Ch.6: A Survey of Common Vegetables

Chapter 6 of the book "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee (1984) Roots and Tubers Lower Stems and Bulbs Stems and Stalks Leaves: Lettuces, Cabbages, and Others Flowers Fruits Used as Vegetables Seaweeds Mushrooms, Truffles, and Relatives

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Part of Public › Books › On Food and Cooking (Harold McGee) · 16 courses

1. [Roots and Tubers] What do roots and tubers such as potatoes, yams and cassava store for the plant?

   Starch: large clusters of the sugars the plant makes by photosynthesis. Stored underground, it is a concentrated food supply that keeps a long time, for the plant and for the billions of people who live on these roots.

2. [Roots and Tubers] Why may cooked tubers have helped early humans when the African savanna cooled, about 2 million years ago?

   Cooking makes tubers far more nutritious, because we cannot digest raw starch well. Raw starch granules resist our digestive enzymes; cooked, gelled starch does not. Fruits had become scarce, while tubers were plentiful for those who dug them up and roasted them in embers.

3. [Roots and Tubers] Why do potatoes cook to a dry, crumbly texture, while carrots, turnips and beets stay moist?

   Potato starch soaks up the water of the cells as it cooks; carrots and beets have little starch. Example: some roots and tubers are a third or more starch by weight.

4. [Roots and Tubers] Which common English words share an Indo-European root, meaning "to swell", with the word tuber?

   Truffle, and also thigh, thumb, tumor and thousand. A tuber is a swollen plant storage organ; a truffle is a swollen underground fungus.

5. [Roots and Tubers] Botanically, what is a potato?

   A tuber: the swollen tip of an underground stem, filled with starch and water. The potato (Solanum tuberosum) is a relative of the tomato, chilli and tobacco.

6. [Roots and Tubers] What are the "eyes" of a potato?

   Buds, which grow into the stem and roots of a new plant.

7. [Roots and Tubers] Where does the potato come from?

   The cool, moist regions of Central and South America. Some of the more than 200 potato species were cultivated 8,000 years ago. Spanish explorers brought it to Europe around 1570. Today the potato leads all vegetables in worldwide production.

8. [Roots and Tubers] What gives potatoes their mild earthy flavor?

   A pyrazine, made by soil microbes and apparently also by the tuber itself.

9. [Roots and Tubers] How do true new potatoes differ from mature potatoes?

   New potatoes are dug young: moist, sweet, low in starch and quick to spoil. They are dug while the vines are still green. Mature potatoes are dug in the fall, after the vines are killed and the tubers have sat in the soil for several weeks to toughen their skin.

10. [Roots and Tubers] At what temperature do potatoes store best?

    7–10 °C (45–50 °F). Warmer, they sprout or rot. Kept in the dark for months, they slowly gain fatty, fruity and flowery flavor notes, made by enzymes from membrane lipids.

11. [Roots and Tubers] What happens to potatoes stored colder than about 7 °C (45 °F)?

    Some of their starch turns into sugar.

12. [Roots and Tubers] Why do potato chip makers keep cold-stored potatoes at room temperature for several weeks before frying?

    To lower their sugar, which would make the chips brown too fast and taste bitter. Cold storage turns some starch into glucose and fructose; this warm "reconditioning" reduces them again.

13. [Roots and Tubers] What are the internal black spots sometimes found in raw potatoes?

    Bruises: a knock damaged the cells, and enzymes turned them dark. The browning enzymes make dark complexes from the amino acid tyrosine. Alkaloid levels, and therefore bitterness, often rise in these spots too.

14. [Roots and Tubers] What colors the flesh of yellow potatoes?

    Carotenoids (lutein, zeaxanthin), which dissolve in fat.

15. [Roots and Tubers] What colors blue and purple potatoes?

    Anthocyanins, water-soluble pigments that are also antioxidants.

16. [Roots and Tubers] Which toxic substances do potatoes contain?

    The bitter alkaloids solanine and chaconine. A hint of their bitterness is part of true potato flavor. Higher levels taste distinctly bitter, burn the throat, and cause digestive and neurological problems, even death.

