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

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.006 7 monitor, evaluate, and report on the world’s major

forms of malnutrition and progress achieved in

combating them.

Given malnutrition’s multicausal nature and multi-

sectoral dimensions, WHO collaborates extensively

with many multilateral (United Nations) and bilateral

(national) development agencies (see examples

above), and international scientific advisory bodies

and NGOs. A world-wide network of 28 distin-

guished collaborating centers in nutrition also

undertake a wide range of nutrition training and

research activities.

006 8 In view of nutrition’s foundational importance for

health and development, WHO focuses on priority

issues in basic nutritional science, nutritional care

throughout the life span from infancy to old age,

and nutrition policies and programs for sustainable

development. Today’s most urgent priority in tackling

these vast nutritional challenges is for WHO to focus

its combined normative and collaborative strength,

particularly through its technical outreach in regions

and countries, to strengthen the ability of all its 191

Member States to effectively prevent, control, reduce,

and, ultimately, eliminate malnutrition in all its forms.

Seealso: Anemia (Anaemia):Iron-deficiencyAnemia;

Carotenoids:Occurrence,Properties,and

Determination; Folic Acid:PropertiesandDetermination;

Food and Agriculture Organization of the United

Nations; Infants:Breast-andBottle-feeding; Iodine:

Iodine-deficiencyDisorders; Legislation:Codex;

Malnutrition:TheProblemofMalnutrition;Malnutritionin

DevelopedCountries

Further Reading

International Digest of Health Legislation database, http://

www.who.int/idhl/

Joint FAO/WHO Expert Committee on Food Additives

http://www.who.int/pcs/jecfa/jecfa.htm

Joint FAO/WHO Expert Committee on Pesticide Residues

http://www.who.int/pcs/jmpr/jmpr.htm

Monitoring Chemical Contaminants in Food http://

www.who.int/fsf/fdstxt1.htm#monitoring

Regional Office for Africa, Harare, Zimbabwe http://

www.whoafr.org/

Regional Office for the Americas, Washington, DC, USA

http://www.paho.org/

Regional Office for the Eastern Mediterranean, Cairo,

Egypt http://www.who.sci.eg /

Regional Office for Europe, Copenhagen, Denmark http://

www.who.dk/

Regional Office for South-East Asia, New Delhi, India

http://www.whosea.org/

Regional Office for the Western Pacific, Manila, Philippines

http://www.who.org.ph/

WHO Department of Nutrition for Health and Develop-

ment, http://www.who.int/nut/

WHO Global Database on Child Growth and Malnutrition

http://www.who.int/nutgrowthdb/

WHO Department of Child and Adolescent Health

and Development http://www.who.int/child-adolescent-

health/

WHO Programme on Cardiovascular Diseases http://

www.who.int/ncd/cvd/index.htm

WHO Food Safety Programme http://www.who.int/fsf/

WHO (1988) Four Decades of Achievement: Highlights of

the Work of WHO. Geneva: WHO.

WHO (1996) Physical Status: The Use and Interpretation

of Anthropometry. Report of a WHO Expert Commit-

tee. World Health Organization Technical Report Series

854. Geneva: WHO.

WHO (1998) Complementary Feeding of Young Children

in Developing Countries: A Review of Current Scientific

Knowledge. Geneva: WHO.

WHO (1998) Preparation and Use of Food-based Dietary

Guidelines. Report of a Joint FAO/WHO Consultation.

World Health Organization Technical Report Series

880. Geneva: WHO.

WHO (1998) HIV and Infant Feeding. Guidelines for De-

cision-makers: A Guide for Health Care Managers and

Supervisors; A Review of HIV Transmission through

Breastfeeding (WHO/FRH/NUT/CHD/98.1). Geneva:

WHO.

WHO (1999) Management of Severe Malnutrition: A

Manual for Physicians and Other Senior Health

Workers. Geneva: WHO.

WHO (2000) The Management of Nutrition in Major

Emergencies. Geneva: WHO.

WHO (2000) Obesity: Preventing and Managing the

Global Epidemic. Report of a WHO Consultation.

World Health Organization Technical Report Series

894. Geneva: WHO.

WHO (2000) Complementary Feeding: Family Foods for

Breastfed Children (WHO/NHD/00.1). Geneva: WHO.

WHO (2000) Nutrition for Health and Development: A

Global Agenda for Combating Malnutrition. Progress

Report (WHO/NHD/00.6). Geneva: WHO.

WHO (2000) Foodborne Disease: A Focus for Health Edu-

cation. Geneva: WHO.

WHO (2001) The Optimal Duration of Exclusive Breast-

feeding: A Systematic Review (WHO/NHD/01.08).

