Thermal processing of food safety and technology

lecture

1. THERMAL PROCESSING
1. Introduction and Principles
1
Contents:
1.Introduction
2.Theory of thermal processing
3.Thermal processing on food quality
2/24/2022
?What is food processing? (By text book defition??)
Commercial food processing is a manufacturing branch that starts with raw
food materials and converts them into intermediate food staffs or edible
products through the application of labor, machinery, energy, processing
technology and scientific knowledge (Connor 1988 with modifications).
2
What is food processing?
Purpose of food processing?
?Food processingis the transformation of raw
ingredientsintofoodby physical or chemical means.
For consumer:
1. It helps in preventing spoilage of food.
2. It helps in retaining the nutritive value
of food.
3. It ensures the availability of food
throughout the year.
4. It ensures the availability of food even
at distant or remote places.
5. It makes the food more palatable.
For food industry:
1. To extend the period during which a food
remains wholesome (shelf-life)by preservation
techniques which inhibit microbiological or
biochemical changes and thus allow time for
distribution, sales and home storage.
2. To increase variety in the diet by providing a
range of attractive flavours, colours, aromas and
texture in food.
3. To provide the nutrientsrequired for health.
4. To generate income for the manufacturing
company and its shareholders.

2. Major processing strategies for food
preservation
1.By addition of heat: thermal processing at elevated temp:
9Cooking
9Blanching
9Pasteurization
9Heat sterilization
2.By removal of heat: processing at reduced temperature.
9Chilling, freezing
9Freeze drying
9Freeze concentration
3.By removal of water: reducing water activity, e.g. drying.
4.By packaging.
5
1. Inactivation; 2. &3.Inhibition; 3. Prevention from contamination
Processing of food
6
A general process for
manufacturing of canned foods
starting from field harvesting
(farm).
Figure 10.1 Typical CanningOperations
(Fellows 2017)
Harvesting
Receiving
rawproduct
Soakingand
washing
Sortingand
grading Blanching
Peeling and
coring
FillingExhaustingSealingProcessing
/Retorting
Warehousing
andpacking
Cooling Labeling
Q: Identify the unit operations and
the major functions of each.
History of food preservation by thermal
processing
?Appertizing: In late 1790?s ?Nicholas Appert, the French
inventor of airtight food preservation, known as "thefather
of canning",observed that food heated in sealed containers
can be preserved ?the Art of Appertizing, canning.
Appertcanning jar
?Pasteurization: In 1860?s, French
scientist Louis Pasteurdiscovered that
microbes were responsible for spoilage of
wine and beer (souring) and invented the
process of pasteurization, heat sterilization
of foods.
7
Objective of thermal processing
?Ensure safety (kill microorganisms)
?Increase digestibility
?Increase shelf life (destruction of enzymes, toxins)
?Add value (texture, flavour, colour)
?Make varieties of new products
?Meet the needs of specific section of population

3. Importance of thermal processing
One of the most important food processing methods.
Advantages:
-the desirable effects on food flavors (e.g. cooking, baking and roasting) that
cannot by other means.
-preservative effect by the destruction of enzymes, microorganisms, insects
and parasites.
-the capability of inactivating anti-nutritional factors (e.g., trypsin inhibitor in
some legumes)
-the capability of improving the availability of some nutrients (e.g.,
digestibility of proteins, gelatinization of starches and release of bound
niacin).
A major disadvantage:
-Destructive effects on heat-sensitive nutrients, flavors, colors, tastes or textures.
9
?Primary objective: to increase palatability
?Baking, roasting -dry heat, 150-200°C
?Boiling, stewing, steaming -boiling water to steam
(~100°C)
?Frying with or without oil, 175 to 225°C
?Some preservative changes
?Destruction of some spoilage and pathogenic
microorganisms
?Inactivation of deteriorative enzymes
?Reduction of water
Processing of food via Cooking
?Prior to drying of fruits
?Primary objective
?Inactivate deteriorative enzymes
?Also kills some spoilage bacteria (reduces microbial load)
?Commercially
?Atmospheric steam or boiling water (~100°C)
?Pressurized steam or hot gas (>100°C)
?Less severe process than canning for example
Processing of fruits and vegetables
via blanching
Processing of fruits and vegetables
via blanching
12
Figure 15. A general flow chart of frozen fruits and vegetables (Mallett, 1993).
www.fao.org/docrep/008/y5979e/y5979e03.htm#TopOfPage
Cold storage and packing
Blanching
(Mainly vegetables)
Freezing
Freezer packages
Hot water, stream
or microwave
Utensils for pre-
freezing
Transportation
Harvesting and transportation
Raw materials
Preparation
Cleaning/washing, peeling, slicing, dicing

