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

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retained was that on the quality of water intended for

human consumption.

0013 By 1980, the original 1969 and 1973 programs had

been virtually abandoned and only a handful of the

proposed vertical directives survived. These directly

related to nine commodities or products, discussed

above.

0014 Following the 1977 initiative, work was concen-

trated on the directive on the labeling, presentation,

and advertising of foodstuffs which was adopted in

1979 and on the specific directives required by the

directive on materials and articles in contact with

food which had been adopted in 1976.

0015 During the early 1980s a number of proposed

directives were under discussion but very few were

adopted. By 1985 only two-fifths of the original 1969

and 1973 harmonization programs had been imple-

mented. The food industry was finding the cost of

compliance extremely high, particularly in the imple-

mentation of the requirements of the vertical direct-

ives that had been adopted.

0016 A major turning point in the progress towards

harmonization and an important influence on the

revised programs for the 1980s was the European

Court of Justice judgment on the ‘Cassis de Dijon’

case. This concerned a German prohibition on the

import of the blackcurrant liqueur Cassis de Dijon

from France as the alcohol content was lower than

that permitted by German law. The court ruled that

the barrier created by the German regulation was

incompatible with the Treaty of Rome, and that

quantitative restrictions on imports should be pro-

hibited. This judgment provided a general principle

for the trade in foods between member states which

was that, provided a foodstuff complies with the

regulations in force in the member state in which it

is manufactured, it should be permitted on the market

of any other member state.

0017 The Cassis de Dijon case is now regarded as being

pivotal in the major change of approach by the Euro-

pean Council and Commission to the harmonization

program. There was a shift away from the vertical

directives and it was determined that future Commu-

nity legislation on foodstuffs should be limited to

provisions that are justified by the need to protect

public health, provide consumers with necessary in-

formation about foods, provide for necessary public

controls, and insure fair trade.

The 1985 Initiative

0018 By the early 1980s it was becoming very clear that,

despite revised timetables and changes to strategy,

the harmonization program was not working and a

completely new approach to food legislation was

required. The need for a new approach to harmoniza-

tion was given an additional impetus in 1985 with the

publication of a White Paper from the Commission to

the Council entitled Completing the Internal Market.

The White Paper addressed the fact that there had

been a significant imbalance between the ‘horizontal’

food legislation (e.g., labeling and additives), where

considerable progress had been made, and the vertical

legislation covering composition, in which there had

been relatively little progress.

0019To overcome these problems it was proposed that a

concept of food legislation was required which only

contained provisions that could be justified as being

essential in the general interest. The main principles

to be followed were the protection of public health

and the assurance of fair trading. The legislation

should also insure that consumers were provided

with information and protection in matters other

than health and that provisions were made for neces-

sary public controls.

0020The new approach, initiated by the 1985 White

Paper, redefined the method of harmonization and

replaced the vertical system with a horizontal one. It

also attempted to simplify the procedures to reduce

the amount of work involved and to speed up the

harmonization process. This new approach was to

be based on three general principles. In summary

these were:

0021that there was the need to distinguish between

what was essential and required harmonization

and what may be left to the mutual recognition of

national legislation and standards;

0022that Council Directives based on Article 100 of the

Treaty of Rome would be restricted to specifying

essential requirements for health and safety which

would be obligatory in all member states and com-

pliance with these would allow free movement of

products within the Community;

0023that, whilst the harmonization of standards would

be encouraged, the absence of a European standard

could not be used as a barrier to trade. In the

absence of a European standard the recognition of

national standards would be the guiding principle.