17. [Roots and Tubers] How much solanine and chaconine (toxic alkaloids) do ordinary commercial potatoes contain?

    2–15 mg per 100 g. Stressful growing conditions and light can double or triple this.

18. [Roots and Tubers] Why is a green surface on a potato a warning sign?

    Light that makes potatoes green also makes them produce more toxic alkaloids. The green is chlorophyll; the alkaloids are solanine and chaconine. Greened potatoes should be peeled deeply or thrown away; strongly bitter potatoes should not be eaten.

19. [Roots and Tubers] What are the two cooking categories of potato?

    Mealy More dry starch in the cells, so denser. When cooked, the cells swell and separate: a fine, dry, fluffy texture, good for frying, baking and mashing. Example: russets, blue and purple varieties. Waxy Neighboring cells stay attached when cooked: a dense, moist texture, and pieces that hold together. Example: true new potatoes and the common red- and white-skinned varieties, used in gratins and potato salads.

20. [Roots and Tubers] How can boiled potatoes be kept from sloughing (shedding their outer layers)?

    Precook them at a low temperature, which strengthens the cell walls. This makes both mealy and waxy potatoes firmer and more coherent.

21. [Roots and Tubers] What causes after-cooking darkening, the bluish-gray patch inside some cooked potatoes?

    Iron and a phenolic compound react with oxygen to form a dark complex. The phenolic is chlorogenic acid. Making the boiling water distinctly acidic (cream of tartar or lemon juice) once the potatoes are half-cooked reduces it.

22. [Roots and Tubers] Why does a baked potato taste richer than a boiled one?

    Browning reactions add malty, "sweet" aromas. Example: methylbutanal, methional. Boiled potatoes taste mainly of the raw tuber's earthy, fatty, fruity and flowery notes, made stronger.

23. [Roots and Tubers] Why do leftover potatoes develop a stale, cardboard-like flavor?

    Once their vitamin C is used up, oxygen turns their aroma molecules into stale-smelling ones. The aroma molecules are fragments of membrane lipids; vitamin C protects them from oxygen for a while; they oxidize into aldehydes. In the refrigerator this takes several days; potatoes kept hot for service go stale within a few hours.

24. [Roots and Tubers] Why do mealy potatoes make smoother mashed potatoes than waxy ones?

    Mealy potatoes fall apart easily; waxy ones need hard work, which makes them gluey. Mealy potatoes separate into single cells that butter and milk coat easily. Working waxy potatoes breaks cells and releases gelled starch.The French pommes purées use waxy potatoes worked very hard with butter to beat in air; American mashed potatoes stir liquid and fat gently into sieved mealy potatoes.

25. [Roots and Tubers] Why do potato chips contain far more oil than thick french fries?

    Oil clings to the surface, and a chip is almost all surface. The oil also soaks into tiny pores at the surface. Chips are about 35% oil, thick fries 10–15%.

26. [Roots and Tubers] Why does a single quick frying give french fries a poor crust?

    The crust is thin, and the moist inside soon softens it. A crisp crust needs a gentle first frying, during which starch dissolves out of the granules in the surface cells and glues their cell walls into a thicker, stronger layer.

27. [Roots and Tubers] What is the two-stage frying method for french fries (sticks 5–10 mm thick)?

    First 120–163 °C (250–325 °F) for 8–10 minutes, then 175–190 °C (350–375 °F) for 3–4 minutes. The first stage can be done ahead for all the fries; the short hot stage browns and crisps them just before serving.

28. [Roots and Tubers] Potato chips are slices about 1.5 mm thick. How many potato cells is that?

    About 10–12 cells. So a chip is a french fry that is all crust and no interior.

29. [Roots and Tubers] Why are kettle-fried chips harder and crunchier than ordinary packaged chips?

    Kettle oil starts cool and heats slowly, giving the starch time to glue the cells together. The oil goes from about 120 °C to 175 °C (250 to 350 °F). Packaged chips fry at a constant 175 °C (350 °F) for 3–4 minutes: the slices dry before the starch absorbs water, giving a delicate, fine-grained crispness.The kettle temperature rises slowly because a batch of cold, wet slices cools the oil, and the heater catches up only as the moisture cooks out.

