Why cocktail foam collapses

Make two identical Sours. Serve one. Leave the other on the bar.

At first, not much seems to happen.

Then look closely.

The foam begins to change. Liquid moves down through it. The surface becomes a little less uniform. Some bubbles disappear; others become larger. What began as a dense, almost matte cap gradually becomes wetter, coarser and thinner.

Behind the bar, we tend to give all of this one name: the foam collapsed.

But that's slightly misleading.

Foam doesn't have one single collapse mechanism. What we see in the glass is the result of several physical processes happening together — particularly drainage, coalescence and disproportionation. Protein-stabilised food foams resist those processes, but they don't stop them forever. [1]

And once you understand those processes, watching a Sour age becomes much more interesting.

Before we can understand why foam disappears, we need to understand what foam is.

It isn't really a white substance sitting on top of the cocktail.

It's a structure.

A liquid foam is gas dispersed through liquid: thousands of bubbles separated by thin liquid films. In protein-stabilised foams, proteins adsorb at the air–water interface and help create stabilising interfacial layers. Protein conformation, flexibility and interactions at that interface contribute to foam stability. [1]

So when you shake a Sour with egg white, you're not simply mixing egg white into the drink.

You're creating thousands of tiny air–liquid interfaces and asking proteins to help keep them apart.

That's rather beautiful when you think about it.

Gravity gets to work almost immediately.

Between the bubbles is liquid. Over time, some of that liquid drains downward through the foam.

As drainage progresses, the liquid distribution and the thin regions separating bubbles change, leaving the structure increasingly vulnerable to destabilisation. Drainage is one of the fundamental processes involved in the ageing and eventual failure of liquid foams. [1]

And here's the interesting bit behind the bar.

A foam can still look tall while already getting worse.

The height may remain impressive, but the texture can be changing underneath.

That makes foam height a surprisingly crude measure of quality.

A huge white head isn't necessarily a good one.

We care about bubble size, texture, drainage, stability and — ultimately — how it feels when the guest drinks it.

Now imagine two neighbouring bubbles.

Between them is a thin liquid film.

If that separating film fails, neighbouring bubbles can merge into a larger bubble.

That's coalescence.

Repeated across a foam, coalescence contributes to a coarser bubble structure and eventually to visible foam deterioration. It is one of the recognised mechanisms of liquid-foam destabilisation. [1]

This gives us another useful distinction: foam can deteriorate before it disappears.

If your Sour still has a centimetre of foam but the surface has gone from tight and satin-like to large and soapy-looking, the foam hasn't remained unchanged just because it's still there.

Which is why we're interested in more than the glamour shot immediately after the pour.

Give it a few minutes.

That's where foam starts telling you what it really is.

This one is less intuitive.

Two bubbles don't necessarily need to physically merge for the foam to become coarser.

Gas can redistribute between bubbles, with smaller bubbles shrinking while larger ones grow. This process is generally described as disproportionation in foam science and contributes to changes in bubble-size distribution as a foam ages. [1]

So when an ageing Sour develops larger bubbles, we shouldn't automatically imagine lots of bubbles physically smashing into one another.

Different destabilisation mechanisms can contribute to what we're seeing.

You don't need to diagnose which process is dominating while you're six tickets deep.

But knowing that more than one exists changes the way you look at the surface.

And it reinforces something we're going to return to throughout this Handbook: bubble size matters.

Now it gets complicated.

Because a cocktail isn't egg white whipped in water.

It's a multicomponent system.

There may be protein, water, ethanol, acid, sugar, flavour compounds, oils and whatever water we introduce through dilution.

Every one of those exists in the same liquid environment in which our foam has to form and survive.

That means seemingly small changes to a spec may affect more than flavour.

But this is also where we need to be disciplined.

Most controlled foam research isn't conducted in Whisky Sours.

A result measured in a model egg-white or protein solution can teach us about an underlying mechanism.

It does not automatically tell us exactly what will happen in your cocktail.

That's where published science should stop pretending to be bartender advice — and where controlled testing becomes interesting.

Sugar gives us a lovely example.

In controlled egg-white protein systems, increasing sucrose increased solution viscosity and decreased foam overrun. At the same time, sucrose could improve wet-foam stability by affecting the continuous phase and interfacial properties. [3]

That's a useful little paradox: a system can make less foam and still make more stable foam.

More isn't automatically better.

And stable isn't automatically beautiful.

A bartender cares about the combination: volume, texture, bubble size, mouthfeel and how the foam develops while the drink is being consumed.

Proteins respond to pH because their charge, interactions and behaviour depend partly on their surrounding chemical environment.

Controlled work involving egg-white proteins has shown that pH can materially alter foamability and other foam properties. [4]

But we're going to resist turning that into: more acid = better foam.

