Lecture 14: Factors Affecting Drying Rate Psychometry

Question 1

What variables represent moisture removal potential?

Answer 1

Mi - Me

Question 2

what variables represent residual moisture at time t?

Answer 2

M- Me

Question 3

What represents MR?

Answer 3

(M-Me)/(Mi-Me)

Question 4

what is MR? what does it express?

Answer 4

residual moisture content ratio

used to express available moisture for drying

Question 5

at the start of drying, what is MR?

Answer 5

1

Question 6

what the end of drying, what is MR?

Answer 6

0

Question 7

what is the graphical relationship or MR and t during constant rate period?

Answer 7

linear

Question 8

what is the graphical relationship or MR and t during the falling rate period?

Answer 8

semi-log

Question 9

what moisture levels represent the constant rate period?

Answer 9

Mi to Mc

Question 10

what moisture levels represent the falling rate period?

Answer 10

Mc to Me

Question 11

define the critical point

Answer 11

• occurs at Mc and tc

when drying shifts from constant to falling rate

Question 12

what are 5 factors that influence drying?

Answer 12

1. state/shape of food: solid, liquid, paste/ particular, granular, sheet
2. operation mode of drying: batch or continuous
3. type and operating conditions: atmospheric, vacuum, freeze, IR, superheated steam
4. heating mode: convection, radiation, conduction
5. energy sources

Question 13

what are some types of drying operating conditions?

Answer 13

atmospheric drying
vacuum
freeze
IR
super heated steam drying

Question 14

what are 3 types of heating modes for drying?

Answer 14

convection
radiation
conduction

Question 15

what are examples of energy sources used for drying?

Answer 15

petroleum products

electricity

other: wind and solar

Question 16

what are factors that affect osmotic dehydration?

Answer 16

• conc of osmotic solute
• temp of osmotic solution
• agitation
• contact time of solute and sample
• type and geomtetry of sample

Question 17

what is the end point for drying used to determine?

Answer 17

the drying time deserved

Question 18

what is considered in determination of the end point for drying?

Answer 18

1. quality parameters
2. microbial stability
3. chemical stability
4. cost consideration

Question 19

define psychometry

Answer 19

study of the thermodynamic properties of air water vapour mixtures

Question 20

what does psychometry help determine?

Answer 20

proper drying conditions

Question 21

what are parameters used in psychometry?

Answer 21

absolute humidity
relative humidity
dry and wet bulb temp
dew point

Question 22

what is dry bulb temp? how is it represented in the psychometry chart?

Answer 22

the temp registered by a regular non-wtted thermometer

horizontal axis

Question 23

what is the absolute humidity/humidity ratio?

how is it represented in the psychometry chart?

Answer 23

moisture content of the air

expressed as mass of water vapour per unit mass of air

vertical axis on right side

Question 24

what equation represents relative humidity?

Answer 24

%RH = (Pwa / Pws) *100

Pwa = partial pressure of H2O vapor in air
Pws = saturated vapour pressure

Question 25

what is the max RH that can occur? what is equivalent when this happens?

Answer 25

max RH = 100%

means that Pwa = Pws

Question 26

what is wet bulb temp?

Answer 26

the lowest temp a thermometer bulb attached to a wet wick can register

Question 27

what is the dew point?

Answer 27

temp that air begins to lose moisture by condensation

Question 28

if air is fully dry, what is the dew point?

Answer 28

there is none (since condensation can’t occur)