Factor affecting growth of microorganisms in food

Md. Humayun Kobir
Dept: Agro Product processing Technology
Jashore University Of Science and Technology,

1. Factors affecting microbial
growth

2. Factors Affecting Growth of Microorganisms
•Growthofmicroorganismsinnatureis
dependentonvariousfactors.
•Thefactorsinfluencingthegrowthof
microorganismsarephysical,chemical
andbiologicalinnature.
•Theimportantfactorswhichcontributeto
thegrowthofmicroorganismscanbe
dividedinto–
1.Extrinsicfactors
2.Intrinsicfactors.
3.ImplicitFactors
4.ProcessingFactors

3. Intrinsic Factors
1.Wateractivity
2.pH
3.Population
4.Oxygen
5.Redoxpotential
6.Nutrients
7.Antimicrobialagents
8.Biologicalstructure

4. Intrinsic Factors
WaterActivity(aw):
•Everylivingorganismhasthedemandforwater,withoutwhichnogrowthcanoccur.
Similarly,microorganismsrequirewaterfortheirgrowth.
•Waterisanexcellentsolventforalllifeprocessesineverylivingorganismfor
biocatalyticactivity.Theamountofwaterrequiredvariesfordifferentorganisms.
•Moistureisrequiredformicrobialgrowthandmetabolism,theexactwaterneededfor
thegrowthofmicro-organismvaries.
•Thiswaterrequirementisbestexpressedintermsofavailablewater(wateractivity).

5. Intrinsic Factors
•Wateractivityforfoodbornebacteriais0.91,foryeastitis0.88,
forfungi0.80.
•Freshfoodwillhaveaw=0.99.
•Ifawislessthan0.6,sporesoffungiandyeastwon’tgerminate.
•Waterinfoodismadeunavailabletomicrobesinthefollowing
ways:
1.IncreasingtheconcentrationsofSaltsandsugars:
•Thesesolutes/ionsfindortiedupwaterinthefoodsandresultin
dryingoffood.
•Herenotonlywateristiedbysolutesbutalsowatertendstocome
outofmicrobialcellsbyprocessofosmosis,ifthereishigher
concentrationofsolutesoutsidethemicrobialcellsthaninside.

6. Intrinsic Factors
2.UseofHydrophiliccolloids:(Gels)
•Makewaterunavailable.e.g.:4%Agarinwater.
3.Waterifcrystallizedisunavailabletomicrobes
•WhenwaterbecomesiceitisnolongerutilizedbyMicroorganisms.Water
activitydecreaseswithdecreaseinwatertemperature.
•Ingeneral,bacteriarequiremorewateractivitythanmoldsandyeasts.Most
bacteriagrowwellinamediumwithawateractivityapproaching1.0.Theygrow
wellinlowconcentrationsofsugarorsalt,althoughthereareexceptions.
•Gramnegativebacteriahavehigherwaterrequirementsthangrampositive
bacteria.Mostofthefoodspoilagebacteriadonotgrowbelowaw0.91,while
spoilagemoldscangrowevenataw0.80.
•Theaerobicfoodpoisoningbacterium,Staphylococcusaureusisfoundtogrowat
awaslowas0.86whileanaerobicClostridiumbotulinumdoesnotgrowbelowaw
0.94.
•GroupofMicroorganismMinimal(aw)value:Bacteria0.91,Yeasts0.88,Molds
0.80,Halophilicbacteria0.75,Xerophilicfungi0.65,Osmophilicyeasts0.60.

7. Intrinsic Factors
pH:
•Proportionaltohydrogenionconcentration(pH).
•BiocatalyticactivitiesareinfluencedbypH.
•GrowthofmicroorganismsisaffectedbythepHofgrowth
environmentsinfood(growthmedium)whichistheresultoflarge
numberofenzymesresponsibleformetabolismandgrowth.
•Everyorganismorenzymehasaminimum,optimumandmaximum
pHrequirementsfortheirsurvival,growthorbiocatalyticactivity.
•InfluenceofpHoffoodnotonlyhaseffectongrowthof
microorganismsbutalsoonprocessingconditions.
•Foodhavingacidiccontentspromotegrowthofacidloving
microorganismssuchasyeasts,moldsandsomebacteria.
•MostofthespoilagebacteriagrowinfoodatneutralpH.

8. Intrinsic Factors
•Foodarepreservedwiththeadditionofpreservativessuchascitricacid,aceticacid
etc.whichcreateacidicenvironmentandprotectfoodtosomeextentfromthegrowth
ofbacteriaandothermicroorganismsrequiringneutraloralkalineenvironment.
•Howevertheremaybechancesofcontaminationandgrowthofacidophilicmoldsor
yeasts.
•Therefore,growthofmoldsisfrequentlyobservedinfruits,fruitjuicesandother
processedacidicfood.
•Foodacidificationbyfermentationinhomefoodpreparationsistheoldestpractice
manhasbeendoing.MostbacteriagrowbestataboutpH7andgrowpoorlyornotat
allbelowpH4.
•Yeastsandmolds,therefore,predominateinlowpHfoodswherebacteriacannot
compete.Thelacticacidbacteriaareexceptions;theycangrowinhighacidfoodsand
actuallyproduceacidtogiveussourmilk,pickles,fermentedmeats,andsimilar
products.

