Food Chemistry — Chapter 7

Dispersed Systems: The Physical Structure of Food

Why is milk white? Why doesn't mayonnaise separate? Why does beer have foam? Why does pudding wobble? All these involve "dispersed systems". In 6 hours we'll master them.

4major types

2main forces (surface + particle)

10+food examples

3hands-on labs

Why does this happen?

Six Everyday Dispersed-System Puzzles

Puzzle 1

🥛 Why is milk white?

Water is clear, but milk is opaque white. Why?

Puzzle 2

🥗 Mayonnaise stays mixed

Oil and vinegar normally separate. Whisk with egg yolk and they become a stable sauce. How?

Puzzle 3

🍺 Beer foam lasts

Beer foam persists for minutes. Why doesn't water do that?

Puzzle 4

🍮 Pudding wobbles

Liquid (milk + egg) becomes a sliceable solid after heating, but still soft and wobbly. Why?

Puzzle 5

🍦 Ice cream contains air

Ice cream has tiny air bubbles, which is why it's smooth. How are they trapped?

Puzzle 6

🧊 Butter has water inside

Butter is "water droplets in oil" — the reverse of milk. How does that stay stable?

Answer: All 6 are dispersed systems — two things that don't easily mix being forced together. After 6 hours, you'll explain every one.

Course Roadmap

Where We're Going in 6 Hours

Hour 1What are dispersed systems? Introducing the 4 types.

Hour 2Surface and interface: surface tension, surfactants.

Hour 3Emulsions: mayonnaise, ice cream, milk.

Hour 4Foams: beer head, meringue, cotton candy.

Hour 5Gels: jelly, tofu, cheese.

Hour 63 hands-on labs + applications integration.

What is a Dispersed System?

"Dispersed Phase" + "Continuous Phase"

Dispersed phase: the thing that gets scattered into small pieces — the "dots".
Continuous phase: the surrounding environment that contains the dispersed phase — the "background".
The two are usually immiscible — like oil in water, or gas in liquid.
We need something to hold them together so they don't separate — that's a surfactant (Hour 2).

Examples: Milk = oil droplets (dispersed) + water (continuous). Balloon = gas (dispersed) + rubber (continuous). Fog = water droplets (dispersed) + air (continuous).

4 Types of Dispersed System

What's Dispersed Decides the Type

Dispersed	Continuous	Type	Food examples
Gas	Liquid	Foam	Beer head, milk foam, whipped cream, meringue
Gas	Solid	Solid foam	Bread, cake, cotton candy, ice cream
Liquid	Liquid	Emulsion	Milk, mayonnaise, butter, salad dressing
Solid	Liquid	Suspension	Fruit pulp in juice, chocolate milk, coffee grounds
Solid	Solid	Solid dispersion	Chocolate, cheese, ham
Liquid	Solid	Gel	Jelly, pudding, tofu, cheese

⚠️ This course focuses on 4 main types: foam, emulsion, suspension, gel. Each gets its own hour later.

The Nature of Food

Why Almost Every Food is a Dispersed System

Reason 1

Nature rarely makes pure substances

Plant and animal cells are made of water + oil + protein + sugar + minerals. Processed food still contains all these mixed together.

Reason 2

Mouthfeel needs multiple layers

Pure water has no texture. Pure oil has no texture. Mixing them + adding bubbles or solids creates richness.

Reason 3

Nutrition needs many things

Protein, fat, sugar, minerals, water — we need all of these. Dispersed systems deliver them at once.

So "food chemistry" is about: ① stabilizing dispersed systems ② understanding why they fail ③ designing new textures.

The World of Scale

Size of Dispersed Phase = Everything

Size	Class	Food example	Property
< 1 nm	Solution	Sugar water, salt water	Clear, fully mixed
1-100 nm	Colloid	Milk (casein micelles), pectin solution	White or semi-transparent, scatters light
0.1-100 μm	Emulsion, suspension	Mayonnaise, fruit pulp	Opaque, settles slowly
> 100 μm	Coarse dispersion	Vegetables in soup, chocolate chips	Visible particles

💡 Smaller = more stable. Food industry uses "homogenization" to shrink milk fat droplets to 0.1-1 μm, so milk doesn't separate for days.

