Oxidation-Reduction Potential in Milk: Impact on Quality and Shelf Life

When a glass of fresh milk sits on a shelf, an invisible chemical balance determines whether it stays wholesome or slowly deteriorates. That balance is captured by a single measurement: oxidation-reduction potential (Eh). It reflects the tendency of milk to gain or lose electrons in chemical reactions – and it directly governs how long milk retains its flavor, safety, and nutritional integrity. Understanding Eh is not just academic; it is central to how modern dairy operations maintain product quality from farm to consumer.

What is oxidation-reduction potential? 🔗

Oxidation-reduction potential, or Eh, is a measure of a system’s ability to either donate electrons (oxidize) or accept electrons (reduce). According to ScienceDirect, a positive Eh value indicates an oxidized state, while a negative value indicates a reduced state. In practical terms, the higher the Eh, the more oxidizing the environment – and the more reactive and unstable the milk becomes.

The major components of milk – fat, lactose, and casein – have relatively little influence on its Eh. Instead, the principal systems that determine Eh in milk are dissolved oxygen, ascorbate (vitamin C), and riboflavin (vitamin B2). The thiol-disulfide system also contributes to Eh in heated milk, such as pasteurized or UHT products.

Normal Eh range in fresh milk 🔗

Research indicates that the redox potential of individual milk samples in equilibrium with air falls in the range of +0.25 to +0.35 V at 25°C at normal milk pH. Eh is also inversely related to pH – rising from about +0.20 V at pH 10 to approximately +0.395 V at pH 3.5 in raw milk. This means that as milk acidifies (as it does during spoilage or fermentation), the Eh value shifts accordingly.

Fresh, high-quality milk typically registers Eh values between +0.2 and +0.3 volts. This range reflects a moderately oxidizing environment that is compatible with milk’s natural biochemistry. Any significant departure from this range – whether due to contamination, mishandling, or microbial growth – signals a shift in quality.

Key factors that influence Eh in milk 🔗

Dissolved oxygen 🔗

Oxygen is the single most direct driver of Eh in milk. The more dissolved oxygen present, the higher the Eh value climbs. During milking, handling, transport, and storage, milk picks up oxygen from the surrounding air. Studies on milk fermentation by lactic acid bacteria confirm that the redox potential decreases in parallel with oxygen consumption – and continues to drop at a slower rate as microbial activity further lowers the oxygen content. This is why dairy processors commonly use nitrogen flushing or vacuum packaging to limit oxygen exposure and maintain a lower, more stable Eh.

Microbial activity 🔗

When bacteria grow in milk, the redox potential changes over time, and the general tendency is for Eh to shift in a negative (reducing) direction. A rapid drop in Eh occurs once aerobic bacteria exhaust the dissolved oxygen. High-quality milk with low bacterial counts maintains relatively stable Eh values, while contaminated milk shows rapid Eh fluctuations as microbial populations grow. This change in Eh forms the chemical basis of the widely used methylene blue and resazurin dye reduction tests for assessing milk’s bacteriological quality – discussed in detail below.

Metal ions – especially copper 🔗

Trace metals are potent catalysts of oxidation reactions in milk. Research published in the International Dairy Journal confirms that the concentrations of copper and ferrous iron in milk are directly linked to its redox potential and the development of oxidized flavor. A classic review in the Journal of Dairy Science concluded that of all factors causing oxidative changes in dairy products, metallic contamination – particularly by copper – is the most important. In cow’s milk, oxidized flavor only develops after a marked rise in Eh triggered by copper-catalyzed oxidation of ascorbic acid. Industry guidance notes that as little as 0.1 ppm of copper, iron, or sulfur in the water supply used for cleaning milking equipment can contribute to flavor defects in milk.

pH and heat treatment 🔗

As noted, Eh is inversely related to pH in raw milk – so any acidification lowers Eh. Heat treatment also changes milk’s redox behavior. Research on antioxidant activity in dairy products shows that pasteurization can both reduce some natural antioxidants and generate new oxidizing molecules through the early stages of the Maillard reaction. Sterilized milk, on the other hand, may actually gain antioxidant activity due to the Maillard reaction between lactose and lysine residues in milk proteins.

How Eh is measured in milk 🔗

Measuring Eh requires electrochemical equipment. The standard method uses a platinum electrode paired with a calomel reference electrode. The platinum electrode responds to the oxidizing or reducing species present in the milk, while the calomel electrode provides a stable reference point. The measured potential difference between the two electrodes gives the Eh value in millivolts or volts.

However, measuring Eh in milk is not always straightforward. ScienceDirect notes that measurement difficulties arise from the sluggishness of some oxidation-reduction systems in reaching equilibrium, the incomplete reversibility of certain reactions, and the risk of back-diffusion of oxygen during the measurement process. These limitations mean that Eh values should be interpreted alongside other quality indicators rather than in isolation.

A newer approach involves cyclic voltammetry, which provides a fingerprint of the electroactive compounds in milk. Research on new approaches for measuring redox state in milk highlights that this technique can capture multiple concurrent oxidation-reduction processes and has even been explored for detecting milk adulteration – for instance, identifying the addition of cow’s milk to goat’s milk.

Eh-based dye reduction tests for milk quality 🔗

One of the most practical applications of Eh in dairy science is dye-based quality testing. Both the methylene blue reduction test (MBRT) and the resazurin reduction test (RRT) exploit the relationship between bacterial activity, oxygen depletion, and Eh change to give a rapid assessment of milk quality.

