Transcript Starchy Staples

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
Dietary fiber
Protein
Fat
Water
Iron, Vitamins C and B6
Vitamins B1,B2,B3
Ca,Mg,P,K,Na
19g
2g
2g
0.1g
75g
>10%RDI
<10%RDI
<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 Asia
A 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 bulbil
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.
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
from Spanish and
exploration. It has
in Africa since it
harsh conditions
and Africa resulted
Portuguese
become a staple crop
grows well under
(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 subSaharan 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 to
reduce the remnant HCN in processed
cassava.
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.