Johanna T Dwyer. Cambridge World History of Food. Editor: Kenneth F Kiple & Kriemhild Conee Ornelas. Volume 1. Cambridge, UK: Cambridge University Press, 2000.
Peanut or groundnut (Arachis hypogaea L.) is a major world crop and member of the Leguminosae family, subfamily Papilionoidae. Arachis is Greek for “legume,” and hypogaea means “below ground.” Arachis, as a genus of wild plants, is South American in origin, and the domesticated Arachis hypogaea was diffused from there to other parts of the world. The origin of Arachis hypogea var. hypogaea was in Bolivia, possibly as an evolutionary adaptation to drought (Krapovickas 1969). Certainly the archaeological evidence of the South American origins is secure. However, the debate about the pre-Columbian presence of New World plants in Asia (especially India) remains unresolved. The other species of Arachis that was domesticated prehistorically by South American Indians was A. villosulicarpa, yet the latter has never been cultivated widely.
As the peanut’s nutritional and economic importance became recognized, it was widely cultivated in India, China, the United States, Africa, and Europe. Thus, the peanut is another of the New World food crops that are now consumed worldwide. The peanut is popular as a food in Africa and in North America, especially in the United States; peanut-fed pigs produce the famous Smithfield ham of Virginia, and peanut butter is extremely popular. There is also much interest in peanut cultivation in the United States.
Botanically, the varieties of peanuts are distinguished by branching order, growth patterns, and number of seeds per pod. The two main types of peanuts, in terms of plant growth, are “bunch or erect,” which grow upright, and “runners or prostrate,” which spread out on or near the ground. Commercially, peanuts are grouped into four market varieties: Virginia, Runner, Spanish, and Valencia. The former two include both bunch and runner plants, and the latter two are bunch plants. Details on the life cycle and growth of the peanut and its harvesting are provided later in this chapter (Lapidis 1977).
Peanuts are also called “groundnuts” because they are not true tree nuts. Peanuts are seeds of tropical legumes with pods that grow underground to protect the plant’s seeds from seasonal drought and from being eaten by animals. Peanuts consumed by humans are dried seeds of the Leguminosae family, as are kidney, pinto, lima, and soy beans, as well as peas and lentils. The dried shell of the peanut corresponds to bean and pea pods. The names “peanut” and “ground pea” (as the food was called when it was first eaten in North America) became popular because the dried seed had a nutlike shell and texture, and it looked like a pea. Peanuts are also named “goobers,” “earth almonds,” “earth nuts,” “Manila nuts,” “monkey nuts,” “pinda,” and pistache de terre. But these terms sometimes also apply to other plants of similar character, such as Voandzeia subterranea, found in West Africa, Madagascar, and South America, and the “hog peanut” (Amphicarpaea menoica), found in North America.
The peanut plant roots at its nodes and is self-pollinating, with flowers that open and die after fertilization. It is unique in that it flowers above the ground, but after fertilization the fruit develops below ground in the soil.
The peanut consists of the germ (heart), two cotyledons (halves of the peanut), the skin, and the shell. The pod is tough and stays closed as the seeds ripen, but the seeds themselves have soft, digestible coats. They have been eaten by humans ever since South American Indians first domesticated them during prehistoric times.
Origins and History
Evidence exists of peanuts having been grown in Peru as early as 2000 B.C. (Sauer 1993). As mentioned, they are believed to have originated in South America, and many wild species of the genus Arachis are found there. Spanish explorers spread the peanut to Europe and the Philippines, and Portuguese explorers took it to East Africa. It reached North America circuitously via the slave trade from Africa, although in pre-Columbian times it probably came to Mexico from South or Central America. The stocks developed in Africa provided the basis for many varieties now grown in the United States. Initially, peanuts were cultivated in the United States for livestock feed to fatten farm animals, especially pigs, turkeys, and chickens. But they gained commercial importance after the Civil War, with much of the credit due to George Washington Carver at the Tuskegee Institute. One of America’s most distinguished African Americans of the nineteenth century, Carver spent his life developing various uses for peanut products, and they became important as a food and as an oil source.