Geneva: WHO.

WHO (2002) Programming of Chronic Disease by

Impaired Fetal Nutrition. Evidence and Implications

for Policy and Intervention Strategies (WHO/NHD/

02.3, WHO/NPH/02.1). Geneva: WHO.

WHO (2002) Keep Fit for Life: Meeting the Nutritional

Needs of Older Persons. Geneva: WHO.

WHO (2002) Nutrient Adequacy of Exclusive Breastfeed-

ing for the Term Infant During the First Six Months of

Life. Geneva: WHO.

World Health Organization, Geneva, Switzerland http://

www.who.int/

WORLD HEALTH ORGANIZATION 6229

WORLD TRADE ORGANIZATION

S D Kalyankar and S P Kalyankar, Marathwada

Agricultural University, Parbhani, Maharashtra, India

C D Khedkar and A S Khojare, College of Dairy

Technology, Warud (Pusad), Maharashtra, India

Emergence of WTO

0001 As per the proposal made by the USA, the world econ-

omy organized around the International Monetary

Fund and International Bank for Reconstruction and

Development. In 1947, the General Agreement on

Tariffs and Trade (GATT) was created and signed by

23 countries on 30 October at the Palais des Nations in

Geneva and came into force in 1948 with its first

meeting at Havana, Cuba. Thereafter, GATT com-

pleted seven rounds. The eighth round, the largest

trade round (1986–1993), was known as the Uruguay

Round (UR). The UR proposals had covered agricul-

ture in a comprehensive manner for the first time, since

the creation of GATT in 1947. The problems of stagna-

tion in agricultural trade were presented in the UR at

Punda del est in 1986 and articulated in the Dunkel

proposals in December 1991. The Dunkel proposals

were eventually ratified, and these form part of the

rules of the World Trade Organization (WTO).

Objectives

0002 WTO is an international body dealing with the rules

of the international trade. The basis for the organiza-

tion are the agreements and negotiations signed by

the bulk of the world’s trading nations, which are

essentially contracts, binding governments to keep

their trade policies within agreed limits. It aims to

facilitate trade flow as freely as possible, as long as

there are no undesirable side-effects, and serves

as a forum for trade negotiations and as a dispute

settlement body. It encourages countries to enter

into free trade gradually through negotiation by

promoting fair competitions and trade without dis-

crimination.

Structure

Membership

0003 WTO currently has 132 member countries. Thirty-

two countries are currently negotiating for accession.

These include China and the Russian Federation, and

a number of developing countries.

Decision-making System

0004WTO has a three-tier system of decision-making. The

decisions are made at three levels: ministerial confer-

ence, general council and other councils, and heads of

delegations.

0005The top-level decision-making body is the ministerial

conference, which meets at least once in a year and can

make decisions on all matters under any of the multilat-

eral trade agreements. WTO has organized two minis-

terial conferences: Singapore (1996) and Geneva (1998).

0006In the intervening period between two conferences,

the general council acts on behalf of the ministerial

conference and carries out the day-to-day work of

WTO. The general council of WTO, together with

other councils for trade in goods, trade in services, and

intellectual property, performs WTO functions. These

councils are responsible for implementing WTO agree-

ments in their respective areas of specialization.

0007The third tier of decision-making system is at the

heads of delegation level. This is most effective in over-

coming inherent impediments for reaching consensus

on trade-related issues. This level is usually represented

by different sets of people, depending on the need of the

nation and their expertise and experience. The usual

process is to hold a meeting of about 40 countries,

which are most interested in a particular issue. Such

meetings are termed ‘green room meetings’.An

example of the success of such a meeting is the market

access negotiation. In the last 5 years of the operations

of WTO, its growing importance and membership have

proved to be effective in this new millennium, and will

grow further with changing requirements. The WTO

structure and decision-making process is adequately

equipped to face future challenges.

Agreements

0008These rules are contained in the following legal in-

struments.

0009Annex 1A. Multilateral Agreement on Trade in

Goods.

0010Annex 1B. General Agreement on Trade in Services

and Annexes.

0011Annex 1C. Agreement on Trade-related Aspects of

Intellectual Property Rights.

0012Annex 2. Understanding the Rules and Procedures

Governing the Settlement of Dispute

0013Annex 3. Trade policy Review Mechanism.

0014Annex 4. Plurilateral Trade Agreement.

0015Member countries are under an obligation to

ensure that their national legislation, regulations,

6230 WORLD TRADE ORGANIZATION

and procedures are in full conformity with the provi-

sions of these agreements. The harmonized rules

stipulate that there should be no unnecessary barriers

to trade and that a country’s exports are not disrupted

by the sudden imposition of higher tarrifs or other

barriers to trade.

