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How Noroviruses Can Contaminate Food
The highly infectious noroviruses multiply only within the cells of their hosts—typically
humans. Nevertheless, they can persist in the environment, such as in shellfish beds, until
consumed by an unsuspecting victim. The CDC estimates that half of all norovirus
outbreaks are due to infected food handlers, highlighting the necessity of good kitchen
Life stages
1 Virus attaches to host cell.
Viral genome is incorporated into
host cell genome.
New viruses are released from host cell.
Contamination
Shellfish bed is contaminated by
human waste.
Unsafe handling
2 Sick employees or patrons
contaminate food surfaces.
3a Sick employees are allowed to
continue handling food and
surfaces, thereby increasing the
contamination risk.
Contamination risk
Low High
hygiene and of keeping sick employees at home. The hardy particles often resist disinfection
and chlorination attempts, but the risk can be reduced by carefully washing fruits and
vegetables, properly cooking oysters and shellfish, cleaning potentially infected surfaces
with bleach, and washing contaminated linens in hot water and detergent.
3a
Contamination risk
Low High
Safe handling
Sick employees stay home;
hands are carefully washed; bleach
is used on kitchen surfaces.
4a
4b
Unsafe cooking
Contaminated food is served raw.
Contamination risk
Low High
Safe cooking
Cooking the shellfish properly
might reduce the risk.
Rotavirus and Hepatitis
Two other kinds of foodborne viruses, rotavirus
and hepatitis, are worth mentioning. The former is
a wheel-shaped virus that primarily affects
children; health officials estimate that most
children have been infected at least once by the
age of four. The Rotavirus genus includes seven
species so fareach denoted by a letter of the
alphabet. Rotavirus A causes 90% of all rotavirus
infections in humans. Only about 1% of the
estimated 3.9 million annual cases of rotavirus
infection in the U.S. are thought to result from
foodborne contamination, but that still translates
to 39,000 incidents annually.
Rotaviruses can cause fever and vomiting, and
they constitute the leading cause of severe diarrhea
among children, a condition that requires
hospital admission in one out of every 40–80
cases. Although deaths from rotavirus infection
are uncommon in developed countries, rotaviruses
kill nearly one million people annually.
Adults who experience repeated exposure to
the same rotavirus strain can develop partial
immunity, which results in less severe infections.
Even asymptomatic adults can shed infectious
particles, however. Hijacked intestinal cells can
rapidly release as many as one billion viral particles
per milliliter (0.2 teaspoon) of feces. And
because rotaviruses require only 10 particles to be
infectious, their next victim is usually not far off.
Hepatitis A, another highly infectious viral
pathogen that propagates primarily through fecal
contamination, is foodborne in about 5% of cases.
It makes a few thousand Americans ill each year.
The resulting liver disease is usually mild but
seems to intensify with age. Infected children are
generally symptom-free. In other victims, symptoms
can include sudden fever, nausea, appetite
loss, and abdominal pain, followed by jaundice.
Severe infections can persist for up to six months.
Hepatitis A is transmitted by direct contact,
usually with those who have poor hygienecommonly,
infected restaurant workers. The disease’s
relatively long incubation period makes tracing
the virus to contaminated foodsoften fresh
produce, shellfish, and icedifficult.
Fortunately, researchers introduced a hepatitis
A vaccine in 1995, and hepatitis A rates have since
dropped considerably in developed countries. In
addition, people who recover from an infection
develop antibodies against the virus that protect
them for life. In many areas, regulations require
vaccination of all food workers. Even where this is
not required, it is an excellent idea.
Like rotavirus infections, astrovirus
infections are foodborne in about
1% of cases. Health agencies
estimate rotaviruses and astroviruses
cause a comparable
number of infections annually in
the U.S. But astroviruses tend to
produce less severe symptoms than
rotaviruses do, and the rate of
hospital admissions for astrovirus
infection is about a quarter that for
rotavirus infection. Astroviruses,
named for their star shape, cause
gastroenteritis and diarrhea; they
primarily infect the young. Childhood
infection can confer longlasting
immunity.
Rotaviruses are some of the less common
foodborne pathogens, but they still sicken
tens of thousands of diners in the United
States each year.
It is not known what time and
temperature combinations are
enough to thermally inactivate
norovirus because, as of this
writing, the studies have not yet
been published. Cooking may or
may not be able to reduce the risk.
Contamination risk
Low High
Contamination risk
Low High
contaminated at the source, such as shellfish
tainted by human feces (see Spreading an Infection
Around, previous page).
Once inside a person, the pathogens can spread
rapidly. One 2008 study estimated that a single
virion (infectious virus particle) of Norwalk virus
is 50% likely to produce an infectionthe highest
infection rate of any known virus. No surprise,
then, that specialists attribute half of all foodborne
gastroenteritis outbreaks to noroviruses.
Recovery from a norovirus infection does not
produce long-term immunity, which means that
Contamination risk
Low High
people can get reinfected repeatedly. No one
knows whether this lack of immunity is caused by
diverse viral strains or by some other feature of the
virus. Intriguing research, however, has indicated
that people with certain blood types may be
resistant to norovirus infection.
Norovirus infections are generally mild and are
only rarely lethal. Symptoms occur after a typical
incubation period of one to two days and include
nausea, vomiting, diarrhea, and abdominal pain;
they usually ease in a few days. But viruses can be
shed for as long as two weeks after recovery.
THE HIST ORY OF
Food Poisoning at The Fat Duck
In early 2009, more than 40 people reported falling ill
after dining at Heston Blumenthal’s three-Michelin-star
restaurant The Fat Duck. Immediately after the incident,
Blumenthal closed the restaurant to figure out what had
gone wrong. Initial inspections focused on ingredients that
could cause food poisoning, such as shellfish, and on the
staff, who could have potentially spread a virus to diners.
Some commentators began to worry that the illness was
caused by one of Blumenthal’s famously unconventional
cooking techniques. Authorities even began investigating
the possibility of sabotage.
Months later, the U.K. government’s Health Protection
Agency determined that the cause of the illness was an
outbreak of a norovirus. Sewage-tainted oysters and razor
clams carried the pathogen into the restaurant kitchen,
infecting both staff and guests. Subsequent testing confirmed
that oyster and razor-clam beds harvested by two
different suppliers to The Fat Duck from coastal waters near
two different parts of Great Britain tested positive for norovirus,
as did stool samples from the ill guests and staff.
Symptoms of food poisoning were ultimately reported by
529 customers, which makes this episode by far the largest
such event in memory to have affected a high-end restaurant.
Officials said that, although the restaurant could have
taken greater steps to prevent this outbreak, there was not
enough evidence to press charges. Indeed, there is very little
that a restaurant can do if its suppliers send it contaminated
oysters, which are generally served raw (as occurred here).
The financial hardship and negative press were perhaps
punishment enough: Blumenthal had to close the restaurant
for nearly three weeks for a top-to-bottom disinfection at
a cost of roughly £160,000 ($240,000).
154 VOLUME 1 · HISTORY AND FUNDAMENTALS
MICROBIOLOGY FOR COOKS 155