Food BiotechnologyDr. Kamal E. M. ElkahloutFood Microbiology 2 Microbial Nutrition and Factors Affecting GrowthIntrinsic and Extrinsic Factors

Factors affecting the growth and survival of micro-organisms in foods

The ability of microorganisms to grow or multiply in a food is determined by the food environment as well as the environment where the food is stored.Intrinsic factors

Factors related to the food itself. (nutrients, water activity, pH, redox potential and oxygen) Extrinsic factors

Environmental conditions where the food is stored.

(Temperature, Relative humidity, Gaseous Environment, packaging conditions)

Intrinsic factors

Nutrients and growthWhen a microbial cell is growing in a food, the nutrients supplied by the food include: carbohydrates, proteins, lipids, minerals and vitamins.

All foods contain these 5 major nutrients but nutrients vary greatly with the type of food.

Example: meat is rich in proteins, lipids, minerals and vitamins but poor in carbohydrates.

Foods from plant sources are rich in carbohydrates but poor in proteins, lipids, minerals and some vitamins.

Microorganisms normally found in food vary greatly in nutrient requirement with bacteria requiring the most followed by yeast and molds

Intrinsic factors

Water activity (Aw)

Water activity (Aw) is a measure of the availability of water for biological functions and relates to water present in a food in “free” form .

In a food system total water is present in a food in “Free” and “Bound” form .

Bound water is the fraction used to hydrate hydrophilic molecules and to dissolve solutes. It is not available for biological function hence does not contribute to Aw.

Water activity (Aw)The Aw of a food can be expressed as: the ratio of water vapor pressure (p) of the food to that of pure water (Po which is 1) i.e. P/Po .

It ranges between 0 -1 or more accurately >0 to <1, as no food can have either 0 or 1 Aw.

The Aw of a food can also be determined from its equilibrium relative humidity (ERH) by dividing ERH by 100.

(% ERH/ 100) (ERH is expressed as percentage ).

Aw and microbial growth

Free water in a food is necessary for microbial growth.It is needed to transport nutrients, remove waste materials, carry out enzymatic reactions, synthesize cellular materials and take part in other biochemical reactions. Each microbial species has an optimum, maximum and minimum Aw level for growth.

Aw of foods can be reduced by removing water (desorption) and increased by the adsorption of water.

Desorption process gives relatively lower values than absorbtion process does at the same moisture content of a food.

Microorganisms can be controlled by reducing the Aw of food.

Aw can be reduced by adding solutes, ions, hydrophilic colloids, freezing and drying.

Intrinsic factors

pH

pH indicates the hydrogen ion concentration in a system. (H+ concentration ).It ranges from 0 – 14 with 7.0 being neutral.

Foods can be grouped as high acid foods (pH below 4.6) and low acid food (pH 4.6 and above).

Fruits, fruit juices, fermented foods (from fruits, vegetables,

meat and milk and salad dressings

(HIGH ACID FOODS)

Most vegetables, meat, fish, milk and soups

LOW ACID FOODS

• pH has profound effect on the growth of microbial cells.• Each species has an optimum and a range of pH for growth:

o Molds and yeasts-able to grow at lower pH than bacteria.o Gram negative bacteria are more sensitive to low pH than Gram

positive bacteria

• pH range:o molds: 1.5-9.0o Yeasts: 2.0-8.5o Gram positive: 4.0-8.5o Gram negative: 4.5-9.0

o However acid tolerant strains (Pediococcus acidilactici) can acquire resistance to lower pH compared with other strains eg Salmonella.

pH and microbial growth

When pH is reduced below the lower limit, microbial cells stop growing and lose viability.

Information on the influence of pH on growth and viability of microorganisms is important in developing methods to prevent the growth of undesirable microorganisms in food.

Intrinsic factors

Redox potential, Oxygen and growth

Redox potential ( Eh) measures the oxidation reduction potential of in a system whereby a substance is oxidized and the other reduced simultaneously.Process involves:loss of electrons from a reduced state (oxidation)gain of electrons by an oxidized substance (reduction)

electron donor reduces oxidized substance (reducing agent).electron recipient is called the oxidizing agent. redox potential is measured as units of millivolts (mV).Oxidized range: + mV Reduced range: - mV

Redox potential in food

Is influenced by its chemical composition, specific processing treatment given and storage condition in relation to air.

Fresh foods of plants and animal origin are in their reduced stage due to the presence of reducing substances e.g. ascorbic acid, reducing sugars and the –SH group of proteins.

once respiration of cells has been stopped ,O2 will diffuse inside and change the redox potential. Processes such as heating, can increase or decrease reducing compounds and alter the Eh.

Redox potential of some foods

Redox potential and microbial growth

On the basis of the growth in presence and absence of free oxygen, microorganisms have been grouped as aerobes, anerobes facultative anaerobes or microaerophiles. Remember

Growth of microorganisms and their ability to generate energy by the specific metabolic reactions depend on the redox potential of foods.

Presence or absence of oxygen and the Eh of food determine the growth capability of a particular microbial group in a food and the specific metabolic pathways used during growth.

Aerobes-need free O2 for energy generation as the final electron acceptor through aerobic respirationFacultative anaerobes can generate energy if free O2 is available or they can use bound O2 in cpds eg NO3 or SO3 as final electron acceptors through anaerobic respirationIf O2 is not available then other cpds are used to accept the electron through fermentation (anaerobic)Anaerobic and facultative anaerobes can only transfer electrons through fermentationAnaerobes such as obligate or strict cannot grow in the presence of even small amount of O2 as they lack superoxide dismutase to scavenge the toxic oxygen free radicals.Aerobic species-molds, yeasts, Bacillus, Pseudomonas, Micrococcus

Anaerobic species-lactic acid bacteria, Clostridium species, Enterbacteriaceae.

Technologies to control the redox potential of food in order to control the growth of microorganisms

Vacuum packaging.Skin tight packaging.Gas flushing.Canning.Antioxidants : These preservatives include natural antioxidants such as ascorbic acid (AA, E300) and tocopherols (E306), as well as synthetic antioxidants such as propyl gallate (PG, E310), tertiary butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA, E320) and butylated hydroxytoluene (BHT, E321).

Extrinsic factors

Extrinsic factors important in microbial growth in a food include the environmental conditions in which it is stored .these include :

Temperature Relative humidity Gaseous Environment

The relative humidity and gaseous conditions affect of storage respectively influence the Aw and Eh of the food.

Extrinsic factorsTemperature

• Foods are exposed to different temperatures from time of production until the time of consumption.

• Microbial growth is accomplished through enzymatic reactions which is depended on temperature.

Remember psychrophiles, mesophiles and thermophiles

• Every 10oC rise doubles the catalytic rate of enzyme and every 10oC decrease reduces it to half.

Extrinsic factors

Relative Humidity• Relative humidity and water are interrelated. • Relative humidity is a measure of water activity of

the gas phase.

• When food commodities have low Aw are stored in high relative humidity, water transfers from gas phase into the food. This causes the otherwise dormant spores of bacteria or fungi to germinate. Once they are actively growing, they produce water as an end product of respiration. Hence they increase the Aw of their own, this favors the growth of high Aw requiring bacteria and increase in spoilage of food.

Extrinsic factors

Gases in atmosphere Oxygen influences the redox potential of microbial associations.

Carbon dioxide regulates cell growth of some bacteria.

If partial pressure of carbon dioxide increases over a critical level, metabolic activity will be retarded.

Retarding effect of CO2 increases with increase in concentrations.

CO2 is used in packaging of some food items in order to control the growth of microorganisms.