8 food biochemistry and food phần 155

Aside from the conventional separation techniques such as solvent and water extraction solid–liquid contacting extraction or leaching, crystallization, precipitation, distillation, and l

technologies in the food processing area have undergone an

explosive growth The competitive nature of the

biotechnolo-gies applied to the pharmaceutical and food industries for

cost-effective manufacturing have provided much impetus for the

development and use of new separation techniques on a large

scale, but at a lower cost Aside from the conventional separation

techniques such as solvent and water extraction (solid–liquid

contacting extraction or leaching), crystallization, precipitation,

distillation, and liquid–liquid extraction, etc., have already been

incorporated in basic food processing and well established and

commercialized A number of newer separation techniques as

promising alternative methods for improved application in food

engineering have been implemented on the commercial scale,

including supercritical CO2 fluid extraction, membrane-based

separation, molecular distillation, and pressured low-polarity

water extraction procedures These newer techniques have made

impressive advances in obtaining adequate segregations of

com-ponents of interest with maximum speed, minimum effort, and

minimum cost at as large a capacity as possible in

production-scale processes These techniques have been implemented for the

purification of proteins, characterization of aromas, whey

pro-tein removal from dairy products, extraction of health-benefiting

fish oil, and clarification of beverages including beer, fruit juices,

and wine The separation processes and technologies for high

value-added products are based on their polarity and

molecu-lar size Many potential high-value products can be developed

from natural resources by different separation technologies and

processes Carotenoids, including lycopene,β-carotene,

astax-anthins, and lutein, make up a world market nearing $1 billion

with a growth rate of about 3% Therefore, efforts to utilize

natu-ral agricultunatu-ral materials for the production of high value-added

products, especially health-promoting foods and ingredients, are

of great interest to the food and biotechnology industries

MAIN SEPARATION PROCESSES IN FOOD

INDUSTRIAL APPLICATIONS

Separation processes such as extraction, concentration,

purifica-tion, and fractionation of nutrients or bioactive components from

agricultural materials are the main processes used to obtain

high-value end products All separations rely on exploiting differences

in physical or chemical properties of mixture of components

Some of the more common properties involved in separation

processes are particles or molecular size and shape, density,

sol-ubility, and electrostatic charge In some operations, more than

one of these physical and chemical properties is involved As a

unit operation, the separation process plays a key function in the

whole procedure for value-added food processing The science

of separation consists of a wide variety of processes, including

mechanical, equilibrium, and chromatographic methods

Mechanical Separation Processes

Centrifugation

The centrifugation process works to separate immiscible liquids

or solids from liquids by the application of centrifugal force A

centrifuge is a spinning settling tank The rapid rotation of the entire unit replaces gravity by a controllable centrifugal or radial force Centrifugal separation is used primarily in solid–liquid or liquid–liquid separation processes, where the process is based

on density difference between the solid or liquids Centrifuga-tion is typically the first step in which the suspended solids

or liquids are separated from the fluid phase Various designs

of centrifuge are used in the food industry such as removal of solids from juice, beverage, fermentation broths, or dewatering

of food materials Most centrifugal processes are carried out on a batch basis However, some automatic and continuous centrifuge equipment and process are applied in the food and biotechnology industry

Filtration

Filtration is the separation of two phases, solid particles or liquid droplets, and a continuous phase such as liquid or gas, from a fluid stream by passing the mixture through a porous medium Filtration finds applications through the food and biotechnology industries Filtration is employed at various stages in food man-ufacture, such as the refining of edible oils, clarification of sugar syrups, fruit juice, vinegar, wine, and beer, as well as yeast re-covery after fermentation Filtration is also carried out to clarify and recover cells from fermentation broths in the biotechnology area

Filtration can be classified into conventional and nonconven-tional filtrations The convennonconven-tional filtration process uses filtra-tion media with coarse selectivity and cannot separate similar size particles It usually refers to the separation of solid, im-miscible particles from liquid streams Conventional filtration is typically the first step in which the suspended solids are sepa-rated from the fluid phase

Membrane Separation

Nonconventional filtration processes use membrane separation technology This nonconventional filtration has evolved into a quite sophisticated technique with the advent of membranes whose pore size and configuration can be controlled to such a degree that the filtration area is maximized while keeping the total volume of the unit small A membrane separation process

is based on differences in the ability to flow through a selective barrier (membrane) that separate two fluids It should permit pas-sage of certain components and retain certain other components

of the mixture Membrane separation processes are classified as microfiltration, ultrafiltration, and reverse osmosis according to the pore size of the membranes

