Exercises — Chapter 31

Kitchen Labs (Full Protocols)

🍳 Lab 31-A: The Cold-Rise Comparison

Goal: Demonstrate that fermentation time and temperature change the flavor of bread, with the same recipe and the same yeast.

Time: 24–30 hours total (2 hours active, mostly waiting); requires planning a day ahead.

⚠️ Allergen flags: Wheat (gluten). Yeast (rare allergen, but possible). The salt is, as always, salt.

Materials - 500 g (about 4 cups) bread flour or all-purpose flour - 350 g (about 1½ cups) water at 24°C (75°F) - 10 g (about 2 teaspoons) fine salt - 3 g (about 1 teaspoon) instant yeast (a quarter packet) - A scale, a mixing bowl, two oiled containers (1 quart each), plastic wrap, a Dutch oven or covered cast-iron pot, an oven that reaches 250°C (480°F).

Protocol

1. Mix the dough. Combine flour, water, salt, and yeast in a large bowl. Mix with a wooden spoon or your hand until no dry flour remains and a rough, shaggy dough forms. Knead for 5 minutes by hand or 3 minutes in a stand mixer with a dough hook. The dough should feel cohesive but does not need to be smooth yet.
2. Bulk-rest 30 minutes. Cover and let the dough sit at room temperature for half an hour to begin gluten development.
3. Divide. Cut the dough into two equal pieces. Place each piece in its own oiled, lidded container.
4. The two paths. - Container A: room-temp ferment. Leave on the counter at 22–24°C (72–75°F) for 2–3 hours, until roughly doubled in size. - Container B: cold ferment. Place immediately in the refrigerator (4°C / 39°F) and leave for 18–24 hours.
5. Shape and bake. When ready to bake, turn each dough out onto a floured surface, fold it gently into a round, let it rest seam-down on parchment for 30 minutes (longer for the cold dough — 60 minutes), then transfer to a preheated Dutch oven at 250°C (480°F). Bake covered for 25 minutes, then uncovered at 230°C (450°F) for another 15–20 minutes until deep brown.
6. Cool fully (at least 1 hour) before cutting.
7. Taste. Cut a slice from each loaf side by side. Smell first; then taste; then chew and notice what comes through the back of the throat.

Expected results - Loaf A (room temp): pleasant, bready, fairly mild flavor; slightly less open crumb. - Loaf B (cold ferment): noticeably more complex; subtle sour notes, deeper "old bread" character; more aroma; often a more open and irregular crumb.

Troubleshooting - Cold dough did not double? It may rise less in the cold than the warm dough. That's expected; the structure develops more than the volume. Trust the time, not the volume. - Loaves are pale? Oven not hot enough, or pan not preheated. The oven needs to be fully preheated for at least 30 minutes with the Dutch oven inside. - Loaves stick to the Dutch oven? Use parchment under the dough.

Classroom variant. This works for a high school chemistry classroom over a weekend with one student volunteer to refrigerate-ferment Container B at home. Have the class taste-test on Monday. Connect to: enzyme kinetics, temperature and reaction rate, volatile compound chemistry. Pat does this with her AP Chem class as the warmup to fermentation week.

🍳 Lab 31-B: Building a Sourdough Starter From Scratch

Goal: Establish a stable wild-fermented yeast and bacteria culture that you can keep indefinitely.

Time: 5–10 days for first activity; ongoing.

⚠️ Allergen flags: Wheat (gluten). Some people use rye flour for the first feeding because it ferments faster — also gluten-containing. For gluten-free starters, use brown rice flour or buckwheat (different microbes, different timing).

Materials - 500 g (about 4 cups) whole-wheat or rye flour for the start; bread flour or all-purpose for ongoing feedings - Bottled or filtered water (chlorinated tap water can suppress wild yeast — let tap water sit uncovered overnight to dechlorinate) - A clean glass jar, 1 quart (1 L), with a loose-fitting lid - A small kitchen scale - A rubber band or marker for tracking volume

Protocol

Day 1. Combine 50 g whole-wheat or rye flour with 50 g water in the jar. Stir vigorously — incorporate air. The mixture should be the consistency of thick paint. Cover loosely. Leave at warm room temperature (24–26°C / 75–79°F) for 24 hours.

Days 2–3. You may see no activity, or you may see a few bubbles. Discard half the contents. Add 50 g flour and 50 g water. Stir vigorously. Cover. Wait 24 hours.

Days 4–5. Activity should be picking up. The mixture will start to smell faintly fruity, then progressively more sour. You may notice it doubling in volume between feedings. Continue feeding once daily: discard half, add 50 g flour and 50 g water.

Days 6–10. A mature starter doubles or triples in volume reliably within 4–8 hours of feeding. The smell is pleasantly sour, slightly yeasty, slightly fruity (from the esters). When your starter passes a "float test" (a small spoonful floats in water rather than sinking), it is ready to leaven bread.

Ongoing maintenance. - For active baking: feed daily at room temperature. - For occasional baking: refrigerate; feed once a week; bring to room temperature and feed once 4–8 hours before using.

