Lecture 3 intrinsic and extrinsic factors

1. LECTURE 3
INTRINSIC AND EXTRINSIC FACTORS
AFFECTING THE TYPE AND NUMBER OF
MICROORGANISMS IN FOODS

2. Microbial growth in food is dependent on
Intrinsic Factors: physical and chemical
properties of the food
Extrinsic Factors: Storage conditions
Implicit Factors: Physiological properties of
microorganisms
Process Factors: heating, cutting,..

3. Intrinsic factors
•pH
•Water activity (a
w)
•Redox potential (Eh)
•Nutrient content
•Antimicrobial constituents
•Biological (antimicrobial) structures

4. Hydrogen ion concentration (pH)
•Every microorganism has a maximal and an optimal pH
for growth. In general yeast and moulds are more acid
tolerant than bacteria.
•The inherent pH of foods varies, although most are
neutral or acidic. Foods with low pH values (below 4.5)
usually are not readily spoiled by bacteriaBUT are
more susceptible to spoilage by yeastand mouldsas
shown in the table below:

8. •AfoodwithinherentlylowpHwouldtherefore
tendtobemorestablemicrobiologicallythana
neutralfood.Theexcellentkeepingqualityofsoft
drinks,fermentedmilks,sauerkrautandpicklesis
duetotheirrestrictivepH.Somefoodshavealow
pHbecauseofinherentacidity;otherse.g.the
fermentedproductshavealowpHbecauseof
developedacidityfromtheaccumulationoflactic
acidduringfermentation.
•MouldscangrowoverawiderrangeofpHvalues
thanmostyeastandbacteriamostfermentative
yeastarefavouredbyapHofabout4.0to4.5asin
fruitjuices.Afilmofyeastgrowswellonacid
foodssuchassauerkrautandpickles.

9. Moisture Requirements: The
Concept of Water Activity:
•Microorganismshaveanabsolutedemandforwater,forwithoutwater
nogrowthcanoccurtheexactamountofwaterneededforgrowthof
microorganismsvaries.Thiswaterrequirementisbestexpressedin
termsofavailablewaterorwateractivity(aw)thevapourpressureof
thesolution(ofsolutesinwaterinmostfoods)dividedbythevapour
pressureofthesolventusuallywater.
•Thewateractivityforpurewaterwouldbe1.00andfora1Msolution
ofidealsolute,theawwouldbe0.9823.Theawwouldbeinequilibrium
witharelativehumidity(RH)oftheatmosphereaboutthefood.
Solutesandionstieupwaterinsolutionthereforeanincreaseinthe
concentrationofdissolvedsubstancessuchassugarsandsaltshasan
effectofdryingofthematerial.
•Notonlywateristiedwiththesolutes,butwatertendstoleavethe
microbialcellsbyOsmosisifthereisahigherconcentrationofsolutes
outsidecellsthaninside.

10. Moisture content/Water activity: a
w
Moisture content = (% water / 100 g of the product)
Ex: bananas 75.7%
asparagus 91.7%
pasta 10.4%
Water activity = index of availability of water to microbial growth
P= vapour pressure above the product
P
0= vapour pressure of pure water
a
w= P / P
0
Relative humidity RH = 100 x a
w

12. The lowest aw values permitting
growth of spoilage organisms
Group of organisms Minimal aw value
Most spoilage bacteria 0.91
Most spoilage yeast 0.88
Most spoilage moulds 0.80

13. Oxidation –Reduction Potential
(Eh)
•Microorganisms display varying degrees of sensitivity to the
oxidation-reduction potential of their growth medium.
•Ehofsubstanceisdefinedastheeasewithwhichthe
substratelosesorgainselectrons.Whenanelementor
compoundloseselectrons,thesubstrateissaidtobe
oxidized,whereasasubstratethatgainselectronsbecomes
reduceCu
Reduction
Oxidation
eCu

14. •Oxidationmayalsobeachievedbyadditionofoxygen.e.g.
•Substancethatreadilygivesupelectrons:Goodreducingagents
Substancethatreadilytakesupelectrons:Goodoxidizingagents
•Whenelectronsaretransferredfromonecompoundtoanother,a
potentialdifferenceiscreatedbetweenthetwocompounds.The
potentialdifferenceismeasuredinmillivolts(mV).Themorehighly
oxidizedasubstanceisthemorepositivewillbeitselectrical
potentialthemorehighlyreducedasubstanceisthemorenegative
willbeitselectricalpotential.
•Whentheconcentrationofoxidantandreductantisequal,azero
electricalpotentialexists.
•Aerobic microorganisms require positive Eh values (oxidized) for
growth, whereas anaerobes require negative Eh values (reduced).
•Among the substances in foods that help to maintain reducing
conditions are SH groups in meats and ascorbic acid and reducing
sugars in fruit and vegetables.CuoOCu 22
2

