Modernist-Cuisine-Vol.-1-Small

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BACTERIA L GROWTH
All bacteria multiply by cell division. When an
individual bacterium reaches a certain point in its
growth, it splits into two separate cells. The time
required for a new cell to begin dividing depends
on local conditionsprimarily, the availability of
nutrients, the acidity (pH), and the temperature.
Bacterial cell division is not as regular as clockwork,
but under the right circumstances, it can
happen in minutes.
Mathematically speaking, the process is known
as geometric growth or exponential growth. It
can be extremely rapid, doubling a bacterial
population with every round of cell division. If
you start with a single bacterial cell, the growth
sequence would be one, two, four, eight, 32, 64,
and so on. After 10 doublings, that single bacterium
would become 1,024. After 20, the population
would exceed one million.
Clostridium perfringens currently claims the
record for fastest known bacterial replication: in
one study, it reached a doubling time of less than
eight minutes in ground beef, meaning it could
theoretically grow by a factor of one million in less
than three hours. Other foodborne pathogens
replicate more slowly in food, but many can still
double in 30 to 50 minutes, resulting in a potential
millionfold increase in 10 to 17 hours.
THE TERMIN OLOGY OF
Measuring Bacterial Reproduction
Researchers measure bacterial reproduction by counting either individual
cells or colony-forming units (CFUs). CFU is the more general category because
it accounts for cases in which the starting point of an infection or outbreak
is not a single cell but rather a connected pair or a chain of cells. A CFU
can even be a bacterial spore.
Because microbe numbers can increase so quickly, most studies measure
the bacterial population by calculating the base-10 logarithm of the CFUs per
gram, or log 10
(CFU)/g. (For liquids, the measurement is typically given in
log 10
(CFU)/ml.) If just 10 CFU (10¹) are present per gram of food, the population
size is 1 log 10
(CFU)/g. If one million (10 6 ) cells are present per gram, the
population is 6 log 10
(CFU)/g. The numeral before the unit thus represents the
population expressed as a power of 10.
The ability of bacterial populations to grow
exponentially if food is improperly handled makes
pathogenic bacteria particularly dangerous. One
of the principal goals in food safety, then, is taking
measures when food is stored, prepared, or served
to prevent this kind of rapid bacterial replication.
Although simple geometric formulas illustrate the
enormous potential for bacterial multiplication,
we can make better mathematical models to
predict replication more accurately over time.
Most chefs will never use these models, but
looking at the calculations can give you a better
idea of how the process works.
Bacterial replication rates depend strongly on
temperature; below a critical threshold, bacteria
simply do not reproduce. The same holds for
replication above an upper threshold. These
critical temperatures vary for different species and
environmental conditions. Some bacteria multiply
at temperatures just above freezing, albeit slowly.
More often, microbe species begin to replicate
somewhere between 3 °C and 12 °C / 37 °F and
54 °F. As the temperature rises above that range,
bacterial reproduction generally accelerates until
it reaches a maximum value.
This temperature dependence is the main
reason that foods are stored in refrigerators and
freezers, where the low temperatures can halt or
dramatically slow the replication of pathogenic
and spoilage bacteria.
If the temperature rises past a certain point, the
bacteria stop reproducing, and at higher temperatures
still they start to die (see The Limits of
Bacterial Reproduction, page 145). As a general
rule, most pathogenic bacteria multiply fastest at
temperatures just below their lethal upper limit,
which leaves a fine line between rapid reproduction
and death. Foodborne pathogens typically
reach their optimal reproductive rate between
37 °C / 98.6 °Fthe normal body temperature of
humansand 43 °C / 109 °F. This is the case for
Escherichia coli O157:H7, for example, as shown in
the chart on the next page. Most pathogens cannot
grow above 55 °C / 131 °F.
Just like any other form of life, bacteria need to
eat, and the availability of nutrients also affects
how fast they reproduce. Once bacteria have
multiplied a millionfold, they can exhaust their
local food source, which causes replication to slow
or even halt. In most food safety scenarios,
however, food provides ample nutrients, so this
limiting factor rarely becomes a practical consideration
in the kitchen.
The pH of food also can greatly affect bacterial
reproduction. Most bacteria multiply fastest in
foods that have a pH near 6.8 (close to the neutral
value, 7.0), but may reproduce in acidic foods with
a pH as low as 4.0 and in alkaline foods with a pH
as high as 8.0. And a few pathogenic species can
multiply at extreme pH values outside this range.
In the chart on the next page, which depicts the
reproduction of E. coli O157:H7 as a function of
both temperature and pH, note the dramatic effect
that a small change in pH, in temperature, or in
both parameters can have on the doubling time of
the population. At lower temperatures, a shift in
pH can extend the required interval for doubling
from 30 minutes to six hours. Put another way, it
can reduce the amount of replication that occurs
in a single day from a factor of some 280 trillion to
a mere factor of 16!
An E. coli cell photographed in the late
stages of cell division has nearly split to
become two.
The addition of nitrates to cured
meats raised concerns in the 1970s
because the compounds can form
into nitrosamines, many of which
are carcinogenic in animals. In
response to this concern, meat
packagers reduced nitrate levels
and began adding vitamin C
(ascorbic acid), vitamin E (alphatocopherol),
and other compounds
that greatly reduce nitrosamine
formation without detracting from
nitrates’ preservative functions.
142 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 143