Transcript Legumes - NIU Department of Biological Sciences

Legumes
Outline
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Legume family: plant characteristics
nitrogen cycle and nitrogen fixation. Haber process
lentils: oldest crop
beans and peas. Two domestication centers for beans
soybeans. Trypsin inhibitor, tofu, soy sauce, rancidity
peanuts: growth habit, peanut butter, GW Carver and JH Kellogg
vegetable oils: triglycerides, fatty acids, cis vs. trans, soap, drying oils
pages
• Chap 13
Legumes
• The legumes are all members of a single plant
family, the Fabaceae.
• Beans, peas, lentils, soybeans, peanuts, alfalfa,
clover, and more.
• Why they are important: nitrogen fixation.
– Bacteria in root nodules convert nitrogen gas into
usable forms.
– This makes legumes the most important source of
protein in the plant world.
– Also makes them good natural fertilizer
Legumes as Plants
• Legumes are dicots: the
sprouting seedling has 2
cotyledon leaves.
– Most of the seed’s protein is
stored in the cotyledons
– Leaves have net vein pattern
• Legume flowers have 5 petals,
but they are bilaterally
symmetric: it’s easy to
recognize a legume flower.
• Fruits are pods (called
“legumes”) containing several
seeds
• Roots have nodules containing
the nitrogen-fixing bacteria
The Nitrogen Cycle
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Nitrogen is part of all proteins, plus many
other biomolecules.
Nitrogen makes up 79% of the atmosphere,
but nitrogen gas, N2, is not usable directly.
Nitrogen is only useful when it has been
“fixed”: converted to ammonia, nitrite, or
nitrate.
– Nitrogen can be fixed by lightning (takes a
lot of energy!) or by certain bacteria, or
artificially.
• Plants convert the fixed nitrogen into
proteins, which are then eaten by
animals.
• Bacteria break down decaying
organisms, releasing some fixed
nitrogen into the soil for reuse , and
converting some back into nitrogen
gas.
Nitrogen Fixation
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Nitrogen gas consists of two nitrogen atoms linked
by a triple bond, which means that it takes a lot of
energy to split the molecule.
Nitrogenase, the enzyme that fixes nitrogen, takes
nitrogen gas plus hydrogen gas plus a lot of energy,
and creates ammonia (NH3). The ammonia can then
be used by the bacteria or the plant host.
– Nitrogenase is very sensitive to oxygen, so it
needs to be kept in tight nodules, with
protective oxygen-absorbing molecules around
it.
– Nitrogenase is found in root nodule bacteria,
plus a number of other, free-living bacteria.
The plants release attractant chemicals into the
soil. The appropriate bacteria find their way to
the root and infect it.
– The root cells respond by growing the nodule
structure, including vascular tissue.
– The bacteria multiply, then undergo internal
changes that end up producing nitrogenase.
Crop Rotation
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Original farming method: slash-and-burn. This means
burning forest or grassland, then farming it until the crop
yields drop significantly, and then burning off a fresh area
and starting over.
– Burned vegetation releases nutrients into the soil
– You need a lot of fresh land, or land unused for many years.
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Crop rotation: planting different crops in different years.
Practiced in many cultures since ancient times to increase
productivity of the land.
– Basic concept: use plants that have different nutritional needs
in different years. An easy way: choose plants from different
families and types.
– If a legume was planted one of those years, nitrogen in the
soil would be renewed. Clover and alfalfa were good for this
– Also, plant pathogens don't see the same host plant every
year.
• During the 1800's, the importance of nitrogen
fertilizer (plus other nutrients) was realized.
Fertilizer
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Two ancient fertilizers: manure (animal waste
products) and plants plowed back into the soil. This
is called "green manure".
Guano: droppings of birds or bats, especially if
protected from rain, worked very well.
– Once a very important commodity. Peru and Bolivia
fought a war with Chile over some guano-covered
islands in the Pacific in 1879-1884. Chile won, and
annexed coastal regions from Peru and Bolivia. This
left Bolivia a land-locked country.
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Fritz Haber, in Germany, invented artificial nitrogen
fixation, using natural gas as the source for hydrogen,
in the early 1900’s. Most nitrogen fertilizer today
comes from this source.
– Many explosives also rely on this process
– He also pioneered the use of poison gas in warfare.
This was one of the really bad things used in World
War 1 (1914-1918).
– Haber was Jewish, and was forced to leave Germany
when the Nazis took over in 1932.
