Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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VODKA

C F Hayman, Hayman Limited, Witham, Essex, UK

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 The exact origin of vodka is unknown, although both

Russia and Poland claim to be the first producers.

Vodka was distilled in Russia in the early twelfth

century and, as with other spirit drinks, began its life

more for medical purposes rather than for enjoyment.

Origin

0002 The word vodka or wodka means ‘little water’.Itis

derived from the Russian word voda. Today the Rus-

sians refer to it as vodoshka. Although the origins of

vodka may date back some eight centuries, it is gen-

erally accepted that vodka as we know it today first

appeared during the sixteenth century.

Characteristics of Production

0003 Vodka is a fairly tasteless and neutral spirit product

which is mainly derived from the fermentation and

distillation of grain. However, some producers will

use other raw materials such as potatoes, sugarbeet,

grapes, or cassava instead of grain, depending on the

local availability and cost. Once the grain or similar

has been converted into neutral alcohol by fermenta-

tion and distillation, the vodka producer will start the

process. Different producers have developed their

own methods of production.

0004 Some distillers will further distil or rectify the

spirit, retaining only the middle part of the distillation

for additional processing. The foreshot and feints are

disregarded. The spirit will be passed through char-

coal or carbon filters which remove the remaining

flavors and odors to give vodka its clean and smooth

character.

0005 The various producers have developed their own,

often secret, ways of filtering but are very concerned

about the contact time of the spirit with the charcoal.

Some distillers have a single filter (Figure 1) while

others require the spirit to pass through a series of

filters. The choice of charcoal varies as it affects the

final product. One producer uses charcoal specially

made from Sussex oak trees; another uses carbon de-

rived from peat found in the Dutch polders. After

filtration, some distilleries will regenerate the charcoal

using steam whilst others discard the spent charcoal.

0006The character of the water added to the finished

vodka to reduce it to the required bottling strength is

very important. The increased technology of water

demineralization plants allows a greater consistency

in quality than ever before. Water quality is of a vital

significance in the final taste of the product.

0007In the European Community, the minimum level of

alcohol acceptable for vodka is 37.5% (v/v), but often

higher-strength vodkas are available, particularly

coming from eastern Europe, e.g., 45–55%.

0008Many vodkas are flavored and this again origin-

ated from both Poland and Russia. One Polish vodka

is flavoured by steeping buffalo grass (Hierochloe

odorata) in the product to give it an additional aro-

matic flavor, and it also takes on some of the green

color from the grass. An old Russian vodka is pre-

pared with an infusion of leaves from apple and pear

trees, to which is added brandy and port-type wine.

This vodka is mellow in taste and pleasing to drink.

Some high-strength vodkas are aged in wood, often

giving them a brown color derived from the cask. In

more recent times, Pepper vodka has been produced,

Active

carbon

Fine gravel (2−5 mm)

Coarse gravel (6−10 mm)

Deaerating

Vodka outlet to tank

Steam inlet

(7 psi)

Pressure

gage

Handgrip

Deaerating

Alcohol supply

Drain

1.1

100 mm

Wire gauze

200 mm

Sieve

fig0001Figure 1 A typical vodka filter. Reproduced from Vodka, En-

cyclopaedia of Food Science, Food Technology and Nutrition, Macrae

R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

6068 VODKA

the pepper enhancing its aroma and giving it a slight

burning sensation on consumption. Other vodkas

flavored with the addition of lemon, peach, and

other fruit flavors are now appearing. (See Barrels:

Wines, Spirits, and other Beverages; Flavor (Flavour)

Compounds: Structures and Characteristics.)

See also: Barrels: Wines, Spirits, and Other Beverages;

Flavor (Flavour) Compounds: Structures and

Characteristics

Further Reading

Begg O (1998) The Vodka Companion. London: Quinted

Publishing Ltd.

Clutton D (1979) Brewers Guardian 108(10): 25.

Pridemore WA (2002) Vodka and violence: alcohol con-

sumption and homicide rates in Russia. American Jour-

nal of Public Health 92(12): 1921–1930.

Rose AM (1977) Alcoholic Beverages. Economic Micro-

biology Series, vol. 1. London: Academic Press.

VODKA 6069

Wafers See Biscuits, Cookies, and Crackers: Nature of the Products; Methods of Manufacture; Chemistry of

Biscuit Making; Wafers

WALNUTS AND PECANS

R B N Prasad, Indian Institute of Chemical Technology,

Hyderabad, India

Introduction

0001 Many of the species of trees in the family Juglanda-

ceae produce edible, oil-rich nuts, and these include

walnuts and pecans. The valuable part of the com-

mercial crop is the whole nut. This article reviews

the varieties, global distribution, commercial import-

ance, morphology, and anatomy of the fruits and

seeds, chemical and nutritional composition, hand-

ling, and storage of walnuts and pecans.

Walnuts

Varieties and Global Distribution

0002 Walnuts are common, large, forest deciduous trees

primarily of temperate areas, but also of subtropical

regions, mostly of eastern North America, Central

America, western South America, and Eastern Asia.

