2. Microbiall Growth ( 3rd semester).pdf

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1. Food
Microbiology
Alishba
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2. Microbial Growth
•Bacteria are single-celled organisms
•Bacteria multiply in a process called binary fission in which
two cells arise from one
•Growth Rate: is the change in cell number per unit time
•The interval for one cell to become two is known as the
generation time or doubling time
•Quickest generation time is 9 mins
•Bacteria typically 15-30 mins
•In extreme cases it can be one year
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3. Growth Curves
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4. Lag Phase
•Considered as the adjustment period when the organism
adapts to new surroundings
•No Microbial growth
•Synthesize enzymes to adapt to the environment
•Recovery from stress or injury
•This period may be extended in unfavorable environments
•In extreme cases the lag phase can last for weeks
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5. Exponential (log phase)
•Growth is stable
•Growth rate is constant for a given bacteria under
specified conditions
•Catabolic processes generate energy
•Anabolic processes build cell structures
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6. •MetabolicactivityishighasDNA,RNA,Cellwall
components,andothersubstancesnecessaryforgrowth
aregeneratedfordivision.
•Itisinthisgrowthphasethatantibioticsanddisinfectants
aremosteffectiveasthesesubstancestypicallytarget
bacteriacellwallsortheproteinsynthesisprocesses
ofDNAtranscriptionandRNAtranslation.
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7. Stationary Phase
•Over time, essential nutrients become depleted or waste
products build up to toxic levels so that logarithmic phase
ceases and results in stationary phase
•No net growth in stationary phase (cell ‘replacing’ but
number not increasing
•Cell functions such as energy metabolism may continue
•Endospore-forming bacteria produce the endospore once the
culture has entered the stationary phase
•pathogenic bacteria begin to generate substances (virulence
factors) that help them survive harsh conditions and
consequently cause disease.
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8. Death phase
•Viable cell count decreases
•Under certain circumstances cell death is accompanied by
cell lysis. They spill their contents into the environment
making these nutrients available to other bacteria. This
helps spore producing bacteria to survive long enough for
spore production. Spores are able to survive the harsh
conditions of the death phase and become growing
bacteria when placed in an environment that supports life.
This image shows bacteria growing
exponentially in a Petri dish. A single
colony can have trillions of bacteria.
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9. Factors Affecting Microbial Growth
•Food Microbiologists must have through understanding of
the factors which influence microbial growth
•Many parameters can affect the growth and survival of
microorganisms. These parameters are divided into:
1-Intrinsic
2-Extrinsic
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10. Intrinsic Parameters
•Are those inherent to the food
These include:
Biological Structures
Nutrient Content
pH and organic acids
Water Activity
Oxidation-reduction potential
Antimicrobial constituents
•Intrinsic parameters have a major impact on the type of
microorganisms that are capable of growing on or in a food
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11. Extrinsic Parameters
•Are those properties of the storage environment which affect
both the food and the microorganisms associated with the food
These include:
Temperature of storage
Relative humidity of environment
Presence and concentrations of gases in the
environment
•Extrinsic parameters effect both the food and
microorganisms and influence the extent and rate at which
microorganisms will grow
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12. Intrinsic Parameters
1-Biological Structure
•Outer barriers against the invasion of microorganisms
(E.gthe skin of fruits and vegetables form a protective
layer to invasion by microorganisms)
•Inner parts of whole healthy tissues are sterile
•Damages during harvesting prprocessing (peeling,
skinning, chopping) expose tissues and increase microbial
loads throughout the product
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13. Biological Structure
•Milkhasnoprotectivebarrier
•Groundmeatspoilsfasterthanwholemeatcuts(grinding
distributessurfacemicroorganismsthroughout)
•Eggsareusuallysterileinsidebutheavilycontaminatedon
theshell,crackintheshellallowsmicrobestoenter
•Salmonellaspp.havebeenshowntogrowontheinteriorof
portionsofcutcantaloupe,watermelon,honeydewmelons
andtomatoesgivensufficienttimeandtemperature.
