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

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See also: Elderly: Nutritionally Related Problems;

Sensory Evaluation: Sensory Characteristics of Human

Foods; Food Acceptability and Sensory Evaluation;

Practical Considerations; Sensory Difference Testing;

Sensory Rating and Scoring Methods; Descriptive

Analysis; Appearance; Texture; Aroma; Taste; Spices

and Flavoring (Flavouring) Crops: Properties and

Analysis; Sucrose: Properties and Determination;

Sweeteners: Intensive; Taste Enhancers

Further Reading

Breslin PAS (1996) Interaction among salty, sour and bitter

compounds. Trends in Food Science & Technology

7(Special Issue on Flavour Perception).

Cardello AV (1996) The role of the human senses in food

acceptance. In: Meiselman HL and MacFile HJH (eds)

Food Choice, Acceptance and Consumption, pp. 1–82.

London: Blackie Academic.

DuBois GE, Walters DE and Kellogg MS (1993) Mechan-

ism of human sweet taste and implication for rational

sweetener design. In: Chi-Tang Ho and Manley CHH

(eds) Flavour Measurement, Basic Symposium Series, pp.

240–266. Chicago, IL: Institute of Food Technologists.

Erickson RP (2000) The evaluation of neural coding ideas

in the chemical senses. In: Physiology and Behavior,

vol. 69, Issues 1, 3–13. New York: ElsevierScience.

Grenby TH (1989) Progress in Sweeteners, pp. 1–394.

London: Elsevier Applied Science.

Halpern BP (1997) Psychophysics of taste. In: Beau-

champ GK and Bartoshuk L (eds) Tasting and Smell-

ing, pp. 77–123. San Diego, CA: Academic Press.

Kawamura Z and Kare MR (1987) Umami. A Basic Taste.

New York: Marcel Dekker.

Lawless HT and Lee CH (1993) Taste. In: Acree TE and

Teranishi R (eds) Flavour Science, Sensible Principles

and Techniques, pp. 23–131. Washington, DC: Ameri-

can Chemical Society.

Mistretta CM and Hill DL (1995) Development of the taste

system: basic neurology. In: Doty RL (ed.) Handbook of

Oflaction and Gustation, pp. 635–668. New York:

Marcel Dekker.

Nabors LB and Gelardi RC (eds) (1991) Alternative Sweet-

eners, pp. 1–459. New York: Marcel Dekker.

Pangborn RM (1987) Selected factors influencing sensory

perception of sweetness. In: Dobbing J (ed.) Sweetness,

pp. 49–66. London: Springer.

Piggott JR (1988) Sensory Analysis of Foods, 2nd edn.,

pp. 1–425. London: Elsevier Science.

Rouseff RL (1990) Bitterness in Foods and Beverages,

pp. 1–356. Amsterdam: Elsevier.

Shallenberger RS (1993) Taste Chemistry, pp. 1–570.

Glasgow: Blackie Academic & Professional.

Walters DE, Orthoefer FT and Dubois GE (1991) Sweeten-

ers: Discovery, Molecular Design and Chemoreception,

pp. 1–333. Washington, DC: American Chemical

Society.

Wendin K (1999) Chemical Senses and Food Quality Per-

ception, pp. 1–52. Gothenburg, Sweden: The Swedish

Institute for Food and Biotechnology.

SEPARATION AND CLARIFICATION

J E Lozano, Plapiqui (Uns-Conicet), Bahı

a Blanca,

Argentina

Background

0001 Every year thousands of different processed food

products are introduced in supermarkets. Food pro-

cessing has three major aims: (1) to make food

toxically and microbiologically safe; (2) to provide

products of the highest nutritional and organoleptic

quality; and (3) to make foods with ease of use.

0002 Most food-processing comprise a series of physical

processes, or unit processes, that can be separated

into a number of basic operations, or unit operations,

which depend upon coherent physical principles.

0003 The main unit operations usually present in a

typical food-processing line, including:

0004Flow of fluid – when a fluid is moved from one

point to another by pumping, gravity, etc.

0005Heat transfer – in which heat is either removed or

added (heating; cooling; refrigeration and freez-

ing).

0006Mass transfer – whether or not this requires a

change in state. Processes that use mass transfer

include drying, distillation, evaporation, crystal-

lization, and membrane processes.