Hour 1 Self-Check

Quick Check

Q1 (Definition)

The "dispersed phase" in a dispersed system is:

Q2 (Classification)

Milk is which type of dispersed system?

Q3 (Scale)

Why do dairies homogenize milk?

Surface Tension

Why Are Water Drops Round?

Water molecules attract each other (via hydrogen bonds). Surface molecules have no water above them — so they're pulled inward. Result: the surface shrinks to the smallest possible size → a sphere.

Surface tension: the force that wants to make a liquid surface as small as possible.
Water has unusually high surface tension (72 mN/m).
That's why water beads up on lotus leaves, why some insects walk on water, why droplets are round.
Food meaning: high surface tension → hard to mix oil and water. To mix them, lower the surface tension → add a surfactant.

Oil vs Water

Why Won't Oil and Water Mix?

Water nature

Polar

Water molecules have positive (H) and negative (O) ends. They link to each other via hydrogen bonds.

Oil nature

Nonpolar

Oil (long hydrocarbon chains) has almost no charge. They cling weakly via van der Waals forces.

Result

Immiscible

Water sticks to water, oil sticks to oil. They reject each other. Pour them together and they automatically separate.

💡 Classic rule: "Like dissolves like". Polar dissolves polar, nonpolar dissolves nonpolar. So sugar (polar) dissolves in water, oil (nonpolar) dissolves in oil.

The Key Player

Surfactants: The Two-Ended "Mediator"

Surfactant: a molecule with two ends of different nature — one hydrophilic, one hydrophobic.
Like a whale: hydrophilic head (lives in water), hydrophobic tail (doesn't mind oil).
Stands at the oil-water interface, lowers surface tension.
Result: oil and water can mix → emulsion forms.
Dish soap works the same way — surfactant wraps around grease, water carries it away.

Natural Food Emulsifiers

What "Mediators" Does Nature Provide?

Phospholipid

Lecithin

Sources: egg yolk, soy.
Structure: phosphate head (hydrophilic) + two fatty acid chains (hydrophobic).
Used in: mayonnaise, chocolate, bread.

Protein

Casein, albumin, whey

Proteins have hydrophilic and hydrophobic regions — can act as emulsifiers.
Examples: milk (casein wraps fat), mayonnaise (egg proteins).

Monoglyceride

MAG

Industrial byproduct from oils. Very common commercial emulsifier.
Used in: margarine, baked goods.

Saponins

Plant defense

Sources: quinoa, onion, soybean.
Natural surfactants — plants use them against pests.
Cause foam when boiling soy milk.

Solid particles

Pickering emulsion

Not just liquids — solid microparticles can sit at interfaces. Examples: cocoa powder, chia seeds, mustard grains.

Commercial

Polysorbate (Tween)

Synthetic surfactants. Common in ice cream, margarine, candies. E numbers E432-E436.

Contact Angle

How "Friendly" Are Liquid and Solid?

Contact angle: the angle between a liquid drop and a solid surface.
< 90°: hydrophilic. Liquid spreads, like water on clean glass.
> 90°: hydrophobic. Liquid beads up, like water on lotus leaves.
Lotus effect: lotus leaves have nano-textured + waxy surfaces, contact angle ~150° → water rolls off, carrying dirt with it.
Food applications: oils don't spread on non-stick pans; liquids enter sponge cake easily because of low contact angle.

The Interface "Skin"

Interfacial Film: The Key Protective Layer

What is it

Interfacial film

The layer of surfactant molecules arranged at the oil-water (or gas-water) interface. Acts like a "skin" protecting the dispersed droplets.

Thickness

Nanoscale

Single-molecule layer thickness ~ 1-3 nm. Invisible, but determines the fate of the whole system.

