Dehydration fundamentals + drying process Flashcards
drying is the process of ______, through ____.
water removal; VAPORIZATION
What are the 3 main methods of drying?
sun (uncontrolled)
dehydration (highly controlled conditions)
solar (controlled, using heat from sun)
How is drying different from concentration or cooking/baking?
concentration can also be accomplished by ADDING solute; not sufficient moisture removal
cooking/baking will have other changes involved; not sufficient moisture removal
What does the process of drying involve, in terms of mass/energy balance?
latent heat supplied from external source to cause vaporization
water loss = mass lost
uses both heat + mass transfer
the two types of drying process:
conventional: evaporate water from liquid/semiliquid/solid/paste by applying heat
freeze-dry: sublimate water from frozen matrix
Why is drying necessary? (4)
- lower Aw
- convert to more stable form
- reduce bulk
- make convenience food
What are other processes that involve water removal, and how do they differ from dehydration? (3)
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)
The Aw represents ______, and can be expressed as:
free water available in food,
ERH/100
Water acts both as a ___ and a ___, making it necessary for what undesirable outcomes?
solvent; reactant
deterioration reactions, bacterial growth
Aw of pure water is:
Aw of dried foods should be:
1
<0.7
What happens for high lipid foods as Aw decreases further and further?
oxidation will increase! Some water is needed to protect lipid from O2 exposure
What are the different stages of drying?
Transition Constant Drying Rate Falling Rate Second Falling rate Equilibrium (end)
The solids % can be described on a ____ basis or ____ basis.
wet; dry
What happens to the moisture lost? Where does the energy lost from the hot air go?
moisture picked up by air
heat from air transferred to food
Compare the inlet air and outlet air temperatures and the food temperature during the phases of drying.
Beginning: Ti>To, Tf=To (saturated active drying)
Middle: Ti>To>Tf (not saturated, restricted drying)
End: Ti=To=Tf (equilibrium reached;no drying)
When does the fastest rate of drying occur?
Constant drying rate (linear)
What type of drying happens during the constant rate, and why?
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)
True/False: no drying happens during the transition period.
False; some drying, not well defined. Adjusting to environment conditions.
length of the constant rate drying depends on ______, and is usually ____.
product type; small
constant rate drying is also known as:_____. How do you determine if this is occurring?
active drying
wet bulb temp of food = wet bulb temp of leaving air
the rate of drying during the constant period depends on:
air conditions (RH, temp, flow rate)
Is the outlet air during the falling period saturated?
No. product not supplying sufficient moisture anymore.
true/false: the falling rate has less drying occurring because of less evaporation occuring on product surface.
False. surface moisture picked up quickly, but DIFFUSION from inside product is limited
(product cannot move moisture for air to pick up)
what happens to rate of migration of moisture in product vs rate of evaporation from surface during the falling period?
migration in product < evaporation from surface
What decreases during the falling period?
saturation of leaving air
moisture loss
diffusion of moisture from inside -> surface
how does the temperature of product vs outlet air compare during falling period?
wet bulb outlet air > wet bulb product
When has the drying process reached the final phase?
When Me = ERH
When the process has reached equilibrium, moisture in product is: _____, and moisture in air is:____.
moisture in product = Me (equilibrium moisture)
moisture in air = ERH (eq. relative humidity)
drying follows a _____ MSI curve, while rehydration follows a ____ MSI curve.
desorption
adsorption
Me vs ERH relationships are called:
moisture sorption isotherms
What factors affect the equilibrium moisture content?
- temp (higher temp -> lower Me)
2. RH of air (higher RH -> higher Me)
The greater the temperature, the (higher/lower) the moisture content at the same RH
lower
True/False: the same MSI can be used for drying and wetting a product
False
The difference between adsorption/desorption curves is known as:
Hysteresis
How does the ERH for adsorption/desorption curves compare? Why?
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)
How is the MSI obtained for a product?
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)
As temperature increases, water activity will ____.
decrease
What are some dessicants that can be used for MSI chambers?
NaCl, KCl, KI, MgCl2, KCO3, CH3COOK, LiCl
