Chapter 29 — Exercises

Kitchen Labs (full protocols)

🍳 Lab 29.1 — Beans, with and without pressure

Goal. Quantify the effect of pressure cooking on a tough food. Compare cooking time, texture, and broth character between traditional simmering and pressure cooking.

Time. About three hours including overnight soaking; the active comparison takes 90-120 minutes.

⚠️ Allergen flags. None; suitable for most diets. Plant-based.

Materials: - 2 cups (~400 g) dried black beans (or any dry bean — pintos, navy, kidney all work) - Two pots: one regular saucepan or stockpot with a lid, one pressure cooker (stovetop or electric) - Water - Salt (1 teaspoon per pot, added at the start) - A timer - A thermometer that reads to at least 121°C (250°F) — optional but informative - A small bowl and fork for taste-testing - Two identical bowls for plating

Procedure:

1. The night before, soak both batches of beans in cold water for 8-12 hours. Drain and rinse before cooking. Use 1 cup of soaked beans per pot.

2. Pot 1 (open simmer). Place 1 cup soaked black beans in a saucepan with 4 cups water and 1 teaspoon salt. Bring to a boil, then reduce to a simmer (water gently bubbling, surface barely roiling). Maintain a steady simmer with the lid slightly ajar.

3. Pot 2 (pressure cooker). Place 1 cup soaked black beans in a pressure cooker with 3 cups water and 1 teaspoon salt. Seal the lid and bring to high pressure (15 psi gauge — most modern cookers default to this). Once high pressure is reached, time 8 minutes at high pressure. Then turn off the heat (or end the program) and allow natural release for 15 minutes. Open the lid carefully.

4. Start both pots at the same time. As you cook, note the cooking time at which beans become tender (a fork should pierce them easily and they should crush smoothly between your fingers).

5. When both pots have reached tenderness, taste a few beans from each. Note texture, mouthfeel, and flavor.

6. Examine the cooking liquid. Is one thicker than the other? Is one cloudier? Save 1/4 cup of each broth in clear glasses to compare side by side.

Expected results: - Pot 1 (simmer): ~70-90 minutes to tenderness. Broth is moderately thick, slightly cloudy. Beans hold their shape but are creamy inside. - Pot 2 (pressure): ~25 minutes total to plate, of which only 8 are at pressure. Broth is thinner and clearer (less starch leaches in the shorter time). Beans are very tender, slightly more intact in shape (because the cooking time is shorter).

Discussion: Both batches reach the same internal tenderness, but the pressure-cooked beans have a different texture (slightly firmer, less starchy mouthfeel) and the broth has a different character (thinner, cleaner). Which would you prefer for a soup? For a refried-beans application? Why?

Troubleshooting: - Pressure-cooked beans not tender at 8 minutes. Add 2-3 more minutes at high pressure. Older beans take longer. - Pressure-cooked beans are split or mushy. Reduce time to 6 minutes; some bean varieties cook faster. - Foaming and clogged vent. Don't fill above 1/3 full with foaming foods like beans. Add a teaspoon of oil to suppress foam.

🍳 Lab 29.2 — Mapping the microwave field with marshmallows

Goal. Visualize the standing-wave pattern in a microwave oven and observe the dielectric heating gradient.

Time. 5 minutes.

⚠️ Allergen flags. Marshmallows contain gelatin (animal-derived); use vegan marshmallows if needed. Marshmallows that brown intensely can scorch.

Materials: - A microwave oven - A microwave-safe plate (large enough to nearly cover the cavity floor) - Enough marshmallows to cover the plate in a single, tightly-packed layer (about 30-40 standard marshmallows or a similar quantity of mini-marshmallows) - A camera or phone for documentation

Procedure:

1. Remove the turntable from the microwave and set it aside. Place the plate of marshmallows directly on the floor of the cavity (which is glass on most microwaves).

2. With the microwave door still open, take a "before" photo from above the plate.

3. Close the door. Heat on high power for 30-60 seconds. Watch through the door (don't put your face on the glass — but standard microwave doors are well-shielded, and observing from a normal distance is safe). Do not exceed 60 seconds; some marshmallows may scorch.

4. Stop the microwave when you can see clear differences in marshmallow swelling — some will be puffy and golden, others will be flat and uncooked.

5. Take an "after" photo. Replace the turntable when done.

Expected results: A pattern of "hotspots" and "coldspots" should be visible — regions where marshmallows have swelled and browned, alternating with regions where they have stayed flat. The pattern reveals the standing-wave structure of the electromagnetic field in your microwave's cavity. The hotspots are typically separated by approximately 6 cm (the half-wavelength of 2.45 GHz microwaves in air).