15. The redox potential (Eh) of a food is determined by the
following:
•The characteristic redoxpotential (Eh)of the original food
•The poising capacity; that is the resistance to changes in potential
to the food
•The oxygen tensionof the atmosphere about the food
•The access that the atmospherehas to the food
•With regard to the Eh of foods, plant foods, especially plant juices
tend to have Eh values from 300mV-400mV therefore aerobic
bacteria and molds are the common cause of spoilage of these
products. Solid meats have Eh values around 200mV, in minced
meat Eh is generally 200mV. Cheeses of various types have Eh
values on the negative side, from -20 to -200mV

16. Nutrient Content:
•In order to grow and function normally the
microorganisms of importance in foods require
the following:
•Water
•Source of energy
•Sources of nitrogen
•Vitamins and related growth factors
•Minerals

17. Assourcesofenergy,foodborne
microorganismsmayutilizesugar,
alcoholsandaminoacids.Somefew
microorganismsareabletoutilize
complexcarbohydratessuchasstarches
andcelluloseassourcesofenergyby
firstdegradingthesecompoundsto
simplesugar.Fatsareusedalsoby
microorganismsassourcesofenergybut
thesecompoundsareattackedbya
relativelysmallnumberofmicrobesin
foods.

18. Antimicrobial Constituents
Some foods cannot easily be attacked by
microorganisms due to the presence of
naturally occurring substances that have
antimicrobial activity
•Example is the presence of essential oils in
some species. Among these are:-
i.Eugenol in cloves
ii.Allicin in garlic
iii.Cinamic aldehyde and eugenol in cinnamon
lactoferrin, conglutinin and lactoperoxidase
system in cow’s milk. Under certain conditions
milk caseinas well as some free fatty acidshave
antimicrobial activity

19. •Eggs contain lysozymes. Thi enzyme
along with conalbumin, provides fresh
eggs with a fairly efficient
antimicrobial system.
•Fruits, vegetables, tea, molasses
contain hydroxycinnamicacid
derivatives which show antibacterial
activity.

20. Biological Structures
Some foodsarenaturally
coveredandthesecovering
provideexcellentprotection
againsttheentryand
subsequent damage by
spoilageorganisms.
Testaofseeds,theouter
coveringoffruitstheshellof
nuts,thehideofanimalsand
theeggshells.

21. •Innuts,theshellpreventtheentry
ofallorganismsbutoncecracked
nutmeatsaresubjecttospoilageby
molds
•Theeggshellandmembranesif
intactpreventtheentryofnearlyall
microorganismswhenstoredunderthe
properconditionsofhumidityand
temperature.

22. •Fruitsandvegetablewithdamaged
coveringundergospoilagemuch
fasterthanthosenotdamaged
•Theskincoveringoffishandmeats
suchasbeefporkpreventsthe
contaminationandspoilageofthese
foodspartlybecauseittendstodry
outfasterthanfreshlycutsurfaces.

23. EXTRINSIC PARAMETERS
These are the properties of the storage
environment that affect both the foods
and their microorganisms. These are:
•Temperature of storage
•Relative humidity of the environment
•Presence and concentration of gases
•Presence and activities of other
microorganisms

24. Individual or groups of microorganisms grow over a wide range
of temperature. Depending on their temperature
requirements for growth, microorganisms can be placed in
three different groups
(i) Psychrotrophs:
Are those organisms that grow well at or below 7
o
C and their
optimum temperature is between 20
o
C and 30
o
C. The
lowest temperature at which a microorganism has been
reported to grow is -34
o
C.; the highest somewhere in
excess of 100
o
C.
(ii) Mesophiles:
Those microorganisms that grow well between 20
o
C and 45
o
C
with optima between 30
o
C and 40
o
C (E.g. Enterococcus
feacalis)
(iii) Thermophiles:
Those microorganisms that grow well at and above 45
o
C with
optima between 55
o
C and 65
o
C
Temperature

25. Micro-organisms -Temperature and Growth
growht
Temp. °C
-10010203040506070
Psychrophile
Psychrotrophe
Mesophile
Thermophile

26. Temperature dependent growth
Start amount
1.000 bacteria per gram food12
6
39
1
2
4
57
8
10
11 12
6
39
1
2
4
57
8
10
11
2.000
bacteria°C
7 °C 12
6
39
1
2
4
57
8
10
11
4.000
bacteria12
6
39
1
2
4
57
8
10
11
8.000
bact.°C
20 °C 4 Million
bact. 260.000
bact. 16.000
bact.