30. [Roots and Tubers] How are soufflée potatoes made to puff up into hollow balloons?

    Thin slices are fried until leathery, cooled, then fried very hot, so steam inflates them. 3 mm slices, first at 175 °C (350 °F), then at about 195 °C (380 °F). The stiffened surfaces hold in the steam, which pushes them apart.

31. [Roots and Tubers] Botanically, what is a sweet potato?

    A swollen true root, of a plant in the morning glory family. The plant is Ipomoea batatas; it is not related to the potato.

32. [Roots and Tubers] Is the orange "yam" sold in the United States a true yam?

    No: it is a moist, orange sweet potato, named "yam" in 1930s marketing campaigns. True yams are tubers of Dioscorea species.

33. [Roots and Tubers] Why do most sweet potatoes get sweeter as they cook?

    An enzyme breaks starch down to maltose, a sugar. Maltose is two glucose molecules joined, about a third as sweet as table sugar. Moist varieties convert as much as 75% of their starch.

34. [Roots and Tubers] Between which temperatures does the sweet potato's starch-to-sugar enzyme work?

    From about 57 °C (135 °F) to about 75 °C (170 °F). At 57 °C the starch granules swell and the enzyme can reach them; at 75 °C heat destroys the enzyme.

35. [Roots and Tubers] Why does slow baking make sweet potatoes sweeter than steaming, boiling or microwaving?

    The root spends longer in the temperature range where its enzyme turns starch into sugar. The range is 57–75 °C (135–170 °F); the sugar is maltose.

36. [Roots and Tubers] At what temperature do sweet potatoes store best?

    13–16 °C (55–60 °F). After harvest they are cured for several days at 30 °C (86 °F) to heal skin damage and develop sugars.

37. [Roots and Tubers] What is hardcore in sweet potatoes?

    A center that stays hard even when cooked; cold storage can cause it.

38. [Roots and Tubers] What gives orange sweet potatoes their heavy, pumpkin-like aroma?

    Fragments of their orange (carotenoid) pigments. Pale and red-purple sweet potatoes have a more delicate, nutty aroma.

39. [Roots and Tubers] How does cassava differ in composition from potatoes?

    Less water and about twice the starch. Example: potatoes are 18% carbohydrate by weight, cassava 36%. Other tropical roots and tubers are similar.

40. [Roots and Tubers] Why must bitter cassava be shredded, pressed and washed, while sweet cassava need only be peeled and cooked?

    Bitter cassava makes cyanide throughout the root; sweet cassava only near the surface.

41. [Roots and Tubers] Small dried balls of cassava starch are called tapioca.

    They turn pleasantly jelly-like when moistened again in desserts and drinks.

42. [Roots and Tubers] Why does raw taro irritate the mouth?

    Needle-shaped crystals pierce the skin, and enzymes attack the tiny wounds. The crystals are calcium oxalate (40–160 mg per 100 g), the enzymes digest protein. Cooking denatures the enzymes and dissolves the crystals.

43. [Roots and Tubers] Why are the juices of true yams (Dioscorea) slippery and frothy?

    They contain saponins, soap-like substances. Yams also have irritating oxalate crystals under the skin, and some varieties the toxic alkaloid dioscorine, removed by grating and leaching in water. True yams can grow to 50 kg and more.

44. [Roots and Tubers] What did the word potato (Spanish patata) originally mean?

    The sweet potato: it comes from batata, the Caribbean Taino word for it. The Quechua word for the true Andean potato was papa.

45. [Roots and Tubers] Why are carrots and parsnips noticeably sweet?

    They contain up to 5% sugars, and less starch than potatoes. The sugars are sucrose, glucose and fructose. Their distinctive aroma is why carrot-family roots are used to add complexity to stocks, soups and stews.

46. [Roots and Tubers] What colors were carrots before the orange carrot appeared?

    Purple, yellow and white. Central Asian carrots have reddish-purple to purple-black outer layers (anthocyanins) around a yellow core. The orange carrot was bred from yellow, white and wild carrots, probably in 17th-century Holland; it is the richest vegetable source of beta-carotene.

