Etiket Arşivleri: Antimicrobial substances

Factors Affecting Microbial Growth in Food

  • 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.