Chapter 28 Exercises — Cold and Ice

Kitchen Labs (Full Protocols)

🍳 Kitchen Lab 28.1 — Pat Hammond's Salt-and-Ice Freezing-Point-Depression Demo

What you'll learn: That dissolved solute lowers the freezing point of water, that this is the same physics governing road salt, ocean water, and the brine of an ice-cream maker, and that you can verify the relationship quantitatively with a $4 budget.

Time: Setup 5 minutes. Demonstration 15 minutes.

Budget: $4 (rock salt or kosher salt, three plastic cups, ice). The thermometers can be borrowed from a chem lab or a kitchen drawer.

⚠️ Allergen flags: None.

⚠️ Safety: The chilled cups can reach -18°C / 0°F. Brief contact with the cup wall is fine; sustained contact (more than 10 seconds) of bare skin against the cup can cause cold-burn discomfort. Use a towel to handle. Do not eat the salty ice — it will not poison anyone, but it will not taste good and the teacher's reputation will suffer. Do not pour salty melt-water down a houseplant.

Equipment:

• 3 disposable plastic cups (16 oz / 480 mL or larger)
• 3 thermometers reading down to at least -20°C / -4°F (chem-lab alcohol thermometers are ideal; instant-read kitchen thermometers work if they read low enough — confirm the rated range)
• Crushed ice or ice cubes, about 4 cups total (1 L)
• Table salt or kosher salt, ¼ cup (60 g)
• Stirring spoons or wooden stir sticks
• Stopwatch or phone timer
• Notebook to record temperatures

Procedure:

1. Set up. Label cups 1, 2, 3. Fill each with the same amount of crushed ice (about 1 cup / 240 mL of ice).
2. Add salt. - Cup 1: no salt. - Cup 2: 2 tablespoons (30 g) salt, stirred into the ice. - Cup 3: 4 tablespoons (60 g) salt, stirred into the ice.
3. Insert a thermometer in each cup. Make sure the thermometer bulb is in the ice slurry, not against the cup wall.
4. Wait 3 minutes. During this time, the salt is dissolving in the small amount of liquid water in equilibrium with the ice, lowering the freezing point of that brine. The brine then melts more of the ice (because the new freezing point is below the cup's current temperature), and melting absorbs heat — the cup cools.
5. Read and record temperatures every minute for 5 minutes. Stir each cup briefly between readings.
6. Tabulate the lowest temperature reached in each cup.

Expected results:

The exact numbers depend on how much liquid water was already mixed with the ice (the brine concentration), how thoroughly the salt was stirred in, and the ambient room temperature.

Discussion prompts:

• Why does Cup 1 sit exactly at 0°C / 32°F regardless of how long you wait? (Answer: ice and water in equilibrium are at the freezing point of pure water by definition.)
• Why is Cup 3 colder than Cup 2? Use the formula ΔT = K_f × m × i to estimate the molality and compare to the predicted depression. (NaCl, K_f for water = 1.86 °C·kg/mol, i = 2 for fully dissociated salt.)
• What temperature do you predict at saturation (about 26 g salt per 100 g water at 0°C)? (Answer: about -21°C / -6°F. This is the practical floor for the salt-ice-cream brine.)
• This is why salt is spread on icy roads. But there's a temperature below which road salt stops working. What is it, and why? (Answer: below about -10°C / 14°F, even saturated salt brine starts to freeze; below -21°C / -6°F, road salt is essentially useless. This is why northern US states and Canadian provinces switch to calcium chloride or magnesium chloride at very low temperatures — those have lower eutectic points.)

Classroom variant (Pat's notes):

This is the lab Pat runs at the end of her colligative-properties unit. She typically does it with one demonstration cup (Cup 3) at the front of the class, with the temperature projected on the document camera, while students fill out a worksheet predicting the temperature based on the formula. Then she has small groups run their own three-cup setup. Total cost for a class of 30: about $6 in salt and ice. Total cost in student attention: priceless. Pat's spouse, who is a farmer, brings rock salt from a 50-pound bag in the barn — it works fine.