47. [Roots and Tubers] Why do orange carrots keep their color in a soup, while purple carrots color the whole soup?

    The orange pigment (carotene) dissolves in fat, the purple one (anthocyanin) in water.

48. [Roots and Tubers] Which compounds give carrots their aroma?

    Mainly terpenes, with notes of pine, wood, oil, citrus and turpentine. White carrot varieties are the most strongly aromatic.

49. [Roots and Tubers] What aroma note does cooking add to carrots?

    A violet-like note, from broken-down orange pigment (carotene).

50. [Roots and Tubers] Why do carrots taste sweeter cooked than raw?

    Heat weakens the tough cell walls, so the sugars inside can reach the tongue.

51. [Roots and Tubers] What does peeling a carrot remove, besides the skin?

    Most of the bitterness and the compounds that make cut carrots brown. Both are concentrated in the thin outer layer; the browning compounds are phenolics. Sunlight, heat or damage make carrots produce a bitter defensive chemical and alcohol. The water-conducting core has less flavor than the outer storage layers.

52. [Roots and Tubers] What is the harmless white fuzz on pre-peeled "baby" carrots?

    Damaged outer cell layers that dry out within hours of processing. "Baby" carrots are cut from mature carrots.

53. [Roots and Tubers] Why are parsnips dug in winter sweeter than those dug in autumn?

    Cold makes the parsnip turn its starch into sugar. It stores more starch than the carrot. Before sugar became cheap, British cooks made cakes and jams from parsnips.

54. [Roots and Tubers] In what form do sunchokes, salsify and burdock store their energy?

    As chains of fructose sugar, not as starch. These lettuce-family roots and tubers also share a mild, artichoke-like taste. The fructose chains (fructosans, inulin) also protect the plant from freezing in winter.

55. [Roots and Tubers] Why can a large serving of sunchokes cause abdominal discomfort?

    We cannot digest their fructose chains, so gut bacteria ferment them and make gas. The gases include carbon dioxide.

56. [Roots and Tubers] What is a sunchoke ("Jerusalem artichoke")?

    The tuber of a North American sunflower. Raw, sunchokes are moist, crunchy and sweet.

57. [Roots and Tubers] What does cooking sunchokes for 12–24 hours at about 93 °C (200 °F) do to them?

    It turns their indigestible fructose chains into sweet fructose; the flesh turns sweet and brown. The flesh also becomes translucent.

58. [Roots and Tubers] Why do salsify, scorzonera and burdock (gobo) turn grayish-brown?

    They are rich in phenolic compounds, which discolor cut surfaces, and the whole root when cooked. They also become fibrous as they grow large and old. The phenolics in gobo are potent antioxidants.

59. [Roots and Tubers] Why do water chestnuts, lotus root, bamboo shoots and beets stay crisp even after long cooking?

    Phenolic compounds cross-link and strengthen their cell walls. They stay crisp even after canning.

60. [Roots and Tubers] Why should fresh Chinese water chestnuts be dipped briefly in boiling water before eating?

    They can carry cysts of a parasitic intestinal worm (a fluke), picked up from contaminated growing water. The cysts infect people who shell the water chestnuts with their teeth. Scrub them before trimming off the tough outer layer, then wash them again.

61. [Roots and Tubers] Why do crosnes (Chinese artichokes) cause gas?

    They contain stachyose, a sugar we cannot digest, so gut bacteria ferment it. Stachyose is two galactoses joined to a sucrose. Crosnes are small tubers of an Asian member of the mint family; they turn mushy when even slightly overcooked.

62. [Roots and Tubers] What is the main culinary virtue of jicama?

    Sturdy crispness: it keeps well, discolors slowly and stays somewhat crunchy when cooked. Jicama is the storage root of a South American member of the bean family.

63. [Roots and Tubers] What gives slices of lotus root their lacy pattern?

    Large empty channels running along the rhizome. The rhizome is an underground stem. Lotus root stays crisp when cooked, and its phenolic compounds make cut surfaces discolor quickly.

64. [Roots and Tubers] Why is oca, a small South American tuber, distinctly tart?