Annex 1A. Multilateral Agreement on Trade in

Goods

1.0016 Agreement on Agriculture;

0017 Agreement on application of sanitary and phyto-

sanitary measures;

0018 Agreement on technical barriers to trade;

0019 Agreement on textiles and clothing;

0020 Agreement on TRIMS;

0021 Agreement on the implementation of Article VI

of GATT 1994;

0022 Agreement on preshipment inspection;

0023 Agreement on rules of origin;

0024 Agreement on import licensing procedures;

10.

0025 Agreement on subsidies and countervailing

measures;

11.

0026 Agreement on safeguards.

Agreement on Agriculture (AOA)

0027 AOA is a set of rules governing international agricul-

ture policy development to liberalize agricultural trade

and reduce the existing distortions in the sector, if any.

The agreement itself has concessions and commitments

of members regarding market access – removing vari-

ous trade restrictions confronting exports, domestic

support programs, and tackling the problem of making

exports artificially competitive. The reform program

requires that countries that have applied quantitative

restrictions to import be permitted to add to existing

import tariffs on the prices of imported products. This

process has come to be known as ‘tariffication.’ Under

the reform program, the countries, which granted sub-

sidies to their farmers will have agreed to reduce by

agreed percentages domestic subsidies that distort

trade and export subsidies.

General Agreement on Trade in Services (GATS)

0028 Services cover a wide range of economic activities

from banking, insurance, and telecommunications

to recreation, cultural, and sporting services. One of

the main characteristics of services is that they are

intangible and invisible. These characteristics influ-

ence the modes in which international trade transac-

tions in goods and services take place. While

international trade in goods involves the physical

movement of goods from one country to another,

relatively few service transactions involve cross-

border movements.

0029Trade in services is growing and accounts at pre-

sent for over 20% of international trade. GATS has

created a framework for bringing this trade under

international discipline. Its provisions apply to all

modes of supply of service in international trade, viz.:

0030cross-border trade in services;

0031establishment of a commercial presence in foreign

country;

0032temporary movement of natural persons to another

country to provide services there; and

0033movement of consumers to the country of import-

ation.

0034The agreement consists of a framework text, which

lays down a set of general principles for measures

affecting trade in services. The annexes applicable to

specific sectors complement the text.

Agreement on Trade-related Aspects of Intellectual

Property Rights (TRIPS)

0035Intellectual property right is the legally enforceable

power to exclude others from using the information

created or to set the terms on which it can be used.

The objective of the agreement on TRIPS is to give

adequate and effective protection to the intellectual

property rights to reward creativity and inventive-

ness. TRIPS sets out minimum protection to be

given to each category of intellectual property in the

national laws of each WTO member. It lays down

procedures and remedies to be provided by each

country for intellectual rights enforcement. The

agreement on TRIPS lays down minimum standards

of protection that countries must provide for intellec-

tual property rights. One of the other important fea-

tures of the agreement is that it also requires countries

to ensure effective enforcement of these rights. Its

rules apply to:

0036copyright and related rights;

0037trademarks;

0038patents;

0039geographical indications;

0040industrial designs;

0041layout designs of integrated circuits; and

0042undisclosed information.

See also: Legislation: History; International Standards

Wort See Beers: History and Types; Raw Materials; Wort Production; Chemistry of Brewing; Biochemistry of

Fermentation; Microbreweries

WORLD TRADE ORGANIZATION 6231

YEASTS

T Dea

k, Szent Istva

n University, Budapest, Hungary

Introduction

0001 Humans have consumed cereals since prehistoric

times. In Mesopotamia, baked pastes of ground

grain were consumed, while, in Egypt, leavened

bread was prepared. For thousands of years, humans

were unaware of the existence of yeasts but were able

to use them not only for breadmaking but also for

brewing. The fundamental role of yeasts and bacteria

in fermentation processes was only discovered in the

last century by Pasteur. Soon after this, Hansen estab-

lished the use of pure cultures for brewing and

baking. Until that time, bakers used spent brewer’s

yeast for leavening, or seeded the fresh dough with a

small part of already leavened dough. The commer-

cial production of yeast for baking purposes started

after about 1850, and since then, yeast technology

has grown into a highly productive and economic

industry. ‘Yeast’ in this article refers solely to the

species Saccharomyces cerevisiae.

The Organism

0002 Baker’s yeast is a biotype of S. cerevisiae that can

metabolize sugars both aerobically, producing the

end products carbon dioxide and water, and anae-

robically, producing ethanol and carbon dioxide.

Thus, baker’s yeast can be propagated in large

quantities under aerobic conditions, and the cell

mass added to dough can produce carbon dioxide

under various conditions to leaven bakery products.