Ion-Exchange and Electrodialysis

Ion-exchange and electrodialysis are distinct separation pro-cesses, but both processes are based on the molecular electro-static charge

The electrodialysis process is based on the use of ion-selective membranes, which are sheets of ion-exchange resin The pro-cess permits the separation of electrolytes from nonelectrolytes

in solution and the exchange of ions between solutions The elec-trodialysis process is used to separate ionic species in the food and biotechnology industries In the electrochemical separation process, a gradient in electrical potential is used to separate ions with charged, ionically selective membranes The electrodialysis process is used widely to desalinate brackish water to produce potable water The process is also used in the food industry

to deionize cheese whey and in a number of pollution-control applications

Ion-Exchange

The ion-exchange process is defined as the selective removal of charged molecules from one liquid phase by transfer to a second liquid phase by a solid ion-exchange material The mechanism

of absorption is electrostatic opposite charges on the solutes and ion-exchanger The feed solution is washed off, followed by desorption, in which the separated species are recovered back into solution in a purified form The main areas using it are sugar, dairy (separation of protein, amino acids), and water pu-rification Ion-exchange processes are also widely used in the recovery, separation, and purification of biochemicals, mono-clonal antibodies, and enzymes

Solid/Solid Separation

Solid/solid separation is a mechanical separation such as in

a milling separation facility Solid/solid separation can be achieved on the basis of particle size from the sorting of large food units down to the molecular level Shape, moisture content, electrostatic charge, and specific gravity may affect the sepa-ration process Screening of materials through perforated beds (wire mesh, cloth screen) produces materials of more uniform particle size For example, screening is usually used to sort and grade many food materials such as fruits, vegetables, and grains

A wide range of geometric designs of flat bed and rotary fixed aperture screens are used in the food industry

Sedimentation (Precipitation or Settling)

The sedimentation process is based on gravitational settling

of solids in liquids Sedimentation processes are slow and are widely used in water and effluent treatment processes

Magnetic Separation

Magnetic separation relies on the behavior of individual parti-cles under the influence of magnetic forces When exposed to a magnetic field, ferromagnetic materials are attracted along lines

of magnetic force from points of lower magnetic field intensity

to points of higher magnetic field intensity

Equilibration Separation Processes

The following processes are the unit operations commonly ob-served in the food industry that involve equilibrium between solid and liquid phases

Adsorption

Absorption is a process whereby a substance (absorbate or sor-bate) is accumulated on the surface of a solid Absorption process

is used to recover and purify the desired products from a mixture

of liquids Adsorption is used for decolorization and enzyme and antibiotic recovery Liquid-phase adsorption is usually used for the removal of contaminants present at low concentrations in process streams

Evaporation

Evaporation is the concentration of a solution by evaporating water or other solvents The process is largely dependent on the heat sensitivity of the material Boiling temperatures can

be lowered under vacuum Most commercial evaporators work

in the range 40–90◦C and minimize the residence time in the heating zone, for the concentration of heat-sensitive liquid foods and volatile flavor and aroma solutions The final product after

an evaporation process is usually in the liquid form

Crystallization

Crystallization processes can be used to separate a liquid mate-rial from a solid Crystallization separates matemate-rials and forms solid particles of defined shape and size from a supersaturated solution by creating crystal nuclei and growing these nuclei to the desired size Crystallization is often used in a high-resolution, polishing, or confectioning step during the separation of biolog-ical macromolecules Crystallization can be affected by either cooling or evaporation to form a supersaturated solution in which crystal nuclei formation may occur Sometimes, it is necessary

to seed the solution by addition of solute crystals Batch and continuous operations of crystallization processes are used in commercial food production

Dewatering (Dehydration or Drying)

Dewatering is the process applied to separate water (or volatile liquids) from solids, slurries, and solutions to yield solid products

Extraction

Solvent extraction is a method for a solvent–solvent or solvent–solid contacting operation

