Download membrane separation process

MIDDLE EAST TECHNICAL
UNIVERSITY
FDE-312
TERM PROJECT
Membrane Applications in Food Industry
Submitted to : Serpil SAHIN
Submitted by : Deniz ARAS
Aysegül YILDIZ
1.INTRODUCTION
MEMBRANE SEPARATION PROCESS
Membrane separations represent a new type of unit operations which, ultimately, is
expected to replace a significant proportion of conventional separation process. The advantage of
membrane separation lies in their relatively low energy requirements, the reason for which is that,
unlike conventional processes such as distillation, extraction & crystallisation, they generally do
not feature phase transitions. If, however, in a membrane process a phase transition does not
occur (as in pervaporation), this advantage does not exist, and economically attractive applications
are restricted to special cases. In gas separations, which in principle do not involve phase
transitions, the energy consumption in a membrane process is in general no less than that, for
instance, in an absorption-desorption process. In fact the two processes are similar, the
membrane corresponding to the absorbing medium. This is insufficiently recognised by scientists &
process engineers working with membranes.
Thus membrane separation is not an ideal new technology, but just another unit operation, whose
attractiveness must be weighed against that of other competitive processes.
For evaluating the advantages & disadvantages of membrane processes it is essential to
have one’s disposal a reliable physical model that permits a sufficiently accurate estimation of the
technical & economic feasibility. Such a model also enables optimisation of the separation
processes & the development of new & suitable membranes & membrane modules.
TYPES OF MEMBRANE SEPARATION PROCESSES
Membrane processes may be classified according to the types of
membranes used. With porous membranes, for example, a distinction is made
between microfiltration & ultrafiltration, depending on the pore size & particle
size involved. As an extension to this, reverse osmosis is sometimes called
hyperfiltration & indeed theories have been developed, such as the “preferential
sorption-capillary flow” model of Sourirajan, in which separation is considered to
take place via pores with dimensions in the range of molecular sizes.
Another common way of classifying the membrane is according to the exerted driving
force. In nonporous (tight) membranes, which are used in, for example, reverse osmosis, dialysis,
pervaporation, vapour permeation & gas separation, different driving forces or phase conditions
are applied (Table 1).
TABLE.1 SEPARATION PROCESSES WITH TIGHT MEMBRANES
PROCESS
Pervaporation
Vapour permeation
Gas permeation
Reverse osmosis
Thermoosmosis
Dialysis
Pertraction
Electrodialysis
DRIVING FORCE
Gradient of vapour pressure
Gradient of vapour pressure
Pressure gradient
Pressure gradient
Temperature gradient
Concentration gradient
Concentration gradient
Gradient in electric potential
Various models suggest fundamental differences between the separation mechanisms of
these processes in tight membranes. Such differences, however, exist only if we consider the
conditions outside the membranes. Within the membranes the transport mechanisms are identical
for the processes mentioned. This has been shown in theoretical studies of Cheng H. Lee & j. G.A.
Bitter.
2.MEMBRANE SEPARATIONS IN FOOD PROCESSING
Membrane separations at the molecular level fit in well with food processing at its many
levels. In an energy-conscious era, any process that efficiently purifies or concentrates liquid
foods at low temperatures while retaining basic qualities is desirable. Such major membrane
separations include ultrafiltration, reverse osmosis &electrodialysis.
Other widely used filtration practices include coarse filtration, for separating particles &
agglomerates in liquid foods & microporous or fine filtration, for removing microorganisms in wine
& pasteurised beer. An alcoholic beverage was probably the first reported substance deliberately
subjected to membrane transfer.
The theory & mechanism of membrane separations have been reviewed extensively
(Madsen ’77; Breslav ’82; Landsdale ’82; Anonymous ’83). The transfer of ultrafiltration & reverse
osmosis from the laboratory bench as tools for biochemists to the pilot plant & industrial areas as
adjuncts to food processing is relatively recent. Commercial units for the membrane separations
of sugars were operable about 1965. Shortly thereafter extensive studies were underway on
whey, fruit juice, milk & egg white.
According to the Bore & Hiddink (1980), the worlds first industrial molecular membrane
separations & concentrating plant for processing whey was established in New Zealand during
1971. By the mid-1970 membrane separations technology had gradually spread worldwide. Since
then, an accelerated scaling up of square meters of membranes in use has occurred in industrial
food processing, associated with an ever-expanding pattern of research.
2.1. ULTRAFILTRATION OF FOODS
Ultrafiltering liquid foods serves 3 major functions: separation, clarification, & selective
concentration. These activities, occurring normally at 10-55C, do not denature proteins.
