Food preservation principles

6.2. Examples of reduction and oxidation processes

Key oxidation-reduction reactions involved in food processing include microbial metabolism, enzymatic activity, lipid oxidation, and flavor development.

1. Microbial metabolism is the oxidation of organic compounds (e.g. carbohydrates, proteins, and lipids) by a variety of aerobe microorganisms, such as bacteria and fungi. This means, these reactions require the presence of oxygen.

An example of microbial metabolism is the fermentation of grains to produce beer and wine. During fermentation, various yeasts, such as Saccharomyces cerevisiae, break down sugar molecules to produce ethanol and carbon dioxide. In this reaction the glucose molecule is reduced, losing 12 electrons, while the oxygen molecule is oxidized, gaining 6 electrons: C₆H₁₂O₆ + 6O₂ + 12e^- --> 2C₂H₅OH + 2CO₂ + 6e^- (glucose + oxygen --> ethanol + carbon dioxide). Microbial metabolism is also used to produce a variety of other food products such as cheese, yogurt, and bread.

Microorganisms like molds and yeasts that metabolize glucose in fruits on the other hand lead to spoilage and bad odors.

2. Oxidation and reduction can also be catalyzed by enzymatic activity. These reactions occur naturally in food processing, such as in the ripening of fruits and the production of beer and wine where enzymes are present. The enzyme that is typically responsible for the reduction of glucose in the presence of water is glucose dehydrogenase. Glucose dehydrogenase is an enzyme found in some bacteria, fungi, and plant cells. It catalyzes the oxidation of glucose by accepting electrons from nicotinamide adenine dinucleotide (NAD). The resulting product is 6-phosphogluconolactone, which is then further metabolized to carbon dioxide, ethanol, and other molecules. In this reaction the glucose molecule is reduced, losing 12 electrons, and the oxygen molecule is oxidized, gaining 6 electrons: C₆H₁₂O₆ + 6H₂O + 12e^- --> 6CO₂ + 12C₂H₅OH + 4ATP + 6e^- (glucose + water --> carbon dioxide + ethanol + ATP)

The enzyme glucose oxidase on the other hand, catalyzes the oxidation of glucose, thereby preventing it from fermenting into alcohol. For example in fruit juice production glucose oxidase can be used as an enzymatic additive to prevent alcohol formation. Gluconic acid and hydrogen peroxide, which are the products of the reaction can act as acidulants, preservatives, and sterilizing agents in the juice. The reaction can be expressed as C₆H₁₂O₆ + O₂ + 2e^- --> C₆H₁₂O₇ + H₂O₂ (glucose + oxygen --> gluconic acid and hydrogen peroxide), whereby the glucose molecule loses two electrons and the oxygen molecule gains two electrons.

3. Lipid oxidation is the oxidative change and breakdown of lipids due to the presence of oxygen, which can produce off-flavors, off-odors, and discoloration in food products. It usually involves the addition of oxygen atoms to polyunsaturated fatty acids, converting them to their oxidized form. For example Linoleic acid (C18:2), a polyunsaturated fatty acid that is found commonly in vegetable oils has an unstable electron distribution. If exposed to air 2 hydrogen atoms are lost, while 2 oxygen atoms are gained, resulting in the formation of the oxidative stable C18:2-hydroperoxide. During the process the following aldehydes are also formed: C₁₈H₃₂O₃ → Acetaldehyde (C₂H₄O) + Formaldehyde (CH₂O) + Propionaldehyde (C₃H₆O). These aldehydes are volatile compounds which contribute to the bad odor and off-flavour of lipid-oxidized foods.