These are the fundamental properties of baker’s

yeast that render it indispensable to the production

of bread. Through this century-old application,

many specific requirements have been imposed by

the producer and the user, and strains have been

selected with improved properties to meet these

requirements. The strains used today are able to

reproduce rapidly under strong aeration and limited

nutrient supply. The yeast cells show remarkable

tolerance to various storage conditions, and survive

drying. Baker’s yeast possesses strong fermentative

activity and flavor development in various condi-

tions of processing.

Production

0003There are at least four major steps in the manufac-

ture of baker’s yeast: preparation, fermentation,

separation, and packaging (Figure 1).

Preparation

0004The manufacture of baker’s yeast starts in two se-

parate areas: in the laboratory with the propagation

of a pure yeast culture, and in the factory with the

preparation of the fermenters and the nutrient

medium.

0005In the laboratory, where pure stock cultures are

maintained, a small flask of sterile fresh culture is

prepared after one or more subcultivations. This

sample is then inoculated into the first pure culture

tank. Two or three tanks of increasing capacity from

50 to 400 l may be applied. The yeast produced in an

earlier stage is used to seed the next stage, while

transfer is made under sterile conditions. In these

early stages of propagation, the main concern is

to maintain purity. The pure culture fermenters are

fed with sterile molasses medium supplemented

with the necessary growth factors, but aeration with

filtered air is not at full capacity in the first batch

fermentation.

0006Before the First World War, grain mash was used

for the commercial propagation of yeast. During the

war, shortages of grain led to its replacement by

molasses, a relatively cheap byproduct of cane and

beet-sugar production. Since then, molasses has

become the traditional source of carbon and energy

for yeast growth. It is usually fortified with a source

of nitrogen, minerals and growth factors. Nitrogen is

added in the form of ammonia, its salts, or urea;

phosphorus is supplied in the form of phosphoric

acid or ammonium phosphate. Depending on the

composition of the raw molasses, certain growth

factors, most frequently biotin, are also added to the

wort. The concentrated molasses is diluted, purified,

supplemented with other nutrients, and sterilized

before use.

Fermentation

0007 Baker’s yeast is usually produced in a multiple-stage

process. During scale-up, strong aeration and incre-

mental feeding are introduced. Full-scale fermenta-

tion is conducted in large (100 m

or larger) tanks.

There is a great variation in the size and shape of

fermenters. However, in their design, an important

requirement is to insure maximum aeration, because

it is the oxygen transfer that is usually the rate-

limiting factor in yeast propagation. Mechanical and

sparger aeration systems are generally used. During

the aerobic growth of yeast, a considerable amount

of heat is liberated, and an efficient cooling system

is also an integral part of the fermenter. The

strict demands for hygiene determine the overall con-

struction of the fermenter and the materials used

therein. Facilities for cleaning in place are always

integrated.

0008After cleaning and disinfection, the fermenter is fed

with water, in which the pure seed yeast is suspended,

then mixed with wort, and the propagation starts

with vigorous aeration. Baker’s yeast ‘fermentation’

is a typical fed-batch process in that, after commenc-

ing the propagation, nutrients are fed incrementally,

maintaining at all times a very low sugar concentra-

tion at full aeration. The protocols for nutrient feed

rate, temperature, pH, and aeration are specifically

set up and strictly controlled to optimize yield, prod-

uctivity, and product quality. Special attention is

paid to prevent underaeration, which leads to exces-

sive alcohol formation and a decrease in productivity.

Instrumental process control and automation are

necessary to produce baker’s yeast economically.

Adequate sensors and computer applications now

make it possible to control the most sophisticated

fermenter systems. Baker’s yeast producers, however,

have to consider the baking quality (stability and

activity) of the product, which can be attained at the

cost of productivity. As a satisfactory compromise, at

the final stage of fermentation, nutrient feeding is

stopped, and aeration is continued for about an

hour. During this ripening period, the properties of

baker’s yeast are improved. Nitrogen starvation

increases stability, but fermentative activity de-

creases. At the end of a typical fermentation, the

yeast solid content may vary between 3 and 8%,

which means a yield of about 20 000–30 000 kg

of fresh yeast in one batch propagation at 28–30



for 12–18 h.

0009Efforts to introduce continuous fermentations

on a commercial scale have remained unsuccessful.

Although continuous systems can be maintained at a

maximum yield, a good product quality can be

achieved only with propagation regimes that do not

easily lend themselves to continuous culture. More-

over, the problem of preventing contamination raises

the cost and makes the process economically

unfeasible.

