Intrinsic & Extrinsic Parameters Affecting Microbial Growth

Terrence Madhujith, PhD

Department of Food Science and Technology

University of Peradeniya

Intrinsic and Extrinsic parameters of foods that affect microbial growth

 Foods are of plant or animal origin

 Especially the plant foods are evolved with natural mechanisms to evade pests and Mos.

 Since some plant foods contain living plant, we can make use of such mechanisms for protecting foods from Mos.

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Intrinsic Parameters

 Inherent properties of plant and animal tissues

◦ pH ◦ Moisture ◦ Oxidation-reduction potential (Eh) ◦ Nutrient content ◦ Antimicrobial constituents ◦ Biological structures

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pH

 The best is the neutral pH for most MOS. – 6.5-7.5

 Few grow below 4

 Bacteria are more sensitive than molds and yeasts

 Pathogenic bacteria are even more sensitive

 The boundaries depend on many factors

 pH range depends on

◦ The microbial species 4

◦ Nutritional level ◦ Presence of other factors e.g. NaCl, toxicants

E.g. In the presence of NaCl, Alcaligenes fecalis grow over a wide range of pH

◦ Type of acid used

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pH

G ro

w th

R a te

No additive

NaCl added

Na citrate added

5 7 9

pH tolerant pattern of Alcaligenes fecalis

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

molds

Lactic acid Bacteria

Satphylococcus aureus

Acetobactor spp

Salmonella spp

E coli

C botulinum

B cereus

V parahaemolyticus

Vibrio sp

pH growth ranges of some foodborne microorganisms

Food pH

Beans 4.6-6.5

Beets 4.2-4.4

Broccoli 6.5

Cabbage 5.4-6.0

Carrots 4.9-5.2

Cauliflower 5.6

Corn 7.3

Onions 5.3-5.8

Potatoes 5.3-5.6

Pumpkin 4.8- 5.2

Tomatoes 4.2-4.3

Fruit pH

Apples 2.9-3.3

Bananas 4.5-4.7

Lime 1.8-2.0

Melon 6.3- 6.7

Orange 3.6-4.3

Grapes 3.4 -4.5

Butter 6.2

Milk 6.5

Cheese 5.0 -6.0

Beef 5.00-6.00

Chicken 6.2

Fish 6.8

Crab 7

Tuna/Salmon 5.2-6.00

Shrimp 7.00

 Fruits, fruit drinks, vinegar and wines fall below the pH that bacteria normally grow

 Better keeping quality of these products are mainly due to the pH effect

 Fruits generally undergo yeast and mold attacks

 this is due to their ability to grow below pH 3.5



 Most vegetables have a higher pH value than fruits and therefore they are more susceptible for bacterial attacks

 Meat from fatigued animals spoils faster than the meat from well rested animals

 This is due to the difference in ultimate pH in meat

 Well rested animals have min. of 1% glycogen which produces enough lactic acid to bring pH down

 After rigor mortis pH goes down from 7.4 to 5.6 in well rested animals

 pH of meat varies between 5.1 and 6.2 depending of the rest given to the animal

 Acids may be ◦ Inherent ◦ Produced by microbes ( bioacidity)

 Cellular medium of microbes is generally neutral

 Exceptions – Methanococcus and Sulfolobus

 The cells need neutrality for functions and reproduction

 When Mos are placed outside of neutrality their growth is dependent on their ability to bring the environment to neutrality

 When the cells are placed in a high acidic environment the cells either have to prevent migration of H+ into the cell or pump out excess into the outside environment

 Organic acids in the non-ionic form are more effective in destroying microbes.

