The physical methods related to microbial control (or growth) are as enumerated under : (a) Heat, (b) Moist Heat, (c) Pasteurization, (d) Dry-Heat Sterilization, (e) Filtration, (f) Cold, (g) Desiccation, (h) Osmotic Pressure, and (i) Radiation.
PHYSICAL
METHODS
The physical methods related to microbial control (or growth) are as
enumerated under :
(a) Heat,
(b) Moist
Heat,
(c) Pasteurization,
(d) Dry-Heat
Sterilization,
(e) Filtration,
(f) Cold,
(g) Desiccation,
(h) Osmotic
Pressure, and
(i) Radiation.
All these
individual methods shall now be treated separately in the sections that follows
:
Heat represents probably the most
common effective, and productive means whereby organisms are almost killed. In fact, it is a usual practice to have the
laboratory media, laboratory glasswares, and hospital surgical instruments
adequately sterilized by heat i.e.,
moist heat in an electric autoclave.
Salient Features. Following
are the salient features of heat
controlled microbes, namely :
(1) Most
economical and easily controlable means of microbial growth.
(2) Usually
kill microbes by causing denaturation of their respective enzymes.
(3) Heat
resistance capacity of the organism must be studied carefully and taken into
consideration.
(4) Thermal Death Time (TDT). TDT is
referred to as the minimal length of time whereby all microbes present in a liquid culture medium will be killed at a
given temperature.
(5) Thermal Death Point (TDP). TDP designates
the lowest temperature at which all
of the microorganisms present in a
liquid suspension will be killed in just 10 minutes. In fact, heat resistance
predominantly varies amongst the different range of organisms ; besides, these
glaring differences may be duly expressed via
the concept of thermal death point
(TDP).
However,
it is pertinent to state here that both TDP and TDT are equally vital,
important, and useful guidelines which essentially indicate the actual
prevailing severity of treatment needed to kill
a given population of organisms.
(6) Decimal Reduction Time [DRT or D-Value]. DRT or D-Value represents a 3rd concept which is directly associated with the organism’s extent of heat resistance. In fact, it is very much
equivalent to the time (minutes), whereby almost 90% of the population of
prevailing microbes at an exact specified temperature shall be killed as
illustrated in Fig. 7.1, having DRT of 1 minute. It is, however, pertinent to
mention here that DRT is of an extreme importance and usefulness in the ‘canning industry’ dealing with fruit
concentrates, fruit pulps, fruit slices, baked beans, corned-beef, fish products, fish chuncks, baby
corns, lentils, and the like.
[Redrawn
From : Tortora GJ et. al. : Microbiology : An introduction., The
Benjamin/Cummings Pub. Co. Inc. New York, 5th edn., 1995].
The curve
in Fig. 7.1 is plotted logarithmically
(as shown by solid line), and arithmatically (as shown by broken line). In this particular
instance, the microbial cells are
found to be dying at a rate of 90% min–1.
It is a
common practice to make use of ‘heat’
in the process of sterilization either in the form of ‘moist heat’ or ‘dry heat’.
It has
been duly proved and established that the so called ‘moist heat’ invariably kills microbes at the very first instance
by the process known as ‘coagulation of
proteins’, that is eventually caused by the specific cleavage of the H-bonds which critically retain the protein in its 3D-structure*. Interest-ingly, one may visualize the phenomenon of protein coagulation/denaturation rather
more vividly in the presence of water.
‘Moist heat’ sterilization may be achieved
effectively by the following widely accepted known methods, such as :
(a) Boiling,
(b) Autoclaving,
and
(c) Pasteurization.
Each of
the aforesaid method of moist-heat
sterilization shall now be treated individually in the sections that
follows :
(a) Boiling
Boiling at 100°C at 760 mm atmospheric
pressure is found to kill particularly several varieties of vegetative states of microbial strains, a good number of viruses
and fungi ; besides their ‘spores’
within a span of 10 minutes only. It is quite obvious that the ‘unpressurized’ i.e., free-flowing steam
is practically equivalent to the prevalent temperature of boiling water (i.e., 100°C). It has been revealed that
the endospores plus certain viruses are evidently not destroyed in
such a short duration of 10 minutes.
Examples.
(a) A typical hepatitis virus may even survive upto a duration of 30 minutes of continuous boiling at an atmospheric
pressure.