The White Paper, and a Commission communication

which followed it, defined a new approach in which

the vertical system was to be replaced by a horizontal

one. The procedures were also simplified to reduce

the work involved and to speed up the harmonization

process. The new program was to be centered on five

framework directives that were considered crucial to

the single market in foodstuffs. These were:

0024additives

0025food labeling and presentation

EUROPEAN UNION/European Food Law Harmonization 2197

.0026 official control of foodstuffs

0027 foods for particular nutritional uses

0028 materials and articles in contact with food

The new proposals required the European Council to

adopt the framework directives that laid down the

general principles and essential requirements for

health and safety. The European Commission had

the responsibility to introduce measures giving the

details and technical specifications that would

enable the requirements to be achieved. For example,

the framework directive on food additives (89/107/

EEC) outlines the general principles for the evalu-

ation and control of food additives based on the

criteria of safety and need. From this are derived a

number of technical directives giving the lists of per-

mitted additives, their conditions of use, and their

purity criteria.

0029 A further, and significant, influence on the need for

a change of harmonization policy was the publication

of a report in 1988 entitled The European Challenge

1992 – The Benefits of a Single Market. The research

was funded by the European Commission and the

chairman of the steering committee of the research

group was Paolo Cecchini, hence the popular descrip-

tion of the paper as the Cecchini Report. This sub-

stantial report evaluated all the industry sectors,

including those involved with food production and

manufacture. It pointed out that the foodstuffs sector

was the biggest contributor to jobs and most value-

added of all the industries in the European Com-

munity, and that it appeared to be well placed to

confront international competition in the 1990s.

However, it then qualified these statements by saying

that appearances may be misleading as the trend

towards global consolidation is dominated by com-

panies from the USA. With the exception of Unilever

and Nestle

, US companies fill the other eight places of

the world’s top 10 food companies.

0030 The second, and very crucial, proviso was that the

Community market for foodstuffs was segmented by

a range of trade barriers that appeared to be on the

increase. The harmonization program, which by

then had been in operation for almost a quarter of a

century, had failed in its primary objectives.

0031 The estimation of the financial cost to the food

industry which resulted directly from nontariff

barriers was in the range of Ecu500–1000 million

annually, not counting the inevitable restrictions in

consumer choice. The report also estimated that the

savings, which would follow the elimination of these

trade obstacles, would represent between 2 and 3%

of the value-added component of the food sector,

which alone accounted for just over 4% of the Com-

munity’s gross domestic product. The savings were

also estimated to equal up to a 2-year gain in the

industry’s productivity.

The 1990s

0032Even with the more simplified and streamlined system

introduced with the 1985 initiative, progress was

slow and much of the proposed legislation was not

adopted at the introduction of the Single Market in

January 1993, 35 years after the signing of the Treaty

of Rome.

0033Between the end of 1991 and early 1993, five

hygiene directives were adopted. Directive 93/43/

EEC covered general food hygiene whilst the other

four were for specific groups of foods such as milk

and milk products; poultry meat; farmed game bird;

and rabbit meats and wild game meats. This plethora

of hygiene requirements was soon found to be overlap-

ping and a few years later moves had already started to

effect a consolidation into a single piece of legislation.

0034Other legislation in the early 1990s covered or-

ganic food production and control, genetically modi-

fied organisms, and the protection of designations of

origin and geographical indications.

0035A communication from the Commission to the

Council and Parliament on the Management of the

Mutual Recognition of National Rules after 1992

was issued in December 1993. This document listed

20 barriers to trade in foodstuffs which had been

identified by member states during the period 1990–

92 and for which many of the Community measures

planned in the 1985 White Paper were yet to be

adopted or to enter into force.

0036By the end of 1994 the greater part of the proposed

harmonization legislation was in place. Two of the

important additives directives, those on colors and

sweeteners, were adopted in June 1994, and although

the directive on additives other than colors and sweet-

eners reached common position in December 1994, it

was only finally adopted in early 1995.

0037Legislation still outstanding by the middle of 1995

included the proposed regulation on novel foods and

the directive on quantity indication (QUID), both

of which achieved common position in June 1995.

However, further delays meant that both the novel

foods regulation and the QUID directive were only

finally adopted in January 1997. By this time draft

directives to amend the directive on sweeteners and

the directive on additives other than colors and sweet-

eners had also been circulated.

