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

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Water Activity and Material Properties

0018 Several properties, such as dielectric properties,

mechanical properties, and viscosity, of amorphous

food materials are greatly affected by the glass

transition. These changes are related to relaxation

times and molecular mobility, which may substan-

tially change and affect food stability over the glass

transition. Such changes may contribute to observed

changes in textural characteristics, e.g., loss of crisp-

ness and sogginess, and reaction rates, e.g., non-

enzymatic browning and oxidation. Furthermore,

the solid, glassy state supports the amorphous state

of food components, but many crystallizing compon-

ents, e.g., sugars, may crystallize in the supercooled

liquid state. Both textural changes and crystallization

of amorphous components contribute to a loss of

flavor and aroma compounds as well as release and

oxidation of encapsulated lipids in numerous dehy-

drated foods and food powders.

001 9 In general, the relaxation times of molecular re-

arrangements in the glassy state are sufficient to

provide stability for low-moisture food systems. The

changes occurring over the glass transition and in the

supercooled liquid state reduce the relaxation times,

and the materials lose their long-term stability. For

example, amorphous, glassy lactose in food systems is

stable, but an increase in water content resulting in

glass transition leads to a rapid decrease in viscosity

and flow of the material (transformation from solid

state to a syrup) accompanied by a loss of structure

(stickiness and collapse) and lactose crystallization.

Crystallization of amorphous sugars is often observed

from water sorption isotherms. When crystallization

occurs, sorbed water is not maintained in the crystals,

and a decrease in water content is recorded. Such

crystallization is time-dependent and detrimental to

the quality of amorphous powders.

Stiffness

0020 A single model based on the Fermi’s distribution

model, as given in eqn (2), allows modelling of the

decrease in relaxation times of mechanical properties

(stiffness) in the vicinity of the glass transition.

According to the model, stiffness can be expressed

in terms of a defined and measurable stiffness param-

eter, Y, which may refer to viscosity, flow, sensory

crispness, etc. The stiffness parameter values can be

modelled as a function of a

, T,orm relative to its

value at a reference state, Y

. A constant, a

, defines

the broadness of the transition, e.g., the water activity

range over which changes in stiffness occur. The ref-

erence value, X

, obtained from b ¼X

, indicates

the value for a

, T,orm that decreases Y to 50%

below Y

 1



¼ b þ

X, ð2Þ

where Y is the stiffness parameter, Y

is the stiffness

parameter at a reference state, X is a

, T, or water

content, m, and b and a

are constants.

0021The term collapse relates to stiffness and covers

various time-dependent structural transformations

that may occur in amorphous food and other bio-

logical materials at temperatures above the T

These changes reflect the effect of changes in relax-

ation times of mechanical properties on the rates of

structural transformations and flow that occur over

and above the glass transition temperature range. The

rates of these changes increase rapidly above the crit-

ical water content or a

during food storage. Sticki-

ness and caking of food powders and collapse of

structure are collapse phenomena that are related to

water activity and glass transition. The main cause of

stickiness is plasticization of particle surfaces by

water, which allows interparticle binding and for-

mation of clusters. Collapse of the structure occurs

when the decrease in viscosity results in flow, and the

material cannot support its own weight. Structural

collapse often leads to a reduction in quality of dehy-

drated, especially freeze-dried, foods, whereas sticki-

ness and caking are detrimental to the quality of

spray-dried foods, such as dairy powders.

Crystallization of Food Components

0022Crystallization of amorphous sugars is known to

result in serious quality losses in food powders. For

example, crystallization of amorphous lactose in de-

hydrated milk products has been observed to result in

acceleration of the nonenzymatic browning reaction

as well as other deteriorative changes and caking.

Crystallization of lactose coincides with an increase

in free fat, which presumably facilitates lipid oxida-

tion in milk powders. Crystallization in low-moisture

carbohydrate matrices, which contain encapsulated

volatiles or lipids, results in a complete loss of flavor

and release of lipids from the matrix. Water plasti-

cization and depression of T

to below the ambient

temperature are responsible for crystallization of

amorphous sugars in foods as a result of increased

free volume and molecular mobility, decreased viscos-

ity, and enhanced diffusion. Crystallization seems to

initiate at T

or corresponding a

and proceed with a

rate determined by the temperature difference T  T

to a maximum extent also defined by the T  T

. The

kinetics of crystallization of sugars and other amorph-

ous food components at a constant temperature above

can be related to water content and a

, which

define the rate controlling T  T

WATER ACTIVITY/Effect on Food Stability 6099

Water Plasticization and Reaction Rates

0023 It has been well established that water as a plasticizer

has a significant effect on molecular mobility and

probably rates of quality changes above a critical,

temperature-dependent a

or water content. A chem-

ical reaction requires sufficient mobility of reactants

and reaction products in addition to the driving force,

e.g., temperature or concentration, of the reaction or

change in quality. Information on factors affecting

rates of various kinetic processes can be used to

manipulate and control rates of changes occurring

in food processing and to develop food products

that are less sensitive to detrimental changes during

storage.

Nonenzymatic Browning

0024 The nonenzymatic browning reaction is one of the

most studied deteriorative reactions affected by water

activity. Other reactions include destruction of vita-

mins, enzymatic changes, and oxidation. The rates of

most reactions are affected by water activity and glass

transition. However, there are no single determinants

of rates of chemical reactions and food stability, al-

though an understanding of both the water activity

and glass transition of a food material is important in

predicting and controlling food stability.