Strength

Mechanical toughness

Strong film: droplets can be squeezed without breaking → stable system.
Weak film: droplets merge on contact → separation.

💡 Why is egg yolk great for mayonnaise? Because lecithin + proteins in yolk form a strong and tough interfacial film — droplets crowd together without merging.

Industry Tool for Picking Emulsifiers

HLB Value: Hydrophile-Lipophile Balance

HLB range	Character	Use	Examples
1-3	Strongly hydrophobic	Anti-foaming	Oleic acid
3-6	Oil-soluble	W/O emulsion (butter)	Monoglycerides
7-9	Wetting agent	Helps liquids spread	Span 80
8-18	Water-soluble	O/W emulsion (milk, mayo)	Tween 80, lecithin
13-15	Detergent	Dish soap, laundry	SDS
15-18	Solubilizer	Dissolves oil in water	Tween 20

Memory aid: Low HLB (oil-leaning) → want butter-type products (water droplets in oil). High HLB (water-leaning) → want milk-type products (oil droplets in water).

Hour 2 Self-Check

Surface and Interface — Check

Q1 (Surface tension)

Why does water bead up on a lotus leaf?

Q2 (Surfactant)

What's the defining feature of a surfactant?

Q3 (HLB)

To make mayonnaise (oil-in-water), choose which HLB?

What is an Emulsion

Making Oil and Water Get Along

Definition

Emulsion

One liquid dispersed as small droplets in another immiscible liquid.
Droplet size: usually 0.1-100 μm.

Requirements

Three ingredients

① Two immiscible liquids (usually oil + water)
② Emulsifier (surfactant)
③ Mechanical force (stir, homogenize)

Significance

Pillar of food industry

Milk, butter, mayonnaise, salad dressing, ice cream, margarine, chocolate — all emulsion systems.

Two Emulsion Directions

O/W vs W/O

O/W (oil-in-water): oil droplets in water. Continuous phase is water → feels "water-based".
W/O (water-in-oil): water droplets in oil. Continuous phase is oil → feels "oil-based".
Quick test: drop into water — dissolves = O/W; floats = W/O.

💡 Milk (O/W) and butter (W/O) differ not just in fat content but in inverted structure. That's why mouthfeel is so different.

Case 1: Milk

Nature's Most Famous Emulsion

Structure

3 layers

① Water (continuous phase)
② Fat droplets (3-5 μm, dispersed)
③ Casein micelles (~100 nm protein clusters)

Why white

Light scattering

Fat droplets and casein micelles scatter visible light in all directions → looks white.
Pure water has nothing to scatter → transparent.

Homogenization

Industrial process

High-pressure homogenizer shrinks fat droplets from 3-5 μm to ~1 μm.
Result: no creaming for days. Un-homogenized milk (raw farm milk) separates.

💡 Why is skim milk less white? Less fat → less scattering → looks slightly blue. Whole milk looks whiter.

Case 2: Mayonnaise

The Science of Mayo

Mayonnaise is the most amazing food emulsion: up to 70-80% oil, yet feels like a water-based sauce.

Water phase: egg yolk + vinegar/lemon + salt + mustard
Oil phase: vegetable oil (sunflower, olive)
Emulsifier: egg yolk's lecithin + proteins
Technique: Add oil slowly, slowly, whisking constantly. Too fast → interfacial film can't form → droplets merge → "broken emulsion".

Saving Broken Mayonnaise

If your mayonnaise breaks (oil separates) — what to do?

① Take a clean bowl, add a fresh egg yolk.
② Slowly pour broken mayo into new yolk while whisking.
③ New yolk provides new emulsifier, rebuilds the film.

Saved! Classic French chef trick.

Case 3: Ice Cream

The Most Complex Dispersed System

4 phases

All at once

① Air (30-50% volume, dispersed)
② Fat globules (O/W emulsion)
③ Ice crystals (frozen water, dispersed)
④ Sugar-protein solution (continuous, unfrozen "serum")

Production key

Stir while freezing

Constant stirring: ① incorporates air ② partially coalesces fat for stable structure ③ keeps ice crystals small (< 50 μm — otherwise it feels gritty).