(b) Likewise,
there are certain microbial endospores
that have been offered resistance to boiling for more than 30 hours.
Conclusion. Boiling for a couple of minutes
will certainly kill organisms present in a Baby’s Feeding Bottle + Nipple, food products, drinking water relatively
safer for human consumption.
The most
reliable sterilization with moist heat
prominently requires such ranges of temperature that are critically above the boiling water i.e., above 100°C. These
high temperatures [120 ± 2°C] are
most conveniently accomplished by moist steam under positive pressure usually
in an ‘autoclave’. One may make use
of ‘autoclaving’ as a means of
sterilization unless the drug substance or material to be sterilized can suffer
serious type of damage either by heat
or by moisture. In fact, higher the
pressure inside the autoclave, the higher will be the temperature inside the
autoclave.
Examples. The following are two typical sets of examples viz.,
(a) Relationship between pressure and temperature
of steam at sea level. It has been adequately proved that—‘the higher the pressure created inside the
autoclave, the higher would be the
attainable temperature inside the autoclave’.
When the
free-flowing stream at a prevailing temperature of 100°C is subjected under
pressure of 1 atmosphere above the sea-level pressure i.e., 15 pounds pressure per square inch (psi), the tempera-ture
inside the autoclave happens to rise upto 121°C, which is an usual and common
parameters em-ployed in the sterilization of food products and surgical
instruments. One may also work at relatively lower/higher pressure (psi) vis-a-vis lower/higher temperatures (°C)
as clearly given in Table : 7.1.
Table 7.1. Relationship Between Pressure and
Temperature of Steam at Sea Level*
Figure
7.2 illustrates the beautiful elaborated diagramatic representation of an
autoclave.
In a
broader perspective, ‘sterilization’
in an autoclave is considered to be
most effective par-ticularly in a situation when the microbes either contact
the steam directly or are adequately contained in a small volume of aqueous
(mostly water) liquid. Importantly, under such a critical experimental
param-eters (i.e., steam at a
pressure of 15 psi at 121°C) all the microbes would be killed while their
endospores in almost within a span of 15 minutes.
Applications of an Autoclave. The
various applications of an autoclave are as enumerated under :
(1) To
sterilize culture media for the identification and propagation of pure strains
of microor-ganisms and yeasts.
(2) To
sterilize various surgical stainless steel instruments that are required for
most of the sur-gical procedures, dental procedures, obstretrics etc.
(3) To
sterilize various types of surgical dressings, gauzes, sutures etc.
(4) To
sterilize a host of IV applicators, equipments, solutions, and syringes as
well.
(5) To
sterilize transfusion equipment(s) and a large number of other alied items that
can con-veniently withstand high pressures and temperatures.
(6) When
the ‘large industrial premises’ make use of the autoclaves, these are knwon as re-torts,
whereas, the small domestic applications invariably employ pressure cookers (both based
on exactly the same principles) for
preparation of food* and canning of processed food products.
Important Aspects. In a
situation, when we essentially look for
extended heat requirement so as
to specifically reach the exact centre
of the solid materials viz.,
canned meats, fish (tuna), due to the fact that such materials fail miserably
to develop the most desired efficient
convection currents which invariably take place in the body of liquids.
Therefore,
the particular heating of large containers/vessels does essentially require
extra time period (in minutes) as given in Table 7.2.
Table 7.2. Overall Effect of Container Size upon
Autoclave Sterilization Times (Minutes) for Liquid Solutions**.
(i) The autoclave sterilization times in
the autoclave very much include the time required for the contents of the
containers to perfectly reach the sterilization temperatures.
(ii) Obviously,
for a very small container this is only 5 minutes or even less, whereas for a 9
L capacity fermentation bottle it might be as high as ~ 70 minutes.
(iii) All
containers that are supposed to be sterilized by ‘autoclave’ are invariably filled only upto 3/4th the total volume i.e., their actual capacity.
Salient Features. The salient features of ‘autoclave sterilization’ are briefly
stipulated as under :
(1) In
order to sterilize duly the surface of a solid, one must allow the ‘steam’ to actually contact the same.
Nevertheless, particular care must be taken to allow the perfect sterilization
of bandages, dry-glasswares, and the like so as to ascertain that steam gets
into contact with all the exposed surfaces.