0025 Nonenzymatic browning is often considered as a

series of condensations that can be assumed bimole-

cular. The reaction occurs between a reducing sugar

and an amino acid or amino group and produces

typical flavors of several foods in processing. When

uncontrolled, the reaction produces unpleasant

flavors and colors, and decreases dehydrated food

quality during storage. The nonenzymatic browning

reaction in several foods exhibits increasing rates

above the critical a

values or water contents corres-

ponding to the glass transition. However, reactions in

amorphous foods are complex and may be controlled

by several other factors, including concentration

changes, pH, and temperature. Established relation-

ships between T

and the rate of the nonenzymatic

browning reaction have, however, suggested that the

reaction may become diffusion-controlled, and the

rate may be affected by the T

. The size of the react-

ants may also be important in diffusion-controlled

reactions. It may be assumed that the rate of diffusion

decreases with increasing size of the diffusant.

0026 The effect of glass transition on the reaction rate

has been related to observed discontinuities in Arrhe-

nius plots suggesting a high activation energy in the

vicinity of the transition. Studies of effects of glass

transition on reaction kinetics and the nonenzymatic

browning reaction in particular are complicated, be-

cause of several factors affecting plasticization and

reaction rates. Several studies, however, have sug-

gested that the reaction rate increases considerably

above the glass transition or the critical water activity.

Oxidation

0027Oxidation is a common reaction in low-moisture

foods. Oxidation of lipids directly exposed to atmos-

pheric oxygen at food surfaces or surfaces of porous

dehydrated materials may occur freely. It is well

known that ‘free fat’ in dairy powders, for example,

is highly susceptible to oxidation causing quality

defects. The stability of dairy powders is related to

the glassy state of lactose, and the lipids are at least

partially encapsulated within the lactose–protein

matrix in the spray-drying process.

Enzymatic Changes

0028Studies of glass transition effects on enzymatic

changes have included, for example, sucrose inverson

by invertase. Sucrose inversion by invertase in low-

moisture systems is dependent on water activity. The

reaction rate increases at water activities above 0.6,

but does not seem to be affected by the glass transi-

tion of the main food matrix. It is possible that the

enzyme mobility is not sufficient to allow the reaction

to occur until the reacting molecules are plasticized

by water to an appropriate extent above 0.6a

Modern Stability Maps

0029Deteriorative changes affected by water activity in-

clude such changes as enzymatic changes, nonenzy-

matic browning, lipid oxidation, microbial growth,

and overall stability. The rate of these changes seems

to be related to water activity. However, water activ-

ity limits for chemical, enzymatic, and mechanical

changes are material-specific, similarly as the ‘mobil-

ization points,’ and they may also be related to

the physical state, molecular mobility, water plasti-

cization and glass transition of amorphous food

solids. Structural transformations, as well as diffu-

sion-controlled deteriorative reactions and changes

affected by crystallization phenomena, occur at in-

creasing rates with increasing water activity above

the critical a

of the material. It is likely that water

contents lower than the critical water content are

needed for maximum stability. Therefore, modern

stability maps provide information on both the water

activity and material physical state on the relative

rates of deteriorative changes (Figure 4). In addition,

state diagrams and sorption isotherms are useful as

stability maps. Knowledge of material properties is

extremely useful in the production of encapsulated

flavors, extruded products, confectionery, develop-

ment of new food products, and avoiding quality

6100 WATER ACTIVITY/Effect on Food Stability

changes that may result from mechanical changes,

e.g., loss of crispness. The temperature-, water con-

tent-, and time-dependent changes that are often

problems in manufacturing and storage of food

powders and other low-moisture foods can be re-

duced by avoiding exceeding their critical values of

water activity based on the T

determination or by

compositional adjustments that provide sufficiently

high values for critical a

and T

. The most important

applications of producing high-quality dehydrated

foods include reduced collapse and improved flavor

retention in dehydration processes. The kinetics of

enzyme activity is important to food quality, and

applying the knowledge in food industry may allow

the design of improved products with extended shelf-

lives or even improved retention of activity, e.g., in

enzyme preparations. Water activity has been success-

fully used in setting limits for microbial stability in

foods and qualitative characterization of relative

rates of deteriorative reactions in low-moisture

foods. The combined use of a

and glass transition

has a high practical applicability as it provides criteria

for critical a

and, together with sorption isotherm,

can be applied to adjust water content at a constant

temperature to achieve the maximum food stability.

See also: Browning: Nonenzymatic; Toxicology of

Nonenzymatic Browning; Crystallization: Basic

Principles; Mycotoxins: Classifications; Oxidation of

Food Components; Spoilage: Chemical and Enzymatic

Spoilage; Bacterial Spoilage; Molds in Spoilage; Yeasts

in Spoilage; Water: Structures, Properties, and

Determination; Physiology; Water Activity: Principles

and Measurement; Yeasts

Further Reading

Barbosa-Ca

novas GV and Welti-Chanes J (eds) (1995) Food

Preservation by Moisture Control: Fundamentals and

Applications. Lancaster, PA: Technomic.

Bell LN (1996) Kinetics of non-enzymatic browning in

amorphous solid systems: distinguishing the effects of

water activity and the glass transition. Food Research

International 28: 591–597.