Expected results - The starter develops over 5–10 days from inert flour-water paste to active, predictable, doubling fermenter. - Wild yeasts (likely Saccharomyces cerevisiae and possibly other species, depending on what is in your kitchen) and lactic acid bacteria (Lactobacillus species) coexist in stable proportions.

Troubleshooting - No activity by Day 4? Try warmer temperatures (proofing box, top of refrigerator, oven with light on). Wild yeast multiplies slowly in cold rooms. - Pink, orange, or green colors? Discard. Start over. These indicate contamination by spoilage organisms — the starter has not yet reached the protective low pH, and unwanted microbes have established. Use a clean jar. - Hooch on top? That brown alcoholic liquid is fine — stir back in, feed.

Classroom variant. Pat starts a class starter on a Monday and lets students observe daily. By the second Monday, the class has a working starter and can bake a loaf together. Connects to microbiology, pH measurement, and the concept of selective culture conditions.

🍳 Lab 31-C: Pour a Beer with Your Eyes Open

Goal: Use the science of this chapter to perceive a beer more carefully than you have before.

Time: 15 minutes.

⚠️ Allergen flags: Most beer contains barley and/or wheat (gluten). Many craft beers contain hops, which are not a common allergen but are mildly sedating in some people. Beer contains alcohol — see the chapter's alcohol-honestly note. Adults of legal drinking age only.

Materials - One pale ale or IPA, one stout or porter, one lager — three contrasting beers - Three clean glasses (a tulip or wine glass shape gathers aromas; a pint glass is fine) - A piece of bread to neutralize between tastes

Protocol

1. Pour each beer into its glass at roughly the same temperature (around 8–10°C / 46–50°F is common).
2. Look. Note color. Pale gold? Amber? Black? The color is mostly the malt — pale malts give pale beers, dark roasted malts give black beers. The Maillard products in the malt are doing the visual work.
3. Smell the foam. The hop aromas (and the yeast esters) accumulate in the head. The foam is mostly proteins from the malt stabilizing CO₂ bubbles. Try to identify three distinct smells per beer.
4. Taste. First sip on the front of the tongue (sweetness from residual sugars). Hold the beer; let it move to the back (bitterness from iso-α-acids). Swallow and exhale through the nose (retronasal aroma — the volatile compounds go up to your olfactory bulb on the way out).
5. Compare. Which beer has the most malt character? The most hop character? The most yeast character (esters, fruity notes)?

Expected results - The pale ale will likely lead with hop aroma and a moderate bitter finish. - The stout will lead with roasted-malt flavors (coffee, chocolate, dark caramel) — these are Maillard products from heavily kilned malts. - The lager will be cleaner, with less yeast-derived fruit, more direct grain and hop character.

Discussion. What you are perceiving is layered chemistry: the substrate (the malted grain, the hops), the metabolism (the yeast strain, the temperature, the time), and the post-fermentation handling (filtering, conditioning, packaging). A skilled beer drinker can taste each of these layers, somewhat the way a wine drinker tastes vineyard, varietal, and vintage.

Classroom variant. This is not a classroom lab for minors. For an adult education or college-level food science course, it works well; have students read the labels and link the style to the malt bill, the hopping schedule, and the yeast strain.

Discussion Questions

1. Saccharomyces cerevisiae can do both aerobic respiration and ethanolic fermentation. Why does it produce vastly more energy from the former, and why do bakers and brewers want the slow, less efficient pathway?

2. A friend tells you that sourdough is "just a kind of yeast bread." Defend or correct this claim using what you know about the microbial community in a starter.

3. Why does a long, cold rise produce more flavor in bread than a quick warm rise? Cite specific chemical mechanisms.

4. Compare two beer styles — say, a German pilsner and an Irish dry stout. Trace the differences back to (a) the malt bill, (b) the hops, (c) the yeast and fermentation conditions. Where does each style get its identity?

5. If sourdough partially degrades gluten, why is it not safe for celiac patients?

6. Lager yeast (Saccharomyces pastorianus) is a hybrid species. What does this mean? How could two species hybridize to produce a third stable, brewing-useful organism?

7. The traditional Andean preparation of chicha de jora sometimes used chewed maize as a starter. What biochemistry was the chewer providing, and how does it map onto industrial brewing?

8. Recent evidence suggests no level of alcohol is "healthy." How does this change (or not change) your view of beer as a food? What does it mean for how this textbook discusses drinking?

9. A bakery owner brags that her sourdough is "ancient" — descended from a starter brought to America in 1820. The starter has been continuously fed for 200+ years. Is the S. cerevisiae in the starter actually 200 years old, in any meaningful sense? Why or why not?

10. Maya kept a starter for 14 weeks before using it. Was the starter "fully alive" the whole time, or did it have to grow and stabilize? What chemistry was happening in those 14 weeks of feeding?

Advanced Sidebars Expanded

Pyruvate Branch Points: Why This Pathway and Not Another?

