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Microbiology and Engineering 2009.4.9
1 CHONNAM NATIONAL UNIVERSITY
1. INTRODUCTION
Because human food sources are of plant and animal origin, it is important
to understand the biological principles of the microbial biota associated with
plants and animals in their natural habitats and respective roles. Although it
sometimes appears that microorganisms are trying to ruin our food sources
by infecting and destroying plants and animals, including humans, this is by
no means their primary role in nature. In our present view-of life on this
planet, the primary function of microorganisms in nature is self-
perpetuation. During this process, the heterotrophs and autotrophs carry
out the following general reaction:
All organic matter
(carbohydrates, proteins, lipids, etc.)
Energy + Inorganic compounds
(nitrates, sulfates, etc.)
It is essentially nothing more than the operation of the nitrogen cycle and
the cycle of other elements. The microbial spoilage of foods may be viewed
simply as an attempt by the food biota to carry out what appears to be their
primary role in nature.
Microbiology fall into four categories; bacteria, yeasts, molds1, viruses.
MICROBIOLOGY
BACTERIA
YEAST
MOLD
VIRUS
Bacteria is the most common food-borne pathogens. Bacteria growth rates,
under optimum condition, are generally faster than those of the yeasts and
molds. Below is a few list of important microorganisms the makes food-
borne occurs.
1 A large group of fungi (like penicillium) that cause mold (as on bread or cheese). A common trigger for
allergies (http://www.biology-online.org/dictionary/Molds)
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2. MICROORGANISMS SOURCE IN THE FOOD
The microorganisms live in many medium such as air, water, soil, plant
materials, and the skin J expected to persist in soils. The bacterial biota of
seawater is essentially Gram-negative, and Gram-positive bacteria exist
there essentially only as transients. Contaminated water has been
implicated in Cyclospora contamination of fresh raspberries.,
Plants and Plant Products. It may be assumed that many or most soil and
water organisms contaminate plants. However, only a relatively smallnumber find the plant environment suitable to their overall well-being.
Those that persist on plant products do so by virtue of a capacity to adhere
to plant surfaces so that they are not easily washed away and because they
are able to obtain their nutritional requirements. Notable among these are
the lactic acid bacteria and some yeasts. Among others that are commonly
associated with plants are bacterial plant pathogens in the genera
Corynebacterium, Curtobacterium, Pectobacterium, Pseudomonas, and
Xanthomonas; and fungal pathogens among several genera of molds.
Food Utensils. When vegetables are harvested in containers and utensils,
one would expect to find
some or all of the surface organisms on the products to contaminate contact
surfaces. As more and more vegetables are placed in the same containers, a
normalization of the microbiota would be expected to occur. In a similar
way, the cutting block in a meat market along with cutting knives and
grinders are contaminated from initial samples, and this process leads to a
buildup of organisms, thus ensuring a fairly constant level of contamination
of meat-borne organisms.
Gastrointestinal Tract. This biota becomes a water source when polluted
water is used to wash raw food products. The intestinal biota consists of
many organisms that do not persist as long in waters as do others, and
notable among these are pathogens such as salmonellae. Any or all of the
Enterobacteriaceae may be expected in fecal wastes, along with intestinal
pathogens, including the five protozoal species already listed.
Food Handlers. The microbiota on the hands and outer garments of handlers
generally reflect the
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environment and habits of individuals, and the organisms in question may
be those from soil, water, dust, and other environmental sources. Additional
important sources are those that are common in nasal cavities, the mouth,
and on the skin, and those from the gastrointestinal tract that may enter
foods through poor personal hygiene practices.
Animal Feeds. This is a source of salmonellae to poultry and other farm
animals. In the case of some silage, it is a known source of Listeria
monocytogenes to dairy and meat animals. The organisms in dry animal feed
are spread throughout the animal environment and may be expected to
occur on animal hides.
Animal Hides. In the case of milk cows, the types of organisms found in rawmilk can be a reflection of the biota of the udder when proper procedures
are not followed in milking and of the general environment of such animals.
From both the udder and the hide, organisms can contaminate the general
environment, milk containers, and the hands of handlers.
Air and Dust. Although most of the organisms listed in Table 22 may at
times be found in air and dust in a food-processing operation, the ones that
can persist include most of the Gram-positive organisms listed. Among fungi,
a number of molds may be expected to occur in air and dust, along
with some yeasts. In general, the types of organisms in air and dust would
be those that are constantly reseeded to the environment. Air ducts are not
unimportant sources see the table below;
Molds are filamentous fungi that grow in the form of a tangled mass that
spreads rapidly and may
cover several inches of area in 2 to 3 days. The total of the mass or any large
portion of it is referred to as mycelium. Mycelium is composed of branches
or filaments referred to as hyphae. Those of greatest importance in foodsmultiply by ascospores, zygospores, or conidia. The ascospores of some
genera are notable for their extreme degrees of heat resistance. One group
forms pycnidia or acervuli (small, flask-shaped, fruiting bodies lined with
conidiophores). Arthrospores result from the fragmentation of hyphae in
some groups
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Table 1. eight important source of bacteria
Note: XX indicates a very importance sourcea
Primary waterb
Primary soilc
Nontuberculous
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Picture 1. Moldy bread2
Yeasts may be viewed as being unicellular fungi in contrast to the molds,
which are multicellular; however, this is not a precise definition, as many of
what are commonly regarded as yeasts actually produce mycelia to varying
degrees.
