Starchy Staples

For better or worse, we get a significant number of our carbohydrate calories from the plants and plant parts we’ll consider today.

The plants we’ll consider store starch in specialized organs that are modified stems or roots. The modified storage organs of stems include:

• stolons – these are aboveground horizontal stems from which buds and roots may emerge at nodes. The best example is strawberries, in which we call the stolons ‘runners’. Another place you see stolons in this area is on beach grasses.

• rhizome – rhizomes are underground stems (and may even have small leaves along them) with adventitious roots developing along the underside, and new shoots arising from nodes. The rhizome may itself be swollen into a storage organ, as in gingerroot. Alternatively, the tips of rhizomes can be expanded into storage organs,

tubers, like the potato.

Since tubers (or expanded rhizomes) are stems, the buds are sites for potential formation of new above ground stems and ‘plantlets’. That’s what the ‘eyes’ of potatoes are.

The roots themselves can be expanded into tubers, as well. The most familiar example of a fleshy, tuberous root is the sweet potato. There are no nodes on tuberous roots.

The taproot (main root) can also become a storage organ. The best examples here are carrots and turnips. Carrots haven’t always been (nor are they all) orange.

Carrots were first domesticated in Afghanistan from a wild Daucus carota that usually has a white root. However, many root colours occur, including red, purple and black, all due to anthocyanin pigments. The orange carrots we eat today were selected in Holland during the 17th century.

There are two other forms of storage organs formed from modified stems: bulbs and corms.

• bulbs – are modified underground, upright stems with a basal plate, growing point, enclosed by thick, fleshy scales.

Examples: onion, garlic, tulip, lily

• corms – corms are also short, thick, underground stems of plants that act as storage organs. Corms mostly are

found in monocots, and consist of one (or occasionally more) internode(s) that is (are) used for storage. Inside a protective skin, it is mostly parenchyma cells. The obvious difference from bulbs is the internal structure – solid in corms and layered in bulbs. Examples of corms: crocuses, taro, Chinese water chestnut

Let’s look, at least briefly, at the major starchy staples:

Potato:

The potato originates in the Andean highlands, at an elevation of around 4000 m. Many types of potatoes still grow in that area. Among them are various types with brightly coloured flesh – blue and red most common.

The colours are anthocyanins, and may have valuable antioxidant properties.

The white potato was being grown in northern South America and Central America in the 16th century when Pizarro and the conquistadors invaded. It was a staple for Inca civilization.

The Spanish took the white potato (Solanum tuberosum) back to Europe. It was used mostly for animal food before the 18th century. Why didn’t Europe take to the potato rapidly?

It is from the family Solanaceae, with tomatoes, bell peppers, eggplant, tobacco, deadly nightshade and jimsonweed. All except tobacco have poisonous leaves due to toxic alkaloids (and you may consider tobacco leaves poisonous, too!).

Glycoalkaloids (particularly solanine) are also present in the tuber when you see green patches.Solanine causes weakness, confusion and sometimes hallucinations in sub lethal toxic doses. It somehow affects mitochondria. The symptoms (but not mechanism) are also effects of jimsonweed consumption.

In jimsonweed the alkaloids are: atropine, scopolamine and hyoscyamine.

Atropine and scopolamine are acetylcholine antagonists (block neuromuscular synapses).

Back to potatoes - There are 200 tuber-bearing species of Solanum. Only 8 have been cultivated. There are, however, almost 6000 cultivars of S. tuberosum. 85% of North American potatoes come from 12 cultivars.

Nutritional values of potatoes (with skins) per 100g:

Carbohydrates 19g Dietary fiber 2gProtein 2gFat 0.1gWater 75gIron, Vitamins C and B6 >10%RDIVitamins B1,B2,B3 <10%RDICa,Mg,P,K,Na <10%RDI

Much of this nutrient value is in the cell layer just beneath the skin.

Sir Walter Raleigh introduced potatoes to Ireland. It grew well there, and provided a ready source of calories for poor Irish Catholics, subjugated by the English.

With this new food, the population of Ireland grew from 1.5 million in 1760 to 8.5 million in 1840. Europe’s population doubled from 140 to 266 million between 1700 and 1800. It wasn’t the potato alone.

• the industrial revolution occurred simultaneously• animal breeding became much more sophisticated• farmers learned about crop rotation • farmers also learned to use manure to fertilize fields

But dependence on a single crop is dangerous.

Potatoes are low in vitamins A and D. A potato-dependent diet can lead to rickets.

Potato blight (Phytophthora infestans) appeared in Europe in 1844, and in Ireland in 1845. 75% of that crop was lost. In 1847 80% was lost. This was the Irish potato famine.

Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in storage.

Prior to the potato blight, half of Irish farms were less than 5 acres, but sufficient to feed a family. The blight meant families could not pay their land tax.

The Penal Laws meant that the Irish lost their land and became tenant farmers on the land. Crop failure meant they could not pay the land rent, either.

As a result of the potato famine, 3 million people died and 4 million emigrated.

Today, Europe and Russia produce over 70% of the world crop. In terms of individual countries Russia ranks 1st, China 2nd, Poland 3rd, and the U.S. 4th.

Potato blight is again important; strains resistant to the dominant fungicide have appeared, and unlike the original one, they reproduce sexually, producing variety.

Resistance to one or more strains of blight has been found in native Andean potatoes, particularly in S. bulbocastanum.

The gene responsible for resistance has been identified, cloned and inserted into a susceptible domesticated cultivar. Is this use of molecular genetic tools (and the production of a GM potato) something we should oppose or support?

Discussion…

The ‘other’ potatoes – sweet potatoes and yams:

Sweet potato (Ipomoea batatas), in the same family (Convulvulaceae) as morning glory, has a tuberous storage root. The plant is a perennial vine.

Sweet potato was domesticated in tropical America at least 5,000 YBP. It spread from the site of origin into the Caribbean and also onto Polynesian Islands and New Zealand.

In North America, we tend to call sweet potatoes ‘yams’, which is incorrect.

Sweet potatoes require a long, warm growing season. The root matures in 2 – 9 months. They are well adapted to tropical growing conditions, but susceptible to chilling injury.

That explains the importance of this crop throughout the tropics. China is the largest producer (of a world total of 127 million metric tons, China produces 105 million), but it has become important in the diet of many African countries (e.g. Burundi, where per capita consumption is 130 kg annually).

There are different flesh colours grown and preferred in different areas. In North America, we want the yellow-orange colour

There are white-fleshed types used in Spanish cooking, and called batatas; the Japanese ‘kotobuki’ is yellow; the Okinawan cultivar is purple fleshed; in New Zealand they grow an old isolate, the kumara, that is red-purple.

Sweet potatoes are rich in dietary fiber and vitamins A, C, and B6. Preliminary evidence suggests that it is a good source of carbohydrates for diabetics; that evidence suggests it stabilizes blood sugar levels and reduces insulin resistance, even though a portion of the carbohydrate is present as sugar (that’s the reason it tastes sweet).

True yams are not species of Ipomoea, but species in the genus Dioscorea. They are large tubers that can be 2.5 m long and weigh 70 kg. They are a staple food in tropical West Africa, China, Southeast Asia, Central America, the Caribbean, and Pacific Islands. There are many species in the genus, but only 10 are major food sources. Cultivation in Africa and Asia may date back to 8000 BC.

Nigeria is the leading producer, growing more than 2/3 of the world’s total crop.

The African yam contains toxins, and preparation involves days of pounding, leaching and boiling to remove them before cooking. Yams are treated in relatively similar ways in Asia.

Not all the chemicals have negative impact. Yams are the original source of diosgenin, used in research to make the first birth control pills. Diosgenin inhibits ovulation. Other saponins (sapogenic glycosides) were used to make cortisone.

Again there are different flesh colours:White (D. rotunda) and yellow (D. cayenensis) are native to Africa.Purple yam (D. alata) is from southeast AsiaA different white yam (D. opposita) is Chinese

Yams vary in toxicity. Some, e.g.. Japanese mountain yam, can be pickled to remove toxic components, then grated and eaten raw.

Elsewhere yams may be boiled, roasted, fried or cooked in soups and stews. The places where yam remains an important food source are mostly West Africa and the Pacific Islands.

The ‘air potato’ (D. bulbifera) is native to Africa and Asia, but has become invasive in Florida. What is eaten (where it’s eaten) is a bulbil, a tuber-like growth at the base of the leaves.

The time to grow a crop of yams is 8 – 12 months. Harvest is difficult because the tubers go so deep into the soil. As a food source, yams have been replaced in recent years by cassava.

the bulbil

Cassava – (or manioc) we know cassava essentially only as the source for tapioca. However, Manihot esculenta is an important food source for many tropical populations.

Its origin is in tropical America, probably Brazil where wild plants can still be found. The species is in the spurge family, Euphorbiaceae, that also includes cactus-like desert species of southern Africa, poinsettia, castor bean and rubber trees.

It was cultivated before recorded history in tropical America (Central and South America) and long before European exploration.

Export to Europe and Africa resulted from Spanish and Portuguese exploration. It has become a staple crop in Africa since it grows well under harsh conditions (flooding, drought).

