Factors Affecting Microbial Growth in Food

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9 yıl önce

Factors affecting microbial growth in food

(a) Intrinsic factors:

These are inherent in the food. They include:

pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure

(b) Extrinsic factors:

Are factors external to the food that affect microbial growth.

Factors affecting microbial growth in food

(a)Intrinsic factors:

These are inherent in the food. They include:

Hydrogen ion concentration (pH)

Moisture content

Nutrient content of the food

Antimicrobial substances

Biological structures

1. Hydrogen ion concentration (pH)
Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5.
Some bacteria can grow within a narrow pH range of 4.5 and 9.0, e.g. Salmonella
Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and 8.5.
pH values of some food products
Microorganisms that are able to grow in acid environment are called acidophilic microorganisms.
These microorganisms are able to grow at pH of around 2.0
Yeasts and molds grow under acidic conditions.
Other microorganisms such as Vibrio cholerae are sensitive to acids and prefer alkaline conditions.
Most bacteria are killed in strong acid or strong alkaline environment except Mycobacteria.
Minimum and maximum pH for growth of some specific microorganism
2. Moisture content
The effect of moisture is in terms of water activity: -the amount of free water in a food medium.
The amount of free water is important for growth of microorganisms.
If there is lack of free water microorganisms will not grow.
Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature. (Aw = VPFood/VPWater)
Moisture content
The water activity of pure water is equal to 1.0
Food products have a water activity of less than 1.0.
A saturated salt solution has a water activity of 0.75.
Salting and drying reduces the water activity of a food product.
Water activity of some food products
Water activity levels
Growth of microorganisms is greatly affected by the level of water activity (Aw) in the food.
Inhibition of growth occurs if the water activity for food is lowered beyond an organism’s minimum level of water activity that is necessary for growth.
Microorganisms have varying minimum water activity requirements that supports their growth in food.
Minimum water activity that supports growth of some microorganisms
3. Nutrients content of the food
Microorganisms require proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins, and minerals for growth.
Various foods have specific nutrients that help in microbial growth.
Foods such as milk, meat and eggs contain a number of nutrients that are required by microorganisms.
These foods are hence susceptible to microbial spoilage.
Antimicrobial substances
Antimicrobial substances in food inhibit microbial growth.
Various foods have inherent antimicrobial substances that prevent (inhibit) microbial attack.
Such inhibitors are like lactenin and anti-coliform factors in milk and lysozyme in eggs.
Biological structures
Some foods have biological structures that prevent microbial entry.
For example, meat has fascia, skin and other membranes that prevent microbial entry.
Eggs have shell and inner membranes that prevent yolk and egg white from infection.
(b) Extrinsic factors
are factors external to the food that affect microbial growth. They include:

1.Temperature of storage,

2.Presence and concentration of gases in the environment

3.Relative humidity of food storage environment.

1. Temperature
The growth of microorganisms is affected by the environmental temperatures.
Various microorganisms are able to grow at certain temperatures and not others.
Bacteria can therefore be divided into the following groups depending upon their optimum tmperature of growth.
(i). Psychrophilic microorganisms
These grow best at about 20oC but also down to -10oC in unfrozen media.
Psychrophilic bacteria can cause food spoilage at low temperatures.
Several of the microorganisms found in the soil and water belong to this group.

…

Factors affecting microbial growth in food

(a) Intrinsic factors:

These are inherent in the food. They include:

pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure

(b) Extrinsic factors:

Are factors external to the food that affect microbial growth.

Factors affecting microbial growth in food

(a)Intrinsic factors:

These are inherent in the food. They include:

Hydrogen ion concentration (pH)

Moisture content

Nutrient content of the food

Antimicrobial substances

Biological structures

1. Hydrogen ion concentration (pH)

Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5.

Some bacteria can grow within a narrow pH range of 4.5 and 9.0, e.g. Salmonella

Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and 8.5.

pH values of some food products

Microorganisms that are able to grow in acid environment are called acidophilic microorganisms.

These microorganisms are able to grow at pH of around 2.0

Yeasts and molds grow under acidic conditions.

Other microorganisms such as Vibrio cholerae are sensitive to acids and prefer alkaline conditions.

Most bacteria are killed in strong acid or strong alkaline environment except Mycobacteria.

Minimum and maximum pH for growth of some specific microorganism

2. Moisture content

The effect of moisture is in terms of water activity: -the amount of free water in a food medium.

The amount of free water is important for growth of microorganisms.

If there is lack of free water microorganisms will not grow.

Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature. (Aw = VPFood/VPWater)

Moisture content

The water activity of pure water is equal to 1.0

Food products have a water activity of less than 1.0.

A saturated salt solution has a water activity of 0.75.

Salting and drying reduces the water activity of a food product.

Water activity of some food products

Water activity levels

Growth of microorganisms is greatly affected by the level of water activity (Aw) in the food.

Inhibition of growth occurs if the water activity for food is lowered beyond an organism’s minimum level of water activity that is necessary for growth.

Microorganisms have varying minimum water activity requirements that supports their growth in food.

Minimum water activity that supports growth of some microorganisms

3. Nutrients content of the food

Microorganisms require proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins, and minerals for growth.

Various foods have specific nutrients that help in microbial growth.

Foods such as milk, meat and eggs contain a number of nutrients that are required by microorganisms.

These foods are hence susceptible to microbial spoilage.

Antimicrobial substances

Antimicrobial substances in food inhibit microbial growth.

Various foods have inherent antimicrobial substances that prevent (inhibit) microbial attack.

Such inhibitors are like lactenin and anti-coliform factors in milk and lysozyme in eggs.

Biological structures

Some foods have biological structures that prevent microbial entry.

For example, meat has fascia, skin and other membranes that prevent microbial entry.

Eggs have shell and inner membranes that prevent yolk and egg white from infection.

(b) Extrinsic factors

are factors external to the food that affect microbial growth. They include:

1.Temperature of storage,

2.Presence and concentration of gases in the environment

3.Relative humidity of food storage environment.

1. Temperature

The growth of microorganisms is affected by the environmental temperatures.

Various microorganisms are able to grow at certain temperatures and not others.

Bacteria can therefore be divided into the following groups depending upon their optimum tmperature of growth.

(i). Psychrophilic microorganisms

These grow best at about 20oC but also down to -10oC in unfrozen media.

Psychrophilic bacteria can cause food spoilage at low temperatures.

Several of the microorganisms found in the soil and water belong to this group.