Yeasts can be differentiated from bacteria by their larger cell size and their
oval, elongate, elliptical, or spherical cell shapes. Typical yeast cells range
from 5 to 8 m in diameter, with some being even larger. Older yeast
cultures tend to have smaller cells. Most of those of importance in foods
divide by budding or fission.
Yeasts can grow over wide ranges of acid pH and in up to 18% ethanol.
Many grow in the presence
of 5560% sucrose. Many colors are produced by yeasts, ranging from
creamy, to pink, to red. The
asco- and arthrospores of some are quite heat resistant. (Arthrospores are
produced by some yeast-like fungi.)
2 http://en.wikipedia.org/wiki/File:Verschimmeltes_Brot_2008-12-07.JPG
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Picture 2. Dried yeasts3
3. MICROBIAL GROWTH
The growth of microbial population can be generalize by using the curve
below (see pic.3). an initial lag phase occurs as organism start to grow and
adapt to new environmental condition. The lag phase is very important
because the maximum extensions of shelf life and length of production run
are directly related to the length of the lag phase. Once adaption has
occurred, the culture enters into the maximum (logarithmic) growth rate,
and control the microbial growth is not possible without major sanitation or
other drastic measures. Numbers can double as fast as 20 to 30 min under
optimum conditions. Toxin production and spore maturation, if possible,
usually occur at the end at the end exponential phase as the culture enters
the stationary phase. At this time, essential nutrients are depleted and/or
inhibitory by-products are accumulated. Eventually the culture dies, the rate
depending on the organism, the medium, and the other environmental
characteristics.
3 http://en.wikipedia.org/wiki/File:Dry_yeast.jpg
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Numberofcelllogarithmic
scale
time
lag
log
stationary
death
picture 3. Typical microbial growth curve4
The growth of microorganism has their own factors which are divided into
two categories, first is intrinsic factor that are function of the food itself and
second is the extrinsic factor that are the function of environmental in which
the food is held.
4. INTRINSIC FACTOR
The parameters of plant and animal tissues that are an inherent part of the
tissues are referred to as intrinsic parameter. These parameters are as
follows:
1. Nutrient content2. Inhibitor3. pH4. Competing organism5. water activity6. Biological structuresEach of these substrate-limiting factors is discussed below, with emphasis
placed on their effects on microorganisms in foods.
4.1. Nutrient
4 1994 ASHRAE Refrigeration Handbook, p.9.1
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Like other living organism, microorganisms require food to grow. Carbon
and energy source are usually supplied in the form of sugars and starches.
Nitrogen requirements are met by the presence of protein. Vitamins and
minerals are also necessary. Lactic acid bacteria have rather exacting
nutritional requirements, while may aerobic spore formers have
tremendous enzymatic capabilities and are capable of growth on the wide
variety of substrates. Cleanable system facilitate the removal of residual
food material and deprive microorganisms of the nutrients required for
growth, thus preventing a buildup of organisms in the environment
4.2. Inhibitors
Inhibitor may be present in the food by naturally or addicted withpreservatives. Preservatives are not substitute for hygienic practices and,
with time, microorganisms may develop resistance. A cleanable system is
still essential in preventing the development of resistance population.
4.3. Competing Microorganisms
The presence of other microorganisms also affect the organisms in foods.
Some organisms produce inhibiting compounds or has fast generation times,
other are better able to use the available nutrients in a food matrix.
4.4. Water Activity
One of the oldest methods of preserving foods is drying or desiccation;
precisely how this method came to be used is not known. The preservation
of foods by drying is a direct consequence of removal or binding of moisture,
without which microorganisms do not grow. It is now generally accepted
that the water requirements of microorganisms should be described in
terms of the water activity (aw) in the environment. This parameter is
defined by the ratio of the water vapor pressure of food substrate to thevapor pressure of pure water at the same temperature: aw = p/po, wherep
is the vapor pressure of the solution and po is the vapor pressure of the
solvent (usually water). This concept is related to relative humidity (RH) in
the following way: RH = 100 aw.13 Pure water has an aw of 1.00, a 22%
NaCl solution (w/v) has an aw of 0.86, and a saturated solution of NaCl has
an aw of 0.75 (Table 2).