Melting problem

Dripping

When melting, ice crystals melt first. If the fat structure isn't strong enough, the whole system collapses → "pool of liquid". Good ice cream melts slowly while keeping shape.

💡 Why is gelato different from American ice cream?
Gelato: low air (20%), low fat, dense — firm texture.
American: high air (50%), high fat, low density — fluffy.

How Emulsions Fail

Four Failure Modes

① Creaming

Density-driven

Density difference makes oil rise (oil is lighter) or solids sink. Common: milk left out separates. Reversible — just shake.

② Flocculation

Clustering

Droplets "stick close" but haven't merged. Films still intact. Stir to disperse.

③ Coalescence

Film breaks

Interfacial film ruptures, two droplets merge into one bigger. Irreversible. Eventually leads to complete separation.

④ Ostwald Ripening

Big eats small

Material dissolves out of small droplets and re-condenses into big ones (high Laplace pressure in small). Slow irreversible change.

Making Emulsions More Stable

Four Protection Strategies

Strategy 1

Smaller droplets

Homogenization shrinks droplets (< 1 μm). Small droplets cream slowly (Stokes' law), system stays stable.

Strategy 2

More emulsifier

Egg yolk + lecithin + protein — thicker, stronger interfacial film. Industry uses combined emulsifiers.

Strategy 3

Add thickener

Xanthan gum, pectin, modified starch — raise viscosity of continuous phase to slow droplet movement. Common in salad dressings.

Strategy 4

Cool storage

Low T → slower molecular motion → slower creaming, more stable film. But too cold causes ice damage.

That's why commercial mayonnaise lasts months refrigerated — multiple strategies stacked together.

Hour 3 Self-Check

Emulsions — Check

Q1 (Type)

Butter is what type of emulsion?

Q2 (Mayo)

If you add oil too fast and mayo breaks, the main reason is:

Q3 (Stability)

Which method does NOT improve emulsion stability?

What is a Foam

Trapping Air Inside Liquid

Definition

Foam

Gas dispersed phase in a liquid (or solid) continuous phase.
Bubble size: ~10 μm-1 mm, larger than emulsion droplets.

Classification

Liquid foam vs Solid foam

Liquid: beer head, milk foam, meringue (short-lived)
Solid: bread, cotton candy, ice cream, sponge cake (set after solidifying)

Significance

Texture revolution

Foam makes food fluffy, light, smooth.
Bread without bubbles = dough lump. Ice cream without bubbles = ice cube.

How Foams Form

Three Ways to Get Air Into Liquid

Method 1

Mechanical force

Whisks, blenders, mixers — forces air in.
Examples: meringue, whipped cream, milk foam (steam + mechanical).

Method 2

Supersaturation release

Pre-dissolve CO₂ under pressure, then release pressure — bubbles emerge.
Examples: beer, champagne, soda, sparkling water.

Method 3

Biological fermentation

Microbes produce CO₂ that comes out of the liquid.
Examples: bread (yeast), fermented kimchi, natto.

Whatever the method, you need a surfactant to wrap the bubbles — otherwise they pop immediately.

Case 1: Meringue

The Science of Meringue

Egg white = ~10% protein, mostly ovalbumin.
Whisking → proteins partially unfold (mild denaturation).
Unfolded proteins crowd into air-water interface → wrap bubbles.
Stability stages: soft peak, stiff peak, broken (over-whipped).

Why meringue fails:

① Tiny bit of yolk fat → fat occupies interface → proteins can't squeeze in → no foam.
② Oily bowl → same thing. So meringue requires a spotlessly clean, fat-free bowl.
③ Sugar added too early → proteins can't fully unfold.

Why add sugar?