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14. 2-Nutrient Content
•The chemical composition of a food influences the type of
microorganisms that will grow and the products that they will
produce during growth
•All microorganisms important to food are chemoheterotrophs
•They use organic compounds as energy and carbon sources
•Food is rich in nutrients
•Food provide microorganisms with water, a source of energy,
carbon and nitrogen sources, and growth factors such as
vitamins and minerals
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15. •Aminoacidsserveasasourceofnitrogenandenergyand
areutilizedbymostmicroorganisms.
•Examplesofmineralsrequiredformicrobialgrowth
includephosphorus,iron,magnesium,sulfur,manganese,
calcium,andpotassium.Ingeneral,smallamountsofthese
mineralsarerequired;thusawiderangeoffoodscanserve
asgoodsourcesofminerals.
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16. •TheGram(+)bacteriaaremorefastidiousintheir
nutritionalrequirementsandthusarenotableto
synthesizecertainnutrientsrequiredforgrowth.
•Forexample,theGram(+)foodbornepathogenS.aureus
requiresaminoacids,thiamine,andnicotinicacidfor
growth
•FruitsandvegetablesthataredeficientinBvitaminsdo
noteffectivelysupportthegrowthofthese
microorganisms.
•TheGram(-)bacteriaaregenerallyabletoderivetheir
basicnutritionalrequirementsfromtheexisting
carbohydrates,proteins,lipids,minerals,andvitamins
thatarefoundinawiderangeoffood
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17. •All microorganisms require water in an available form
to grow and metabolize
•Availability of water is measured by water activity (a
w)
•a
w= Vapor pressure of food substrate/ Vapor pressure of
pure water at the same temperature
•aw = p/po
•a
wof pure water is 1.0 and the aw of a completely
dehydrated food is 0.00
•% moisture of foods is not the same as a
w
3-Water Activity17

18. 4-Oxidation-Reduction Potential (Redox)
•Redox potential of a substrate is the ease with which the
substrate is able to gain or lose electrons
•A compound that loses electrons reducing agent
•A compound that gains electrons oxidizing agent
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19. Redox Potential of a Food Depends on:
•Availability of Oxygen
•pH (alkaline more negative)
•Poising capacity (resistance to change, extent to which
food can buffer changes)
•Access of food to the atmosphere
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20. Microorganisms
•Microorganisms are classified with respect to their oxygen
requirements or tolerance
•Obligate Aerobes: Oxygen is required
•Microaerophilic: Oxygen is required at levels lower than
atmospheric
•Obligate Anaerobes: Oxygen is harmful or lethal
•Facultative Anaerobes: Grow well in the presence or absence of
oxygen
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21. Logarithmic
Oxygen is Toxic/Redox Potential of Foods
•InthepresenceofOxygensuperoxideradicals(extremely
toxic)aregenerated
•Organismscapableofgrowthinthepresenceofoxygenhave
theenzymesystemstodetoxifytheoxygenbyproducts
Catalase
peroxidase
superoxidedismutase
•Most fresh plant and animal foods have low, well-poised
Redox potential in their interior
•Exterior surfaces or chopped up products have higher Redox
potential (exposed to air), i.e the higher the oxygen
concentration the higher the Redox potential
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22. 5-pH and Organic acids
•Mostimportantintrinsicfactor
•Infoods,weakacids(organicacids)aremostimportant
becausetheyaremorecommoninnatureandaremore
effectiveantimicrobialagents.