The cited work is not a controlled Sour experiment. It examined protein systems under defined pH conditions.

A cocktail contains multiple components, and changing citrus changes far more than pH alone.

So the useful conclusion is: acidity is a real variable in protein foaming, but the effect of changing acidity in a finished cocktail should be demonstrated rather than assumed from a simplified laboratory system.

That's a perfect Handbook experiment.

Same Sour. Same everything we can control. Change the acid systematically.

Then watch what happens.

Anyone who has made meringue has probably heard that even a little yolk can cause trouble.

There is good evidence behind that observation.

In a controlled 2009 study of liquid egg white, yolk contamination at just 0.022% by weight on an as-is basis caused significant reductions in foaming capacity and foaming speed. The neutral-lipid fraction of the yolk caused the major detrimental effect observed in that experiment. [5]

That's a remarkable number.

But don't turn it into another bad rule.

It does not establish that every cocktail ingredient containing fat at some particular concentration will destroy your foam.

Yolk is a specific material and cocktails are much more complicated systems.

What it tells us is that lipid contamination can strongly interfere with the foaming performance of liquid egg white under controlled conditions.

If one spec foams beautifully and another doesn't — and the second introduces a lipid-rich ingredient — that's worth investigating.

This deserves a small detour because bartenders use the word constantly.

In cocktail practice, dilution usually means the water incorporated into the drink as ice melts during chilling and mixing.

During shaking we're simultaneously mixing, aerating, chilling and diluting.

And chilling and dilution are intimately connected.

Professional cocktail experiments by Dave Arnold and collaborators explored the relationship between shaking, ice, temperature and dilution and found that, under the conditions they tested, chilling and dilution tracked closely together. [6] [7]

So dilution isn't simply accidental damage inflicted on a cocktail while we're trying to make it cold.

It is part of the physical process of preparing the drink.

Ethanol clearly changes the physical and chemical environment of a cocktail.

But for this article, we're stopping there.

We do not currently have evidence strong and cocktail-specific enough in this source set to turn that general fact into a reliable bartender rule about how changing ABV affects an egg-white Sour's foam.

It would be easy to write: high alcohol destabilises foam.

It sounds plausible.

It may even match some practical observations.

But sounding plausible isn't our standard.

So for now: ABV is a variable we want to test, not a rule we're ready to teach.

Here's another distinction worth keeping.

Food-foam research distinguishes between the ability of a system to create foam and the stability of the resulting foam. Changes in formulation can improve one while worsening the other — the sucrose study above is a particularly clear example. [3]

That's enormously useful behind a bar.

A technique can give you an impressive head immediately after pouring without necessarily giving you the texture or longevity you want.

So asking which shake makes the most foam isn't quite the right question.

We want to know: how much foam? What bubble size? What texture? How quickly does it drain? How does it feel against the lip? How does it look after two minutes? Five? And what did the extra technique cost us during service?

That's why dry shake, reverse dry shake and single shake deserve proper treatment rather than a winner-and-loser comparison.

There's an assumption worth challenging here too.

Pasteurised liquid egg white isn't necessarily one functionally identical material regardless of how it was processed.

Pasteurisation uses heat for microbiological control, and heat treatment can change egg-white protein structure and aggregation. A 2024 study found that the temperature–time pasteurisation treatments it tested substantially reduced foaming capacity while slightly increasing foam stability. The researchers associated the loss of foaming capacity particularly with heat-induced protein aggregation and structural changes. [8]

Earlier work by Wang and Wang also found that the effect of heat treatment on liquid egg-white foaming depended on temperature and holding time. In their experiments, some controlled industrial processing and pasteurisation conditions did not produce the same deterioration, illustrating why the word pasteurised alone is not enough to predict exact foaming behaviour. [5]

That's important.

It means “pasteurised” doesn't tell a bartender everything they need to know about foaming performance.

Processing matters.

The resulting protein system matters.

And ultimately, performance in the actual cocktail matters.

This is also why we shouldn't assume that every liquid egg-white product will behave identically behind the bar.

Because foam is fighting its natural tendency to become less foamy.

The moment we finish shaking, the structure starts evolving.

Liquid drains through it.

Films separating bubbles can fail and bubbles can coalesce.

Gas can redistribute through the foam and the bubble-size distribution can change.

Protein layers at the air–water interface help resist destabilisation, while the formulation and physical environment influence how the foam behaves. [1]

That's why the goal isn't: make foam that never collapses.

That foam doesn't exist.

The goal is to create the right foam for the drink — the volume, bubble size, texture, stability and mouthfeel we want — and have it remain beautiful for as long as that drink needs it.

And once you start looking at a Sour that way, something rather nice happens.

You stop seeing white stuff on top of a cocktail.

You start seeing a structure.

And then you start wondering what else you can change.