Acidity and pH in Milk: Indicators of Freshness and Quality

Milk quality isn’t always visible to the naked eye. A glass of milk can look and even smell normal, yet already be on its way to spoilage. That’s where acidity and pH come in – two closely related but distinct chemical measurements that act as reliable early-warning indicators of milk’s freshness, safety, and suitability for processing. Understanding how they work, what causes them to change, and how the dairy industry measures them is fundamental to ensuring that milk reaching consumers is safe and of high quality.

What is pH and why does it matter in milk? 🔗

The pH scale runs from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, and values above 7 indicate alkalinity. Fresh cow’s milk has a pH between 6.5 and 6.8, placing it just on the slightly acidic side of neutral. This is a normal, healthy characteristic – not a sign of spoilage. The slight acidity is a result of milk’s complex chemical composition, including dissolved carbon dioxide, phosphates, citrates, and proteins.

Of these components, casein – the dominant protein in milk, accounting for roughly 80% of its protein content – plays the biggest role. Casein acts as a natural buffering agent, helping resist rapid shifts in pH even when small amounts of acid are introduced. Whey proteins and dissolved minerals like phosphates and citrates contribute to this buffering capacity as well. This is why milk doesn’t immediately turn sour the moment a single bacterium enters it – the buffering system temporarily resists change.

Natural acidity vs. developed acidity 🔗

Acidity in milk is classified into two types: natural (inherent) acidity and developed acidity. Both contribute to the total or titratable acidity of milk, but they have very different implications for quality.

Natural acidity 🔗

Natural acidity is present in freshly drawn milk from a healthy cow. It arises from the milk’s inherent chemical constituents – casein and whey proteins, carbon dioxide in solution, phosphates, citrates, and small amounts of naturally occurring lactic acid. This is not caused by microbial activity and does not indicate any quality problem. According to the Dairy Knowledge Portal, fresh milk’s titratable acidity, which accounts for both natural and developed components, normally ranges from 0.10% to 0.20% lactic acid, with the natural portion accounting for 0.10% to 0.12%.

Developed acidity 🔗

Developed acidity is a different matter entirely. It occurs when bacteria present in milk – particularly Lactobacillus species – begin fermenting lactose (milk sugar) and converting it into lactic acid. This fermentation process leads to a progressive decrease in pH over time, and as lactic acid accumulates, the milk begins to taste sour, develop off-odors, and eventually curdle. The rate of this process depends heavily on storage temperature, hygiene during milking, and the initial bacterial load of the milk. A pH drop below 6.4 is a strong indicator of active lactic acid fermentation and early spoilage – a reading that dairy processors treat as a red flag.

It is worth noting that certain conditions unrelated to spoilage can also shift milk pH. Colostrum – the first milk produced by a cow after calving – is more acidic than normal milk. Conversely, milk from cows suffering from mastitis (udder infection) tends to have a higher pH, often above 6.8, because blood components entering the milk raise its alkalinity. So both unusually low and unusually high pH values can signal that something is wrong.

Measuring acidity: pH vs. titratable acidity 🔗

Two primary methods are used in the dairy industry to measure acidity in milk. They are complementary – not interchangeable – and each provides different information.

pH measurement 🔗

pH is measured using a calibrated pH meter with an electrode probe inserted directly into the milk sample. Any significant deviation from the normal pH range of 6.7 to 6.9 may indicate spoilage or contamination. For accurate readings, milk samples should be at room temperature before testing, and pH meters must be regularly calibrated using standard buffer solutions (typically pH 4.0 and 7.0). pH strips can be used for quick field assessments, though they are less precise than electronic meters.

pH measurement is fast and useful as an initial screening tool. However, it only reflects the current hydrogen ion concentration in milk – it does not capture the full picture of all acidic components present.

Titratable acidity (TA) 🔗

Titratable acidity provides a more comprehensive measure of all the acidic substances in milk. The test involves titrating a known volume of milk (typically 10 ml) against a standard 0.1N sodium hydroxide (NaOH) solution, using phenolphthalein as an indicator. The titration continues drop by drop until a faint but persistent pink colour appears, marking the endpoint (approximately pH 8.3). The volume of NaOH consumed is used to calculate the percentage of lactic acid equivalent in the sample.

According to AgriMoon, the standard formula used is:

% Lactic Acid = (Volume of NaOH × Normality × 0.009 × 100) / Weight of sample

Fresh milk typically shows a titratable acidity of 0.13% to 0.16% lactic acid equivalent. Values rising above 0.18% are a warning sign, and anything beyond 0.20% is generally classified as developed acidity from bacterial action, making the milk unsuitable for standard processing.