Buera MP and Karel M (1995) Effect of physical changes

on the rates of nonenzymatic browning and related

reactions. Food Chemistry 52: 167–173

Labuza TP (1980) The effect of water activity on reaction

kinetics of food deterioration. Food Technology 34:

36–41, 59.

Lievonen SM, Laaksonen TJ and Roos YH (1998) Glass

transition and reaction rates: nonenzymatic browning in

glassy and liquid systems. Journal of Agricultural and

Food Chemistry 46: 2778–2784.

Matveev YuI, Grinberg VYa and Tolstoguzov VB (2000)

The plasticizing effect of water on proteins, polysacchar-

ides and their mixtures. Glassy state of biopolymers,

food and seeds. Food Hydrocolloids 14: 425–437.

Nelson KA and Labuza TP (1994) Water activity and food

polymer science: implications of state on Arrhenius and

WLF models in predicting shelf life. Journal of Food

Engineering 22: 271–289.

Peleg M (1993) Mapping the stiffness–temperature–mois-

ture relationship of solid biomaterials at and around

their glass transition. Rheologica Acta 32: 575–580.

Roos YH (1993) Melting and glass transitions of low

molecular weight carbohydrates. Carbohydrate Re-

search 238: 39–48.

Roos YH (1995) Phase Transitions in Foods. San Diego,

CA: Academic Press.

Roos YH, Karel M and Kokini JL (1996) Glass transitions

in low moisture and frozen foods: effects on shelf life and

quality. Food Technology 50(11): 95–108.

Roos YH (2000) Water activity and plasticization. In: Eskin

NAM and Robinson DS (eds) Food Shelf Life Stability.

Boca Raton, FL: CRC Press.

Roos YH, Leslie RB and Lillford PJ (eds) (1999) Water

Management in the Design and Distribution of Quality

Foods. Lancaster, PA: Technomic.

Shimada Y, Roos Y and Karel M (1991) Oxidation of

methyl linoleate encapsulated in amorphous lactose-

based food model. Journal of Agricultural and Food

Chemistry 39: 637–641.

Slade L and Levine H (1991) Beyond water activity: Recent

advances based on an alternative approach to the assess-

ment of food quality and safety. Critical Reviews in Food

Science and Nutrition 30: 115.

Water activity0 1.0

Water content

Relaxation time/reaction rate

Relaxation times

Oxidation

Nonenzymatic browning

Glass transition

range

Water sorption

fig0004 Figure 4 Food stability map taking into account water activity

and physical state effects on relaxation times and reaction rates.

WATER ACTIVITY/Effect on Food Stability 6101

WATER PLANTS

T M Koyama, Ninon University, Fujisawa City, Japan

Lotus (

Nelumbo nucifera

Gaertner)

(Nymphaeacea: Waterlily Family)

0001 Lotus is a well-known water plant in Asia, and is

widespread from Iran and the Caspian Sea eastwards

to China and Japan, and southeastwards to India and

Indo-China, then to northern Australia. In India,

Thailand, China, and Japan, it is the sacred flower

of Buddhism, and is the national flower of Thailand.

0002 It is a large, emergent herb, growing in marshes and

shallow ponds with rhizomes creeping in mud and

sending out leaves and flowers over the water. The

slender rhizomes spread extensively, branching and

rooting at nodes, which bear a single leaf and a flower

at the axil of some leaves; the round leaf blades are

deeply glaucous-green, 70 cm or more across, slightly

bowl-shaped and have some 20 nerves radially

arranged. They are peltate on a petiole up to 2 m in

length, usually sparsely bearing brownish spines. The

large flowers are 10–20 cm across, appear in summer

in temperate regions, are subtended by four large

bracts, and terminate in a scape (about 2 m long)

which is sparsely spiny. The petals are many, origin-

ally deep pink but sometimes white. In the center of

the flowers, there are many yellow stamens and an

obconical receptacle with many holes, each contain-

ing an ovary. The receptacle develops into a large,

obconical, spongy structure, flat on top, and matur-

ing some 20–30 nuts. The nuts are ellipsoidal or

obovoidal, deep brown in color with hard shells,

and about 2 cm long.

0003 The history of the lotus can be traced back to the

diluvian epoch, the formation of which, in central

Japan, contains the fossil rhizomes and fruits of

lotus. Records show that it was grown in ancient

Egypt and that a white-flowered variety was grown

in India as early as 500 bc. It reached Japan during

the seventh century, at which time it was used mostly

as an ornamental. Now, in Japan, there are over 100

cultivars of lotus, including some 10 varieties as food

crops, showing varying colors (white, pink-tinged,

pink-striped, pink-variegated, yellow, orange–yellow,

etc.) and shapes (double-flowered and polycephalous)

of flower in the ornamental forms. In Japan, it was

also hybridized with the North American yellow-

flowered lotus, N. lutea Persoon, to produce another

ornamental lotus.

0004The edible parts of the lotus are principally its

rhizomes and seeds, though in China, immature

receptacles are occasionally eaten boiled or fried as

a delicacy, and whole plants are used as a Chinese

medicine against high blood pressure.