Discussion: Why do microwaves include rotating turntables? What other strategies do microwave manufacturers use to even out the field? (Some use "mode stirrers" — rotating metal blades inside the magnetron-to-cavity waveguide — for the same purpose.)

Variant for classrooms: This is a memorable demo for students. Pat Hammond runs it for AP Chemistry every spring. She measures the distance between hotspots, then has students calculate the corresponding wavelength and frequency, and confirms the result matches the labeled 2.45 GHz on the back of the microwave. It is one of the only experiments in a high school chemistry curriculum that directly measures the wavelength of an invisible electromagnetic wave.

🍳 Lab 29.3 — Induction speed test (if you have access)

Goal. Compare the time to boil 1 liter of water on three heat sources: gas, electric resistance, and induction.

Time. 20 minutes.

⚠️ Allergen flags. None.

Materials: - 1 liter of cold tap water - An identical pot (induction-compatible — verify by sticking a magnet to the bottom) for each test - Access to gas, electric resistance, and induction burners (or whichever subset you have) - A timer - A thermometer (optional but informative)

Procedure:

1. For each burner, weigh out 1 liter of cold water (mass = ~1000 g if at room temperature).
2. Place the pot on the burner and turn the burner to maximum power.
3. Start the timer the moment the burner is on.
4. Stop the timer when the water reaches a rolling boil (large bubbles continuously breaking the surface).
5. Record the time. Allow the burner to cool. Repeat with the next heat source using the same pot.

Expected results: - Gas (residential, ~14,000-18,000 BTU): 4-7 minutes - Electric resistance coil: 5-8 minutes - Induction (residential, ~2,000-3,500 W): 2.5-4 minutes

Discussion: Why is induction faster despite typically having a similar or lower wattage rating to a gas burner? (Hint: efficiency. Most of a gas burner's heat goes into the air around the pot, not the pot itself.) What does the responsiveness mean in practice — try turning each burner from full power to off, and observe how quickly the pot stops boiling.

Discussion Questions

1. The author argues that microwaves do not "cook from the inside out," but many people believe they do. Where does this misconception come from? What part of the actual physics resembles "inside-out cooking" enough to fuel the myth?

2. Pressure cooking accelerates Maillard-free reactions (collagen breakdown, bean softening, starch gelatinization) but cannot produce browning. Why? Could you design a hybrid cooking method that combines pressure cooking with Maillard browning? What would it look like?

3. Compare the energy efficiency of the cooking methods discussed in this chapter. Which is most efficient for boiling water? For roasting a chicken? For melting butter? Does efficiency always correlate with speed?

4. Imagine you are preparing a weeknight dinner with thirty minutes of total time. You have an Instant Pot, an induction burner, a microwave, and an air fryer. Plan a meal that uses all four appliances in parallel and explain the role of each.

5. Pat Hammond's marshmallow microwave experiment reveals that the field inside a microwave is not uniform. How might this information change how you use your microwave? What types of food are most affected by field non-uniformity?

6. Pressure cookers from the 1960s had a real, if rare, history of failure. Modern ones do not. What changed? Use this case to discuss how engineering safety improves through redundancy.

7. The Instant Pot became a cultural phenomenon despite using physics that was understood in 1679. Why did it take so long for the home version to dominate? What other "old physics, new convenience" appliances do you predict?

8. Why does the boiling point of water rise with pressure? Use the concept of vapor pressure to explain. (Advanced: write the Clausius-Clapeyron equation and use it to estimate the boiling point of water at 30 psi gauge.)

9. Some "induction-ready" cookware is not labeled clearly. How can you test a pan for induction compatibility? What metals work, and why?

10. The chapter argues that air fryers are essentially small convection ovens. Do you agree? What, if anything, is genuinely new about air fryers, beyond the marketing?

Advanced sidebars expanded

A. The Clausius-Clapeyron equation and pressure cooking

The relationship between vapor pressure and temperature for a liquid in equilibrium with its vapor is given by the Clausius-Clapeyron equation:

ln(P₂/P₁) = -ΔHvap/R × (1/T₂ - 1/T₁)

Where P is pressure, T is absolute temperature (Kelvin), R is the universal gas constant (8.314 J/mol·K), and ΔHvap is the molar enthalpy of vaporization (for water, approximately 40,650 J/mol at 100°C).

