The inherent safety, convenience, availability, nutritional content, aesthetic appeal, and variety that typify food supplies are a hallmark of modern life. Chemical engineering knowledge can be accredited with improving the conversion of raw foodstuffs into safe consumer products of the highest possible quality. Among those, membrane-based separation and other filtration techniques are most common. Chemical engineers have applied their expertise to chemically synthesize fertilizers, herbicides, and pesticides that promote crop growth and protect crops from weeds, insects, and other pests (AlChe, 2009). It's so easy to feel the contribution of chemical engineering in food sector if we observe the morning tea to late night beverage. All processed food stuffs around us is more or less subject to different types of filtration. One of the fastest growing parts of the whole sector is the mineral water and soft drinks sector, which has a sizeable requirement for fine filtration (Sutherland, 2010).
Modern techniques are used to improve the flavour, texture, nutritional value, safety, appearance and overall aesthetic appeal of various foods. Modern food processing can also improve the quality of life for people with food allergies and for diabetics. Chemical engineers have made lasting assistances to limiting spoilage and giving foods greater shelf life using the process of pasteurization. Foods processed using aseptic packaging retains their vitamins, minerals, and desired textures, colours, and flavours more effectively than those processed with traditional canning. Nestlé, Wal-Mart, Unilever, PepsiCo etc. are some world famous food companies without whom modern life cannot be imagined.
Filtration in food processing
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Filtration is a process where solid particles present in a suspension are separated from liquid or gas employing a porous medium (Srikanth, 2012). Surface, depth and cake filtration are different types based on filtration mechanism. Two types of filtration theory are widely known which are gas filtration theory and liquid filtration theory. These theories have also some limitations. This article includes some advances filters, different types of filter media and their applications in food industries. There are some criteria for choice of filter medium such as particle size that has to removed, permeability of clean medium, solid holding capacity of the medium, flow resistance of medium etc. Filter aid is a very important factor which forms a surface deposit to screen out the solids. It also prevents the plugging of the supporting filter medium (Srikanth, 2012).
Chemical engineers have invented a variety of engineered processes that allow food processors to remove impure substances to improve food quality, safety, and aesthetics. Today's membrane-based separation is used widely to remove impurities during food processing by pressure to force unwanted substances in food ingredients to pass through a semi permeable membrane and it is also applied majorly in the dairy industry, mainly as a processing phase in production soft cheeses. Chemical engineers strive to maximize the available surface area in filter, reduce membrane pore size, minimize the pressure drop the fluid will experience when flowing through the unit and maximize cost-effectiveness. For food industries, there are some specific requirements to choose filter media such as dissipation of electrostatic charges, high abrasion resistance, available clean-in-place system etc. In this dissertation, some food processing industries have been presented where different types of filtration are the key factor. Among those cane and beet sugar industry, starch and sugar industry, beverage industries like wine, beer etc. are notable. Filtration should be the most prior subject to enhance food safety management of an industry. It can help to remove physical, chemical and other microbiological contaminants with great efficiency. Filtration performance and monitoring are also very important for food safety.
This also focuses on advantages of self-cleaning filters over manual and mechanical cleaning. Advances in filtration technology include the development of continuous processes to replace old batch process technology (Patel R. et al, 2010). Different self-cleaning filters reduce product loss, required minimal operator intervention and improve flow consistency and with the use of latest filter and filter media it is possible to apply it in various food, starch and sugar industries, which reduces the time as well as give better quality products.
FOOD PROCESSING INDUSTRIES
Chemical engineering innovation in food production
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The food industry is a complex, global collective of diverse businesses that supply much of the food energy consumed by the world population. Only subsistence farmers, those who survive on what they grow, can be considered outside of the scope of the modern food industry. The inherent safety, convenience, availability, nutritional content, aesthetic appeal, and variety that typify food supplies are a hallmark of modern life, but this was not always the case. Before modern engineering advances were widely adopted by the food industry, the variety of foods available at stores were determined by what was produced locally, since transportation limitations dictated the distance that perishable foods could travel (AIChe, 2009).
Chemical engineering know-how can be credited with improving the conversion of raw foodstuffs into safe consumer products of the highest possible quality. Chemical engineers routinely develop advanced materials and techniques used for, among other things, chemical and heat sterilization, advanced packaging, and monitoring and control, which are essential to the highly automated facilities for the high-throughput production of safe food products (AIChe, 2009). Chemical engineering unit operations and procedures, established for other industrialized reasons, are used by the food industry like drying, milling, extrusion, refrigeration, heat and mass transfer, membrane-based separation, concentration, centrifugation, fluid flow and blending, powder and bulk -solids mixing, pneumatic conveying, and process mode ling, monitoring, and control. Among these, membrane-based separation and other filtration techniques are mostly common (AIChe, 2009).
