Drying curves Flashcards
The point at which constant rate ends and falling rate begins is known as:
critical point (tc, Mc)
What does the residual moisture ratio describe?
the remaining moisture in food that can be removed by drying
During the constant period, moisture levels vary from ___ to ____.
During the falling period, moisture levels vary from ___ to ___.
Mi to Mc
Mc to Me
Graphically speaking, M = Mc at t = ?
M = Me at t = ?
What are these points known as?
M = Mc at t=tc (critical point) M = Me at t=infinity (equilibrium; not reachable since log curve)
What is “MR?” What are the values at beginning vs end of drying (theroretically)?
Residual Moisture ratio (M-Me)/(Mi-Me)
beginning: 1
end: 0
How is MR determination different for constant vs falling rate periods?
constant: (M-Me)/(Mi-Mc)
falling: (M-Me)/(Mc-Me
ratio = 1 when M=Mc)
What is the relationship between MR and time for the constant vs falling rate drying?
constant: linear
falling: semi-logarithmically
What processing factors affect drying? (5)
state/shape of food operation: batch vs continuous operating conditions/type heating mode energy source
What types of drying operations can be used, and what factors are considered for each?
atmospheric - (temp, RH, flow rate) vacuum - (temp, pressure) freeze dry - (temp, pressure) IR - (IR source, temp) superheated steam dry - (temp, flow rate)
What factor is important for all types of drying?
temperature
Pressure is an important factor for what types of drying? (2)
vacuum, freeze dry
What are the different possible heating modes? What is most common for drying?
convection
conduction
radiation
most common: convection
using dry steam instead of hot air is known as ____ drying.
superheated steam drying
In what type of drying is RH an important consideration?
atmospheric
solar dehydration is a form of ____ heat transfer.
Give examples of the other 2 types.
radiation
Conduction: drum drying
convection: Hot air/hot dry steam
What is the difference between normal and superheated steam?
normal: around 100C - moist
superheated: around 150C - dry steam, able to pick up moisture
What can be used as energy for drying processes? What are their uses? (3)
petroleum (gas) - to heat steam, air, etc
electricity - for IR, RF, MW, ohmic (direct heating methods)
other: wind, solar (used in tropical areas)
What are some methods affecting osmotic drying? (6)
concentration of osmotic solute temp of solution agitation contact time solute type product shape/size
What is the main problem with osmotic drying? How can this be reduced?
increase of solutes in product (migrate in)
- apply osmotic solution through SPRAY instead of submerging
- use high mol. wt solute (less migration)
What is the process of osmotic drying?
submerge product in solution of osmotic solute
higher osmolarity of solution will draw water out from product
reduced water content in product
What needs to be assessed at the end point of drying?
- quality - color/texture preservation, rehydration ability
- microbial stability - need to lower Aw to set limit
- chemical stability - prevent oxidation
What is a problem with using some fruit types in osmotic drying? How is this addressed?
waxy coating - prevent effective moisture loss
remove by peel/cut/alkali (NaOH)
What is sometimes done for dried products, due to cost considerations?
Do not dry fully, use dessicant (silica pack) in package, product will continue to lose moisture
What is psychrometry?
study of thermodynamic properties of air/water/vapour mixtures
The common psychrometric parameters:
RH
wet/dry bulb temp
Absolute H
dew point
What is absolute humidity, and how is it expressed?
moisture content of air
mass of water vapor per unit mass of air
The curves on the psychrometric chart represent: ___.
What is it?
relative humidity
% ratio of partial pressure of water vapor in air vs saturated vapor pressure
What is on the horizontal axis of a psychrometric chart? What is on the right vertical axis?
dry bulb temp
absolute humidity/humidity ratio
The max value for RH is ____. What would this indicate?
100%
max saturation - highest possible humidity at that temperature
RH has no meaning if:
it is not paired with another variable (temperature)
What happens if you attempt to add more moisture at an RH of 100%?
Air already saturated, cannot hold any more. Will just condense
What is wet bulb temp?
lowest temp on thermometer attached to WET WICK, when in equilibrium with surrounding air
Psychrometric charts are used for estimating the processes of:
heating cooling humidification dehumidification dehydration
What happens to dew point if the air is FULLY DRY? Why?
does not exist
no moisture left for air to lose
The lower the wet bulb temp, the ____ the air. Why?
dryer
air picks up moisture -> lower temp
Dew point is:
temperature at which cooled air will start condensing (losing moisture)
___ will increase as the temperature is lowered, until it reaches ____. The temperature at that point is known as ____, and _____ will occur.
RH, 100%
dew point
condensation/moisture loss
What is the difference between adiabatic and isothermal processes?
adiabatic: no heat transfer, temperature can change
isothermal: no temperature change, exchange of heat w/ outside environment
How can the psychrometric chart be used for drying processes?
determine appropriate temp/humidity of incoming air