Walnuts belong to the genus Juglans, which consists

of about 15 species.

0003 The most important species, producing the com-

mon, Persian, or English walnut, is J. regia L. It was

probably moved by migrating populations from an-

cient Persia to Greece and later distributed through-

out the Roman empire. The early colonists from the

UK brought seeds to North America, and the settlers

called the resulting trees ‘English walnuts’ to distin-

guish them from the native American black walnuts.

The English walnut is cultivated commercially in

the USA, France, Italy, former Yugoslavia, Poland,

Germany, Czech Republic, Bulgaria, Turkey, Chile,

Northern India, China, and Australia.

0004The second principal variety is the black walnut

(J. nigra L.), which is native to the USA and is found

distributed along the coast, west to Michigan and

throughout most of the midwestern states.

0005A third walnut variety, less important from a com-

mercial standpoint, is the butternut (J. cinerea L.).

This tree is native to North America and is found in

the eastern regions.

0006Other popular species of walnut trees include

the following: Arizona (or Nogal) walnut, J. major

Helier (distributed in New Mexico and Arizona);

the California walnut, J. californica Walt. (southern

California); the Chinese walnut, J. cathayensis Dode

(central and western China); the Hinds walnut,

J. hindsii Sarg. (California); the Texas walnut,

J. rupestris Engelm. (south-western USA to Mexico);

the Manchurian walnut, J. mandshurica Maxim.

(eastern Commonwealth of Independent States and

northern China); and the Japanese walnut, J. sieboldi-

ana Maxim. (Japan). Some of these species are

favored for garden and park plantings.

Commercial Importance

0007Although the most important and best known com-

mercial species is undoubtedly the English walnut,

many commercial varieties of different species are

grown around the world. China is the leading produ-

cer of walnuts, at 300 000 mt in the year 2000, out of

1 183 898 mt of the world production. The other

important walnut-growing countries are the USA

(222 260 mt), Iran (142 906 mt), Turkey (122 000),

India (31 000 mt), France (27 780 mt) and Romania

(25 000 mt). The nuts of all species are edible, and

the young green fruits, before the nuts harden, are

also eaten pickled in vinegar. Mature nuts can be

eaten raw or salted, and are excellent for dessert,

in icecreams and candy, and also in baking and

confectionery. Walnuts are a part of holiday celebra-

tions around the world and especially in places with

German tradition. Families sit around the table and

crack walnuts, eating the fresh meats. Although the

black walnut has a richer flavor than the English

walnut, it is not as popular because its shell is much

thicker and harder to remove without breaking the

kernels. The kernel forms 42–60% of the nut. About

300–600 t of walnut oil is produced from waste ma-

terial from shelling and processing plants, e.g., the

inedible kernels rejected during shelling and frag-

ments of kernels recovered from shells. Walnut oil

makes a good salad oil, and can also be used in the

preparation of soap, and as a drying oil in paints.

Defatted walnut cake is useful as an animal feed.

0008 The walnut shells, reduced to flour of various mesh

sizes, have found applications as fillers in synthetic

resin adhesives, plastics, and industrial tiles, and as an

insecticide diluent. About 50% of the annual produc-

tion of walnut shell is used in the plastics industry

alone. The shell powder is also used for drilling mud

in oil fields, and as an abrasive in polishing metal

castings. Dyes prepared from walnut shells and leaves

are still used in Turkey, and the high tannin content of

green walnut shells is useful in leather manufacture. A

pharmaceutical grade of activated charcoal can be

prepared from the inedible parts of the fruit. Various

parts of the walnut tree, such as bark, leaves, shells,

fruits, and kernels, have been used in folk medicine

throughout the world. The timber of walnuts pos-

sesses excellent mechanical strength and very good

shock resistance, and hence it is much valued for

veneers, furniture, boat-building, and gunstocks.

Morphology and Anatomy of the Fruits and Seeds

0009 Walnuts are usually tall, broad-headed, deciduous

trees that grow 10–50 m in height. They have rough

bark, and compound pinnate leaves that exude an

aromatic fragrance when crushed or bruised, a prop-

erty which varies in degree from one species to

another.

0010 The English walnut is a large, deciduous tree

attaining over 30 m in height and with a trunk of

6 m in girth; the bark is gray, smooth on young

trees, and fissuring with age. The staminate (male)

flowers of this species are borne on twigs of the

previous season’s growth, and the pistillate (female)

flowers, usually few, are borne on the twigs of

the current season’s shoots. Pollination occurs natur-

ally via the wind, but numerous cultivated varieties

have recently been developed by other means. The

green fruit is 3.7–5.0 cm in length with a subglobose,

gland-dotted, and glabrous structure. The fruit is a

drupe, and consists of an outer layer and an inner nut,

inside which is the single seed containing abundant

oil. When the seed is ripe, the husk opens and the

ovoid-shaped stone falls to the ground. The surface of

the stone is wrinkled, showing the suture of the

carpels in the median vertical plane. The walnut has

an outer green husk which is a perfect seal from

insects. Inside there is a hard shell. Nature has created

a perfect package. The seed consists mainly of two

large cotyledons storing fatty substances.