Furthermore, the low temperature conserve energy & reduce costs. An absence of a phase change
in the ultrafiltration & reverse osmosis processing of liquid foods further favours cost reduction.
The time, temperature, pressure & flow rate depend on the nature of the food & its
condition, the membrane type, equipment design & various dual or multiple interactions of food
components. Single-strength fruit juices may be ultrafiltered at 10C but legally can not be
diafiltrated, whereas sugar solutions are clarified by ultrafiltration at 60C or higher. To produce
fruit juice concentrates requires prior aroma stripping of hot juices followed by ultrafiltration at
50C or higher. The permeate may then be diafiltered & vacuum concentrated.
A large number of foods in growing list lend themselves to ultrafiltration. (Table 2).
Clarification is the most important reason for applying this type of membrane separations to fruit
juices. The concentrated pectin obtained as a by-product often is used in making jellies & jams.
Milk & milk products, with their large amounts of protein & lactose, are not clarified by
ultrafiltration but are selectively concentrated.
TABLE.2 SOME FOODS OR FOOD-RELATED MATERIALS PROCESSED
BY MOLECULAR MEMBRANE SEPARATION
Fruit & vegetable juices
Gelatin
Thin sugar juices
Enzyme preparations
Maple sap
Blood plasma
Vinegar
Carbohydrates
Coffee
Cereal & oilseed proteins
Beer
Fresh fluid milk
Wine
Cheese
Gums
Whey
Egg
Fermented milks
Pectins
Dehydrated milks
Natural colorants
Ice cream
2.1.A Ultrafiltration of Plant Foods
 Fruit Juices
Traditional methods for clarifying fruit juices require pressing extraction followed by a
juice pretreatment with an enzyme, pectinase, to reduce or eliminate pectin. Then centrifugation
or standard filtration is applied.
Apple, pear, orange, grape & other fruit juices have been effectively clarified without
fining agents by ultrafiltration. Little or no pectinase is required & clarified juice as permeate is
very clear, because pectin, starch & impurities are concentrated in the retentate. Microorganisms
do not pass through UF membranes, so the resulting permeate, under ideal conditions, is free of
microbial cells. The flavour & colour of the ultrafiltered clarified fruit juice are excellent & yield
is high. Even apple pomace after extraction can be purified by ultrafiltration.
Optimal conditions for processing apple juice by ultrafiltration depend in part on
pretreatment & the type of equipment. The yield of permeates for clarified apple juice on an
industrial scale is about 97%, with a throughput of up to 20,000 litres of juice per hour.
Fruit juices, after clarification by ultrafiltration, can be concentrated by reverse osmosis,
vacuum evaporation or a combination of two. Concentrates of about 72% soluble solids have been
obtained.
Fruit juices other than apple juice are readily clarified by ultrafiltration, but one problem
associated with fruit processing, in general, is the short harvesting time, This places a burden on
biotechnological processes which require a large capital investment, but apparently the advantages
of ultrafiltration outweigh this factor.
Wines & Other Alcohol Beverages
Wines are yeast-fermented products of grapes. Fruit wines are derived from various
macerated fruits or their juices to which sugar & yeast nutrients have been added followed by an
aerobic fermentation. Several problems in their production include oxidation & browning in white
wines & tartrate crystal formation in white & red wines.
The oxidation & browning defects of white wines have traditionally been controlled by adding up to
100ppmof free SO2 in the form of sodium metabisulphite or gaseous sulfur dioxide to grape juice
& young wine. Recently, complaints by sensitive consumers who suffered mild to serious
physiological reactions after ingesting vegetable salads treated with metasulphites to maintain
colour & crispness have alerted regulatory authorities to the possibility that sulphite-bearing
beverages may have similar effects.
Potassium tartrate crystallisation of untreated wines becomes appearent in the home &
restaurant, where temperature fluctuates. A traditional preventive practice, cold stabilisation,
assures the stability of the wine by holding it at –8 to-6C for 2 or 3 weeks, at the winery. This
treatment forces the precipitation of potassium tartrate, a natural ingredient & the stable
commercial wines are then filtered & bottled. An alternate method requiring less refrigerated
space is suggested through ultrafiltration. When young wine is ultrafiltered, water, alcohol &
soluble salts permeate, leaving behind a retentate with aroma compounds. The permeate is
exposed to chill temperatures & following potassium tartrate crystallisation, it is recombined with
the retentate & bottle.
Mostly, however, ultrfiltration serves a useful clarifying function by giving wines a fine
polish.