Cassava has a high percentage of starch (30% of fresh weight), and also vitamin C (25 mg/100 g), calcium (50 mg/100g), and phosphorus (40 mg/100g), but little protein (1%).

There isn’t much protein in the root, but the leaves have about 30% protein. For these leaves to be a major protein source, they need to be supplemented with a source of methionine.

Cassava is a carbohydrate staple for ~1 billion people.

Cassava produces starch energy so efficiently that the U.S. D.O.E. is sequencing the genome in the belief it could become a source of renewable energy.

The cassava root must be processed to be consumed. The epidermal layer accumulates cyanogenic glycosides. Initial digestion of them releases highly toxic hydrocyanic acid.

There are two ‘varieties’ (not biologically, but practically) – a sweet form that can be eaten by peeling the root, then cooking it by boiling, steaming or frying. It has fairly low levels of the cyanogenic glycosides.

The other form is called ‘bitter’; it has high levels of the glycosides. To prepare it for safe consumption it is peeled, dried, soaked, boiled, grated, drained, and fermented.

In South America it is peeled, presoaked, grated, then squeezed to express liquid that contains most of the glycosides. Then it is roasted to dryness, to be reconstituted when needed.

A problem is that the amount of cyanogenic glycosides increases when the plant is under drought stress. In sub-Saharan Africa, where the protection offered by sufficient sulfur (from protein) in the diet against small to moderate glycoside intake is frequently absent, a disease called “konso” occurs. The motor parts of the cortex are affected, causing an irreversible paralysis; the disease name means ‘tied legs’.

The Rockefeller Foundation is supporting genetic research to reduce cyanogenic glycoside production and the problem where cassava is so important as a source of calories. Australian research is trying toreduce the remnant HCN in processedcassava.

Jerusalem artichoke – is a misnomer for the tuber of a North American sunflower, Helianthus tuberosus.

It was harvested by native Americans, and carried back to Europe by Samuel Champlain in the early 17th century. The common name may result from it being called sunflower in Italian (girasole).

The tubers store the carbohydrate inulin instead of starch. From commercial crops the inulin is used as a source for fructose production.

They’ve been promoted, but not yet produced, as a potential source of biofuels.

The tubers break down when cooked (e.g. in a stew) into a tender, tasty root vegetable particularly healthy for diabetics.

The plants are easy to grow, and will return from even small bits of tuber left after digging up a crop. That also makes them weedy for many gardeners. The quality of the tubers apparently also declines when they are left, rather than replanted in fertile soil.

They are also notorious as a cause of flatulence.

Taro - is a corm harvested from Colocasia esculenta. We know it from Hawaiian tradition, where it is sweetened and called poi, and from Chinese cooking, where a kind of dim sum called ‘wu kwok’ is a deep fried, egg-shaped ball of taro with a little pork and spice inside (or, from a fancier dish I call ‘duck with yam’ that has a layer of duck meat and skin atop a layer of mashed taro, the whole lightly breaded and deep fried).

Leaves and flowers are also eaten in some places. The plant is actually inedible if ingested raw because of needle-shaped raphides in the plant cells.

Raphides are a type of elongated crystalline form of calcium oxalate aggregated in bundles within a plant cell. Because of the needle-like form, large numbers in the tissue of, say, a leaf can render the leaf unpalatable to herbivores.

Taro leaves are a good source of thiamin, riboflavin, iron, P, K, Cu, Mn, and zinc, and a very good source of vitamins B6, C, and niacin.

Taro corms are very high in starch, and are a good source of dietary fiber.

Wu kwok

Finally, a couple of oddities:

Oca – otherwise called New Zealand yam, biologically it is Oxalis tuberosa. Its origin is probably Peru, from where it was exported to Europe. In Europe it was a failure, due to specific environmental needs. In New Zealand it was and remains successful.

What is used is a starchy tuber that is said to resemble a small, shrivelled carrot. They come in a variety of colors, including red, white, purple and yellow. Like potatoes, the consumable part is mostly carbohydrate, but is very high in sugar content (thus quite sweet).

Preparation includes use raw, pickled, boiled, fried, and in soups or stews.

Jicama – is a tropical vine grown for its tuberous root. Jicama comes originally from tropical Mexico and Central America. Spanish explorers carried it to Asia and the Pacific Islands, as well as using it on their ships. It stores well, retaining crispness and moistness (like a fresh carrot). Both on the ships and currently it is eaten raw, for example in salads. It can also be cooked, for example in a mixed vegetable stir fry.

After spreading widely, jicama was ‘re-introduced’ to South America in what was then French Guiana in the mid-1800’s and is still grown in the South American tropics.