0038A number of directives required under the frame-

work directive on foods for particular nutritional uses

were still outstanding by mid-1995, as were proposals

for the fortification of foods and food supplements.

A proposal for a directive on food claims was

2198 EUROPEAN UNION/European Food Law Harmonization

withdrawn and a new proposal on unit pricing

introduced.

0039 During 1994 the Commission received a total of 35

complaints about obstacles to trade in foodstuffs in

contravention of Article 30 et seq of the Treaty of

Rome. An analysis of these complaints showed that

the principal obstacle was the national requirements

of prior authorization procedures, whether or not

they involved the issuing of certificates, designed to

control the composition of the foodstuffs, in particu-

lar those containing nutrients such as vitamins and

trace minerals and/or additives.

0040 Other examples of complaints listed in the Com-

mission’s report for 1994 were the requirement that

an accompanying note be produced when sugar is

imported into Italy (this was in order to control its

use in wine), the ban on the use of sorbic acid in milk-

chocolate fillings in France, and the requirement

imposed by the Spanish authorities that an official

health certificate of the country of origin be produced

as evidence that products have been manufactured in

accordance with the rules in force in the country

of origin and that their consumption presents no

health risks.

Post-2000

0041 At the end of 1999 it was apparent that the program

was still incomplete. Although some directives and

regulations had been under discussion for a number

of years, they had still not been adopted. These in-

cluded a directive on sports nutrition products, one

on fortified foods, and a controversial directive on

food supplements. There were also a number of out-

standing technical directives and regulations, such as

those on the control of additives used in flavorings,

and further controls on the labeling of foods contain-

ing genetically modified organisms.

0042 In December 1999 the European Commission pub-

lished its White Paper on food safety. This reiterated

previous statements that the European Union’s (EU’s)

food policy must be built around high standards of

food safety that serve to protect, and promote, the

health of the consumer. It also stated that the central

role of the Internal Market was to insure that con-

sumers were offered a wide range of safe and high-

quality products coming from all member states. This

principle should also apply to food imported from

countries outside the EU.

0043 The White Paper stated that both EU and member-

state food safety systems had been under unpre-

cedented pressure during the food and animal feed

emergencies in the second half of the 1990s. These

emergencies had exposed weaknesses in the systems

both at EU and member-state level and the White

Paper assessed the requirement for action by the

responsible authorities (the Commission, member

states, and European Parliament) to reinforce, im-

prove, and further develop existing food safety

systems.

0044The paper outlined the requirement for a farm-to-

table policy that included traceability of animal feed,

human foods, and their ingredients.

0045It was also determined that risk analysis must form

the foundation on which the future food safety policy

should be based. This was developed into the three

components of risk analysis, which are risk assess-

ment, risk management, and risk communication.

The paper also stated that, where scientific evidence

is insufficient, inconclusive, or uncertain, measures

based on the ‘precautionary principle’ should apply.

0046A new legal framework of the control of human

food and animal feed was laid out in the paper. The

inclusion of animal feed was on the premise that the

safety of food from animal origin begins with safe

animal feed.

0047The Commission proposed that there should be a

comprehensive piece of legislation to recast the differ-

ent food control requirements already in existence.

This would take into account the general principle

that all parts of the food production chain should be

subject to official controls. An annex to the paper

gave an action plan on food safety, which included a

number of initiatives relating to food and animal feed

safety. This plan covered 84 proposals for regula-

tions, directives, and supporting procedures. The

timetable given in the plan required that the majority

of the legislation would be adopted by the Commis-

sion by the end of 2000, with final adoption by the

Council and European Parliament by the first half of

2002. By the end of 2001 many had already fallen

behind the timetable.

0048The rationale for the establishment of a European

Food Authority was given in detail. The Food Author-

ity was envisaged as having particular responsibilities

for both risk assessment and communication on food

safety issues.

