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  • 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.

    2

  • Intrinsic Parameters

     Inherent properties of plant and animal tissues

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

    3

  • 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

    5

  • 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