In addition, commercial mills that crushed peanuts for oil were developed independently in Asia and Europe. Europe’s inability to meet a demand for olive oil led to a market for peanut oil there, with the peanuts coming mainly from West Africa, and then from India after the opening of the Suez canal. Peanuts subsequently were cultivated in all tropical and subtropical parts of the world.
Unlike most legumes, peanuts store oil instead of starch. During the early growth of the cotyledon storage cells (up to about 30 days after the peg or gynophore strikes the soil), starch granules predominate, and lipid and protein bodies are few. After this stage, however, to about 45 days, both lipid and protein bodies increase rapidly, and from 45 to 68 days, protein bodies and especially lipid bodies continue to expand. The plant’s protein and fat come from these bodies in the peanut cotyledon. In the final stage, there is little further growth of the cotyledon (Short 1990; Weijian, Shiyao, and Mushon 1991). Most peanuts require 140 to 150 frost-free days to mature, but such factors as growing season, location, and time of fruit set also influence the time required to reach maturity (Cole and Dorner 1992).
Peanut Pathogens and Pests
Approximately a quarter of the peanut fruit and vine crop is lost because of plant disorders wrought by insects, bacteria, fungi, nematodes, and viruses. Sclerotina minor, the cause of sclerotina blight, and Cercospora arachidicola, the cause of early leaf spot, are two important peanut pathogens. These are controlled by herbicides. Unfortunately, resistance to one is often associated with high susceptibility to the other, and resistance to S. minor is also associated with small seed size and an undesirable shade of tan color for the Virginia peanut type (Porter et al. 1992). Bacterial wilt is caused byPseudomonas solanacearum. Fungal species, including Aspergillus, Rhizopus, Fusarium, and others, cause various diseases.
The peanut root-knot nematode (Meloidogyne arenaria [Neal] Chitwood race 1) is a major pest in the peanut-producing areas in the southern United States. These microscopic worms greatly reduce yields but can be controlled with fumigants and nematicides. Efforts are now moving forward to select M. arenaria -resistant species of peanuts, because chemical controls of the pest are becoming more limited (Holbrook and Noe 1992).
Tomato spotted wilt virus (TSWV) decreases seed number and weight. Other viruses cause such diseases as spotted wilt and chlorotic rosettes. Insects that attack peanuts include the corn rootworm, which causes rot, and the potato leafhopper, which secretes a toxic substance, damaging the leaves.
Staphylococcus aureus brings about microbial degradation of fat in peanuts, but the major pathogen with relevance to human health is a fungal aflatoxin. It is a carcinogenic metabolite of Aspergillus flavus and Aspergillus parasiticus, which may cause or promote liver cancer in humans, especially when infected nuts are eaten in large quantities.
Although neither pathogen nor pest, drought is another major limiting factor in peanut production, and efforts are now progressing to develop drought resistance in some varieties (Branch and Kvien 1992).
The six leading peanut-producing countries of the world are India, China, the United States, Nigeria, Indonesia, and Senegal. World production of peanuts in the shell for 1992 was 23 million metric tons, with Asia and Africa producing 90 percent of the total (FAO 1993).
In the United States, the state of Georgia leads the nation in peanut production, followed by Texas, Alabama, and North Carolina (United States Department of Agriculture 1992). The most famous peanut producer in the United States is former President Jimmy Carter.
Peanuts need hot climates with alternating wet and dry seasons and sandy soils. Ideally, rainfall or moisture from irrigation should total at least an inch a week during the wet season. Peanuts are planted after the danger of frost is gone, when soil temperatures are above 65° F. The soil is usually treated with herbicides, limed, fertilized, and plowed before planting. Insecticides may then be applied, and herbicides are applied between preemergence and cracking time (postemergence). Postemergence practices involve cultivation, insecticides if needed, and herbicides for weed control. Calcium sulfate is provided to maximize peanut fruit development. This addition of calcium is important in peanut fertilization, because insufficient levels can lead to empty pods with aborted or shriveled fruit (Cole and Dorner 1992). Peanuts are usually rotated with grass crops such as corn, or with small grains, every three years. This rotation reduces disease and soil depletion. Efforts have been made to intercrop peanuts with other plants, such as the pigeon pea or cotton, but these have not been successful.