0007Other operations requiring energy, such as mech-

anical separation (filtration, centrifugation, sedi-

mentation, and sieving); size adjustment by size

reductions (slicing, dicing, cutting, grinding) or

size increase (aggregation, agglomeration, gel-

ation); and mixing, which may include solubiliz-

ing solids, preparing emulsions or foams, and dry

blending of dry powders (flour, sugar, etc.).

0008For example, the manufacture of concentrated

clarified apple juice includes several unit operations

SEPARATION AND CLARIFICATION 5187

such as size reduction, pressing, heating, pumping,

and evaporating, in sequence.

Separation Processes

0009 Separation technologies, including extraction and

sedimentation, are widely used in the food industry

and basic knowledge on separation science is required

to design a process properly. Separation in food usu-

ally implies pumpable suspension of solids in a liquid

(water solution of true solutes, like sugars) or wet

solid (e.g., fruit pulp). Separations can be achieved

on the basis of density or size and shape, and may be

classified into settling (or flotation), filtration, and

expression methods (Figure 1). Washing, a typical

food process, may also be considered a separation

process in which soil or some other undesirable com-

ponent is eliminated by the combined use of water

and mechanical methods.

Washing

0010 Washing of foods includes soaking, brushing, and the

application of chlorine or water spraying. Heavy

spray and rotary brush wash applications may also

be required to remove any rot. Many products such as

mechanically harvested berries are air-cleaned on

mesh conveyors or vibrator passing over an air jet.

Washers are conveyor belts or roller conveyors with

water sprays, reel (cylinder) type with internal spray

(Figure 2), brushes, and/or rubber rolls with or with-

out studs. Vibratory-type washers are very effective

for berries and small fruits. Brushes are effective to

eliminate rotten portions of fruits, thus preventing

problems with yeast toxins.

Settling

0011Settling is the use of the force of gravity to cause

particles to settle down the liquid food. Although

settling does not give a complete separation, it is

often the best way to process very large volumes of

a food suspension and remove most of the liquid. The

concentrated suspension can then be filtered with

smaller equipment and the cloudy liquid can be clari-

fied if necessary. Settlers can also be used for classify-

ing particles by size or density, which is not usually

possible with filtration.

0012Gravitational separation Separation by the action

of gravity, or sedimentation, can be represented by

Stoke’s law:

g 

 



18

ð1Þ

where V

¼the velocity of particles (m s

1

); r

specific gravity of liquid media (kg m

3

); r

¼specific

gravity of particles (kg m

3

); D

¼ mean diameter of

particle (m); m ¼ viscosity of liquid media (Pa s

1

);

and g ¼ gravitational constant (m s

2

). Solid–liquid

separation by screening is also possible for some food

suspensions, including coarse particles (fruit juice

from belt presses), but it is not widely applicable.

Continuous

Batch

Pressing

Gravitational

Pressure

Cross-flow

units

Flow-through

filters

Centrifugal

Vacuum

Filtration

Membrane

filtration

Settling

Flotation

Gravitational

Centrifugal

Basic food

separation

methods

Washing

fig0001 Figure 1 Separation methods commonly used in the food

industry.

Fruit in

Water in

Fruit out

fig0002Figure 2 Reel washer with internal spray.

Bins in

Bins

out

Flotation

fig0003Figure 3 Example of the use of flotation for separation of fruits

from container (bins).

5188 SEPARATION AND CLARIFICATION

0013 Flotation A typical example of flotation as a separ-

ation process is unloading fruits into a silo system.

Harvesting containers, known as bins, are commonly

used to transport fruit from the orchard to the

processing plant. Once in the plant, bin dumping

unloading and soil and foreign material separation

can be performed by flotation (this is good for fruits

with density < 1, like apples), as Figure 3 shows.

0014 Centrifugation Centrifugal separation is a process

which is quite often used in the food industry. In the

dairy industry, for example, uses of centrifugal force

include butter oil purification (separation of serum

phase from anhydrous milk fat); clarification (re-

moval of solid impurities from milk); separation of

fat from whey; skimming and standardizing.

0015 Centrifugation is based on the same Stoke’s law

presented in eqn (1) by changing the force of gravity

g by (ro

), where r ¼radius of axis of rotation; and

o ¼angular velocity. Stoke’s equation (eqn (1)) indi-

cates that a fat globule with a diameter of 2 mm will

rise four times faster than a fat globule with a

diameter of 1 mm.