4. ?A heat treatment which kills part of the microbial population
present in a food
?Min changes in sensory or nutritive value
?Pasteurization of milk
?Primary objective is to kill pathogenic micro-organisms; shelf
life is extended due to a reduction in spoilage organisms,
deteriorative enzymes
?Target pathogens: was Mycobacterium tuberculosis (TB), now
Coxiellaburnetti(Q fever)
Processing of fluid foodsvia pasteurization
?Two equivalent processes in terms of microbial kill
?LTH (low temp, hold) = 63°C for 30 min
?HTST (hi temp, short time or flash) = 72°C for 15 sec
?However, LTH is more detrimental to nutritional and sensory
properties
?Don?t drink raw milk -
?Other pathogens -Ontario study (1997) Listeriamonocytogenes,
salmonella, VerotoxogenicE. coli.
Processing of fluid foodsvia pasteurization
?Beer
?Intention to kill spoilage organisms-
primarily wild yeasts, gives a shelf life of 6
months for bottled beer vs 1 month for draft
?Juices
?Eggs -recommended that pasteurized eggs be
used for salad dressings and sauces to kill
salmonella
?Can now do ?in shell?
Processing of fluid foodsvia pasteurization
16
Figure 69.2 General flowchart for commercial fluid milk processing procedures
(Adapted from Q69:1 FDA Workshop, St. Louis, MO, 2000).
Processing of fluid foods(e.g. milk) via pasteurization

5. Thermal sterilization
?Sterilizationis the removal/destruction of microorganisms such
as yeasts, molds and bacteria in nutritional and biological objects.
?The term ?sterilization?in the food industry refers to the
achievement of commercial sterility, at which the potentially
harmful microorganisms are eliminated and not able to grow in
the food under normal non-refrigerated storage and distribution.
?Compared with other processes available such as microfiltration,
radiation and chemical treatment, heating is the most efficient,
cost-effective, and convenient means for food sterilization, with
relatively simple and easily controlled processes.
17
?A heat treatment sufficient to destroy ALL
microorganisms capable of growth under the conditions
of storage
?This is not true sterility as some nonpathogenic spore
forming bacteria may eventually grow under optimum
conditions
?Severity of the process required for safety and shelf
stability is dependent on the acidity of the food product
being canned.
?Low-acid foods (pH > 4.5)
?Acid foods (pH 3.7 to 4.5)
?High-acid foods (pH < 3.7)
Processing of foodsvia Sterilization
?Factors Influencing Severity
?Severity of thermal process required to produce
commercial sterility depends on:
?Nature and heat resistance of the microbes present
?Initial microbial load
?Nature of the food (e.g. pH, chemical composition,
water activity?)
?Conditions of storage
Processing of foodsvia Sterilization Conc.
?Two ways of thermal processing for food preservation:
1)In-container sterilization/retort processing: heating the food
products packaged in hermetic/sealed containers (e.g. tin
cans, glass jars and flexible pouches). Common and for
longer shelf-life
2)In-flow sterilization: directly heating the food products, and
then packaging aseptically. Less common and for shorter
shelf-life. Thus, usually combine with Aseptic packaging.
?Food processed exterior to package
?This must be done in an aseptic environment
?Packaging must be sterilized prior to packaging
Processing of foods via Sterilization Conc.