⚠️ Cleanup: Pour the salty water down a sink (not a houseplant). Rinse the cups. The thermometers, if alcohol-filled, are robust at this temperature range. If they cracked, dispose of broken glass appropriately and check that no mercury was involved (older school thermometers may contain mercury; modern lab thermometers do not).

🍳 Kitchen Lab 28.2 — Maya's Family Hand-Crank Ice Cream

What you'll learn: That fast cooling plus continuous churning is the recipe for creamy texture, that the salt-and-ice brine in a hand-crank ice-cream maker is a working demonstration of freezing-point depression, and that even an inexpensive home setup can produce excellent results — but the differences from commercial ice cream are themselves a lesson in physics.

Time: 4 hours total: 2 hours custard chilling, 30 minutes setup, 25 minutes churning, 1+ hour ripening in freezer.

⚠️ Allergen flags: Dairy (cream, milk), egg (yolks). For non-dairy adaptations: a coconut-milk-and-cashew-cream base works (but the mouthfeel will be different — coconut fat behaves differently from milk fat). For egg-free: a starch-based thickener (cornstarch and a stabilizer like guar gum) replaces the yolks but the texture will be subtly different.

⚠️ Safety: The custard cooking step requires temperatures up to 82°C / 180°F. Use a heatproof spatula, do not stop stirring, and do not let the custard simmer (curdling). The hand-crank churning is mechanical work — children can crank with adult supervision; be aware that the salt-ice brine is cold enough to give brief frostbite if mishandled. Wash all equipment thoroughly after.

Equipment:

• Hand-crank ice cream maker with wooden bucket, metal canister, dasher, and crank (or an electric ice cream maker with frozen canister, scaled to similar capacity)
• Heavy-bottomed saucepan, 2 quart / 2 L
• Whisk, heatproof spatula, fine-mesh strainer
• Mixing bowl over an ice bath for cooling
• Instant-read thermometer
• 4 lb (1.8 kg) crushed ice (or about 8 trays of cubes broken up)
• 1 cup (240 g) rock salt (or kosher salt; rock is traditional and slightly slower-dissolving, which works in our favor)
• Storage container for the finished ice cream, 1 quart (1 L)

Custard base ingredients (makes about 1 quart / 1 L finished ice cream):

• 2 cups (480 mL) heavy cream (about 36% fat)
• 1 cup (240 mL) whole milk
• ¾ cup (150 g) granulated sugar
• 4 large egg yolks
• 1 teaspoon (5 mL) pure vanilla extract
• Pinch of salt (about ¼ teaspoon / 1.5 g)

Procedure:

1. Make the custard. In a heavy-bottomed saucepan, warm the cream, milk, sugar, and salt over medium-low heat, stirring, until the sugar dissolves and the mixture reaches about 70°C / 160°F (steaming, not simmering). In a separate bowl, whisk the yolks well. Slowly drizzle about 1 cup of the warm milk mixture into the yolks while whisking — this is tempering, raising the yolk temperature without scrambling. Then pour the tempered yolk mixture back into the saucepan, whisking. Cook over medium-low heat, stirring constantly with a heatproof spatula, until the custard thickens enough to coat the back of a spoon and reaches 82°C / 180°F. Do not let it boil.
2. Strain and chill. Pour the custard through a fine-mesh strainer into a bowl set over an ice bath. Stir occasionally as it cools. Once room temperature, stir in the vanilla, cover, and refrigerate at least 2 hours, ideally overnight. The custard must be cold (4°C / 39°F or below) before churning. Skipping this step is the #1 reason home ice cream is icy.
3. Set up the maker. Place the canister in the wooden bucket. Pour the cold custard into the canister to no more than three-quarters full (it will expand during freezing). Insert the dasher. Close the canister lid. Pack the gap between the canister and bucket with crushed ice in alternating layers with rock salt — about a 6:1 ratio of ice to salt by volume. Pack tightly. Add cold water to bring the brine level up to about three-quarters of the canister height; this improves heat transfer.
4. Crank. Steady, even cranking — about 60 rotations per minute. The first 5 minutes will be easy. The crank gets progressively harder as the cream firms up. Continue cranking for about 20–25 minutes, until the crank is too hard to turn easily.
5. Open and check. The ice cream should be soft-serve consistency — it will hold a peak briefly but is too soft for scooping. This is correct for the churning stage.
6. Ripen. Transfer the ice cream to a clean storage container, press a sheet of plastic wrap directly against the surface, close the lid, and place in the coldest part of the freezer for at least 1 hour (4 hours is better). This ripening phase lets remaining water freeze, the fat network firm up, and the texture stabilize.
7. Serve. Scoop and eat immediately. The texture peaks within a day; within a week it is still good; beyond two weeks, expect noticeable graining (no commercial stabilizer in this formula).