•Organic acids, e.g. acetic acid (vinegar), citric acid in
citrus fruits
•Dissociate depending on the solution pH
•HA H
+
+ A
-
Acidic EnvironmentAlkaline Environment
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23. Effect on Microorganisms
•Un-dissociatedacidscancrossthecellmembrane
•Onceinsidethecell,sincetheinternalpHishigherthanthe
externalpH,theseacidsbecomedissociatedintoH
+
andA
-
•Theacidificationoftheinternal,i.ecellcontentsresultsin:
Breakdownofmembranetransportfunctions
Inhibitionofessentialbiochemicalpathways
Inhibitionofcellgrowth
Eventuallydeath
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24. •MostmicroorganismsgrowatpHvaluesaroundneutrality
•YeastsandmouldscangrowatabroaderpHrange
•FewpathogenswillgrowbelowpH4.0
•AnadversepHmakesbacterialcellsmoresensitivetotoxic
agentsandenvironmentalconditions
•Rateofmicrobialgrowth,survivalduringstorage,heatingor
dryingisdecreasedwithdecreasingpH
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25. pH and Food
•pHoffoodcanbenatural(citric)orderivedfromthe
additionofacidingredients(citric,acetic)orbymicrobial
fermentation(lacticinyogurt)
•Fruits,ingeneral,havelowerpHthanmeatsorvegetables
•MicrobialgrowthcaninfluencepHbyeitherraisingor
loweringit
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26. pH and Food
•Food can be broadly categorized on the basis of their pH
High Acid pH < 4.5 Lemons
Raspberries
Medium Acid pH 4.5-5.0 Bread, cheese,
carrots
Low Acid pH 5.0-7.0 Meat, Milk, most
vegetables
Alkaline pH > 7 Eggs
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27. 6-Antimicrobial Constituents
•Natural
•Chemical
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28. Natural Antimicrobial Constituents
•Naturally occurring anti-microbial components in plants
•Naturally occurring antimicrobials in foods
•Anti-microbial agents derived from microorganisms
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29. Plant Antimicrobials
•Essentialoils
Eugenolincloves,cinnamonandsage
Allicinfromgarlicandonions
Thymolfromoregano
•GrampositivebacteriamoresensitivethanGramnegative
bacteria
•Thoughttoactoncellmembrane
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30. Naturally Occurring antimicrobials in foods
•Eggs
Egg white has several inhibitory agents that include:
e.g. conalbumin
Lysozyme: weakens cell wall of Gram positive bacteria
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31. •Milk
-Lactoperoxidase system
SCN
-
+ H
2O
2 OSCN
-
+ H
2O
•OSCN
-
(Hypothiocyanite)isanoxidizingagentthat
cankillorinhibitthegrowthandmetabolismofawide
rangeofGrampositiveandGramnegativebacteria
•Usedasanalternativetorefrigerationforrawmilkin
developingcountries
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32. Extrinsic and Other Factors Affecting
the growth and Survival of
Microorganism in Food
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33. Extrinsic Parameters
1-Temperature of Storage
•A major tool for the control of microbial growth in foods is the
control of temperature through refrigeration, freezing, or heating
•Microorganisms are placed into one of three broad groups
Psychrotrophs: can grow at temperatures < 7
o
C
Mesophiles: Optimum 30-40
o
C, no growth at
refrigerated temps
Thermophiles: Optimum 55-65
o
C
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34. •Thetemperaturerangebetween5and63
o
Cis
consideredthe“dangerzone”becauseactivegrowth
ofbacteria,yeastsandmoldscanoccurincluding
mostpathogensfoundinfood
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35. Danger Zone
Destruction of bacterial
spores
All bacteria except bacterial
spores are killed
DANGER ZONE
Active growth of bacteria,
yeasts and molds, including
pathogens
Slow growth of
psychrotrophic microbes
> 100
o
C
63-100
o
C
5-63
o
C
< 5
o
C
KEEP HOT FOOD
HOTTER THAN
63
0
C
KEEP COLD
FOOD COLDER
THAN 5
o
C
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36. Temperature of storage/Time and Food Safety
•Products held within the danger zone for greater than 2
hours will enable significant microbial growth
•Central catering facilities are significant contributors to
food borne illness
•Moldsaretypicallyabletogrowoverawiderrangeof
temperaturesthanbacteria
•Yeastscangrowwithinthepsychotropicandmesophilic
temperaturerangesbutdonottypicallyinthethermophilic
range
Atypicalhomerefrigeratorshouldrunat5to7
o
C,butmost
runashighas10
o
C!
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37. 2-Relative humidity
•The relative humidity of the environment surrounding the
food during storage is important in maintaining the aw of a
food
•In controlling surface microbial growth
•High relative humidity increases water activity of foods
•Enhances the inactivation efficacy of ozone and chlorine
dioxide
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38. 3-Gases in the environment
•The mixture of gases surrounding a food will affect the
redox potential on the surface of the food and therefore affect
the growth of microorganisms
•Packaging has introduced controlled gaseous atmospheres
•A controlled atmosphere is anagricultural storage methodin
which the concentrations of oxygen, carbon dioxide and
nitrogen, as well as the temperature and humidity of a storage
room are regulated. Both dry commodities and fresh fruit and
vegetables can be stored in controlled atmospheres.