0011The macroscopic and microscopic structures of the

walnut (J. regia) seed are given in Figure 1a–c. Most

of the endocarp consists of stone cells with brown

walls. The outermost of these have very thick walls

with slit-like pits, but towards the interior the walls

are thinner, with relatively large pits. Inside the endo-

carp there is a zone of flattened parenchyma cells with

brown walls. The outer epidermis of the testa consists

of rather radially elongated cells up to 90 mmin

length. In some places, this layer is biseriate; pigment

is found in some of these cells. It has been reported

that raised stomata are present in the outer epidermis.

The rest of the testa consists of three or four rows of

flattened parenchyma cells with brown walls, and

also some vascular tissue. The endosperm is normally

represented by a single layer of cells, up to 16 mmin

radial diameter, with small aleurone grains and oil

drops. The parenchyma cells of the cotyledon possess

intercellular spaces and oil drops, and contain aleur-

one grains, up to 10 mm in diameter (Figure 1a–c).

0012The black walnut tree grows up to 50 m in height

and has brownish bark with fissuring. The drupelike

fruits are solitary or in pairs, 2.5–5.0 cm in length,

globose, oblong and pointed at the apex, or slightly

pyriform with a thick husk. The nut is oval or oblong,

slightly flattened, 3.0–3.8 cm in diameter, deeply

divided on the outer surface into thin or thick, often

interrupted, irregular ridges.

Chemical and Nutritional Composition

0013Chemical characteristics of oil Walnut oil is gener-

ally produced from the inedible meat rejected during

shelling, plus meat fragments recovered from the

shell. The oils from the different species of walnut

bear a close resemblance to each other in terms of

both physical and chemical characteristics. They are

light yellow in color with a greenish tinge and a faint,

nutty odor. The range of chemical characteristics of

walnut oil is given in Table 1.

0014Fat content Walnut kernels generally contain about

60% oil, but vary from 52% to 70% depending on

the variety. The major constituents of the oil are

triacylglycerols. Free fatty acids, diacylglycerol,

monoacylglycerol, sterols, sterol esters, phospho-

lipids, and vitamins are present in minor quantities.

The triacylglycerol moiety of the oil is a mixture of

6072 WALNUTS AND PECANS

(a)

(b)

cot

end

par

cot

end

(d) (e)

fig0001 Figure 1 (a–c) Walnut (Juglans regia). (a) Nut. (b) Longitudinal section of nut showing: end, endocarp; par, partition; cot, cotyledone,

t, testa. (c) Transverse section: abbreviations as in (b); es, endosperm;  250. (d–f) Pecan (

Carya illinoensis). (d) Nut. (e) Longitudinal

section of nut; (f) Transverse section of endocarp, testa, endosperm, and cotyledon;  270. Reproduced from Vaughan JG (1970)

Juglandaceae. In: The Structure and Utilization of Oilseeds, pp. 113–116. London: Chapman and Hall, with permission.

WALNUTS AND PECANS 6073

triunsaturated and nonsymmetrical diunsaturated

glycerides that form up to 83–95% of the total frac-

tion. The 2-position of the triacylglycerol contains

primarily linoleic acid. The fatty acids of the walnut

oil are predominantly unsaturated and consist mainly

of linoleic and oleic acids (Table 2). The linoleic acid

content in English walnut is higher than that in black

walnut. (See Fatty Acids: Properties; Triglycerides:

Structures and Properties.)

0015 Walnut oil contains about 0.9–1.3% phospho-

lipids; the composition of individual components

of total phospholipids is: phosphatidylcholine, 18–

20.4%; phosphatidylinositol, 28.2–45.0%; phospha-

tidylethanolamine, 15–30.5%; cardiolipin, 0–20.4%;

phosphatidylserine, 0–5%; phosphatidic acid, 0–7%;

lysophosphatidylcholine, traces to 2.0%; lysopho-

sphatidylethanolamine, traces to 2.0%.

0016 Walnut oil contains at least 29 volatile compon-

ents, such as terpenes, alcohols, and carbonyls, and

the characteristic odor of the oil is attributable to the

collective effect of a number of constituents. (See

Sensory Evaluation: Aroma.)

0017 Protein content Walnut kernels contain about 14.5–

24% of protein, and this is enriched up to 61–66% in

dry defatted cake. Full details of the nature of the

protein are not available, but the molecular weight

of the three main proteins was estimated to be about

43 000, 39 000, and 25 000. The content of the sulf-

hydryl and disulfide bonds in the walnut protein was

reported to be higher than that of soybean and yeast

protein. Walnut cake contains more arginine and less

lysine than does casein and soybean cake. The walnut

kernel protein contains 18 amino acids, and glumatic

acid (2.809 g per 100 g of kernel) is the main amino

acid, followed by arginine (2.103 g per 100 g of

kernel). The essential amino acid composition is

given in Table 3.(See Amino Acids: Properties and

Occurrence.)

