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
To
increase homogeneity of a mixture by reducing non uniformity or gradients in
composition, properties or temperature
Control
rates and heat transfer
To
achieve dispersion
To bring physical or structural changes
To promote chemical reactions
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.
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.
HIGH
SHEAR MIXING AND EMULSIFICATION
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.
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.
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.
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.
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 :
Ø
The fill-up volume (should not be more than
50-60% of the total blender volume)
Ø
The residence time.
Ø
The rotation speed (increasing the speed above
the optimum speed causes adhesion of the powder on the walls of the mixer)
Ø
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
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.
CONSTRUCTION
Ø
It consist of a vessel having propeller with
long blades.
Ø
The clearance (distance between propeller
blades and walls of vessel) is low.
Ø
There is a closing lid that closes the vessel
after material to be mixed is added.
Ø
For introduction of material/granulating agent,
a funnel is used.
Ø
For the purpose of granulation, a chopper is
present on side wall.
WORKING
Ø
The material to be mixed is introduced in the
mixer. The centrally mounted propeller blade at the bottom of the mixer rotates
at high speed, throwing material towards the mixture bowl wall by centrifugal force.
Ø
The material is then forced upward before
dropping back down towards the centre of the mixer.
Ø
The particulate movement within the bowl tends
to mix the components quickly owing to high shear forces (arising from the high
velocity) and expansion in the bed volume that allows diffusive mixing.
Ø
After mixing, the granulating agent (water or
alcohol) is then added through the funnel.
Ø
It will produce wet mass that will go to the
side wall of mixer because of propeller.
Ø
On sides, chopper with vertical, short and
sharp blades, are present that is rotating at higher speeds than that of the
propeller and will break the wet mass to produce granules.
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.
CONSTRUCTION
Ø
It consist of conical vessel fitted at the base
with a rotating screw, which is fastened to the end of rotating arm. Accessory
equipments include, lump breaker (attached at the bottom, of the conical
chamber), temperature monitor, infrared moisture analyzer.
WORKING
Ø
The powder to be mixed and liquid granulating
agents are added through the inlet.
Ø
The screw is moving in a planetary motion and
also lifting the material to be blended from bottom to the near top, where it
cascades back into the mass, thus imparts 3 dimensional mixing.
Ø The mixer then combines
i.
Convective mixing
ii.
Shear mixing
iii.
Diffusive mixing
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.
CONSTRUCTION AND WORKING
Ø
It consist of double trough shaped stationary
bowl.
Ø
Two sigma shaped blades have very low clearance
and are connected to a fixed speed drive. Mixer is loaded from the top and
unloaded by titlting the entire bowl.
Ø
The blades rotate tangentially at different
speeds, one about twice than the other (2:1), which allows movement of powder
from sides to centers.
Ø
The materials also moves top to downwards and
gets sheared between the blades and the wall of the trough resulting cascading
action.
Ø
Perforated blades can be used to break lumps
and aggregate which create high shear forces.
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