Expected results:

The texture is creamier than supermarket ice cream but slightly less smooth than commercial premium ice cream. You will feel some structure on the tongue — a kind of grainy creaminess that is not a defect but an honest signal of large-ish ice crystals (perhaps 50–80 μm), churned to the limit of what hand power and salt-ice brine can achieve. The flavor will be intense, because there are no air-puffing additives; you are tasting essentially undiluted custard. Many tasters prefer this to commercial premium.

Discussion prompts:

• Estimate the overrun of your finished ice cream: weigh 1 cup of the unfrozen custard, weigh 1 cup of the finished ice cream, and calculate (custard weight − ice cream weight) / ice cream weight × 100%. Compare to commercial ice cream.
• Why does the cranking get progressively harder? What is happening at the molecular level?
• If you wanted a smoother texture, what would you change? List at least three options and predict their effects.
• Compare the texture of your homemade ice cream after 1 day, 1 week, and 2 weeks. What does this tell you about Ostwald ripening and recrystallization?

🍳 Kitchen Lab 28.3 — Directional-Freezing Clear Ice for Cocktails

What you'll learn: That cloudiness in standard ice cubes is an artifact of how the cube freezes (impurities and air pockets trapped during simultaneous all-around freezing), and that controlling the direction of freezing pushes those impurities out of the part you want clear.

Time: 24–36 hours of freezer time; 10 minutes of carving.

⚠️ Allergen flags: None.

⚠️ Safety: Carving frozen ice with a knife requires care. Use a serrated bread knife (a long-bladed serrated edge gives you saw-like control) or a dedicated ice pick. Keep the carving hand on the back of the blade; the cutting hand must not be in the path of the blade. Carve on a stable surface, ideally a wooden cutting board.

Equipment:

• A small plastic or foam-insulated cooler with a removable lid, around 6–10 quart (6–10 L) capacity
• Distilled or filtered water (mineral content reduces clarity)
• A serrated bread knife or ice pick
• Towels
• A storage container or bag for the carved cubes

Procedure:

1. Fill the cooler with cool water (room temperature is fine; cold tap water also works) to about 2/3 full.
2. Place the cooler — uncovered, lid off — in your freezer. The lid-off configuration is crucial. With the lid off, the freezer's cold air can only reach the top of the water. Heat escapes upward; the water freezes from the top down. The unfrozen water below stays liquid longer, and as the freezing front descends, it pushes dissolved air and impurities ahead of it. The bottom of the cooler will be the last part to freeze, and that part will be cloudy with concentrated impurities.
3. Wait 24–36 hours. Check periodically. You want the top 2/3 to be solidly frozen and the bottom 1/3 to remain liquid (or a slushy with cloudy ice). The exact timing depends on your freezer temperature; -18°C / 0°F is typical and will give you the right window in around 30 hours.
4. Remove the cooler. Pour off the unfrozen water at the bottom. Knock the ice block out of the cooler — a few seconds of warm tap water on the outside of the cooler, or running it briefly under warm water, releases the block.
5. Trim away the cloudy portion at the bottom. This is the part with the concentrated impurities. The top 2/3 should be glass-clear.
6. Carve cubes or balls. Score the ice block with a serrated bread knife along your desired cube lines, then strike along the score with the bread knife or an ice pick to split. For spheres, use a commercial ice ball mold and a piece of clear ice cut to fit. Keep the carved pieces in a freezer bag in the freezer until use.

