How Heat Affects Milk: Changes in Protein, Acidity, and Flavor

Every time milk is heated – whether during pasteurization, UHT processing, or simple boiling at home – a series of chemical changes unfold inside it. These aren’t random. Heat systematically alters milk’s proteins, mineral balance, acidity, and flavor compounds in ways that directly impact its nutritional value, processing behavior, and sensory qualities. Understanding these changes is foundational to producing consistent, high-quality dairy products.

What happens to milk proteins when heated?
Casein: the heat-stable fraction
Whey proteins: heat-sensitive and reactive
How denaturation affects dairy product quality
Effect of heat on milk acidity and mineral balance
Changes in pH during heating
Calcium and phosphate redistribution
How heat generates cooked flavor in milk
Sulfhydryl groups and volatile sulfur compounds
The Maillard reaction and broader flavor changes
Why these changes matter in dairy processing

What happens to milk proteins when heated?

Milk contains two major protein families: caseins (about 80% of total protein) and whey proteins (about 20%). These two groups respond to heat in fundamentally different ways.

Casein: the heat-stable fraction

Caseins exist naturally as large colloidal structures called casein micelles, stabilized by calcium phosphate and a surface layer of κ-casein. These micelles are remarkably resistant to heat. Research published in Frontiers in Nutrition confirms that caseins are relatively stable at high temperatures, and do not denature in the same way as globular proteins. Even at typical pasteurization temperatures, their core structure remains largely intact. This thermal stability is one reason why casein-based products tolerate heat processing well.

However, at higher temperatures – particularly above 80-90°C – casein micelles are not entirely unaffected. They can interact with denatured whey proteins, and their surface chemistry changes, which ultimately affects rennet coagulation, gel formation, and cheese-making performance.

Whey proteins: heat-sensitive and reactive

Whey proteins – primarily β-lactoglobulin (β-Lg) and α-lactalbumin (α-La) – are far more heat-sensitive. A study in the journal Foods (MDPI) confirms that denaturation of these heat-labile proteins is initiated when temperatures exceed approximately 62°C for α-La and 75°C for β-Lg. Once these thresholds are crossed, the proteins begin to unfold, exposing hydrophobic regions and reactive chemical groups that were previously buried inside their native structure.

β-Lg plays a particularly important role in this process. It contains a free sulfhydryl (-SH) group and two disulfide bonds. When unfolded by heat, its exposed thiol group triggers a cascade of thiol-disulfide exchange reactions. Research reviewed in the International Journal of Food Science shows that in the early stages of heating, β-Lg interacts with κ-casein on the micelle surface via hydrophobic attractions, followed by covalent -SH/S-S interchange reactions. The result is the formation of whey protein-casein complexes that significantly alter milk’s functional properties.

α-La, by contrast, lacks a free thiol group. Despite this, it can still undergo denaturation through disulfide bond exchange and can form complexes with β-Lg through noncovalent interactions. A study published in PMC found that both denaturation degree and the combination of whey protein with casein increase significantly as both temperature and heating time increase, following a quadratic regression relationship.

How denaturation affects dairy product quality

The degree of whey protein denaturation has direct, practical consequences for dairy manufacturing. A study in the International Dairy Journal reports that heating milk above 75°C promotes protein and moisture retention in cheese curd, resulting in higher cheese yield. However, this same heat treatment has a detrimental effect on rennet-induced coagulation kinetics and curd contraction. The balance between these outcomes depends on both the temperature and the pH at the time of heating.

For yogurt, the picture is different. A review in the International Journal of Food Science (Wiley) shows that heat-induced denaturation of whey proteins – and their subsequent binding to casein micelles – is actually desirable in yogurt production. It creates a denser protein network that improves gel body, texture, and water-holding capacity. This is why yogurt milk is routinely heat-treated at 85-95°C for 5-10 minutes before fermentation.

Effect of heat on milk acidity and mineral balance

Heating milk doesn’t just affect proteins – it also disrupts the delicate equilibrium of mineral salts and alters the acidity of milk in ways that are both scientifically important and practically significant.

Changes in pH during heating

Fresh milk has a slightly acidic pH of around 6.6 to 6.8. When heat is applied, pH drops. A study published in the Journal of Dairy Science (Cornell University) established that during heating of milk, pH decreases linearly with increasing temperature, directly linked to shifts in calcium phosphate equilibrium. Using established coefficients, the pH of milk at 80°C can be about 0.58 pH units lower than the same milk at 0°C – a substantial shift.

This pH drop during heating is generally reversible if temperatures do not exceed 100°C and no significant protein or lactose degradation occurs. At higher temperatures – such as those used in UHT or sterilization – lactose begins to degrade through the Maillard reaction, producing organic acids (principally formic acid) that contribute further to the decline in pH. Research by P.F. Fox in the Journal of Dairy Science quantified three main contributors to this pH decline during severe heat treatment: production of organic acids from lactose (contributing about 50%), precipitation of calcium phosphate as tertiary phosphate with release of H⁺ ions (20%), and hydrolysis of casein phosphate followed by its precipitation (30%).