Over the years, cleverly engineered solutions have increased the production of processed fruits and vegetables, dairy, meat and poultry, and seafood products, and have allowed more widespread distribution of such foods. The following are some of the most revolutionary improvements in food processing noted in the "Milestones of the Twentieth Century" by the Institute of Food Technologists (AIChe, 2009).
1900s: Vacuum packaging, which removes the oxygen from inside the food package, was invented to prolong the shelf life of foods, and the widespread practice of freezing foods began with fruit and fish. The first ready-to -eat cereals using many chemical engineering unit operations appeared as well (AIChe, 2009).
1920s: Fast-freezing practices for foods were first commercialized by Clarence Birdseye, whose name has become practically known with frozen foods. Birdseye found that by blanching vegetables (cooking them for a short time in boiling water) just before freezing, the process could deactivate certain enzymes that cause off-colours and off-flavours, thereby enhancing the quality of the thawed vegetables. The first commercial use of "puffing" to produce such cereals as Cheerios and puffed rice also began (AIChe, 2009).
1930s: Freeze-drying processes were pioneered in this decade, and frozen foods are dried after deep freezing, in which the entrained water is removed by a process known as sublimation by heating the frozen product in a vacuum chamber. Freeze-dried foods in turn become shelf-stored foods that quickly regain their original flavour, aroma, size, shape, and texture after rehydration. The removal of water slows spoilage, thus providing longer shelf life, and reducing the weight of the food, which makes it cheaper and easier to transport (AIChe, 2009).
1940s: The advent of automated processes to concentrate, freeze, and dehydrate foods enabled a greater variety of foods to be mass-produced and packaged for shipment overseas to military personnel during World War II. Disease-free packaging extremely improved food quality, safety, and nutrient retention (AIChe, 2009).
1950s: During this era, monitored-atmosphere packaging using plastic increased the shelf life of fresh foods. The process controls oxygen and carbon dioxide levels inside the packaging environment to reduce respiration by fruits and vegetables (similar to human breathing) and reduces the amount of off-gas ethylene produced, which delays maturing and damage (AIChe, 2009).
1960s: The first commercial-scale producing machine began producing cold-dried foods and coffee. Advances in aseptic processing allowed shorter heating times for sealed food containers (AIChe, 2009).
1970s: The period of the 1970s saw growing usage in the chemical process industries (paint, textile, oil recovery, pulp and paper). In this decade, the major effects of this technology is in the food and biotechnology processing industries, where ultrafiltration and cross-flow microfiltration are finding increasing uses as a gentle and efficient way of fractionating, concentrating and clarifying a variety of food from milk products, fruit juices and alcoholic beverages to fermentation broths, protein fractions and wastewaters (Cheryan M, 1986).
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1980s: Advanced-atmosphere packaging began to be used widely during this era and It is a more progressive difference of controlled-atmosphere packing, in which the "head space" atmosphere within a food package or the transportation/ storage vessel is modified by flushing it with a blend of inert (nonreactive) gases (AIChe, 2009).
1990s: High-pressure processing was commercially applied first to fresh packaged foods to kill microorganisms that cause spoilage without altering flavours, texture, or appearance (AIChe, 2009).
After 2000: Recent food trends are actually based on fat calculation but tasty, healthy and doctor-designed. Different types of cupcakes, cheese, pizza, fast foods etc. are people's first choice. Also various types of grain made foods are getting popularity day by day (AIChe, 2009).
Advances in chemical fertilizers, herbicides, and pesticides
Early mankind experimented with human and animal wastes, seaweed, ashes, menhaden (a fish used by coastal Native Americans), and other substances to fertilize crops and increase yields. Chemical engineers have applied their expertise to chemically synthesize fertilizers, herbicides, and pesticides that promote crop growth and protect crops from weeds, insects, and other pests. Today, the use of these products is more important than ever to meet the needs of an ever-expanding population (AIChe, 2009).
Nitrogen is the most plentiful part of the air we breathe, present at 79% by volume and a prime nutrient (most often in the form of ammonia). Modern fertilizers stem from a chemical engineering breakthrough pioneered by Fritz Haber in 1908 that developed a process to synthesize ammonia by reacting hydrogen and nitrogen. In 1918, he was awarded the Nobel Prize in Chemistry for this discovery (AIChe, 2009).