Only about 15 percent of peanut flowers produce fruit. The harvest includes both mature and immature varieties, as all fruits do not mature at the same time, and about 30 percent is immature at harvesting. Maturity can be estimated in a variety of ways. The “shell-out method” for recognition of maturity has to do with the darkening of the skin (testa) and the inside of the hull. The “hull scrape method” is done by scraping the outer shell layer (exocarp) to reveal the color of the middle shell (mesocarp), which is black in the mature peanut. Peanut harvesting involves removing plants from the soil with the peanuts attached (the upright plant is better suited to mechanical harvesting). A peanut combine is used to remove the pods from the plant.
After harvesting, the peanuts are cleaned by removing stones, sticks, and other foreign material with a series of screens and blowers. For safe storage the peanuts are dried with forced, heated air to 10 percent moisture.
Cleaned, unshelled peanuts can be stored in silos for up to six months. Shelled peanuts are stored for a lesser time in refrigerated warehouses at 32-36° F and 60 percent relative humidity, which protects against insects. A high fat content makes peanuts susceptible to rancidity, and because fat oxidation is encouraged by light, heat, and metal ions, the fruit is best stored in cool, dry places (McGee 1988). On the whole, however, unshelled peanuts keep better than shelled.
Peanuts may be processed shelled or unshelled, depending on the desired end product. Those left unshelled are mainly of the Virginia and the Valencia types. They are separated according to pod size by screening; discolored or defective seeds are removed by electronic color sorting, and the stems and immature pods are removed by specific gravity (Cole and Dorner 1992). Peanuts that are to be salted and roasted in the shell are soaked in a brine solution under pressure, and then dried and roasted.
Peanuts to be shelled are passed between a series of rollers, after which the broken shells and any foreign materials are removed by screens and blowers. Next, the shelled peanuts are sorted by size. Any remaining foreign materials and defective or moldy seeds are removed by an electronic eye, which inspects individual seeds. Ultraviolet light is useful for detecting aflatoxin contamination.
Peanuts are frequently blanched to remove the skins and hearts. This can be done by roasting (259-293° F for 5 to 20 minutes), or by boiling, after which they are rubbed to remove the skins. Then the kernels are dried to 7 percent moisture and stored, or converted into various peanut products.
Another method – dry roasting – is popular because it develops a desirable color, texture, and flavor for peanut butter, candies, and bakery products. In this case, unblanched peanuts are heated to 399° F for 20 to 30 minutes, then cooled and blanched.
Shelled peanuts are usually dry roasted in a gas-fired rotary roaster at 399° F, then cooled to 86° F, after which they are cleaned and the skins removed for making peanut butter. Oil-roasted peanuts are placed in coconut oil or partially hydrogenated vegetable oil at 300° F for 15 to 18 minutes until the desired color is achieved, whereupon a fine salt is added. Roasting makes the tissue more crisp by drying and also enhances flavor because of the browning reaction. Relatively low temperatures are used to avoid scorching the outside before the inside is cooked through. The roasting of peanuts also reduces aflatoxin content. For example, roasting for a half hour at 302° F may reduce aflatoxin B1 content by as much as 80 percent (Scott 1969).
And finally, peanut oil is extracted by one of three different methods: hydraulic pressing, expeller pressing, or solvent extraction.
Traditionally, peanuts were used as a source of oil and, even now, most of the world’s peanut production goes into cooking oils, margarines, and shortenings, as well as into the manufacture of soap and other industrial processes. Also called “arachis oil,” “nut oil,” or “groundnut oil,” peanut oil is a colorless, brilliant oil, high in monounsaturates. Virgin oil is mechanically extracted (expeller pressed at low temperature [80-160° F]), and lightly filtered. This method provides the lowest yield but the highest-quality edible oil.