0049From early 2000 onwards two subtle changes were

taking place in the European food law. One was the

development of legislation not directly related to the

original harmonization program but dealing with

emerging issues. This included detailed legislation

on contaminants in foods such as mycotoxins and

dioxins.

0050The other change that has been taking place is the

move from directives to regulations for a wider spec-

trum of food law. This is an interesting move as it

effectively speeds up the legislative process. Directives

require member states to modify their internal legisla-

tion to incorporate the requirements of the directive

EUROPEAN UNION/European Food Law Harmonization 2199

within a given period, which can be quite long.

Regulations, however, require the member state to

introduce the legislation as the unaltered text of the

regulation and the time allowed is relatively short,

often a matter of months. As a consequence, regula-

tions allow for a more rapid implementation of the law.

General Food Law

0051 One of the most glaring omissions, which had still not

been resolved by the end of the 1990s, was the legal

definition of a food and a single document giving the

principles and philosophy of European food law.

Although the precepts of the legal aspects of Euro-

pean control had been introduced into individual

directives and regulations, there was no single official

document to cover these aspects. For many years the

concept of a directive on general food law had been

under discussion, with the objective of producing a

piece of legislation to clarify the parameters for the

structure and operation of European food law.

0052 It was not until 1997 that the European Commis-

sion published its Green Paper on food law, which

laid down the common principles underlying the food

legislation and established food safety as the primary

objective of food law. The Green Paper was followed

at the end of 1999 by the Commission’s White Paper

on Food Safety. Chapter 6 of the White Paper advo-

cated a new legal framework for food safety em-

bodied in legislation covering general food law.

0053 Just a year later, at the end of 2000, the Commis-

sion published a proposal for ‘a Regulation laying

down the general principles and requirements of

food law, establishing the European Food Authority,

and laying down procedures in matters of food

safety.’ Article 2 of the proposal gave a definition of

food for the first time. In essence, this covered any

substance or product, whether processed, partially

processed, or unprocessed, intended to be, or reason-

ably expected to be, ingested by humans. It included

drink, chewing gum and any substance, including

water, intentionally incorporated into the food during

its manufacture, preparation, or treatment. Exclu-

sions from the definition included medicinal prod-

ucts, cosmetics, tobacco and tobacco products, and

narcotic or psychotropic substances.

0054 As stated in its title, the proposed regulation also

contained the requirement for the establishment of

a European Food Authority, which would have the

objective of providing scientific advice and scientific

and technical support for EU legislation in all fields

that have a direct or indirect impact on the safety of

human food and animal feed.

0055 The mission for the Authority as given in the pro-

posal was the contribution to a high level of protection

of human life and health and in this respect to take

account of animal health and welfare, plant health,

and the environment. This was to be carried out in

the context of the operation of the internal market.

0056The Authority was to comprise a management

board, an executive director and staff, an advisory

forum, a scientific committee, and permanent scien-

tific panels. The proposal specified eight scientific

panels, which would be required to cover the various

aspects covered by the Authority’s remit. The panels

were to be composed of independent scientific experts.

0057A further part of the proposed regulation specific-

ally related to food safety and covered the requirement

for a rapid alert system, emergencies, and crisis man-

agement. This initiative was mainly as a consequence

of a number of European food scares during the late

1990s such as bovine spongiform encephalopathy

(BSE) in cattle and the Belgian and Dutch dioxin crises.

0058The proposals also included the requirement for full

traceability of food, animal feed, and food-producing

animals. The responsibilities of food and animal feed

operators are given and the concept of the precaution-

ary principle is laid down in law.

0059These proposals were finally adopted in January

2002 as Regulation (EC) N



178/2002.

See also: Legislation: History; Additives; Contaminants

and Adulterants

Further Reading

Berry Ottaway P (1995) Harmonisation of European Food

Legislation. FT Management Report. London, UK:

Pearson Professional.

Cecchini P, Catinat M and Jaquemin A (1988) The Euro-

pean Challenge 1992 – The Benefits of a Single Market.