9. Intrinsic Factors
Population:
•Ahighinitialbacterialloadincreases
thelikelihoodthatspoilagewilloccur
undermarginalcircumstances.
•Thisfactisofmajorimportancetothe
processorofrefrigeratedfoods,the
shelf-lifeofwhichisenhancedbygood
sanitation.
•Ahighlevelofsporesalsoincreasesthe
possibilitythatafewwillsurviveto
spoilheatprocessedproducts

10. Intrinsic Factors
Oxygen:
•Oxygen is essential for growth of some microorganisms; these are called
aerobes. Others cannot grow in its presence and are called anaerobes. Still
others can grow either with or without oxygen and are called facultative.
•Strict aerobes grow only on food surfaces and cannot grow in foods stored
in cans or in other evacuated, hermetically sealed containers.
•Anaerobes grow only beneath the surface of foods or inside containers.
Aerobic growth is faster than anaerobic.
•Therefore, in products where both conditions exist, such as in fresh meat,
the surface growth is promptly evident, whereas subsurface growth is not.

11. Intrinsic Factors
OxidationReductionPotential(Eh):
•Itistheoxygentension.
•Itisoxidizingpoweroffood.
•Itisdefinedaseasewithwhichfooddonatesoracceptsprotonsor
electrons.

12. Intrinsic Factors
•Reducingfoods:-arethosefoodswhichdonateelectronsandhelpother
foodsbecomingreduced.
•Oxidizingfoods:-arethosefoodswhichacceptelectronsandhelptheother
foodsbecomeoxidized.
•Oxidationandreductionpotentialismeasuredbypotentiometer.
•IfORis+veitisaerobic.
•IfORis–veitisanaerobic.
•Ifitisslightly+veor–veitismicroaerophilic.
•FruitsandvegetableshaveOR+ve(+300to+400mv)henceaerobescause
spoilage.
•Meathas-200to+200mvthekindofspoilageisaerobictoanaerobic.
•Cheese=-20to-200mv.Thereforeithasmicroaerophilictoanaerobic
spoilage.

13. Intrinsic Factors
Nutrients:
•Differentfoodswillhavedifferentnutrientsfore.g.Milkisacompletefood
withallessentialnutrientsfoodsrichinallnutrientswillattractmore
microorganisms.
•Microbeshavecapacitytoadopttoparticularfoodbyproducinggrowthfactors.
•Fungiingeneralhavethehighestcapacitytosynthesizegrowthfactorsfollowed
byyeast,Gram-veandGram+vebacteria.
•Nutritionalqualityoffoodintermsoftheirchemicalcomposition,nutritive
valueornutrients,theirproportionandgrowthpromotingabilityareimportant
forgrowthofmicroorganisms.
•Carbohydratesmayincludepolysaccharideslikecellulose,starchanddifferent
sugars.
•Dependingonthetypeofcarbohydrateandotherenergycompoundspresent,
therewillbevariationintermsofmicroorganismsgrowinginfood.

14. Intrinsic Factors
•Mostofthesugaryfoodhavegrowthofyeastsandbacteriaandstarchyfoodpromote
growthofmoldswhichareabletohydrolyzestarchbyproducingamylases.
•Cellulolyticandpectinolyticmoldsgrowonfoodcontainingthesematerials.
•Carbonforgrowthusuallycomesfromcarbohydratesandotherorganiccompoundsin
mostoftheheterotrophicmicroorganisms.
•Microorganismsdifferintheirabilitytousevariousnitrogenouscompoundsassource
ofnitrogenforgrowth.
•Manyorganismsareunabletohydrolyzeproteinsduetolackofproteolyticenzymes.
Bacteriaaremoreproteolyticthanfungiingeneral.
•Foodcontaindifferentvitamins,mineralsandothergrowthfactorsandtheir
compositionandcontentmayvary.
•FreshplantandanimalfoodcontainBcomplexgroupofvitaminsbutstalefoodlack
someofthesevitamins.
•MeatsarehighinBvitaminsandfruitsarelow,butfruitsarehighinascorbicacid

15. Intrinsic Factors
•Theconcentrationofkeynutrients(carbohydrates,proteinsetc.)can,tosomeextent,
determinetherateofmicrobialgrowth.
•Themorethenutrients,themorethemicrobialgrowth.
•Bacterialgrowthcanbewellexplainedbyusingbacterialgrowthcurve.

16. Intrinsic Factors
Presence or Anti-microbial Substances:
•If they are present in food lower loads or microbes which are at resistant types will be
found.

17. Intrinsic Factors
BiologicalStructure:
•Outerskinoffruits,vegetableswhich
haveathickrindwillhavelow
microbialloadanyinjurytothese
naturalstructureswillleadto
microbialcontamination.