Working with industrialist Carl Bosch, Haber scaled up the successful Haber-Bosch process that allows ammonia to be produced cost-effectively in commercial quantities for use in nitrogen fertilizers. Haber's original reaction was carried out under high pressures. The improved ammonia synthesis process carries out the reaction at lower pressures and temperatures, which helps save money by reducing the amount of energy required by the process. (In general, higher pressures and temperatures require more expensive materials and specialized designs.) Many people consider the Haber-Bosch process one of the most monumental chemical-engineering achievements of all time, thanks to its direct impact on global food production (AIChe, 2009).
Pesticides and herbicides
Chemists and chemical engineers have also been instrumental in the discovery, synthesis and commercial-scale manufacture of numerous chemical compounds that function as pesticides (to kill insects) and herbicides (to kill weeds). Various pesticides and herbicides work in different ways. For example, chemical engineers discovered that when glyphosate (the primary ingredient in Monsanto's widely used herbicide Roundup) is applied to a crop, it inhibits a specific growth enzyme called the EPSP synthase. Glyphosate is rapidly metabolized by weeds, and unlike many other earlier herbicides, it binds tightly to soil so that it does not accumulate in runoff to contaminate surface waters or underground aquifers. According to its manufacturer, it eliminates more than 125 kinds of weeds, but does not affect mammals, birds, fish, or insects (AIChe, 2009).
Advanced food processing techniques
Techniques to improve the flavours, texture, nutritional value, safety, appearance and overall aesthetic appeal of various foods involving cooking over fire, smoking, steaming, baking, fermenting, sun drying, or preserving with salt or spices were already being practiced before recorded history.
Today, imaginative and effective engineered approaches many drawn directly from the chemical engineers' toolbox routinely add nutrients, improve aesthetic appeal (in terms of a food's flavours, texture, and appearance), enable longer distance transport (leading to multi-seasonal availability), extend shelf life, and remove microorganisms that contribute to spoilage and are responsible for food-borne illnesses. Modern food processing can also improve the quality of life for people with food allergies (by removing or neutralizing the proteins and other substances that create allergic reactions in certain people) and for diabetics (by reducing sugar content and providing sugar-free alternatives).
The roasting of coffee beans requires exceptionally precise control of the chemical and physical reactions over time. Depending on the progressive bean temperature experienced during roasting, final flavours characteristics can vary widely. Chemical engineers have devised ways to make timely adjustments to the roaster to moderate airflow rates and manipulate bean temperatures without changing the flavours (AIChe, 2009).
Sterilizing and packaging perishable foods
Sterilization is a key aspect of any food -packaging operation. The ability to sterilize foods to protect them against spoilage by oxidation, bacteria, and moulds has always presented an important engineering challenge. Throughout history, people have experimented with the use of dehydration, smoking, salting, pickling, candying and the use of certain spices. Chemical engineers have made enduring contributions to reducing spoilage and giving foods greater shelf life. They include high-temperature pasteurization and canning, refrigeration and freezing, chemical preservatives (using such compounds as sulphite, sodium nitrite, ethyl formate, propionic acid, sorbic acid, and benzoic acid), and irradiation (AIChe, 2009).
In the early years, no one knew how Nicolas Appert's process preserved foods successfully, but the ability to can foods meant that Napoleon's army fighting a long way from home could be fed properly and safely and that British sailors could maintain a healthier diet by feasting on fruits, vegetables, and meats while on long voyages overseas.
More than 50 years later, Louis Pasteur (1822-1895) solved the mystery by demonstrating that the growth of microorganisms is the primary cause of food spoilage and food -borne illnesses and that a high percentage of them could be killed by heating liquids to about 130Â°F (55Â°C) or higher, for relatively short periods, without altering the chemical makeup of the food. This simple process became known as pasteurization and was quickly and widely adopted (AIChe, 2009).
First introduced in the U.S. in the early 1960s, it provides major advantages over traditional canning. It allows many products once considered perishables such as milk and juice to be packaged, distributed, and stored for months or longer without the need for refrigeration, irradiation, or chemical preservatives. In general, during aseptic packaging, both the food and packaging are sterilized at high temperatures for very short periods. The sterile container is then filled in a sterile atmosphere (AIChe, 2009).
