MODERN "MIXING MACHINES" USED IN FOOD INDUSTRY
MODERN "MIXING MACHINES" USED IN FOOD INDUSTRY
Mixing or blending is an unit operation in which a uniform mixture is obtained from two or more components, by dispersing one within the other.
Like emulsion, the larger components are sometimes called continuous phase and the smaller components are called dispersed phase in mixing also Mixing has no specific preservative effect and is intended solely as a processing aid or to alter the eating quality of foods. It has wide applications in many food industries where it is used to combine ingredients to achieve different functional properties or sensory characteristics for example to obtain sensory characteristics in doughs and control of sugar crystallization in ice creams etc.
In some foods adequate mixing is necessary to ensure that the proportion of each component complies with the legislative standards.Extruders and some type of size reduction equipments also have a mixing action.
OBJECTIVES OF MIXING
Ideal mix –?When one particle lay adjacent to a particle of the other component (each particle lies as closely as possible in contact with a particle or the other component)
Random mix –?A mix where probability of selecting a particular type of particle is the same at all positions of the mix, and is equal to the proportion of such particles in the total mix.
Segregation or de-mixing –?A mix where particles differ in size, shape or density. These differences make this particles behave differently when forced to move and hence tends to separate.?
At the heart of transforming raw ingredients into food for human consumption is the mixing operation. One of its main tasks, which other food processing steps also share, is to establish consistency. Whether a food product requires small-scale mixing by hand or high volume blending of multiple ingredients, at-home cooks and process engineers alike know the importance of proper mixing. Even with the right amount of ingredients and flavors, a great recipe will not transform into good food unless the components are well-mixed. Taste, texture, color, appearance – these are all crucial parameters intimately influenced by the mixing process. Consumers expect that the food products they patronize will be exactly the same as the one they had last. It is easy to understand that within the food industry a high level of consistency is required not just batch-to-batch but facility-to-facility. In this market, consistency is the backbone of consumer loyalty.
Various types and styles of mixing equipment are utilized within the food industry. Their use and application are determined by the phases being mixed (liquid-liquid, solid-liquid, or solid-solid) as well as physical characteristics of the end product (like viscosity and density). In reality, many mixing technologies overlap in use and function such that certain applications can actually be successfully produced by two or more types of mixing systems. In these situations, economics rule out the more costly initial investments, but differences in efficiencies must also be taken into account. Proper mixer selection is vital to process optimization.
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RIBBON BLENDER (DRY BLENDING)
The?Ribbon Blender?is well-proven equipment popularly used in the food and beverage industries.
A ribbon blender consists of a Ushaped horizontal trough and an agitator made up of inner and outer helical ribbons that are pitched to move material axially in opposite directions, as well as radially. The ribbons rotate up to tip speeds of approximately?300 ft/min. This blender design is very efficient and cost effective for mixing dry applications such as cake and muffin mixes, flour, bread improvers, cereals, trail mixes, snack bars, spices & herbs, tea (leaves or iced tea powders), coffee (whole or ground beans), and other beverage blends including whey protein shakes, chocolate drinks, powdered juices, energy drinks, etc.
When dry blending food products, relatively small amounts of liquid may be added to the solids in order to coat or absorb coloring, flavoring, oils or other additive solutions. Liquid ingredients can be added through a charge port on the cover but for critical applications, liquid addition is best accomplished through the use of spray nozzles installed in a spray bar located just above the ribbon agitator. Liquid flowrate, as well as blender speed, are fine-tuned during liquid addition to avoid flooding or formation of wet clumps of powder.
Although dry blending is its more popular function, the ribbon blender is also used in the preparation of flowable slurries or pastes, say in food extrusion operations. Food extrusion is a processing technology employed for a wide variety of end products, from pasta to ready-to-eat cereals, from snack chips to pet food. The function of the ribbon blender in the extrusion process is to homogeneously mix two or more grains, flours, oil, sugar, emulsifiers, extrusion aides and other powders. Once the constituents are blended, water is usually added to the batch in order to raise the existing moisture content to the proper level for extrusion.
For blends that require a gentler mixing action, the?Paddle Blender, Vertical Blender or Tumble Blender?are considered by food manufacturers.
A?horizontal paddle?blender also utilizes a?U-shaped?trough. The agitator consists of several paddles and has less surface area at the periphery than a ribbon, thus providing lower shear and less heat development.
In comparison, the blending action of a vertical blender’s slow turning auger is far gentler than that of any horizontal blender. The auger screw orbits a conical vessel wall while it turns and gently lifts material upward. As materials reach the upper most level of the batch, they cascade slowly back down in regions opposite the moving auger screw.
The tumble blender is a rotating device that commonly comes in double-cone or V-shaped configurations. Asymmetric vessels designed to reduce blend times and improve uniformity are also available. Generally, tumble blenders operate at a speed of?5 to 25 revolutions per minute. Materials cascade and intermix as the vessel rotates. Mixing is very low-impact.
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HIGH SHEAR MIXING AND EMULSIFICATION
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High Shear Mixers (HSM’s)?utilize a rotor/stator assembly which generates intense shear necessary to puree solid ingredients in the preparation of dressings, sauces and pastes. This type of device is also used in the food industry for the production of syrup solutions, beverage emulsions and dispersions.