0005In China and Japan, lotus cultivated in paddies

begins producing tuberous apices of rhizomes in late

autumn and early winter. These fat, tuberous apices

of rhizomes are much thicker than the leaf-bearing

parts of the rhizome, with one to three internodes up

to 30 cm long by 10 cm wide, bearing some 10 pipes

as air spaces; these are called the ‘lotus roots’ and are

widely marketed. Recently, water-boiled lotus roots

have been exported from China as a canned vege-

table. In Japan, lotus roots are eaten boiled and

seasoned with soy sauce and sugar, or fried as tem-

pura (Japanese fritters), or sometimes pickled, while

in China, occasionally, a kind of arrowroot starch is

produced, which is used with spice and pork or with

pork spareribs for making a popular soup.

0006After removal of the dark brown, hard shell, the

obovoidal-to-round seed is covered with thin brown

testa and is 13–17 mm long. To prepare the edible

parts, which are the thick cotyledons, rich in raffi-

nose, the testa and hypocotyle, which is bitter, must

be removed. Slightly immature seeds have a very

sweet taste and can be eaten raw. The cotyledons of

mature seeds are hard and must be boiled prior to

eating. Water-boiled lotus seeds are canned and

exported. They are also candied (i.e., preserved in

sugar) and eaten as a dessert sweet.

0007The yellow-flowered lotus, N. lutea Persoon, from

the eastern USA, is the American counterpart species

of lotus. Its rhizomes are not as thick as those of the

Asian lotus, but it has been eaten by American

Indians in the past. Euryale ferox Roxb., of the

same family is an annual water plant occurring in

India, China, and southern Japan. It has large, orbicu-

lar leaf blades, 30–100 cm across, floating on the

water surface, and smaller flowers of some 4 cm in

diameter. A rather sweet starch extracted from the

seeds is used for making a kind of rice-cake in

Japan. In Central China, the caudices and leaf petioles

(with the skin removed) of this species are sometimes

used as vegetables.

Water Chestnut, also called Water

Caltrops or Trapa Nut (

Trapa natans

L.)

(Onagraceae: Willow Herb Family)

0008Water chestnut is an aquatic annual herb that grows

in ponds or marshes with its roots in the muddy soil at

6102 WATER PLANTS

the bottom of the water and with its upper leaves

floating on the water surface. Its slender stems are

of varying length, depending upon the depth of the

water, and bear feathery submerged leaves at the

nodes. The leaves of a normal kind, with a rhombic

blade shallowly serrate on the upper margins, are

gathered in a rosette at the tip of the stem; they are

deep green and shiny on the upper surface, slightly

hairy and often colored with purplish brown beneath.

The leaf petioles are swollen at the midway, enclosing

spongy aerial tissue, which helps the blade to float on

the water surface. Flowers are borne at the leaf axils

of floating leaves, and are pedicelled. They have a

four-part calyx, four white petals, four stamens and

a pistil. Fruits are nut-like, 4–5 cm wide and purple–

brown with two 1-cm-long, horn-like spines, one on

each side.

0009 The nuts are the edible part of water chestnut, and

are eaten boiled, or occasionally roasted; alterna-

tively, the dried nuts are ground into a starchy flour

to prepare a kind of gruel or for fermentation into a

local liqueur. Young nuts are sometimes eaten raw,

with the greenish skin removed. In China, the fruits

are listed as a Chinese medicinal plant, to be used as

an antipyretic and as an astringent.

0010 The use of the water chestnut as a food can be

traced back to Neolithic times, and the fruits are

found abundantly in Tertiary sediments. People of

Central Europe and Asia eat the typical water chest-

nut, collecting the fruits from wild plants, but in

central and southern China and in India, the variety

T. bispinosa (Roxb.) Kakino is cultivated in paddy

fields. Related species, which are occasionally culti-

vated in China and Korea, are T. bicornis Osbeck and

T. quadrispinosa Roxb. While the former species

bears larger fruits of 7–8 cm in width, the latter

bears four-horned fruits. In northeast China, there

occurs another wild species, T. maximowiczii Korsh.,

of which the smaller nuts (c.1–1.5 cm in width) are

also eaten locally, and sometimes used for fermenta-

tion into a local spirit.

Chinese Water Chestnut (

Eleocharis

dulcis

Trinius cv.

tuberosa

(Roxb.) T.

Koyama) (Cyperaceae: Sedge Family)

0011 The Chinese water chestnut, also known by the Chi-

nese names ‘Ma-ti’ or ‘Pitzi’, is a stem vegetable crop

derived form its wild stock, E. dulcis Trinius, which is

sporadically, but widely, distributed from Madagas-

car through the Indian subcontinent and southern

China to northern Australia and the Pacific Islands,

and also to southern Japan. It is rather widely culti-

vated in both mainland China and Taiwan, and some

parts of Thailand, not only for local consumption but

also as a source of preserved (canned) vegetables for

export.

0012Grown in paddy fields, often as a substitute crop of

rice (where rice is overproduced), plants of the Chi-

nese water chestnuts form a tuft of many slenderly

tubular stems, which are 60–90 cm high, deeply

green, hollow inside and transversely septate. Leaves

of a ‘normal’ kind disappear, being reduced to pale or

purple–brown sheaths surrounding the base of the

stems. Small flowers are borne in cylindrical spikes

at the apices of some stems; the spikes bearing

yellow–brown scales imbricatedly. The base of the

plants send out slender, elongated, underground

runners that form a tuber at the apex. Tubers are

depressed-globose, 2–5 cm in diameter and 1.5–3cm

high, and are clothed with thin, purple–brown to

black–brown scales. Being a high polyploid with

approximately 150 chromosomes, the cultivated

Chinese water chestnut does not produce seeds and

must be propagated by tubers.