The original technology superheated steam to sterilize cans. Pressurized heat exchangers and holding tubes allows the foods and beverages to be sterilized at around 300Â°F. Foods processed using aseptic packaging retains their vitamins, minerals, and desired textures, colours, and flavours more effectively than those processed with traditional canning. In 1989, aseptic-packaging technology was voted the food industry's top innovation of the last 50 years by the Institute of Food Technologists (AIChe, 2009)
Some world famous food companies
In terms of corporate size, food manufacture has no companies to match the giants of other sectors (Sutherland, 2010). Although still by far the largest of the food producers, Nestlé, with annual sales in 2009 of about $95 billion (well down on 2008), is only a quarter of the size of the largest petroleum companies such as Exxon or Shell (Sutherland, 2010). (There are, of course, food retailers much larger than Nestlé, especially Wal-Mart whose 2009 sales of $400 billion made it the third largest company in the world in terms of turnover, with Carrefour a long way behind at second in the list of retailers, at $130 billion (Sutherland, 2010).
The next largest company classified as a food producer is Unilever, with total 2009 sales of $53 billion (although the Unilever picture is complicated by its extensive range of non-food household goods businesses) (Sutherland, 2010). Unilever is closely followed by Cargill, the largest private company in the USA, and by Archer-Daniels-Midland, although both of these are large natural product commodity dealers as well. Then come ConAgra, Kraft Foods, Danone, Kellogg, General Mills, and H J Heinz (Sutherland, 2010).
For some time, the leading beverage companies have been the soft drink makers Pepsico (2009 sales of $43 billion) and Coca Cola ($32 billion), some distance ahead of the brewers (Sutherland, 2010). This picture changed in 2008 with the purchase of Anheuser-Busch by InBev (itself the fairly recent merger of Interbrew and AmBev) to create a company larger than Coca Cola (although still behind Pepsi) and second in size of the brewers is now SABMiller (a 2002 creation), followed by Heineken and then Carlsberg (Sutherland, 2010). Further consolidation in the beverage sector is being driven by a search for markets, because beer drinking can be very regional. Thus, Heineken has acquired the beer business of Femsa in Mexico - which holds 40% of its domestic market and nearly 10% of that in Brazil and one of the fastest growing parts of the whole sector is the mineral water and soft drinks sector, which has a sizeable requirement for fine filtration (Sutherland, 2010).
Filtration is a process whereby solid particles present in a suspension are separated from the liquid or gas employing a porous medium, which retains the solid but, allows the fluid to pass through. It is a common operation used widely in sterile products, bulk drugs and in liquid oral formulation. The suspension to be filtered is known as slurry. The porous medium used to retain the solids is known as filter medium and the accumulated solids on the filter are referred as filter cake and the clear liquid passing through the filter is filtrate (Srikanth, 2012). The pores of the filter medium are smaller than the size of particles to be separated. Filter medium like filter paper or muslin cloth is placed on a support. When feed is passed over the filter medium, the fluid flows through it by virtue of a pressure differential across the filter. Gravity is acting on the liquid column; the solids are trapped on the surface of the filter medium. After a particular point of time, the resistance offered by the filter cake is high that stops the filtration (Sambhamurthy, 2005).
Types of filtration
Based on the mechanism, there are 3 types of filtration. They are surface filtration, depth filtration and cake filtration.
It is a screening action by which pores or holes of the medium prevent the passage of solids. The mechanisms, straining and impingement are responsible for this type of filtration. For this purpose, plates with holes or woven sieves are used (Matteson, 1987). An example is a cellulose membrane filter.
This filtration mechanism retains particulate matter not only on the surface but also at the inside of the filter. This is aided by the mechanism entanglement. It is extensively used for clarification. Ceramic filters and sintered filters are examples of depth filtration (Stephan, 2003).
Case study of depth filtration (Carey, 2008):
Several forces have driven changes in filtration technology during the last few decades, including environmental concerns, the health and safety of winery workers and wine quality. The major active component in traditional depth filtration is diatomaceous earth, which has several major problems. First, it is difficult to dispose because it does not decompose. Second, it can cause symptoms similar to coalminers 'black lung' disease when inhaled over long periods of time. To overcome these demerits, cross flow filtration and ultra-filtration are being practiced in recent years.
Cross flow filtration:
It ranges between ultra-filtration and reverse osmosis and the nominal pore size of the membrane is typically below 1 nanometer (http://en.wikipedia.org/wiki/Nanofiltration , 2012). Nano filtration membranes are still subject to scaling and fouling and often modifiers such as anti-scalants are required for use (Hillie, 2007).
It is a pressure driven membrane transport process that has been applied on both the laboratory and industrial scale. It is becoming a powerful separation tool for the rapidly growing biotechnology industry (Goldsmith et al., 1974).
By this mechanism, the cake accumulated on the surface of the filter is itself used as a filter. A filter consists of a coarse woven cloth through which a concentrated suspension of rigid particles is passed so that they bridge the holes and forma bed. An example is cake made from diatomite. This cake can remove sub micro meter colloidal particles with high efficiency.