Lentils
• Lentils are probably the earliest domesticated
legume. Carbonized seeds from 11,000 years
ago in the Middle East, with domesticated
forms from 8000 years ago.
• They play a role in the Bible:
– Jacob and Esau were the twin sons of Issac,
leader of the Israelites. Esau was born first, so he
would become the leader next: his birthright.
– One day, Esau came home hungry, and saw
Jacob cooking some red lentil stew.
– Esau asked for the stew, but Jacob asked for his
birthright in return.
– Esau was very hungry and felt that was more
important, so he sold his birthright for a meal of
lentils.
Beans and Peas
• There are many different species called “bean” or “pea”.
– Mung bean, fava bean, lima bean, kidney bean
– Chick pea, black-eyed pea, cowpea
• Beans contain up to 25% protein.
– But low in two essential amino acids, methionine and cysteine.
– Grain crops contain plenty of these amino acids, so grain plus bean
makes a complete protein. For example, corn and beans, or wheat
are peas.
• We will focus on the common bean, Phaseolus vulgaris
and the common pea, Pisum sativum.
Beans
• Most dry beans in the US are
Phaseolus varieties: red beans, black
beans, pinto beans, kidney beans, navy
beans
– Also, snap beans: string beans , green
beans, and wax beans are immature pods
of the same species.
• Domesticated in two places: Mexico
and Peru. Crosses between types from
these two centers are difficult.
• Tend to produce intestinal gas: beans
contain some sugars we can’t digest,
but our intestinal bacteria can, which
produces large amounts of carbon
dioxide plus “odorants”.
Peas
• Domesticated in western Asia (Turkey and Syria), from
wild peas growing there.
• Dried peas were a mainstay of peasants in Europe in the
Middle Ages, cooked into a thick broth. Here’s a
child’s nursery rhyme referring to that:
– Pease porridge hot, pease porridge cold, Pease porridge in
the pot, nine days old;
– Originally “pease” was the word that referred to a group
of peas (like “oatmeal”). The singular form “pea” was a
later invention.
• Eating peas as a fresh green vegetable didn’t get started
until the 1600’s in France, when it became “both a
fashion and a madness” in the court of Louis XIV.
• Sugar snap peas: you eat the whole pod with green peas
in it, date from about 1979.
• The science of genetics was started by Gregor Mendel’s
research on peas in the 1870’s.
Fava Beans and Favism
• Fava beans are also called broad beans, are from the
species Vicia fava, which is native to north Africa and the
eastern Mediteranean region.
– Often eaten as a snack food after being salted and spiced.
• Fava beans contain several chemical compounds that are
strong oxidants. Red blood cells are very sensitive to
oxidative stress, especially if they lack the enzyme glucose
6-phospate dehydrogenase (G6PD). In this case, eating
the fava beans causes the blood cells to rupture, which
creates a sudden onset of severe anemia. This bean-caused
illness is called G6PD deficiency, or favism.
– Also, the hemoglobin is released into the blood where it is
converted into a yellow pigment called bilirubin. This
creates jaundice (yellow pigment in the eyes and skin), and
can lead to kidney failure, especially in newborns.
Inheritance of G6PD Deficiency
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G6PD deficiency (favism) is a hereditary condition that
affects about 400 million people, mostly in Africa and
eastern Mediterranean. It is one of the most common
human genetic diseases.
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It is associated with malaria resistance: the malaria parasite is killed
when the blood cells rupture.
• Usually affects males, since the gene for G6PD is
on the X chromosome and males only have 1 X,
while females have 2 X’s.
– --that is, males have one X chromosome and one Y
chromosome: they are XY, while females have 2 X
chromosomes: they are XX.
• Most commonly, the mother is a carrier: one of her
X’s has a good copy of the G6PD gene and the
other X has a bad copy.
– Favism is genetically recessive, so a carrier is not affected
• Her sons have a 50% chance of inheriting the X
with the bad G6PD gene.
– They have no health problems unless they eat fava beans or
certain medicinal drugs.
Malaria
• Malaria is a disease native to Africa, and it
probably has been with us since long
before Homo sapiens arose as a species.
– Close relatives infect the other Great Apes.
– There are many genetic variants in humans
known to create malaria resistance: sickle cell
hemoglobin in the best known.
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Like many parasites, Plasmodium changes its
surface proteins frequently to evade the
immune system. This also makes it difficult to
develop a vaccine.
The first drug acting against Plasmodium was
quinine, produced by the bark of a South
American tree.
