Method for determining lethality of thermal processes

1. Methods of determining lethality of
thermal processes
“IMPROVED GENERAL
METHOD”
Samuel J
521FP6001

2. CONTENTS
• Introduction
• Brief about bacteria
• Methods to determine lethality
– Graphical Method
– Improved graphical method

3. Introduction
• Thermal processing of foods
–Inactivation of microorganisms
–Preserve the food for as long as possible
• Methods
–Aseptic processing
–Pasteurization
–Sterilization

4. Bacteria

5. Factors affect the heat resistance of
cells
• Temperature time relationship
• Initial concentration of spores
• Previous history of the vegetative cells and spores
• Composition of the substrate in which cells or spores are
heated

6. Effect of temperature of heating on time
Temperature (oC) Thermal death time or time
to destroy all spores, min
100 1200
105 600
110 190
115 70
120 19
125 7
130 3
135 1

7. Effect of initial no of spores on time
Initial concentration of TDT min at 121 oC spores, no/ml
• 50000 14
• 5000 10
• 500 9
• 50 8
• Source: Bigelow and Esty (1920)

8. Heat resistance of bacteria
• Cocci usually are more resistant than rods
• Higher the optimal and maximal temperatures
for growth, the greater the resistance to heat
• Bacteria that clump considerably or form
capsules are more difficult to kill than those
which do not
• Cells high in lipid content are harder to kill
than are other cells

9. Thermal death times of bacterial cells
Bacterium Time (Min) Temp
(oC)
• Salmoella typhi 4.3 50
• Staphylococcus aureus 18.8 60
• Escherichia coli 20-30 57.3
• Streotococcus thermophilus 15 70-75
• Lactobacillus bulgaricus 30 71

10. Shape of Bacteria
 On basisof shape, bacteriaareclassified as follows…
1. Cocci
2. Bacilli
3. Vibrios
4. Spirilla
5. Spirochetes
6. Many

11. Cocci
 Cocci are small, spherical oroval in shape
 In greek ‘kokkos’ meansberry
 E.g. Micrococcus

12. Bacilli
 They are rod inshapes.
 It is derived from greek word ‘Bacillum’ meaning
stick.
 Some of the bacilli the length of the cell may be
equal to width those are called coccobacilli
 E.g. Bracella

15. 1
5

16. 1
6

17. Spores
 Many bacterial species produce spores inside the cell & outside
the cell.
 Inside the spores are called endospores & outside the spores are
called exospores. E.g Bacillus anthracis, Bacillus subtilis etc.
 Spores are extremely resistant to desiccation, staining, radiation,
disinfecting chemicals & heat.
 Each bacterial spore on germination forms a single vegetative cell.
 They remain viable for long time & help bacteria to survive for
long period under unfavourablecondition.

18.  Endospores are thick-walled, highly refractile bodies that are
produced one percell.
 All the endospores contain large amount of DPA (dipicolinic
acid).
 It occurs in combination with large amount of calcium, which
is present in central partof the spore (core).
 That calcium & DPA complex play important role in the heat
resistant of endospores.
 Endospores consists of a core orenvelopeor protoplast.
 In the core or protoplast consist of DNA & ribosomes, t-RNA &
enzymes.
 The spore envelop consist of the inner membrane, outer
membrane, cortex & sporecoat.
 In some species have the outer layer called exosporium which
bears ridges & fold.

20. Thermal death times of bacterial spores
Bacterium Time to kill at 100 oC (Min)
• Bacillus anthracis 1.7
• Bacillus subtilis 15-20
• Clostridium botulinum 100-330
• Clostridium calidotolerans 520
• Flat sour bacteria over 1030

21. Bacteria used as test organisms
Bacteria used Processing or test purpose
• Clostridium sporogenes Inoculated pack
studies
• Bacillus pumilus Cobalt or gamma
radiation sterilization
• Bacillus stearothermophillus Steam sterilization
• Bacillus subtilis Steam Sterilization

22. Graphical Method
• The level of sterilization is expressed as a
treatment time and temperature for
– each type of product,
– shape, and
– size of container.
• If the product is treated at a fixed temperature, the
treatment time can be directly obtained from
Equation.
• However, the temperature of the product varies,
not only with position, but also with time.

23. • For this reason, it is commonly established that
time should be measured from the instant
when the work temperature is reached until the
end of the heating process.
• If temperature varies with treatment time,
integration is needed to obtain the reduction
degree required:

24. General method
• Target Microorganism Z = 10 oC
• Thermal death time (TDT) @ 121.1oC
= 2.52 min
• F121.1 = 2.52 min
FT = F121.1 * 10(121.1-T/10)
• TDTT = TDT121.1 * 10(121.1-T/10)

26. Improved general method
• Target Microorganism Z = 10 oC
• Thermal death time (TDT) @ 121.1 oC
= 1 min
• F T ref(F121.1) = 1 min
• FT = F121.1 10(121.1-T/10)
• FT /F121.1 = 10(121.1-T/10)
• F121.1 /FT = 1/10(121.1-T/10)
• F121.1 /FT = 10(T-121.1/10)
• 1/FT = 10(T-121.1/10)
• 1/FT = L
L = 10(T-121.1/10)

28. COMPARISON
General Method
• T. Microorganism Z = 10 oC
• Thermal death time (TDT)
@ 121.1 oC = 2.52 min
• F121.1 = 2.52 min
• FT = F121.1 * 10(121.1-T/10)
• TDTT = TDT121.1 * 10(121.1-
T/10)
Improved general method
• T. Microorganism Z = 10 oC
• Thermal death time (TDT)
@ 121.1 oC = 1 min
• F T ref(F121.1) = 1 min
• FT/F121.1 =10(121.1-T/10)
• F121.1 /FT = 10(T-121.1/10)
• 1/FT = 10(T-121.1/10)
• L = 10(T-121.1/10)

29. • C:UserssamuelseekerDesktopGraphical
Method.xlsx

32. A
B
C
G E
D
F

35. FO value for different process times
• ABC = 12.56 min
• ADE = 8.9 min
• AFG = 5.6 min

36. Plot graph FO vs T
13 .
12
11
FO 10 - - - - - - - - - - - -
9 .
8
7
6 .
28 30 32 34 36
Time

38. From graph
• FO = 33.5 min

39. References
• Food Microbiology W.C. Frazier
• Unit operations in Food Engineering Albert
Gustavo
• Handbook of Food Engineering Practices
R Paul Singh