0013The edible part of the Chinese water chestnut is the

tuber (see Table 1). It is a considerably popular ‘root’

vegetable (strictly speaking, a stem vegetable) among

the Chinese, and is also used in Thailand, the

Philippines and Indonesia, but not in the traditional

Japanese kitchen. There, they eat the tubers boiled

and seasoned in various vegetable dishes, and extract

fine-quality starch grains. In addition, minutely

chopped tubers are mixed with ground meat when

making Chinese meatballs. The tubers, rich in starch,

are crispy and slightly sweet, making a good ingredi-

ent in vegetable salads; hence the water-boiled tubers

are exported to various parts of the world as a canned

vegetable.

tbl0001Table 1 Chemical components of Chinese water chestnut

tubers (with skins removed)

America data

Chinese data

Major components (%)

Water 78.3 86.0

Protein 1.4 1.2

Fat 0.2 0.1

Carbohydrate 19.0 11.5

Fiber 0.8 0.1

Ash 1.1 1.1

Minerals (mg per 100 g)

Calcium 4.0 1.0

Phosphorus 65.0 64.0

Iron 0.6 0.9

Vitamin C (mg per 100 g) 4.0 7.0

Watt BK and Merrill AL (1963) Composition of foods. USDA Agricultural

Handbook No. 8, pp. 1–189. Washington, DC: US Department of Agriculture.

Chang WH and Chow LT (1980) Studies on the chemical and nutritional

compositions of 12 unexploited economic plants. National Science Council

Monthly 8: 827–832.

WATER PLANTS 6103

0014 In 1947, importation of canned Chinese water

chestnut into the USA reached 1154 tonnes per year.

The yield of this variety in China is 18–37 tonnes per

hectare. The wild stock of Chinese water chestnut

differs from the cultivated in having much smaller

tubers (only as large as 10 mm across) and due to

the fertile nature (seed-bearing) of the flowers. Rum-

phius in 1750 had noted that the natives of Java were

eating the tubers of this wild Chinese water chestnut,

which were seen in morning markets between August

and December. The cultivated Chinese water chest-

nut, apparently developed from the wild stock in

northern Vietnam or adjoining southern China, has

two obvious forms: the red-skin type (comparatively

large, soft tubers with reddish–purplish skin), and the

black-skin type (comparatively small tubers with a

harder flesh and covered with black–brown skin).

The former is for vegetable use, whereas the latter is

used for extracting high-quality starch grains called

the Ma-ti powder; the latter is principally used both

for cooking and as a stomach powder in Chinese

medicine.

Arrowhead (

Sagittaria trifolia

L. cv.

sinensis

(Makino) T. Koyama)

(Alismataceae: Arrowhead Family)

0015 A marsh plant of Chinese origin, arrowhead is culti-

vated in the paddy fields of temperate China and

Japan for the tubers. The cultivated arrowhead, cv.

sinensis, is much larger than its wild counterpart,

with its leaves attaining a height of 100 cm. Several

leaves grow from a short rhizome; they have a long

petiole, emergent from the water. The leaf blades are

sagittate (arrow-shaped) with two rather broad and

elongated basal lobes, and are 10–15 cm long. Culti-

vated arrowhead seldom bears flowers. The scape, if

produced, is normally lower than the leaves, and

bears three branches at several nodes. Flowers are

monoecious and unisexual, and the staminate flowers

are borne on the upper part of the raceme, the pistil-

late ones on the basal part of the raceme. Flowers are

about 2.5 cm across, and have three green sepals and

three white, roundish petals. Staminate flowers have

many yellow stamens. Pistillate flowers have a pistil

of many carpels, each maturing into an achene.

0016 The short rhizome sends out several slender under-

ground runners, and each develops a tuber at its tip

from late summer to autumn. Tubers are globular to

somewhat depressed-globular, 3–5 cm across, and

their white or bluish surfaces are zonate, bearing

deciduous, membranous scales. In winter, the tubers

bear a beak-like terminal bud at the apex.

0017The edible parts of the arrowhead are the tubers,

which are considerably rich in starch (24.8%); the

other components are water (68.2%), protein

(4.4%), fat and oil (0.23%), fiber (0.4%), and inor-

ganic salts (1.37%). Arrowhead tubers are eaten

boiled and seasoned in both Chinese and Japanese

cooking.

Water Shield (

Brasenia schreberi

Gmelin)

(Nymphaeceae: Water Lily Family)

0018This is a cosmopolitan water plant occurring in Asia,

North America, Australia, and Africa, but is eaten

only in Japan. The slender rhizome is located in the

mud of ponds or lakes, and stretches out its stems into

the water. The leaves are alternately arranged on the

stems, with the blades floating on the water surface.

The blades are broadly elliptic, 5–10 cm long,

‘freshly’ green and shiny above, and tinged with

purple beneath. Their petioles are peltately attached

to the center of the blade. Flowers appear during the

summer. They are about 2.5 cm across and are a

purplish brown color with six petals, some 16

stamens and six to 18 carpels. Sac-like fruits are

indehiscent, and usually mature in the water.