Many new drugs have been developed as the
parasite acquire resistance through natural
selection
Current drug target: the plastid. It is essential
for the parasite, and humans don’t have one.
Soybeans
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Soybeans were domesticated in China in ancient times.
– Soybeans were one of the five sacred plants, as proclaimed by
the Emperor Shennong in 2853 BC: soybeans, rice, wheat,
barley, and millet.
– The Emperor Shennong is semi-legendary (we don’t have his
birth certificate), the founder of agriculture in China.
– He also identified hundreds of medicinal herbs by personally
testing them on himself. He is thus considered the founder of
traditional Chinese medicine.
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Not grown much in the US until the 1920’s.
– Soybeans like the same climate as corn: warm, temperate
climate with moderate rainfall.
– They add fixed nitrogen to the soil; corn then uses this
nitrogen.
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Soybeans are an important oil crop and livestock feed.
Most American soybeans go to these two uses: first extract
the oil, then feed the dried remains to animals (high protein
content)
Also used for human food: bean sprouts, tofu, textured
vegetable protein, soy milk, soy sauce
Soybeans as Food
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Soybeans contain a trypsin inhibitor, which
makes them indigestible: trypsin is one of our
body’s main protein-digesting enzymes.
– Thus it is necessary to cook soybeans before
eating them.
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Soy milk is made by grinding the dry beans
with water, then boiling it to destroy the
trypsin inhibitor, and filtering out any
insoluble products.
Tofu (bean curd) is made by coagulating soy
milk and then pressing it into molds.
– Coagulating the soy milk means denaturing
the proteins so they are no longer soluble.
This is usually done by adding gypsum
(calcium sulfate), which also makes tofu a
good source of calcium.
– Tofu is quite bland, but it readily picks up
flavors from other ingredients.
Rancid Fats
• Off-flavors in soybean products are caused by the enzyme
lipoxygenase, which combines unsaturated fatty acids (in the
oil) with oxygen to make compounds that taste and smell bad:
they are rancid. Rancid fats are bad for your health.
– Keep air and light away from vegetable oil, and don’t expect it
to last for a long time.
– Many products contain anti-oxidants (like BHT)
• Soybeans have a lot of lipoxygenase, but it can be inactivated
by a quick pulse of high temperature at the beginning of the
grinding process.
– Lipoxygenase can also be eliminated through genetic
engineering.
• This same problem occurs in whole wheat flour: the embryo
(germ) contains lipoxygenase, which causes the flour to go
bad. In contrast, white flour is made from the endosperm
alone, with the germ removed; it has a much longer shelf life.
Soy Sauce
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Soy sauce is a dark brown savory liquid that is the main
condiment used in East Asian cooking. It is a
fermented food: in the absence of air, microorganisms
have partly digested it and converted it to other
molecules
– There are many varieties of soy sauce: most countries in
East Asia use some variant: China, Japan, Korea,
Vietnam, Malaysia, Phillipines, Indonesia
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– There are several other fermented soy products
used as condiments: miso and tempeh
Soy sauce is made by fermenting soybeans with the
fungus Aspergillus in salty water. Traditionally, this
process takes up to 3 years.
– The ancient Romans used a condiment made from
fermented fish guts.
• Modern industrial production: break up soy
proteins with acid, add food coloring and artificial
flavoring.
Peanuts
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The peanut is also known as groundnut in many parts of
the world. Also “goober peas” and several other names.
Peanuts were domesticated in Peru, and spread
throughout the world by European traders after the
Spanish conquest.
They quickly became very popular in Africa, and they
were brought into the US by Africans being enslaved in
the 1700’s.
– Thought to be fit only for pigs and poor people until about
1850’s.
– Became a popular snack during the Civil War (1861-1865),
and then at baseball games and circuses after the war.
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Starting in 1903, George Washington Carver at the
Tuskeegee Institute improved the plant through selective
breeding, plus invented many peanut-based products.
– He encouraged the use of peanuts in a crop rotation with
cotton. This greatly enriched the soils in the South, which
were exhausted of nitrogen by too much cotton cultivation.
– Mechanical harvesting also started around 1900.
– This is the real beginning of widespread peanut cultivation
in the US.
Peanuts as Plants
• Domesticated peanut is a tetraploid: it
has two sets of chromosomes from
two different, closely related plants.
As with wheat, humans noticed this
unusual plant and saved it for
cultivation.
• After the flowers are pollinated, the
flower stalks elongate and turn
downward. They bury into the ground
a few inches, and the fruit develops
underground.