Theory of filtration
Depending on dispersing medium filtration theory is divided in two parts;
Gas filtration theory
It mainly includes filtration of aerosols and lyosols. There are several mechanisms for this theory. They are as follows (Wilson & Cavanagh, 1969).
The trajectories of individual small particles do not coincide with the streamlines of the fluid because of Brownian motion. With decreasing particle size the intensity of Brownian motion increases and, as a consequence, so does the intensity of diffusion deposition.
This mechanism involves the finite size of particles. A particle is intercepted as it approaches the collection surface to a distance equal to its radius. A special case of this is the so called sieve effect or sieve mechanism.
The presence of a body in the flowing fluid results in a curvature of the streamlines in the neighbourhood of the body. Because of their inertia, the individual particles do not follow the curved streamlines but are projected against the body and may deposit there. It is obvious that the intensity of this mechanism increases with increasing particle size and velocity of flow.
Every particle has a definite sedimentation speed due to gravity. As a consequence, the particles deviate from the streamlines of the fluid and owing to this deviation; the particles may touch a fibre.
Both the particles and the fibres in the filter may carry electric charges. Deposition of particles on the fibres may take place because of forces acting between charges or induced forces.
Liquid filtration theory
The term solid-liquid filtration covers all processes in which a liquid containing suspended solid is freed of some or the entire solid when the suspension is drawn through a porous medium (Melia & Weber, 1972).
Kozeny - Carman equation
= â€¦â€¦â€¦â€¦. (1)
A = filter area
V = total volume of filtrate delivered
t = filtration time
âˆ†P = pressure drop across cake and medium
r = specific cake resistance
Âµ = filtrate viscosity
l = cake thickness
L = thickness of cake equivalent to medium resistance (http://www.chemeng.ed.ac.uk/).
This equation does not take into account of the fact that depth of the granular bed is lesser than the actual path traversed by the fluid. The actual path is not straight throughout the bed, but it is sinuous or tortuous (Chowdiah et al., 1981).
This Law considered that filtration is similar to the streamline flow of a liquid under pressure through capillaries.
= â€¦â€¦â€¦â€¦. (2)
Cake resistance, RM = â€¦â€¦â€¦â€¦â€¦â€¦â€¦â€¦. (3)
Specific cake resistance, Î± = Î±×³âˆ†Pâ€¦â€¦â€¦...................... (4)
The filter resistance is much less than the cake resistance (RC << RM)
The filter medium acts as a mechanical support for the filter cake and it is responsible for the collection of solids (Srikanth, 2012). Minimum cake thickness of discharge for different types of filter is presented in Table 1 (Subramanyam et al., 2005).
Table 1: Minimum cake thickness for discharge (Andrew et al., 2002)
Minimum design thickness
Materials used as filter media (Rushton, 2008)
Different types of materials used as filter media for various applications industrially and domestically are presented in Table 2.
Table 2: Type of filter media, features and their application (Patel R. et al, 2010).
Type of filter media
Metal fibre media (non-woven metal fibre)
Excellent durability, co abrasion resistance
Polymer & gas industry
Multilayer sintered mesh
It can be reused
Stainless steel (plain, twill & Dutch type)
Water proof inside & plastic woven cloth outside
Oil, chemical, food, pharmaceutical & aviation industry
Anthracite filter media
It has high efficiency
Filter media treated by graphite
Made up of fiberglass
Used in cement & steel industry. Used as filter cloth for air filter
Activated carbon fabric (non-woven type)
Little air current resistance, strong strength
Used in air conditioner as auto air filter or carbon air filter
High biological activity
Sanitary sewage & industrial waste processing
Aramide filter fabric
Easiness of cake peeling, high stability, anti-distortion
Used in ore dressing, chemical & brewing industry, equipped in filter presses, vacuum filters etc.
Autoroll filter media
It has metal structure, saves energy & work stably
Used in air filtrate
Laminating PTFE membrane
Felt type of filter
Used in cement company & incineration fields
Pocket type of filter
Air conditioner & electronic industry, food industry, applied to the pre-filtration of coarse efficiency
Woven materials such as felts or cloths
Fig 1: Monofilament woven cloth
Woven material is made of wool, cotton, silk and synthetic fibres. Synthetic fibres have greater chemical resistance than wool or cotton. The choice of fibre also depends on the physical state and chemical constitution of the slurry. It includes mainly of two types.