Solid–Liquid Extraction, e.g., Organic–Aqueous Extraction

The solid is contacted with a liquid phase in the process called solid–liquid extraction or leaching in order to separate the de-sired solute constituent or to remove an unwanted component from the solid phase Solid–liquid extraction or leaching is a

separation process affected by a fluid involving the transfer of

solutes from a solid matrix to a solvent It is an operation

ex-tensively used to recover many important high-value food

com-ponents such as oil from oilseeds, protein from soybean meal,

phytochemicals from plants, etc Solid–liquid extraction is also

used to remove undesirable contaminants or toxins from food

materials

Liquid–Liquid Extraction, e.g., Two-Phase

Aqueous Extraction

Liquid–liquid extraction separates a dissolved component from

its solvent by transfer to a second solvent, mutually

nonmisci-ble with the carrier solvent The liquid–liquid extraction as a

technology has been used in the antibiotics industry for several

decades and now is recognized as a potentially usefully

separa-tion step in protein recovery on a commercial producsepara-tion

Supercritical Fluid Extraction

Supercritical CO2fluid extraction is mostly for high value-added

products that are sensitive to heat, light, and oxygen The

extrac-tion process is implemented in a supercritical region where the

extraction fluid (e.g., CO2) has liquid-like densities and solvating

strengths but retains the penetrating properties of a gas

Distillation

Distillation is a physical separation process used to separate the

components of a solution (mixture) that contains more

compo-nents in the liquid mixture, by the distribution of gas and liquid

in each phase The principle of separation is based on the

dif-ferences in composition between a liquid mixture and the vapor

formed from it, because each substance in the mixture has its

own unique boiling point

Chromatographic Methods

Chromatography is a separation technique based on the uneven

distribution of analytes between a stationary (usually a solid) and

a mobile phase (either a liquid or a gas) Chromatography can be

conducted in two or three dimensions Two-dimensional

chro-matography, for example paper chrochro-matography, is almost solely

used for analytical purposes Column chromatography

separa-tion is the most common form of chromatography used in the

food processing field for further purification of high value-added

components Many variations exist on the basic chromatography

methods Some well-established methods are listed below

Liquid–Solid Absorption Chromatography

(e.g., Hydroxyapatite)

In absorption, molecules in a fluid phase concentrate on a solid

surface without any chemical change Physical surface forces

from the solid phase attract and hold certain molecules

(sub-strate) from the fluid surrounding the surface

Affinity Chromatography

Affinity chromatography exploits the natural, biospecific actions that occur between biological molecules These inter-actions are very specific and because of this affinity separation process are very high resolution methods for the purification of food products such as proteins Affinity chromatography has a gel surface that is covered with molecules (ligand) binding a particular molecule or a family of molecules in the sample

Size-Exclusion Chromatography

Size-exclusion chromatography, also known as gel permeation chromatography when organic solvents are used, is used to control the pore size of the stationary phase so as to separate molecules according to their hydrodynamic volume and molec-ular size

Ion-Exchange Chromatography

Ion-exchange chromatography and a related technique called ion-exclusion chromatography are based on differences in the electric charge density of their molecules during charge–charge interaction of molecules It is a variation of absorption chro-matography in which the solid adsorbent has charged groups chemically linked to an inert support Ion-exchange chromatog-raphy is usually used for recovery and purification of amino acids, antibiotics, proteins, and living cells

Hydrophobic Interaction Chromatography

Hydrophobic interaction chromatography is based on a mild ad-sorption process, and separates solutes by exploiting differences

in their hydrophobicity

Reverse Phase Chromatography

Reverse phase chromatography is based on the similar principles

of hydrophobic interaction chromatography The difference is that the solid support is highly hydrophobic in reverse phase chromatography, which allows the mobile phase to be aqueous

ENGINEERING ASPECTS OF SEPARATION PROCESSES

Much of the recent process development for the separation of valuable components from natural materials has been directed toward high value-added products For the recovery of valuable components from raw materials, applications of separation tech-nologies to recover major or minor components from agricul-tural commodities is usually a solid–liquid contacting operation

As research moves into commercial production, there is a great need to develop scalable and cost-effective methods of separa-tion technologies The successful and effective development of separation technologies is a critical issue in the chain of value-added processing of agricultural materials such as developments

in functional food ingredients, nutrients, and nutraceuticals

Separation operations are interphase mass transfer processes because they involve certain heat, mass, and phase transfers,

as well as chemical reactions among food components The engineering properties of the targeted food components via separation systems include separation modeling, simulations, optimization control studies, and thermodynamic analyses The principles of mass conservation and component transfer amounts are used to analyze and design industrial processes Molecular intuition and a thermodynamic approach constitute powerful tools for the design of a successful separation process The fol-lowing issues of engineering properties are important for process optimization and simulation