Expected results:

Glass-clear ice cubes, 2 inches (5 cm) on a side or larger, with no visible cloudiness. They look beautiful in a glass and they melt slowly because of their low surface-area-to-volume ratio.

Discussion prompts:

• Why does an open-topped freezing setup give clear ice but a standard ice tray (which freezes from all sides simultaneously) gives cloudy ice?
• Predict what happens if you try this with sparkling water vs. distilled water vs. tap water with high mineral content. Test if you have time.
• What is the role of dissolved air in cloudy cubes? (Answer: dissolved gases come out of solution as the water freezes; they form microbubbles that scatter light. Directional freezing pushes them ahead of the freezing front.)
• A serious cocktail bar might use a Clinebell ice block machine — a $4,000+ device that does the same physics on a much larger scale. Why is an insulated cooler a good home substitute? (Answer: same principle, slower, smaller scale, but the physics is identical.)

Discussion Questions (for class or self-study)

1. The colligative properties cluster. Freezing-point depression is one of four "colligative properties" of solutions — properties that depend on the number of solute particles, not their identity. The other three are boiling-point elevation, vapor-pressure lowering, and osmotic pressure. For each of the four, name one cooking phenomenon where you have already encountered it. (Hint: pasta water for one of them.)

2. The texture trade-off. Commercial ice cream uses stabilizers and artificial small-air bubbles to stretch shelf life and reduce cost. Homemade ice cream is freer of additives but stales faster and is denser. Make a case for each as the "better" approach. Then make the case that there is no universal answer.

3. The supercooling demonstration. Pat's bottle-tap demo works for pure water but not for the salty water in your kitchen ice-cream maker. Why? What does this tell you about the role of nucleation sites in real food systems?

4. Why does kulfi work without churning? Indian kulfi is frozen without agitation. Yet it is dense and rich rather than gritty. Explain in terms of (a) reduced milk's low water content, (b) high sugar concentration, and (c) the use of small molds. How does each contribute to a tolerable texture without churning?

5. The freezer-burn distinction. Many cooks confuse freezer burn with "freezing damage." They are physically different processes. Explain each in your own words and describe how to prevent each.

6. The flash-frozen-fish argument. A friend insists that "fresh fish is always better than frozen." Make the counter-argument using the science of this chapter. What is the role of (a) initial freezing rate, (b) storage temperature, (c) packaging, and (d) thaw method?

7. Ice in cocktails. A bartender at a serious cocktail bar uses one large clear cube in a whisky glass. A different bartender at a high-volume nightclub uses crushed ice in a similar drink. Each is correct for their context. Why?

8. The chocolate-in-the-freezer problem. Why is the freezer a poor place to store fine chocolate? What conditions, specifically, cause sugar bloom on chocolate? (Hint: this question previews 🔗 Chapter 20.)

9. Glass transition. What is the glass transition temperature of an ice-cream serum, and why is it relevant to long-term ice cream storage? Why does a commercial freezer often run at -25°C / -13°F when the storage temperature listed on the carton is -18°C / 0°F?

10. Liquid nitrogen ice cream — when is it worth it? LN₂ produces the smoothest possible ice cream, but at significant cost and risk. For a restaurant, when is it worth doing? For a home cook, when is it not?

Advanced Sidebar Expansions

For the food-science student and chemistry teacher who want more.