The American Dairy Products Institute (ADPI) uses a standardized titration method where 1.0 mL of 0.1N NaOH corresponds to 0.009008 g of lactic acid – the stoichiometric basis for these calculations. Titratable acidity is considered more reliable than pH alone because it accounts for all acidic constituents in milk, including those that contribute to buffering but don’t directly affect the pH reading.

The clot-on-boiling (COB) test 🔗

While titratable acidity is a precise laboratory measurement, the clot-on-boiling (COB) test is a rapid, practical field test used to assess whether milk can withstand heat treatment without coagulating. It is particularly important at milk reception points at dairy plants, where large volumes of raw milk need to be quickly screened before processing.

The procedure is simple: approximately 5 ml of milk is placed in a test tube and heated in a boiling water bath until the sample itself boils. The milk is then examined for the formation of clots or visible curd.

The science behind the test is straightforward. When titratable acidity exceeds approximately 0.18% lactic acid, the casein proteins in milk become destabilized. Under the heat stress of boiling, these proteins aggregate and form visible clots. This coagulation is essentially a preview of what would happen if this milk were pasteurized or processed – it would curdle in the equipment, causing defects and product losses.

Results are interpreted as follows: a negative COB test (no clots form) indicates acceptable acidity and good thermal stability – the milk is suitable for further processing. A positive COB test (clots are visible) signals excessive developed acidity. Such acidic milk cannot withstand heat treatment and is unsuitable for liquid milk processing or further dairy manufacturing.

It’s worth noting that at high altitudes, water and milk boil at a lower temperature than at sea level, which can affect the sensitivity of the COB test – something field technicians in mountainous regions need to account for.

Why monitoring acidity matters throughout the dairy chain 🔗

Acidity and pH are not just quality indicators – they are safety parameters. Milk that has developed significant acidity due to bacterial activity may carry pathogenic microorganisms alongside the spoilage bacteria, even if the two processes are not always directly linked. Regularly monitoring these parameters helps dairy operations catch quality deterioration before it compounds into a public health issue.

Beyond raw milk reception, pH control is critical in downstream dairy processing. The pH level directly influences the texture, flavor, and shelf life of finished dairy products – cheese varieties require carefully controlled acidification to achieve the right texture and taste, while fluid milk must maintain low acidity to ensure processing stability and consumer acceptability. Butter cream is maintained at specific pH values depending on whether sweet or sour butter is being made.

Good storage practices also directly control acidity development. Refrigeration temperatures below 4°C significantly slow bacterial growth and the resulting lactic acid production, extending the window during which milk remains within acceptable acidity limits. Clean milking equipment, hygienic handling, and first-in-first-out inventory rotation are all practices that reduce the rate at which developed acidity accumulates in stored milk.

A quick summary of key acidity benchmarks in milk 🔗

For easy reference, the critical values used in dairy quality assessment are:

pH of fresh milk: 6.6 – 6.8 (normal range); below 6.4 signals early souring; above 6.8 may indicate mastitis or colostrum contamination.

Titratable acidity of fresh milk: 0.13% – 0.16% lactic acid equivalent (normal); above 0.18% indicates quality decline; above 0.20% is classified as significant developed acidity.

COB test threshold: Positive result (clot formation) typically occurs when acidity exceeds 0.18% lactic acid, indicating the milk will not survive heat processing without coagulating.

These benchmarks are widely used by dairy industry professionals in India and globally as the basis for accepting or rejecting incoming milk supplies, and for routing milk to different product streams depending on its acidity profile.

What do you think? If two milk samples have the same pH but different titratable acidity values, what does that tell you about the composition of the milk – and which test would you trust more for making a processing decision? Also, given that temperature is the biggest driver of developed acidity, how should cold chain infrastructure in rural dairy collection systems be prioritized to protect milk quality from farm to plant?

References

1. https://nutri.it.com/what-ph-is-milk-understanding-dairy-acidity-and-freshness
2. https://www.dairyknowledge.in/dkp/content/titrable-acidity-milk
3. https://safemilklabs.com/understanding-the-ph-of-milk-and-milk-products-a-key-indicator-of-quality-and-freshness/
4. https://wellness.alibaba.com/nutrition/ph-level-of-milk-explained
5. https://www.sigmaaldrich.com/US/en/technical-documents/protocol/analytical-chemistry/photometry-and-reflectometry/ph-of-milk-and-milk-products
6. https://agrimoon.com/acidity-test/
7. https://adpi.org/methodsofanalysis/analytical-method-007/
8. https://unacademy.com/content/kerala-psc/study-material/dairy-microbiology/clot-on-boiling-test/