture

17
Q

What can be used as energy for drying processes? What are their uses? (3)

A

petroleum (gas) - to heat steam, air, etc
electricity - for IR, RF, MW, ohmic (direct heating methods)
other: wind, solar (used in tropical areas)

18
Q

What are some methods affecting osmotic drying? (6)

A 
concentration of osmotic solute
temp of solution
agitation
contact time
solute type
product shape/size
19
Q

What is the main problem with osmotic drying? How can this be reduced?

A

increase of solutes in product (migrate in)

  • apply osmotic solution through SPRAY instead of submerging
  • use high mol. wt solute (less migration)
20
Q

What is the process of osmotic drying?

A

submerge product in solution of osmotic solute
higher osmolarity of solution will draw water out from product
reduced water content in product

21
Q

What needs to be assessed at the end point of drying?

A
  1. quality - color/texture preservation, rehydration ability
  2. microbial stability - need to lower Aw to set limit
  3. chemical stability - prevent oxidation
22
Q

What is a problem with using some fruit types in osmotic drying? How is this addressed?

A

waxy coating - prevent effective moisture loss

remove by peel/cut/alkali (NaOH)

23
Q

What is sometimes done for dried products, due to cost considerations?

A

Do not dry fully, use dessicant (silica pack) in package, product will continue to lose moisture

24
Q

What is psychrometry?

A

study of thermodynamic properties of air/water/vapour mixtures

25
Q

The common psychrometric parameters:

A

RH
wet/dry bulb temp
Absolute H
dew point

26
Q

What is absolute humidity, and how is it expressed?

A

moisture content of air

mass of water vapor per unit mass of air

27
Q

The curves on the psychrometric chart represent: ___.

What is it?

A

relative humidity

% ratio of partial pressure of water vapor in air vs saturated vapor pressure

28
Q

What is on the horizontal axis of a psychrometric chart? What is on the right vertical axis?

A

dry bulb temp

absolute humidity/humidity ratio

29
Q

The max value for RH is ____. What would this indicate?

A

100%

max saturation - highest possible humidity at that temperature

30
Q

RH has no meaning if:

A

it is not paired with another variable (temperature)

31
Q

What happens if you attempt to add more moisture at an RH of 100%?

A

Air already saturated, cannot hold any more. Will just condense

32
Q

What is wet bulb temp?

A

lowest temp on thermometer attached to WET WICK, when in equilibrium with surrounding air

33
Q

Psychrometric charts are used for estimating the processes of:

A 
heating
cooling
humidification
dehumidification
dehydration
34
Q

What happens to dew point if the air is FULLY DRY? Why?

A

does not exist

no moisture left for air to lose

35
Q

The lower the wet bulb temp, the ____ the air. Why?

A

dryer

air picks up moisture -> lower temp

36
Q

Dew point is:

A

temperature at which cooled air will start condensing (losing moisture)

37
Q

___ will increase as the temperature is lowered, until it reaches ____. The temperature at that point is known as ____, and _____ will occur.

A

RH, 100%
dew point
condensation/moisture loss

38
Q

What is the difference between adiabatic and isothermal processes?

A

adiabatic: no heat transfer, temperature can change
isothermal: no temperature change, exchange of heat w/ outside environment

39
Q

How can the psychrometric chart be used for drying processes?

A

determine appropriate temp/humidity of incoming air

food processing (17 decks)

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