Aldershot, UK: Wildwood House.

Commission recommendation of 20 September 1965 con-

cerning prior communication to the Commission in

draft form of certain laws, regulations and adminis-

trative instructions. OJ 65* 160 of 29 September

1965, p. 2611. Luxembourg: Office for Official Publi-

cations of the European Communities.

Communication from the Commission to the Council and

to the European Parliament (1985) The Completion of

the Internal Market: Community Legislation on Food-

stuffs. COM(85)603 Final. Brussels.

Communication from the Commission to the Council and

European Parliament (1993) The Management of the

Mutual Recognition of National Rules after 1992.

COM(93)669 Final. Luxembourg: Office for Official

Publications of the European Communities.

Council resolution of 28 May 1969 drawing up a pro-

gramme for the elimination of technical barriers to

trade in industrial products. OJ69 C 076 of 17 June

1969, p. 1. Luxembourg: Office for Official Publications

of the European Communities.

2200 EUROPEAN UNION/European Food Law Harmonization

Council resolution of 17 December 1973 on industrial

policy. OJ73 C 117 of 31 December 1973, p. 1. Luxem-

bourg: Office for Official Publications of the European

Communities.

European Court of Justice (1979) Case 120/78. European

Courts Reports p. 649.

Goodburn K (ed.) (2001) EU Food Law. Cambridge, UK:

Woodhead/CRS Press.

Green Paper on General Principles of Food Law in the EU

(1997) European Commission COM(97)176 Final.

Proposal for a regulation of the European Parliament and

Council laying down the general principles and require-

ments of food law, establishing the European Food Au-

thority and laying down procedures in matters of food

safety (2000) European Commission COM(2000)716

Final. Luxembourg: Office for Official Publications of

the European Communities.

Report from the Commission to the Council and European

Parliament (1995) The Single Market in 1994.

COM(95)238 Final. Luxembourg. Office for Official

Publications of the European Communities.

Treaty establishing the European Economic Community,

Rome, 25 March 1957. London: HMSO.

White Paper on Food Safety (2000) European Commission

COM(2000)1. Luxembourg: Office for Official Publica-

tions of the European Communities.

White Paper from the Commission to the European Council

(1985) Completing the Internal Market. Luxembourg:

Office for Official Publications of the European Com-

munities.

Evaporated Milk See Condensed Milk

EVAPORATION

Contents

Basic Principles

Uses in the Food Industry

Basic Principles

P H Ferguson, Cornwell Products, Edenbridge, UK

This article is reproduced from Encyclopaedia of Food Science,

Background

0001 Evaporation is the removal of water, or another

solvent, from a solution by the application of heat –

primarily the latent heat of vaporization – to the

solution. Thus, it can be seen that evaporation

involves a heat-transfer process that heats a liquid in

order to generate a vapor, which in turn has to be

separated and, in nearly all cases, subsequently con-

densed. The theory of evaporation therefore is based

upon the principles of heat transfer and, in particular,

their application to the boiling (evaporation) of

fluids, and the condensation of vapours. (See Heat

Transfer Methods.)

0002 Considering the process economics aspect, the

heat-transfer step is important as its efficiency is

the factor having the greatest influence on both the

capital and the operating costs. The vapor–liquid

separation is also critical as the effectiveness with

which it is carried out influences the product losses

(by way of vapor entrainment), pollution of the con-

densate steam byproduct, and potential corrosion

problems of the condenser surfaces. The subsequent

condensation of the evaporated vapor also requires

careful consideration, because not only does it have a

bearing on the operation of the evaporator system,

but perhaps more importantly, it significantly affects

the system’s operating economy.

0003Conventionally, the heat required is provided by

low-pressure steam (generated from a range of

primary fuels), although in practice, this requirement

can be minimized by the reuse of vapors from

the evaporation process itself. Less common than

these indirect heating techniques are direct heating

methods (e.g., solar heating of salt brines).