Monofilament woven cloth: The yarns of a monofilament fabric are not only impermeable but also fairly smooth and cylindrical. Orifice analogy and drag theory approaches have been the most successful in predicting the resistance of these materials to fluid flow. Multifilament woven cloth: The chief difficulty encountered when dealing with multifilament media is the highly complex geometry of the fibres and yarns that make up the cloth. Even in a fabric of apparently simple weave and construction, such as a plain-weave, continuous-filament cloth, some of the flow takes place in the highly tortuous channels present in the yarns (Wardsworth, 2007).
Criteria for choice of filter medium
There are three criteria for choice of filter medium.
Size of particle retained by the medium
The permeability of the clean medium
The solid holding capacity of the medium and the resistance to fluid flow of the used medium (Purchas, 2000).
Measurement of pore size and particle retention
In some cases, the desirable component in the slurry is the liquid, which may be required in clarified form e.g., beverage filtration; here the choice of deep-bed elements of pre-coated candles of large solids-holding capacity may be indicated. While, where the solids are valuable, a sieve like mechanism is favoured, so that information about the pore size of the medium may be of more direct use in media selection. The pore structure of the medium will determine the feasibility of a separation (Lach & Wright, 2004).
It forms a surface deposit which screens out the solids, also prevents the plugging of the supporting filter medium and used as filter media in recoat filtration (Srikanth, 2012). The ideal characteristics of filter aid materials are chemically inert to the liquid being filtered and free from impurities, low specific gravity (so that filter aids remain suspended in liquid), porous rather than dense (so that previous cake can be formed) and recoverable (Hunt, 2001) & (Srikanth, 2012).
Different forms of equipment are employed for filtration. The factors which should be considered, while selecting the equipment and operating conditions are given below.
Material related properties of the fluid (Shirato, 1978).
Nature of the solids (particle size, shape, size distribution etc.)
Concentration of solids in suspension
Quantity of material to be handled
Equipment and process related properties
Flow rate of process fluid
The limit to size of particles passing through the filter
Filter should be sterilized by heat, radiation or gas
It should be economical
Advances in filtration technology are making new products possible in food and beverage. Micro filtration has served the food industry in a variety of areas for years, but refinements in membrane technology and a better understanding of the impact membranes have on the molecules that pass through are opening up a new world of possibilities. For example, bacteria and spoilage organisms in milk are easily removed by micro filters with pore sizes ranging from 0.1 to 20 microns. Ultra filtration units with pores ranging from 0.01-0.2 microns have been shown to affect the appearance and sensory properties of fluid milk because of the protein molecules that can be retained and then added back (Higgins, 2003).
FILTRATION IN FOOD INDUSTRIES
Advances in food purification
Contamination by bacteria, mould, and other microorganisms is by far the most important cause of food-borne illnesses, so chemical engineers have been working hard to commercialize effective technologies to control such microbes as Escherichia coli, Salmonella, and other disease-carrying pathogens. Such systems typically use high temperatures, high pressure or high vacuum, and chemical preservatives. In all cases these techniques must be able to kill microorganisms at sufficient levels, be controlled in large scale operations, be cost effective and economical, and function without damaging meat proteins and creating unwanted changes in food appearance, taste, texture, colour, or nutritional value (AlChe, 2009).
Membrane based separations
Membrane separation processes are centred on the capability of semipermeable membranes of the applicable physical and chemical nature to differentiate between molecules primarily on the basis of size and to a lesser extent, on shape and chemical composition. A membrane's role is to act as a selective barrier, enriching certain components in a feed stream, and depleting it for others. Membranes are made from natural or synthetic polymers or inorganic materials in the form of flat sheets or self-supporting hollow tubes, across or through which the feed solution to be separated is pumped under pressure. The chemical nature and physical properties of the membrane control which components are retained and which permeate through the membrane (Cheryan M, 1986).
Chemical engineers have invented a variety of engineered processes that allow food processors to remove these substances to improve food quality, safety, and aesthetics. Today's membrane-based separation is used widely to remove impurities during food processing by pressure to force unwanted substances in food ingredients to pass through a semi permeable membrane. It is also used broadly in the dairy industry, principally as a processing step in making soft cheeses and in separating and recovering soluble whey proteins (milk albumins and globulins) from whey, a necessary by-product in the process for producing hard cheeses. The recovered whey protein, typically as a concentrate or dried, has considerable commercial value as an additive to a wide variety of other food products (AlChe, 2009).
Membrane-based separation systems are classified as reverse osmosis, microfiltration, ultra filtration, or nano filtration, based on the size and structure of the membrane pores (this dictates the size of the solid particles or liquid droplets to be removed). Numerous designs are available, including tubular, hollow -fibre, plate-and-frame, and spiral-wound configurations. Chemical engineers strive to develop advanced membrane-based separation systems to maximize the available surface area, reduce membrane pore size (for the more precise removal of smaller contaminants), minimize the pressure drop the fluid will experience when flowing through the unit, and maximize cost effectiveness (AlChe, 2009).