0019The stems, petioles, and lower surface of the leaf

blades are covered with a semitranslucent, agar-like

substance that is secreted from glands on the plant

surfaces. The young shoot apices, in particular, are

copiously covered with this gelatinous substance, and

this portion is eaten as a delicacy, but rather com-

monly, in Japan. They are used in Japanese-style clear

soup, salads, etc., or they are stored in vinegar and/or

in salt. Recently, water-boiled shoots of this plant

have been preserved in bottles and cans, and widely

sold.

See also: Ethnic Foods; Vegetables of Temperate

Climates: Oriental Brassicas

Further Reading

Chang WH and Chow LT (1980) Studies on the chemical

and nutritional compositions of 12 unexploited eco-

nomic plants. National Science Council Monthly 8:

827–832.

Rumphius GE (1750) Herbarium Amboinense, vol. 6, p. 7.

Watt BK and Merrill AL (1963) Composition of foods.

USDA Agricultural Handbook No. 8, pp. 1–189. Wash-

ington, DC: US Department of Agriculture.

6104 WATER PLANTS

WATER SUPPLIES

Contents

Water Treatment

Chemical Analysis

Microbiological Analysis

Water Treatment

J Manning, Wessex Water, Bath, UK

Introduction – Why Treat the Water?

0001 The aim of all water providers throughout the world

must be to collect, treat, and distribute a safe supply

of drinking water to their consumers that complies

with all legislative requirements in a cost-effective

way. The natural water cycle provides the raw mater-

ial but treatment processes are needed to remove

chemical and bacteriological contaminants that

would otherwise be damaging to health or affect the

esthetic appearance of the product at the customer’s

tap. Technical advances in treatment processes and an

understanding of the effects of raw-water contamin-

ation on consumers’ health will continue to make

cost-effective solutions to treatment more accessible.

This should mean that compliance with legislation

becomes an easier task, but is also likely to mean

that quality standards will continue to become more

stringent in the UK and Europe.

Water Sources

0002 As the aim is to provide safe clean water, it makes

sense to take the raw material from the point in the

water cycle where water is least contaminated, in

sufficient quantity and well placed geographically,

for the supply to reach the customers. Sources can

be derived from deep aquifers using boreholes, shal-

low groundwater such as springs, surface water col-

lected in reservoirs from lowland areas, or less

polluted upland areas, or direct abstractions from

rivers. Contaminants can come from agricultural

sources, surface water drainage or runoff, and sewage

treatment. Industrial pollution can also affect water

supplies. Harmful bacteria and viruses will be found

if there is fecal contamination of the source.

0003 Generally, raw groundwater from boreholes is of

good quality as the ground above the aquifer provides

a filtration medium to remove particles such as

bacteria. However, the time for water to travel

through strata can be sufficiently long to dissolve

other soluble contaminants such as naturally occur-

ring calcium and magnesium (hardness) but also

pesticides and nitrate from agricultural activities.

Wherever water is stored for a long period of time,

be it as naturally occurring aquifer or an artificial

impounding reservoir, reduced oxygen concentra-

tions will dissolve iron and manganese and produce

hydrogen sulfide (characterized by a rotten-egg

smell). This will result in a dirty and foul-smelling

water source, although this can still be successfully

treated. The long travel time from surface to aquifer,

with naturally occurring treatment on the way, means

that groundwater sources are very consistent in their

quality and hence desirable for ease of treatment. A

large proportion of the UK’s water supplies is ab-

stracted from boreholes in both chalk and greensand

aquifers.

0004Springs collect water from the ground into horizon-

tally laid pipework, and as this is very much closer

to the ground surface than a borehole, they are

more at risk from variable quality and surface water

intrusion.

0005Reservoir, lake, and river water sources, by their

nature, are generally contaminated with fecal mater-

ial, colored yellow or brown by the organic debris of

rotting vegetation and turbid from mud and silt, and

algae which will grow in suspension. The quality of a

surface water source can vary within hours, depend-

ing on weather conditions. This is especially true in

river intakes and seasonal changes must be expected

in reservoirs and lakes. Wet winter conditions will

cause runoff of agricultural contaminants such as

pesticides and nutrients that can cause excessive

algal growth (eutrophication). This can be controlled

to some extent by mixing of the reservoir using com-

pressed air lines or wind-powered mixing.

0006Storage reservoirs provide protection against short-

ages of rainfall but also blending of source waters to a

more consistent quality suitable for treatment. A stor-

age or retention time in excess of 7 days can provide

some settlement and a significant reduction in bacter-

ial numbers.

WATER SUPPLIES/Water Treatment 6105

0007 With improvements in technology, we are now

seeing a greater variety of source waters becoming

available for treatment. For example, reverse osmosis

is widely used for desalination of sea water where

other sources of fresh water are not available. Many

manufacturing industries will use complex treatment

processes to recover or reuse effluent produced in

manufacturing processes.

Treatment Processes

000 8 A summary of some commonly used treatment pro-

cesses is shown in Table 1. The treatment objectives

of a water treatment plant will be site-specific and

depend on the raw water quality, and the quantity

and standard of the treated water quality that it is

required to produce. The main objective will always

be to provide water that is microbiologically safe

for the consumers to drink. This is achieved using

appropriate disinfection processes either to kill or

remove bacteria, viruses, and cysts. The control

of the disinfection process is therefore considered

critical.