General Principles of Heat Transfer

0004Heat can be transferred by conduction, convection,

or radiation. The last means of heat transfer is not

usually significant in industrial evaporation systems.

EVAPORATION/Basic Principles 2201

Heat Transfer by Conduction

0005 The quantity of heat transferred between two points

(see Figure 1) is defined as

Q ¼KA

, ð1Þ

where Q is the quantity of heat, K is the thermal

conductivity, A is the surface area through which

heat is passing, T is the temperature difference

between the two points, and X is the distance between

two points (the negative sign shows that the heat

flow is in the opposite direction to the temperature

gradient).

0006 Thermal conductivity is a property of the material

through which the heat is passing, and can vary from,

for example, 15.93 for stainless steel, 0.62 for water,

to 0.024 for air, a difference factor of nearly 700.

(Units are J s

1

.) Interestingly, stainless steel

(the most common material of construction for food

industry evaporators) has one of the lowest thermal

conductivities for metals, whilst water has one of the

highest thermal conductivities for liquids.

0007 Another useful parameter to consider is thermal

resistance:

R ¼

: ð2Þ

Where we are considering heat transfer through a

composite wall (i.e., several layers of different mater-

ials), thermal resistances are cumulative in their effect

on heat transfer. Hence, if one layer is a material with

a high thermal resistance (e.g., an insulating material

or fouling scale), the overall heat transfer will be low,

even if other layers have a low resistance (e.g.,

metals).

0008 In the instance of conduction through the wall of a

tube (Figure 2), the surface area across which the heat

transfer takes place is the tube wall itself. The equa-

tion for determining the heat load transferred is:

Q ¼K2rl

, ð3Þ

where r is the tube radius, l is the length of the tube,

and W is the wall thickness.

Heat Transfer by Convection

0009 Heat transfer by convection occurs as a result of the

movement of fluid on a macroscopic scale, in the

form of eddy and circulation currents. This con-

vective movement can take two forms:

0010 Natural convection: these currents arise from the

heating process itself, because of the differences in

density between the heated fluid and the surround-

ing colder fluid, which causes the former to rise,

and the latter to sink to take the place of the

former.

0011Forced convection: in this type of convection, the

currents are created by an external device, e.g., a

circulation pump causing turbulent flow in a pipe.

0012When heat transfer occurs from a surface into the

body of a fluid, natural convection currents are

weakest at the surface, which is covered by what is

effectively a static film. Consequently, heat transfer

across this film can only occur by conduction, and, as

mentioned above, thermal conductivity in fluids is

low. Hence, the main resistance to heat transfer into

fluid in a pipe is this film adjacent to the pipe wall. An

increase in the velocity of the fluid moving through

the pipe will reduce the thickness of this static film

and give rise to an overall increase in the heat transfer

into the fluid.

fig0001Figure 1 Heat transfer by conduction between two points.

Reproduced from Evaporation: Basic Principles, Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R, Robin-

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

fig0002Figure 2 Heat transfer through the wall of a tube. Reproduced

from Evaporation: Basic Principles, Encyclopaedia of Food Sci-

ence, Food Technology and Nutrition, Macrae R, Robinson RK and

Sadler MJ (eds), 1993, Academic Press.

2202 EVAPORATION/Basic Principles

001 3 In theory, the heat load transferred across this film

is defined as in eqn (1). However, in practice, it is

difficult to calculate the film thickness, X, and so the

following relationship is used:

Q ¼hAT, ð4Þ

where h is the heat-transfer coefficient.

0014 Thermal resistance is therefore the reciprocal of the

heat-transfer coefficient, i.e., 1/h.

0015 Forced convection is of more importance than

natural convection in industrial food processes and

equipment, where the fluids are under turbulent flow

conditions. It should be noted, however, that evapor-

ators can be classified as either ‘natural circulation’ or

‘forced circulation’ systems. In the former case, the

eddy and circulation currents described above are

greatly enhanced by the currents caused by the rising

bubbles generated in the boiling processes.