6. Instantaneous heating
Two Methods for Sterilization
Long heating and coolingprofiles
?Typically done in either a metal can or a retoratable
pouch.
In packaging Processing
?Also called Appertization
?A brief history lesson
?Nicolas Appert, 1809, won a prize from Napoleon
for discovering way to preserve food by enclosing
in hermetically sealed containers
Canning
?Impermeable to recontamination of bacteria
?Air tight seal
Canning ?Hermetic Seal

7. Canning
?Batch retort
?Must be done in pressurized environment to preserve
package integrity
?Very little industrial application
?Labour intensive
?Natural convection
?Continuous retort
?Still must be pressurized
?Induces forced convection
Retorts
Batch Retort Continuous Retort

8. ?Sterilize with a heat exchanger or steam injection
?Put in sterilized package in a sterile environment
?Drink boxes (tetra pack, Combibloc)
?Cups (Bosch)
?Packages sterilized with H
2O
2and UV light
Aseptic Processing Aseptic Plant
Filling Apparatus
Flexible retort
pouch
Glass
bottles/jarsTin cans
32
Aseptic cartons
Thermal sterilized food packages

9. ?Various degrees of heat treatment depending on the nature of
the product, and whether other preservation methods are used.
?Chemical changes during food storage can be controlled by
9appropriate heat treatment.
9maintaining proper storage temperature.
9addition of food preservatives (e.g. antioxidants).
?Enzymes in the food must also be inactivated by heat.
Means for food preservation
33
As microbial action is the main cause of food spoilage,
sterilization, the destruction of microorganisms is the chief aim of
thermal processing.
Inactivation of enzymes
?Enzymes are naturally present in foods, which can cause food
spoilage such as
9oxidation of fats,
9hydrolysis of proteins,
9destruction of vitamins, and
9development of undesired color.
?Normally, the heat processes which render the canned foods
commercial sterility are also adequate to inactivate the enzymes.
?However, problem may arise in UHT process (132-150?for a
few seconds).
Enzymes must be
inactivated to
preserve foods.
34
Important factors for
Thermal destruction of microorganisms
1)Degree of bacterial contamination-Bacteria are killed by
heat at a logarithmic rate.
2)Processing temperature -Usually for every 10qCincrease
in the processing temperature, the heating time required is
reduced 10 times.
3)pH of the food -As pH is lowered, less heat is required to
kill microorganisms.
4)Food composition-such as fat content, sugar and salt
concentration.
Microorganisms for food spoilage
?Microbial classes: Yeasts and molds (belonging to fungi) and
bacteria
?Major spoilage bacteria:
?Clostridium botulinum6?"?9?:existing in soil and water; labeled as the
deadliest in the world; mainly found in canned and bottled products.
?Salmonella spp."?K?"?9?: commonly associated with seafood, poultry, and
uncooked eggs.
?Shigellaspp.: often found with water and fresh fruit and vegetables exposed to
fecal contamination.
?Escherichia coli: one of the most well known bacteria; resides in the intestines of
human and animals. Some strains are dangerous to health.
(Ref: Wilhelm LR et al. 2004)

10. ?Clostridiumbotulinum6?"?9?
?Itisthemostheatresistantandisalsopathogenic;a
spore-former.
?mesophilic,anaerobic
37
?Destruction ofC. botulinum is a minimumrequirement of heat
sterilization. Normally, foods receive more than this minimum
treatment as other more heat-resistant spoilage bacteria may also
be present.
Cl. botulinum: the marker bacterium for food
sterilization
?Under anaerobic conditions in a sealed container, can grow
to produce a potent exotoxin, botulin6?"?9?"3P.
?inhibited in foods with pH < 4.5
?a potential hazard in foods with pH > 4.5
Thermal resistance of microorganisms
Microbial classes based on optimal growth temp:
?Psychrophilic (low-temp growth) organisms: -10 qCto 10qC
?Mesophilic (medium-temp growth) organisms: 10qCto 40qC
?Thermophilic (high-temp growth) organisms: 40 qC to 80qC
38
(Ref: Wilhelm LR et al. 2004)
Thermal resistance of microorganisms
39
D and Z are useful parameters for evaluating the heat resistance of
microorganisms as well as enzymes and nutrients in food.
2. Theory of thermal sterilization
?Thermal death rate of microbes and
enzymes follows the first order kinetics,
known as the logarithmic order of death,
as given by,
(2)
N= number of microbes = N
tat time t and N
0 at time 0
k= death rate constant or rate constant for microbial inactivation
Thermal Death Kinetics
40
Integration:
(1)
N
t
N
0
N
t
Thermal death time trend