Example. Aluminium foil does not
allow the passage of steam to pass across (i.e., impervious), and hence must be avoided to wrap such materials meant to be
sterilized ; instead, one may freely make use of brown wrapping paper (cellulose).
(2) Trapped Air. All necessary precautions and
requisite care must be taken to get rid of any trapped air strategically located at the bottom of a ‘dry container’, due to the fact that
the ‘trapped air’ shall not be
replaced by ‘steam’ at any cost,
which being lighter than air. However, one may just visual-ize imaginatively
the so called ‘trapped air’ as a mini-hot air oven, that would eventually
require not only a higher temperature
but also a much longer duration to sterilize materials.
Based on
the actual experience one may specifically tackle such containers which have a
ten-dency to trap air must be
positioned in a ‘tipped state’ in
order that all the steam shall ultimately help to force out the air.
Note. Importantly, such products which obstruct
penetration by moisture viz., petroleum jelly, mineral oil (furnace oil) are
not usually sterilized by the same methods as adopted to sterilize aqueous
solutions.
Pasteurization refers to ‘the process of heating of a fluid at a moderate temperature for a
definite period of time to destroy undesirable microorganisms without changing
to any extent the chemical composition.’
Example. In pasteurization of milk,
pathogenic organisms are invariably destroyed by heating at 62° C for a duration of 30 minutes, or by ‘flash’ heating to higher temperatures for less than 1 minute,
which is otherwise known as high-temperature
short time (HTST) pasteurization.
In a
broader perspective the pasteurization of milk, effectively lowers the total
bacterial count of the milk by almost 97 to 99%, due to the fact that the most
prevalent milk-borne pathogens viz.,
Tubercle bacillus*, and Samonella, Streptococcus, and Brucella organisms,
fail to form ‘spores’, and are quite sensitive
to heat.
It may,
however, be observed that several relatively heat-resistant (thermoluric) microorganisms do survive
pasteurization, and these may
ultimately fail to :
·
Cause refrigerated milk to turn sour (spoil) in a
short span of time, and
·
Cause any sort of disease in humans.
Ultra-High-Temperature (UHT) Treatments. Sterilization
of milk is absolutely different from pasteurization.
It may be duly accomplished by UHT
treatments in order that it can be most easily and conveniently stored even
without any sort of refrigeration. So as to maintain the first order ‘organoleptic characteristic features’** of fresh milk and to avoid attributing
to the milk a prevalent cooked taste, the
UHT system gained reasonable
qualified success and hence due recognition across the globe, whereby the
liquid milk never touches a surface hotter than the milk itself during the
course of heating by steam.
Methodology. The various steps involved are as
follows :
(1) Milk
is allowed to fall in a thin-film
vertically down through a stainless-steel (SS) chamber of ‘superheated steam’, and attains 140°C in less than 1 second.
(2) Resulting
milk is adequately held for a duration of only 3 seconds duly in a ‘holding
tube’.
(3) Ultimately,
the pre-heated milk is cooled in a ‘vacuum
chamber’, wherein the steam simply flashes
off.
(4) The
above stated process [(in (3)] distinctly enables the milk to raise its
temperature from 74—140°C in just 5 seconds, and suddenly drops back to 74°C
again.
Summararily,
the very concept of equivalent
treatments* clearly expatiates the particular rea-sons of the various
methods of killing microbes, such as :
Pasteurization : At 63°C for 30 minutes ;
HTST-Treatment : At 72°C for 15 seconds ;
UHT-Treatment : At 140°C for < 1 second ;
It is a
well known fact that microorgansims get killed by dry heat due to the oxidation effects.
Direct Flaming. Direct flaming designates
one of the most simple method of
dry-heat sterili-zation. In reality, the dry-heat sterilization is mostly
used in a ‘microbiology laboratory’ for
the steri-lization of the ‘inoculating
loops’, which is duly accomplished by heating the loop wire to a ‘red-glow’, and this is 100% effective
in actual practice. Likewise, the same principle is even extended to the process of ‘inceneration’ to sterilize as well as dispose of heavily
contaminated paper bags, cups, and used dressings.
Hot-Air Sterilization. It may be
regarded as another kind of dry-heat
sterilization. In this particular
process, the various items need to be sterilized are duly kept in an electric oven, preferably with a
stainless-steel chamber inside, and duly maintained at 170°C for a duration of
approximately 2 hours (to ensure complete sterilization).
