Week 1 Module: What Is Edible Matter?

Research question#

When a diner in Chengdu says boba (珍珠, tapioca pearls) has good “QQ” — springy, resistant, bouncing back after a bite — what are they actually measuring? This module asks whether a texture word from a kitchen can be mapped onto a property a rheometer reports in pascals, and if so, which one.

Specimen tapioca pearl (boba), cooked, at serving temperature

Where this sits#

This is the opening module of Stage 1, Measuring Edible Matter. Before the course can talk about structure (Stage 2) or transformation (Stage 3), it needs a shared, testable vocabulary for what a food material is. Every later module reuses the framing built here: a food is a material occupying one of a small number of physical states, and its behaviour under load, heat, or moisture is what a model predicts — not an adjective.

The model: states of edible matter#

Cooked and processed foods mostly sit in four regimes, each with a distinct mechanical signature:

• Fluid — deforms continuously under any shear stress (chili oil).
• Viscoelastic solid — has both an elastic (spring-like, energy-storing) and a viscous (dashpot-like, energy-dissipating) response (dough, tapioca gel).
• Amorphous glass — a disordered solid below its glass transition temperature, rigid and often brittle (a dried spring roll wrapper — see Module 9 and Module 11).
• Crystalline/semi-crystalline solid — ordered at the molecular scale (ice, some fats).

A boba pearl at serving temperature is a physical hydrogel: a swollen, crosslinked tapioca-starch network holding a large fraction of water. Its “QQ” character comes from two separable quantities:

G′ (storage modulus)

the elastic component — how much energy the pearl stores and returns on deformation. Higher G′ reads as "springy" or "resistant."

G″ (loss modulus)

the viscous component — how much energy is dissipated as heat rather than returned. Higher G″ relative to G′ reads as "soft" or "mushy."

tan δ = G″/G′

the ratio diners actually feel: a low tan δ pearl snaps back; a high tan δ pearl deforms and stays deformed.

“QQ” texture, in this framing, is a high G′ with a low tan δ — a gel that is both stiff and highly elastic, not merely stiff. A pearl that is just as stiff but has a higher tan δ reads as a more dissipative gel — a less elastic response, “firm” but not “QQ” — the two are mechanically distinguishable even though both would be called “firm” in English.

Figure — the complex modulus as a vector

Material states

4

Loss ratio

tan δ = G″⁄G′

QQ target

high G′, low tan δ

What you measure and analyse#

Using a supplied oscillatory-rheology dataset (Laboratory Portfolio, dataset analysis component), you will read G′ and G″ against time for tapioca pearls cooked for three different durations, compute tan δ for each, and rank the three by how close they sit to a target “QQ” window rather than by G′ alone.

Activity#

Pick two Chinese foods you’d both call “chewy” in English — a candidates list: hand-pulled noodle dough (拉面), century egg white (皮蛋), dried squid (鱿鱼干). For each, argue from the material-state list above which state it occupies, and predict qualitatively whether it has a higher or lower tan δ than tapioca pearls. You are not expected to have measured this — the point is to practise turning a texture word into a testable material claim, which every later module in this course requires.

References#

• Steffe, J.F. (1996). Rheological Methods in Food Process Engineering, 2nd ed. Freeman Press.
• Walstra, P. (2003). Physical Chemistry of Foods. Marcel Dekker.

The spec

What you bring to the session. Written so a reader can tell whether it has been met without asking you.

• state, in one sentence, what "QQ" (Q弹) texture means as a physical property
• name the four material regimes a cooked food can occupy (fluid, viscoelastic solid, amorphous glass, crystalline/semi-crystalline solid) and give a Chinese food example of each
• explain why "chewy" is not one property but at least two (elasticity and toughness)

Teaching team

• Idris Fenn — tutor