Theories Behind the Churning Process in Butter Production

Churning is one of those deceptively simple dairy processes – you agitate cream, and out comes butter. But what exactly happens at the microscopic level when millions of fat globules collide, merge, and eventually form butter grains? Over the years, three major theories have attempted to answer this question: Fisher and Hooker’s Phase-Reversal Theory, Rahn’s Foam Theory, and King’s Modern Theory. Each offers a different lens through which to understand the physical and chemical transformation of cream into butter.

What is churning? 🔗

Churning is the process of agitating cream at a suitable temperature until the fat globules stick together, form increasingly larger masses, and eventually separate almost completely from the liquid portion known as buttermilk. During churning, paddles or rotating mechanisms within a butter churn vigorously move the cream, causing it to thicken and then “break” into two distinct components – butter grains and buttermilk.

At this stage, cold water (around 10°C) is often added to help the cream break cleanly. This is called break water, and its volume should generally not exceed 25% of the total cream volume. The churning continues until the butter granules reach roughly the size of pea grains. The entire process typically takes 10 to 20 minutes, depending on factors like cream temperature, fat content, and the intensity of agitation.

For this process to work well, the cream must have good churnability – meaning it converts easily and completely into firm butter grains within a reasonable time. The body and texture of the resulting butter depend heavily on how effectively this conversion takes place.

The sequence of events during churning 🔗

Before diving into the three theories, it helps to understand the general sequence of events that scientists agree upon. First, agitation incorporates air bubbles into the cream, increasing the total volume and creating a large air-plasma interface. Second, frictional forces partially disrupt the protective membranes around fat globules, causing some liquid fat to leak out and spread as a thin film over the air bubble surfaces. Third, this fat film acts as a foam depressant, causing the air bubbles to burst. The leaked liquid fat also serves as a cementing material, joining fat globules together. Finally, butter grains form and float in the remaining plasma – the buttermilk.

Now, let’s look at how the three major theories explain this transformation differently.

Fisher and Hooker’s phase-reversal theory 🔗

This was one of the earliest scientific explanations of churning. Cream is naturally an oil-in-water (O/W) emulsion – tiny fat globules are dispersed throughout a water-based liquid. Fisher and Hooker proposed that churning reverses this arrangement entirely, converting cream into a water-in-oil (W/O) emulsion, which is what butter essentially is. This concept of “phase reversal” gives the theory its name.

How phase reversal works 🔗

According to this theory, as cream is agitated, fat globules collide with each other repeatedly. These collisions cause the globules to merge (coalesce) and clump together, forming progressively larger fat masses. As these fat units grow, their combined surface area relative to their volume decreases. Eventually, the fat mass becomes too large for the water phase to remain stable around it. At this point, the fat-in-water emulsion suddenly breaks, producing butter grains surrounded by free buttermilk.

There is observable evidence that supports this idea. As churning progresses, you can often hear a distinct change in the sound within the churn. The mixture also visually shifts – from smooth and uniform cream to a clearly separated mixture of visible butter grains floating in thin buttermilk.

Limitations of the phase-reversal theory 🔗

Despite its elegance, this theory has a significant flaw. If churning were truly a complete phase reversal, the finished butter should be a perfect water-in-oil emulsion with no intact fat globules remaining. However, microscopic studies have revealed that many fat globules in worked butter remain intact. This means butter is not a true W/O emulsion as the theory assumes – it is actually a more complex mixture containing intact globules, free fat, water droplets, and air.

Rahn’s foam theory 🔗

Otto Rahn proposed a fundamentally different explanation. Instead of focusing on phase reversal, Rahn argued that foam formation is the essential mechanism behind churning. His theory centers on the idea that agitation creates a foam structure within the cream, and it is the eventual collapse of this foam that brings fat globules together to form butter.

The role of foam in churning 🔗

According to Rahn, when cream is agitated, air gets incorporated into the liquid, forming countless tiny air bubbles. These bubbles create a foam structure. Fat globules in the cream tend to concentrate at the surfaces of these air bubbles due to surface tension effects. Rahn also proposed that a “foam-producing substance” present in cream gradually solidifies during agitation, contributing to the foam’s initial stability.