 E.g. malic, tartaric, citric, oxalic acids

 H+ interact with enzymes in cytopolasmic membrane

 Thus, when MOs grow on either side of the optimum range, an increase in lag phase will result

 Lag phase represents the time required by the MOs to convert the environment suitable for the microbes

time

P o p

u la

ti o n

 Adverse pH makes cells more vulnerable for other parameters

 Cl. acetobutyliticum neutralizes acidity by converting butyric acid into butanol

 Enterobactor aerogenes produces acetone from pyruvic acid

Effects of pH

 Morphology of the cell is affected

 Dysfunction of biomoleucles - ATP, DNA, proteins and enzymes

 Absorption of cations is affected – e.g. K+ - glucose metabolism

 High energy expenditure

 Increased lag phase

 Moisture content itself does not correlate well with microbial activity

 Instead, water activity (aw) is a very good parameter that decides the microbial activity

Water Activity

 aw = p/po  p = partial pressure of water of the food

 po= partial pressure of pure water at the same temperature

 Water activity is related to RH

 ERH = 100 X aw

 Pure water has aw of 1

 22% NaCl solution = 0.86; saturated NaCl solution = 0.75

Microorganism aw

Cl. botulinum type E 0.97

Cl. botulinum type A and B 0.94

Pseudomonas 0.97

E coli 0.96

Acinetobactor 0.96

Bacillus subtilis 0.95

Vibrio parahaemolyticus 0.94

Staphilococus aureus 0.86

Penicilium patulum 0.81

Aspergillus 0.70

Rhizopus stolonifer 0.93

Mucor 0.93

Microorganism aw

Halophilic bacteria 0.75

Xerophilic molds 0.61

Osmophilic yeasts 0.61

Candida 0.92

Xeromyces 0.61

Most spoilage bacteria 0.90

Most spoilage molds 0.80

Most spoilage yeasts 0.88

 Water activity of most fresh fruits and vegetables is above 0.99

 In general, bacteria require higher aw than molds and yeasts

 Most spoilage bacteria don’t grow below 0.91

 G(-)ves need higher water activity than G(+)ve bacteria

 However, spoilage molds can go up to 0.80

 Staphylococcus aureus can grow at 0.86

 Cl. botulinum does not grow below 0.94

 Lowering aw results in increasing the length of the lag phase

 All chemical and biochemical reactions depend on water and therefore microbial functions cease at low aw

 With reduction of aw in the envt. The no. of groups of MO capable of growing reduces

 Certain MOs can cope with the reduction of aw by generating osmo- regulatory compounds

 E.g. many bacteria, molds and yeasts produce polyols

 Some Mos need low aw envts. For optimum growth ◦ Halophilic ◦ Osmophilic ◦ Xerophilic

 E.g. Halophilic bacteria (Halobacteria) need high salt concentration for their growth

 E.g. Halobacteria and Halococci produce KCl as the osmo-regulator – they live in salt water, slat pans, salt lakes etc.

 Some produce proline

 The limiting value of aw for any MO is 0.60

 Below this level, cells can not live

 Cell wall of bacteria and fungi provides the necessary support to avoid cell rupture up to a certain extent

 At the point where the cell can no longer cope with the water influx the cell enlarges and bursts

Interaction of aw with other

parameters  aw interacts with

◦ Temperature ◦ pH ◦ Nutrient content etc.

 Effect of temperature on aw is prominent

 The ability of MOs to grow is reduced as aw is lowered at any temperature

 The range of aw over which MOs can grow

is high at optimum temperature

 Nutrients increases the range of aw

10 0 20 30 40 50

0.80

0.85

0.90

0.95

1.0

Temp

aw

Penicillium expansum

Aspergillus flavus

Effect of Low Water Activity

 Prolongs lag phase

 Reduce growth rate, size and final population

 Effects depend on other factors such as temp, pH, Eh etc.

 E.g. when temp and pH are made unfavorable the min aw for growth goes up.

Water activity of some foods

Food aw Fresh F&V, meats, fish milk, beverages

>0.98

Evaporated milk, tomato paste, cheese, bread

0.93-0.98

Sweetened condensed milk 0.85 - 0.93

Dried fruits, flour, cereals, jam jellies, nuts, cakes, IMFs

0.6 – 0.85

Chocolates, candies, honey, biscuits, crackers, milk powder, dried vegetables

Oxidation Reduction Potential