Sugar:
① Adsorbs at interface, making the film thicker
② Stabilizes foam (slower collapse)
③ Adds toughness (harder to over-whip)

That's why Italian meringue (with sugar syrup) is more stable than French (with sugar).

Similarly, lemon juice or cream of tartar (lowering pH) stabilizes — proteins stack better near pI.

Case 2: Beer Foam vs Milk Foam

Two Liquid Foams Compared

Beer foam

Barley proteins lead

Source: CO₂ from fermentation + barley proteins.
Helper: hops' iso-α-acids stabilize the foam.
Killer: oil on glass rim (lipstick, soap residue) → foam vanishes instantly.
Therefore: beer glasses must be spotless.

Milk foam

Whey proteins lead

Source: steam injected into cold milk → foam + heating.
Key: whey proteins denature, casein helps too.
Temperature: too hot (>75°C) → over-denaturation → rough foam.
Therefore: latte foam is kept at 60-65°C.

Case 3: Solid Foams

From Liquid Foam to "Set" Foam

Cotton candy

Sugar spinning

Melted sugar shot out of a spinning machine, sugar threads solidify in air → masses of bubbles between sugar fibers.

Bread

Yeast + gluten

Dough ferments → yeast produces CO₂ → gluten network traps gas. Baking solidifies → solid foam (the honeycomb interior).

Sponge cake

Egg foam + flour

Whipped egg + flour + baking. Heat coagulates proteins and gelatinizes starch, locking bubbles in place.

Ice cream

30-50% air!

Stir-while-freezing locks air bubbles. Without air = ice block.

Macarons

Bake-set meringue

Meringue + almond + sugar. Foam solidifies during baking, producing the hollow shell texture.

Soufflé

Whip + bake

Meringue folded into sauce, baked → bubbles expand, protein sets → fluffy. Collapses after leaving oven (bubbles shrink).

Why Do Foams Collapse?

Three "Foam Killers"

① Drainage

Gravity drains

Gravity pulls liquid out from between bubbles. Film between bubbles thins → easy to break.
That's why beer becomes liquid below, dry foam on top after some time.

② Ostwald

Big eats small

Small bubbles have high gas pressure (Laplace); gas diffuses to bigger bubbles → small bubbles shrink and disappear, big ones grow.
That's why meringue's bubbles become uneven over time.

③ Coalescence

Film breaks

Film ruptures, two bubbles merge into one. Eventually foam dies.
Fat contamination = instant film rupture = instant foam death.

The Secret of Bread Bubbles

How Bread Gets Fluffy

① KneadFlour + water + salt + yeast → knead repeatedly. Gluten (protein network) forms, can stretch.

② FermentYeast metabolizes sugar in flour → releases CO₂ and alcohol. CO₂ trapped by gluten; dough expands 2-3×.

③ BakeDough goes in oven: gas expands further (one last rise), yeast dies at ~60°C, proteins set, starch gelatinizes. Bubbles fixed in place.

④ CoolGas shrinks after baking, but the structure is already solid. Forms the honeycomb solid foam.

💡 Try this: under-kneaded dough = no gluten network = CO₂ escapes = flat hard bread.

Hour 4 Self-Check

Foams — Check

Q1 (Meringue)

Why does a tiny bit of egg yolk ruin meringue?

Q2 (Beer)

What happens when there's grease (lipstick) on the rim of a beer glass?

Q3 (Bread)

Why does well-kneaded bread become fluffy?

What is a Gel

A "Net" That Catches Water

Definition

Gel

3D molecular network + lots of trapped liquid (usually water).
Network is only 1-5% of weight, rest is water — yet it acts like a solid.

Property

Semi-solid semi-liquid

Shape like a solid (doesn't flow), but >95% water inside. Can be cut, can wobble, can spring.

Classification

Network material

Polysaccharide gels: pectin, agar, carrageenan, gelatin.
Protein gels: egg, tofu, cheese, yogurt, ham.
Mixed gels: yogurt + pectin, etc.