It has
been adequately observed that the longer
the period plus higher temperature
are needed profusely due to the fact that the heat in water is more rapidly passed onto a ‘cool body’ in comparison to the heat in air.
Example. The experience of exposing the ‘finger’ in a boiling water at 100°C (212°F) vis-a-vis exposing the
same ‘finger’ in a hot-air oven at
the same tempearture for the same
duration.
Filtration may be defined as ‘the process of removing particles from a
solution by allowing the liquid position to pass through a membrane or other
particle barrier’. In reality, it essentially contains tiny spaces or holes which exclusively allow the liquid
to pass but are too small to permit the passage of the small particles.
In other
words, one may also explain ‘filtration’
as the process of a liquid or gaseous substance via a screen-like material having suitable pores small enough to
retain the microorganisms (bacteria). A vacuum
which is formed in the ‘receiver flask’
actually aids by means of gravity to suck the liquid via the filter medium engaged. However, in actual practice the
phenomenon of filtration is
invariably em-ployed to sterilize the specific
heat sensitive substances, namely : culture
media ; vaccines ; enzymes ; and several
antibiotic solutions.
High-Efficiency Particulate Air (HEPA) Filters.
HEPA-Filters are mostly used to get rid of practically all microbes that happen to be larger than 0.3 μm in
diameter.
Examples. HEPA-Filters are
largely used in :
(a) Intensive-Care Units [ICUs] in
specialized hospitals treating severe Burn
cases.
(b) In Sterile Zones of
High-Value Antibiotic Preparations, Packaging, IV-injections, and other such
sensitive sterile preparations.
Membrane Filters. In the
recent past, technologically advanced
membrane filters made up of either
Cellulose Esters or Plastic Polymers have been employed
profusely for the laboratory and industrial applications as shown in
Fig. 7.3 and 7.4.
Explanation for Fig. 7.4 :
(1) The
sample to be filtered is duly loaded into the ‘upper chamber’, and consequently forced through the strategically
placed membrane filter.
(2) The
pores present in the membrane filter
are definitely much smaller in comparison to the microorganisms ; and,
therefore, the microorganisms present are obviously retained upon the surface
of the filter.
(3) Sterilized
sample (free from microbes) may now be decanted conveniently from the ‘lower chamber’.
Specifications of Membrane Filters. Membrane
filters usually have a thickness of 0.1 μm, and having almost uniform pores. However, in certain commercially available brands, the film
is duly irradiated so as to generate
extremely uniform holes, where the radiation particles have made its passage, are critically etched in the
plastic. The pores of membrane filters usually range between 0.22 to 0.45 μm, intended for microorganisms.
Note. (1) Certain highly flexible microbes viz. spirochaetes, and the wall-less
bacteria viz., mycoplasma, may sometimes pass through such membrane filters.
(2) To retain certain viruses and large-sized
protein molecules are duly retained by such filters with pore size as small as
0.01 μm.
It has
been critically observed that the overall effect of ‘low temperature’ upon the microorgan-isms exclusively depends on
the specific organism and the intensity of the application.
Example. At temperatures ranging between
0–7°C (i.e., the ordinary refrigerator), the actual rate of metabolism of
majority of microorganisms gets reduced substantially to such an extent that
they are rendered incapable of either
synthesizing toxins* or causing reproduction.**
Thus, one
may conclude that ‘ordinary
refrigeration’ exerts a distinct bacteriostatic
effect i.e., stops the
multiplication vis-a-vis growth of
microbes.
Psychotrophs***, however, are found to grow
appreciably but slowly particularly at the
refrigerator temperature conditions ; and may change the very appearance and taste of food products after a certain lapse of time.
Salient Features. The
various salient features of microbes
in a ‘cold’ environment are as
follows :
(1) A few
microbes may even grow at sub-freezing
temperatures (i.e., below the
freezing temperature).
(2) Sudden
exposure to sub-freezing temperatures
invariably render bacteria into the ‘dormant-state’;
however, they do not kill them (bactericidal effect) ultimately.
(3) Gradual Freezing is
observed to be quite harmful and detrimental to microorganisms, per-haps due to
the fact that the ice-crystals which eventually form and grow do disrupt the cellu-lar as well as the molecular structure of the
microorganisms.
