IntrinsicParameters of Foods That Affect

Microbial GrowthDr. AJAYI

FST 314

INTRINSIC PARAMETERS

• The parameters of plant and animal tissues that are an inherent part of the tissues are referred to as intrinsic parameters.

• These parameters are as follows:

• pH

• Nutrient content

• Moisture content

• Oxidation-reduction potential (Eh)

• Antimicrobial constituents

• Biological structures

pH• Most microorganisms grow best at pH values around 7.0 (6.6-7.5)

• Bacteria tend to be more fastidious (critical) in their relationships to pH than moulds and yeasts, and pathogenic bacteria being the most fastidious.

• The excellent keeping quality of some food products is due in part to their pH

• Examples: fruits, soft drinks, vinegar, and wines have pH below the point at which bacteria normally grow

• Fruits initially undergo mould and yeast spoilage

• Most vegetables have higher pH values than fruits, and, consequently, vegetables should be subject more to bacterial than fungal spoilage.

• Most of the meat and seafood have a final ultimate pH of about 5.6 and above. This makes these products susceptible to bacteria as well as to mould and yeast spoilage

• Adverse pH affects at least two aspects of a respiring microbial cell:

I. The functioning of the enzymes

II. The transport of nutrients into the cell

NUTRIENT CONTENT

• In order to function properly, microorganisms require

1. Water

2. Source of energy

3. Source of nitrogen

4. Vitamins and related growth factors

5. Minerals

• Moulds have the lowest requirement for nutrient content, followed by yeast, Gram negative (pink colour) and Gram positive (purple colour) bacteria

• Primary source of nitrogen utilized by heterotrophic (require external supply of energy) microbes are amino acids.

• As a source of energy, foodborne microorganisms can utilize sugars, alcohols and amino acids

• Only a few mirobes are able to utilize complex carbohydrates such as starches and cellulose as sources of energy

• Microorganisms may require vitamins B in low quantities

• Generally, Gram positive bacteria are the least synthetic. Therefore, the bacteria must be supplied with one or more of these compounds for growth

• Gram negative bacteria and moulds are able to synthesize most of or all of their requirements

• Both groups of bacteria may be found growing on foods low in vitamins B

MOISTURE CONTENT• It is generally accepted that the water requirements of microorganisms

should be described in terms of the water activity (aw) in the environment

• The preservation of foods by drying is a direct consequence or removal of moisture, without which microorganisms do not grow

• Bacteria require higher water activity for growth than fungi. Gram negative bacteria require higher moisture content (aw) than Gram positives

• Most spoilage bacteria do not grow at aw below 0.91. however, spoilage moulds can grow at low aw of 0.80.

• Staphylococcus aureus can grow at aw of 0.86 an exception

Relationship among aw, temperature and nutrition• Certain relationship exist among aw, temperature and nutrition.

1. At any temperature, the ability of microorganisms to grow is reduced as the aw is lowered.

2. The range of aw over which growth occurs is greatest at the optimum temperature for growth.

3. The presence of nutrients increases the range of aw over which the organisms can survive. If the nutrients are manipulated, there is increased proliferation of microorganisms

Effects of low aw

• The general effects of lowering aw below optimum is to increase the length of the lag phase of growth and to decrease the growth rate and size of final population of microorganisms

• The predictable effects of lowered aw on microorganisms include:

1. Accumulation of proline amino acid

2. Increases in amino acids in some salt – tolerant bacteria

Oxidation-reduction potential (Eh)

• This is defined as the ease with which the compound/substrate loses or gains electrons

• When an element or compound loses an electron, the compound is said to have been oxidized. And if a compound gains an electron it is said to have been reduced.

• When electrons are transferred from one compound to another, a potential difference is created between the two compounds.

• The higher the reducing potential of food, the greater the chance of spoilage and the higher the oxidizing potential the less chance of spoilage.

Example, meat has Eh of -200 mV while plant foods have Eh value of 300-400 mV.

Meat spoils quicker than plant foods because it has a higher reducing potential

Antimicrobial Constituents of Foods

• The stability of some foods against the attack by microorganisms is due to the presence of certain naturally occurring substances. These include:

1. Essential oils possess antimicrobial activity. Ex. Garlic and Onion has Allicin; Cloves have Eugenol; and Cinnamon has Cinnamic aldehyde.

2. Bovine milk contains several antimicrobial substances such as lactoferrin, conglutinin and lactoperoxidase system

3. Eggs contain lysozyme, providing an efficient antimicrobial system

4. Cell vacuoles of some plants such as cabbage, turnips and broccoli have glucosinolates which have antibacterial and antifungal capability

Biological Structures• Natural covering of seeds provide excellent protection against entry and

subsequent damage by spoilage organisms.

• These categories are structures such as

I. Testa of seeds

II. Outer covering of fruits

III. Shell of nuts

IV. Hide/skin of animals

V. Outer shell and membranes of eggs if intact