The document discusses the microbiology of vegetables and fruits, highlighting the differences in microbial growth environments due to their distinct compositions and pH levels. Vegetables are primarily affected by lactic acid bacteria and coliforms, while fruits tend to harbor fermentative microorganisms due to their higher carbohydrate content. The text also addresses food safety concerns, linking outbreaks of E. coli and other pathogens to agricultural practices and handling of both vegetables and fruits.

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This is module 6, Vegetable and fruit microbiology.
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The first part of this section is the microbiology of vegetables
including intrinsic
properties of vegetables, pathogens and spoilage organisms.
The microbial growth environment of vegetables is considerably
different from
meats, poultry, or seafood. Vegetables have low proteins and
lipid content,
especially compared to meats. Vegetables are made up of
mostly simple and
complex carbohydrates.
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The carbohydrate content of vegetables is diverse.
Carbohydrates include
polysaccharides, oligosaccharides, monosaccharides, sugar
alcohols, and esters.
Microbial growth using these compounds requires the
appropriate enzymes and

2. transport proteins. Often microbial communities become
commensal when
extracellular enzymes are excreted by some genera breaking
down complex
polysaccharides making the simple sugars available to all.
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Most vegetables are in the neutral pH range from pH 6-7. The
chart indicates the
pH averages for many types of foods, including both vegetables
and fruits.
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In general the biota of vegetables are reflective of the soil they
are grown in,
although exceptions occur. The actinomycetes are the most
predominant
microorganisms of soils, but they are rarely found on
vegetables. The lactic acid
bacteria are not common to soils, but they are one of the most
predominant
microbial biota of vegetables. Like most other foods, the
surface of vegetables is
highly contaminated with microorganisms, while the interior is
sterile or mostly
sterile. Vegetables also possess a tough outer skin. This may
keep many
miroorganisms from entering the interior.
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3. The lactic acid bacteria are a group of Gram positive bacteria,
non-respiring (no
ETS), non-spore forming, cocci or rods. They produce lactic
acid as the major end
product of carbohydrate fermentation. They can be found in
rotting vegetation, but
not usually in soil itself. Once they get access to a new plant
they have the unique
ability to adhere to plant surfaces. This provides them with a
biota advantage over
other microorganisms. The lactic acid bacteria may be 50% of
the total vegetable
biota.
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Coliform bacteria are also part of the vegetable biota. The
genus Enterobacter is a
common soil coliform. The remaining are generally fecal in
origin, including E. coli.
They find their way into soils from manure, both accidental and
intentional. Note
that the majority of E. coli are non-pathogenic.
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Soils are the natural reservoir for most spores from
sporeformers. There, they
survive in the latent form until conditions become conducive to

4. growth.
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The most significant vegetable spoilage microorganisms are
those that produce
polysaccharide and oligosaccharide degradation enzymes.
These include cellulase,
pectinase, alpha and beta amylase, and similar enzymes. The
polysaccharides
provide the backbone structure to vegetables. Degrading that
backbone leads to a
loss of shape and texture.
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There is one major sporeformer pathogen that can contaminate
vegetables from
soils. Clostridium botulinum is gram positive and during
growth can form the most
potent neurotoxin on the planet. Consuming spores is a
common daily event for
humans, however the spores themselves are not toxic. C.
botulinum spores can
survive normal cooking temperatures and can start to grow once
a cooked food
cools. C. botulinum related to soils are different from those
found in the ocean
related to seafood. Bacillus cereus is a less common
sporeformer, but it is
commonly found in starch based plants. It can cause a vomiting
and/or diarrhea

5. illness when present in large numbers (approximately 10,000 or
more per gram).
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Because of their handling methods spices are the greatest source
of spores for any
food products. Some companies sell gas sterilized spices where
ethylene or
propylene oxide is used to kill spores. However, with the
benefit of spice sterility
comes the question of toxic residues or byproducts of ethylene
or propylene oxides.
Incidently, propylene oxide is one of the pasteurization
treatments being used on
nuts and grains to kill Salmonella. Speaking of Salmonella…
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Salmonella does not grow at low water activities. However,
recently it has been
found to be the source of foodborne illnesses in low water
activity and dried foods.
It is found in cocoa (and then in chocolate), peanut butter, dried
vegetable proteins,
and in many dry grains.
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In 2006 a major E. coli O157:H7 outbreak occurred in baby

6. spinach sold washed
and raw. The exact cause is not known, but it has been linked
to just one large
spinach processor in California together with three spinach
farms. The E. coli
O157:H7 may have come from the irrigation water or from feral
pigs that defecated
in the fields. There were hundreds of illnesses and at least four
deaths. The photo
above is of a Utah boy who became ill from the spinach and
suffered hemolytic
uremic syndrome. He almost lost his life and he will certainly
need kidney
transplants when older.
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The second part of this section is fruit microbiology.
The microbial growth environment of fruits is similar to
vegetables in that they also
have low protein and lipid contents. The big difference is that
fruits have an
elevated carbohydrate content, especially that of
monosaccharides.
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7. Most fruits have a pH below 4.6. This is important considering
that the botulism
microorganisms cannot grow at or below this pH level. There
are some exceptions,
including Asian pears, bananas, some mangos, mangosteen, figs,
melon, papaya,
and watermelon.
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In general the biota of fruits reflect the intrinsic properties of
fruits. Essentially they
are fermentative microorganisms that rapidly grow on simple
sugars and those
microorganisms that tolerate the lower pH levels of many fruits.
These include
yeasts and fermentative bacteria. Unlike vegetables, the lactic
acid bacteria are not
predominate microorganisms on fruits because they require
external sources of B
vitamins. Fruits do not contain significant levels of B vitamins.
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E. coli O157:H7 caused several major outbreaks in
unpasteurized raw apple juice.
Most likely orchards were spread with manure to encourage
better tree growth.
Apples were both picked and collected from the ground (drops).
Many of these
drops were contaminated with E. coli O157:H7. In response, all
juice must be

8. pasteurized or contain a safety warning. In addition, many
orchards no longer use
drops for juice that will not be pasteurized.
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Several outbreaks of waterborne pathogens have occurred in
fruits such as
strawberries. In 1997 a large outbreak in strawberries due to
Cyclospora (a
unicellular parasite) occurred in the USA. In 1997 strawberries
were contaminated
with Hepatitis A that resulted in many illnesses. Many of these
outbreaks have been
from imported fruits. The result is that now many large fruit
companies have
instituted good agricultural practices in countries that will
export fruits to the USA.
The use of potable water and worker hygiene are just two
efforts that contribute to
safer fruits (and vegetables).
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