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Unformatted text preview: Nt/No = 0.1, therefore D = 2.303/k.
D is most easily determined/visualized graphically by plotting the number of survivors
(log scale, Y-axis) against time of heating (arithmetic scale, X-axis) (Figure 9-7). This
transforms the exponential curve into a straight line allowing D to be easily obtained
from the graph. In the example Figure 9-7, D ~ 2 min. As might be expected, D
decreases with increasing temperature (Figure 9-8). 1000 100 Number of viable
cells or endspores
10 1 Time (min)
D ~ 2 min Figure 9-7: Graphical estimation of decimal reduction time FOOD-296 105 Number of viable
cells or endspores 104 Log scale
103 102 D70 D60 D50 Time (min) Figure 9-8: Effect of temperature on the decimal reduction time (D) Why don’t all cells in a population die instantly when treated with lethal heat or
chemicals? The reason is based, in part, on the random probability of the agent causing
a lethal "hit" in a given cell. Cells contain thousands of different proteins and thousands
of molecules of each. Not all proteins and not all genes in a chromosome are damaged
by an agent at the same time. Damage accumulates. Only when enough molecules of an
essential protein or a gene encoding that protein are damaged will the cell die. Cells that
die first are those that accumulate lethal hits early. Members of the population that die
later have, by random chance absorbed more hits on nonessential proteins or genes,
sparing the essential ones. Why, if 90% of a population is killed in 1 minute, aren't the
remaining 10% killed in the next minute? It seems logical that all should have
succumbed, yet after the second minute, 1% of the original population remains alive.
This can also be explained by the random hit concept. Although there are fewer viable
cells after 1 minute, each has the same random chance of having a lethal hit as when the
treatment began. Thus, death rate is an exponential function, much like radioactive
decay is an exponential function. A final consideration involves the overall fitness of
individual cells. It is mistaken to assume that all cells in a population are physiologically
identical and therefore equally susceptible to the chemical or physical agent. Practical
considerations Sterilization using high temperature is not straight-forward because
the use of very high temperatures may destroy the material being sterilized (eg. food)
the death rate, k, constant is different for different cells (eg. vegetative cells and
endospores have different sensitivities to high temperature).
the number of viable cells remaining a certain time after heat treatment depends on the
number originally present (see equation above).
Because the number of viable cells remaining after heat treatment depends on the
number of cells initially present and their identity, it is impossible to specify a time
required for sterilization (the time required to reduce the number of viable cells to <1) that
can be applied in all circumstances without first knowing
what kinds of cells are present.
how many of each type of cell...
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This note was uploaded on 10/25/2013 for the course MICB 201 taught by Professor Davidturner during the Fall '12 term at UBC.
- Fall '12