11. D-Value: Decimal Reduction Time
?D is the time required to kill 90% of the microorganisms or to
achieve 10 times reduction of the viable microbial population at
a given temp.
?D is also a quantitative index for the thermal resistance of a
given microorganism, which varies with temperature.
Since ln (y) = 2.303 log (y),D = 2.303/k from previous Eq. 2.
(3)
?The decimal reduction time or the D-value is defined by,
41
?Eq.3 is rewritten as,log N
t= -t/D + log N
0, so that plot log N
t
versus tis a straight line with the slope = ?1/D .
HKC
0
P
0
r
LF
P
&
In class exercise: D-Value Calculation
?Calculate the D Value at 60
o
C for C. burnetiiin
milk from the data table provided
Time
(min)
Concentration
0 100,000
2 50,000
4 25,000
6 12,500
8 6,250
10 3,126
0
20,000
40,000
60,000
80,000
100,000
120,000
0 2 4 6 8 10 12
Time (min)
Population
Time
(min)
Concentration
0 100,000
2 50,000
4 25,000
6 12,500
8 6,250
10 3,126
Time (min)
Log Population
3
3.5
4
4.5
5
5.5
0 2 4 6 8 10 12
In class exercise: D-Value Calculation
?Calculate the D Value at 60
o
C for C. burnetiiin
milk from the data table provided
Time (min)
Log Population
3
3.5
4
4.5
5
5.5
0 2 4 6 8 10 12
In class exercise: D-Value Calculation
?Calculate the D Value at 60
o
C for C. burnetiiin
milk from the data table provided

12. D-Value D-Value
D-Value
?D
90of grey line is
5 min
?D
90 of red line is 9
min
?Which organism
is more heat
sensitive?
D-Value
?Usefulness of D-values:
?if we know the D-value at a particular temp for our
target organism and the # of organisms present, we
can calculate how long to heat the product at this
temperature to destroy the entire population.

13. Fellows, 2000
Thermal Death Rate Curve
Figure 1.45 Thermal death rate curve (Fellows 2017)
49
log scale
3
1
0
2
log N
Regular
scale
N
HKC
?
?
?
?
LF
5
?
t
HKC0
PLF
5
?
PEHKC0K
)RUDOLQHDUHT
UL=TE>
'y
'x
b Slope a ='y/'x
y
x
HKC
?
?
?
?
LF
5
?
t
PL&HKC
0
r
0
P
= one log-
cycle reduction
50
Effect of temperature on microbial destruction
?Microbial destruction is faster at higher T, so that D decreases as
T increases, e.g. D at 121qC |1.3 min and |5.6 min at 115 qC.
Stoforos Fig 2.1 Thermal death rate curve. Graphical determination of
decimal reduction times (D) at two temp.
Thermal Death Time Constant: the Z-value
?Z-value:Thermal death time constant or thermal resistance
constantis defined as the qC (or qF)temperature increase
required to attain a ten-fold or one log-cycle reduction in D-
value.
51
log scale
Figure 1.46 TDT curve. (Fellows 2017)
Thermal Death Time Curve
Take reference T
o= 121
o
C:
log D
T= F(1/Z)T + b
log D
o= F(1/Z)T
o+ b
D
T=D
12110
(121-T)/z
HKC
&
6
&
K
LF
:6F6K;
<
HKC
&
6
&
sts
LF
:6Fsts;
<
HKC
&
6
&
sts
LF
:6Fsts;
<
Slope =F
HKCsr
<
LF
s
<
= one log-
cycle reduction
Z-value

14. Z-value
?Why is it better to process for short times at
elevated temperatures compared to low
temperature for longer time
Z-Value
?According to the first-order death kinetics, N
t
= 0 only whent ?f: it is not practical to
achieve complete sterilization,
D
t
N
N
t