(4) Life-Span of Frozen Vegetative Microbes—Usually
remain active for a year upto 33% of the
entire initial population, whereas
other microbial species may afford relatively very scanty survival rates.
In order
to have both normal growth and adequate multiplication the
microorganisms do re-quire water.
Desiccation represents a typical state of microbes in the absence of water ; however, their growth and
reproduction remain restricted but could sustain viability for several years.
Interestingly, as soon as ‘water’ is
duly made available to them the said organisms resume their usual growth and
divi-sion as well. This highly specific ability has been adequately employed in
the laboratory manipulations whereby the microbes are carefully preserved by lyophilization.*
It has
been duly observed that the ensuing resistance of the vegetative cells to
undergo the phenomenon of desiccation changes with the specific species as well as the microorganism’s environment.
Example : Gonorrhea** organism, Neisseria
gonorrhoeae (Gonococcus), possess
an ability to withstand dryness only
upto a duration 60 minutes hardly ; whereas, Tuberculosis*** bacterium, Mycobacterium
tuberculosis (Bacillus) may even
remain completely viable for months together at a stretch.
Important Points : Following
are certain important points which should always be borne in mind :
(a) An
invariably susceptible microbe is found to be appreciably resistant when it
gets duly embedded in pus cells, mucous secretions, and in faeces.
(b) In
contract to microbes the viruses are usually found to be quite resistant to the
phenomenon of ‘desiccation; however, they do not exhibit resistance comparable
to the bacterial endospores.
(c) Importantly,
in a typical hospital environment (setting) the presence and subsequent ability
of some particular dried bacteria and endospores do remain absolutely viable,
such as : beddings, clothings, dust particulate matters, and above all the
disposable (used) dressings from patients may contain infectious organisms
strategically located in dried pus, faecal matter, mucous secretions, and
urine.
Osmotic pressure refers to–‘the pressure which develops when two solutions of different
concentrations are duly separated by a semipermeable membrane’.
In actual
age-old practice, the preservation of food products viz., pickles, fruits, are duly accom-plished by the use of
high-concentrations of salts and sugars which eventually exert their
effects on account of the osmotic
pressure. The most logical and probable underlying mechanism being the
creation of an extremely hypertonic environment due to the presence of these substances
(salts and sugars) at high concentrations that enables water to leave the microbial cell precisely. In fact, the
preservation afforded by the osmotic
pressure very much resembles to that caused by desiccation (see Section 7.2.2.7), besides, the glaring fact that
both processes evidently deny the
microbial cell of the requisite quantum of moisture essentially required
for its normal growth. Dehydration of the microbial
cell actu-ally renders the plasma
membrane to shrink away from the
respective cell-wall (i.e.,plasmolysis), whereby the consequent
cell stops growth (and hence reproduction), and it may not cause an instant
death. In a broader perspective, the fundamental principle of osmotic pressure is largely exploited
in the prolonged preservation of food products.
Examples : (a) Concentrated Salt
Solutions (Brine Solution) may be used profusely in the preservation and cure of meats, fish, vegetables, pickles etc.
(b) Concentrated Sugar Solutions (Sugar Syrup) may be
employed, extensively in the preser-vation of lime juice, fruits etc.
Radiation refers to — ‘any form of radiant energy emission or divergence, as of energy in
all directions from luminous bodies, radiographical tubes, particle
accelerators, radioactive ele-ments, and fluorescent substances’.
It has
been established beyond any reasonable doubt that radiation exerts its various effects on the cells, depending upon
its wavelength, intensity, and duration as well. Generally, one may come across
two kinds of radiation which would cause a bactericidal
effects on microbes, or usually referred to as the ‘sterilizing radiation’, namely :
(a) Ionizing
Radiation, and
(b) Nonionizing
Radiation.
Each of
the aforesaid types of radiation shall be treated individually in the sections
that follows :
The ionizing radiation normally possess a
wavelength distinctly shorter in comparison to the nonionizing radiation (size
< 1 nm) e.g., γ-rays, X-rays, or high-energy electron beams.
Figure
7.5 vividly depicts that the said ionization
radiation invariably carries a significant quan-tum of energy ranging
between 10–5 nm (γ-rays) to 10–3 nm (X-rays).
γ-Rays : These are emitted by radioactive
cobalt (Co),
X-Rays : These are produced by X-ray
machines, and
Electron Beams : These are generated by
accelerating electrons to high energies in special machines.