Existing membrane-based systems that produce drinking water from unpurified sources often use reverse osmosis membranes (as does this self-hydrating pouch), but they typically require a pump to force the water across the membrane. By comparison, this design relies on a process called forward osmosis, in which during operation water is pulled through the membrane without a pump lightening the load combat troops must carry (AlChe, 2009).
The drivers of the process are specific ingredients in the beverage powder or dehydrated food contained in the self-hydrating pouch. The water moves across the membrane in the direction of charged ions such as salts, sugars and amino acids in the foods to equalize the osmotic pressure inside the pouch (AlChe, 2009).
Specific Requirements for Filter Media in the Food Industry
The filter media which are used in food industries are selected based on some criteria such as dissipation ofÂ electrostatic charges, high abrasion resistance, suitability for clean-in-place systems etc. A brief description of these points has been presented as follows.
Dissipation ofÂ electrostatic charges
When filtering the flammable/ explosive powders there is risk of explosion initiated by a spark. Static charges can built up on the dust cake or filter media and subsequently may discharge causing a spark followed by explosion. It is therefore essential to dissipate this static charge. To give filter medium good antistatic properties, the only way is to blend evenly conductive fibres elements in filter media.
Stainless steel fibre and carbon coated fibre blends are the most prevalent ways to create filter media most conductive to do the job. Care must be taken that the cage and support housing are properly earth in these conditions. Filter bags made from media with conductive scrims can be supplied.
High abrasion resistance
A substance that abrades or wears down is called abrasive material. The transport of fragile, abrasive or lumpy materials presents a challenge that many companies have to face daily with the risk of product damage and system wear.
The developed technology system for the dense phase pneumatic conveyance ensures high performance in the transport of delicate and abrasive materials, such as glass, sand, and corundum. The low speed combined with a reduced amount of compressed air or other gaseous fluid, avoids pipes wear and preserves the integrity of the product. It also contributes greatly to reduced maintenance costs. The materials that tend to clog are able to flow homogenously within the pipe until the selected destination. The use ofÂ specific boosters makes it possible to reduce the friction inside the pipes in order to safeguard the materials properties and avoid lumps. (http://www.air-tec.it/en-US/Markets/Lump-Breaker-Abrasive-Materials.aspx)
Suitability for clean-in-place systemsÂ
Clean-in-Place (CIP) is a way of cleaning the inner surfaces of process equipment, filters, pipes, vessels, and associated fittings, without disassembly. Up to the 1950s, sealed systems were taken apart and cleaned manually. The advent of CIP was a boon to industries that needed frequent internal cleaning of their processes. Industries that rely heavily on CIP are those requiring high levels of hygiene, and include dairy, beverage, brewing, processed foods, pharmaceutical, and cosmetics. The benefit to industries that use CIP is that the cleaning is faster, less labour intensive and more repeatable (http://en.wikipedia.org/wiki/Clean-in-place).
Since the 1950s, CIP has evolved to include fully automated systems with programmable logic controllers, multiple balance tanks, sensors, valves, heat exchangers, data acquisition and specially designed spray nozzle systems. Simple, manually operated CIP systems can still be found in use today. Higher temperature and chemical detergents are frequently applied to improve cleaning effectiveness. CIP has more recently been applied to groundwater source boreholes used for high end-uses such as natural mineral/ spring waters, food production and carbonated soft drinks (CSD).
Boreholes that are open to the atmosphere are prone to a number of chemical and microbiological problems, so sources for high end-use are often sealed at the surface (headworks). An air filter is built into the headworks to permit the borehole to inhale and exhale when the water level rises and falls quickly (usually due to the pump being turned on and off) without drawing in airborne particles or contaminants (spores, moulds, fungi, bacterium, etc.).
In addition, CIP systems can be built into the borehole headworks to permit the injection of cleaning solutions (such as sodium hypochlorite or other sanitizers) and the subsequent recirculation of the mix of these chemicals and the groundwater. This process cleans the borehole interior and equipment without any invasive maintenance being required. (http://en.wikipedia.org/wiki/Clean-in-place)
USES OF PROCESS FILTRATION IN FOOD INDUSTRIES
In the food and beverage industries, membrane filtration is an advanced technology for clarification, concentration, fractionation (separation of components), desalting and purification of a variety of beverages. It is also applied to improving the food safety of products while avoiding heat treatment. Some examples of final products using this technique are fruit and vegetable juices, like apple or carrot; cheeses, like ricotta, ice cream, butter or some fermented milks; skimmed or low-lactose dairy products; micro filtered milk; non-alcoholic beers, wines and ciders etc.