Case 1: Polysaccharide Gels

The Jelly Family

Name	Source	Mechanism	Texture
Gelatin	Animal collagen	Cooling → α-helix re-coils	Melts at body T (soft, smooth, wobbly)
Agar	Red algae	Cooling → double helix	Firm, elastic, doesn't melt
Carrageenan	Red algae	Cooling + K/Ca ions	Soft to firm, adjustable
Pectin	Fruit peels (apple, citrus)	Sugar + acid + heat	Jam, gummies
Gellan	Bacteria	Ions + cooling	Very transparent, strong
Modified starch	Corn, tapioca	Gelatinize + cool	Pudding, soup

💡 Why does gelatin jelly melt in your mouth but agar doesn't? Gelatin's network melts at ~35°C, so it dissolves on your tongue. Agar needs 80°C to melt — chews like rubber.

Case 2: Protein Gels

From Liquid to Sliceable Shape

Heat-set

Egg, custard

Proteins heat-denature → unfolded chains link → network → traps water.
Temp: egg white ~60-65°C, yolk ~65-70°C.

Acid-set

Yogurt, paneer

Acid (lactic or lemon) drops pH to casein's isoelectric point (~4.6) → loss of charge → proteins aggregate → network.

Enzyme-set

Cheese

Chymosin (rennet) cuts casein at a specific spot → micelles destabilize → calcium bridges form network → solid curd.
Connection to Ch.6 enzyme course.

Salt-set

Tofu

Soy milk + CaSO₄ or MgCl₂ → ions bridge soy proteins → tofu gel.
Different salts = different texture: CaSO₄ = firm; MgCl₂ (nigari) = silky.

Cold-set

Surimi (fish paste)

Fish + salt → cold → proteins network. Then heat to fix.
Used for imitation crab, fish balls, fish cakes.

Mixed

Ham, sausage

Ground meat proteins + salt + heat → "binding" structure. Without salt, no binding.

Deep Dive: Cheese

The Gel Journey from Milk to Cheese

① MilkContains casein micelles (suspended) + whey proteins + lactose + calcium phosphate

② Add rennetCuts κ-casein → micelles destabilize → aggregate → gel block forms

③ Cut + drain wheyCut the gel → whey drains, leaving curd. Smaller cuts → drier curd

④ Salt + pressSalt controls moisture and microbes, adds flavor. Press shapes the cheese

⑤ AgeMicrobes + leftover enzymes slowly transform proteins and fats → develop flavor, soften texture. Weeks to years

💡 Secrets to different cheeses: ① rennet amount ② cut size ③ heating intensity ④ salt content ⑤ aging time and microbes. Same milk → Mozzarella, Cheddar, Brie, Parmesan — all via gel processing differences.

Case 3: Mixed Gels

Real Foods Are Mostly "Composite Systems"

Pudding

Custard

Milk + egg + sugar + heat.
Protein gel (egg) + some starch (if corn flour added).
Texture: soft, smooth, sliceable.

Panna cotta

Italian dessert

Cream + milk + sugar + gelatin.
Polysaccharide gel (gelatin) traps milk fat + water.
Texture: silky, melts in mouth.

Mousse

Chocolate or fruit

Chocolate or fruit purée + meringue (foam) + whipped cream.
Foam + weak gel combination — light and airy.

Yogurt

Cultured milk

Acid-set milk → protein gel.
Commercial yogurt often adds pectin or modified starch to prevent whey separation.

Fish balls

Asian comfort food

Fish paste + salt + starch.
Protein network + starch gelatinization = Q-bouncy texture.

Tiramisu

Italian masterpiece

Mascarpone + egg yolk + cream + meringue.
FOUR dispersed structures at once!

Physical Properties of Gels

Why Bouncy, Why Crumbly?

Density

Denser = stronger

Higher polysaccharide concentration → denser network → harder, stronger gel.
Ex: jelly with 1% gelatin = soft; 3% = rubber-like.

