Picking For The Right Mix

Selecting the right equipment to do the job can have wider implications than keeping within budget or getting the level of throughput to meet demands. This is particularly so in the food industry with its unique challenges in terms of variety of ingredients, complex combination of processes and of course, the hygiene and quality issues. Here, we consider how choosing the right mixer can be a recipe for success rather than an instrument of failure when all aspects of operations are taken into consideration.

As ingredient blending can have a massive impact on both end-product quality and other downstream processes, it is essential that manufacturers make the right mixer choices. Yet, even experts agree this is no ‘piece of cake’ with the different mixers on the market, a heightened pressure to get products to market quickly and the complexity of process requirements.

Working in partnership with a mixer manufacturer and/ or a food processing specialist is a route chosen by many, including the largest food manufacturers for whom making the wrong mixing decision could be catastrophic.

With the increasing complexities of food ingredient mixing, what are the options available and opportunities for manufacturers to exceed process expectations and gain production advantages by making the right decisions when choosing an ingredient mixer?

The first step in making the right mixer choice is to have a basic understanding of the nature of the ingredients to be mixed and what you need as an end result. In order to ensure correct mixer specifications, manufacturers should be asking questions such as: Do any of the powders contain a dust explosion hazard? What degree of cleaning is desired between batches? How will material be fed into this process? How many ingredients are to be mixed together—are they all powders or are liquids being introduced too?

The complexity of mixing processes is escalating today as companies continuously strive to achieve new products, energy efficiencies, contamination risk free production and economies of volume and automated production. In order to meet these requirements, modern mixers are now often expected to not only deliver accurate mixing of multiple ingredients, but to also coat, wet and agglomerate as well, making decisions even harder.

Prior to specifying equipment, it should be determined if a batch or continuous process will be required. Traditionally, batch mixing is the most frequently used in food manufacturing. In batch mixing, the whole batch can be identified and is traceable for quality control. However, with demands for volume production, continuous mixing has the advantage.

In a batch mixer, all particles can move in every possible direction, i.e. they are set in random motion within the boundaries of the vessel. Eventually these individual particles form a uniform mixture.

In continuous mixing, the random motion is confined to a radial direction with the mixer producing almost plug flow conditions. Mixing thoroughness depends on residence time.

It is not uncommon within the food industry to incorporate both modes of mixing in a hybrid mixing station, where minor ingredients may be premixed in a batch blender and the output treated as a single component for introduction alongside a major component to a continuous mixer.

There are three common batch mixing requirements to consider for mixer selection, and each have their specific production advantages to be gained.

Choosing the right machine means all three processes can be achieved in one single machine—delivering the advantages of reduced product handling, production time savings, reduced contamination risk, capital cost economies over the purchase of three machines, plus the potential for space savings in the production area.

Typically, a conical mixer with an orbital rotating screw offers this 3-in-1 benefit. The addition of a high speed rotating paddle or intensifier enables small amounts of liquids or dry powders to be blended into the main powder bulk to create coated products. With a slight design modification and application of vacuum, this mixer can also become a vacuum dryer. Hence mixing, coating and drying can be carried out in one processing vessel.

Many food ingredients today sit firmlyin the ‘difficult to handle’ category due to their cohesive, heat sensitive or fragile natures combined with the increasing requirements for quality texture, flavour, colour and blend accuracy. For such applications, what type of mixer would be best?

Mixing sensitive ingredients provides one of the greatest threats for manufacturers to waste high value ingredients and create inconsistent product batches that, should they be released to market, can damage brand perception. It is important therefore to consider different solutions to the very specific problems presented by these types of ingredients.

For cohesive or heat sensitive ingredients that benefit from short but intensive mixing cycles, a high shear conical mixer with fast rotating paddles may be the solution. The conical design forces the material from the bottom to the top of the mixing vessel for efficient mixing with a gentle action on materials, yet without product damage resulting from mechanical or thermal stress. The benefits of a cooling jacket are also well worth considering the need to further protect heat-sensitive products.

Alternatively, for higher volumes (up to 100,000 litres), the low impact mixing action of a conical mixer is the proven and valued solution in the production of baby food. Mixing accuracy, easy product discharge with no product retention, ease of cleaning for reduced risk of contamination and degradation free production are key factors in selecting this machine for such a sensitive application.

Such a mixer may feature:

• a large manway for easy access and inspection
• sanitary screw with rounded edges
• hinged bottom section, pneumatically operated with quick release fasteners for easy access and inspection of vessel internals
• pneumatically operated ball segment valve

The cohesive and agglomeration characteristics of powders make mixing and blending difficult, and when you introduce liquids, all too often lumps causing unevenness in the mixture lead to the need for an additional de-lumping process. For such applications, a mixer with variable shear mixing can solve the problem.

With a double screw combination and a large diameter in these conical blenders, transport capacity can increase up to eight times higher than traditional conical mixers. The high rotational speeds create a fluid bed at the top of the mixer into which liquids can be introduced. The ratio of liquid, powder and air results in a homogenous powdery material containing liquids, fats or oils without any lumps. Therefore, lump breakers are not needed for breaking up lumps which would normally be created with other types of mixers when liquids are injected into powders.

A mixer such as this can therefore be used in applications such as injecting vegetable oil into flour, or for lump free mixtures in bakery products.

Homogeneity is a key quality for continuous mixing systems. To achieve this, attention needs to be paid to the accuracy and control of the feed system for the powders, solids and liquids. This is particularly so where the residence time in the blender unit is very small. Time and money spent on an accurate and reliable feed system would be well spent.

Products that would require continuous mixing would be those such as instantised products, like soups, beverages and sauces. The rise of ‘just add water’ provisions such as soups, coffee, cocoa and fortified drinks continues, as manufacturers identify more opportunities for value added instant, non-stir products and consumers access ‘food to go—in an instant’.

Many dry powder food ingredients do not have the good instantising properties required for lump free dispersion. Therefore, food manufacturers need to optimise the powder properties of their ingredients to achieve better wettability, solubility and dispersing characteristics. Agglomeration can address this by creating an open structured composite particle, rather like a raspberry which enables liquids to wet all parts of the solid effectively and quickly to give good instantising properties to the material.

A good continuous agglomeration process should be able to deliver food products with a uniform and consistent particle size range from 200 to 1,500 microns at high outputs. This would achieve excellent product uniformity due to the avoidance of demixing in the almost instantaneous blending of all ingredients. Even better would be if the machine had a mixer wall that is continuously mechanically flexed to avoid build up and can be applied to efficiently disperse small amounts of liquids or solids to a bulk carrier.

With stringent requirements for contamination free food products, it is important to consider the cleaning options available in relation to your equipment choice. Today’s equipment manufacturers incorporate ‘clean designs’ into their equipment portfolio, which incorporate highly polished surfaces and crevice-free construction to facilitate full discharge of material without residual heel. Special discharge valves and positioning of discharge outlets are also now considered standard in food industry equipment.

The majority of manufacturers choose air or water for equipment cleaning with internally fitted ‘clean-in-place’ and ‘sterilise-in-place’ systems as the desirable options. With the benefits of reduced downtime, these automated systems can be uniquely designed for specific mixer applications to ensure accurate cleaning protocols and levels are achieved.

The escalating complexity of mixer is today further compounded by the desirability for production flexibility; dedicated machines being an expensive option in an industry that is driven by changing food trends. Continuous systems are generally designed for a specific application. They are more difficult than batch systems to adapt to other purposes. Flexibility is one of the main features of the batch mixer. Batch mixers can cope with frequent formulation changes.