• 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.

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