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Week 3:Thermal Processing Principles - Coggle Diagram
Week 3:Thermal Processing Principles
Microbial Survival Curve: thermal destruction of microorganisms (m/o) shows that they follow a first order reaction hence indicating a logarithmic order of death.
(i.e. logarithm of the no. of m/o that survive during a particular heat treatment at particular temperature when plot against heating time will give a straight line.
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The microbial population reduction (N) as a function of time (t) can be described using a first-order model :
๐๐/๐๐ก=โ๐๐ whereby k: first-order rate constant
The basic model above is being used to describe survivor curves obtained when microbial populations are exposed to elevated temperatures.
influenced by external agents such as temperature. As the magnitude of external agents increases, the rate of microbial population reduction also increases.
Thermal Resistance of Microorganisms (D value & z value) :check:
D Value
The decimal reduction time ,D, is the time required to cross one log-cycle (decimal reduction) or time required for 90% reduction in the microbial population at a specific temperature
Z Value
Thermal resistance constant (z) is defined as the temperature increases required for one log cycle reduction in D values or 90% reduction in D value.
z=(๐2 โ๐1)/logโกใ๐ท๐1โlogโก๐ท๐2 ใ
Thermal Death Time (F) :check:
Thermal death time, F :time required to achieve a stated reduction (required to achieve desired shelf life e.g. in terms of food safety/spoilage to reduce pathogens or spoilage m/o) in microbial population at a given temperature.
Formulae
F = m D
whereby m = number of decimal reduction
Lethality (F0) :check:
Calculation of Lethality (General Method of Process Calculation) :check:
Lethal Rate: converts the actual heating time of a process at a specified temperature to the time required at the reference temperature that will achieve the same destruction of m/o.
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a dimensionless number
L = 10^([๐โ๐0)/๐ง]) whereby To = Reference temperature
Purpose: To determine the appropriate processing time/temperature conditions to achieve the desired lethality or to estimate the process lethality of a given set of processing parameter.
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Desired degree of lethality (F0) is pre- established for the product
process parameters are established to ensure the product thermal centre achieves this pre-determined value.
two classes
(2) Formula Method
Integrate the lethal effects by using parameters obtained from heat penetration data together with several mathematical procedures
Advantages
Used widely for process calculation due to convenience
No need to obtain experimental heat penetration information for each set of process conditions
More flexible
Limitation: Not easy to apply for products with non-linear heating curves.
(1) General Method/Improved General Method
Integrate the lethal effects by either graphical or numerical integration procedure using the time-temperature data obtained from actual test containers that are processed under actual commercial processing conditions.
Advantages
Takes account of the entire heating & cooling effects including any changes in heat penetration rate (e.g. product gelation or liquefaction)
No assumption made in relation to the nature of heating/cooling curve (i.e experimental data are taken directly, converted to lethal rates and integrated with respect to time.
More accurate than Formula methods
Limitation: Less flexible since process time is specific for a given set of processing conditions only.
Improved general method
can be calculated using the integration of lethal rate over the whole processing time whereby the whole area under lethal rate curve is known as the lethality.
Area is the F0
Counting squares:
Count the number of squares under the curve only. For squares falling on the curve, count those with more than half of the square falling inside the curve
Lethality (F0): time at the reference temperature that achieves the same degree of destruction of micro-organisms as the process under study :check:
Formula
F0 = F value at reference temp (To)
F0 = F 10^([๐โ๐0)/๐ง])
F = F0 10^([๐0 โ๐)/๐ง])
Alternatively, it can be defined as an equivalent heating of 1 min at the reference temperature
Spoilage Probability: used for the estimation of the number of spoiled containers within the total batch of processed products.
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p = 1/๐ = "No" /"10 F/D "
whereby p = probability of having 1 container with spoilage within the total number of processed cans.
Whereby No = initial population of m/o
Nf = final population of m/o
Principles :check:
Aim: destroy pathogenic/spoilage microorganisms and enzymes that result in food deterioration during storage.
NOT designed to destroy ALL microorganisms in packaged food since such processing will result in poor product quality.
6 Factors
Type, initial load and heat resistance of target m/o, spores, enzymes
Nature of food (e.g. pH, Aw)
Heating medium and conditions (e.g. air, steam, temperature of retort)
Thermophysical properties of food (e.g. solid, liquid, viscosity)
Container shape and size
Storage conditions after heat treatment
Thermal Centre
Some factors affecting the centre
Mode of heat transfer
Viscosity (When decreases, the thermal centre drops from centre)
Shape/size of containers
Theoretically, thermal centre of conduction heating is at the geometric centre while convection heating product is slightly below.
Important to locate the thermocouple tips at thermal centre for solid products (by conduction) to prevent significant heating lags.