0016 Modern centrifuges are self-cleaning, allowing a

continuous separation/clarification process. Centri-

fuges have also been used in preparing fruit juice

from which sediment and microorganisms can be

removed centrifugally in a clarifier, which are disk-

type centrifuges that employ forces of 5000–10 000

times gravity.

Pressing

0017 Solid–liquid separation is concerned in this article

with mechanical processes for the separation in

foods of water (including soluble solids) and finely

divided insoluble particles. Expression is the removal

of relatively small content of liquid from compres-

sible milled food, by mechanical means. The way to

separate most water and soluble solids depends on the

food.

0018 For citrus fruits there are three main types of

extractors: (1) The FMC-type (FMC Corporation,

Lakeland, USA) citrus juice extractor which basically

consists of a pair of cups that cut two holes in the

fruit, separate the peel, and move the peeled fruit into

a strainer tube where the juice is separated from the

seeds and the rest of the fruit; (2) the Brown-type

extractor (Automatic Machinery & Electronic, Los

Angeles, USA); in which fruit is cut in half, the halves

placed in rubber cups and the juice removed by rotat-

ing plastic reamers; and (3) the rotary press, in which

fruit is cut in halves and juice extracted in rotary

cylinders.

0019 Most systems for extracting juices from apple and

similar fruit and vegetable pulps use some method of

pressing juice through cloth of various thickness, in

which pomace is retained. These systems are called

filter presses and usually come in three main types: (1)

rack-and-cloth press; (2) horizontal pack press; and

(3) continuous belt press.

0020In a rack-and-cloth press the milled fruit pulp is

placed in a nylon, Dacron or polypropylene cloth

to form a ‘cheese’ with the help of a cheese frame-

work. Layers of pulp cheeses, separated by racks

made of hardwood or plastic, are stacked up to

1 m or more in height depending on the maturity

of the fruit and the size of the racks (Figure 4).

Rack-and-cloth presses are efficient but very labor-

intensive in operation, cleaning and repairing.

0021Cage presses are horizontal presses with enclosed

cages of several cubic meters in which pressing

takes place. The cage is filled with a complex filter

system consisting of grooved flexible rods filled

with sleeves of press cloth material (Figure 5a).

Juice passes from the pulp, through press cloth

sleeves, along grooves in the flexible rods, and

out to collecting channels in the ends of the cage

Pulp

cheese

Racks

Press

Expressed juice

fig0004Figure 4 Rack-and-cloth press.

Pulp in

Juice out

(a)

(b)

fig0005Figure 5 Hydraulic press: (a) loading; (b) pressing/extraction.

SEPARATION AND CLARIFICATION 5189

and in the piston (Figure 5b). The drum may be

rotated, breaking up the pulp, and adding more

water, permitting a second pressing with more

juice extraction. The whole process may be auto-

mated. Although some cleaning labor is saved,

rods and sleeves require a considerable amount

of maintenance.

0022 Continuous belt presses are based in the Ensink

design for paper pulp pressing. This type of press

offers a truly continuous operation (Figure 6). In

belt presses, a layer of mash (pulp) is pumped on to

the belt entering the machine. Press aid may be

added for improved yield.

0023 Screw presses. A typical screw press consists of a

stainless-steel cylinder screen enclosing a large-

bore screw with narrow clearance between screw

and cylinder. Adjustable backpressure is usually

provided at the end of the chamber. Breaker bars

must be incorporated to disrupt the compressing

mash.

0024 Diffusion extraction. Typically used for the extrac-

tion of sugar from sugar beets (Figure 7), diffusion

extraction is a typical countercurrent-type process.

It is desirable to retain the same driving force,

DC (concentration of soluble components in solids

vs concentration of soluble components in liquid).

In order to maintain a constant DC throughout the

extraction process, it is necessary to carry out a

continuous weighing of feed and to control the

water flow to the counterflow extractor by

means of a control loop. The diffusion extraction

process is influenced by a number of variables

(temperature, raw product size, water, etc.)

Extracted products are usually passed through a

conventional press system and the very dilute

juice may be returned to the extractor.

0025Hydraulic pressing does not usually requires the

addition of press aids, unless exceptionally overma-

ture fruit is used. For continuous screw presses, how-

ever, it is usually necessary to add 1% (w/w) or more

of cellulose. Mixing off cellulose and fruit occurs in

the mill and subsequent pumping to press.

