Mixing & Blending Tips for Processors: 10 Things you Need to Know

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Mixing & Blending Tips for

Processors10 Important Things You Need to KnowScott Jones

12/4/2012

To become proficient and cost-effective in processing mixing and blending this white paper outlines

those areas you need to focus on such as: characterization of powders, mixing of powders and liquids,

continuous mixing, energy efficiency and more.


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Characterization of powders

Particle properties may include size distribution, shape, particle density, composition and internal

structure. Bulk properties include flowability, bulk density, mixture quality, segregation tendency,

dustiness and rheology (physics of flow and change shape.) Fluctuations in raw material composition,

feed rate and process variables will also impact particle properties which can lead to off-spec. product

and equipment downtime. Particle and bulk property measurement is important for a number of

reasons. Firstly, these properties are often quality parameters, such as particle size or mixture quality.

Secondly, the measurement of key properties can describe what is happening in a process.

One of the major reasons for production formulation in powder form is simply to prolong the shelf-life

of the ingredient by reducing water content; otherwise, the ingredient will be degraded and broken

down in its natural biological environment. Another important reason is simple transport economics as

reducing the water content reduces the mass and thus cost of ingredient material to be transported.

Overall, the major function of the powder form is to maintain the stability of the ingredient functionality

until it is required for utilization, which is usually in some sort of wet formulation.

The major functionalities of food ingredients can be broadly classified as:

Physical / chemical: for example, gelation, emulsification, foaming, pH control

Nutritional: for example, vitamins, nutraceuticals

Organoleptic (perceived by a sense organ): for example, color, taste, smell, texture

Mixing of powders and liquids

Mixing of powders and liquids can be classified into 2 categories. The first category is where powder is

mixed with the liquid-based material to form a wet formulation, usually as a dispersion or solution.

These processes are of major importance as they represent the final application of most food powders.

The rate-limiting step is often the initial step of wetting the powder with the liquid. This becomes more

problematic as solid concentrations increase. in solution or dispersion. Energy requirements may

increase exponentially at higher solids concentrations, therefore mixing system designs that to minimize

the energy requirements becomes very important.

The second category is where the liquid, which contains an ingredient, wets the surface of the powder

particle and, in so doing, attaches the ingredient onto the surface of the powder particle. This is basically

a coating operation to produce a mix. It is usually done by contacting a fine spray of liquid droplets onto

the particles. In this category, obtaining a uniform "coating" of the liquid ingredient may prove

challenging especially as liquid/powder ratio decreases. Specification and position of spray nozzles is

challenging. With liquid addition, the material may become stickier and stick onto the equipment, giving

rise to crust development within the equipment which is undesirable. Agglomeration may also occur,

which may be desirable or undesirable and controlling it is another challenge.


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Some may argue that in most powder mixing processes the mixing time and the energy consumption are

not of relevant importance. Much more important are the questions "Are there any dead zones in the

mixer?" or "Is there a tendency of demixing?" This is strongly influenced by different mobilities of the

mixed components, which depend on the powder properties.

Continuous mixingBatch mixers are facing more and more competition from continuous mixing processes. Batch mixing is

often good, however the subsequent process stages of emptying, transport, storage and packing offers a

substantial chance of segregation and this may result in demixing. Agglomeration, demixing, and static

cling are processes that oppose mixing. Agglomeration prevents the spread of materials. Demixing can

occur through a number of different mechanisms. Differences in size can lead to demixing, smaller

particles sieve through larger particles. In a box of cereal, fines go to the bottom of the box in this

instance.

Continuous mixing offers advantages over batch mixing and requires considerably less space.

Continuous mixing can reduce storage requirement in silos, segregation can be limited because theproduct can be taken directly to the next processing stage, lower potential risk (e. g. explosion) caused

by a smaller hold up, and less cleaning requirement in continuous production. However, there are

downsides to the technology as well, mainly the requirement of more elaborate feeders and control

systems.

Continuous mixers can do a good mixing job, but their effectiveness depends on being able to precisely

control the feed rates of the ingredients to be mixed. The average residence time of the particles in the

mixer related to the period length of the entering mass flow fluctuation is the main influencing

parameter of the mixing quality.

Energy efficiency

Most powder operations are not particularly energy efficient. Drying is an energy intensive unit

operation in which there is always a need for strategies that can reduce energy demand. Grinding and

crushing operations commonly used in the size reduction of food materials are very energy inefficient. It

is also a well-known fact that less that 5% of the input energy in many crushing and grinding operations

is used in breaking particles; the rest is dissipated as frictional heating. Innovations and their

implementation are usually driven when energy costs increase significantly.

In another example, control of solids content or viscosity of the concentrate leaving an evaporator can

lead to energy cost reductions. If the solids content is too low, then more energy is required in drying,

and if it is the viscosity of the concentrate is too high it will disrupt the atomization process and the

drying process.

Mixer designs with shorter mixing times and reduced energy requirement are desirable to achieve a

specified mixing job.


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Industrial issues and problems

Food powders are usually considered as lower value, thus there is great difficulty in adding cost to

powders which has an inhibiting effect on innovation and problem solving in food powder production.