0018In addition to the normal amino acids, walnut

kernels are found to contain a sulfur amino acid,

taurine (2-aminoethanesulfonic acid) in concentra-

tions of about 15–46 nmol g

1

, depending on the

variety. In humans, taurine deficiency may lead to a

tbl0001 Table 1 Chemical characteristics of walnut and pecan oils

Property Walnuts Pecans

Specific gravity (at 15



C) 0.919–0.929 0.911–0.914 (25



Refractive index (n



C) 1.469–1.478 1.469–1.471

Acid value 0.5–10.0 1.1–10.5

Saponification value 189–197 189–198

Iodine value 132–162 77–106

Polenske value 0.2–0.3 0.3

Unsaponifiable

matter (g 100 g

1

)

0.2–1.0 0.35–0.67

Solidification point (



C) 12 to 28 18

Data compiled from: Krischenbauer (1960) In: Fats and Oils ^ An Outline of

their Chemistry and Technology, pp. 198–203. New York: Reinhold; Eckey EW

(1954) In: Vegetable Fats and Oils, pp. 378–382. New York: Reinhold;

Jamieson GS (1943) In: Vegetable Fats and Oils, pp. 158, 333–334. New York:

Reinhold; Mehran M (1974) Oil characteristics of Iranian walnuts. Journal

of the American Oil Chemistry Society 51: 477–478; Council of Scientific and

Industrial Research (1959) The Wealth of India ^ Raw Materials,vol.V,

pp. 298–305. New Delhi: Council of Scientific and Industrial Research.

tbl0003Table 3 Essential amino acid component of walnut and pecan

kernels

Amino acid Amino acidcontents (g 100 g

1

)

Walnuts Pecans

Ariginine 2.103–3.661 0.848–1.185

Histidine 0.359–0.680 0.198–0.273

Isoleucine 0.566–0.978 0.322–0.533

Leucine 0.992–1.704 0.520–0.773

Lysine 0.285–0.721 0.287–0.435

Methionine 0.247–0.473 0.112–0.186

Phenylalamine 0.628–1.107 0.392–0.564

Threonine 0.230–0.589 0.253–0.389

Tryptophan 0.175–0.321 0.093–0.199

Valine 0.723–1.286 0.345–0.525

Data compiled from: USDA Database for Standard Reference Release 11

September 1996 and Release 13 November 1999; Considine PEDM and

Consodine GD (eds) (1982) Pecan and walnut. In: Foods and Food

Production Encyclopaedia, pp. 1345–1350. New York: Van Nostrand

Reinhold; Young CT (1996) Nuts. In: Kroschwitz J and Howe-Grant M (eds)

Kirk-Othmer Encyclopaedia of Chemical Technology, 4th edn, vol. 17, pp.

544–579 New York: John Wiley.

tbl0002Table 2 Fatty acid composition of walnut and pecan oils (g per

100 g total fatty acids)

Fattyacid Walnuts Pecans

Myristic 0–2

Palmitic 2.9–8.2 3.3–7.1

Palmitoleic 0–0.1

Stearic 0.6–2.2 0.9–5.5

Oleic 9.0–39.2 40.6–88

Linoleic 42.5–76 13.5–38

Linolenic 2.0–18.0 0.8–2.0

Gadoleic 0–0.4 0.21–0.25

Data compiled from: Mehram M (1974) Oil characteristics of Iranian

walnuts. Journal of the American Oil and Chemistry Society 51: 477–478;

Jamieson GS and Mekinney RS (1936) Fatty acid composition of black

walnut oil. Oil and Soap 13: 202; Rockland LB and Benedict CD (1970)

Occurrence of fatty acid methyl esters in walnut kernel and other oils.

Journal of Agriculture and Food Chemistry 18: 223–228; Savage GP, Dutta PC

and Mc Neil DL (1999) Fatty acid and tocopherol contents and oxidative

stability of walnut oils. Journal of the American Oil and Chemistry Society 76:

1059–1063; Hilditch TP and Williams PN (1964) In: the Chemical Constitution

of Natural Fats, 4th edn; London: Chapman & Hall. Young CT (1996) Nuts. In:

Kroschwitz J and Howe-Grant M (eds) Kirk-Othmer Encyclopaedia of

Chemical Technology, 4th edn, vol. 17, pp. 544–579. New York: John Wiley;

French RB (1962) Analyses of pecan, peanut and other oils by gas

chromatography and ultra violet spectrophotometry. Journal of the

American Oil and Chemistry Society 39: 176–181; Jamieson GS (1943)

Vegetable Fats and Oils, p. 158. New York: Reinhold; USDA Database for

Standard References Release 11 September 1996 and Release 13

November 1999.