Filtration

0026Filtration is also a mechanical process designed for

clarification by removing insoluble solids from a

valuable liquid food, by the passage of most of the

fluid through a porous barrier which retains most of

the solid particulates contained in the food. The piece

of equipment by which filtration is performed is

named the filter. The filter medium, which may be a

screen, cloth, paper, or bed of solids, is the barrier

that lets the liquid pass while most of the solids are

retained. The liquid that passes through the filter

medium is called the filtrate. Filtration and filters

can be classified in several ways, including:

0027Driving force: The filtrate is induced to flow

through the filter medium by:

0028Hydrostatic head (gravity)

0029Pressure (upstream filter medium)

0030Vacuum (downstream filter medium)

0031Centrifugal force across the medium

0032Filtration mechanism:

0033Cake filtration (solids are retained at the sur-

face of a filter medium and pile upon one

another)

0034Depth or clarifying filtration (solids are

trapped within the pores or body of the filter

medium)

0035Operating cycle:

0036Intermittent (batch)

0037Continuous rate.

0038These methods of classification are not mutually

exclusive. Thus filters are usually divided first into the

two groups of cake and clarifying equipment, then

into groups of machines using the same kind of

driving force, then further into batch and continuous

classes. Some filtering devices usually employed in the

food industry are described below.

0039Pressure filters The main advantages of pressure

filtration compared to other filtering methods are

Pulp in

Bagasse out

Juice out

fig0006 Figure 6 Typical fruit belt press.

Water

Sugar beets

Warm-water

inlet

Sugar beets in

Sugar solution

outlet

fig0007 Figure 7 Diffusion extraction process.

5190 SEPARATION AND CLARIFICATION

that cakes are obtained with very low moisture con-

tent; clean filtrates may be produced by recirculating

the filtrate or by precoating; and solutions may be

polished (finished) to a high degree of clarity. Among

the disadvantages it must be noted that cloth washing

is difficult and precoat is required; the operator

cannot see the forming cake and is unable to carry

out an inspection while the filter is in operation; the

internals are difficult to clean, and this may be a

problem with food-grade applications.

0040 With the exception of the rotary drum pressure

filter, these types of filters are semicontinuous ma-

chines in which wash and cake discharge are per-

formed at the end of the filtration cycle. Since the

operation is in batches, intermediary tanks are re-

quired. The collection of filtrate depends on the oper-

ating mode of the filter, which can be constant flow

rate, constant pressure, or both, with the pressure

rising and flow rate reducing during filtration. The

filtration rate is mainly influenced by the properties

of food (particle size and distribution, presence of

gelatinous solids like pectin, liquid viscosity, etc.)

Although continuous-pressure filters are available,

they are mechanically complex and expensive, so

they are uncommon in the food industry.

0041 Filter Aid and Precoating Filter aid and precoating

are often mentioned as in the case of pressing, in

connection with pressure filtration. Filter aid is used

when the pulp or turbid liquid food is low in solids

content, with fine and muddy particles that are diffi-

cult to filter. To enhance filtration, coarse solids are

used with large surface area that capture and trap in

their interstices the slow-filtering particles from the

suspension, producing a porous cake matrix.

0042 On the other hand, precoating is the formation on

the filter plates of a defined thick medium of a known

permeability. Precoating prior to filtration serves

when the particles to separate are gelatinous and

sticky, forming a barrier that avoids cloth blinding.

The filter aids and precoating materials most com-

monly employed in the food industry are: diatom-

aceous earth (silicaceous skeletal remains of aquatic

unicellular plants); perlite (glassy crushed and heat-

expanded volcanic rock); cellulose (fibrous, light-

weight, and ashless paper); and special groundwood.

0043 Types of pressure filters Pressure filters usually

found in the food industry are:

0044 Filterpress, also named plate-and-frame, consists

of a head and follower contained between a pack

of vertical rectangular plates supported by side or

overhead beams (Figure 8). The head serves as a

fixed end to which the feed and filtrate pipes are

connected and the follower moves along the

beams and presses the plates together during the

filtration cycle by a hydraulic or mechanical mech-

anism. Each plate is dressed with filter cloth on

both sides and, once pressed together, they form a

series of chambers that depend on the number of

plates. The plates generally have a centered feed

port that passes through the entire length of the

filterpress, so that all the chambers of the plate

pack are connected together.