Disinfection

0009Chlorination is the most commonly used disinfection

process in the UK. Chlorine is effective because it is

toxic, highly oxidizing, soluble in water, and its ger-

micidal effectiveness lasts for days. It is also widely

available and therefore cost-effective.

0010Chlorine will react in water to form hypochlorous

acid, which can then dissociate into hydrogen and

hypochlorite ions, according to Eqn (1).

þ H

O ¼ HOCl þ H

þ OCI



ð1Þ

This reaction is very important, as the disinfecting

power of HOCl, hypochlorous acid, is about 40–80

tbl0001 Table 1 Summary of treatment processes

Example Uses

Disinfection

Chlorination, ozonation, chlorine

dioxide, ultraviolet irradiation

Oxidants used to kill bacteria and viruses

Coagulation

Aluminum sulfate, ferric chloride,

or polyaluminium chloride

Combines small, suspended, and colloidal particles into larger, flocculated

particles by chemical attraction. Algae, iron, manganese, color, and turbidity can be

entrained in floc particles which are then removed by solid/liquid separation

processes

Solid/liquid separation

Microstraining Large (20–100 mm) particles such as algae and leaves removed by straining

Dissolved air flotation Removal of coagulant sludges by floating on a curtain of small air bubbles

Sedimentation Removal of coagulant sludges by settlement. Suited to sludge with high inorganic

content

Slow sand filtration Removal of colloidal, organic, and suspended material by straining and biological

activity on the surface of sand filter beds

Rapid gravity filtration Removal of suspended material by straining and entrapment within the depth of sand

filter beds

Membrane micro- and ultrafiltration

(size exclusion)

Removal of submicron-sized particles using membranes manufactured from

polymeric material

Membrane nanofiltration and reverse

osmosis (diffusion control)

Removal of submicron particles and dissolved inorganic species of high

molecular weight

Membrane ion exchange or electrodialysis

(charge-controlled)

Removal of salts, i.e., desalination of sea water or nitrate removal

Adsorption

Granular-activated carbon filtration

(fixed bed) or powdered activated

carbon (requires filtration process)

Adsorption of pesticides and volatile organic compounds, removal of taste and

odors

Aeration

Packed towers or cascades Removal of H

S, volatile organic compounds, or carbon dioxide for corrosivity control.

Oxidation of iron and manganese

Chemical addition

Acid and alkali for pH adjustment Mineral acid and/or sodium hydroxide, sodium carbonate, or lime for optimum

coagulation

Phosphoric acid Provides phosphate coating on lead pipes to control lead concentrations in water

Softening

Lime addition Removal of calcium carbonate by precipitation at high pH. Reduces scale formation in

downstream structures

6106 WATER SUPPLIES/Water Treatment

times that of OCl



, hypochlorite. Chlorination is

more effective at pH values below 6 as there is a

high proportion of HOCl present and very little

OCl



. The quantity of HOCl and OCl



that is pre-

sent in the water is known as the free chlorine. The

effectiveness of chlorination will depend on factors

listed in Table 2.

0011 Chlorine reacts within minutes with a variety of

substances, including nitrite and ferrous ions. The

demand for chlorine from these reactions must

be satisfied before the slower reactions with the

bacteria cell walls can take place and hence provide

disinfection.

0012 To insure the disinfection process has taken place

satisfactorily, continuous monitoring of the residual

chlorine concentration will normally be used along

with regular analysis for fecal and heterotrophic

bacteria.

0013 For surface water, where there may be higher con-

centrations of organic contaminants or ammonia,

superchlorination, followed by dechlorination, may

be used. The chlorine concentration is held for some

time at a value much higher than that expected to be

found at the customer’s taps – around 2–5mgCl

1

This will quickly satisfy the chlorine demand of the

water, and the concentration is then reduced to

an acceptable level by adding sulfur dioxide. Treated

water will leave the plant with a residual of around

0.3 mg Cl

1

to control any subsequent regrowth

of bacteria in the distribution pipework to the

customers.

0014 Surface waters often contain humic and fulvic acids

from the decomposition of vegetation. This reacts

with chlorine to form trihalomethanes (THMs) such

as chloroform, which are suspected carcinogens. The

formation of THMs can be controlled by insuring

chlorination takes place after removal of the precur-

sor compounds. Concern in forming these byproducts

has led to an increase in alternative disinfectants such

as ozone, ultraviolet radiation, and chlorine dioxide.

0015 Research over the last decade has increased aware-

ness and concern over organisms that are harmful to

health and not affected by normal disinfection tech-

niques. Cryptosporidium and Giardia are both diffi-

cult to identify analytically, resistant to chlorine, and

form very small (2–5 mm) oocysts outside the host.

Fecal in origin, they are commonly found in rivers,

lakes, and surface water sources, but because of their

persistence in the environment and very small size

they can also be found in groundwater sources. Alter-

native treatment strategies based on removing the

organisms, using barrier treatment, rather than

killing them are required. Accepted practice is to use

well-controlled coagulation and rapid gravity filtra-

tion, preferably with an additional solids removal

stage such as DAF (dissolved air flotation) or sedi-

mentation, or using membrane micro- or ultra-

filtration.

Coagulation and Flocculation

0016Suspended, colloidal, and dissolved material is com-

monly found in surface water, such as fine clay par-

ticles, algae, and color from humic substances. These

dissolved substances and submicron-sized particles

can react with coagulant to form larger, agglomerated

solid particles that can more easily be removed by

solid–liquid separation processes. The surfaces of

very small particles are charged and the repulsive

forces between them help to keep them in suspension.