6074 WALNUTS AND PECANS

decreased electroretinogram (ERG) and to pigmen-

tary degeneration of the retina. Hence walnut kernels

have the potential to replace meat as a source of

dietary taurine. (See Protein: Requirements.)

0019 Carbohydrate The total carbohydrate content in

different varieties of walnut varies from 12.1 to

18.3 g 100 g

1

kernels. English walnut is rich in

carbohydrate compared to the black variety. The

fiber content in walnut varieties is about 1.7–6.5 g

100 g

1

kernel. Some varieties of walnut kernels con-

tain large amounts of sugar(s). (See Carbohydrates:

Classification and Properties.)

0020 Vitamins Walnut kernels are a good source of vita-

mins. Walnuts contain about 30–300 IU of vitamin A,

0.22–0.45 mg thiamin, 0.10–0.16 mg riboflavin, and

0.7–1.105 mg niacin 100 g

1

of kernel. The richest

sources of vitamin C are the immature fruits or their

green hulls, and the leaves of walnuts. Unripened

walnuts are reported to have a very high vitamin C

content (2400–3700 mg 100 g

1

), displaying an activ-

ity 40–50 times as high as that of oranges and lemons.

However, the dry walnut kernels are reported to con-

tain only 0.4–8.5 mg 100 g

1

after ripening. Reports

of large amounts of vitamin C in green walnuts have

yet to be fully confirmed.

0021 Walnuts contain about 7.3–28.7 mg a-tocopherol,

1.0–8.2 mg b-tocopherol, 205–375.8 mg g-tocopherol

and 28.0–62.1 mg d-tocopherol g

1

of its oil. Walnut

kernels are also found to contain vitamin K

(0.9–1.1 mg 100 g

1

). Refer to individual vitamins.

0022 Phenolic acids In food products, the presence of

phenolic acids has been associated with astringency,

discoloration, inhibition of enzyme activity, and anti-

oxidant properties. Defatted kernels are found to

contain some phenolic acids, namely phenylacetic,

protocatechuic, syringic, vanillic, gallic, caffeic, and

ferulic acids, in very small quantities ranging from

0.02 to 0.20 mgg

1

of kernels. (See Phenolic Com-

pounds.)

0023 Pectic substances Walnuts contain about 2% (dry

matter in the edible portion) pectic substances

(expressed as anhydrogalacturonic acid). Pectic sub-

stances, which make up a component of dietary fiber,

are basically polymers of galacturonic acid and may

have important physiological and nutritional conse-

quences, such as hypocholesterolemic effects, in-

creased excretion of fecal sterols, and the capacity to

bind bile salts. Pectic substances may also slow down

the absorption of soluble carbohydrates, causing a

smaller increase in the postprandial level of blood

sugar; hence walnut kernels could play a role in the

design of pectic-rich diets if required for medical

reasons.

0024Juglone Juglone (5-hydroxy-1,4,-naphthoquinone),

a yellow-brown compound, was shown to be a host

factor that may be associated with resistance of

walnut to scab caused by fungus. Juglone is present

in considerable amounts in all green and growing

parts of the trees and unripe hulls of the nuts. Juglone

was also reported in the surface waxes of English

walnut leaves (1800 mgg

1

of fresh leaf) and fruits

(97 mgg

1

of fresh fruit) along with hydrocarbons,

wax esters, aldehydes, fatty alcohols, and fatty

acids. However, juglone levels in kernels was either

very low or absent.

0025Minerals Walnuts are considered to be a good

source of dietary minerals. English and black walnuts

contain almost similar quantities of minerals, except

calcium, which is present only in black walnuts.

Potassium, phosphorus, magnesium, and iron are

found in significant quantities in these nuts. The min-

eral composition of walnuts is given in Table 4. Refer

to individual minerals.

Nutritional Quality

0026Walnut kernels exhibit high nutritive value compared

to cereals and legumes and therefore find a great

tbl0004Table 4 Minerals of walnut and pecan kernels

Minerals Mineral content (mg 100 g

1

)

Walnuts Pecans

Calcium Traces–108 21–139

Copper 0.31–3.2 0.8–1.4

Iron 2.1–7.6 1.9–4.8

Magnesium 131–169 120–170

Manganese 2.4–2.9 1.7–5.3

Phosphorus 309–570 271–610

Potassium 328–687 330–660

Sodium 1–16.5 0–1

Sulfur 104–140 0–113.0

Zinc 2.3–3.6 5.3–10.4

Data compiled from: Senter SD (1976) Mineral composition of pecan nut

meats. Journal of Food Science 41: 963–964; Hammar HE and Hunte JH

(1946) Chemical composition of pecan nut kernels. Plant Physiology 21:

476–478; Young CT (1996) Nuts. In: Kroschwitz J and Howe-Grant M (eds),

Kirk-Othmer Encyclopaedia of Chemical Technology, 4th edn, vol. 17,

pp. 544–579. New York: John Wiley; Considine PEDM and Consodine GD

(eds) (1982) Pecan and walnut. In: Foods and Food Production

Encyclopaedia, pp. 1345–1350. New York: Van Nostrand Reinhold; Council

of Scientific and Industrial Research (1959) The Wealth of India ^ Raw

Materials, vol. V, pp. 298–305. New Delhi: Council of Scientific and

Industrial Research; Composition of Foods, Agriculture Hand Book no. 8

(1963) Agriculture Research Service, US Department of Agriculture;