?
?
?
?
?
?
?
?
0
log
55
?Practically, a food product is processed to an acceptable level
RIVSRLODJHKD]DUGNQRZQDV?commercial sterility?In a batch
of food product, the microbial population is reduced from an
initial value N
0to a desired value N
f,
N
0?N
f m = log (N
0/N
f)
where mis called the reduction exponent, the fold of log cycle reduction by
the process.
Commercial sterility
?For example, the reduction from 10
4
spores/can to 10
-3
spores/can, m= log10
4
/10
-3
= 7, is a 7Dthermal processing.
56
Thermal death time: the F-value
?Thermal death time F is defined as the time required to achieve
a stated reduction in the microbial population at a given T.
F = m ×D = log (N
0/N
f) ×D = D (log N
0?log N
f)
?F-value is usually expressed as multiples of D-values.
?For example, in a given process, the number of microbes is
initially N
0= 10
2
per volume and reduced to N
f= 10
-2
per volume
at the end of 40 min period, m = log (10
2
/10
-2
) = 4, F = 4D = 40
min.
?F-value is a useful parameter for comparing heat processing
operations.

15. ?Reference F-value(F
0): for a process that operates at a
reference T, targeting on an organism with a given Z-value.
Process F
0values (min)
Vegetable in brine
Cream soups
Meat in gravy
3-6
4-5
12-15
57
Reference F-value: F
0
Typical F
0values at 121
o
C:
?Generally, the reference temperature is 121
o
C(250
o
F) and
reference Z-value is 10
o
C
10
121F
Microprocessor for recording
temperature and calculation of F0
Thermal Death Time
F-Value
?F=thermal death time, m is the number of
decimal reductions required, and D is the
decimal reduction time.
?If a 12 D process is required, how long would
C. burnetiiin milk have to be processed for at
60
o
C (Dvalue=6.64min)
?For a microbial population, Frepresents the thermal death timeof
the microbial population in a food product.
?For a thermal process, F refers to the sterilization value(SV) of
the process (SV = F/D). SV is a useful parameter for comparing
sterilization processes.
59
Practical meaning of D, Z and F
1. An organism has a F
0-value = 3 min means that it takes 3 min at 121°C to
destroy the organism which has a Z-value = 10°C to achieve commercial sterility.
2. A thermal process has a F
0= 3 min means that the combined time-temperature
effect of the process is equivalent to heating at 121°C for 3 min targeting on an
organism having a Z-value of 10°C.
?D-value and Z-value are used to quantify the heat resistance of a
microorganism and its temperature dependence, respectively.
Lethal rate and Sterilization value
?Lethal rate (L) is the rate of microbial cell death at T?
relative to the rate at a reference temperature.
Since F = m ×Dand if the reference T is 121 qC,
121 /121 121
10
TZ
TT
FD
L
FD


60
?The area under the lethal rate curve is the actualsterilization
value (F) or lethality of a process. The F value of a thermal
process must be tm ×D in order to achieve commercial
sterility.
121
0
t
Ldt mDt?
= F
121
(F
processtF
required)
?The F value of a process is formally defined as the equivalent processing time
of a hypothetical thermal process at a constant, reference temp, Tref, that
produces the same destructive effect as the actual thermal process.

16. 61
Calculation of sterilization value of a process
F
121/F
T= D
121/D
T= 10
(T-121)/Z
, T
ref =121°C(= 250°F)
0
0
t
FLdt ?
Time, min
121/
10
TZ
L


F
121/
10
TZ
L


Time, min
F
(A) Constant temp (B) Varying temp
Therefore, the sterilization value of a process is the integrated
effect of temperature and time on microorganisms.
F is equal to the total area under the curve:
(The cooling time data are neglected as a safety margin.)
62
(121)/(121)/
0
00
1,2...
10 10
i
tt
TzTz
in
FLdt dt t


| ' ???
Calculation of F value of a process with varying
temp by graphical integration
- 121/
10
TZ
L


Graphical
integration
3. Thermal processing on product quality
63
Chemical changes caused by thermal processing
?Thermal processing also inevitably sacrifices nutritive value in
some cases, and may cause harmful effects on the color, flavor
and texture. Specifically, heating can,
-destroy some heat-labile vitamins.
-reduce the biological value of proteins (e.g. owing to
destruction of amino acids or Maillardbrowning reaction).
-remove volatile aromatic compounds.
-promote lipid oxidation.
?Temperature effect: Arrhenius equation
/
00 0
ln ln log log
2.303
a
ERT aa
EE
kke k k k k
RT RT

o o
Kinetics of thermal destruction
00
2.303
ln ; log ;
2.303
CCkt
kt D
CC k
?? ??
?? ??
?? ??
k=reactionconstantatT(TinK);
k
o=Arrheniusconstant:frequencyfactor;
E
a=theactivationenergy(J/molorcal/mol);
R=gaslawconstant=8.314J/mol.K.
?Thermal destruction kinetics of many nutrients such as vitamins,
aroma compounds and pigments also follows the first-order
kinetics.