    Oxalic acid, typical of its family (the wood sorrels, Oxalis).

65. [Lower Stems and Bulbs] Why do beets, turnips and radishes cook faster than potatoes?

    They store little starch, so they are less dense, cook faster and stay moist. Onions are the same. These vegetables grow at or just below ground level.

66. [Lower Stems and Bulbs] Which part of the beet plant is the beet "root" mainly?

    The lower stem. People first ate the leaves of Beta vulgaris (chard), and only later the underground part of special varieties.

67. [Lower Stems and Bulbs] How sugary are table beets compared with sugar beets?

    Table beets about 3% sugar, sugar beets 20%. Sugar beets were selected in the 18th century; some animal-feed beets have 8%.

68. [Lower Stems and Bulbs] Which pigments color beets red, orange and yellow?

    Betains (betalains), a group distinct from anthocyanins. They are water-soluble and stain other ingredients.

69. [Lower Stems and Bulbs] Why do beets with alternating red and white rings look best raw?

    Cooking lets the red pigment leak out of its cells into the white rings. The red rings are phloem tissue, the white ones xylem.

70. [Lower Stems and Bulbs] Why can eating beets turn urine red?

    Normally stomach acid and gut iron decolorize it, but sometimes it passes through intact. McGee: "a startling but harmless event".

71. [Lower Stems and Bulbs] Which molecule gives beets their earthy smell?

    Geosmin. It was long thought to come only from soil microbes, but the beet root seems to produce it too.

72. [Lower Stems and Bulbs] What is celeriac (celery root), botanically?

    The swollen lower stem of a variety of celery. It tastes like celery, has the same aromatics, and contains 5–6% starch.

73. [Lower Stems and Bulbs] What happens when turnips are cooked too long?

    They taste of overcooked cabbage and turn mushy. The turnip (Brassica rapa) is a cabbage-family plant with the family's sulfury aroma; it consists of lower stem and taproot.

74. [Lower Stems and Bulbs] What makes radishes pungent?

    A mustard oil, formed by an enzyme when the cells are damaged.

75. [Lower Stems and Bulbs] How can the pungency of radishes be reduced?

    Peel them (much of the enzyme is in the skin), or cook them (which stops the enzyme). Cooked radishes taste sweeter and milder, like turnips.

76. [Lower Stems and Bulbs] What is an onion or garlic bulb made of?

    Swollen leaf bases ("scales") wrapped around a central bud, storing energy for next season. Onion and garlic bulbs, cut and whole.

77. [Lower Stems and Bulbs] In what form do onions and garlic store energy?

    As chains of fructose sugars, not as starch. Sunchokes and their lettuce-family relatives store energy the same way.

78. [Lower Stems and Bulbs] Why do onions turn markedly sweet with long, slow cooking?

    Heat breaks down their stored chains of fructose into sweet sugars.

79. [Lower Stems and Bulbs] How does a raw onion produce its sharp sulfur flavor when cut?

    Cutting lets an enzyme reach sulfur-containing molecules and split them into irritating ones. In the intact cell, the precursors float in the cell fluid and the enzyme is held apart in the vacuole. Many of the products are unstable and keep changing into other compounds. The system evolved to deter animals from eating the plant.

80. [Lower Stems and Bulbs] Why do chopped, pounded and pureed garlic taste different?

    The more the tissue is damaged, the more the flavor-making enzyme reactions run. The flavor also depends on how much oxygen reaches the tissue and how long the reactions run.

81. [Lower Stems and Bulbs] Why does garlic taste so much more potent than onions?

    It produces about a hundred times more of the first sulfur reaction products.

82. [Lower Stems and Bulbs] Why should chopped onions that will be eaten raw be rinsed?

    To wash off the sulfur compounds on the cut surfaces, which grow harsher with time and air.

83. [Lower Stems and Bulbs] What is the onion lacrimator?

    A volatile sulfur compound that makes our eyes water. It apparently acts directly on nerve endings in the eyes and nose, after which it decomposes to hydrogen sulfide, sulfur dioxide and sulfuric acid ( H2S\mathrm{H_2S}H2​S , SO2\mathrm{SO_2}SO2​ , H2SO4\mathrm{H_2SO_4}H2​SO4​ ).