Cane, Beet, & fruit Rind Sugar Juices
Sugar juices should be free of gum, starch, & protein impurities before they are
concentrated into syrups and crystallised into sugars; otherwise darkening of juice colour &
turbidity result.
Traditionally, lime (CaO) is applied to pH 7.0-7.5 sugar juices & heated to 100c to precipitate
turbidity-forming components. The resulting precipitate is centrifuged & the remaining
turbidity-forming substances removed by individual treatments. Dextrans, for example, produced
by Leuconostoc mesenteroides bacteria, are hydrolysed by dextranases.
Industrial ultrafiltration to clarify cane sugar juices is conducted with polysulfone
membranes, Most of the juice is separated in this manner, but more can be extracted through
diafiltration. The ultrafiltered juices produced using lower lime levels are clear & less pigmented
than those processed traditionally.
Purifying cane sugar juice by ultrafiltration may not provide significant advantages over
traditional procedures under all conditions. After purifying cane sugar juices & 30 Brix molasses
by ultrafiltration, Gandana (1981) concluded that the relatively low fluxes make the process
uneconomical.
Clarification of thin sugar beet juice at an industrial level has been studied intensively by
the Danish Sugar Corporation (DDS). Processing was conducted at pH 6.5 & 80C using a
polysulfone membrane with inlet pressures of 4.5 bar. Initially, clay & other soil impurities & pulp
impede the effectiveness of prefilters & membranes. The situation is somewhat analogous pore
plugging occurring during cane sugar juice clarification caused by fiber & wax accumulations
originating from sugar cane stalks. Using prefiltering screens led to more effective cleaning of
membranes & improved clarification, resulting in clear, low-colour sugar beet juices after
ultrafiltration. The small volume of concentrate that formed during the process was utilised as
cattle fodder.
Citrus fruits such as grapefruit contain bitter components & 5-8% sugar solids in their
peels. In an interesting application combining ultrafiltration & ion exchange, Breslau et al (1976)
separated sugars, 64% invert & 34% sucrose, from the rind & peels of citrus fruits while excluding
the bitter components. An essential step for making the above process function properly was to
prescreen & filter the pulp 6 suspended materials from the sugar extract. Ultrafiltration with a
hollow fibre unit initially gave a straw-coloured bitter permeate. The bitter components were
removed by passage of the permeate through a cationic exchanger & than an anionic exchanger.
Although lighter in colour & very sweet, the citrus rind juice still contained residual flavour.
Passage through a mixed bed column of cationic & anionic resins produced a crystal-clear, sweet
juice which was vacuum evaporated to approximately 63% soluble solid syrups.
Vegetable Proteins
For several decades activity has been directed at isolating vegetable proteins to be solubilized in
alkali & spun into anlog foods. Soy protein was mostly involved because of its relatively low cost.
Spun foods made from soy protein isolates & texture foods developed from soy flour have not met
with overwhelming success.
Ultrafiltration-reconstituted oilseed protein flour or flakes result in permeates streams
edible protein with a high biological oxygen demand.
Rapeseed, peanut, cottenseed, & potato subjected to ultrafiltration have produced
concentrates of quality equal or almost equal to that of protein isolates. The membrane separation
technology applied is simpler and, apparently, less costly than traditional practices involving
extraction and precipitation.
Colorants
Government restrictions on a number of synthetic red dyes used in foods and beverages
have instigated an intense search for natural colorants. Juice from red beets apparently is a
satisfactory replacement obtained by solid- liquid extraction of the minced beet. These extracts
are clarified by ultrafiltration and concentrated by reverse osmosis.
2.1.B Ultrafiltration of Animal Foods
 Animal Proteins
Egg white or albumin, gelatin and bovine blood plasma produced under hygenic conditions
are excellent natural foods, or components of foods, for which many uses exist. Traditional
processes for the separation, purification and concentration of these animal protein-bearing
materials can be cumbersome, and the carbohydrate residue and other impurities induce many
undesirable colours.
Concentration of 12% total solids egg white to 23% total solids by a combination of
ultrafiltration and reverse osmosis and of 24% total solids whole egg to 45% total solids by
ultrafiltration alone are now attainable using industrial processes. Fermentation of egg white, a
nominal step in traditional processes, reduced the flux of ultrafiltration and increased process
capacity.
Gelatin is obtained by extraction from the bones and tissues of slaughtered domestic
animals. The extracts contain considerable macromolecular material, which is removed by
ultrafiltering.