• The fruits (the peanut shell) contain
one or two seeds, which are what we
eat.
• At harvesting, the whole plant is
removed from the ground, and the
peanuts are removed by mechanical
pickers
Peanuts as Food
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In the US, the main uses of peanuts are as a snack food, as
peanut butter, and as peanut oil.
The Aztecs of Mexico used to grind up peanuts into a paste,
but the modern use of peanut butter comes from J.H. Kellogg,
who also invented breakfast cereal, in 1897.
– Kellogg was trying to improve the diet of the American
worker, which was very meat-rich (more suitable for hard
work on the farm than for city life.
– He was a Seventh-day Adventist who ran a sanitarium in
Battle Creek Michigan. He believed in vegetarianism, no
alcohol or tobacco, and vigorous exercise. Also yogurt
enemas. A good movie: The Road to Wellville
– Both breakfast cereal (corn flakes, initially) and peanut
butter were invented as health foods.
Peanut butter manufacture is quite simple: after shelling, the
peanuts are dry-roasted, then ground up with salt and an antioxidant, and sometimes sugar. Packaging under a vacuum also
reduces oxidation and rancidity.
Vegetable Oils
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Humans cannot make all the fatty acids we need for our cell membranes.
Thus, there are two essential fatty acids: linoleic acid and linolenic acid.
These are easily obtained from vegetable oils.
Peanuts and soybeans are two crops used for vegetable oils. Other important
oil crops include palm, rapeseed, sunflower, coconut, and olive.
– Rapeseed is commonly known as “canola”, which comes from Canadian oil, low
acid. It is in the mustard (Brassica) family, and grows best in cool conditions. It
has low levels of saturated fats, making it more healthy than most vegetable oils.
• Much vegetable oil is used for cooking, but other uses include soap,
oil-based paint and ink, and biodiesel fuel
• Basic process is quite simple: press the seeds until the oil comes out.
Water also comes out: it gets separated from the oil with a centrifuge.
• The modern industrial process involves extracting the oil by treating
ground sees with a petroleum-based solvent like hexane, then
removing the hexane.
Fat Chemistry
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Vegetable oils are trigylcerides: 3 fatty
acids attached to glycerol. Animal fats are
also triglycerides. The difference between
them is in the fatty acids.
Animal fats are solid because their fatty
acids are saturated: all possible positions on
the carbon chains are taken up by
hydrogens instead of double bonds between
the carbons.
– This makes the fatty acid chains pack
together tightly.
Vegetable oils are liquid because their fatty
acids are unsaturated: instead of hydrogens,
some carbons have double bonds, which
gives them an irregular shape that doesn’t
pack together well.
More Fat Chemistry
• The unsaturated fatty acids in vegetable oils
have a particular configuration:
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– The carbon chains of the fatty acids bend at
the double bonds, which is called cis
configuration.
Solid vegetable oils, like margarine, are made by
hydrogenation: adding hydrogen to the double
bonds
– Unfortunately, this process also rearranges
the remaining double bonds into trans
configuration, which has proven to be
unhealthy.
Soap
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The ancient Romans didn’t use soap for cleaning
themselves. Instead, they soaked in pools of hot water,
then scraped the dirt and grease off with a dull knife.
Soap was not widely used in the Western world until the
late 1700’s. The relationship between being clean and
being healthy wasn’t recognized until microorganisms
were discovered.
Soap is a salt of a fatty acid. It cleans by having one
end of the molecule (the salt end) soluble in oil and
grease and the other end (the fatty end) soluble in water.
This causes oily dirt to disperse in water, so it can be
washed away.
It is made by treating fatty acids with caustic soda or lye
(sodium or potassium hydroxide).
– This was originally done by boiling animal or vegetable
fats with wood ashes (which contain lye).
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Different vegetable oils provide different qualities. For
example, palm oil makes soap harder and coconut oil
makes lots of lather.
Drying Vegetable Oils
• Vegetable oils that are used as food are "non-drying". Other
vegetable oils dry to form a waterproof film.
• These drying oils can be used to protect wood or leather (e.g.
varnish, linseed oil and tung oil)
• When mixed with pigments, oil paints are produced.
– Also for ink: soybean oil
• Linoleum, used on floors was once made from linseed oil
– But today, plastics made from petroleum are used.
• Also for diesel fuel: “biodiesel” is made by reacting vegetable
oils (or animal fats) with alcohol.
– Easy to do on the farm as a do-it-yourself source of fuel.