In glycolysis, pyruvate is the universal three-carbon hub. Different organisms (and the same organism in different conditions) take pyruvate to different end-products — that is, glycolysis is a shared backbone, and what happens next depends on the cell.

• Yeast (anaerobic): Pyruvate → acetaldehyde → ethanol. Two enzymes: pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH).
• Lactic acid bacteria: Pyruvate → lactate, directly. One enzyme: lactate dehydrogenase.
• Animal muscle (anaerobic): Same as LAB — pyruvate → lactate. This is what makes your muscles burn during sprinting.
• Human gut bacteria: Various — propionate, butyrate, etc., depending on species.
• Aerobic respiration: Pyruvate → acetyl-CoA → into the citric acid cycle (Krebs cycle) → all the way to CO₂ + water.

The key point: the difference between the products of fermentation is one or two enzymes, not a whole different metabolism. The yeast and the Lactobacillus in your sourdough starter are running the same first ten steps. They diverge at pyruvate. This is why fermentation across organisms is best understood as a family of related metabolisms, and why the same substrate (sugar) yields beer or yogurt or pickles depending on which microbe you let win.

Wild Yeast Selection in Sourdough

Why does a sourdough starter end up with a specific community of microbes that is repeatable and stable? The answer is selective conditions:

• Low pH (around 3.5–4.5). Most spoilage organisms cannot grow below pH 4.5. Yeasts and lactobacilli can. So once a starter has acidified, the field has been narrowed.
• Available oxygen at the surface, anaerobic in the depth. Yeasts (facultative aerobes) can use both; obligate aerobes are excluded.
• Carbon source: starch and maltose, no free glucose. Microbes that cannot use maltose are at a disadvantage. Lactobacillus sanfranciscensis is particularly successful because it specializes in maltose and shares glucose with the yeast.
• Temperature. Mesophilic range (15–35°C / 59–95°F). Thermophiles and psychrophiles are excluded.

The result is that, despite the random microbial input from flour, hands, and air, sourdough starters in different parts of the world converge on similar communities — Saccharomyces cerevisiae or close relatives, L. sanfranciscensis or close relatives, sometimes with regional differences (the famous San Francisco starter is dominated by L. sanfranciscensis, named after the city). The starter is not random; it is a selectively shaped community, and the conditions you provide do most of the shaping.

Why Beer Foam Lasts and Champagne Foam Disappears

Both beer and champagne are carbonated by dissolved CO₂. Both produce a foam when poured. But beer foam (the head) can last for several minutes; champagne foam dissipates in seconds. Why?

The answer is surface stabilization. Beer foam is stabilized by proteins from the malt — particularly a class of proteins called hordeins that survive the brewing process and adsorb at the bubble-air interface. These proteins form a viscoelastic film around each bubble that resists drainage and rupture. The foam is essentially a protein-stabilized two-phase system, similar to a meringue (Ch 12).

Champagne is made from grape must, which has very little protein. The bubbles in champagne are not stabilized by a protein network; they are pure CO₂ in liquid, with only the surface tension of the wine itself holding them together. They burst quickly.

This is why a beer with high protein content (a stout, a hefeweizen) has a thicker, more lasting head than a low-protein beer, and why champagne — despite having more dissolved CO₂ pressure — produces a fleeting foam.

🥖 Mastery Food Checkpoint

For the bread track

This is the chapter where the "rising" of bread finally has a clean explanation at the cellular level. The yeast is doing glycolysis. The CO₂ is the gas of leavening. The ethanol mostly evaporates during baking. The flavor compounds (esters, alcohols, organic acids, aldehydes) are the side products of yeast metabolism, plus — in sourdough — lactic acid and acetic acid from the bacteria. Long, slow, cold fermentation produces more of these compounds. Write a one-page summary of "what is in my bread" — listing every category of microbe, every category of metabolic byproduct, and what each contributes to flavor and structure.

For the cheese track

The lactic acid bacteria you saw active in sourdough are the same family of organisms that make cheese, yogurt, and pickles. This chapter has introduced them in their bread role; Chapter 32 will introduce them in their dairy role. The fermentation chemistry is closely related — pyruvate to lactate via lactate dehydrogenase. Watch for the connection in Chapter 32.

For the chocolate track

The yeast we have studied here also appears in cacao fermentation (Ch 34), where it ferments the pulp around the cacao seed. Different conditions, similar metabolism. Note this when you reach Chapter 34.

For the fermented vegetables track

The lactic acid bacteria of sourdough are close relatives of the bacteria that ferment cabbage into sauerkraut and kimchi. In Chapter 33 we will study them in more detail. Notice how a sourdough's sour smell and a sauerkraut's sour smell share notes — that's the same lactic acid pathway.

For the coffee track

Chapter 34 will cover coffee fermentation, where a community of yeasts and bacteria ferment the pulp around coffee beans before they are dried and roasted. The microbiology is similar in spirit to a sourdough — wild fermentation, multiple organisms, end products that build flavor. Look for the parallel.