Milk
Milk products concentrated by ultrafiltration include whole milk, skim milk, whey and
buttermilk.
In milk the presence of 3.5% total protein and 4% fat make concentration more difficult
than in most other food beverages, because of magnified concentration polarisation and increased
membrane fouling. Milk processing also is subject to extremely strict sanitation regulations and
equipment must be designed with sanitation as a top priority.
Essentially all present milk ultrafiltration is undertaken using polysulfone membranes and
the configuration of membranes and the design of support equipment are the same as for other
foods, except when modified to meet specific sanitary code requirements.
Fresh Farm Milk and Products
Concentration of milk on farms saves on transportation costs, as a large fraction of the
milk is retained as permeate. This permeate containing lactose, soluble salts, nonprotein nitrogen
and vitamins, is fed to farm animals by blending it with feeds at an established 3-5% savings in
total feed costs.
Ultrafiltered retentates made at the farm would be utilised in a region of many cheese
factories or at factories producing ice cream, skim milk, butter and buttermilk.
Ultrafiltration techniques applied at farms appear technically feasible and valuable dairy
products can be produced by mechanically separating the retentates or utilising them directly in
cheese making. However, the anticipated heavy investment costs for membrane equipment and
increased demands for labor and sanitazing materials applied to small production units must be
carefully considered.
Fermented Milk Foods
One desirable characteristic of a good yogurt is high viscosity. This is attained by optimum
lactic acid development, stabiliser supplementation, and in many instances addition of skim milk
powder. The latter increases the viscosity of yogurt, but it also increases calories and lactose.
Milk mixes for yogurt can be ultrafiltered at 52 C to raise their protein content enough to obtain
high viscosity without an accompanying increase in lactose. It was found that yogurt obtained by
direct ultrafiltration was higher in curd firmness and viscosity with less lactose and more distinct
flavor than yogurt supplemented with skim milk powder.
Natural Cheese
When membrane technology applied to liquid foods was in its infancy, the close parallel
relationships between molecular separation by ultrafiltration and coarse sieve separation of solids
in cheese making were discerned. In traditional cheese making no molecular membranes are used,
but the screening of whey and curds after milk coagulation and curd setting gives a
macromolecular concentrate of fat, protein and insoluble salts in a decreasing volume of liquid or
serum. This serum also contains entrapped low molecular weight solutes such as lactose and
calcium. In the retained curd the insoluble components rise to high levels, while enough lactose still
remains to provide an essential energy source for the lactic acid bacteria involved in the
fermentation. Maubois, Mocquot and Vassal patented an original concept of cheese making utilising
highly concentrated ultrafiltered milk which largely reflected this relationship. Their process,
known as the MMV process, was identified by the first letters of their last names. Many European
firms have adapted their concept of cheese making.
2.1.C MMV Process of Cheese Making
The concept behind the MMV process is to obtain highly concentrated retentates through
direct ultrafiltration of milk. This permits fat, protein and insoluble salts, but not lactose, to
concentrate. The proteins of the retentate consist of casein, albumin, and globulin, because these
can not permeate the membrane; in standard cheese making only casein is retained. Retentates
produced from skim milk with prsesnt standard polysulfone membranes can attain a concentration
of about 21%total protein (7:1), and from whole milk 16% total protein (5:1). Because of the
selective nature of ultrafiltration, these retentate concentrates in liquid from approach, or can be
made to approach through proper milk standardisation, nearly the exact general composition of
selected cheeses. For these reason they are called precheeses, exhibiting high viscosity &
pseudoplastic properties.
Direct ultrafiltration of milk by the MMV process is conducted either in a batch or
continuous mode using any of the different membrane configurations, tubular, spiral wound, hollow
fibre, or plate.
The liquid pre-cheese , or retentate , is pumped to a dispersion unit where rennet, starter
culture, coloring matter, salt and microbial spores are metered in automatically. Then the
pre-cheese, fully primed,is filled directly into metal or plastic forms to give within 15 min or less a
firm wheel or block of curd. This young cheese is removed and placed in a temperature-controlled
ripening room.
Advantages of the MMV ultrafiltration process for making cheese include the following: a)
Albumin and globulin, ordinarily lost to whey in traditional processes , are retained and may
increase cheese yield by 10-20%.; b) approximately 75% less rennet is required ; c) cheese vats are
not essential, potentially enabling the operation to become continuous in a closed , highly sanitary
environment; d) better control of cheese weight is established; e) little or no whey is obtained.