The coagulation mechanism is essentially twofold.

First, the coagulant forms highly charged hydrolyzed

ions in water which destabilize the charge on the

surface of the particles. Second, once the charge has

been destabilized, attractive forces, which cause the

particles to agglomerate, replace repulsive forces and

larger particles called floc are formed. This agglomer-

ation is referred to as flocculation. There are many

complex chemical and physical reactions taking place

during the coagulation/flocculation process, making

exact conditions difficult to predict. The factors

affecting efficiency are summarized in Table 3.

0017Predicting coagulation conditions is very difficult

and specific to each water source. Conditions can

change quickly with changing raw-water quality.

tbl0002 Table 2 Factors affecting the efficiency of chlorination

Parameter Effect onchlorination Typical value for good disinfection

Turbidity Absorbs chlorine and protects entrapped bacteria 0.4 NTU or less

pH Alters the proportions of hypochlorous acid and hypochlorite

ion present

pH 6 –7.5

Contact time Allows the oxidizing reactions to take place 30–60 min

Chlorine dose rate High concentrations will increase reaction rates 0.2–2 mg Cl

1

Organic contaminants Reacts to form less effective chloroorganic compounds Less than 5 mg C l

1

as total organic carbon

Temperature Increases the decay rate of free chlorine to chloride Up to 20



Ammonia Reacts to form chloramines Trace

NTU, nephelometer turbidity units.

WATER SUPPLIES/Water Treatment 6107

Solid–Liquid Separation

0018 After coagulation, the flocculated particles must be

removed. Removal of gross solids can be achieved

with sedimentation or dissolved air flotation and re-

sidual smaller particles can be removed using rapid

gravity or other filtration techniques.

0019 Sedimentation Flocculated particles are denser than

water and will settle readily by gravity. Generally,

plants are designed to operate continuously as upflow

clarifiers. Water enters at the bottom of the tank at a

low-enough velocity (1–3m h

1

) for the particles

to settle to the bottom, where the sludge can be

removed. The tanks combine the flocculation and

removal mechanisms in one plant as flocculation

occurs when water entering the bottom of the tank

has to travel through the sludge ‘blanket’ settled at the

tank base.

0020 Dissolved-air flotation Algae often have low spe-

cific gravity and algae-laden waters are therefore

suited to dissolved-air flotation because of their ten-

dency to float. A recirculated stream of clarified

water (8–10% of the flow) is saturated with dissolved

air under pressure using compressed air introduced

into packed pressure vessels. This is returned to the

inlet of the flotation tank with the flocculated water

where, on releasing the pressure, a curtain of very

small bubbles forms as the air comes out of solution.

These rise to the surface, attaching floc particles on

the way, to form a blanket of sludge that floats on the

surface. Periodically, the sludge is removed by

scraping the surface. Both sedimentation and dis-

solved-air flotation will remove coagulant sludge

to around 0.2–1.0 mg l

1

of aluminum or iron.

Sand filtration will be required to remove residual

coagulant.

0021Rapid depth filtration Depth filtration is achieved

by passing water that contains particles through a bed

of filtration media, usually sand, at relatively high

velocity or filtration rate. At a filtration rate of 3–

10 m h

1

, the particles are drawn into the medium

rather than remaining on the surface of the bed. This

has two advantages; a smaller structure is required

because of the high hydraulic capacity and the pres-

sure drop or head loss is reduced by avoiding the

build-up of a blinding layer of particles on the sur-

face. Filters can be constructed as either operating

under gravity or enclosed and under pressure. A sche-

matic diagram of a rapid gravity filter is shown in

Figure 1.

0022As particles block the filter, head loss will increase

until either flow from the filter reduces or a break-

through of particles is seen in the filter outlet water as

turbidity. Before either of these happens, the filter

should be washed. The bed is partly drained and air

tbl0003 Table 3 Factors affecting the efficiency of coagulation/flocculation

Parameter Effect on coagulation/flocculation Typical value for coagulation/flocculation

Coagulant type Ability to form large

polymeric hydrolyzed ions

Aluminum sulfate or iron (III) chloride, polyaluminum chloride

pH Affects charge destabilization

reactions; coagulants become

soluble at very high and low pH

pH 6.0–7.5, depends on coagulant used

Coagulant dose rate Insufficient will cause small floc

and ineffective removal, excess

may cause dissolution of

aluminum/ferric

or excess solids

1–10 mg l

1

(as aluminum or iron) depending on raw-water

conditions

Mixing speed Coagulation requires immediate

contact, i.e., rapid mix; flocculation

requires slow mixing to allow

particles to coalesce without

breaking up

Velocity gradients G, 1000 s

1

for coagulation, 20–200 s

1

for

flocculation

Contact time Sufficient time is needed to allow

reactions to take place

Seconds for coagulation, 5 –20 min for flocculation

Organic contaminants Typically, large molecules with charged

surfaces and therefore more difficult

to remove

High concentrations will require high coagulant doses

and lower pH

Alkalinity Alkalinity (bicarbonate/carbonate) is

used in the hydrolysis reaction.

Low-alkalinity water will form

weak floc

Greater than 30 mg CaCO

1

alkalinity

Temperature Affects reaction rates 5–25



6108 WATER SUPPLIES/Water Treatment