Bond strength

Covalent vs non-covalent

Covalent bonds (e.g., S–S in proteins) → permanently strong.
Non-covalent (H-bond, ionic) → weaker but reversible.

Water content

Too much = soft

More water → network stretched → soft and fragile.
Less water → network tight → firm and tough.

Temperature

Gels change with T

Gelatin: melts > 35°C.
Agar: melts > 80°C.
Protein gels: don't easily melt, but soften.

Ions

Ca²⁺, Mg²⁺ etc.

Some gels (carrageenan, alginate) need Ca²⁺ or K⁺ to form. Adding Ca tunes the strength.

Texture words

Sensory science

Bouncy, smooth, crispy, melts in mouth, jelly-like, meaty — these are all physical structure descriptions.

The Science of Mouthfeel

"The Collapse Process in Your Mouth"

First bite

Surface resistance

Teeth's first contact → hardness, elasticity, smoothness.
Apple crunchy, gummy soft, jelly bouncy.

During chew

Structural breakdown

Food breaks → releases liquid/fat → aromas to nose.
Juiciness, creaminess, smoothness — all from this stage.

Before swallow

Melt or emulsify

Food mixes with saliva → swallowable bolus forms.
Chocolate's "melt in mouth" = fat melts at 32°C.

💡 Food scientists use texture analyzers to measure hardness, elasticity, chewiness, recoverability — and correlate with sensory scores. Turn "bouncy" into a quantifiable number.

Hour 5 Self-Check

Gels — Check

Q1 (Definition)

The essence of a gel is:

Q2 (Gelatin vs Agar)

Why does gelatin jelly melt in mouth but agar doesn't?

Q3 (Cheese)

The key to different cheese textures is:

Lab 1

Make Your Own Mayonnaise

Materials

What you need

1 fresh egg yolk
1 tsp mustard (natural emulsifier)
1 tsp lemon juice or white vinegar
Pinch of salt
200 ml neutral oil (sunflower, canola)
Whisk, deep bowl

Steps

5 minutes

① Yolk + mustard + lemon + salt, whisk together
② Add oil drop by drop, whisking constantly
③ After half the oil is in and sauce thickens, add oil faster
④ Whisk to very thick sauce

Observe

Record

① What happens if oil added too fast? (Emulsion "breaks")
② Why are yolk + mustard both used? (Two emulsifiers stack)
③ Finished mayo is 80% oil yet feels like a water-based sauce — why?

💭 If it breaks: get a clean bowl + a fresh yolk. Slowly pour broken mayo into new yolk while whisking — saved!

Lab 2

Meringue Comparison

Materials

What you need

3 eggs (separate whites and yolks)
Sugar, salt, lemon juice, cream of tartar
4 clean bowls, electric mixer
Deliberately oil one bowl (rub a drop of oil)

4 treatments

Setup

① Plain egg white (control)
② Egg white + drop of yolk
③ Egg white + oily bowl
④ Egg white + 1 tsp lemon juice
Whip each 3 min to "stiff peak"

Observe

Compare

① Which fails to whip? Why? (Both yolk and oil kill foam)
② Which is most stable? Why? (Lemon juice lowers pH, proteins stack better)
③ Invert bowl — stiff peaks shouldn't fall out

💭 After 30 min, observe again: which collapses first? Which mechanism (drainage / Ostwald / coalescence)?

Lab 3

Jelly vs Tofu: Two Gels

A. Home jelly

Polysaccharide gel

Materials: 100 ml fruit juice, 5 g gelatin powder (or 1.5 g agar powder), a little sugar
Steps: ① Heat juice ② Add gelatin and stir to dissolve ③ Pour into mold ④ Refrigerate 2 hours
Compare: split into two cups, one gelatin and one agar — compare mouthfeel and melt temp
Advanced: add fresh pineapple → gelatin won't set (pineapple's protease cuts the gelatin protein)

B. Home tofu

Protein gel

Materials: 500 ml soy milk (unsweetened, unsalted), 1.5 g glucono-delta-lactone (GDL, from pharmacy)
Steps: ① Heat soy milk to 80°C ② Stir in GDL ③ Pour into container ④ Rest 30 min
Compare: split, one batch GDL, one batch 1% MgCl₂ (nigari) — compare texture
Observe: the journey from soy milk → soy curd → tofu

💭 After both: ① Jelly vs tofu have very different mouthfeel — why? ② Pineapple + gelatin = fails, pineapple + soy milk = fine. Why?