Methylene blue reduction test 🔗

Methylene blue is blue in its oxidized form (at Eh ≈ +0.3 V) and becomes colorless when reduced. A foundational study in the Journal of Dairy Science showed that bacteria first consume dissolved oxygen, and only then does the dye itself get reduced to its colorless form. The time taken for decolorization is roughly proportional to the number of bacteria present – milk that decolorizes in under 30 minutes is considered unsatisfactory. The test is straightforward: a measured amount of methylene blue solution is added to milk and incubated at around 37°C, and the color is observed at intervals.

Resazurin reduction test 🔗

The resazurin test follows a similar principle but offers a more detailed visual scale. Resazurin starts blue at Eh ≈ +0.3 V, shifts to pink (resorufin) as Eh drops to about +0.2 V, and finally becomes colorless at high bacterial loads. This progressive color change – blue → purple → pink → colorless – provides more nuanced quality grading than the simple decolorization seen in MBRT. The test can also be run in a rapid 10-minute format at the milk reception dock, making it particularly useful for high-throughput dairy operations.

Both tests have important limitations: they are not suitable for pasteurized milk (heat treatment interferes with dye reduction chemistry), and they reflect overall bacterial metabolic activity rather than identifying specific pathogens.

Eh and milk flavor: the oxidation connection 🔗

The most commercially significant consequence of Eh changes in milk is oxidized flavor. Research in the Journal of Dairy Science identifies the phospholipid fraction of milk fat as a primary target of oxidation – leading to metallic, cardboard-like, or tallowy off-flavors depending on the nature of the oxidative change. Grass-fed cows produce milk that is less susceptible to oxidized flavor, partly because their feed is rich in reducing substances such as ascorbic acid and carotenoids that help buffer the Eh.

Research on the antioxidant capacity of milk confirms that oxidation processes shorten shelf life and deteriorate taste by triggering the appearance of unpleasant aftertastes. Importantly, protein oxidation and lipid oxidation can occur independently of each other. Elevated concentrations of natural antioxidants – such as whey proteins, vitamins A, C, and E, and β-carotene – help prolong the lag phase before oxidative damage sets in. Studies on dairy powder oxidative quality further highlight that lipid oxidation is a chain reaction involving initiation, propagation, and termination stages, producing volatile aldehydes, ketones, and alcohols responsible for off-flavors in processed dairy products.

Controlling Eh in dairy processing 🔗

Managing Eh effectively across the dairy supply chain requires attention at multiple points.

Minimizing oxygen exposure 🔗

Limiting dissolved oxygen is the most direct way to keep Eh within a desirable range. Closed-system milking equipment, nitrogen flushing of storage tanks, and vacuum or barrier packaging all reduce oxygen ingress. Research on dissolved oxygen and milk fermentation kinetics confirms that nitrogen-flushed milk supports faster, more controlled acidification by lactic acid bacteria compared to oxygen-exposed milk – an important consideration for yogurt and cheese production.

Managing metal contamination 🔗

Eliminating copper and iron contamination from milking equipment and water supplies is critical. Practical dairy guidelines recommend testing water sources for copper, iron, and sulfur levels, replacing copper pipelines with plastic tubing where water is acidic (pH below 7.0), and switching to iodine-based sanitizers when chlorine residues on equipment surfaces are suspected of contributing to oxidation.

Antioxidant buffering 🔗

Natural antioxidants present in milk – including ascorbate, riboflavin, and thiol groups – act as Eh buffers by absorbing electrons before oxidation can damage fats or proteins. Some processors supplement milk with vitamins E or C for additional protection. Electroreduction has also been tested as a method of actively lowering milk’s Eh, though research shows that the effect is temporary, with Eh returning to positive values within a few days.

Eh monitoring in fermented dairy products 🔗

In cheese making, Eh undergoes a dramatic shift. The Eh of fresh milk is around +150 mV, while that of cheese is approximately −250 mV. This shift to a strongly reducing environment is linked to the fermentation of lactose to lactic acid by starter cultures. At such low Eh, cheese becomes essentially anaerobic, restricting growth to facultative and obligate anaerobes. Research published in the Journal of Dairy Research found that Enterococcus faecalis and Lactococcus lactis are among the most potent Eh-reducing bacterial species in dairy fermentations – making their selection in starter culture development an important quality control consideration.

Why Eh matters for dairy quality assurance 🔗

Eh occupies a unique position in dairy quality science because it integrates multiple quality-relevant variables – oxygen content, microbial load, metal contamination, and antioxidant status – into a single measurable value. As noted in redox research on milk, knowledge of the actual redox conditions is important for interpreting milk quality and freshness. Milk spoilage is related to oxidation-reduction reactions that alter a range of compounds, and a clear correlation exists between the degree of oxidation and the sensory quality of cheese and other dairy products. Integrating routine Eh monitoring into quality control programs – alongside standard microbiological and compositional testing – provides dairy processors with an early warning tool that can prevent costly quality failures before they reach the consumer.

What do you think? Given that copper contamination at even 0.1 ppm can trigger oxidized flavor in milk, how realistic is it for small-scale dairy farms to consistently monitor and control metal ion levels in their water and equipment? And as fermented dairy products like cheese depend on a dramatic Eh reduction by starter cultures, should Eh be adopted as a standard process control parameter in every commercial cheesemaking facility?

References

1. https://www.sciencedirect.com/topics/immunology-and-microbiology/oxidation-reduction-potential
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