Lecture 5: Thermal Process Calculations (General Methods)

Question 1

what are the two factors that describe the TDT curve?

Answer 1

reference TDT and z value

Question 2

in the TDT equation: Fo^18 = 0.2min, what does each component mean?

Answer 2

Fo is the reference temperature at T of 250degF = 0.2min

z value is 18degF

Question 3

does destruction kinetics or thermal processing have a CUT time?

Answer 3

thermal processing has a CUT, but destruction kinetics does not

Question 4

what are main differences between destruction kinetics and thermal processing

Answer 4

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

Question 5

in destruction kinetics, Fo is what?

for canning, Fo is what?

Answer 5

in destruction kinetics: Fo = TDT

canning: Fo = process lethality

Question 6

what is process lethality?

Answer 6

represents the equivalent heating time of the entire process expressed as minutes at 250F

Question 7

describe differences with heating by conduction vs by convection

Answer 7

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

Question 8

in convection heating, where is the hottest and the coldest points?

Answer 8

hottest point: around middle and 1/10th from the bottom

cold: near the top area

Question 9

what factors does product temperature profile depend on?

Answer 9

1. heating process: still vs agitated retort
2. heating medium: steam, water, with/without overpressure, steam-air
3. heating conditions: retort temp, initial temp loading pattern
4. product type: solid, liquid, etc…
5. container: type, shape, size

Question 10

from a safety aspect, what is the objective of thermal processing?

Answer 10

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

Question 11

from a quality point of view, what is the objective of thermal processing?

Answer 11

• 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

Question 12

what is PE?

Answer 12

process establishment

Question 13

what is PV?

Answer 13

process verification

Question 14

what are the three categories of process calculations?

Answer 14

1. general methods: graphical and numerical integration
2. formula methods
3. umerical methods

Question 15

what is the original general method based on?

Answer 15

destruction data on a selected test microorganism under the processing conditions

Question 16

what data is needed for the original general method?

Answer 16

1. thermal resistance data (TDT and z value)

2. time-temperature data in product

Question 17

the reciprocal of TDT is _____

Answer 17

rate of destruction

thus:
rate of destruction = 1/TDT

Question 18

the extent of destruction is____

Answer 18

rate of destruction x time (at temperature T)

Question 19

how is sterilization value related to TDT?

Answer 19

SV = sum of (1/TDT)

aka, SV is the sum of all extends of destruction

Question 20

what does SV = 1 or t = TDT mean?

what is the minimum required time of destruction?

Answer 20

destruction is complete (achieved full sterilization)

min required is SV of 1 + safety margin.

a 100% safety margin means SV = 2

Question 21

what units does SV have?

Answer 21

no units

Question 22

what does SV = 2 mean?

Answer 22

achieved sterilization twice

Question 23

what is the minimum D for sterilization of C.bot?

Answer 23

12D

Question 24

the general method is based on integrating ____ vs ____

Answer 24

1/TDT vs time

Question 25

how to you find the sterilization value graphically?

Answer 25

SV is the area under the curve of 1/TDT vs time

Question 26

what does the sterilization value of a process indicate?

Answer 26

the total extent of destruction at the cold point of the an over the entire process in relation to a target microorganism

Question 27

what is the equation of SV?

Answer 27

SV = heating time at T / TDT at T

Question 28

what are limitations of the general method

Answer 28

• tedious and limited to experimental set up
• need heat penetration data for every condition
• no generalization
• based on an indicator microorganism

Question 29

what is Fo/F defined as?

Answer 29

lethal rate or L value

Question 30

what units does the lethal rate have?

Answer 30

no units

Question 31

what is the difference with the improved general method and the old general method?

Answer 31

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

Question 32

the area under the lethal rate and time curve is equal to what?

Answer 32

Fo (process lethality)

Question 33

what is the advantage of using the improved general method?

Answer 33

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