84. [Lower Stems and Bulbs] Does cutting garlic make you cry, as cutting onions does?

    No: garlic makes almost none of the tear-causing compound. Onion, shallot, leek, chive and rakkyo make it in quantity.

85. [Lower Stems and Bulbs] How does chilling onions in ice water for 30–60 minutes reduce tears when cutting them?

    Cold slows the enzyme and keeps the irritating molecules from escaping into the air. It also moistens the papery skin, which becomes less brittle and easier to peel.

86. [Lower Stems and Bulbs] Which ways of cooking onions produce the smell of overcooked cabbage?

    Baking, drying and microwaving, which form trisulfides.

87. [Lower Stems and Bulbs] Which way of cooking onions gives the strongest flavor?

    Frying in fat at high temperature. Mild garlic compounds stay mild in butter but turn rubbery and pungent in unsaturated vegetable oils.

88. [Lower Stems and Bulbs] Why is garlic cooked whole only mildly pungent, with sweet, nutty notes?

    Heating the uncut cloves stops the flavor-making enzyme before it can act. Example: blanching whole cloves. Pickled garlic and onions are mild for a similar reason.

89. [Lower Stems and Bulbs] Why do onions and garlic brown so readily when fried?

    They contain sugars and fructose chains. These also give cooked onions a caramel note.

90. [Lower Stems and Bulbs] What makes a "sweet" onion sweet-tasting?

    Less sulfur pungency, not more sugar: it is grown in sulfur-poor soil. It has half or less of the usual sulfur compounds; it is usually a yellow spring onion. So "mild" would be the more accurate name.

91. [Lower Stems and Bulbs] How do spring onions differ from storage onions?

    Spring onions are picked young: mild, moist and perishable. Storage onions are sharper and keep. Storage onions are harvested mature in fall, are drier, and keep for months. Spring onions are planted in late fall and harvested in spring and early summer.

92. [Lower Stems and Bulbs] Why do red onions lose much of their color when cooked?

    The red pigment sits only in the surface layers of each scale, so cooking dilutes it. The pigment is a water-soluble anthocyanin. Yellow onions owe their color to phenolic flavonoids.

93. [Lower Stems and Bulbs] Why does garlic burn much faster than onion when fried?

    Garlic is drier and richer in sugars. A garlic clove is under 60% water, onion scales 90%, and garlic has far more fructose and fructose chains. A clove is a single swollen storage leaf around the young shoot; an onion has many layered scales.

94. [Lower Stems and Bulbs] "Elephant garlic" is actually a variety of leek.

    It forms a garlic-like bulb cluster of up to 450 g, with a milder flavor than garlic.

95. [Lower Stems and Bulbs] How does refrigerated storage change the flavor of garlic?

    Less garlicky, more generally oniony. Cold growing conditions give a more intense garlic flavor; garlic is moistest soon after harvest, late summer to late fall.

96. [Lower Stems and Bulbs] Why is peeled garlic stored under oil a danger?

    Botulism bacteria can grow in the airless oil.

97. [Lower Stems and Bulbs] How can peeled garlic be stored under oil safely?

    Soak it in vinegar or lemon juice for several hours first, and refrigerate it.

98. [Lower Stems and Bulbs] Why does acid-pickled garlic sometimes turn bluish-green?

    A reaction involving one of its sulfur flavor precursors. Blanching the garlic before pickling reduces it.

99. [Lower Stems and Bulbs] What are the two main sources of garlic breath?

    Thiols left in the mouth Relatives of skunk spray, such as methanethiol. The browning enzymes of raw fruits and vegetables turn them into odorless molecules, as do mouthwashes with strong oxidizing agents. Example: eat an apple or a salad. Methyl allyl sulfide from digestion Formed as garlic passes through the digestive system; it peaks in the breath 6 to 18 hours after the meal and is probably beyond our reach.

100. [Lower Stems and Bulbs] Why do leeks need careful washing?

     Growers heap soil around them to lengthen the white part, which leaves grit between the leaves. Leeks are grown for their leaves, not for a storage bulb.