Cristina VP and Maria ETI (1981) Mineral elements in walnuts. Anales

Bromatologia 33: 69–76; USDA Database for Standard References Release

11 September 1996 and Release 13 November 1999.

WALNUTS AND PECANS 6075

importance in food industry. A high level of fat, con-

taining unsaturated fatty acids with considerable

amounts of natural antioxidants like tocopherols

and phenolic acids, enhances its importance as a

good edible oil source. Defatted walnut kernels are

useful as a dietary supplement as they contain a very

high content of digestible protein and all the essential

amino acids, taurine, and significant amounts of vita-

mins and minerals. Walnuts are claimed to have a

hypocholesterolemic effect on the basis of richness

in magnesium and fiber, and a low lysine and arginine

ratio. The positive nutritional advantages of walnuts

in lowering blood cholesterol come from the high

levels of mono- and polyunsaturated fatty acids and

possibly the tocopherol content. A diet that includes

moderate quantities of walnuts without an overall

increase in total dietary fat and calories decreases

serum cholesterol levels and favorably modifies the

lipoprotein profile in normal men. Walnut kernels

are a good source of energy and provide about

2686 kJ 100 g

1

.(See Energy: Measurement of

Energy Expenditure.)

Industrial Uses of Oil

0027 Walnut oil is similar to linseed oil in composition, and

its drying properties have been extensively used in

paints and artists’ colors from early times. In com-

parison with black walnut oil, English walnut oil

contains more linoleic acid and has more active

drying properties. Walnut oil is also used in the

manufacture of soaps, varnishes, alkyd resins, and

styrenated oils. In addition, since the Second World

War, walnut oil has been consumed as a specialty

food oil.

Handling and Storage

0028 Rancidity is one of the first signs of deterioration in

walnuts because they are rich in oil containing unsat-

urated fatty acids. High temperature and humidity, as

well as sunlight, favor the development of rancidity in

kernels, and the best storage conditions are therefore

low temperature, low humidity, and little or no light.

Aflatoxin presents another problem. In general, afla-

toxin production occurs during the harvest period

after the nuts have begun to dry; the nuts must there-

fore pass quickly through this critical moisture zone.

Storage of nuts under proper temperature and humid-

ity will prevent further contamination, and there are

many well-established methods of destroying afla-

toxins in the kernels and oils. (See Mycotoxins:

Occurrence and Determination; Oxidation of Food

Components.)

0029 The optimum moisture level for the storage

of walnuts is 3.2–3.8%, and stability of the shelled

nuts is significantly reduced at higher or lower

moisture levels. Very elaborate machinery is used for

cleaning, grading, bleaching, cracking, and packaging

of edible tree nuts, including walnuts. Skin color of

the nuts is very important, and methods such as the

photometric estimation of absorbance of a methanol

extract of the whole nuts are useful for determining

the rate of darkening of walnuts at regular intervals

during storage. Walnuts are bleached for market by

dipping them for 5–10 s in an aqueous solution of

bleaching powder and sodium carbonate, with a

small amount of sulfuric acid to clear the liquid. In

another method, walnuts are exposed to ethylene

(one part ethylene to 1000 parts air) to preserve the

natural color of the kernel. This treatment prevents

the hulls from sticking and staining the shells. After

hulling and washing, the walnuts are placed in bins

and dried with warm air (not over 43



C) to reduce

the moisture content. The best storage conditions

for walnuts are 0–2



C, accompanied by ventilation

with air of relative humidity in the range 65–75%.

The storage methods adopted for pecan nuts are also

useful for walnuts. (See Preservation of Food; Storage

Stability: Parameters Affecting Storage Stability.)

Pecans

Varieties and Global Distribution

0030The species Carya illinoensis Koch, or C. pecan

Marsh, are known internationally as pecans. Botan-

ically pecan is classified as a hickery (carya) and is a

member of the walnut family (Juglandaceae). Native

to North America, the pecan is grown mainly in

southern and central USA and northern Mexico,

with limited cultivation in Australia, South Africa,

Brazil, and Israel. Commercially, well over 300 var-

ieties of pecan are offered. Pecan is one of the major

commercial edible nuts, and is produced in quantities

exceeding 153 500 t year

1

in 2000–2001 in the USA.

Commercial Importance

0031The nuts of pecan have been of value for human

consumption in North America for the last three

centuries. Pecan is next in importance to the English

walnut for its valuable nuts; hence more and more

orchards are being planted, not only in southern USA,

but also in other countries. The coarse, brittle wood is

not as strong as walnut, but it is used occasionally to

make tool handles and agricultural implements, and

as fuel.

