Lecture 10: Food Freezing Flashcards
what are the 5 freezing time definitions?
- total time from start to finish. Depends on initial and final temp
- time taken by food from initial temp to reach a target temp (-10 or -18C)
- time from food reaching odegC to -10C or -19C
- time from initial freezing temp to 10C below initial FP
- time to cross zone of max ice crystal formation (-1C to -5C)
which is the most accepted freezing time definition?
- time taken by food from initial temp to reach a target temp (-10 or -18C)
in the 1st freezing rate definition, how is the rate defined?
what two variables need to be known to determine the rate?
as the rate of propagation of a frozen layer from the surface towards he center
need to know
- freezing time
- sample thickness
according the the first freezing rate definition, what are the 2 ways that rate can be defined?
- half-thickness: freezing from both sides
2. thickness: freezing from one side
according to the 2nd freezing rate definition, what rates are considered to be slow, moderate and fast?
slow: rate < 1cm/h
moderate: rate = 1-5cm/h
fast: rate >5cm/h
what is the 3rd freezing rate definition based on?
the time to cross the zone of max ice crystal formation (-1 to -5degC)
in the 3rd freezing rate definition, what is considered quick or slow freezing?
quick: if time is < 30 min
slow: if time is > 30 min
what is the estimation of freezing time and rate important for?
process
product
package
equipment design
what are the two methods for determining freezing time and rate?
- numerical solution: using heat transfer equations involving phase change problems
- empirical equations: based on assumptions and approximations
what are the three periods in freezing time?
what fraction of the total time does each take up?
- pre-cooling: 1/5th of total time
- phase change: 7/10th
- tempering: 1/10th
what is the earliest model to predict freezing times?
plank’s model
which period does plank’s model ONLY estimate?
the phase change period
what do the more recent empirical models, after plank’s model incorporate?
all three periods (pre-cooling, phase change and tempering)
what are the assumptions in plank’s model?
- product to be frozen is free water
- FP of product is 0degC
- freezing process is initiated w/ product at 0degC (as water) and terminated when water is converted to ice at 0degC
- all heat is removed in the form of latent heat
- model is based on rate of progression of ice front from the surface
- freezing is terminated when the ice front reaches the midpoint or end/bottom
the plank model is based on solving 3 heat transfer relationships, which are?
- convection heat transfer at surface
- conduction heat transfer at frozen region
- latent heat absorption at front ice