Available in batch (vertical) or inline (horizontal) configurations, high shear mixers are comprised of a rotor that turns at high speed within a stationary stator. As the rotating blades pass each opening in the stator, they mechanically shear particles and droplets, and expel material at high velocity into the surrounding mix, creating hydraulic shear. As fast as material is expelled, more is drawn into the rotor/stator generator, which promotes continuous flow and fast mixing.
?A major development in HSM design is the?SLIM (Solids/Liquid Injection Manifold) Technology, a high speed powder induction system available on Ross High Shear Mixers. The modified rotor/stator assembly is specially designed to create negative pressure (vacuum) behind the rotor, which can be used as the motive force to suck powdered (or liquid) ingredients directly into the high shear zone.
The?SLIM?is particularly useful in inducting hard-to-disperse thickening agents such as CMC, xanthan gum, gum Arabic, guar, carrageenan and alginates into a liquid phase. These powders are notorious for driving up processing costs. Even with a strong vortex in an open vessel, they resist wetting out and often float on the surface for hours. Using the SLIM, solids are combined with the liquid stream and instantly subjected to intense shear. In other words, solids and liquid meet at precisely the point where turbulent mixing takes place. When solids and liquids are combined and mixed simultaneously, agglomerates are prevented from forming because dispersion is virtually instantaneous.
The inline configuration of the SLIM is a great improvement in design compared to earlier venturi or eductor systems. In these systems, the process liquid is pumped at high velocity into a venturi chamber and passes into the inline mixer. The combination of the pump, venturi and the pumping action of the mixer creates a vacuum in the venturi chamber. Powder fed through an overhead hopper is drawn by this vacuum into the eductor where it joins the liquid flow. A rotor/stator then mixes the powder and liquid, and propels the flow downstream.
While this set-up eliminates the dusting and floating issues of batch systems, it also presents serious limitations. With three separate devices in series, maintenance – in terms of?labor, required expertise and spare parts?— is intensive. Balancing the performance of the pump, eductor and mixer is often difficult, and in many applications, downtime is quite high. But the most serious limitation relates to the inherent operating limitations of the venturi or eductor. Clogging is routine. The system is temperamental and requires a lot of operator experience and attention to operate successfully. Since the feed rate of the eductor relies on the vacuum created by a fast-moving stream, it is also extremely viscosity-dependent. As the viscosity of the stream rises, velocity falls and the efficiency of the eductor drops offs steadily until it finally stops.
The SLIM design is a breakthrough based on one simple idea —?eliminate the eductor.
In the older powder induction designs, solids are combined with the moving liquid stream in the eductor, and then mixed farther down the line. That distance between the eductor and the mixer is critical. Material that had been combined but not yet mixed intimately could clog the pathway before reaching the rotor/stator mixer where agglomerates could be disintegrated and small particles are forced into a dispersion that could flow quickly without problems. In addition, clumps produced in the venturi chamber could solvate to form a tough outer layer which prevents complete wetting of the interior particles. While product can be recirculated several times to improve initial dispersion, the high shear conditions usually needed to break up tough agglomerates can also overshear already hydrated particles resulting in a permanent viscosity loss.
Food companies are not only faced with the challenge of dispersing gums, thickeners and other “difficult” ingredients into a liquid stream. Another common and critical requirement is the need to reach a high level of solids loading in the final batch. Because the SLIM system combines and mixes solids and liquids simultaneously, it is able to operate at extremely high feed rates without choking.
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ULTRA-HIGH SHEAR MIXING (CONTINUOUS PROCESS)
For applications that still fall short of the desired particle size distribution even at the maximum speed setting of the rotor/stator mixer, a move to a higher energy?HSM design?is recommended.
The X-Series head?consists of concentric rows of intermeshing teeth. The product enters at the center of the stator and moves outward through radial channels in the rotor/stator teeth. Tolerances are extremely close and the rotor runs at very high tip speeds typically up to?11,300 ft/min. This combination subjects the product to intense shear in every pass. The gap between adjacent surfaces of the rotor and stator can be set as close as?0.003”?and is adjustable for fine-tuning shear levels and flow rates.
The QuadSlot mixing?head is a multi-stage rotor/stator with a fixed clearance. This generator produces higher pumping rates and requires higher horsepower compared to an X-Series rotor/stator set running at similar speeds.
The MegaShear head?operates at the same tip speed as the X-Series and QuadSlot heads, but is even more aggressive in terms of shear and throughput levels. It consists of parallel semi-cylindrical grooves in the rotor and stator towards which product is forced by high velocity pumping vanes. Different streams are induced within the grooves and collide at high frequency before exiting the mix chamber.
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DUAL-SHAFT AND TRIPLE SHAFT MIXERS (HIGH VISCOSITY BATCH MIXING)
Dual-Shaft and Triple Shaft Mixers?are used in the food industry for batching medium to high viscosity applications such as candy syrups, beverages, nutraceuticals, sauces, pastes, peanut butter, and other spreads.
