Legumes

Outline

• 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• 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• 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• 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.

• 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• 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.

• 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• 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.

– 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.

• 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• 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.

• 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.

• 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

• 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.

• 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• 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

– 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• 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.

• 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• 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 anti-oxidant, and sometimes sugar. Packaging under a vacuum also reduces oxidation and rancidity.

Vegetable Oils• 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• 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:– 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• 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).

• 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.