Extra cost cannot be added in. Many of the processes used today were designed for processors 10-50

years ago.

Sticky powders and cleaning

Sticky powders may stick to process equipment during processing. This may cause crusting. As a result,

there is a need to periodically clean the equipment. Wet cleaning may not be desirable as it may

introduce an opportunity for microbial growth; however there is an option of using wet cleaning

followed by efficient drying.

Handling and transport

Another important powder issue includes the ability to handle and transport ingredient powders. Food

powder handling and processing consists of a variety of operations including powder storage, transport,

mixing, mixing with liquids, particle size control, particle separation and coating. It also concerns theproperties of particles and powders and how this affects their bulk behavior.

The major functionalities of handling and transport processes include:

Storage (e.g. silos, IBCs, bags, cans)

Transport (e.g. feeders and conveyers)

Packaging equipment (batching, bagging)

Processors can take a number of steps to help minimize powder mixing problems. One process

guideline is to feed the mixer in an appropriate manner. Minor ingredients should not be added first to a

completely empty vessel. This precaution prevents the material from getting into the nooks and

crannies of the vessel, which are sometimes difficult to reach by mixing. Different feeds should be

layered to avoid this problem. Also mixing during the addition of materials is always a good idea (if it can

be executed) because it prevents stratification.

Processing

Food manufacturing and the demand for diversity in food products has driven a substantial market

increase for food ingredients. Most ingredients are supplied in powdered form and therefore powder

technology is an increasingly important issue both to food ingredient manufacturers and food

producers.

There arent many food powders that are directly consumed by the final consumer in powder form only.Most are incorporated as ingredients into some sort of wet formulation somewhere along the

processing chain before finally being ingested by the consumer.

There are a number of process technologies applied to food powders that provide properties such as

improved shelf-life, ingredient stability and final powder application characteristics. Some of these

processes may include particle coating and particle size control. Particle coating technology is now


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increasingly being considered by the food industry to produce a wide variety of encapsulated versions of

powdered food ingredients and additives, such as preservatives, fortifiers, flavors and spices.

Superior product quality can be achieved by manipulating the particle size through size-reduction,

agglomeration and sieving. For instance, agglomeration can improve mixture quality and reduce

segregation, reduce dust formation problems and improve ingredient sinkability in liquids. The chemicaland physical state of the components in the powder will influence the cohesive nature, stickiness and

caking characteristics of the powder, which will influence its flow characteristics. The major

functionalities of processing that give powder its properties include:

Powder production processes

o Dryingo Comminution (dividing into small parts)

Crystallization/precipitation followed by drying

o Mixingo Separationo Agglomeration/granulationo Coating/encapsulation

Health and food safetyThis safeguard is becoming more and more important in the manufacture of powdered products. Batch

mixing plants are rarely used for processing only one single product. However, frequent product

changes are a source of potential danger.

Another issue of critical importance in relation to food powders is powder safety. This involves

prevention of contamination with undesirable organisms and chemical components in the raw

materials, during production and right through to final application of the powder. These derive frompotential sources of microbial and chemical contamination of food powders or how undesirable

compositional change of the powder may occur from production to consumption. In addition, with good

segregation of product and personnel flows, there is a reduced risk of contamination.

Food powders frequently contain proteinaceous material so that allergic reaction and sensitization

needs to be eliminated or minimized. As more active agents are used, personal hazard from enzyme

action, microbial action or accidental dosage of ingredients such as antioxidants or carcinogenic agents

must be eliminated. Contamination of food ingredient and animal feed powders with pathogens,

viruses, hormones, pharmaceuticals and other undesirable chemicals is of great concern to processors.

A solution is the use of hygienically designed equipment that prevents dead-spots where material canbuild up.

Fire and explosion hazards

There is a real issue in the handling and processing of food powders because most food powders are

flammable and can produce explosions. Reducing risk of dust layer formation and consequently a

reduced risk of contamination and secondary explosion is important.


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The explosive characteristics of the raw materials are often unknown to the operator; they are in fact

not always available to the manufactures of the raw materials. Batch mixing plants in particular may

process hundreds of different raw materials and accurate analysis with knowledge of the characteristic

data is urgently required for safe design.

Outside ignition sources (foreign bodies, metals) must not be introduced into the mixing chamber.Suitable organizational procedures are required here. Safety equipment such as control screens, metal

separation or metal detection are installed in the material feed.

Most solid processes in the food industry are subject to fire hazard safeguards and governed by the

standards and best practices of agencies such as the National Fire Protection Association (NFPA) or the

European Explosives Directive (ATEX.) Specific safety safeguards in processing include:

Standards for combustible dust hazard process safety

Dust explosion fundamentals: ignition criteria and pressure development

Hazard assessment and mitigation technologies

Advances in explosion protection technology

Explosion protectionquantifying the effectiveness of explosion protection measure.

Best practice guidance for Identifying and mitigating hazards

Explosion-proof agitator motors and variable frequency drive are one example of dust explosion

prevention measures.

Mixing & Blending Tips for Processors: 10 Things you Need to Know

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