LPT 321: Meat Science (1+1)

FACTORS INFLUENCING GROWTH OF MICRO ORGANISM 

• The growth of microorganisms in food is affected by a few inherent characteristics of the food, referred to as intrinsic parameters, and also by the conditions of environment in which the food is stored or held referred to as extrinsic parameters.

Intrinsic parameters that affect microbial growth

• pH of the food- Most microorganisms grow well at a pH of around 7 (6.6 – 7.5), while only few of them grow at pH below 4. Bacteria are more sensitive to ph requirements in comparison to yeast and moulds, which manage to grow across a pH range of 0 – 11.
• Moisture content - It is common knowledge that drying of food enhances its shelf life. The water requirement of microorganisms is generally described the water activity (a_w) of the food. Water activity is the ratio of the vapour pressure of the food substance to the vapour pressure of pure water. Most spoilage and pathogenic bacteria in meat require a water activity in equal to more than 0.9, and the most tolerant bacteria is Staphylococcus aureus which continues to grow at a a_w of 0.86. Most spoilage yeasts require a a_w of 0.88, while spoilage moulds manage to grow at a a_w of 0.80. If foods are dried to a final a_w of 0.60 or lesser, the product becomes shelf stable.
• Oxidation reduction potential (Eh), which is the ease at which a substrate gains or loses electrons, where gaining an electron is referred to as reduction, and losing an electron is known as oxidation. Aerobes require oxidized conditions for growth (a positive Eh value), while anaerobes require reduced condition for growth (a negative Eh value). Microaerophiles, require slightly reduced conditions whereas facultative anaerobes grow in both reduced and oxidized conditions.
• Nutrient content- Microbes require the following in adequate amounts for their normal growth:
• A source of energy
• A source of nitrogen
• Vitamins and related growth factors and
• Minerals.
• Antimicrobial constituents in food accord protection for foods against microbial growth. Examples of such substances include eugenol in cloves, allicin in garlic, allyl isothiocyanate in mustard, cinnamic aldehyde and eugenol in cinnamon and lactoferrin, conglutinin and Lactoperoxidase system in milk.
• Biological structures- Certain biological structures such as rind of fruits, shell of nuts and fascia in carcasses accords protection against microbial invasion to a certain degree.

Extrinsic parameters that affect microbial growth

• Temperature of storage – Most microorganisms in food are mesophiles, which grow well at temperatures between 30 - 40⁰C, while some of them optimally need a temperature range of 20 -30⁰C, but grow well at 7⁰C or below and are known as psychrotrophs. Some microorganisms optimally need a temperature range of 55 - 65⁰C, but grow well at 45⁰C or above and are referred to as thermophiles.
• Gases in the environment - It is common knowledge that oxygen is required for aerobes and anaerobes do well in the absence of oxygen.
• Relative humidity - Relative humidity has a role to play in both surface spoilage as well as deep seated spoilage.