0016 Whenever possible, streamline flow conditions

should be avoided or at least minimized, as the heat

transfer coefficients of fluids (in convective heat

transfer) are much greater than thermal conductivity

factors (in conductive heat transfer). With very vis-

cous fluids (e.g., food pastes and slurries), turbulent

flow can only be produced by a high-pressure drop

across the heat-transfer device (e.g., by the input of a

large quantity of pump energy).

0017 In a tubular heat exchanger (e.g., an evaporator),

where perhaps one fluid is flowing inside a pipe and

being heated (or cooled) by another outside, the con-

sideration must be of heat transfer both inside and

outside the tube. In the latter case, flow can be either

lengthwise along the tube (in either direction) or at

right angles to the single tube or tube bundle.

0018Also, those fluids passing along the length of the

heat exchange tube (either inside or outside) will

experience either an increase or decrease in tempera-

ture, obviously because of heat transfer. This means

that in order to correctly quantify this heat transfer, it

is first necessary to define the difference in tempera-

ture between the fluids, given that it will not only vary

at different points along the length of the tube, but

will also vary depending on whether cocurrent or

countercurrent flow is being employed. It is necessary

to calculate an ‘average’ value for the temperature

difference, and the factor normally employed is the

logarithmic mean temperature difference (LMTD):

LMTD ¼

 T

ðÞT

 T

ðÞ

log

 T

ðÞ= T

 T

ðÞ

ðcocurrent flowÞ

ð4aÞ

 T

ðÞT

 T

ðÞ

log

 T

ðÞ= T

 T

ðÞ

ðcountercurrent flowÞ,

ð4bÞ

where T is the temperature (see Figure 3 for a

definition of subscript numbers).

Heat-transfer Aspects of Evaporation and

Boiling

0019As already stated, evaporation is the removal of water

(or other solvent) from a solution by boiling the

liquor in a suitable heat exchanger (i.e., the evapor-

ator) and removing the resultant vapor.

0020In boiling liquids (e.g., inside an evaporator tube),

the heat-transfer coefficient (h) depends very much on

the temperature difference (T) between the heating

surface and the boiling liquid.

0021Boiling can be classified into three types: inter-

face evaporation, nucleate boiling and film boiling

(Figure 4).

Interface Evaporation

0022Here, relatively few bubbles are formed on the

heating surface, and they move to the vapor–liquid

interface by natural convection, causing negligible

agitation in the liquid. Full boiling is often suppressed

owing to the hydrostatic head of the liquid above

the heating surface. This phenomenon is particularly

likely at the bottom of a vertical evaporator tube.

0023The heat-transfer coefficient increases slightly with

increasing temperature difference.

Nucleate Boiling

0024Here, a large number of bubbles form on the heating

surface, and their escape to the vapor–liquid interface

Cocurrent flow

Temperature

Distance along tube

Countercurrent flow

Temperature

Distance along tube

fig0003 Figure 3 Logarithmic mean temperature difference. Repro-

duced from Evaporation: Basic Principles Encyclopaedia of Food

Science, Food Technology and Nutrition, Macrae R, Robinson RK

and Sadler MJ (eds), 1993, Academic Press.

EVAPORATION/Basic Principles 2203

brings about significant turbulence in the liquid. This

in turn means that the heat-transfer coefficients are

greater, and rise more rapidly with increasing tem-

perature difference, than in the instance of interface

evaporation. The maximum heat-transfer coefficient

is reached at the top end of this boiling category.

Film Boiling

0025 Film boiling occurs at higher temperature differences,

when the quality of vapor being generated at the

heating surface is so great that the bubbles start to

coalesce to form a vapor film over the heating surface.

This in turn means a large drop in heat-transfer

coefficient as the latter is obviously much lower for

vapors than for liquids, and therefore the quantity of

heat transferred falls in proportion.

