quality change in storage Flashcards
What happens to volume of food when dried? What are the exceptions?
Shrinkage - cells collapse, volume loss of 70-90%
Freeze dry: no volume loss, since water removed directly from matrix
vacuum/foam drying - VOLUME INCREASE!
How can we reduce the loss of volatiles during drying? (2)
lower temperature
vacuum drying
What are some undesirable changes that occur with drying?(3) The extent to which they occur depends on: (2)
- loss of texture
- lack of complete rehydration
- loss of volatiles
depends on product type, and drying mode
What could happen if you use air that is too dry? Describe what happens
Why is this bad?
Case hardening: initial drying is very fast, but moisture from inside is slower to diffuse
outer layer is dry and hard, but inside still moist - trapped inside!
quality defect:
-> deterioration during storage
What might still occur in storage of dried foods? This leads to: ______ (2)
chemical/enzyme/microbial activity
leads to off flavor, discoloration
sulfites can be used for what in dried food? (2)
as antimicrobial
as bleaching agent
How can we reduce the deteriorative reactions in dried foods? (6)
blanching (for enzymes) antioxidants antimicrobials (SO2) sulfite dip - bleaching vacuum pack inert gas/CO2 pack
What 2 forms of moisture migration need to be considered for stability of a dried food?
- moisture transfer through package
2. moisture equilibration inside package
The thicker the surface film, the less the _____.
vapor flux
What Law describes the moisture migration from outside to a food in a package?
Fick’s law of diffusion (movement of moisture from wet outside environment to inside dry environment, across a BARRIER FILM)
An increase in _____ (3) would cause more vapor flux in fick’s law.
surface area
diffusivity
difference between outside and inside partial pressures of vapor (difference in moisture)
A decrease in ______ (2) would cause more vapor flux in fick’s law.
thickness
Temperature
The 3 types of water in food:
monolayer (I) - STRONGLY bound to specific sites
multilayer (II) - less strong, form multiple layers after monolayer is covered (non-solvent, non-freezable)
free (III) - loosely associated, mobile
monolayer water has the lowest ___ and ___, while free water is at the highest.
Aw, moisture content
Should monolayer water be removed? Why or why not?
No
- expensive!
- need layer to protect fat from oxidation
The ___ isotherm is used to determine monolayer water.
Monolayer water is represented by:
BET (Brunauer Emmet Teller) isotherm
m1
2 common moisture sorption isotherm models:
Rockland
Henderson
Using the BET isotherm, first the equation is _____, and then ____ is used to find the monolayer water.
linearized
slope and intercept
What is the difference between the 2 common MSIs?
Henderson: ln(1-a) = -cm^n
Rockland: log log (1/(1-a)) = n log m + Y
What is the significance of “n” and “Y” in the rockland MSI?
represents parameters and constant
- apply for LOCAL ISOTHERMS (each section of the plot will need specific n,Y pair)
At what Aw ranges is the moisture content most sensitive to change? When is it the least?
How does this show in a MSI?
moisture content sensitive to changes in HIGH Aw, or LOW aw
least sensitive in middle range (STABLE REGION)
FORMS SIGMOID CURVE
what does a stability isotherm relate?
Aw vs slope of MSI
The stability isotherm is at its lowest in: _____
intermediate Aw; stable region
What is the salwin concept?
Moisture in different components packaged together will migrate until Aw has equilibrated
How is the combined stability isotherm for multiple components determined?
add together slope values (y values) for components