Foam breakdown and butter formation 🔗

As churning continues, the foam-producing substance assumes a solid character, and the foam structure becomes increasingly unstable. When the foam finally collapses, the fat globules that were concentrated at the air-liquid interfaces are forced into direct contact with each other. This close proximity allows them to coalesce, and butter grains begin to form.

Rahn’s theory provides a useful explanation for why the intensity and duration of churning matter. Too little agitation means the foam doesn’t develop properly. Too much agitation too quickly may break the foam before enough fat globules have concentrated at the bubble surfaces.

Limitations of the foam theory 🔗

The main weakness of Rahn’s theory is that it assumes foam is absolutely essential for churning. However, in some continuous butter-making processes, foam formation is not required at all. Butter can be produced in systems where air incorporation is minimal or absent, which directly contradicts the central premise of Rahn’s theory.

King’s modern theory 🔗

Recognizing the strengths and weaknesses of both earlier theories, King proposed a more comprehensive explanation that incorporates elements of both phase reversal and foam formation, while adding new insights about fat globule membrane disruption. King’s theory is widely considered the most accurate description of the churning mechanism.

The starting point: fat at churning temperature 🔗

King’s theory begins with an important observation about the state of fat in cooled cream. At churning temperature (typically 10-15°C), the fat inside each globule exists partly in solid crystalline form and partly as liquid fat. The globules are arranged in clusters or clumps within the cream. This mixed solid-liquid state of the fat is crucial to the entire process.

Step-by-step mechanism according to King 🔗

King described the churning process as a series of well-defined stages:

Cluster disruption and foam formation: Agitation breaks up the natural fat globule clusters and simultaneously incorporates air into the cream, forming foam. The globules become concentrated in the thin liquid film surrounding the air bubbles, bringing them into close contact with each other.

Membrane disruption: As globules move over one another within the foam film, and as they collide directly, the protective phospholipid-protein membrane (the milk fat globule membrane or MFGM) surrounding each globule gradually wears away. The mechanical forces – friction, pressure, and shear – stretch and eventually tear these membranes. This is the key step that distinguishes King’s theory from the earlier two.

Coalescence and grain formation: Once the protective membranes are compromised, exposed fat from different globules can merge directly. Liquid fat leaks out and acts as a cementing agent, binding globules into progressively larger particles. These particles eventually become visible as butter grains. The grains also trap some air from the collapsing foam within their structure.

Working of butter grains: After the grains are formed and buttermilk is drained off, the grains are worked (kneaded). During working, the remaining intact globules are forced to move over each other under friction and pressure. Some yield up their liquid fat, and others break open entirely. Eventually, enough free liquid fat is released to form a continuous fat phase that encloses all the water droplets, air bubbles, and remaining intact fat globules – producing the final butter structure.

Why King’s theory is widely accepted 🔗

King’s approach works because it does not rely on a single mechanism. It acknowledges that both foam formation and phase reversal play roles, but identifies membrane disruption as the central event driving butter formation. This explains why butter contains a mix of intact and broken fat globules, free fat, water droplets, and air – something neither of the earlier theories could fully account for.

Comparing the three theories 🔗

Each theory captures part of the truth about what happens during churning. Fisher and Hooker correctly identified that a fundamental change in the emulsion type occurs – from oil-in-water to a system dominated by fat. Rahn correctly recognized that foam plays an important role in concentrating fat globules and facilitating their contact. King combined these insights with the critical observation that the fat globule membrane must be physically disrupted for coalescence to occur.

The key difference lies in what each theory considers the primary driver. For Fisher and Hooker, it is emulsion inversion. For Rahn, it is foam collapse. For King, it is membrane disruption enabled by both mechanical forces and the concentrating effect of foam. In modern dairy science, King’s integrated approach remains the most widely referenced framework for understanding butter formation.

Factors that influence churnability 🔗

Understanding the theories also helps explain why certain conditions affect how easily cream converts to butter. Here are the key factors:

Temperature: This is the single most critical variable. At the right churning temperature (typically 10-15°C or about 50-60°F), the fat is in an optimal mix of solid and liquid states. Too cold, and the membranes become rigid and hard to disrupt. Too warm, and the fat is too soft to form firm grains. Research published in the Journal of Dairy Science has confirmed that cream feed temperature significantly influences the water content and quality of the resulting butter.