Not all cheeses presently can be made by the MMV concept. These include hard cheeses
such as Swiss, grana, and cheddar, whose protein levels are high, often above 25%, and lower
protein soft cheeses , like cottgage cheese , with complicating post-cooking , washing, and
creaming steps.
2.1.D. Whey Ultrafiltration
Whey is the serum portion obtained after the coagulation of milk and contraction of curds
in cheese and casein manufacture. In its natural liquid state whey contains 6-6.5% total solids and
many nutrients and has a high BOD. Cheese factories have had difficulty disposing of liquid whey.
In modifying processes to make whey more attractive , spray-dried powders and ultrafiltered
whey protein concentrates (WPC) have been produced . The WPC also can be produced efficiently
by pre-concentrating whey with reverse osmosis followed by ultrafiltration and vacuum
evaporation.
2.2 REVERSE OSMOSIS OF FOODS
Reverse osmosis concentrates liquid food components by remooving water at low
temperatures. The removal of water does not require a phase change, thus conserving energy.
Desalination of salt water is another major application of reverse osmosis, but equally interesting
is the reduction of alcohol in beers using special reverse osmosis membranes.
Ultrafiltration and reverse osmosis are often coordinated as a dual process for
liquid foods. The selective nature of ultafilration helps remove specific components for ultimate
recovery, and contaminants for purification of the concentrate. Reverse osmosis generally serves
in collaboration with vacuum evaporation to concentrate the permeates. Sole concentration of
permeate or retentates by reverse osmosis or by vacuum evaporation depends on the starting
solids level, the concentration desired and the relative cost advantages.
Reverse osmosis reduces the need for floor space and operates down to extremely low
temperatures, e.g 0-7C in processing alcohol beverages.
2.2.A Reverse Osmosis of Plant Foods
Fruit and Vegetable Juices
Most fruit juices including apple, orange, and pear juices which develop high osmotic
pressures can be concentrated to relatively high solids by reverse osmosis. Limitations are set by
concentration polarisation, the fouling of membranes and the need for coarse filtering and
prescreening of fibres, pulp and particles. Pectin is one of the fouling components in fruit juice.
Among vegetable juices , tomato juice and tomato serum have been concentrated by
reverse osmosis with the use of cellulose acetate membranes.
Alcoholic Beverages
In the brewing industry, reverse osmosis is influencing change. Ethyl alcohol passes along
with water through cellulose acetate membranes. This characteristic has led to the making of
low-or reduced-alcohol beers and ales.
Sugar Beet Juice
Sugar beet juice concentration by reverse osmosis is performed more easily by polysulfone
membranes than cellulose acetate, although some clarification problems persist because sugar
beets come from the soil.
 Coffee
Another plant-derived liquid food now being concentrated industrially by reverse osmosis
is instant coffee. Coffee extract for the instant product contains approximately 13% total solid.
The extract is concentrated by reverse osmosis at 70C to 36% total solids without a significant
loss of solids. Thereafter the concentrate is vacuum evaporated to 48% total solids, dried and
packaged.
Maple Syrup
Reverse osmosis can concentrate maple sap and increase soluble solids to a level that is
economical for vacuum or alternate evaporation.
2.2.B Reverse Osmosis of Animal Foods
Egg
Whole egg has a high fat concentration and is preferably concentrated by ultrafiltration,
but the almost fat-free egg white is amenable to concentration by reverse osmosis after first
being ultrafiltered.
Milk
Whole milk can be concentrated without any foiling or flux reduction than is encountered
with skim milk during reverse osmosis; however rancidity can be seen when making milk powder
from reverse osmosis concentrated milk.
Cheese and Milk Products
Reverse osmosis retentates , unlike ultrafiltration retentates uniformly reflect the
concentration of all the components. Reverse osmosis milks can possess good flavour and stability
but realistically they should be compared to low temperature vacuum-evaporated milks as their
general compositions at a given concentration are the same.
Limitations in the use of reverse osmosis and vacuum evaporated milks for cheese and
other dairy products relate to excess lactase buildup.
2.2.C. Reverse Osmosis of Whey
Reverse osmosis utilising low temperatures was accepted widely by the cheese industry
after demonstrations that it could more economically raise the normal 6.5% total solids of whey to
12% prior to vacuum evaporation.
3.REFERENCES
 McGregor W. Courtney, Membrane Separations in Biotechnology, 1986, Marcel Dekker,
INC.
 Bitter J.G.A, Transport Mechanisms in Membrane Separation Processes, 1991, Plenum
Press.
 Cecille L., Toussaint J. C., Future Industrial Prospects of Membrane Processes, 1988,
Elsevier Applied Science.