This type of mixing system is comprised of two or more independently-driven agitators working in tandem. A low speed anchor compliments one or two stationary high shear devices, such as an open disc-style disperser blade or a high shear mixer rotor/stator assembly. On its own, a disperser blade will produce acceptable flow patterns in batches up to?around 50,000cP; the rotor/stator up to?around 10,000cP. Hence, for higher viscosities, there is a need for a supplemental agitator to improve bulk flow, deliver material to the high speed devices and constantly remove product from the vessel walls for better heat transfer.
The most common low speed agitator designs are the two-wing and three-wing anchor. For added efficiency, especially in terms of axial flow, a three-wing anchor can be modified to feature helical flights in between wings. In combination, high shear devices and an anchor will process products that are several hundred thousand centipoise.
One user of a Dual-Shaft Mixer produces fortified peanut butter. The process starts with shelled and roasted peanuts. Vegetable oil is added to the whole peanuts and the two ingredients are creamed together in the Dual-Shaft Mixer equipped with a two-wing anchor and a high speed disperser blade. Powdered milk, vitamin and mineral mix and sugar are added through a charge port and mixed into the paste.
The fortified peanut butter is then passed through an inline rotor/stator mixer to reduce the size of the peanuts and granular sugar, producing a very smooth paste. This method of mixing not only ensures homogeneity but also prevents separation during storage. During discharge, the finished product is pumped by the inline mixer into large plastic drums and packed into plastic bottles for distribution.
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DOUBLE PLANETARY MIXING
As product viscosity continues to build up, a multi-agitator mixing system will eventually fail to produce adequate flow as can be characterized by an anchor simply carving a path through the batch (instead of moving product from the walls and into the center) or by high-temperature zones right near the disperser and rotor/stator assemblies. At this point, agitators with a fixed axis of rotation will no longer suffice and a move to a planetary mixer is recommended. The agitators of a planetary mixer rotate and travel through the mix vessel, passing through every point within the batch, not just along the periphery. Highly viscous materials must literally be carried from the vessel wall to the batch interior.
While single planetary mixers are commonly utilized for processing viscous food applications such as dough, their use has certain limitations when it comes to products that are too sticky, too heavy, or those that tend to climb up the stirrer. In these cases, the double planetary is a more practical choice.
It can be equipped with traditional rectangular stirrer blades, finger blades or High Viscosity “HV” blades. The latter is a patented blade design which generates a down-thrust mixing action owing to its precisely angled helical contour. This sweeping curve firmly pushes the batch material forward and downward, a unique mixing mechanism that solves the ‘climbing’ problem commonly experienced with highly filled materials. In addition, the HV blades do not have a lower crossbar so they can be cleanly lifted out a very viscous batch and can pierce right through it just as easily.
TUMBLING MIXER
In this type of mixer, the movement of whole mixer is responsible for mixing action of solid.?
Tumbling mixer usually consist of a metallic vessel which rotates on its horizontal axis at optimum speed by means of motor. The mixing vessel is usually made up of stainless steel and have door where we can load and unload materials. The door is lined with rubber which provides a perfect seal after closure.?
The degree of mixing/blending achieved by using tumbling mixer in carrying out a mixing operation is dependent on :
??The fill-up volume (should not be more than 50-60% of the total blender volume)
??The residence time.
??The rotation speed (increasing the speed above the optimum speed causes adhesion of the powder on the walls of the mixer)
??Inclination angle of the mixer.
Tumbling mixers are available in variety of shapes and sizes, which include:
Twin shell or V shaped mixer?–?
The V-blender, consists of two hollow cylindrical vessels that are joined at an angle of 750 to 900 which is mounted on trunnion to allow it to tumble. The free fall of the material within the vessel, and the repetitive converging and diverging motion, coupled with increased frictional contact between the material and the vessel’s long, straight sides as the mixer tumbles, split the material and recombines them continuously results in a homogenized blend. Removal of the blended material from the V-blender is normally through the apex port which is fitted with a discharge tube. A V- blender can be modified by providing it with a high-speed intensifier bar also known as lump breaker running through trunnion into the vessel along with spray pipes for liquid addition. This modified v-blender is called V-Blender with intensifier bar.
DOUBLE CONE MIXER
The double cone blender is an efficient and multipurpose tumbler blender for mixing dry powders and granules homogeneously. It is made of two conical shaped stainless vessels (available in different capacity ranging from 5 – 200 kg or even more) that are separated by a cylindrical section. It is mounted at the center of the container between two trunnions that allow the blender to turn end over end.
Double cone blender has no dead spots mixing and it is easily cleanly after use. Double cone blender is not a suitable choice of blender for very fine particles and particles with greater particle size difference due to less shear.
HIGH SHEAR MIXER-GRANULATOR?
It is so called because mixing mainly occurs by shear mixing mechanism and at the same time granulation is carried out.
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NAUTA MIXER
?It is a vertical screw mixer. Originally designed as a powder and semi-solid mixer but now a days also used as a mixer granulator.
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SIGMA-BLADE MIXER
It is used for foods with semi-solid or plastic consistency. The intermeshing of sigma blades creates high shear and kneading action which mixes the components.
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