0032The nuts are eaten raw or salted, and are excellent

for dessert, icecreams, cakes, bread, candies and other

confectionery, and in vegetarian croquettes and sand-

wiches. Pecans have a mild, sweet, walnut-like flavor.

6076 WALNUTS AND PECANS

About 300 t of pecan oil is produced annually from

the pecan-shelling industry. The oil is refined and

used for edible purpose or for the production of

soap cosmetics and some drugs; the cake is used in

animal feed.

0033 Waste materials from the pecan-shelling industry

have shown commercial possibilities for the recovery

and production of oil, tannin, shell flour, and acti-

vated charcoal. Tannic acid produced from pecan

shells is used by the tanning industry and by the oil

industry to control the viscosity of drilling muds. The

shell flour of pecan, in common with that of walnut,

has many industrial applications.

Morphology and Anatomy of the Fruits and Seeds

0034 The pecan is a large, deciduous tree which grows up

to 9–12 m high – some specimens may reach a height

of 50 m – and the trunk can be up to 2 m in diameter,

with gray, furrowed bark.

0035 The male flowers are borne in three-branched pen-

dulous catkins, the female in (two to 10) spikes. The

fruit is a drupe (3–7 cm long) which, at maturity,

splits into four valves to reveal the smooth, brown

kernel. The nut is ovoid to ellipsoidal, nearly cylin-

drical or slightly four-angled towards the pointed

apex, rounded and usually apiculate at the base. The

brown shelled nut is about 3.5 cm in length and

is wrapped in a husk which usually splits and falls

away. If the husk does not break away, the nuts

are usually defective. The nuts contain 39–55%

meat, most of which can be separated in a stable

form, but a small portion (about 3%) remains with

the shell.

0036 The macroscopic and microscopic structures of the

pecan nut are shown in Figure 1d–f. The endocarp of

the pecan nut has essentially the same structure as the

walnut endocarp, except that some of the outermost

stone cells have diamond-shaped crystals in their

cavities. Some of the cells of the outer epidermis, in

a transverse section of the testa, are square-shaped

and others tangentially elongated, with a radial diam-

eter of up to 18 mm and a tangential diameter of

11–32 mm. Some of these cells contain a brown pig-

ment, which may also be found in the guard cells of

the stomata. The rest of the testa consists of vascular

tissue and parenchyma, some cells of which contain

pigment. The endosperm of the pecan nut is the same

as that of the walnut. The embryo is also similar, but

the aleurone grains are smaller, about 5 mm in diam-

eter, and there may be minute starch grains present

(Figure 1d–f).

Chemical and Nutritional Composition

0037 Chemical characteristics of oil On account of the

value of the nuts for edible purposes, pecan oil is

not prepared commercially. However, the unusable

fine nut fragments which accumulate at shelling

plants can be utilized for the production of oil. The

chemical characteristics of pecan oil are given in

Table 1.

0038Fat content Pecan kernels contain 60–75% oil de-

pending on the cultivars. Triacylglycerol forms more

than 95% of total lipids, while monoacylglycerols,

diacylglycerols, complex lipids, free fatty acids, and

sterols are present at levels of less than 1%. The fatty

acids of pecan oil are mainly unsaturated, with oleic

acid as the predominant fatty acid (unlike walnut oil,

in which linoleic is the major acid; Table 2).

0039Protein content The protein content of pecan

kernels varies from 7.8 to 17% depending on the

cultivar. The pecan kernel protein also contains 18

amino acids and glutamic acid is the main amino acid

(1.829 g 100 g

1

kernels), followed by arginine

(1.177 g 100 g

1

kernels) in pecan kernels. The essen-

tial amino acid content of the kernels is given in

Table 3.

0040Carbohydrate content The total carbohydrate and

fiber contents of pecan kernels are about 13.9–18.3

and 1.6–9.6 g 100 g

1

, respectively. The total redu-

cing sugar content is lower in pecan than most other

nuts; the amount of sucrose is 2.0 g 100 g

1

kernel.

0041Juglone Juglone is also present in all green and

growing parts of the trees and unripened hulls of

pecan nuts. However, the kernels contain minute

amounts of juglone.

0042Vitamins Pecan nuts contain about 77–130 IU vita-

min A, 0.66–0.86 mg thiamin, 0.11–0.13 mg ribofla-

vin, 0.89–1.167 mg niacin, and 1.1–2.0 mg ascorbic

acid 100 g

1

kernel. The nuts contain 170 mg

g-tocopherol and 15 mg a-tocopherol g

1

. The

d-isomer is present in minute quantities.

0043Minerals Although some minerals, such as copper,

iron, cobalt, and chromium, decrease the stability of

vegetable oils, many minerals are essential for normal

metabolic functions and are required components in a

balanced diet. Pecan kernels are rich in dietary min-

erals as they contain phosphorus, potassium, magne-

sium, and sulfur in fairly large quantities (Table 4),

and others, such as iron, copper, zinc, and molyb-

denum, in microquantities.

0044Tannins The integument of the pecan contains up to

25% tannin, and hence the crop is a very good source

WALNUTS AND PECANS 6077

of commercial vegetable tannin. Tannic acid pro-

duced from pecan shells is used by the tanning indus-

try. It is also used by the oil industry to control

the viscosity of drilling muds. (See Tannins and Poly-

phenols.)

0045 Phenolic acids Pecan kernels also contain phenolic

acids, namely gallic, gentisic, vanillic, protocatechuic,

p-hydroxybenzoic, and p-hydroxyphenylacetic acids.

Coumaric acid and syringic acids are present in trace

amounts. The phenolic compounds were located only

in the testa of the kernel. The total phenolic acid

content in the defatted kernels was found to be

about 171.4 mgg

1

; gallic acid was predominant and

constituted about 78% of those present. The content

of all the major phenolic acids except p-hydroxy phe-

nylacetic acid progressively decreases during storage,

and thus they appear to function as antioxidants to

maintain the chemical integrity of the kernel. (See

Antioxidants: Natural Antioxidants.)

Nutritional Quality

0046 Pecan nuts are mainly used as appetizers and add-

itions to salads, cakes, candies, and cookies or as

emergency rations for persons expending great

amounts of energy, rather than a basic diet. These

kernels are high in fat content with a calorie count

close to butter. Pecan kernels are nutritious and a

good source of energy and vitamins. Defatted kernel

powder is useful as a dietary supplement as it contains

all the essential amino acids and significant amounts

of minerals and natural antioxidants like phenolic

acids and tocopherols. The oil is used in cooking

and salads and has good keeping qualities. Pecan

kernels are a good source of energy and provide

about 2891 kJ 100 g

1

.(See Energy: Measurement of

Food Energy.)

Industrial Uses of Oil

0047 Like walnut oil, pecan nut oil is used in food, soap,

enamel paints, oil colors, and printing inks. The re-

fined oil is a clear, neutral, edible oil but is sold mostly

for special uses to the pharmaceutical, essential oils,

and cosmetics industries. In the foreseeable future,

pecan oil will remain a byproduct oil, since the main

part of the crop is too valuable in the form of nuts or

nut meats.

Handling and Storage

0048 More than 50% of pecan kernels offered for sale may,

in any one year, fail to meet minimum US Department

of Agriculture (USDA) requirements because of dis-

coloration, rancidity, and staleness. Hence it is very

important to harvest at the appropriate time, and

store the crop under proper conditions to maintain

the quality of the nuts.

0049Early harvest has distinct advantages as a means of

avoiding unfavorable weather conditions, and the

color and flavor scores of early advanced pecans

that are dried and refrigerated promptly change at

a much slower rate during storage than do late-

harvested crops. A moisture content of 4–5% is rec-

ommended to maintain the color.

0050Very elaborate machinery is used for cleaning,

grading, bleaching, cracking, and packing the pecan

nuts. In commercial shelling plants, in-shell pecans

are conditioned prior to cracking and shelling to im-

prove the yield of halves. In general, conditioning can

be achieved by soaking pecan nuts in large vats of

water containing chlorine at 1000 p.p.m. for 1–2h,

and then holding them for 12–24 h before cracking.

Another method used by processors is soaking the

pecans in water (85



C) for 3–5 min, followed by

holding for 12–24 h before cracking. The most effect-

ive process for improving the yield of halves from

shells, and for maintaining the quality of the kernels,

is the 3-min atmospheric steam process. Immediately

after cracking, the moisture content is reduced to less

than 4.5% for pecan halves and less than 3.5% for

pieces in order to maintain quality.

0051Acidification of pecan kernels with dilute acid so-

lutions significantly lightens the color of the kernels,

and phosphoric acid has proved to be the most effect-

ive. Dilute solutions of citric acid can also enhance the

color but, unlike phosphoric acid, they adversely

affect the taste. Treatment with dilute hydrochloric

acid, ammonia, and sulfur dioxide can improve the

color but responses are variable. The color of the

pecan kernels is considered a primary quality param-

eter of the shelled nuts and, in the USA, they are

graded into four color classes by the USDA: light,

light amber, amber, and dark amber.

0052Rancidity is the major problem for pecan kernels

during storage, as the oil is rich in unsaturated fatty

acids. The development of rancidity also influences

the flavor of pecans, and they can be kept for about

3–4 months only, at ordinary temperatures, without

developing rancidity. However, they may be stored,

soon after harvest, at 3



C for 1 year, without

developing off-flavors at 4.5



C for 3 years and at

17.5



C for more than 5 years. It is believed that

kernels do not become rancid more rapidly after

being removed from storage than fresh pecans ex-

posed to the same conditions.

0053In pecans, molds which are capable of producing

aflatoxins are of great concern to processors and

marketers because fungi like Penicillium, Aspergillus

and Fusarium species are capable of growing on

pecans at 0



C and, to a lesser extent, at 6–5



6078 WALNUTS AND PECANS