Refined oil is typically produced by solvent extraction. It is made from crushed and cooked peanut pulp, which is then chemically treated in order to deodorize, bleach, and neutralize the flavor of the oil. In the United States, only low-grade nuts are used for oil production. The fatty acid composition is quite variable for a number of reasons, such as genotype, geography, and seasonal weather (Holaday and Pearson 1974). When refined oil is stored at low temperature, a deposit is formed, and hence it cannot be used in salad oils and dressings.
Only peanuts that are free from visible mold and subject to less than 2 percent damage are used for edible purposes. In the United States and Western Europe, most peanuts to be eaten go into the “cleaned and shelled” trade and are consumed as roasted and/or salted nuts, as peanut butter, or as a component of confections.
Because of its high protein and low carbohydrate content, peanut butter was first developed in 1890 as a health food for people who were ill. It is a soft paste made from Virginia, Spanish, or other types of peanuts. The skin and germ are removed, and the kernels are dry roasted and ground. Salt, antioxidants, flavors, and sugars (dextrose or corn syrup) may be added after grinding. Hydrogenation and/or the addition of emulsifiers prevents separation. “Crunchy style” peanut butter has bits of roasted nuts mixed into it.
Peanut butter is approximately 27 percent protein, 49 percent fat, 17 percent carbohydrate, 2 percent fiber, and 4 percent ash. Its sodium content is approximately 500 mg per 100 g. Peanut butter has good stability even after two years of light-free storage at 80 degrees Fahrenheit (Willich, Morris, and Freeman 1954), and keeps longer if refrigerated. But sooner or later, it becomes stale and rancid.
Peanuts are frequently employed in the cuisines of China, Southeast Asia, and Africa. The residual high-protein cake from oil extraction is used as an ingredient in cooked foods and, in Chinese cooking, is also fermented by microbes.
In recent years, peanuts have been added to a variety of cereal- and legume-based foods to alleviate the problem of malnutrition. Moreover, peanuts in the form of flour, protein isolate, and meal in a mixed product have desirable sensory qualities (Singh and Singh 1991). Peanut flour is made by crushing the shelled, skinned nuts, extracting the oil, and grinding the crushed nuts. In India, the flour is used in supplementary foods, weaning foods, and protein-rich biscuits (Achaya 1980).
In addition, the peanut plant itself has a high nutritional value and can be used for livestock feed or plowed back into the soil to aid in fertilization of future crops (Cole and Dorner 1992). Nonedible nuts are processed into oil, with the cake used for animal feed. Peanut shells, which accumulate in abundance, can be used as fuel for boilers (Woodroof 1966).
Having a higher percentage of protein by weight than animal foods and beans (ranging from 22 to 30 percent), peanuts provide an excellent, inexpensive source of vegetable protein for humans. A 1 ounce serving of oil- or dry-roasted peanuts provides 7 to 8 grams of protein, or 11 to 12 percent of the U. S. Recommended Dietary Allowance (RDA).
The protein quality of the peanut also is high, with liberal amounts of most of the essential and nonessential amino acids (the limiting amino acids in roasted peanuts and peanut butter are lysine, threonine, methionine, and cystine). For this reason, U. S. government nutritional guidelines include peanuts along with other high-quality protein foods, such as meat, poultry, fish, dry beans, and eggs. In the last few decades, cereal-and legume-based plant food mixtures using peanuts have grown in popularity, especially in developing countries, because of the excellent nutritional value of peanut proteins and their low cost.
New methods for determining free amino acids in whole peanuts are now available (Marshall, Shaffer, and Conkerkin 1989); peanuts have an amino pattern similar to that of high-quality proteins, and are much higher in protein than other staple plants, save for legumes.
Peanut proteins include the large saline-soluble globulins, arachin and conarachin, and the water-soluble albumins. The relative protein content of peanuts may vary with variety, strain, growing area, and climate. Arachin constitutes about 63 percent, and conarachin 33 percent, of the total protein in peanuts. The remaining 4 percent consists of other proteins, including glycoproteins, peanut lectin (agglutinin), alpha beta amylase inhibitor, protease inhibitors, and phospholipase D.