For water at sea level: P₁ = 101,325 Pa (1 atm), T₁ = 373.15 K (100°C). What pressure corresponds to T₂ = 121°C = 394.15 K?

Solving: ln(P₂/101,325) = -(40,650/8.314)(1/394.15 - 1/373.15) = -(4,890)(-0.0001428) = 0.698

P₂ = 101,325 × e^0.698 = 101,325 × 2.01 = 203,664 Pa ≈ 2 atmospheres absolute, or approximately 1 atm (15 psi) gauge.

This matches the standard "high pressure" setting on a pressure cooker. The equation also predicts boiling-point shifts at altitude — invaluable for high-altitude baking and brewing.

B. The dipole moment of water

A water molecule has a permanent electric dipole moment of approximately 1.85 debye (D), one of the highest of any small molecule. The asymmetric arrangement of two hydrogens at ~104.5° from each other on the oxygen creates a permanent charge separation: the oxygen end carries a partial negative charge of roughly -0.66 elementary charges, balanced by +0.33 on each hydrogen.

When water is exposed to an electric field of frequency f, the molecule rotates to align with the field. As the field oscillates, the molecule oscillates. Energy lost to rotational friction (water's viscosity, hydrogen-bond breaking, intermolecular collisions) becomes heat. The frequency at which absorption is maximized depends on the relaxation time of these motions in liquid water — about 10^11 Hz (~30 GHz) at room temperature. Microwave ovens at 2.45 GHz operate well below the absorption peak, but at the chosen frequency the penetration depth into food is convenient (a few centimeters), which gives reasonable cooking uniformity.

C. Skin depth and induction

When an alternating magnetic field is applied to a conductor, the induced currents are not distributed uniformly; they concentrate near the surface. The characteristic depth at which current density falls to 1/e of its surface value is called the skin depth, given by:

δ = √(2ρ/(ωμ))

Where ρ is the resistivity of the metal, ω is angular frequency (2πf), and μ is the magnetic permeability.

For iron at induction frequencies (~30 kHz), skin depth is about 0.5 mm. So the heating happens almost entirely in the bottom 0.5 mm of the pan, which then conducts heat upward through the rest of the pan body. This is why heavy-bottomed induction-compatible pans work well — the thick bottom distributes the heat evenly. Thin-bottomed pans can develop hotspots directly above the coil pattern.

🥖 Mastery Food Checkpoint

Bread track. This chapter introduces an unusual technique: pressure-cooker bread. The dough cooks in 25-30 minutes in a sealed pressure environment, but emerges with no crust. To get a normal-looking, normal-tasting loaf, the bread must be transferred to a hot oven or broiler at the end. Try the experiment if curiosity strikes — but bread on the bread track is properly the subject of Chapter 17 (gluten and structure) and Chapter 31 (yeast biology). Pressure-cooker bread is a curiosity, not a destination.

Cheese track. Modern microwave techniques are sometimes used for accelerated yogurt incubation (the Instant Pot's yogurt setting holds a steady 40-43°C, perfect for Streptococcus thermophilus and Lactobacillus delbrueckii). Chapter 32 will return to this — yogurt is a fermentation technique and Part V is its proper home. The induction burner is the gold standard for cheese-making temperature control: precise, responsive, easy to hold a steady 32°C for cultured milks.

Chocolate track. Microwave melting of chocolate is reliable for small quantities, but requires care. Heat in 30-second pulses, stir between each. The polar molecules in chocolate (water in cocoa solids, polar groups in lecithin) absorb microwaves; the cocoa butter does not absorb much directly but heats by conduction from the absorbing components. Chapter 20's tempering discussion will rely on careful temperature control more than fast heating; an induction burner with a flat-bottomed bowl over a water bath is the cleanest setup.

Fermented vegetables track. Pressure canning (Chapter 36) is the safe method for low-acid vegetable preservation — but you generally do not want to pressure-can fermented foods, because the heat will kill the live cultures that make fermentation interesting. Refrigeration is the standard preservation method for live ferments. The microwave can be useful for brief reheating of fermented soups (warming to serving temperature without killing too much culture), but more than 60°C will pasteurize the food.

Coffee track. Induction is the dark-horse hero for the coffee track. Precise water temperature control (boiling vs 200°F vs 195°F) makes a real, perceptible difference in extraction; an induction burner with an electronic kettle on top, or an induction-compatible kettle with a built-in temperature sensor, lets you nail the temperature within a degree. Chapter 21 will return to this.