Lecture 5: Thermal Process Calculations (General Methods) Flashcards
what are the two factors that describe the TDT curve?
reference TDT and z value
in the TDT equation: Fo^18 = 0.2min, what does each component mean?
Fo is the reference temperature at T of 250degF = 0.2min
z value is 18degF
does destruction kinetics or thermal processing have a CUT time?
thermal processing has a CUT, but destruction kinetics does not
what are main differences between destruction kinetics and thermal processing
destruction kinetics:
- has an instant CUT time
- small sample size
- constant temperatures
- heating bath is mostly pre-stabilized at target temp
thermal processing
- for food in cans and jars, etc…
- larger sample size
- shows heating and cooling lags
- destruction occurs over a wide range of temp
- cooked in pressurized retorts
in destruction kinetics, Fo is what?
for canning, Fo is what?
in destruction kinetics: Fo = TDT
canning: Fo = process lethality
what is process lethality?
represents the equivalent heating time of the entire process expressed as minutes at 250F
describe differences with heating by conduction vs by convection
conduction
- in tightly packed solid products
- little space for movement
- heat transfers via molecules without moving
convection
- in liquid products
- allows for movement of particles
- temperature of the product on bottom increases first
in convection heating, where is the hottest and the coldest points?
hottest point: around middle and 1/10th from the bottom
cold: near the top area
what factors does product temperature profile depend on?
- heating process: still vs agitated retort
- heating medium: steam, water, with/without overpressure, steam-air
- heating conditions: retort temp, initial temp loading pattern
- product type: solid, liquid, etc…
- container: type, shape, size
from a safety aspect, what is the objective of thermal processing?
to monitor the temp and establish the process at the coldest point in the container so that the desired minimum heat lethality with respect to pathogen destruction applies to every point in the container
from a quality point of view, what is the objective of thermal processing?
- to integrate the destruction achieved during heating and cooling
- since the product near or at the surface of the container will be more cooked than the product near the center, we want to minimize the bulk cooking
what is PE?
process establishment
what is PV?
process verification
what are the three categories of process calculations?
- general methods: graphical and numerical integration
- formula methods
- umerical methods
what is the original general method based on?
destruction data on a selected test microorganism under the processing conditions
what data is needed for the original general method?
- thermal resistance data (TDT and z value)
2. time-temperature data in product
the reciprocal of TDT is _____
rate of destruction
thus:
rate of destruction = 1/TDT
the extent of destruction is____
rate of destruction x time (at temperature T)
how is sterilization value related to TDT?
SV = sum of (1/TDT)
aka, SV is the sum of all extends of destruction
what does SV = 1 or t = TDT mean?
what is the minimum required time of destruction?
destruction is complete (achieved full sterilization)
min required is SV of 1 + safety margin.
a 100% safety margin means SV = 2
what units does SV have?
no units
what does SV = 2 mean?
achieved sterilization twice
what is the minimum D for sterilization of C.bot?
12D
the general method is based on integrating ____ vs ____
1/TDT vs time
how to you find the sterilization value graphically?
SV is the area under the curve of 1/TDT vs time
what does the sterilization value of a process indicate?
the total extent of destruction at the cold point of the an over the entire process in relation to a target microorganism
what is the equation of SV?
SV = heating time at T / TDT at T
what are limitations of the general method
- tedious and limited to experimental set up
- need heat penetration data for every condition
- no generalization
- based on an indicator microorganism
what is Fo/F defined as?
lethal rate or L value
what units does the lethal rate have?
no units
what is the difference with the improved general method and the old general method?
the improved method has
- construction of a hypothetical thermal destruction curve
- calculation of the lethal rate for each datapoint
- plotting lethal rate vs time curve
the area under the lethal rate and time curve is equal to what?
Fo (process lethality)
what is the advantage of using the improved general method?
the Fo (process lethality) calculated can be used to determine the extent of destruction of any microbe
the old GM was specific for each microbe