18. Environmental factors
1.Relativehumidity
2.Temperature
3.Gaseousatmosphere

19. Environmental factors
Relativehumidity:
•Relativehumidityandwateractivityareinterrelated.Whenfood
commoditieshavinglowwateractivityarestoredinanatmosphereofhigh
RH,waterwilltransferfromthegasphasetothefood.
•Itmaytakeaverylongtimeforthebulkofthecommoditytoincreasein
wateractivity.
•Oncemicroorganismshavestartedtogrowandbecomephysiologically
activetheyusuallyproducewaterasanendproductofrespiration.
•Storageoffreshfruitsandvegetablesrequiresverycarefulcontrolof
relativehumidity.ItRHistoolow,manyvegetableswilllosewaterand
becomeflaccid.
•Itistoohighthencondensationmayoccurandmicrobialspoilagemaybe
initiated.

20. Environmental factors
Temperature:
•Microbialgrowthcanoccuroveratemperaturerangefromabout-8°Cupto100°C.
•Psychrophilicbacteria(-15to10°Corevenlower)
•Psychotropicbacteria(0°Ctomesophilictemp.)
•Mesophilicbacteria(20-45°C)
•Thermophilicbacteria(>45°C)
•Microorganismscanbeclassifiedintoseveralphysiologicalgroupsbasedontheir
cardinaltemperatures.
•Lowtemperatureaffectstheuptakeandsupplyofnutrientstoenzymesystemswithin
thecell.
•Manymicroorganismsrespondstogrowthatlowertemperaturebyincreasingthe
amountofunsaturatedfattyacidsintheirmembranelipidsandthatpsychrotrophs
generallyhavehigherlevelofunsaturationinafattyaciddecreasesitsmeltingpointso
thatmembranescontaininghigherlevelsofunsaturatedfatlyacidwillremainfluidand
hencefunctionalatlowertemperatures.

21. Environmental factors
Gaseousatmosphere:
•Oxygencomprises21%oftheearth’satmosphereandisthemost
importantgasincontactwithfoodundernormalcircumstances.
•TheinhibitoryeffectofCO2onmicrobialgrowthisappliedinmodified
atmospherepackingoffoodandisanadvantageincarbonatedmineral
watersandsoftdrinks.
•MoldsandbacteriaaresensitivetoCO2condensation.
•CO2dissolvesinwatertoproducecarbonicacidwhichdecreasespHand
partiallydissociatesintobicarbonateanionsandprotons.
•CO2alsoaffectssolutetransport,inhibitionofkeyenzymesinvolving
carboxylation,decarboxylationreactionsinwhichCO2isareactantand
reactionwithproteinaminogroupscausingchangeintheirpropertiesand
activity.

22. implicit factors
1.Synergism:Itistheconditionwhentwoormoredifferent
specieshelpeachothertopropagateortodevelop.
2.Antagonism:Itistheconditionwhentwodifferentspecies
opposetoeachothertopropagateortodevelop.
3.Commensalism:itistheconditionwhenonespeciesdoesn’t
affectothersinpropagation.

23. processing factors
1.Slicing
2.Washing
3.Packing
4.Irradiation
5.Heatingandcooling

24. processing factors
1.Slicing:
▪Slicingmethodandthicknesshasaneffectonmicrobialgrowthon
fruitsandvegetables.
▪Maintainingofverysmallerthicknessduringcuttingresultslarge
surfaceareaofsamplewhichishelpfulformicrobes.
2.Washing:
▪Soakingwatershouldbeofpotablequality.
▪Washingreducesthemicrobialloadinfoods.
▪Contaminatedwaterisanimportantriskfactorformicrobial
propagation.

25. processing factors
3.Packaging:
▪Packagingpreventsmicrobesfromenteringintothecontent.
▪Packagingwithinertgasesandcarbondioxideprovidegood
protectingbehavior.
▪Protectiondegreechangeswiththepackagingmaterialsand
thickness.

26. processing factors
4.Radiation:
▪Electromagneticwavesofdifferentlengthsdisplaydifferenteffecton
microorganisms
a.Infrared–nodirectlethaleffect(heat!)(wavelength700nmto1
mm)
b.Visiblelight–positiveandnegativeeffectonsomeactivityofcells
c.Ultraviolet–strongmutagenicandlethaleffects(lowpenetration)
d.Ionicradiation-strongmutagenicandlethaleffects(highpenetration)
e.Microwaveradiation-themicrowaveregionofthespectrumoccupies
frequenciesbetween109Hzupto1012Hzandsohasarelativelylow
quantumenergy.

27. processing factors
5.Heatingandcooling:
•Thereare2maintypesofheattreatment,
➢LowTemperature
➢HighTemperature
•Pasteurizationinactivateandcontrolpathogenicmicrobesinfoods
whilesterilizationkillsbothsporeformingandnon-sporeforming
microbes.
•Sterilizationkillsallthemicrobes.
•Chillingslowsdownwhilefreezingstopsthemicrobialactivity.

28. Thanks