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Table 2. relationship between water activity and concentration salt solution
The water activity (aw) of most fresh foods is above 0.99. The minimum
values reported for the growth of some microorganisms in foods are
presented in Table 3. In general, bacteria require higher values of aw for
growth than fungi, with Gram-negative bacteria having higher requirements
than Gram positives. Most spoilage bacteria do not grow below aw = 0.91,
whereas spoilage molds can grow as low as 0.80
Table 3. Approximate Minimum awValues for Growth of Microorganisms
Important in Foods5
5 James M Jay,2005; Modern food microbiology 7 th edition,
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4.5. PH
It has been well established that most microorganisms grow best at pH
values around 7.0 (6.67.5), whereas few grow below 4.0 . Bacteria tend to
be more fastidious in their relationshipsto pH than molds and yeasts, with
the pathogenic bacteria being the most fastidious. With respect to pH
minima and maxima of microorganisms, those represented in picture 4,
should not be taken to be precise boundaries, as the actual values are
known to be dependent on other growth parameters. For example, the pH
minima of certain lactobacilli have been shown to be dependent on the type
of acid used, with citric, hydrochloric, phosphoric, and tartaric acids
permitting growth at a lower pH value than acetic or lactic acids
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Picture 4. Approximate pH growth ranges for some food-borne organisms
5. EXTRINSIC FACTORS
The extrinsic factor of that influence the growth of microorganisms are
temperature, environmental relative humidity, and oxygen levels.
Refrigeration and ventilation system play a major role in controlling these
factors.
5.1. Temperature.
Microorganisms are capable of growth over a wide range of temperature.
Minimum growth temperatures for a variety of spoilage and pathogenic
bacteria of significance in food are summarize in table 4.
Table 4. minimum growth temperature for some bacteria in foods
Organism Possible significant Approximate
min growth
temp,oC.
Staphylococcus aureus Food-born disease 10
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Salmonella spp. Food-born disease 5.5
Clostridium botulinum Food-born disease
proteolic 10
nonproteolic 3
Lactobacillus and
leuconostoc
Spoilage of cooked sausage 3
Listeria monocytogenes Food-borne disease 1
Acinetobacter spp Spoilage of precooked
food
-1
Pseudomonads Spoilage of raw fish, meats,
poultry, and dairy products
-1
The lowest temperature at which a microorganism has been reported to
grow is 34C; the highest is somewhere in excess of 100C. It is customary
to place microorganisms into three groups based on their temperature
requirements for growth. Those organisms that grow well at or below 7C
and have their optimum between 20C and 30C are referred to as
psychrotrophs. Those that grow well between 20C and 45C with optima
between 30C and 40C are referred to as mesophiles, whereas those that
grow well at and above 45C with optima between 55C and 65C are
referred to as thermophiles.
5.2. Environmental relative humidity
As previously discussed as an essential intrinsic growth factor, is also major
extrinsic factor. Environmental water acts as a vector for transmission of
microorganisms from one location to another through foot traffic or
aerosols.
5.3. Oxigen
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Microorganisms
classified by oxygenrequirement
AEROBIC
ANAEROBIC
By controlling the oxygen level to prolong storage life by retarding growth of
spoilage organisms in addition to influencing ripening processes. Vacuum
packing of food also uses this extrinsic growth factor by inhibiting the
growth of strict aerobes.
6. DESIGN FOR CONTROL OF MICROORGANISMS
DESIGN
Prevention ofContamination
Prevention ofGrowth
Destruction ofOrganisms
Prevention ofGrowth
Water control
sanitation
Humidity control
freezing
Picture 5. Food temperature chart6
High temperatures destroy most bacteria. The higher the temperature,
6 Taken fromhttp://www.doh.state.fl.us/Environment/community/food/temp.htm
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the quicker and more effective the kill. Low temperatures prevent bacteria
from reproducing.
Bacteria thrive in a temperature range from 5 degrees C to 60oC. This
range includes room temperature and body temperature. In this range
bacteria reproduce wildly and have the greatest potential for
contamination spread and infection.
Hot foods are generally cooked between 62.7 and 100oC. It is important
that center of grilled meat reaches at least 62.7oC. Verify this with a
meat thermometer. The minimum storage temperature for hot food is
160 degrees C. Cold foods should be kept at a range of -17.7 to 5oC. The
maximum storage temperature for cold food is 5 oC. Frozen foods should
be kept frozen between -17.7 and - 34oC.
In the ASHRAE standard 52.1-1992, dust has to be suppressed to prevent
the contamination of food, food is expose with air, the filtration of air is
important by using 95% filters that dust would be sufficiently removed
from the contamination. HEPA (high-efficiency particulate air) filters
provide sterile air and are used for cleanrooms.
Picture 6. HEPA filter7
7 Taken fromhttp://en.wikipedia.org/wiki/File:HEPA_Filter_diagram_en.svg
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