17. E
aand Z ?temperature dependence
?Smaller Z or larger E
ameans the reaction is more temperature
dependent or sensitive.
Heat effect on various contents of food
?From the D values, nutrients and quality factors are more heat
resistant than spores.
?From the Z values (or E
a), the nutrients and quality factors
show very different temperature dependence than spores.
Quality loss is less temperature sensitive.
?For example, thermal destruction of bacteria spores (Z=10°C) is
more temperature dependent than thermal destruction of
thiamin (Z=30°C) in milk.
?To destroy bacterial spores to achieve commercial sterility,
HTST process will give a better quality retention.
D and Z values in the above table
Optimization of a thermal process
?Aproperheattreatmentcanimprovepalatabilityofthefood,
improvethekeepingqualityandmicrobiologicalsafety,and
destroytoxicoranti-nutritionalfactors.
?Anoptimumthermalprocessshould
-destroy microorganisms, enzymes, and anti-nutritional factors.
-retain maximum nutrients and sensory quality.
?To optimize a thermal process for a product, it is necessary to
study the kinetics of the heat destruction of spores, enzymes,
anti-nutritional factors, nutrients, and sensory properties.
Temperature and time effects on nutrients
68
Thermal death time F versus T for
microbes (solid line) and quality attributes
(dash lines) (Heldman and Hartel).
A: 90% quality reduction;
B to E: lower degree of quality
reduction due to shorter time.
The relatively flat slope to that of F
for microbes: slower increase with
temperature increase
?Z values of most nutrients > those of microbes and enzymes:
better retained by the use of higher T and shorter-time heat
processing. This is basis for the modern quick blanching, high-T
and short-time (HTST) and UHT thermal processes.

18. Temperature and time effects on nutrients
69
Fellows Figure 12.9 Rates of microbial and nutrient destruction in conventional
sterilization (canning) and UHT.
T1 T2T3T4
t1
t2
t3
t4
To achieve the same sterilization value (thick line),
the nutrient (thiamin)loss (thin lines):
T1: 30% loss; T2: 20% loss
T3: 10% loss; T4: 1% loss
Summary and study guide
1.Major thermal processing methods for preservation and
sterilization of food products.
2.Kinetics/theory of thermal destruction of microorganisms
3.Important heat sterilization parameters, definitions,
mathematical and graphical expressions, calculations, and
practical meanings for the microorganisms/processes:
Decimal reduction time D-value; Thermal resistance constant Z-
value; Thermal death time F-value; Sterilization value/Lethality;
Lethal rate L
4.Thermal effects on food quality
70
As#3 Sterilization: 4?VIURP*HDQNRSOLVDQGIURP:LOKHOP
Examples: Geankoplis Ex9.12-1, Ex9.12-3. Toledo Ex 9.2, 9.3, 9.4, 9.5, 9.6,
9.10, 9.11.Wilhelm Ex7.2, 7.3, 7.5.
71
Fellows 11.1Raw milk containing 4u10
5
bacterial cells/ml is pasteurized at
77qC for 21 s. The average D-value of the bacteria present in the milk is 7 min
at 63qC and their z-value is 7qC. Calculate (1) the number of viable bacteria
that will remain after pasteurization; (2) the processing time required at 63qC to
achieve the same degree of lethality.
Given Z = 7 qC, D = 7 min at 63qC; Processing at T = 77 qC for t = 21/60 =
0.35 min
Exercises
72
D
126
D
122
Effect of temperature on destruction of microorganisms
?Microbial destruction is faster at higher T, so that D decreases as
T increases, e.g. D at 126qC |1.7 min and |5.0 min at 122 qC.