5.Factors Affecting the Growth of Microorganisms

Factors affecting the growth of microorganisms

Factors affecting the growth of M.O divided to

A) Intrinsic factors

Factors inherent to food itself 1-Water activity2- pH3- Oxidation reduction potential4- Nutrient content5- Antimicrobial constituents6-Biological structure

Factors affecting the growth of M.O divided to

B) Extrinsic factorsFactors inherent to the environment

1- Temperature

2- Gaseous environment

3- Relative humidity

4- Microbial interaction.

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Intrinsic factors: • water activity• pH• oxidation reduction potential• nutrient content• antimicrobial constituents • biological structure

B) Extrinsic factors Factors inherent to the environment •temperature

•gaseous

•relative humidity

•microbial interaction.

Total moisture of a food product composed of

1- free moisture

available for bacterial use

2- bound moisture

moisture that is

unavailable for

bacterial use.

Water activity (aw(

Water activity (aw(

Vapor pressure of water divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one. aw = vapor pressure of food substrate (solution)

vapor pressure of pure water

Related to relative humidity: RH= 100 x aw

Substance aw

Distilled Water 1

Tap water 0.99

Raw meats 0.97 - 0.99

Milk 0.97

Juice 0.97

Saturated NaCl solution 0.75

Point at which cereal loses crunch 0.65

Honey 0.5 - 0.7

Dried fruit 0.5 - 0.6

aw requirements for microbial growth

Each microorganism has its own minimum water activity value below which it will not grow.

bacteria > yeast > mold

Microorganism Inhibited aw

Clostridium botulinum E 0.97

Pseudomonas fluorescens 0.97

Escherichia coli 0.95

Salmonella 0.95

Vibrio cholerae 0.95

Bacillus cereus 0.93

Listeria monocytogenes 0.92

Staphylococcus aureus 0.87

Most Fungi 0.70

No microbial proliferation 0.60

Bound moisture in foods can be increased by

(Reducing the aw):

1. Formation of gels or gums. Add flour, pectin, etc.

2. Addition of solutes (salt, CHO)

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Changes in free moisture within a substrate

1. evaporation

2. water of respiration

3. product degradation ( gel decomposition)

4. freezing: as ice is formed outside the cell, solute concentration within the cell increases.

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Water activity is temperature dependent.

Freezing: as ice is formed outside the cell, solute concentration within the cell increases. -1C = 0.990 -5C = 0.953 -10C = 0.907 -20C = 0.823

At constant temperature the moisture content of a food changes until it comes into equilibrium with the water vapor in the surrounding air.

Proper storage (packaging) of these foods is required in order to prevent mold growth.

Equilibrium relative humidity (ERH)

Microorganisms growing at marginal aw (high osmotic pressure)

1. Halophiles:Tolerate or require salt ( Na+, K+) for

growth. Many originate from marine (hypersaline)

environments.

2. Osmophiles:• Grow in foods with high osmotic pressure• Sugars syrups, honey, soft drinks, candy,

jams jellies etc. • Most are yeasts.3. Xerophiles: • Dry loving. Grow to an (aw = 0.65).• Many are fungi particularly storage fungi.

Food Perishability:

1- Perishable foods contain high levels of free moisture. They normally require preservatives and / or

refrigeration. Fruits, vegetables, fresh meat, fish

2-Non perishable foods have low levels of free moisture. aw < 0.6 would be

required

Intermediate moisture foods (IMF)

–Have moisture content 15-50% (aw 0.6-0.85)

–Shelf stable at room temp 20-22C.–Includes jams, sausages, soft centered candies, dried fruit–Moisture has been withdrawn via drying and/or addition of ( humectants) solutes including sugars, salts ( glycerol, sorbitol, sucrose).

Intermediate moisture foods (IMF)

–Drying food down to a level where both M.O and enzymatic activity is halted or reduced. –Most important spoilage organisms are yeast and molds–IMF contain fungistats: sorbate and benzoic acidsProper storage (packaging) of these foods is required in order to prevent mold growth.

Preservation By Drying

Microbial growth and enzymatic activity depend on free water

M.O need water for nutrient transport, nutrient metabolism and removal of cellular waste

Drying food down to a level where both MO and enzymatic activity is halted or reduced

For all microbial growth (aw< 0.60)

Water is Removed:

Under natural field conditions– grains, raisins, seeds

Cooking/bakingControlled dehydration processes

treatment of foods prior to drying includes:

1- Peeling, cutting, trimming of fruits and vegetables: may reduce microbial load

2- Light colored fruits and vegetables may be treated with SO2 (1,000-3,000 ppm). Reduces enzymatic browning by polyphenol oxidase; also reduces MO on surface. Preserves vitamin C

3-Blanching or scalding ( 80 -100C) of vegetables is performed to inactivate some of the enzymes that could be problematic during drying/storage of the product

Most meats are cooked prior to dehydration ( chicken cubes used in dried soup mixes)

Eggs (albumen) are usually desugared prior to drying in order to prevent browning

Dehydration Principle:

Apply Heat / Remove WaterEnhanced by:

– Increased surface area– Increased temperature– Increased air velocity– Reduced humidity– Reduced pressure (vacuum)

Spoilage Of Dried Foods

dried foods do not undergo microbial spoilagechemical changes occur in foods containing

fat and oxygen (oxidative rancidity)reducing sugars ( carbonyl group) in dried

foods react with the amino group of amino acids and proteins Maillard reaction ( non enzymatic browning) bitterness

loss of vitamins ( C), discoloration and structural changes can occur during drying/storage

Freeze-Drying

It is a combined method of preservation based on the dehydration of food in the frozen state through vacuum sublimation of its ice content.

The moisture content is reduced to a level 2-8% or aw 0.10-0.25

It should be packed in an inert atmosphere in vapor-impermeable packaging

Exam I

Wednesday, June, 27th, 2012

Time:

Location: