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0036 In the thick ascending limb potassium is mainly

reabsorbed. The mechanism of active transport

includes apical uptake by Na

-Cl



cotransport

and passive paracellular exit across the basolateral

membrane by diffusion. Driven by transepithelial po-

tential difference, reabsorption also occurs through

the paracellular pathway.

0037 The role of this limb in potassium secretion has also

been recognized. The K

secretion through apical K

channels is important for net Na

and Cl



reabsorp-

tion through Na

-Cl



cotransport, since luminal

and Cl



concentrations are higher than luminal

levels.

0038 Distal tubule and collecting duct These tubules are

the major determinants of urinary K

, since they are

able either to reabsorb or secrete potassium at rates

which depend on K

intake and other factors.

0039 Potassium secretion occurs in ‘principal cells’ by

active uptake across the basolateral membrane by

-ATPase and passive diffusion into the lumen

across the apical membrane by K

channels or using a

-Cl



cotransport (Figure 2). K

transport follows

this route towards the lumen thanks to a favorable

electrochemical gradient and the greater K

permea-

bility of the apical membrane in comparison with

the basolateral one. High Na

concentration in the

lumen favors K

secretion, as the entrance of Na

into the cell generates a potential difference that tends

to make the interior positive, so K

diffuses in the

opposite direction. Simultaneously, the enhanced de-

livery of Na

into principal cells facilitates the action

of Na

-ATPase, and the accelerated potassium

uptake favors its secretion towards the lumen.

0040 Potassium reabsorption mechanisms have been

detected in the intercalated cells of the distal tubule

and collecting duct. It appears that there is a compon-

ent of potassium reabsorption even during net secre-

tion. Several isoforms of H

-ATPases are able to

reabsorb K

in exchange for H

. The Na

-K

-Cl



contransport may also introduce K

into the cell

while the basolateral K

channels enable potassium

to pass from the cell into the interstitial space. Proton

secretion by H

-ATPase, together with that by

-ATPase, contributes to urine acidification. These

enzymes participate in the acid–base balance and

potassium status.

Potassium Balance and Homeostasis

0041 Intra- and extracellular potassium concentrations must

be maintained within narrow limits, despite wide fluc-

tuations in dietary intake. This is achieved, on the one

hand, by ‘internal balance’: distribution of K

between

the intracellular fluid and ECF carried out mainly by

-ATPase, and on the other hand, by ‘external

balance’: maintenance of the amount of K

in the body

through equilibrium between K

intake and excretion,

which is achieved principally in the kidney but also in

the intestine. After a meal, the absorbed K

rapidly

enters the ECF. The subsequent rise in plasma [K

]is

attenuated by a rapid cellular K

uptake (which occurs

within minutes). To maintain external K

balance, all

3Na

2Cl

−

2Cl

−

ATP

Reabsorption

secretion

Secretion

Reabsorption

Intercalated cell

Principal cell

(a)

(b)

(c)

Apical Basolateral

fig0002Figure 2 Principal mechanisms of potassium transport in the

major tubular segments of the nephron: (a) proximal tubule;

(b) thick ascending limb; (c) distal tubule and collecting duct.

ATP, adenosine triphosphate.

POTASSIUM/Physiology 4655

of the K

absorbed in excess must be excreted slowly

by the kidneys.

Potassium Homeostasis

0042 To maintain the equilibrium previously described,

several factors which act at different levels contribute

to potassium homeostasis (Figure 3).

0043 Internal balance A rise in plasma [K

], especially

with hyperkalemia, and the hormones insulin, epi-

nephrine (adrenaline: by activating b

-receptors),

and aldosterone promote K

uptake by muscle, livers,

bone, and red blood cells. In contrast, a decrease in

plasma [K

], such as with hypokalemia, and stimula-

tion of a-adrenergic receptors induce K

transport

from cells to the ECF. Moreover, hyperkalemia stimu-

lates insulin, aldosterone, and epinephrine secretions,

while hypokalemia has the opposite effect. These are

the major physiological factors involved in internal

balance homeostasis.

0044 Furthermore, other factors which are not normal

homeostatic mechanisms influence K

movements

across the plasma membrane: metabolic acidosis pro-

motes exit of K

from cells, whereas metabolic alkal-

osis favors its uptake. Increased osmolality of the ECF

enhances K

release by cells. Cell lysis (severe trauma,

burns, etc.) induces K

release and may produce

hyperkalemia. During intense exercise, K

is also

released from skeletal muscle cells.

0045 External balance There are two hypotheses con-

cerning the homeostatic mechanisms which regulate

external K

balance:

0046The first involves a peripheral mechanism without

central nervous system (CNS) intervention. In this

case [K

] would stimulate urinary K

excretion

and aldosterone secretion directly.

0047The second attributes K

regulation to a reflex

mechanism initiated by K

receptors in portal

vein and liver stimulated by high [K

]. Afferent

fibers would transmit signals to CNS –‘K control

center’–and efferent limbs, mediated by several

hormones and other factors, would modulate

urinary K

excretion in response.

0048In any case, plasma [K

] and aldosterone are the

major physiological regulators of K

excretion. High

] increases aldosterone secretion and both act

synergistically, stimulating K

elimination by the in-

testine (particularly increasing colonic secretion) and,

above all, by the kidney (promoting secretion in the

distal tubule and collecting duct). Renal secretion is

due to stimulation of the Na

-ATPase pump, in-

creased permeability of the apical membrane to K

and greater K

uptake across the basolateral

membrane. Hypokalemia decreases K

secretion by

antagonist action. Under repletion conditions, potas-

sium transported by Na

-ATPase and H

ATPase recycles back into the lumen through K

channels; however, when potassium levels are low,

little potassium is wasted through luminal channels

and passage of K through basolateral channels pre-

dominates.

0049Aldosterone secretion is favored by angiotensin II.

On the other hand, aldosterone stimulates Na

and

Exercise

Cell lysis

Hyperosmolality

Metabolic acidosis

Metabolic alkalosis

Epinephrine (adrenaline)

Insulin

Aldosterone

Secretion

Absorption

Glucocorticoids

Tubular fluid [Na

]

Flow rate

Secretion

Nephron

Filtration

Hypokalemia [K

] Hyperkalemia

Small intestine

Colon

External balance

Internal balance

Cell

ADH

Aldosterone

Metabolic alkalosis

Metabolic acidosis

fig0003 Figure 3 Potassium homeostasis. ADH, antidiuretic hormone. Continuous arrow, increase; dashed arrow, decrease.

4656 POTASSIUM/Physiology

water reabsorption and consequently decreases

tubular flow at first. However, flux is restored in

time, permitting aldosterone to increase K

renal ex-

cretion.

0050 Antidiuretic hormone (ADH) increases K

secre-

tion in the apical membrane of the principal cells in

exchange for Na

absorption. However, because

ADH also decreases tubular flow, changes in ADH

levels do not substantially alter urinary K. K

secre-

tion rises along the distal tubule in a flow-dependent

manner and it is influenced by dietary potassium

intake. This means that diuretics may enhance urin-

ary K

excretion. However, K

excretion and distal

flow rate are not completely coupled and, the K

balance is maintained through compensatory mech-

anisms when necessary.

0051 Physiological [Na

] does not alter K

secretion but

high [Na

] in tubular fluid increases K

secretion,

whereas a fall in concentration has the opposite

effect. Acid–base balance is another factor that

modulates K

secretion: alkalosis increases secretion,

whereas acidosis decreases it. Glucocorticoids are

kaliuretic, but their effects appear to be a conse-

quence of an increase in the glomerular filtration rate.

0052 K

itself exerts a feedback regulation over most of

its own control factors. Together, all these mechan-

isms allow the human body to maintain normal

plasma [K

]. However, the equilibrium can some-

times be altered, causing either hyperkalemia (extra-

cellular [K

] over 5.5 mmol l

1

) or hypokalemia

(extracellular [K

] below 3.5 mmol l

1

0053 Hyperkalemia may be the consequence of a K

shift from cells to ECF or the effect of excessive

retention. The first signs are flaccid paralysis,

natriuresis, and other minor effects. The most import-

ant consequences are ventricular fibrillation and

cardiac arrest. Hyperkalemia may be:

0054 Without potassium excess: caused by K

shift from

cells (acute acidosis, hyperosmolality, insulin defi-

ciency, b-adrenergic blockers, vigorous exercise, etc.)

0055 With potassium excess: impaired renal excretion

(renal failure, aldosterone deficiency, primary

tubule dysfunction). Inappropriately high oral or

parenteral K

administration.

0056 Hypokalemia may be the result of either intracellular

shift or K

depletion, or both. Among its clinical

sequelae are hyperpolarization of membrane, which

affects nerves and muscle activity, and produces renal,

cardiac, and metabolic alterations. The hypokalemia

may be caused by:

0057 Intracellular shift: alkalosis, insulin excess, familial

periodic paralysis, catecholamine increase, intoxi-

cations, etc.

0058Extrarenal causes: insufficient K

intake, diarrhea,

vomiting, skin loss, etc.

0059Excessive renal losses: renal tubular acidosis,

hypermineralocorticoidism, syndrome of choride

depletion, diuretic condition.

See also: Body Composition; Colon: Structure and

Function; Diarrheal (Diarrhoeal) Diseases; Dietary

Requirements of Adults; Electrolytes: Water–

Electrolyte Balance; Acid–Base Balance; Hormones:

Adrenal Hormones; Pituitary Hormones; Hypertension:

Hypertension and Diet; Legumes: Dietary Importance;

Potassium: Properties and Determination; Renal

Function and Disorders: Kidney: Structure and Function;

Nutritional Management of Renal Disorders; Vegetarian

Diets; Water: Physiology

Further Reading

Aizman R, Grahnquist L and Celsi G (1998) Potassium

homeostasis: ontogenic aspects. Acta Pediatric 87:

609–617.

Berne RM and Levy MN (1998) Physiology, 4th edn, pp.

699–754. St Louis: Mosby.

Eaton SB, Eaton III SB, Konner MJ and Shostak M (1996)

An evolutionary perspective enhances understanding of

human nutritional requirements. Journal of Nutrition

126: 1732–1740.

Giebisch G (1998) Renal potassium transport: mechanisms

and regulation. American Journal of Physiology

274(Renal Physiology 43): F817–F833.

Johnson LR, Alpers DH, Christensen J, Jacobson ED and

Walsh JH (1994) Physiology of the Gastrointestinal

Tract, 3rd edn, pp. 2027–2171. New York: Raven Press.

Navarro MP and Vaquero MP (1998) Potassium. Physi-

ology, dietary sources and requirements. In: Sadler M,

Caballero B and Strain S (eds) Encyclopedia of Human

Nutrition, pp. 1573–1580. London: Academic Press.

Nicholes CG and Lopatin AN (1997) Inward rectifier po-

tassium channels. Annual Reviews of Physiology 59:

171–191.

Nilius B, Viana F and Droogmans G (1997) Ion channels in

vascular endothelium. Annual Reviews of Physiology

59: 145–170.

Shils ME, Olson JA, Shilke M and Ross AC (1999) Modern

Nutrition in Health and Disease, 9th edn, pp. 105–139,

1217–1225. Philadelphia: Lippincott/Williams &

Wilkins.

Suter PM (1998) Potassium and hypertension. Nutrition

Reviews 56: 151–153.

Suter PM (1999) The effects of potassium, magnesium,

calcium, and fiber on risk of stroke. Nutrition Reviews

57: 84–91.

Wang W, Hebert SC and Giebisch G (1997) Renal K

channels: structure and function. Annual Reviews of

Physiology 59: 413–436.

Yokoshiki H, Sunagawa M, Seki T and Sperelakis N (1998)

ATP-sensitive K

channels in pancreatic, cardiac, and

vascular smooth muscle cells. American Journal of

Physiology 174 (Cell Physiology 43): C25–C37.

POTASSIUM/Physiology 4657

POTATOES AND RELATED CROPS

Contents

The Root Crop and its Uses

Fruits of the Solanaceae

Processing Potato Tubers

The Root Crop and its Uses

M V Rama and P Narasimham,FormerlyofCentral

FoodTechnologicalResearchInstitute,Mysore,India

Introduction

000 1 The potato (Irish potato, white potato, Solanum

tuberosum L.) is of ancient origin. It has originated

and was first domesticated in South America, even

before the appearance of maize. It was initially intro-

duced to Spain and the UK, and gradually to parts of

Asia and North America. It has spread around the

world over the past 400 years, and gained recognition

as an inexpensive and nutritive food in the eighteenth

century. It is now among the 10 major food crops

of the world and grown in over 140 countries.

This article reviews the geographical distribution,

varieties, commercial importance, morphology, and

anatomy of the tuber, chemical composition, nutritive

value, and postharvest handling and storage of

potatoes.

Geographical Distribution

000 2 The potato (S. tuberosum L.) belongs to the Solanaceae

family and is largely grown in cool regions where the

mean temperature during the growing season does not

exceed 18



C. World production of potato accounts for

47%from37% acreage of total rootand tubercrops.It

is the fourth most important food crop in the world,

next to wheat, maize, and rice in global tonnage

(Table 1). The principal producers are the European

and Asian countries, North America, and the Andean

countries of Latin America (Table 2). Production

shows a yearly increase in both the developing and

developed countries, more significantly in the former.

Russia, China, USA, Poland, India, Germany, UK,

France, Turkey, and Canada are some of the highest

potato-producing countries of the world (Table 3). The

USA and Canada together account for 8% of the world

production from 4% of acreage.

Commercial Varieties and Transgenic

Potatoes

0003There are more than 150 wild species of potato found

in Central America, Mexico, and the USA. S. tuber-

osum L. (tetraploid) represents the cultivated species

and there are seven other cultivated species, including

S. ajanhuiri, S. goniocalys, S. phureja, S. stenotomum

(diploids), S.  juzepczukii, S.  chaucha (triploids),

S.  curtilobum (pentaploid), which are grown in

different parts of Peru, Bolivia, Ecuador, and Vene-

zuela. Many improved local varieties have been de-

veloped and introduced in several countries and these

may show increased yield, disease resistance, better

tuber shape, texture, and quality for processing as

well as flavor.

tbl0001 Table 1 World production of potatoes compared to major tuber crops and cereals

Crop Area harvested Yield Production

(10 0 0 ha ) (kg ha

1

)(10 0 0 Mt )

Potato 17 586 164 063 288 522

Cassava 16 601 99 983 165 986

Yam 3802 96 122 36 545

Sweet potato 9002 133 215 199 919

Wheat 213 790 27 313 583 918

Rice 513 177 38 437 588 766

Maize 139 878 43 209 604 400

Datafromwww.fao.org–FoodandAgricultureOrganizationStatisticaldatabase,provisional1999productionandproductionindicesdata.

4658 POTATOES AND RELATED CROPS/The Root Crop and its Uses

000 4 In recent years, attempts have also been made

worldwide to generate transgenic potato plants and

assess the impact of transgenic expressions on diverse

parameters such as yield, quality, carbohydrate

metabolism, stress physiology, and pest and disease

resistance. The outcome of the transgenic approach is

promising and already insect- and herbicide-resistant

potato plants are in commercial use.

Commercial Importance

0005 The potato is grown as an early or late crop for seed

or ware purpose. Even in comparatively harsher cli-

mates, it produces more dry matter (DM) and protein

per hectare than major cereal crops of the world. It is

a major food crop in the temperate zone and is either

a staple food or merely a vegetable in tropical and

subtropical countries. Tubers are consumed in vari-

ous forms after cooking or processing. In the UK,

USA, and India 65–90% of the production is con-

sumed as food, and a small quantity is used for indus-

trial purposes. In The Netherlands, a large percentage

is used for starch production. Starch, alcohol, glu-

cose, and dextrin are the industrial products of

potato. World production of potato starch is about

2  10

t. Potatoes are generally boiled, cooked in oil,

or baked. Dehydrated, frozen, and canned products

are also popular. In the USA, 50% of the potato crop

is consumed in processed form; the most relished is

potato chips. Varieties with high DM content are

preferred for potato chips. Frozen products include

hash browns (patties), French fries, croquettes

(puffs), and mashed potato, while dehydrated prod-

ucts include granules, flakes, diced chunks, and juli-

enne strips. The common canned products are new

potatoes, hash, stew, and soups. Papa seca and chuno

are the traditional dried potatoes, which form a vital

part of the diet in the highland areas of Peru and

Bolivia. Dough-like sliceable, storage-stable potato

products have also been developed. (See Starch:

Sources and Processing.)

0006The waste water collected from peeling, cutting,

and blanching in the processing industries has been

used to recover highly nutritive protein of 80–85%

purity. The use of this recovered protein of high

biological value is limited as human food, due to the

presence of antinutritional factors, but the residual

water can be used in preparing a growth medium for

single-cell protein production. Potato peel itself is

superior to wheat bran, containing high amounts of

tbl00 02 Table 2 Potatoproductioninpartsoftheworld

PartsoftheworldAreaharvestedYieldProductionPercent

(10 0 0 ha ) (kg ha

1

)(1000 Mt) production

World 17 586 164 063 288 522 100

Africa 811 111 790 9072 3.1

Asia 9171 148 288 92 058 31.9

Europe 6208 156 305 143 349 49.7

North Central America 306 918 324 28 134 9.7

South America 1030 135 924 14 008 4.8

Oceania 58 326 278 1899 0.6

Developing countries 7585 141 777 107 550 37.3

Developed countries 10 000 180 969 180 972 62.7

Datafromwww.fao.org–FoodandAgricultureOrganizationStatisticalDatabase,provisional1999productionandproductionindicesdata.

tbl0003 Table 3 Major potato-producing countries of the world (production 1000 Mt)

Country Year

19 9 5 1997 19 9 9

Former Soviet Union 39 909 37 039 30 300

China 45 754 45 534 43 477

USA 20 122 20 861 21 840

Poland 24 891 20 776 26 000

India 17 401 19 240 22 100

Germany 10 888 12 438 12 074

UK 6396 7154 7000

France 5882 6500 6500

Turkey 4750 5000 5315

Canada 3374 4050 4260

Datafromwww.fao.org–FoodandAgricultureOrganizationStatisticalDatabase,provisional1999productionandproductionindicesdata.

POTATOES AND RELATED CROPS/The Root Crop and its Uses 4659

minerals and fiber. The water extract of the peel is a

nonmutagenic antioxidant with potential antimicro-

bial activity. Snakin-1 (SN-1), a peptide isolated from

the potato tuber, is shown to be active against potato

bacterial and fungal pathogens. Further, potato plant

having tuber as the sink for carbon and nitrogen is said

to serve as a model species for various experiments

involving bacteria-mediated gene transfer systems.

(See Alkaloids: Properties and Determination.)

0007 Tuber is also used for propagation. Tuber seed is

the costliest input, accounting for nearly 40% of the

cost in potato production; the availability of an

adequate quantity of virus-free seed stock is still a

constraint for increased production, especially in de-

veloping countries. Hence, in recent years commer-

cial crops are being raised using true potato seed

(TPS), which has several advantages over traditional

tuber seeds (Table 4). However, usage of TPS is

still limited in developed countries, due to erratic

germination and low yield. A host of hybrid TPS

families with desirable attributes has been developed

by the International Potato Research Center (CIP),

Peru, and Central Potato Research Institute (CPRI),

India, using S. andigena and S. tuberosum as parental

lines.

Morphology and Physiology of the Tuber

0008 The potato is an underground, modified, fleshy stem,

with a shortened axis, developing from the subapical

region of a diageotrophic rhizome or lateral shoot.

The tuber end that is attached to the rhizome is called

the stem end or heel end, and the other end is the bud

end or rose end. There are as many as 20 eyes in the

axils of leaf scars (eye brows) arranged spirally

around a tuber. Each eye has one main bud and

several small lateral buds. Physiologically, the

youngest bud is the last formed apical bud

(Figure 1a). Tubers sprout after completing the inher-

ent dormancy period, which lasts from 6 to 16 weeks

after harvest, depending upon the variety. Tubers

exhibit apical dominance. Studies have shown that

the balance between the two hormones, abscisic acid

(ABA) and gibberellic acid (GA

), in the tubers deter-

mines the extent of the dormancy period. Dormancy

breaking is also related to protein degradation and

mobilization of nitrogen reserves for sprout forma-

tion. Morphological characters of the tuber such as

shape (round, oval, or elongated), size (small,

medium, or big), skin color (light yellow to black or

purple), and depth of eye (deep or shallow) determine

its acceptability for various end uses.

tbl0004 Table 4 Advantages of using true potato seeds (TPS) for potato production

Parameters Advantages

Seed rate 25 g of TPS against 2 t seed tubers per hectare

Profit Planting material cost is very low; saves cost on transport, cold storage, and needs less

storage room

Productivity More productive due to higher yield and more dry matter

Technical feasibility Can be effectively used in different agroclimatic zones

Less risky/environment-friendly Built in broad-based resistance to late blight: reduced usage of pesticides

Adapted from Upadhya MD (1994) True potato seed: propagule for potato production for the 21st century. In: Shekhawat GS, Khurana P, Pandey SK and

Chandla VK (eds), Potato: Present and Future, pp. 15–22. Shimla, India: Indian Potato Association.

Bud end

Skin

Cortex

Vascular ring

Perimedullary zone

Medullary rays

Lenticels

Pith

Bud end

Apical bud

Lateral bud

Expanded scale leaf

Lenticels

Eyes

Stolan scar

Stem end

(a)

(

)

fig0001Figure 1 A potato tuber: (a) morphological details; (b) longitu-

dinal section.

4660 POTATOES AND RELATED CROPS/The Root Crop and its Uses

Anatomy of the Tuber

0009 The potato tuber is protected by the outermost skin

or periderm, consisting of six to 10 layers of suberized

cells. Skin color depends on the anthocyanin concen-

tration in the periderm and peripheral cortex. The

active periderm of a young tuber can be easily

removed, and tuber enlargement is associated with

sloughing off the periderm, which is then replaced

by a new ‘cork’ layer, formed from beneath. Similarly,

wound healing takes place in damaged tuber tissue by

the formation of a wound periderm, which is more

impervious than normal skin. A number of lenticels

are found in the periderm, and these pores also facili-

tate the exchange of gases and the entry of pathogens.

Inside the periderm is the parenchymatous cortex

(0.3–1.0 cm thick) in which food material is stored

in the form of starch granules. Cortex (potato flesh)

may be white or various shades of yellow depending

upon the variety. Yellow-fleshed varieties are some-

times highly prized. The parenchymatous perimedul-

lary zone is seen between the vascular ring and the

medullary ray (Figure 1b). Xylem is visible as a ring,

while phloem forms many bundles in the cortex and

perimedullary zones. Medullary rays run from the

stem end to the eyes. The pith present at the center

of the tuber is translucent, as it has less starch.

Chemical Composition

0010 The chemical composition of potato varies with variety,

soil type, cultural practice, maturity stage, disease, and

storage conditions. The distribution of DM and chem-

ical compounds (Figure 2) is extremely heterogeneous

within a tuber. The data in Tables 5 and 6 shows potato

to be a good source of carbohydrates, vitamins, and

minerals. The small fraction of protein present in the

tuber consists of many essential amino acids.

Carbohydrates

0011Starch, sugars, and nonstarch polysaccharides consti-

tute the carbohydrate fractions of potato tuber.

(a) (b) (c) (d) (e)

(f) (g) (h) (i) (j)

fig0002 Figure 2 Pattern of distribution of chemical compounds

(shaded areas) within a potato tuber. (a) Starch; (b) sugars; (c)

proteins and amino acids; (d) fat; (e) crude fiber; (f) vitamins; (g)

minerals; (h) organic acids; (i) phenol compounds; (j) alkaloids.

Adapted from Rastovski A, Van Es A et al. (1987) Storage of Pota-

toes. Past Harvest Behaviour. Store Design, Storage Practice, Hand-

ling. Wageningen, The Netherlands: Center for Agricultural

Publishing and Documentation.

tbl0005Table 5 Chemical composition of the dry matter (22.5%) of the

potato tuber

Chemical composition Content

Crude fiber 2.2

Starch 74.2

Total sugar 1.3

Reducing sugar 0.6

Fat 1.0

Total nitrogen (N) 1.2

Protein N 1.0

Protein fractions (% of total protein N)

Albumin 48.9

Globulin 25.9

Prolamin 4.3

Glutelin 8.3

Minerals

Calcium 0.02

Magnesium 0.08

Potassium 1.47

Sodium 0.02

Phosphorous 1.87

Iron (p.p.m.) 15.70

Vitamins (mg 100 g

1

)

Thiamin 0.73

Ascorbic acid 92.08

Nicotinic acid 10.08

Riboflavin 0.12

Values given as percentages, unless otherwise stated.

Data adapted from Rastovski A, Van Es A et al. (1987) Storage of Potatoes.

Post Harvest Behaviour, Store Design, Storage Practice, Handling.

Wageningen, The Netherlands: Center for Agricultural Publishing and

Documentation.

tbl0006Table 6 Essential amino acid composition of potato tuber (dry-

weight basis)

Amino acid Mean concentration

(mg g

1

tuber)

Histidine 2.5

Isoleucine 3.4

Leucine 5.0

Lysine 4.9

Methionone þ cysteine 1.9

Phenylalanine þ tyrosine 7.0

Threonine 3.1

Tryptophan 1.3

Valine 5.7

Average of lowest and highest values reported.

Reproduced from Jadhav SJ and Kadam SS (1998) Potato. In: Salunkhe DK

and Kadam SS (eds) Handbook of Vegetable Science and Technology:

Production, Composition, Storage and Processing, pp. 11–69. New York:

Marcel Dekker.

POTATOES AND RELATED CROPS/The Root Crop and its Uses 4661

Starch, being the major carbohydrate (16–20% on a

fresh-weight basis or fwb), constitutes 60–80% of the

dry matter, and it is composed of amylose and amy-

lopectin in a 3:1 ratio. Potato starch gelatinizes above



C. The total sugar content ranges from 0.1% to

0.7% (fwb), and this is chiefly associated with matur-

ity, senescence, and sprouting. The major sugars of

potato are glucose, fructose, and sucrose. Trace

amounts of melibiose, raffinose, stachyose, glycerol,

galactinol, and glucosyl have also been identified.

Tubers containing more than 2% (fwb) reducing

sugars give dark-colored chips owing to Maillard

reactions and are not suitable for processing. In

mature stored tubers, starch and sugars exist in a

state of dynamic equilibrium. The nonstarchy poly-

saccharides, such as cellulose, hemicellusose, and

pectic substances, constitute about 1.2% (fwb)

and are present in the cell walls and middle lamellae.

They contribute to the final texture of cooked potato,

and act as a source of dietary fiber. (See Browning:

Nonenzymatic; Carbohydrates: Requirements and

Dietary Importance; Classification and Properties;

Starch: Structure, Properties, and Determination.)

Proteins and Amino acids

0012 Potato is considered to be low in protein (2% fwb),

but is rich in lysine compared to cereal proteins, while

the concentration of sulfur amino acids is less than in

cereals. The protein is concentrated more in the

cortex and pith. The levels of proteins such as albu-

min, globulin, prolamine, and glutelin are 48.9%,

25.9%, 4.3%, and 8.3% of total protein respectively.

Nearly 75% of the nonprotein nitrogen (NPN) occurs

as free amino acids and amides. Two-thirds of the

NPN fraction is composed of free amino acids and

21 of the amino acids have been identified. The essen-

tial amino acids and their concentrations are given in

Table 6. Sprouting, storage, diseases, and fertilizer

applications influence the concentration of free

amino acids in the tuber. Amides like glutamine and

asparagine occur in almost equal amounts. (See Pro-

tein: Chemistry; Requirements.)

Vitamins and Minerals

0013 Potato contains substantial quantities of vitamins B

and C (Table 5). Vitamin C is present in both oxidized

and reduced forms. Freshly harvested tubers may

contain 20 mg of ascorbic acid per 100 g; losses of

vitamin C occur during long-term storage (40–60%),

cooking, and processing (20%). Ascorbic acid con-

tent of potato is higher than that of several other

vegetables like carrots, pumpkins, onions, and green

beans. The vitamin B group comprises thiamin, ribo-

flavin, nicotinic acid, and pyridoxine; folic acid along

with pantothenic acid is also present in potato. The

ash content is about 1% (fwb), which is equivalent to

4–6% of the DM content. Fat-soluble vitamins occur

in traces or are absent. This necessitates supplemen-

tation of other food sources rich in vitamin A in a

potato diet. The major elements present are phos-

phorus, potassium, magnesium, sodium, and calcium,

with wide ranges in their amounts. Potato is a poor

source of calcium and sodium. A small percentage of

phosphorus (25%) occurs as insoluble phytic acid.

Others, such as boron, copper, zinc, iodine, alumi-

num, arsenic, nickel, and molybdenum, are found

in trace amounts. (Refer to individual minerals and

vitamins.)

Lipids and Organic Acids

0014Approximately 0.1% (fwb) lipid is found in potato; it

is concentrated in the periderm. Linoleic, linolenic,

and palmitic acids are the major fatty acids. A

number of organic acids are present in potato in

varying quantities; they contribute to the flavor and

buffering of the potato sap. The major organic acids

are citric, oxalic, fumaric, and malic acids. Other

than phytic acid, nicotinic and chlorogenic acids

have also been reported. Chlorogenic acid reacts

with ferric iron, forming a complex, which causes

darkening after cooking. Enzymatic browning in cut

and homogenized potato tissue is caused by the oxi-

dation of tyrosine. Both enzymatic and nonenzymatic

browning can be inhibited by treatments with sulfur

dioxide and sulfites. Other phenolic compounds

found in potato are polyphenols, flavones, anthocya-

nins, and tannins. Phenols are again associated with

after-cooking discoloration of tubers, particularly at

the stem end. Tannins being localized in the periderm

impart tan coloration to the skin. (See Acids: Natural

Acids and Acidulants; Fats: Classification; Fatty

Acids: Properties; Phenolic Compounds; Tannins

and Polyphenols.)

Enzymes and Pigments

0015The enzymes reported in potato include amylase,

glyoxalase, phosphorylase, tyrosinase, peroxidase,

catalase, aldehydrase, phosphatase, sistoamylase, and

zymohexase. The phosphorylase and amylase systems

form sugars at low temperatures. The probable role of

D-enzyme (EC 2.4.1.25; 4-alpha-glucanotransferase)

in starch metabolism has been suggested. Transgenic

potato plants with reduced D-enzyme activity have

been obtained. Polyphenol oxidase, peroxidase, and

catalase enzymes involved in the oxidation of phenols

bring about browning of the freshly cut surface of the

potato. The yellow color of potato flesh is attribut-

able to carotenoids such as a-carotene, auroxanthin,

4662 POTATOES AND RELATED CROPS/The Root Crop and its Uses

violaxanthin, lutein, isolutein, and neo-b-carotene.

Flavonols, flavones, flavin, and neoxanthin are also

found in small quantities. Red-skinned potatoes have

anthocyanin in the periderm and outer cortical cells.

Chlorophyl is present in tubers exposed to light, and

these green potatoes lose their market value. (See

Carotenoids: Occurrence, Properties, and Determin-

ation; Chlorophyl; Colorants (Colourants): Proper-

ties and Determination of Natural Pigments;

Enzymes: Functions and Characteristics.)

Antinutritional Factors

0016 The glycoalkaloids, proteinase inhibitors, and the

lectins are the major antinutritional factors present in

the tubers. The synthesis of toxic glycoalkaloids such

as a-solanine and a-chaconine, which are believed to

be a part of a disease-resistance mechanism, takes

place in damaged and light-exposed tubers; a normal

tuber contains an insignificant amount (5–10 mg

100 g

1

) of these alkaloids. They are recognized as

natural products with insect deterrent and antifungal

activity, concentrated more in the peel, sprouts, and

around the eyes, and absent in the pith. Tubers develop

bitterness and off-flavors when the alkaloid content is

25–80 mg 100 g

1

, and consumption of tubers with

more than 20 mg of glycoalkaloid causes fatal illness.

It can be eliminated to an extent of 60%, by peeling the

tuber, while its accumulation can be controlled by

waxing, oil and water dipping, and storing in the

dark. The nutritional significance of two other toxic

substances, proteinase inhibitors and lectins, has not

been much studied, but they are reported to be heat-

labile. (See Alkaloids: Toxicology.)

Nutritive Value

0017 Potato is one of the richest sources of energy and has a

greater capacity to supply energy than any other food

crop. In some developing countries, potato is con-

sidered merely a vegetable rather than a staple food

because of the belief that excessive consumption

causes flatulence in humans. Experiments have

shown that cooked potato starch is easily digestible,

and hence forms a valuable food, even for infants.

Potato has a slightly lower energy content (335 kJ

100 g

1

) than other roots, tubers, cereals, and

legumes (Table 7), but this is advantageous in over-

coming the problem of obesity in the developed

world. On the other hand, large quantities of potato

have to be consumed in developing countries to meet

their populations’ daily energy needs. It has been

calculated that 100 g of potato can supply 5–7% of

the daily energy and 10–12% of the daily protein

need of children aged 1–5 years.

0018Potato protein has a biological value equal to that

of soya bean protein and the ratio of total essential

amino acids to total amino acids is so balanced that it

can meet the needs of infants and small children. It

has been calculated that 100 g of potato can supply

3–6% of proteins, the recommended daily allowance

(RDA) for adults (depending upon sex and body

weight), and one medium-sized potato provides

15 mg of vitamin C, which is about 20% of the

recommended allowance (75 mg) per person per day.

Eating the tuber with the skin on increases the dietary

fiber intake. It is also a most valuable food for those

who suffer from excess acidity of the stomach as it has

an alkaline reaction. Potato fat is too low to have any

nutritional significance, but it does contribute to-

wards palatability. The tuber also provides most of

the trace elements needed to maintain good health,

and although potato is not a primary source of iron,

100 g of cooked potato can supply between 6% and

12% of the daily iron requirement of children or

adult men. About 7% of the US RDA of phosphorus

for both children and adults is available from 100 g

potato. A low percent of phytic acid present in potato

tuber, compared to other staple cereals and vege-

tables, is an advantage as it allows greater availability

of free calcium, iron, zinc, and phosphorus for ab-

sorption by the human intestine. (See Amino Acids:

Metabolism; Ascorbic Acid: Physiology; Dietary

Fiber: Physiological Effects; Energy: Measurement

of Food Energy; Iron: Physiology; Protein: Quality.)

tbl0007 Table 7 Composition of potato (raw and dried) and other major cereals and root crops (per 100 g edible portion)

Crop Energy (kJ) Moisture (%) Crude protein (g) Fat (g) Total carbohydrate (g) Crude fiber (g) Ash (g)

Potato (raw) 335 78.0 2.1 0.1 18.5 2.1 1.0

Potato (dried) 1343 11.7 8.4 0.4 74.3 8.4 4.0

Rice 1523 12.0 6.8 0.5 80.2 2.4 0.6

Wheat 1389 12.3 13.3 2.0 70.9 12.1 1.7

Sweet potato 485 70.2 1.4 0.4 27.4 2.5 1.1

Yam 444 72.0 2.2 0.2 24.2 4.1 1.0

Cassava 609 62.6 1.1 0.3 35.2 5.2 0.9

Adapted from Woolfe JA (1987) The Potato in the Human Diet. Cambridge: Cambridge University Press.

POTATOES AND RELATED CROPS/The Root Crop and its Uses 4663

Handling and Storage

0019 In most countries, potatoes are only harvested at

certain times of the year, and harvested tubers must

be stored for at least a few months. Harvesting and

subsequent processes such as assembling, grading,

bagging, transport, and marketing, may cause

damage to the tubers; such tubers lose water quickly

and are highly susceptible to microbial spoilage.

Bruising also causes a physiological disorder called

black spot which is as the result of oxidative biochem-

ical reactions involving chlorogenic acid, polypohe-

nol oxidase, amino acids, and tyrosine, resulting in

melanin formation (enzymatic browning); bruised

black-spotted tubers have less market value and

shorter shelf-life. Degree of dormancy, rate of tuber

respiration, chemical composition, and ability of

wound healing influence the spread of disease-

causing organisms through the tuber tissue. Thus

curing, a process which promotes wound healing, is

necessary before potato storage. Complete curing

takes 3–6 days at 20



C and 85–90% relative humid-

ity (RH), but is faster at higher temperature. The

physiological disorders, diseases, and pests of the

tubers in the field and in storage are listed in Table 8.

0020 Good storage conditions should prevent excessive

sprouting, root development, moisture loss, sugar

accumulation, greening, and temperature damage.

The postharvest losses in potatoes are estimated to

vary from 5 to 40%. For short periods, the tubers can

be stored in clamps, in specially designed simple

sheds, pits, cellars, or buildings with controlled tem-

perature and humidity. Modern potato storages are

equipped with computer-based control systems to

monitor and maintain optimum levels of temperature

and humidity with air envelop facilities. Long-term

storage without sprouting and minimum loss of mois-

ture is possible at 4–5



C, but temperatures below



C lead to the formation of reducing sugars, and

this is not a desirable feature for industrial processing.

Tubers can be reconditioned by transfer to a higher

temperature, but the sweetness developed by long-

term, low-temperature storage (senescent sweetening)

cannot be eliminated completely; nearly 30% of the

starch is lost as sugars, reducing tuber quality, par-

ticularly for processing.

0021 The freezing of tubers takes place at 1



Cto

2



C, and frozen tubers soon become soft and un-

usable. Potato stores are artificially heated using oil

or gas heaters when the outside air temperature is

lower than the required temperature. On the other

hand, when potatoes are stored above 30



C, accu-

mulation of carbon dioxide leads to death of cells,

causing black heart. In temperate regions, where the

ambient temperature falls below 4



C, potatoes are

cooled with outside air. In the tropics and subtropics,

refrigeration is used. The recommended temperatures

for tubers to be used for different purposes are as

follows:

0022Seed potato 2–4



0023Fresh consumption 4–5



0024Chipping 7–10



0025French frying 5–8



0026Granulation 5–7



(See Storage Stability: Mechanisms of Degradation.)

0027Sprouting increases weight loss, softens the tuber,

and favors accumulation of glycoalkaloids, thus

decreasing marketability and nutritive value. Apart

from low-temperature storage, other methods of

achieving sprout inhibition include the use of maleic

hydrazide (MH), isopropyl-N-phenylcarbamate (IPC)

and isopropyl-N-(3-chlorophenyl)-carbamate (CIPC),

tetrachloronitrobenzene (TCNB), naphthalene acetic

acid (NAA), methyl ester of naphthalene acetic acid

(MENA), and irradiation. Time, method of applica-

tion, and concentration of the chemical are important

factors, as treatment may have adverse effects such as

tbl0008Table 8 Potato tuber diseases and pests in the field and during

storage

Disease Causalagent/organism

Nonparasitic diseases

Soft rot, mahogany browning,

and net necrosis

Low-temperature storage

(chilling injuries)

Black heart and hallow heart Low oxygen supply

Black spot Bruising

Pathological diseases

Bacterial

Bacterial soft rot Erwinia carotovora

Brown rot Pseudomonas

solanacearum

Bacterial ring rot Corynebacterium

sepedonicum

Common scab Streptomyces scabies

Fungal

Powdery scab Spongophora subterranea

Dry rot Fusarium coeruleum

Late blight Phytophthora infestans

Early blight Alternaria solani

Charcoal rot Macrophomina phaseoli

Wart Synchytrium endobioticum

Gangrene Phoma exigua

Pink rot Phytophthora erythroseptica

Insect (pest)

Potato tuber moth Gnoremoschema operculella

Viral

Tobacco rattle, mop top,

yellow viruses

dwarf diseases

Adapted from Pushkarnath (1976) Potato in Sub-Tropics. New Delhi: Orient

Longman; and Rich AE (1983) Potato Diseases. New York: Academic Press.

4664 POTATOES AND RELATED CROPS/The Root Crop and its Uses