Lecture 13: Dehydration Fundamentals and Drying Process Flashcards
what is drying?
process of moisture removal
- vaporization (not a physical separation process)
- latent heat is supplied by external means
- involves simultaneous heat and mass transfer
compare sun drying, dehydration and solar drying
sundrying:
- unpredictable method
- used for cereal grains, coffee and nuts
dehydration:
- controlled application of heat for selective removal of moisture
- controlled heat and mass transfer
solar drying:
- controlled drying using heat from sun
define conventional drying:
removing moisture from a liquid, solid of paste material by evaporation of water into vapour via heat
define freeze drying
removal of water directly from frozen solid state (ice) to vapour, by sublimination
define concentration
evaporation of moisture from a liquid to convert it to a more concentrated form by the application of heat
less energy intensive than drying
not the same as drying
what are other methods of removing moisture, that is NOT considered drying?
how do they compare to drying?
filtration
sedimentation
centrifugation
these are much less energy intensive and more economical than drying
define osmotic drying
how does this compare to drying?
involves removal of moisture by liquid diffusion
more economical than drying
what are 4 reasons for drying?
- preservation by lowering water activity
- conversion of available food surplus to stable forms
- reduction of weight and volume
- convenience foods
define water activity
indicator of free or availability of water, which depends on moisture content, moisture state and temp
what is the equation of water activity?
Aw = ERH / 100 = P/Po
where P = vapour pressure of water in solution
Po = vapour pressure of pure water at the same temp
what is ERH?
equilibrium relative humidity
what is an example of conversion of available food surplus to stable forms, using drying?
liquid milk (perishable) to milk powder (stable)
Describe the temperatures of air inlet, outlet and of food at the beginning of the air drying process.
Describe the saturation level of the air
Describe the type of drying that is occurring
Ti > To
Tf = To
air is saturated
active drying is occurring
describe the temperatures of air inlet, outlet and of food at the major part of the air drying process
Ti > To
Ti > To > Tf
air is not saturated
restrictive drying occurs
describe the temperatures of air inlet, outlet and of food at the end of the air drying process
Ti = To = Tf
no drying is occurring
equilibrium is reached
what is the equation of moisture (%dry base)?
%DB = (M/S) x 100
what is the equation of moisture (%wet base)?
%WB = (M/(M+S)) x 100
What equation is used to convert dry to wet base?
X = W/ (1-W)
what are the periods in drying?
- transitional
- constant rate drying
- falling rate
- no significant drying
describe the transitional phase of drying
- first phase
- drying is adjusting to environmental conditions
- period is not well defined
- depends on specific condition: rh, temp, air flow
describe the constant rate drying phase
- 2nd phase
- moisture removed per unit time is constant
- free passage of moisture from food to air (no resistance in moisture transfer)
- rate of drying depends on air conditions
- length of period depends on type of product
- air leaving drying is mostly saturated
- rate of moisture migration from within > evaporation rate at surface
- active drying occurs
- product leaving air will be at wet bulb temp
describe the falling rate period
- 3rd period
- moisture removed per unit time continuously decreases
- limited passage of moisture (due to controlled diffusion from food interior to surface
- surface moisture evaporates quickly
- air leaving dryer will be less saturated w/ time
- rate of moisture migration from within < evaporation rate at surface
- period depends on product condition
- product and air leaving will be higher than wet bulb
describe the final phase of drying
- no significant drying occuring
- equilibrium is reached
- moisture in food = moisture in air
- limiting moisture content to which the product can be dried
- equilibrium moisture content depends on: temp of air and RH of air
in the transitional period, what factors is this period dependent on?
specific conditions: ie RH, temp, air flow, etc…
why is the transitional period not well defined?
drying is still adjusting to environmental conditions
in constant rate drying, describe the moisture removal rate
constant
in constant rate drying, describe the product to moisture transfer rate
no resistance
there is free passage of moisture from food to air
in constant rate drying, what does the rate of drying depend on?
air conditions (RH, temp, flow rate)
in constant rate drying, what does the length of the period depend on?
the type of product being dried
in constant rate drying, compare the rate of migration of moisture from within to the evaporation rate at surface
rate of migration of moisture from within > evaporation rate at surface
in constant rate drying, describe the level of saturation of air leaving the dryer
mostly saturated, since the product gives all the moisture that air can carry
in the falling rate period, describe the moisture removal rate
continuously decreasing
in the falling rate period, why is there limited passage of moisture from food interior to surface?
due to controlled diffusion
in the falling rate period, compare the rate of migration of moisture from within to the evaporation rate at surface
how is this different than in constant rate drying?
rate of migration of moisture from within < evaporation rate at surface
because surface moisture is evaporating quickly
in the falling rate period, describe the saturation of the air leaving the dryer
will be less saturated with time
in the falling rate period, what does the length of period depend on?
product condition
in the final phase of drying, describe the relationship of moisture in food to moisture in air
equivalent
Me = ERH
in the final phase of drying, ____ has been reached.
What does this moisture content signify?
equilibrium
this moisture content is the limiting moisture content to which the product can be dried
in the final phase of drying, what does the equilibrium moisture content depend on?
- temp of air (inc temp = dec Me)
2. relative humidity of air (inc RH = incr Me)
in the final phase of drying, what happens to equilibrium moisture when you increase temp?
inc temp = dec Me
in the final phase of drying, what happens to equilibrium moisture (Me) when you increase RH?
incr RH = incr Me
what is ERH? what does this also represent?
equilibrium relative humidity
also represents water activity
what is the Me and ERH relationship also called?
MSI: moisture sorption isotherm
Me and ERH are both specific to _____
temperature
when does adsorption occur?
describe the Aw during adsorption
during wetting or rehydration
when added moisture is free (ie. high Aw)
when does desorption occur?
describe the Aw during desorption
during drying
when moisture is confined due to tissue collapse/blocking (ie. when Aw is low)
when does hysterisis behaviour occur?
at any Me: when ERH during adsorption > ERH during desorption
what is moisture sorption isotherm?
- same product at different moisture contents
- plot of moisture content vs Aw
how is MSI obtained?
from drying/desportion and rehydration/adsorption
using a water activity meter to determine the Aw at different moisture contents