0026 The critical temperature difference at which the

heat transfer coefficient is at its maximum depends

on the physical properties of the boiling fluid and

also on the geometry of the heating surface.

0027 With even further increases in temperature differ-

ence, the heat-transfer coefficient eventually starts to

rise again, as heat transfer by radiation (across the

vapor film) starts to become a significant part of the

total heat transferred. These very high temperature

differences are rarely found in practice, however, and

most evaporation systems are designed to operate at,

or just below, the temperature difference at which the

heat transfer coefficient becomes a maximum.

Boiling-point Rise

0028 In addition to the nature of the boiling regime pre-

sent, another characteristic of evaporation systems is

the boiling-point rise (BPR). The boiling point of a

solution (e.g., a liquid foodstuff) is always higher

than that of pure water, and the magnitude of this

temperature elevation is proportional to the concen-

tration of solids in the solution – hence, as the solids

increase during the process of evaporation, the

boiling point will increase. By way of example, the

BPR of milk is only 0.5



C at 16% total solids, rising

to 2



C at 50% and 5



C at 80% total solids.

0029The consequence of this phenomenon is that the

temperature difference between the heating medium

(e.g., steam at 110



C) and the boiling liquid (e.g.,

milk at 50% total solids; BPR ¼ 2



C; boiling point

¼ 102



C at atmospheric pressure) is less (110



C 

102



C ¼ 8



C) than in the case of pure water (110



 100



C ¼ 10



C). The 20% drop in the temperature

difference, from 10



Cto8



C, means a pro rata drop

in heat transfer, as it is directly proportional to the

temperature difference. The resultant vapor will be at

the temperature of the boiling liquid and hence will

be superheated by the amount of the BPR (i.e., 102



 100



C ¼ 2



C).

0030In practice, the sensible heat associated with this

superheating is often harder to recover than the latent

heat itself and effectively is partly or totally lost,

causing a loss in energy economy.

Heat-transfer Aspects of Condensation

0031When a saturated vapor is brought into contact with a

cooler surface, and falls below its dew point, vapor

will condense on that surface, giving out its latent

heat of vaporization.

0032This condensation can occur in two ways:

0033Dropwise condensation: here, the condensing

vapor forms a large number of small individual

drops on the surface, which do not wet the surface

but, after growing slightly, fall away and hence

expose fresh surface for further similar condensa-

tion to take place on. This type of condensation

can result in very high heat-transfer rates, as heat

does not have to flow through a film of liquid by

conduction. However, dropwise condensation is

rarely found in practice and in fact has only been

definitely demonstrated with pure steam, under

specialized conditions.

0034Film condensation: in this form of condensation,

the condensing vapor forms a continuous film on

the cooler surface, and subsequent condensation

actually takes place on this film of liquid. If the

surface is vertical, the condensate film flows down-

wards, the flow being predominantly streamline,

and hence heat-transfer will be by conduction. It

can be assumed that the temperature of the film at

the cool surface is equal to that of the surface itself,

and the film temperature at the vapor surface is at

the same temperature as the vapor.

Interface Nucleate Film

boilingboilingevaporation

Temperature difference, T

Heat transfer coefficient, h

fig0004 Figure 4 Types of boiling. Reproduced from Evaporation:

Basic Principles, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

2204 EVAPORATION/Basic Principles

Another aspect of vaporization to consider is that, in

most evaporation systems encountered, the vapors

will contain a small quantity of noncondensable gas

(e.g., air that has either entered the product or via

leaks into the main body of the evaporator, most of

which will often be operated under varying levels of

vacuum). This mixture of gas and vapor, when cooled

in the condenser, will give rise to a layer of condensed

vapors only, on the heat exchange surfaces of the

condenser. Heat transfer to this surface will, there-

fore, be not only by the vapor condensing, and hence

releasing its latent heat, but also by the transfer of

sensible heat from the noncondensing gas. The design

of the condenser will need to take this factor into

account.