Depending on the cultivar, the fat content of peanuts ranges from 44 to 56 percent. Over 85 percent of the fat in peanuts is unsaturated; an ounce of peanuts contains 14 grams of fat, of which about a third is polyunsaturated and over half is monounsaturated. More precisely, peanuts have a polyunsaturated to saturated fat ratio of 2:3; a high proportion of their total fat is monounsaturated (49 to 54 percent), and a low percentage (14 to 15 percent) is saturated (McCarthy and Matthews 1984).
Monounsaturated fats help to lower LDL (low density lipoprotein) cholesterol when they replace saturated fats in the diet, and thus can help reduce risks of coronary artery disease that are associated with hyperlipidemia.
Calories, Carbohydrates, and Cholesterol
Well over three-quarters of the calories in peanuts are from fat, with the remainder from protein and carbohydrate, although the content of the latter varies with variety and growing conditions. Peanuts usually have about 20 percent carbohydrates, most of which are sucrose (4 to 7 percent) and starch (0.5 to 7 percent). Peanuts have no cholesterol.
The dietary fiber content of peanuts is approximately 7 percent by weight. The percentage of edible fiber is 3.3 (Ockerman 1991a), and of water-soluble fiber 0.77. The latter two percentages were determined by enzymatic figures (Deutsche Forschungsanstalt für Lebensmittelchemie 1991).
In their raw state, peanuts are very low in sodium. Unsalted dry-roasted nuts, and “cocktail” nuts, contain no sodium in a 1-ounce serving. However, whole peanuts are usually served salted. A 1-ounce serving of lightly salted peanuts contains less than 140 milligrams of sodium, which is the U. S. Food and Drug Administration’s current definition of a low sodium food. But other peanut products, such as “regular salted” nuts, contain higher amounts of sodium.
Vitamins and Minerals
Peanuts are good sources of riboflavin, thiamine, and niacin, and fair sources of vitamins E and K. They are also relatively high in magnesium, phosphorous, sulfur, copper, and potassium.
In the case of niacin, peanuts are rich sources of tryptophan (an essential amino acid that can be converted into niacin) and, in addition, are relatively rich sources of preformed niacin itself, a 1-ounce serving providing 20 percent of the U.S. RDA.
Nutrients and Processing
Under processing conditions in developing countries, dry roasting preserves both the storage stability of peanuts and their nutritional value to a greater extent than oil roasting (DaGrame, Chaven, and Kadam 1990). With roasting, the thiamine content decreases and the color darkens; hence color gives an indication of the extent of thiamine loss. The proteins, vitamins (except thiamine), and minerals are very stable during processing. But blanching or mechanical removal of skin further reduces thiamine content because thiamine is concentrated in the skins (Woodroof 1966).
Enhancing Protein Quality
Plant proteins, like those in peanuts, which are rich in essential amino acids and nitrogen and low in only a few amino acids, help improve the overall quality of diets, especially diets based on plant proteins. Protein supplementation involves adding to the diet small amounts of a protein that is a rich source of those amino acids that would otherwise be lacking. Protein complementation involves combining protein sources so that they mutually balance each other’s excesses or deficiencies (Bressani 1977).
These principles have been used to produce cereal-legume multimixes for humans (Bressani and Elias 1968), and the cuisines of several countries that have traditionally relied on plant protein foods as staples also employ these same principles to good effect, so that protein quality is rarely a problem.
A Weaning Food in Cereal Multimixes
Infants and young children, as weanlings, are growing rapidly and require plenty of high-quality protein. Yet, for cultural and economic reasons, protein-rich animal foods are frequently not readily available in many developing countries. A quarter of a century ago, cereal and cereal-legume multimixes (including peanuts) began to be used to provide a high-protein and high-calorie weaning food for children in this age group. These multimixes can be produced at the local level, are economical, and have excellent results in supporting child growth.
