Fish Microbiology and Quality Assurance

2.1.4. Redox potential

Unit 2 - Growth and survival of microorganisms in food

2.1.4. Redox potential (Oxidation – Reduction potential / O-R potential/ Eh)
Microorganisms show varying degree of sensitivity to O-R potential (Eh) of growth medium. Redox reaction occurs as a result of transfer of electrons between atoms or molecules. O-R potential of a substrate is defined as the ease with which the substrate loses or gains electrons. Substrate is oxidized when it loses electrons and is called as good reducing agent. Substrates that gain electrons become reduced and are a good oxidizing agents.

Cu ------> Cu + e

Oxidation may also achieved by addition of oxygen.

2 Cu + O₂ -------> 2 CuO

Transfer of electros from one compound to another creates a potential difference between two compounds and is measured by an instrument and expressed as millivolts (mv). Highly oxidized substances have more electric potential (positive potential) and reduced substances negative electrical potential. Zero electric potential when oxidation and reduction are equal. O/R protential of a system is expressed by Eh. Aerobic microorganisms require positive Eh for growth and anaerobes, negative Eh. Reducing conditions in food is maintained by -SH groups in meat and ascorbic acid and reducing sugars in fruits and vegetables.

Factors influencing O/R potential of a food

• The characteristic of O/R potential of the original food.
• Poising capacity – (Resistance to change in potential of food)
• Oxygen tension of the storage atmosphere of food
• Access that the atmosphere has to the food
• Microbial activity

Eh requirement of microorganisms

• Aerobic microorganisms require oxidized condition (+ Eh) for growth. Ex. Bacillus sp
• Anaerobes require reduced condition (-Eh). Ex. Clostridium sp
• Microaerophils are aerobes growing at slightly reduced condition. Ex. Lactobacillus, Campylobacter.
• Facultative anaerobes have capacity to grow both under reduced and oxidized condition. Eg. Yeasts.
• Plant foods have positive Eh (fruits, vegetables) and spoilage is mainly caused by aerobes (bacteria and molds).
• Solid meat and fish have negative Eh (-200 mv), and minced meat positive Eh (+200 mv).

Microorganisms and Eh of food

• Microorganisms affect Eh of food during growth. Aerobes reduce the Eh of environment due to oxygen utilization. Growth medium becomes poorer in oxidizing and richer in reducing substances.
• Microorganisms reduce Eh by releasing metabolites. Hydrogen sulphide released by anaerobic microorganisms reacts with oxygen and creates reduced condition.
• Presence or absence of appropriate quantity of oxidizing/ reducing agents in the medium influences growth and activity of all microorganisms.