0045Vertical and horizontal pressure leaf filters consist

of a vessel that is fitted with a stack of vertical

Feed

Filtrate

Feed

Filtrate

out

Filtrate

out

Plate

fig0008Figure 8 Filterpress operation sketch and details of filtering

plate.

Feed in

Filtrate out

Plates

fig0009Figure 9 Horizontal plate filter.

SEPARATION AND CLARIFICATION 5191

(Figure 9) or horizontal leaves that serve as filter

elements. The leaf is constructed with ribs on both

sides to allow free flow of filtrate towards the neck

and is covered with coarse mesh screens that sup-

port the finer-woven metal screens or filter cloth

that retain the cake. The space between the leaves

may vary from 30 to 100 mm depending on the

cake formation properties and the ability of the

vacuum to hold a thick and heavy cake to the leaf

surface. They may be applied to polishing liquid

foods with very low solids or for cake filtration

with a solids concentration < 20–25%. In contrast

to filterpress, pressure leaf filters may be readily

jacketed for applications whenever hot or cold

temperatures are to be preserved. The cloth mesh

screens that cover the leaves may be accessed more

easily on horizontal than on vertical tanks.

0046 Candle filters may be used in applications that

require efficient low-moisture cake filtration or a

high degree of polishing (finishing). Candle filters

are available in many sizes, containing up to 250

or more filtering elements (Figure 10). The candle

filter consists of three major components: the

vessel, the filtering elements (candles), and the

cake discharge mechanism. Vessel configuration

may be with a conical bottom for cake filtration

and polishing, or with a dished bottom for slurry

thickening. This last vessel is scarcely or never

used in the food industry.

0047 The filtering element generally consists of the fil-

trate core and the filtering medium. The core serves

for filtrate passage and to support the filter medium.

In the food industry the core is a bundle of perforated

stainless-steel tubes. The types of filter medium avail-

able are diverse, such as porous ceramics, woven

mesh screens, sintered metal tubes, or synthetic filter

cloths. Finally, there are two methods of discharging

the cake at the end of the cycle: snap blow or vibrat-

ing mechanism. The advantages of a candle filter are

the excellent cake discharge capacity and its mechan-

ical simplicity.

0048Vacuum filters Vacuum filters are simple and

reliable machines and therefore have gained wide

acceptance in the food industries. Among the differ-

ent types and sizes for standard vacuum filters, the

following are included: drum; disk; horizontal belt;

tilting pan; and table filters.

0049Drum filters are usually utilized in the food indus-

try. Advantages and disadvantages of vacuum filtra-

tion as compared to other separation methods are as

follows:

0050Advantages: continuous operation; very effective

polishing (finishing) of solutions (on a precoat

filter); and easy control of operating parameters

such as cake thickness.

0051Disadvantages: higher residual moisture in the cake

and difficult to clean (mainly as required for food-

grade applications).

0052Precoat filters are used when liquid foods (e.g.,

clarified apple juice) require a very high degree of

clarity. To polish the solution the drum deck is pre-

coated with an appropriate medium. A scraper blade,

also called the doctor blade, moves slowly towards

the drum and shaves off a thin layer of the separated

solids and precoating material. This movement con-

tinuously exposes a fresh layer of the precoat sur-

face so that when the drum submerges into the

tank it is ready to polish the solution. On precoat

filters the entire drum deck is subjected to vacuum

(Figure 11).

0053Membrane processes The membrane is an interface

between two bulk phases. In the food industry,

membrane processes include a wide range of unit

operations, from sieving to reverse osmosis. However,

during conventional filtration, only the separation of

coarse particles to micron range was considered. This

operation requires frequent cleaning due to a build-up

of a cake layer which results in increased resistance to

filtration. For separation of fine solids from liquids,

cake filtration or cross-flow filtration should be con-

sidered. In this section only cross-flow membrane

filtration will be considered. In cross-flow filtration

(Figure 12) the bulk phase under pressure flows along

Bunch of

candles

Filtrate out

Vessel

fig0010 Figure 10 Candle-type filter.

5192 SEPARATION AND CLARIFICATION

the surface of the membrane, sweeping and at least

partially removing the cake layer. This allows rela-

tively high permeate fluxes over long periods. Reverse

osmosis (RO), ultrafiltration (UF), and microfiltra-

tion (MF) are typical cross-flow filtration methods

that use membranes with varying pore sizes to effect

separation on the basis of size and shape. RO has

membranes with the smallest pore and is used to

separate water from other solutes.