Similarly, many protein-rich cereal- and legume-based foods containing peanuts are now in wide-spread use in developing countries for alleviating problems associated with protein calorie malnutrition. Peanuts, which are rich in oil and in protein, and are also tasty, are particularly valuable for these purposes (Singh and Singh 1991).
It is unfortunate that peanuts are not for everyone. The cotyledons, axial germ tissue (hearts), and skin of peanuts contain allergens, and some, but not all, of these are still present after roasting. Because the allergens do not have a characteristic odor or flavor, they cannot easily be detected by peanut-sensitive individuals; thus, labeling of peanut-containing products is essential, save for pure peanut oil, which is not allergenic.
The many different peanut allergens that exist all contain protein. These allergens have been demonstrated by the use of radioallergenabsorbent test (RAST) inhibition, and by immunologic techniques, such as crossed radioimmunoelectrophoresis (CRIE), two-dimensional electrophoresis, and immunoblotting to isolate and characterize the peanut allergens. Then sera from peanut-sensitive individuals is used to determine if specific IgE binding to the various isolated subfractions exists. Since the isolation and characterization methods may affect the physical structure of the protein or its subfractions, different techniques may give different results. Nonetheless, at present, it is clear that multiple allergens exist in peanuts.
A well-characterized peanut allergen recently identified in patients with atopic dermatitis and positive peanut challenges is called Ara h I (Arachis hypogaea I) in the official nomenclature (Burks et al. 1991). Highly atopic infants and children appear to be particularly likely to form IgE antibodies that respond to peanuts, as well as to other food proteins (Zimmerman, Forsyth, and Gold 1989). Such children begin producing IgE antibodies to respond to inhalant allergens during their first and second years of life; they are defined as highly atopic because their serum IgE levels are 10 times those of normal infants, and their RAST tests are positive on multiple occasions.
Diagnosis. Diagnosis of peanut allergy is difficult because standardized peanut extracts do not yet exist. A RAST can be used on the sera of already sensitive persons for whom a skin test would be dangerous. Double-blind placebo-controlled challenges are definitive but are not often needed. If such double-blind challenges are done, provisions need to be made to cope with emergencies that may arise if an anaphylactic reaction occurs. The allergenicity of hydrolyzed peanut protein must be further studied. It is not clear at what level of hydrolysis allergenicity is lost.
Prevalence of peanut sensitivity. Peanut sensitivity is less prevalent among humans than are, for example, sensitivities to milk and eggs, but peanuts are especially dangerous for a number of reasons. One important reason is that peanuts occur in small amounts in so many different foods and recipes, ranging from satay sauce and “vegeburgers” to main dishes and spreads, making it difficult for those who have the allergy to avoid them (Smith 1990).
The allergy occurs in vegetarians as well as omnivores (Donovan and Peters 1990), and cross-reactivity with other legumes appears to exist. In addition, individuals who are allergic to other foods besides legumes are sometimes allergic to peanuts. Finally, highly atopic individuals, such as asthmatics, and those who suffer from atopic dermatitis or from multiple other food allergies, are likely to be at particular risk.
Signs of peanut allergy. The signs of peanut sensitivity range from urticaria (hives) to angioedema and asthma, and occasionally even to anaphylaxis and death (Lemanske and Taylor 1987; Boyd 1989). Crude extracts of proteins in both raw and roasted peanuts, as well as purified peanut proteins, such as arachin, conarachin, and concanavalin A reactive glycoprotein, are all allergenic in some persons (Barnett, Baldo, and Howden 1983).
