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Legumes: Important Protein Source and Natural Fertilizer, Slides of Biology

An overview of legumes, their role in the nitrogen cycle, and their importance as a source of protein and natural fertilizer. Topics include the characteristics of legume plants, nitrogen fixation, crop rotation, and specific legumes such as lentils, beans, peas, soybeans, and peanuts. The document also covers the production of vegetable oils from legumes.

Typology: Slides

2012/2013

Uploaded on 01/09/2013

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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

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

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 (NH 3 ). 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.

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.

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.

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

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 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.