0054 High pressures (up to 10 000 kPa) are required to

overcome the high osmotic pressure of solutes (sugars

and salts). UF has membranes with larger pores and

will retain proteins, lipids, and colloidal salts, while

allowing smaller molecules to pass through to the

permeate phase. Required working pressures during

UF are in the range 130–1400 kPa. MF, with pores

> 0.1 mm, is used to separate fat from proteins and

to reduce microorganisms from fluid food systems.

This process requires the lowest operating pressure

(< 350 kPa).

0055 Table 1 lists examples of commercial uses of mem-

brane processes in different food industries. However,

there are still a number of technical obstacles that still

need to be resolved, including inadequate processing

rates (flux) and separation capabilities (rejection and

selectivity), insufficient membrane or module life,

and incompatible materials.

Clarification

0056Many liquid foods are colloidal suspensions which

must be unstabilized to give a completely transparent

liquid. This process is known as clarification

(Table 2). Usually, colloidal state is only defined in

terms of particle size; the lower limit is generally

taken as 1–5 nm (10–50 A

) and the upper size limit

as 1–5 mm. When the dispersed phase is a solid and

the continuous phase is a liquid, the system is defined

as a sol. A typical example of a food sol is a cloudy

fruit juice, which is a colloidal system where the

continuous medium is a solution of pectin and sugars,

and the dispersed (colloidal) matter is mainly formed

by cellular tissue comminuted during fruit processing.

Pectin is basically a biopolymer made by d-galacturo-

nic acid units.

0057Particles in a cloudy juice adhere together and form

aggregates of increasing size (flocculation) which may

Precoat

Doctor blade

Vacuum pump

Filtrate

Barometric leg

Receiver

Feed

fig0011 Figure 11 Vacuum drum precoat filter.

Retentate

Membrane

Permeate (J )

Permeate

Feed

Rejected solutes

Diffusive flux

Counter-

diffusive flux

fig0012Figure 12 Typical cross-flow membrane process and details of

acting fluxes.

SEPARATION AND CLARIFICATION 5193

settle due to gravity. If flocs change to a much denser

form, it is said to undergo coagulation which is an

irreversible process. For a clear juice, these suspended

particles have to be removed. It may seem simple

merely to filter them out, but unfortunately some

soluble pectin remains in the juice, making it too

viscous to filter quickly. A dose of commercial

enzyme is the accepted way of removing unwanted

pectin. Enzyme depectinization has two effects: it

degrades the viscous soluble pectin and it also causes

the aggregation of cloud particles. Figure 13 shows

how this may happen: pectin forms a protective coat

around proteins in suspension. In an acidic environ-

ment (apple juice typically has a pH of 3.5), pectin

molecules carry a negative charge. This causes them

to repel one another. Pectinase degrades this pectin

and exposes part of the positively charged protein

beneath. The electrostatic repulsion between cloud

particles is thereby reduced, so that they clump

together.

0058 These larger particles will eventually settle out, but

to improve the process, flocculating, also named

fining, agents (Table 1) can be added. Fining agents

work either by sticking to particles, making them

heavy enough to sink, or by using charged ions to

cause particles to stick to each other, making them

settle to the bottom. What is left is a transparent but

by no means clear juice. A second centrifugation and

subsequent filtration (fining) are needed to give the

clear juice that many consumers prefer.

0059Another potential contributor to the haziness of

juice is starch. Unripe apples, for example, may con-

tain up to 15% starch. Although the first centrifuga-

tion (before the juice reaches the clarification tank)

removes most of the starch, about 5% usually

remains. This can be broken down using a specific

enzyme (amyloglucosidase) added at the same time as

tbl0002 Table 2 Fruit juice clarification agents

Name Description

Sparkolloid A natural albuminous protein extracted from kelp and very finely powdered

Gelatin Mixture of gelatins and silicon dioxide, with the active ingredient being animal collagen

Kieselsol Suspension of small silica particles usually used in combination with gelatins. This fining aids in pulling

proteins out of suspension

Bentonite Refined clay (montmorillonite) presented as powder or coarse granules. Bentonite must be added to hot

water: stir well and let it stand for 36–48 h before use, so that it swells and becomes almost gelatin-like

