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 flavours, a great recipe will not transform into good food unless the components are well-mixed.
Taste, texture, colour, appearance —these are all crucial parameters intimately influenced by the mixing process. Consumers expect that the food products they patronise 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 utilised 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 optimisation.
The ribbon blender is a well-proven equipment popularly used in the food and beverage industries. A ribbon blender consists of a U-shaped 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 and herbs, tea (leaves or iced tea powders) , coffee (whole or ground beans), and other beverage blends including whey protein shakes, chocolate drinks, powdered juices and energy drinks.
When dry blending food products, relatively small amounts of liquid may be added to the solids in order to coat or absorb colouring, flavouring, 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 utilises a U-shaped trough. The agitator consists of several paddles and has less surface area at the periphery than a ribbon, therefore 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 & Emulsification
High Shear Mixers (HSMs) utilise 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 solids/liquid injection manifold (SLIM) technology, a high speed powder induction system. 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 an 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 labour, 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 off s 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.
Evolutionary improvements in mixing technologies present an opportunity for food companies to periodically update processes, upgrade efficiencies, improve product consistency and strengthen research and development efforts.
It is recommended to plan a thorough testing program with a reliable and experienced equipment manufacturer before committing to a specific type of mixer system.
Confirm your mixing strategy by trying a variety of potential candidates utilising your own raw materials and simulating operating conditions as close to your actual process as possible. The rewards will be sweet and fulfilling.