Natural history of peanut allergy. Allergic reactions to peanuts usually begin early in life and persist. Studies over a period of several years have now been completed on individuals who exhibited symptoms of peanut allergy in childhood after a double-blind, placebo-controlled challenge, and reacted positively to puncture skin tests at the same time. Most such individuals had avoided peanuts since diagnosis, but those who inadvertently ingested peanuts 2 to 14 years later had reactions. This, coupled with a continued skin reactivity to peanut extract in puncture tests, suggests that peanut-sensitive individuals rarely lose their sensitivity with time (Bock and Atkins 1989). Fatal reactions to peanuts can also occur after many years of abstinence (Fries 1982). Fortunately, peanut oil, at least the usual grade sold in the United States and Europe, which contains no detectable protein, is not allergenic (Taylor et al. 1982). Unfortunately, in other countries the oil may contain enough of the protein to cause allergic reaction.
Allergy treatment. Avoidance of products containing peanut protein is the surest way to avoid peanut allergy. But certain foods may be accidentally contaminated with peanut protein, so that even products supposedly peanut free may be dangerous. Because the prevalence of peanut allergy is high, both labeling and label reading are important. Treatment of peanut sensitivity with immunotherapy has not proved helpful. If a sensitive person does ingest peanuts, self-administered epinephrine may help.
Peanut anaphylaxis is a medical emergency (Sampson 1990). One common cause is consumption of a product containing deflavored and colored peanut protein reformulated to resemble other nuts (Yunginger et al. 1989). In one case, an “almond” icing that was actually made from peanuts led to a fatal reaction (Evans, Skea, and Dolovich 1988). Other possible hidden sources of peanuts are egg rolls, cookies, candy, pastries, and vegetable burgers. Chinese food and dried-food dressings that contain peanuts have also been causes of anaphylactic shock (Assem et al. 1990).
Peanut allergy is probably the major cause of food-related anaphylaxis in the United States. Only a few milligrams will cause reactions in some persons. Those who are at risk of anaphylactic reactions to peanuts should wear medic-alert bracelets and carry preloaded epinephrine syringes and antihistamines. If treatment is needed, repeated doses of epinephrine, antihistamines, corticosteroids, mechanical methods to open airways, oxygen, vasopressors, and intravenous fluids may be necessary to prevent a fatal reaction (Settipane 1989).
Cross-reactivity in allergenicity. Peanuts cross-react in vitro with other members of the Leguminosae family, especially with garden peas, chickpeas, and soybeans, although clinical sensitivity is not always observed (Toorenenbergen and Dieges 1984; Barnett, Bonham, and Howden 1987). Reactions to nonlegume nuts, however, are relatively rare among those allergic to peanuts.
Aflatoxins are naturally occurring environmental contaminants that often infest peanuts. These carcinogenic mycotoxins arise from a fungus (Aspergillus flavus) that colonizes peanuts under certain environmental conditions or improper storage conditions that permit fungal growth. The aflatoxins B 1 and B2are most common in peanuts.
Aflatoxin-contaminated food has been shown to be associated with liver cancer in both humans and several experimental animals. The problem appears to be most severe in Africa and other parts of the world where environmental and storage conditions favor the mold’s growth and where processing methods that could identify and eliminate contaminated seeds are still not in place. But it is also a problem in the Orient, where people prefer the flavor of crude peanut oil. This form of the oil is high in aflatoxins.
In highly industrialized countries, aflatoxin contamination generally occurs before the harvest; in developing countries, contamination during storage is an additional and seemingly greater problem. In the former case, insect damage to the pods and roots permits seed contamination by the mold, especially during growing seasons where there is a late-season drought, which increases stress on the plant and also helps to exclude competition by other fungi. In a three-year study it was found that pods are more susceptible to contamination than roots (Sanders et al. 1993). Shriveled peanuts have the highest content of aflatoxin B1 (Sashidhar 1993).
Under favorable environmental conditions, Aspergillus flavus produces millions of spores, which can be carried by the wind for miles (Cleveland and Bhatnagar 1992). Insect damage promotes Aspergillus flavus growth when the spores land at a site of insect injury and when the moisture content exceeds 9 percent in peanuts, or 16 percent in peanut meal, and temperatures are 30 to 35° C. Therefore, the only sure method to avoid aflatoxins is to prevent their formation by careful harvesting and quick drying and storage. Food additives, such as sodium bisulfite, sorbate, proprionate, and nitrite, reduce aflatoxin production. Also, certain food components and spices, such as peppers, mustard, cinnamon, and cloves, may inhibit mycotoxin production (Jones 1992).
