Lecture 15: Quality Changes During Storage

Question 1

what are 6 changes that occur during dehydration?

Answer 1

1. shrinkage
2. case hardening
3. loss of texture
4. lack of complete rehydration
5. loss of volatile
6. chemical, enzymatic and microbial activity

Question 2

how does shrinkage occur during dehydration?

Answer 2

• when moisture evaporates, cells collapse and shrink

Question 3

what percentage of volume loss occurs during air drying?

Answer 3

70-90% loss

Question 4

what is a solution to shrinkage during dehydration?

Answer 4

freeze drying

Question 5

what is freeze drying?

Answer 5

• protects product from shrinkage during dehydration, since no free H2O means no mobility
• ice escapes directly as vapour when everything is frozen

Question 6

what causes case hardening?

Answer 6

when increasing the rate of drying = increases the rate of surface moisture evaporation = decreases rate of moisture loss from inside

Question 7

when does case hardening occur?

Answer 7

when drying the surface causes hardness, which taps moisture inside

this is a defect and causes deterioration during storage

Question 8

what are solutions to case hardening?

Answer 8

• drying slower at lower temp
• moderate drying rate
• incr humidity = slower drying rate

Question 9

what does the loss of texture during dehydration depend on?

Answer 9

mode of drying and type of product

Question 10

how can you reduce the loss of volatiles during dehydration?

Answer 10

by using low temp or vacuum drying

Question 11

how are volatiles lost during dehydration?

Answer 11

along with vapour

Question 12

what are examples of chemical, enzymatic and microbial changes that occur in dehydration?

Answer 12

• discolouration and off flavors
• inactivation of enzymes (From blanching)
• bleaching discolouration (from sulphite dip)
• microbial agents

Question 13

what are 2 factors that are considered in the storage stability of dried foods?

Answer 13

1. moisture migration (transfer of moisture through packaging)
2. moisture equilibrium within package

Question 14

what causes moisture migration to affect storage stability of dried foods?

which law can model this?

Answer 14

when product is dry and picks up moisture from outside if environment is wet

• causes moisture transfer inwards
• can be modelled by Fick’s law of gas

Question 15

what is Fick’s law of gas?

Answer 15

• models inward moisture transfer through packaging

J = (AD(Pi-Pa)) / (deltax * R* T)

Question 16

what are the 3 forms of water?

Answer 16

monolayer
multilayer
free water

Question 17

describe monolayer water

Answer 17

• water is bound very strongly

- offers protective coating which prevents fat oxidation

Question 18

what equation can you use to determine monolayer water?

Answer 18

BET isotherm

Question 19

describe multilayer water

Answer 19

• less strongly bound
• present in multiple layers after monolayer surfaces are covered
• non0freezable
• non-solvent H@O

Question 20

describe free water

Answer 20

• mobile water
• loosely associated
• unbound

Question 21

what are the two common MSI models?

Answer 21

henderson

rockland

Question 22

what is the henderson MSI model equation?

Answer 22

ln (1-a) = -cm^n

Question 23

what is the rockland MSI model equation?

Answer 23

log log (1/(1-a) = n log (m+y)

Question 24

what type of isotherm does the rockland equation provide?

how do you determine the whole isotherm?

Answer 24

provides local isotherms from the n and y parameters

need several local isotherms to quantify the whole isotherm

Question 25

what does the rockland MSI model apply to?

Answer 25

the early MI (up to Aw = 0.5)

Question 26

compare the conventional MSI plot with the delta m / delta a plot

Answer 26

conventional MSI

• moisture sensitive to changes in Aw on both ends (where the is fast rates of change)
• least sensitive at intermediate Aw (ie. the stable region where rates of change is slow)

delta m / delta a plot:
- can identify the stable region better

Question 27

what is the stable region in the MSI plot?

Answer 27

the intermediate section

when the rates of change of m is very slow

almost a plateau