Ice Cream & Yogurt Structure

Ice Cream — Four Phases in One Product

Ice cream contains four distinct physical phases that interact dynamically:

The fat network — partial coalescence

During freezing in the scraped-surface heat exchanger (continuous freezer), the combination of shear, cold and air incorporation causes partial coalescence of fat globules. Native fat globules don't fully merge; instead they form clusters that attach to air cell surfaces, stabilising the foam.

This is why ice cream uses unhomogenised cream or only gently homogenised mix — the intact MFGM allows partial coalescence. Fully-homogenised emulsions don't form the fat network and give a wet, slimy ice cream that melts to liquid quickly.

Air cells — the overrun lever

Overrun is the % volume increase due to air incorporation. Standard ice cream: 90–110% overrun (so 1L mix produces 1.9–2.1L ice cream). Premium / super-premium: 25–50% (denser, richer). Soft-serve: 30–50%.

Higher overrun = more air = lower density = lower per-litre cost (fewer ingredients per packaged litre). But too much air gives a foamy, melt-too-fast product. The structure must balance lightness against the stability of the air cells.

Ice crystals — smaller is better

Mouthfeel depends critically on ice crystal size. Crystals larger than ~50 µm feel gritty; under 25 µm feel smooth. Controlled by:

• Freezing rate — faster = smaller crystals; modern scraped-surface freezers exit at −5 to −7°C with crystals 20–30 µm
• Stabilisers — locust bean gum, guar, carrageenan inhibit crystal growth during storage
• Storage temperature — cycling between −25 and −18°C causes crystal growth ("Ostwald ripening"); steady −25°C preserves texture
• Recovery from melting — product that melts partially and refreezes will have grown crystals; visible "sandiness"

Yogurt — The Casein Gel

Yogurt structure is fundamentally a casein gel formed by acid coagulation:

1. Starter culture (Streptococcus thermophilus + Lactobacillus delbrueckii subsp. bulgaricus) ferments lactose to lactic acid
2. pH drops from 6.6–6.8 to ~4.6 (casein isoelectric point) over 4–6 hours at 42–43°C
3. Caseins precipitate and link via Ca²⁺ bridges into a 3D network
4. Fat globules and whey proteins are entrapped within the casein network
5. Cooling to <10°C stops fermentation and stabilises the gel

Heat treatment — the structure-builder

Yogurt milk is pasteurised at higher temperature than fresh milk — typically 85–95°C for 5–10 minutes. This denatures whey proteins (mainly β-lactoglobulin), which then bind to the casein micelles via disulphide bonds. The denatured whey proteins act as bridges within the gel, dramatically increasing firmness and reducing syneresis.

Skip the high-heat step and yogurt is weak, syneretic and not commercially viable.

Stirred vs set yogurt structure

Syneresis — the major defect

Syneresis is the spontaneous expulsion of whey from a gel — the watery layer on top of yogurt. It is caused by:

• Insufficient solids in the mix (target SNF > 8.5%)
• Insufficient heat treatment (poor whey protein integration)
• Wrong starter culture or under-fermentation
• Mechanical disturbance during gel set
• Excessive cooling rate breaking the structure

Solutions: increase milk solids (SMP or condensed skim addition), check heat treatment, validate culture activity, stabiliser addition (pectin, modified starch).

Cream Whipping — Air in Fat

Whipping cream foam is built by partial coalescence of intact fat globules around air bubbles — same physics as ice cream foam, but in a non-frozen system:

• Fat content — minimum 35% for stable whipping; 38–42% optimal
• Intact MFGM — never homogenise whipping cream; the original fat globule membrane allows partial coalescence
• Crystallised fat — cold ageing for 4–24 hours at 4°C allows fat crystallisation; uncrystallised fat doesn't whip
• Cold temperature — below 7°C ideal; warm cream won't whip
• Mechanical work — vigorous whisking incorporates air and shears fat globules together

Stable whipped cream holds 80–120% overrun (so 1L cream becomes 1.8–2.2L whipped). Stability over 24 hours requires fat globule integrity; over-whipping shears too much fat and causes "buttering" (fat lump formation).