In traditional cheese making, the milk is first coagulated by addition of starter culture and rennet. The concentration takes place when the whey is drained off and the curd forms. This whey contains approximately 25% of the protein content and approximately 10% of the fat content of the milk. This means that, in traditional cheese making, only about 75% of the protein content and about 90% of the fat content of the milk is utilised (P.S. Nielsen, 1988).
Ultra filtration of milk represents the first real innovation in the history of cheese making, offering substantial advantages to both manufacturers and consumers. During the cheese making process some of the nutrients found in milk are lost in the whey (e.g. carbohydrates, soluble vitamins and minerals). These losses have a considerable impact on the economics of the processing operation. Ultra filtration is an effective means of recovering the by-products, which can be used for further food formulations. At the same time the result is cheese products of higher nutritional value at a better price. Another application in cheese is the use of microfiltration to remove undesirable micro-organisms from the milk used in the production of raw milk cheeses. (http://www.eufic.org/article/en/food-technology/food-processing/artid/membrane-filtration-food-quality/)
Micro filtered milk
Classical techniques used to improve shelf-life and safety of milk is based on heat treatments, like pasteurization and sterilization. Those techniques modify some sensory properties of milk, for example its taste. Microfiltration creates an option to heat usage to reduce the presence of bacteria and enhance the microbiological safety of dairy products whilst conserving the taste. Fresh micro filtered milk has a longer shelf life than traditionally pasteurized fresh milk. There is also a new development in membrane technology manufacture, which leads to a similar hygienic safety as "thermisation" of skimmed milk at 50Â°C. This will allow the commercialization of new milk, which can be stored at room temperature for six months and with a taste similar to fresh pasteurized milk.
Cane and beet sugar industries
The sugar industry in developed countries has been under pressure for some time due to high energy and labour costs and environmental challenges. Many technologies are being constantly explored to improve sugar yields and quality with reduced energy consumption. Membrane filtration technology offers economic and technical advantages, when used either as a standalone process or in combination with other more established technologies such as ion exchange and chromatographic separators.
Ultra filtration or micro filtration process in cane sugar production acts as a pre-treatment prior to other separation technologies by removing impurities from the raw juice including starch, dextran, gums, waxes, proteins and polysaccharides (Patel R. et al, 2010).
Starch and sugar industry
In a very short duration, cross flow membrane filtration has become a mainstream unit operation in the starch and sweetener industry. Membrane filtration processes, namely reverse osmosis, nano filtration, and microfiltration by their versatility have gained acceptance. Microfiltration of saccharification tank liquor removes unliquified starch, polysaccharides, proteinaceous matter and other impurities. The process has been successfully applied to sweeteners derived from various starch sources-corns, wheat, tapioca, potatoes or cassava. The process eliminates use of diatomaceous earth in rotary vacuum filters, while at the same time producing a superior quality product. Microfiltration is used for clarification of maltodextrins, depyrogenation of dextrose, final filtration of dextrose and fructose syrups. Reverse osmosis is used for concentration of dilute sugar streams and in some cases as a pre-concentration step prior to an evaporator (Patel R. et al, 2010).
Dewatering of fruit purees
A fruit puree is similar to a fruit juice in that it is extracted from a fruit by a mechanical process. But a puree is different from juice in that it contains fruit pulp and is thicker than juice. Puree and (more rarely) mash are general terms for cooked food, usually vegetables or legumes, that have been ground, pressed, blended, and/or sieved to the consistency of a soft creamy paste or thick liquid. Purees of specific foods are often known by specific names, e.g., mashed potatoes or apple sauce. Purees generally must be cooked, either before or after grinding, in order to improve flavour and texture, remove toxic substances, and/or reduce their water content.
Fig 2: Fruit puree
To produce frozen fruit puree natural, exotic fruits are selected, washed, properly sanitized, pulped, pasteurized and packaged. The fruits are typically picked at the optimum time for peak flavour and sugar content. The fruits are washed, pressed and filtered prior to being pasteurized, which reduces the use of the practice of thin film evaporation. Fruit juice production has been greatly simplified by the use of the ultra press process. This process utilizes formed-in-place metallic membranes on sintered stainless steel tubes for simultaneous pressing and ultra-filtration of fruit purees. The fruit purees are enzymatically depectinized and pumped directly through the metallic membrane system in a single pass to obtain highly clarified juice which can be aseptically packaged (Thomas R.L et al., 1989).
Fig 3: Simple flow diagram for fruit puree production