1 Chemical equilibration—binary, ternary, and multicom-ponent systems in solid–liquid contacting operation,

2 Diffusivity (pressure diffusion, thermal diffusion, gaseous diffusion) and convection,

3 Solubility of targeted components under different separa-tion operating condisepara-tions,

4 Iso-electric points and charge dependence on pH,

5 Chemical interaction kinetics (colloid formation and affinity),

6 Physical properties of particles of the food material,

7 Flux and fouling properties in membrane separation pro-cesses,

8 Solvent selection, recycling, and management,

9 Nature of solvents, optimum composition of mixed sol-vents for certain nutrient separations, and solvent residues

in food products are factors to be optimized

10 Some solvent treatments such as evaporation, concen-tration, de-watering, de-coloring, toxicological anal-yses, waste minimization, recycling, and disposal are necessary

SEPARATION SYSTEM DESIGN

Technical Request for a Separation System

An important consideration in determining the appropriateness

of a separation technique and system is the actual purity require-ment for the end products In design of organic solvent (toxic chemical)-free separation technologies and systems, there are several essential technical approach requirements for technol-ogy advances, such as combination of new techniques available for system optimization, product design and reasonable sepa-ration and purification steps, environmentally friendly process, less air pollution and industrial waste (e.g., energy, greenhouse gases emission, reduction of waste water production), and eco-nomic feasibility and raw material selection

Food Quality and Separation System

The major issues related to product quality after separations are the impacts of processing on bioactive compounds and the nutritional aspects of foods, as well as the quality character-istics To meet the food safety regulations, no toxic chemical solvent residues are permitted in the end food products, e.g.,

“green” food products Nutrition and health regulations must be

met Also important are a high stability of nutrients and bioac-tive components, processes operating at low temperatures to reduce thermal effects, processes that exclude light to reduce light induced (UV) irradiation effects, and processes that ex-clude oxygen to reduce oxygen effects And the final product must maintain uniformity and quality consistency, and purity can meet food grade or pharmaceutical grade requirements

Scaling Up Technology for Industrial Production

Scaling up of a natural product separation process is by no means a simple affair The enormous variations from process

to process necessitate careful attention to details at all stages of product development When the technology in a food process

is designed for industrial-scale production, an important area for consideration is the balance of capital and operating costs

as the scale of the separation operation increases Scale up of separation technology also involves optimization with respect to increasing the efficiency of each stage, giving rise to increasing demands on the accuracy of the assay system

NEW TECHNOLOGY DEVELOPMENT

Extraction of health-promoting components from plant materi-als has usually been accomplished by conventional extraction processes such as solid–liquid extractions employing methanol, ethanol, acetone, or hexane and also through steam distillation

or evaporation processes to remove solvents from extracts Cur-rently, the demand for natural bioactive compounds is increasing due to their use by the functional food and pharmaceutical in-dustry Thus, there has been increasing interest in the use of en-vironmentally friendly “green” separation technologies able to provide high quality–high bioactivity extracts while precluding any toxicity associated with the solvent Some of the motivations

to employ “green” separation technologies as a viable separation technique are (a) tightening government regulations on toxic-chemical solvent residues and pollution control, (b) consumers’ concern over the use of toxic chemical solvents in the process-ing, and (c) increased demand for higher quality products that traditional processing techniques cannot meet

One of the most important considerations in developing new extraction processes is the safety aspect In this sense, a variety

of processes involving extractions with supercritical CO2fluid extraction, membrane-based separation, molecular distillation, and pressured low-polarity water extraction, etc., are generally recognized as “green” separation technology and are consid-ered clean and safe processes to meet the requirements (Ib´a˜nez

et al 1999, Fernandez Perez et al 2000, Herrero et al 2006, Chang et al 2008) They have been developed and are regarded

as innovative emerging separation technologies that meet food quality and safety requirements These processes can be used

to solve some of the problems associated with conventional or-ganic solvent-oriented separation processes Operation parame-ters and other factors related to the quality of the original plant, its geographic origin, the harvesting date, its storage, and its pretreatment process prior to extraction also influence the