Visual screening of peanuts will reveal the conidial heads of the Aspergillus flavus fungus. Another technique is to screen unshelled nuts for the presence of bright greenish yellow fluorescence (BGYF) under ultraviolet light, using electronic color-sorting techniques. However, neither of these techniques screens out all aflatoxin contamination. For greater precision, chemical tests are used that include thin layer chromatography (TLC) and high performance liquid chromatography (HPLC) (Beaver 1989).
Immunological methods, such as ELISA or affinity column methods, are also useful, although precautions must be taken in performing the analysis since aflatoxins are highly carcinogenic (Wilson 1989). Much research is now in progress on the development of standardized methods for determining aflatoxin in peanut products. Other research concentrates on eliminating the contamination or inactivating it.
Chlorine gas can inactivate one aflatoxin, aflatoxin B 1, and the resulting compounds do not appear to be mutagenic (Samarajeewa et al. 1991). Ammonia and ozone treatments of peanuts also appear to work. As yet, however, these methods are still experimental.
The liver cancer in question is thought to be caused by hepatitis B, with other factors, such as aflatoxin, acting as promoters or as less-potent initiators. In most case-control studies, primary hepatocellular carcinoma is highly associated with antibodies to hepatitis B and C virus; other risk factors, such as peanut consumption (presumably with aflatoxin contamination), smoking, and drinking, are less highly associated (Yu et al. 1991). A recent large study in mainland China showed high correlations of liver cancer with hepatitis B surface antigen (HBsAg+) carrier status, whereas lesser associations were seen with alcohol use and cadmium of plant origin, but none with the measure of aflatoxin exposure that was used (Campbell et al. 1990). However, smaller studies, especially of peanut oils contaminated with aflatoxin, continue to suggest that aflatoxin may be involved in hepatobilliary cancers (Guo 1991).
Aflatoxin: Food safety. In recent years, the European Community has increasingly collaborated on food safety tests, and a great deal of effort has been devoted to developing more sensitive, rapid, and standardized methods for detecting aflatoxin levels in peanut products (Van Egmond and Wagstaffe 1989; Patey, Sherman, and Gilbert 1990; Gilbert et al. 1991). Thanks to this collaboration, standardized methods and materials are now available for assuring valid and reliable food safety testing. Current guidelines are that no more than 20 parts per billion of aflatoxin are permitted.
Developing countries and the aflatoxin problem. As we have mentioned, the problem of aflatoxin contamination is particularly serious in developing countries because of storage and processing problems. Thus, by way of example, aflatoxin levels in Pacific countries such as Fiji and Tonga are high, and so are liver cancer rates. Therefore, in addition to improving inspection methods, techniques must be developed for decreasing the carcinogenicity of aflatoxin-contaminated foods for humans and for animals. One technique that has been helpful in reducing the carcinogenicity of aflatoxin-contaminated groundnut cakes is ammoniation (Frayssinet and Lafarge-Frayssinet 1990). However, more such solutions are needed.
Because peanuts are a low-acid food, incorrect commercial canning of peanuts can cause botulism, as a recent case in China showed (Tsai et al. 1990). On a more positive note, peanuts are often used by dentists as foods for masticatory tests for jaw and muscle function (Kapur, Garrett, and Fischer 1990), because they are easy to standardize and familiar to patients (Wang and Stohler 1991). Moreover, roasted peanuts are among the least likely of popular snack foods to cause dental caries (Grenby 1990). Peanuts remain available dry, oil roasted, unsalted, lightly salted, or salted, as well as in a variety of flavors today in the United States (Nabisco Foods Group, Planters Division 1990). And finally, the not fully mature (or green) peanuts can be used as a vegetable that is much appreciated in the southern United States. Green peanuts are boiled in a weak brine solution and usually consumed immediately. If refrigerated, they will last for five days. They can also be frozen or canned.