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

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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.