Separation and Filtration

0010At the end of each fed-batch propagation period, the

yeast cells are recovered from the spent medium by

centrifugation. Water wash is applied between two

passages through centrifugal separators. A yeast

cream is obtained with 18–20% dry weight, which

can be stored in agitated tanks at 2–4



C for a few

days without any loss of quality.

0011The yeast cream is further concentrated by filtra-

tion on rotary vacuum filters or filter presses. Filtra-

tion yields a yeast cake of about 27–30% dry matter

content. (See Filtration of Liquids.)

Pure yeast culture

Seed propagation,

scale up

Molasses treatment

Preparation of

ingredients

1st stage fermentation

Filtered air

2nd stage fermentation

Separation

(yeast cream)

Cold storage

Filtration

(yeast cake)

Compression

packaging

Drying,

packaging

Active dried yeast (ADY)

Instant dried

east

(

IDY

)

Fresh compressed

east

fig0001 Figure 1 Flow chart for the production of baker’s yeast.

6234 YEASTS

Packaging

0012 After filtration, the yeast cake is mixed with oils,

emulsifiers, and a small amount of water, then com-

pressed and extruded into blocks, or granulated for

bulk distribution. The oil and emulsifiers improve

product appearance and aid the formation of blocks

(extrusion, cutting). (See Emulsifiers: Uses in Pro-

cessed Foods.)

The Product

Compressed Yeast

0013 This is the form of baker’s yeast produced by

the process outlined above. Compressed yeast is the

traditional form of baker’s yeast, which is available to

wholesale bakers in 0.5–2.0-kg blocks, while smaller

(10–50 g) blocks are prepared for households. Com-

pressed yeast wrapped with waxed paper and stored

at 4



C keeps for a few weeks. A granulated pressed

cake form of the same product packed in 10–20-kg

bags can also be prepared for large-scale bakeries.

0014 Compressed yeast cells are alive. They use their

reserve carbohydrates (glycogen, trehalose) for energy

to survive. Storage under refrigeration retards metab-

olism, and wrapping inhibits drying out. Under im-

proper storage conditions, deterioration processes

(autofermentation, autolysis) may start, resulting in

heat build-up and loss of yeast activity.

Dried Yeast

0015 In a dried form, baker’s yeast possesses a longer shelf-

life than compressed yeast. Dried yeast retains

stability even when stored at room temperature. It

offers benefits by reducing the cost of refrigeration,

transport and storage, but drying increases the ex-

penses of the manufacturer. The disadvantage for

the user arising from the lower activity of heat-

stressed cells is compensated by the ready availability

of dried yeast. In all, there is an increasing preference

and a growing share in the market for dried yeast.

There are two forms of dried yeast. The first, intro-

duced about 50 years ago, active dried yeast (ADY)

needs to be rehydrated in warm water before use.

Developed only in the last decade, instant dried

yeast (IDY) does not require rehydration and can

be mixed directly with flour in making dough. The

early procedures of yeast propagation for drying

are basically similar to those of the traditional baker’s

yeast fermentation. However, specific yeast strains

selected to withstand drying stresses are used, and

the final stages of propagation are set in order to

increase yeast resistance to drying. The feeding sched-

ule and maturation period are controlled to produce

yeast with lower protein but higher trehalose and

lipid contents.

0016Preparation of the yeast for drying begins with

extrusion of the compressed yeast cake into slender

strands (1–3-mm diameter) that are cut into short

pieces. These particles are dried in a hot air current;

the early procedure of tunnel drying has been mostly

replaced by tumble or rotating driers and, increas-

ingly, by fluidized bed driers. Only the latter are

suitable for the production of IDY. Airlift driers

employ a blast of hot air at a velocity sufficient to

suspend the yeast particles in a fluidized bed. Air

temperatures of 160



C can be used for quick drying

but, after loss of the free cell water content (at about

35% moisture), temperatures should not exceed



C in order to minimize cell-membrane damage

and reduction of enzyme activity. Cells are rapidly

killed at temperatures exceeding 50



C. (See Drying:

Spray Drying.)

0017The moisture content of ADY ranges from 6 to 8%,

whereas that of IDY is only between 4 and 6%. ADY

possesses only one-third to one-half of the leavening

activity of fresh compressed yeast (Table 1). The in-

stant drying procedure allows the production of yeast

with a leavening activity comparable with that of

compressed yeast.