Industry Applications Overview

Dispersed Systems in Industry

Dairy

Homogenization

Shrink milk fat droplets to ~1 μm. No separation for days. Also smoother mouthfeel.

Bakery

Dough fermentation control

Temperature, humidity, yeast level all affect bubble formation. Commercial bread uses "pre-ferment" and "cold-proofing" for precise control.

Ice cream

Ice crystal and air

Ice cream machines work at −5°C with continuous stirring. Overrun (air %) 30-100%, determines firmness and cost.

Dressings

Emulsion stabilization

Xanthan gum, modified starch, lecithin — keep vinaigrettes from separating, easy to pour and spread.

Plant meat

Meaty texture

Soy protein extrusion + gel + fat droplets = meat-like bite. Designing "juicy when bitten" is an industry challenge.

Molecular cuisine

Cross-disciplinary

Reverse spherification (alginate + Ca), spray drying, liquid nitrogen — creating new dispersed structures.

Food Design

What Food Scientists Actually Do

4 dimensions

Considered together

① Taste: sweet, salty, sour, bitter, umami + aroma
② Nutrition: protein, fat, sugar, fiber, vitamins
③ Structure: dispersed systems (this course)
④ Preservation: antimicrobial, antioxidant, packaging

New trends

Plant-based alternatives

Using plant proteins to mimic meat, milk, egg dispersed structures. Beyond Burger, Oatly, plant butters are great examples.

Challenge

Health vs mouthfeel

Lower fat, lower sugar → dispersed structure suffers. Skim milk looks duller. Maintaining mouthfeel while cutting calories is a hot industry research area.

Summary Table

4 Types of Dispersed System Compared

Type	Dispersed	Key stabilizer	Failure mode	Examples
Emulsion	Oil (liquid) in water	Surfactant + interfacial film	Creaming, coalescence, Ostwald	Milk, mayonnaise, butter
Foam	Gas in liquid / solid	Proteins / surfactants	Drainage, coalescence	Beer foam, meringue, bread
Suspension	Solid in liquid	Thickener, stirring	Sedimentation	Fruit pulp, cocoa drink
Gel	Liquid in 3D network	Polysaccharide / protein network	Syneresis, melting, cracking	Jelly, tofu, cheese, pudding

💡 Real foods often combine multiple systems. Ice cream = emulsion (O/W) + foam (~30% air) + gel (partial freeze) + suspension (ice crystals) — all 4 at once!

Final Self-Check

6-Hour Total Review

Q1 (Categories)

Which is NOT a dispersed system?

Q2 (Emulsion)

Why does mayonnaise, which is 80% oil, feel like a water-based sauce?

Q3 (Combined)

Ice cream is a combination of which dispersed systems?

Closing

Food Isn't Just Chemicals Anymore

Next time you eat ice cream, make mayo, or eat cheese fondue — you'll see the "physical structure" behind the food. One piece of food = one carefully designed dispersed system.

Remember 1

4 dispersed systems

Emulsion, foam, suspension, gel — the basic vocabulary of food physical structure.

Remember 2

Surfactants are key

Without them, oil and water won't mix, air can't be trapped, particles settle. Natural emulsifiers (lecithin, proteins) make everything possible.

Remember 3

Multi-system reality

Real foods (ice cream, bread, cheese, tiramisu) are multiple dispersed structures combined. The food scientist's job is to design them.

Final assignment: choose any food and write a 250-word "dispersed system analysis" — its dispersed phase, continuous phase, key stabilizer, and possible failure modes.