Butter — The Inverted Emulsion

Butter is the opposite of milk — in milk, fat is dispersed in water; in butter, water droplets are dispersed in fat. This is a water-in-oil emulsion stabilised by:

• The fat crystal network — partial fat crystallisation creates a structural matrix
• Surface-active proteins from the buttermilk phase stabilise the water droplets
• Working during buttermaking creates uniform droplet distribution

Butter's texture depends on the ratio of solid-to-liquid fat at serving temperature. At 5°C: ~50% solid fat (firm, hard to spread). At 20°C: ~20% solid fat (soft, spreadable). The polymorphic crystal structure (α, β', β) influences texture; β' gives the best functional properties.

Cheese — The Casein Network with Embedded Fat

Cheese structure is a casein protein matrix with fat globules trapped inside the protein network. Key structural features:

• Casein network — formed by rennet (Cheddar, Gouda) or acid (cottage cheese, cream cheese) coagulation
• Fat globules — trapped intact or partially coalesced depending on whether milk was homogenised
• Whey channels — small interstitial spaces between casein strands; their size and connectivity affect moisture and texture
• Ageing modifications — proteolysis breaks down the casein network over weeks to years, softening and developing flavour

Pasta filata cheeses (mozzarella, provolone) have a distinctive structure: casein realigned into long parallel fibres during the hot-water stretching step. This gives the characteristic stretch and melt of pizza cheese.

Frequently Asked Questions

Why does ice cream contain so much air?

Air (overrun) gives ice cream lightness, scoop-ability and the soft mouthfeel consumers expect. Standard ice cream is 50% air by volume (100% overrun). Premium ice cream uses less air (25–50% overrun) for a denser, richer product. The air must be stabilised by a fat network or it would simply escape.

What causes large ice crystals in ice cream?

Large crystals (causing "sandy" or "icy" texture) form when freezing is too slow, storage temperature cycles (e.g. freezer thawing in transport), or insufficient stabiliser. Smooth ice cream requires fast freezing in scraped-surface heat exchangers, storage at steady −25°C, and stabilisers like locust bean gum.

Why does yogurt sometimes have watery layer on top?

This is syneresis — whey expelled from the casein gel. Causes include insufficient milk solids (SNF below 8.5%), inadequate heat treatment (90°C/5min minimum for stirred yogurt), wrong starter culture, or mechanical disturbance during gel set. Solutions: increase milk solids (SMP addition), check heat treatment regime, add stabilisers.

Why is yogurt milk heated higher than regular milk?

Yogurt milk is heated to 85–95°C for 5–10 minutes to denature whey proteins (mainly β-lactoglobulin). The denatured whey proteins then bind to casein micelles and act as bridges in the final gel, dramatically increasing firmness and reducing syneresis. Standard HTST pasteurisation alone gives a weak, syneretic yogurt.

Why does whipped cream sometimes turn to butter?

Over-whipping shears the fat globules so vigorously that they fully coalesce rather than partially. The fat then separates from the water phase as a butter mass. Stop whipping when peaks hold; aim for 80–120% overrun. Once buttering starts, the cream cannot be recovered.

Why is butter solid at room temperature but melts in your mouth?

Butter contains a mixture of milk fat triglycerides with different melting points. At 5°C (fridge): ~50% solid fat. At 20°C (room): ~20% solid fat. At 37°C (body): completely liquid. The wide melting range gives butter its characteristic mouthfeel — firm enough to handle, but liquefying on the tongue.

What gives mozzarella its stretch?

During the pasta filata stretching step, casein protein realigns into long parallel fibres rather than the random network of typical cheeses. At pH 5.1–5.3 the casein structure is loose enough to flow but cohesive enough to form fibres. Cooling locks this structure in place, giving mozzarella its distinctive stretch and string on pizza.

Further reading: John Watson publishes articles on dairy industry topics on LinkedIn. Browse all articles by John Watson on LinkedIn →

See related: Ice cream production, Yogurt production, Cream production, Butter making, Cheese making, Homogenisation, Mozzarella cheese, Frozen yogurt, all dairy science information, consultancy services.