0018ADY can be stored without refrigeration. During

storage, ADY loses its activity by 1% per month if

packed under vacuum or under nitrogen. On storage

in air at ambient temperature, the loss of activity is

faster. In order to restore activity, rehydration of ADY

should be carried out by adding warm (40



C) water

to the yeast in a 4:1 ratio. During rehydration,

20–30% leakage of intracellular materials occurs,

which leads to a loss of fermentation activity. A

tbl0001 Table 1 Main characteristics of commercial baker’s yeast products

Product Form Method of drying Dry (%) Protein(%)

Gas production (%)

Compressed yeast Blocks or granules None 30 52 100

Active dried yeast Irregular spheres Drum 94 40 45

Irregular particles Belt 94 40 40

Instant dried yeast Short rods Fluidized bed 96 48 80

Nitrogen 6.5.

Related to the activity of compressed yeast.

YEASTS 6235

further disadvantage of this leaching is that it releases

reducing substances, such as glutathione, which may

cause slackening in the dough.

0019 IDY particles are highly porous and easy to

rehydrate. This allows immediate use without prior

rehydration. However, air may also access the cells,

resulting in rapid oxidation and loss of activity.

Hence, IDY must be packed under vacuum or in a

nitrogen atmosphere, and must be used within a few

days after opening the package.

0020 Addition of a variety of agents to the yeast cake

prior to drying can improve activity and stability of

dried yeast. Emulsifiers (e.g., 1% sorbitan esters)

facilitate rehydration of IDY, and antioxidants (e.g.,

0.1% butylated hydroxyanisole) increase the stability

of ADY. (See Antioxidants: Synthetic Antioxidants.)

Applications

0021 The application of baker’s yeast is indispensable to

the production of leavened baked products, such as

breads, rolls, pastries, doughnuts, etc. Recipes and

technologies for these products vary world-wide,

but the essence of the process is the same, in that

after mixing flour, water, yeast, salt, and optional

ingredients, the dough undergoes panary fermenta-

tion before baking. The primary role of baker’s yeast

in the baking industry will be illustrated using as an

example the predominant product, white bread.

Breadmaking

0022 Conventional breadmaking technology involves

sponge dough. This dough comprises about two-

thirds of the total flour mixed with water, salt, and

yeast, and is left for a fermentation period of 4–5h.

The sponge is then added to the balance of flour,

water, and all remaining ingredients and thoroughly

mixed mechanically until it is transformed into a

smooth dough. The characteristic rheological proper-

ties of the dough are due to the structure of gluten,

a cross-linked network formed from wheat proteins

and lipids. This allows the elasticity of dough

to retain gas evolved by yeast and thus to leaven.

(See Wheat.)

0023 The dough undergoes a series of mechanical oper-

ations (divided into pieces, rounded, and moulded)

while being allowed to rest between these procedures

for short periods. During these proofing periods,

fermentation proceeds, and leavening continues.

After the final proof, loaves are placed into a hot

oven for baking. Within the loaf, gas expands,

steam and alcohol evaporate to form holes in the

coagulated matrix of gluten, and the characteristic

structure of the crumb sets. While the temperature

in the center of the loaf remains below 100



C, the

surface reaches 140



C, to form a hard, brown-

colored crust. The baked bread is left to cool before

the finishing operations (slicing, wrapping) and dis-

tribution. (See Bread: Breadmaking Processes.)

0024The conventional sponge dough technology

requires about 8 h to finish, and several alternative

methods have been developed to shorten this period

(Table 2). In the straight dough method, all the ingre-

dients are mixed at the start, and one bulk fermenta-

tion period of 2–4 h is allowed for leavening. In the

short-time dough process, only 15–30 min are allot-

ted for the dough to rest, and intense mechanical

working brings about the structure of the dough.

Time is also saved by the continuous mix processes,

in which a ferment or brew is first prepared from

yeast with little or no flour (liquid ferment), and

after about 2 h of fermentation, the dough is mechan-

ically developed in a continuous mixer. Bulk fermen-

tation of the dough can be replaced by intense

mechanical working and/or the addition of chemical

improvers in other process variants. Improvements

in equipment design have brought about savings in

labor, better control and automation, effective sanita-

tion, and greater processing flexibility of bread-

making technology.

Role of Yeast

0025Yeast plays three major functions in the dough:

leavening, maturing, and flavor development.

0026Leavening The increase of dough volume is due

to the production of carbon dioxide during yeast

fermentation of the carbohydrates available in the

flour. Dry flour contains approximately 1–8% fer-

mentable sugars (glucose, fructose, sucrose), whereas

maltose is produced from starch granules by wheat

amylases after wetting the flour. (See Flour: Dietary

Importance.)

0027Yeast has to adapt to the mostly anaerobic environ-

ment in the dough as well as to the fermentation of

maltose after depletion of available free sugars. Yeast

also has to tolerate a certain degree of osmotic pres-

sure exerted by salt (and sugars, if added). The con-

centration of solutes is higher at the first stage of

dough preparation when only half of the regular

water is added. In certain formulae, sucrose or

high-fructose syrup is used to sweeten the dough.