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39. Gases in the environment
•Vacuum packaging
•Modified atmosphere
•Antimicrobial
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40. Vacuum Packaging
•All the air is removed from pack leading to anaerobic
conditions
•Inhibits growth of aerobes and slows oxidation reactions
•Facultative and anaerobes still grow so refrigeration has to be
performed
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41. Modified Atmosphere Packaging
•“The packaging of a perishable product in an atmosphere
which has been modified so that its composition is other than
that of air”
•The air is removed and replaced with a specific gas
mixture
•Gases used:
Carbon Dioxide
Oxygen
Nitrogen
Argon
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42. Carbon Dioxide
•20-30%hasshowninhibitoryeffectonmicroorganisms
•MoreeffectiveagainstGramnegativeaerobic
microorganisms
•Lowtemperaturesincreaseinhibitoryeffectsduetogreater
solubilityofCO
2atlowertemperatures
•IncreasesofCO
2willcauseapopulationshift,i.eGram
positivewillbecomepredominant
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43. Oxygen
•Introduced at levels of 70-100%
•Inhibits aerobes and anaerobes
•Meat products discolor in the absence of oxygen
•Not totally effective at inhibiting E. coli and Listeria
monocytogenes
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44. Inert Gases
•Argon
•Nitrogen
•Used to displace oxygen
•Typically used in headspace of dried snacks and coffee
http://www.flairpackaging.com/category/gourmet_snack_packaging/ConsumerDeman
dDrivesChangesinGourmetSnackPackaging
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45. Combination of Gases
•Typically use a combination of CO
2/O
2/N
2
•Preserves product sensory quality
•More effective at retarding microbial spoilage and growth
of pathogens
•Less expensive
https://www.sorbentsystems.com/gas-flush.html
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46. Inordertocheckthemixtureratioof
theprotectivegasesinsideapackage,a
self-adhesiveseptumisappliedtoa
samplingpoint.Thesuctionneedleof
the gas analyser
istheninsertedthroughtheseptuminto
theheadspaceofthepackage.Asimple
touchonthetouch-screenofthedevice
willensurethattherequiredsampleis
automaticallydrawnin.Withinavery
shorttime,theoxygenandcarbon
dioxidecontentismeasured,the
nitrogencontentcalculated,andthe
resultsareshownonthedisplay
Analysis of headspace volume
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47. Other factors
1.Intendedend-use
Afoodproductthatdoesnotrequiretime/temperaturecontrol
forsafetyatonepointinthefoodproductionmayrequiresuch
controlatanotherpoint,dependingonitsintendeduse.
Forexample,athermallyprocessedfoodthatishot-filledinto
itsfinalpackagingmaynotrequirerefrigerationifspore-
formingpathogensarenotcapableofoutgrowthbutmay
requirerefrigerationoncethefooditemisremovedfromits
originalpackaging.
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48. Other factors
2.Producthistory
Therearefoods,suchaswhitebread,thathavealonghistory
ofsafestorageuseatambienttemperatures.Producthistory,
alone,shouldnotbeusedasthesolefactorindetermining
whetherornotafoodneedstime/temperaturecontrolfor
safety,unlessavalidscientificrationaleisprovided.
3.Interactionoffactors
Althoughthereisalong-standingrecognitionofinteractions
andthehurdletechnologyeffectofinhibitoryfactors,the
currentdefinitionof“potentiallyhazardousfoods”considers
pHanda
w
onlyasindividualindependentfactors.Thepanel
believesthatpHanda
w
interactionsmustalsobetakeninto
consideration.
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49. Bacterial Hazards -Examples
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50. 50

51. References
Foodbornepathogensandfoodsafety(2016).EditedbyMd.
LatifulBari&DikeO.Ukuku
ThomasBintsis.,Foodbornepathogens(2017).AIMS
Microbiology.
EvaluationandDefinitionofPotentiallyHazardousFoods.
Comprehensivereviewsinfoodscienceandfoodsafety.
Chap 2 -Hazards -Biological, Chemical, and Physical.
Retrieved from
http://seafoodhaccp.cornell.edu/Intro/blue_pdf/Chap02Blue.pd
f
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