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 activity 2- pH 3- Oxidation reduction potential 4- Nutrient content 5- Antimicrobial constituents 6-Biological structure

Factors affecting the growth of M.O divided to B) Extrinsic factors Factors inherent to the environment 1- Temperature 2- Gaseous environment 3- Relative humidity 4- Microbial interaction.

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 Saturated NaCl solution 0.75 Point at which cereal loses crunch 0.65 Honey 0.5 - 0.7 Dried fruit 0.5 - 0.6

Total moisture of a food product = free moisture (available for bacterial use) + bound moisture (moisture that is unavailable for bacterial use).

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)

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 Escherichia coli 0.95 Salmonella Vibrio cholerae Bacillus cereus 0.93 Listeria monocytogenes 0.92 Staphylococcus aureus 0.87 Most Fungi 0.70 No microbial proliferation 0.60

Equilibrium relative humidity (ERH) 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.

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.

Intermediate moisture foods (IMF) Have moisture content 15-50% (aw 0.6-0.85) Shelf stable at room temp 20-22C. 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 acids Proper 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/baking Controlled 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 -100C) 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 Water Enhanced by: Increased surface area Increased temperature Increased air velocity Reduced humidity Reduced pressure (vacuum)

Spoilage Of Dried Foods dried foods do not undergo microbial spoilage chemical 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