Increasing the osmotic stress not only reduces

the fermentation rate but also induces glycerol

production.

0028Maturation To some extent, both yeast itself and

its fermentation activity play important roles in

developing the texture of the dough, called matur-

ation. This involves complex changes, including

6236 YEASTS

the mechanical forces of mixing leading to gluten

formation. Carbon dioxide developed during yeast

fermentation would not produce gas cells without

the ability of viscoelastic gluten films to retain the

gas. In fact, it is the air bubbles preformed in the

dough during mixing into which the carbon dioxide

diffuses. The rheological properties of the dough are

influenced by the fermentation products (ethanol, pH

decrease) in a way not yet clearly understood. Re-

duced compound (e.g., glutathione) liberated from

yeast cells may split the disulfide bonds between

gluten molecules, leading to the cleavage of the gluten

structure.

0029 Taste and Flavor The characteristic and appealing

bread aroma would not develop without yeast. It is

difficult to characterize the complex nature of bread

aroma and to determine the precise role of yeast

fermentation in its development. More than 200 vola-

tile compounds have been identified by gas chroma-

tography, and many of these organic esters and acids,

alcohols, and carbonyl compounds are formed as

byproducts of yeast fermentation. Other compounds,

such as amino acids, originate from the yeast cells. In

addition to fermentation, bread aroma is determined

by the process of baking, which leads to crust

browning. This is a Maillard-type reaction, and its

extent is influenced by the fermentation products of

yeast and its cell constituents. (See Browning: Non-

enzymatic; Flavor (Flavour) Compounds: Structures

and Characteristics.)

Sour Dough

0030Before baker’s yeast was available commercially, part

of leavened dough was added as an inoculum to the

fresh dough. Acidification normally took place in this

old dough, hence the name: sour dough. In recent

years, consumption of sour-dough breads has greatly

increased. The use of sour dough is necessary for

the development of characteristic properties of rye

breads. Sour doughs contain both heterofermentative

lactic acid bacteria and yeasts, and the mixed popula-

tion usually comprises several different species of

both groups. Unlike baker’s yeast consisting over-

whelmingly of a single yeast species, S. cerevisiae,

even commercial sour-dough starters are not made

of defined pure cultures. Lactobacillus brevis and

L. sanfrancisco are characteristic lactic acid bacteria

in sour dough, whereas Candida milleri and S. exiguus

are the predominant yeast species.

Yeast Starter Cultures

0031While about 1.5 million tons of baker’s yeast are

produced annually throughout the world, the esti-

mated production of yeast starter cultures is less

then 1000 tons, although these are made commer-

cially in many countries. Wine starter cultures are

used to initiate the fermentation of must instead of

a natural (spontaneous) process. The use of yeast

starter cultures in wine-making has increased strongly

over the last decade. Freeze-dried or active dry yeast

starters are made of selected strains of various useful

tbl0002 Table 2 Schematic comparison of breadmaking processes

Time (h) Sponge dough Straight dough Continuous mix Short-time dough

Mixing

Sponge

Dough

Dividing

Rounding

1st proof

Molding

2nd proof

Bakin

Mixing

Dough

Dividing

Rounding

1st proof

2nd proof

Baking

Bakin

Liquid ferment

Mixing

Developing

Dividing

Rounding

Mixing

Proofing

Baking

High-speed mixing

Dividing

Rounding

1st proof

Molding

2nd proof

YEASTS 6237

technological properties, such as a tolerance of high

concentrations of ethanol, sugar, and sulfur dioxide,

low temperature, etc. Their use is also advantageous

in sparkling-wine production.

0032 The use of dried yeast in breweries is not common,

although it may offer advantages such as yeast avail-

ability, flexibility, and cost-effectiveness in particular

for producing specialty beers in small quantities.

Future Developments

0033 Although baker’s yeast is one of the oldest biotechno-

logical products that is used in large quantities and

has been the subject of basic biochemical and genetic

studies, there have remained a number of unresolved

questions about its physiology relevant to production

and application.

0034 The carbohydrate metabolism of S. cerevisiae is

under strict regulation, and it has been long recog-

nized that a high sugar concentration triggers alco-

holic fermentation, even under fully aerobic

conditions (the so-called ‘Pasteur effect’). Hence,

baker’s yeast is produced with strong aeration and

limited sugar addition in a fed-batch process.

