Lecture 15: Quality Changes During Storage Flashcards
what are 6 changes that occur during dehydration?
- shrinkage
- case hardening
- loss of texture
- lack of complete rehydration
- loss of volatile
- chemical, enzymatic and microbial activity
how does shrinkage occur during dehydration?
- when moisture evaporates, cells collapse and shrink
what percentage of volume loss occurs during air drying?
70-90% loss
what is a solution to shrinkage during dehydration?
freeze drying
what is freeze drying?
- protects product from shrinkage during dehydration, since no free H2O means no mobility
- ice escapes directly as vapour when everything is frozen
what causes case hardening?
when increasing the rate of drying = increases the rate of surface moisture evaporation = decreases rate of moisture loss from inside
when does case hardening occur?
when drying the surface causes hardness, which taps moisture inside
this is a defect and causes deterioration during storage
what are solutions to case hardening?
- drying slower at lower temp
- moderate drying rate
- incr humidity = slower drying rate
what does the loss of texture during dehydration depend on?
mode of drying and type of product
how can you reduce the loss of volatiles during dehydration?
by using low temp or vacuum drying
how are volatiles lost during dehydration?
along with vapour
what are examples of chemical, enzymatic and microbial changes that occur in dehydration?
- discolouration and off flavors
- inactivation of enzymes (From blanching)
- bleaching discolouration (from sulphite dip)
- microbial agents
what are 2 factors that are considered in the storage stability of dried foods?
- moisture migration (transfer of moisture through packaging)
- moisture equilibrium within package
what causes moisture migration to affect storage stability of dried foods?
which law can model this?
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
what is Fick’s law of gas?
- models inward moisture transfer through packaging
J = (AD(Pi-Pa)) / (deltax * R* T)
what are the 3 forms of water?
monolayer
multilayer
free water
describe monolayer water
- water is bound very strongly
- offers protective coating which prevents fat oxidation
what equation can you use to determine monolayer water?
BET isotherm
describe multilayer water
- less strongly bound
- present in multiple layers after monolayer surfaces are covered
- non0freezable
- non-solvent H@O
describe free water
- mobile water
- loosely associated
- unbound
what are the two common MSI models?
henderson
rockland
what is the henderson MSI model equation?
ln (1-a) = -cm^n
what is the rockland MSI model equation?
log log (1/(1-a) = n log (m+y)
what type of isotherm does the rockland equation provide?
how do you determine the whole isotherm?
provides local isotherms from the n and y parameters
need several local isotherms to quantify the whole isotherm
what does the rockland MSI model apply to?
the early MI (up to Aw = 0.5)
compare the conventional MSI plot with the delta m / delta a plot
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
what is the stable region in the MSI plot?
the intermediate section
when the rates of change of m is very slow
almost a plateau