Expansion 1 — Colligative properties as a four-property family

The four colligative properties of dilute solutions:

• Vapor-pressure lowering: ΔP = X_solute × P°_solvent. Adding a non-volatile solute lowers the vapor pressure of the solvent in proportion to the mole fraction of solute.
• Boiling-point elevation: ΔT_b = K_b × m × i. Adding solute raises the boiling point.
• Freezing-point depression: ΔT_f = K_f × m × i. The topic of this chapter.
• Osmotic pressure: π = M × R × T × i. The pressure required to prevent solvent flow across a semi-permeable membrane.

All four arise from the same root: dissolved solute reduces the entropy difference between the pure solvent phase and the solution phase, shifting the equilibrium temperatures and pressures of phase changes. The closeness of these four phenomena explains why they all appear together in food science: salt brining (osmotic pressure), pasta water salinity and boiling rate (boiling point elevation), pure-water condensation onto a cold beer glass (vapor pressure), and ice cream brine (freezing point depression). Same physics. Four faces.

Expansion 2 — The Williams-Landel-Ferry equation

For ice-cream texture stability, the kinetics of recrystallization (Ostwald ripening) below the glass transition follow the Williams-Landel-Ferry (WLF) equation:

log(rate) = log(rate at T_ref) − [C₁ × (T − T_ref)] / [C₂ + (T − T_ref)]

Where T_ref is a reference temperature (often T_g + 50°C), and C₁, C₂ are empirical constants. The practical takeaway: at temperatures within a few degrees of T_g, the rate of recrystallization changes by orders of magnitude per degree. A commercial freezer at -28°C and a home freezer at -18°C are not "almost the same"; the rate of recrystallization in the home freezer can be 100× higher. This is the quantitative form of the qualitative observation that home freezers are bad for long-term ice-cream texture.

Expansion 3 — The cooling rate / crystal size relationship

The mean crystal size r̄ in a frozen system can be modeled as:

r̄ ≈ (D × t)^(1/3)

Where D is the diffusion coefficient of water in the unfrozen serum and t is the time spent in the freezing window (between the freezing point and T_g). Faster cooling → less time in the window → smaller r̄. This is also why the "cube root" relationship makes a 10× faster cool produce only a ~2× reduction in crystal size — the relationship is steep but not linear. To achieve dramatic reductions, you need huge cooling rate increases — which is what liquid nitrogen offers.

Mastery Food Checkpoint — Chocolate and Cheese Tracks

Chocolate track: This chapter is mostly a "what not to do" for chocolate. The key facts: (1) freezing damages cocoa butter's Form V crystallization (the form that gives chocolate its snap), pushing it toward less stable forms; (2) thawing frozen chocolate causes condensation that triggers sugar bloom on the surface; (3) wrapped, room-temperature storage at 18°C / 64°F is the correct approach. The full chocolate chapter (Chapter 20) takes up the six crystal forms and the tempering process in detail; this chapter just establishes that the freezer is not your friend for finished chocolate. The exception: chocolate-coated frozen products (chocolate-dipped ice cream bars, frozen ganache), where the chocolate has been formulated specifically to survive the freeze-thaw cycle (typically with added cocoa-butter equivalents and emulsifiers).

Cheese track: Most cheese should not be frozen, period. The freezing damages the casein protein network and the fat structure, and the thawed cheese is crumbly, watery, and texturally compromised. Hard, low-moisture cheeses (parmesan, aged cheddar, pecorino) tolerate freezing for grating use only — the texture is destroyed but the flavor is preserved, so frozen-then-grated parmesan over pasta is acceptable when fresh is unavailable. Soft cheeses (brie, camembert, fresh mozzarella, ricotta) lose their texture entirely on freezing and should never be frozen. Cream cheese is an exception that can survive freezing for use in cooked applications (a frozen-and-thawed cream cheese is fine baked into a cheesecake; less fine on a bagel). The general rule: cheese is alive (microbially and texturally), and the freezer kills it. Refrigeration, properly wrapped, is the correct storage approach.