Isinglass Protein produced from sturgeon swim bladders. It is presented either as a fine powder or as dry hard fragments

tbl0001 Table 1 Use of membrane processes in different food industries

Ultrafiltration Reverse osmosis

Beverage industry Sterilization of draft beer Juice concentration in fruit wines

Wine clarification Tartrate removal

Fractionation of whey to produce protein

concentrate

Concentration of whey

Dairy industry Fractionation of milk for cheeses Concentration of milk

Fats and oils industry Recovery of oilseed protein and tocopherol and

oil from waste water

Fruit/vegetable industry Juice clarification Sugar concentration

Juice concentration

Meat processing Concentration of egg white, gelatin, and blood

serum proteins

Sugar/sweetener industry Grain milling Concentration of maple sap

Clarification of dextrose Recovery of sugar from rinse water

Semistable

colloid

+ +

Positively

charged particle

Pectinase

Pectin layer

fig0013Figure 13 How the breakdown of pectin leads to the formation

of aggregated cloud particles during fruit juice clarification.

5194 SEPARATION AND CLARIFICATION

the pectinase. Optimization of fining and ultrafiltra-

tion steps can help retard or prevent postbottling haze

development. Figure 14 shows a typical fruit juice

clarification process.

0060 Prepress treatment with pectinase takes anything

from 15 min to 2 h depending upon the enzyme, the

reaction temperature, and the fruit chosen. Enzyme

treatment is considered to be complete once the vis-

cosity of the juice has returned to its original level or

less. It is important that the pulp is not broken down

too much. Other polysaccharides (like araban, a poly-

mer of the pentose arabinose) may appear as a haze in

fruit juices and other beverages a few weeks after it

has been concentrated. Although commercial pecti-

nase preparations often contain arabanase, certain

fruits (like pears) are rich in araban and may require

the addition of extra arabanase to the clarification

tank.

Clarification by Ultrafiltration

0061 The application of UF as an alternative to conven-

tional processes for the clarification of fruit and vege-

table juices has been demonstrated. However, the

acceptance of UF in the food-processing industry

is not yet complete, because there are problems with

the operation and fouling of membranes. During UF,

two fluid streams are generated: the ultrafiltered

solids free juice (permeate), and the retentate with

variable content of insoluble solids and macro-

molecules.

0062 Permeate flux (J) results from the difference be-

tween a convective flux from the bulk of the retentate

to the membrane and a counterdiffusive flux or out-

flow by which solute is transferred back into the bulk

of the fluid (Figure 12). The value of J is strongly

dependent on hydrodynamic conditions, membrane

properties, and operating parameters. The main

driving force of UF is the transmembrane pressure

(DPTM), which, for example, in the case of hollow

fiber ultrafiltration systems, can be defined as:

PTM ¼ P

þ P

ðÞ=2  P

ext

ð2Þ

where P

¼ pressure at the inlet of the fiber, P

outlet pressure, and P

ext

¼ pressure on the permeate

side. In practice, the J-values obtained with apple

juice are much less than those obtained with water

only. This phenomenon is attributable to various

causes, including the concentration polarization

boundary layer (defined as a localized increase in

concentration of rejected solutes at the membrane

surface due to convective transport of solutes) and

plugging of pores due to fouling. Where some of

these phenomena are reversible and disappear after

cleaning of the UF membranes, others are definitively

irreversible.

0063Flux decline due to this phenomenon can be re-

duced by increasing flow velocity on the membrane.

Traditionally, correlations of J with DPTM have been

determined by parameter fitting of the experimental

data.

0064As a summary, membrane processes are limited by

the following factors:

0065They must be combined with other processes or

steps.

0066The process is practically limited to low-viscosity

liquids and foods.

0067Ceramic membranes are versatile, but expensive

to install.

0068Although the membrane process is easy to under-

stand, very careful design and trained and skilled

operators are needed to insure long-term filtering

capacity.

0069A generalized application of membrane technology

in the food industry may require improvements in

thermal stability and chemical resistance; a more

Reception

tank

Juice from

presses

Clarification

agents

Centrifuge

Precoat

filter

Fining

Clarified

juice

fig0014 Figure 14 Fruit juice conventional clarification line.

SEPARATION AND CLARIFICATION 5195

controlled pore structure and narrow pore size distri-

bution; a higher permeate flux; an improved micro-

biological resistance; and more controlled fouling.

See also: Colloids and Emulsions; Filtration of Liquids;

Membrane Techniques: Principles of Reverse Osmosis;

Applications of Reverse Osmosis; Principles of

Ultrafiltration; Applications of Ultrafiltration