However, cultures adapted to these conditions are

required to perform leavening in drastically different

conditions prevailing in the dough, i.e., anaerobically

and with sugars in excess. Moreover, when glucose

and fructose present initially are exhausted, yeast

cells have to adapt the metabolism of maltose origin-

ating from flour starch. Hence, baker’s yeast with a

good leavening capacity should have a high glycolytic

activity, a high potential of maltose fermentation, and

an ability to synthesize enzymes under anaerobic con-

ditions to adapt rapidly to changing substrates. In

attempts to improve these characteristics, overexpres-

sion of certain glycolytic enzymes, modulation of the

balance between glycolysis and gluconeogenesis, and

induction or derepression of maltose utilization

enzymes have been targeted using genetic transfor-

mants, producing mutants and constructing hybrids.

Recombinant DNA technology provides further ways

to enhance biomass yield on molasses and alternative

carbon sources. Molasses contains raffinose, of which

only the fructose part can be utilized by baker’s yeast.

Brewer’s yeast, a close relative, produces melibiase,

permitting complete fermentation of the trisacchar-

ide. Baking yeast strains with this ability have been-

constructed, resulting in an increase in biomass yield

up to 8%. A genetically modified baker’s yeast,

developed at Gist-Brocades (Delft, The Netherlands)

in 1990, was the first to be approved for use.

Currently, strains constructed by intraspecific hybrid-

ization and gene transfer are not regarded as true

genetically modified organisms (GMOs); neverthe-

less, commercial use of genetically manipulated

yeast, or any other organisms, raises issues of safety,

regulation, and labelling by authorities and receives

serious concern from the public. To date, successful

strategies of genetic engineering to introduce foreign

genes into S. cerevisiae enabling the utilization of

alternative carbon sources have remained at labora-

tory scale.

0035Recombinant DNA techniques have been applied

to incorporate and express amylase genes from

bacilli, molds, and different yeast species into baker’s

yeast, enabling starch hydrolysis. S. cerevisiae strains

have also been created that are able to utilize lactose,

pentoses from hemicellulose, and even cellulose.

Whey, which is rich in lactose, would be a good

alternative carbon source, but baker’s yeast lacks

the necessary enzymes to utilize lactose. Laboratory

strains of S. cerevisiae have been constructed by both

interspecies protoplast fusion with the yeast Kluyver-

omyces lactis and genetic transformation of genes

from the bacterium Escherichia coli or the mold

Aspergillus niger.

0036The keeping quality of baker’s yeast is a complex

property that is not well characterized in terms of

physiology and biochemistry. Factors influencing

cryoresistance, osmotolerance, and tolerance to

drying have a great impact on the loss of leavening

performance of ADY in frozen doughs and sweet

doughs. In this respect, the cryoprotective effect of

trehalose, the osmoprotective role of glycerol, and the

stress tolerance attributed to membrane composition

have received most attention. Molecular genetic stud-

ies have allowed a deeper insight into the regulation

of trehalose, glycerol, and phospholipid metabolism,

the knowledge on which strategies to improve keep-

ing quality of baker’s yeast can be based. Conditions

of yeast propagation and storage have been optimized

to produce yeast cells with higher contents of treha-

lose, phospholipids, unsaturated fatty acids, and

ergosterol. Glycerol accumulation has been achieved

in genetically altered strains.

0037Strain development by recombinant DNA technol-

ogy may have a further impact on the application of

baker’s yeast. The supplementation of bread doughs

with enzymes has increased greatly and has become

a standard practice in breadmaking. The use of

amylases, often together with protease, lipase, lipoxi

genase or other enzymes, improves the functional

properties of dough, decreases the mixing time,

and contributes to flavor development. These com-

mercial enzymes are produced with filamentous

fungi or bacilli, and may contain impurities adversely

affecting bread quality or acting as allergens. Possi-

bilities exist to genetically engineer baker’s yeast

6238 YEASTS

secreting one or more enzyme additives during

the process of breadmaking. Success achieved with

laboratory strains may be impractical, however, at

the industrial level. Moreover, the approval for

commercial use of genetically manipulated yeast

expressing heterologous proteins is still problematic

and has met with public resistance. Nevertheless,

taking measures to prevent any possibility of harmful

consequence of genetic manipulation, recombinant

strains will have an important role in future develop-

ments in the production and application of baker’s

yeast.

See also: Antioxidants: Synthetic Antioxidants; Bread:

Breadmaking Processes; Browning: Nonenzymatic;

Drying: Spray Drying; Emulsifiers: Uses in Processed

Foods; Filtration of Liquids; Flavor (Flavour)

Compounds: Structures and Characteristics; Flour:

Dietary Importance; Starter Cultures; Wheat: The Crop;

Grain Structure of Wheat and Wheat-based Products;

Wines: Production of Table Wines; Production of

Sparkling Wines; Yeasts