Fat percentage of cream: Cream used for butter making typically contains around 30-40% fat. Higher fat content means more fat globules available for coalescence, generally resulting in faster churning and higher butter yield.

Fat globule size: Larger fat globules have thinner membranes relative to their volume and are easier to disrupt during churning. Studies have shown that reducing fat globule size increases churning time and leads to greater fat loss in buttermilk, because smaller globules are harder to destabilize.

Chemical composition of fat: The ratio of saturated to unsaturated fatty acids in milk fat varies with season and animal diet. Feed that is rich in pasture grasses tends to produce softer fat (more unsaturated fatty acids), which churns more easily at lower temperatures. Winter feed produces harder fat that requires higher churning temperatures.

Acidity and viscosity of cream: Slightly acidic cream (as in cultured butter production) churns faster because the lower pH weakens the fat globule membrane proteins. Higher viscosity also affects how globules interact during agitation.

Speed and nature of agitation: The churn speed and design determine how much mechanical energy is transferred to the cream. Too slow, and insufficient membrane disruption occurs. Too fast, and the foam may break prematurely or the butter may be overworked, leading to excessive moisture retention.

Practical significance for butter makers 🔗

These theories are not just academic exercises. They directly inform how dairy plants and even home butter makers optimize their process. For instance, understanding that membrane disruption requires a specific combination of solid and liquid fat within globules (as King explained) is exactly why temperature control is so critical in commercial butter production.

Modern continuous butter-making machines – such as the Fritz-type continuous churns widely used in industry – are designed based on the principles described in these theories. They control the shear rate on the beater, cream flow rate, and temperature to achieve consistent phase inversion and optimal water content in the finished butter. The maximum permitted water content in butter is 16%, as regulated by the FAO’s international standards.

Even at the cottage or homestead level, knowing that frozen and thawed cream churns faster (because freezing disrupts fat globule membranes in advance), or that adding break water helps the cream “break” more cleanly, are direct applications of the science these theories describe.

From cream to butter: the complete picture 🔗

Bringing all three theories together, the full picture of churning looks something like this. Cream starts as a stable oil-in-water emulsion, with fat globules protected by phospholipid-protein membranes. Agitation breaks up fat globule clusters and incorporates air, forming foam. Fat globules migrate to foam bubble surfaces, where they come into close contact. Mechanical forces gradually wear away the protective membranes. Exposed fat from adjacent globules merges, with liquid fat acting as cement. Butter grains form, trapping some air and water. The buttermilk is drained, and working the grains releases more free fat until a continuous fat phase encloses everything – water, air, and remaining intact globules – to produce finished butter.

Each theory contributed a piece of this understanding. Together, they give us a remarkably detailed picture of one of humanity’s oldest food processing techniques.

What do you think? Given that King’s Modern Theory combines elements of both earlier theories, do you think future research in emulsion science could lead to an even more refined explanation of the churning process? And how might a deeper understanding of fat globule membrane behaviour open the door to new types of butter or dairy spread formulations?

References

1. https://en.wikipedia.org/wiki/Churning_(butter)
2. http://dairy-technology.blogspot.com/2014/01/churning-and-its-theories.html
3. https://www.agricultureinindia.net/dairy-science/butter/churning-of-cream-into-butter-top-3-theories/20154
4. https://www.studocu.com/in/document/sam-higginbottom-university-of-agriculture-technology-and-sciences/food-science/theories-of-churning/22784841
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC4921067/
6. https://www.slideshare.net/slideshow/churning-of-butter-factors-affecting/267349792
7. https://www.sciencedirect.com/science/article/abs/pii/S0958694620302338
8. https://books.lib.uoguelph.ca/dairyscienceandtechnologyebook/chapter/butter-manufacture/
9. https://www.sciencedirect.com/science/article/abs/pii/S0958694607001422
10. https://www.sciencedirect.com/science/article/abs/pii/S0963996917302302
11. https://www.fao.org/home/en