Calcium and phosphate redistribution

In fresh milk, calcium and phosphate exist in a carefully maintained equilibrium between soluble (free) and colloidal (micelle-bound) forms. Heating disrupts this balance. A study in Dairy Science & Technology found that at any given milk concentration, levels of soluble calcium and soluble phosphate decrease within the first few minutes of heating – as these minerals transfer from the soluble phase to the colloidal phase, forming insoluble calcium phosphate deposits within or around casein micelles.

This redistribution of calcium phosphate has measurable consequences. It reduces the concentration of free ionic calcium in milk serum, which affects rennet coagulation, heat stability, and the behavior of milk during concentration and drying. In cheese-making, this shift in mineral balance can lengthen coagulation time. In concentrated dairy products, it can contribute to fouling of heat exchanger surfaces and age gelation during storage.

How heat generates cooked flavor in milk

One of the most immediately noticeable effects of heating milk is the change in its flavor. Fresh milk has a mild, clean taste. Heated milk, particularly when treated at high temperatures, develops what is commonly described as a cooked, slightly sulfurous, or eggy aroma. This is not accidental – it results from specific chemical reactions triggered by heat.

Sulfhydryl groups and volatile sulfur compounds

The primary source of cooked flavor is the liberation of free sulfhydryl (-SH) groups during whey protein denaturation. According to a review of sulfur compounds in foods published by the American Chemical Society, heat denaturation of sulfur-bearing whey proteins liberates free sulfhydryl groups that are widely considered responsible for the cooked flavor in heat-processed milk. Once exposed, these groups react with other compounds to generate volatile sulfur molecules.

Research published in the Journal of Dairy Science confirmed that milk serum proteins – particularly β-Lg and milk fat globule membrane proteins – are the primary source of volatile sulfur compounds in thermally processed milk. Among these, hydrogen sulfide (H₂S) and carbon disulfide (CS₂) were identified as the specific compounds contributing to eggy and sulfur/burnt flavors respectively. Importantly, sensory panelists found that higher processing temperatures (direct steam injection ultra-pasteurization > indirect ultra-pasteurization > HTST pasteurization) produced progressively stronger sulfur/burnt and eggy flavor intensities.

The amino acid cysteine, present in whey proteins, acts as the direct precursor to H₂S: its free sulfhydryl group can be oxidized to release hydrogen sulfide. Methionine, another sulfur-containing amino acid, can undergo heat degradation to form methional, which further oxidizes into additional sulfur compounds. A review of volatile sulfur compounds in UHT milk published in PubMed identified several such compounds in processed milk, including methanethiol, dimethyl sulfide, carbonyl sulfide, and dimethyl disulfide – many of which increase during heat processing and are linked to consumer complaints about off-flavor.

The Maillard reaction and broader flavor changes

Beyond sulfur compounds, heating also drives Maillard browning reactions between the amino groups of proteins and the reducing sugars in milk (primarily lactose). These reactions generate a wide range of flavor molecules including furanones, pyrazines, and aldehydes that collectively produce a sweet, caramel-like or cooked note, particularly noticeable in sterilized and UHT milk.

A study in Food Science & Nutrition (PMC) confirmed that volatile aldehydes and ketones – including heptanal, nonanal, 2-heptanone, and 2-nonanone – are important components of oxidized flavor in heated milk, and their concentrations increase measurably with greater heat intensity. The Maillard reaction also leads to some loss of available lysine, slightly reducing the nutritional quality of the protein at very high temperatures.

Why these changes matter in dairy processing

The heat-induced changes in milk are not simply chemical curiosities – they have direct, practical implications at every stage of dairy manufacturing.

In cheese making, excessive whey protein denaturation interferes with rennet coagulation and extends the time needed to form a firm curd. Processors must balance the safety benefits of heat treatment against its impact on rennet activity and curd quality. In yogurt production, the same denaturation that disrupts cheese-making is deliberately induced to create a smoother, firmer gel texture. For UHT and sterilized milk, controlling the development of cooked flavor is a key quality challenge, since consumers are sensitive even to trace concentrations of hydrogen sulfide and other volatile sulfur compounds. And in infant formula and milk powder manufacturing, Maillard reactions at high temperatures must be carefully controlled to preserve lysine bioavailability and minimize browning.

The shifts in calcium and phosphate solubility affect heat exchanger fouling, powder reconstitution, and the stability of concentrated milk products. Managing these mineral equilibria through pH adjustment or the use of sequestering salts is a standard tool in modern dairy processing.

Heat is, in short, a double-edged instrument in dairy technology – indispensable for safety and many product attributes, but transformative in ways that must be understood and controlled.

What do you think? Given that higher processing temperatures produce more cooked flavor compounds, how should dairy processors balance the trade-off between microbial safety and sensory quality in products like UHT milk? And with whey protein denaturation being both desirable (in yogurt) and undesirable (in cheese), how do you think processing conditions should be tailored differently for each product?

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Table of Contents

What happens to milk proteins when heated? 🔗

Casein: the heat-stable fraction 🔗

Whey proteins: heat-sensitive and reactive 🔗

How denaturation affects dairy product quality 🔗

Effect of heat on milk acidity and mineral balance 🔗

Changes in pH during heating 🔗

Calcium and phosphate redistribution 🔗

How heat generates cooked flavor in milk 🔗

Sulfhydryl groups and volatile sulfur compounds 🔗

The Maillard reaction and broader flavor changes 🔗

Why these changes matter in dairy processing 🔗