27. Thepsychrotrophsfoundmostcommonlyonfoodsare
thosethatbelongtothegeneraPseudomonasand
Enterococcus.Theseorganismsgrowwellat
refrigeratortemperaturesandcausespoilageofmeats
poultry,eggsandotherfoodsnormallyheldatthis
temperature.
Thermophilicbacteriaofimportanceinfoodsbelongto
thegeneraBacillusandClostridium.Althoughafew
speciesofthisgeneraarethermophillic,theyareofgreat
importanceinthecanningindustry.

28. Moldsareabletogrowoverawiderrangeoftemperature
thanbacteriamanymoldsareabletogrowatrefrigeration
temperatures(e.g.somestrainsofAspergillusineggs,
sidesofbeefandfruits.
Yeastgrowoverthepsychrotrophicandmesophillic
temperaturerangesbutgenerallynotwithinthe
thermophillicrange
Temperatureofstorageisthemostimportantparameter
thataffectsthespoilageofhighlyperishablefoods.

29. Relative Humidity of Environment
(RH)
•Whenfoodswithlowa
wvaluesare
placedinenvironmentsofhighR.Hthe
foodspickupmoistureuntil
equilibriumhasbeenestablished.
•Likewisefoodswithahigha
wlose
moisturewhenshouldbearinmind
thatthereisarelationshipbetween
R.Handtemperaturethehigherthe
temperature,thelowertheR.Hand
viceversa.

30. •Foodsthatundergosurfacespoilage
frommoulds,yeastsandcertain
bacteriashouldbestoredunder
conditionoflowR.H
•Improperlyrappedmeatssuchas
wholechickensandbeefcutstendto
suffermuch spoilageinthe
refrigeratorbeforedeepspoilage
occurs.ThisisduetohighR.Hofthe
refrigeratorandthefactthatmeat
spoilagemicroorganismsareaerobicin
nature.

31. Presence of Concentration of Gases in
The Environment
Controlledatmosphere(modifiedatmosphere
storage)
Thisistheatmospherecontainingincreased
amountsofCO
2uptoabout10%CO
2is
appliedfrommechanicalsourcesorbyuse
ofsolidCO
2
Modifiedatmosphereisemployedin
storageoffruitse.g.applesand
pears.CO
2retardfungalrottingoffruits
CO
2atmospheresextendthestoragelifeof
meatcarcasses.Ingeneraltheinhibitory
effectsofCO
2atlowertemperatures,and
thepHofmeatsstoredinhigh-CO
2
environmentstendstobeslightlylowerthan
thatofair–storedduetocarbonicacid
formation.

32. Gram-negativebacteriaaremore
sensitivetoCO
2thangram-positive.
Ozone(O
3);whenaddedtofood
storageenvironmenthas a
preservativeeffectoncertainfoods.
Atlevelsofseveralpartspermillion
(ppm)O
3hasbeenfoundtobe
effectiveagainstavarietyof
microorganismsO
3shouldnotbe
usedonhigh-lipidcontentfoods
becauseitwouldcauseanincreasein
rancidityasitisastrongoxidizing
agent

33. Presence and Activities of Other
Microorganisms
Some food borne organisms produce substances
that are either inhibitory or lethal to others.
These substances include antibiotics, bacteria,
hydrogen peroxideand organic acids.
Antibiotics:
•These are secondary metabolites produce by
microorganisms that inhibit or kill a wide
spectrum e.g. nicin and natamycin.

34. General microbial interference
•Refers to general nonspecific inhibition or
destruction of one microorganism by other
members of the same habitat or environment
Lactic antagonism:
•This is a specific example of microbial
interference, it is the phenomenon of a lactic acid
bacterium inhibiting or killing closely related and
food poisoning or food spoilage organisms when in
mixed culture.
•In this case the bacteriocins, pH depression,
organic acids, H
2O
2diacetyl and other products
effect inhibition of pathogens and food –spoilage
organisms.

35. COMBINED INTRINSIC AND EXTRINSIC
PARAMETERS THE HURDLE CONCEPT
•Inthehurdleconcept,multiplefactorsortechniques
areemployedtoeffectthecontrolofmicroorganisms
infoods,whileunderintrinsicandextrinsic
parameters;theeffectofsinglefactorsonthe
growthofmicroorganismsispresented.
•Thehurdleconceptisappliedinfoodpreservationis
alsodescribedindifferentwayssuchas;Barrier
Technology,CombinationPreservationorCombined
Methods.
•Inthistechnique,inordertogrowtheorganismsmust
“hurdle”aseriesofbarriers.Alargenumberof
factorsareknownthatcanbeappliedtofoodsystems
ashurdles.

36. Example of the Hurdle
Effect
•Inpreventinggerminationof
sporesofphotolyticorgroup1
strainsofClostridiumbotulinum,
amongtheintrinsicandextrinsic
parametersthatareknownareas
follows:pH<4.6,aw<0.99,10%
Nacl,120ppm,NaNO2,incubation
temperatureand<10
o
C.