Visible light plus other forms of radiant energy
invariably radiate via space as waves
of various lengths.
Ionizing radiation viz., γ-rays and
X-rays possess a wavelength shorter than 1 nm.
Nonionizing radiation viz., UV-light has a wavelength ranging between 1–380 nm, where the
visible spectrum commences.
Salient Features. The
various salient features of the Ionizing Radiation are as stated under
:
(1) The γ-rays usually penetrate deeply but would essentially
require reasonably longer dura-tion, extended to several hours, for the
sterilization of relatively large
masses.
(2) High-energy electron beams do
possess appreciably lower penetrating power ; however, need only a few seconds of exposure to cause sterilization.
(3) Major
causative effect of ionizing radiation
being its distinct ability to the ionization of water, which in turn gives rise
to highly reactive hydroxyl radicals [OH•]*.
Interestingly, these radi-cals critically interact with the cellular organic
components, especially the DNA, and thereby kill the cell ultimately.
(4) High-energy electron beams (ionizing radiation)
has
recently gained an enormous world-wide acceptance, recognition, and utilities
for the exclusive sterilization of such substances as : pharmaceuticals, disposable dental materials, and disposable medical supplies. A few
typical examples are : plastic syringes, catheters, surgical
gloves, suturing materials.
Note. Radiation has virtually replaced ‘gases’ for
the ultimate sterilization of these items.
Predominantly
the nonionizing radiation possesses
a distinct wavelength much longer than that of the corresponding ionizing radiation, invariably greater
than about 1 nm.
Example : UV-light : The most
befitting example of the nonionizing
radiation is the UV-light, which
is able to cause permanent damage to
the DNA of exposed cells by virtue
of creation of newer additional bonds between the ‘adjacent thymines’ strategically present in the DNA-chains, as illus-trated in Figure :
7.6. The said figure evidently shows the formation of a thymine dimer after being exposed duly to the UV-light whereby the adjacent
thymines may be rendered into a cross-linked
entity. Importantly, in the absence
of the visible light, this
particular mechanism is usually employed by
a cell to afford the repair of the prevailing damage caused.
In
reality, these ‘thymine dimers’ are
found to cause effective inhibition in correcting replica-tion of the DNA in
the course of division (reproduction) of the cell. It has been duly established
that the UV-wavelengths at nearly 260 nm
are most effective and useful for
killing microbes due to the fact that
these are exhaustively absorbed by the cellular DNA.
Advantages of UV Light : are as
given under :
(a) It
controls and maintains the miroorganisms in the air.
(2) A ‘UV-Radiation Lamp’ or a ‘Germicidal Lamp’ is abundantly and
profusely employed in a variety of such sensitive areas as : operation theaters, hospital rooms,
nurseries, and cafeterias.
(3) UV Light or Radiation is
invariably employed to sterilize a plethora of highly sensitive biological products commonly used in
the therapeutic armamentarium, such
as : serum, toxins, and a variety of vaccines.
(4) UV Light is also employed to sterilize the
drinking water in homes, hospitals, and public places.
(5) UV Radiation is also used for the
sterilization of the ultimate treated
‘municipal-waste waters’ for agriculture and horticulture purposes.
Disadvantages of UV Light : These are
as stated under :
(1) UV Radiation is found to be not very
penetrating in nature ; and, therefore, the microorgan-isms intended to be
killed should be exposed almost directly to the UV-rays.
(2) Besides,
such microbes that are adequately shielded (protected) by means of textiles, col-oured, glass, and paper (i.e., textured cellulose
materials) are observed to be least affected by the UV radiation.
(3) Serious Problem. In fact, UV light poses a serious problem in
causing permanent damage to human
eyes on direct exposure, besides, prolonged exposure may even cause sun burns as well as skin cancers.
Note : (1) Antimicrobial effect of UV sunlight is
on account of the exclusive formation of the ‘singlet oxygen in the cytoplasm’.
(2) Microwaves (in the microwave oven) do not
exhibit any direct effect on the microbes, but kill them indirectly by heating
the food stuff.
A
comprehensive summary of the various physical methods invariably utilized for
the effective control of the microbial growth has been duly recorded in Table :
7.3.
Table : 7.3. Comprehensive Summary of Various
Physical Methods Utilized for the Effective Control of Microbial Growth
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