separation operations and the final composition of the extracts

obtained

Supercritical CO 2 Fluid Technology

Changes in food processing practices and new opportunities

for innovative food products have spurred interest in

supercrit-ical CO2 fluid extraction Supercritical CO2 fluid technology

has been widely used to extract essential oils, functional fatty

acids, and bioactive compounds, and also been applied in

re-cently developed extraction and fractionation for carbohydrates

(Glisic et al 2007, Shi et al 2007a, Monta˜n´es et al 2008, Mitra

et al 2009, Monta˜n´es et al 2009, Sanchez-Vicente et al 2009,

Shi et al 2010a, 2010b) Supercritical CO2 fluid extraction is

a novel separation technique that utilizes the solvent properties

of fluids near their thermodynamic critical point Supercritical

CO2is being given a great deal more attention as an alternative to

organic chemical industrial solvents, and increased

governmen-tal scrutiny and new regulations restricting the use of common

industrial solvents such as chlorinated hydrocarbons It is one

of the “green” separation processes that provides nontoxic and

environmentally friendly attributes and leaves no traces of any

toxic chemical solvent residue in foods

When CO2 fluid is forced into a pressure and temperature

above its critical point (Fig 40.1), CO2becomes a supercritical

fluid Under these conditions, various properties of the fluid are

placed between those of a gas and those of a liquid Although

the density of a supercritical CO2fluid is similar to a liquid and

its viscosity is similar to a gas, its diffusivity is intermediate

between the two states Thus, the supercritical state of a fluid

has been defined as a state in which liquid and gas are

indis-tinguishable from each other or as a state in which the fluid

is compressible (i.e., similar behavior to a gas), even though

possessing a density similar to that of a liquid and with similar

Temperature (°C)

Supercritical region

Tc=31.1°C

Pc=7.38MPa

Liquid phase

Solid phase

Gas phase

T C

P C

Triple point

Figure 40.1 Supercritical pressure–temperature diagram for

carbon dioxide.

solvating power Because of its different physicochemical prop-erties, supercritical CO2provides several operational advantages over traditional extraction methods Because of their low viscos-ity and relatively high diffusivviscos-ity, supercritical CO2fluids have better transport properties than liquids They can diffuse eas-ily through solid materials and therefore give faster extraction yields One of the main characteristics of a supercritical fluid is the possibility of modifying the density of the fluid by chang-ing its pressure and/or its temperature Since density is directly related to solubility (Ravent´os et al 2002, Shi and Zhou 2007, Shi et al 2009a), by altering the extraction pressure, the solvent strength of the fluid can be modified

The characteristic traits of CO2 are inertness, nonflamma-bility, noncorrosiveness, inexpensive, easily available, odorless, tasteless, and environmentally friendly Its near-ambient criti-cal temperature makes it ideal for thermolabile natural products (Mendiola et al 2007) CO2has its favorable properties and the ease of changing selectivity by the addition of a relatively low amount of modifier (co-solvent) such as ethanol and other polar solvents (e.g., water) CO2may be considered the most desirable supercritical fluid for extracting natural products for food and medicinal uses (Shi et al 2007b, Kasamma et al 2008, Shi et al 2009b, Yi et al 2009) Other supercritical fluids, such as ethane, propane, butane, pentane, ethylene, ammonia, sulphur dioxide, water, chlorodifluoromethane, etc., are also used in supercritical fluid extraction processes

In a supercritical CO2fluid extraction process, supercritical

CO2fluid has a solvating power similar to organic liquid solvents and a higher diffusivity, with lower surface tension and viscosity The physicochemical properties of supercritical fluids, such as the density, diffusivity, viscosity, and dielectric constant, can be controlled by varying the operating conditions of pressure and temperature or both in combination (Tena et al 1997, Shi et al 2007a, 2007b, 2007e, Kasamma et al 2008, Shi et al 2009b) The separation process can be affected by simply changing the operating pressure and temperature to alter the solvating power

of the solvent After modifying CO2with a co-solvent, the ex-traction process can significantly augment the selective and sep-aration power and in some cases extend its solvating powers to polar components (Shi et al 2009a)

Supercritical CO2 fluid extraction is particularly relevant to food and pharmaceutical applications involving the processing and handling of complex, thermo-sensitive bioactive compo-nents, and an increased application in the areas of nutraceuticals, flavors, and other high-value items such as in the extraction and fractionation of fats and oils (Reverchon et al 1992, Rizvi and Bhaskar 1995), the purification of a solid matrix, separation of tocopherols and antioxidants, removal of pesticide residues from herbs, medicines, and food products, the detoxification of shell-fish, the concentration of fermented broth, fruit juices, essential oils, spices, coffee, and the separation of caffeine, etc (Perrut

2000, Gonz´alez et al 2002, Miyawaki et al 2008, Martinez et al

2008, Liu et al 2009a, 2009b)

This technology has been successfully applied in the ex-traction of bioactive components (antioxidants, flavonoids, ly-copene, essential oils, lectins, carotenoids, etc.) from a variety

of biological materials such as hops, spices, tomato skins, and