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Getting the perfect mix

14 February 2021

Stuart Rigby offers advice on choosing the most efficient and flexible solution for your mixing application. 

The idea of mixing is to rapidly blend two or more components which may be liquid/solid or liquid/liquid and are often difficult to combine. Depending on the ingredients and results required, certain powders, starches and hydrocolloids require different levels of mixing/shearing to fully distribute them. This is also the case for ingredients such as fruit, vegetables and meat which require a different level and type of agitation and mixing depending on how sturdy they are, and also the liquid base they are in. Choosing the right mixing equipment and paddles/agitator blades is critical, as is the speed of the addition of the ingredients.  

Aggressive or gentle mixing? A variable speed and multi-directional agitator offers a good solution for a wide range of ingredients. For a more gentle agitation, the rpm of the agitator can be reduced in a forward direction and the ingredients can be mixed and folded in without any damage, especially if curved-shaped agitator paddles are used. If more aggressive agitation is required, by increasing the rpm of the agitator and setting it to run forward and then reverse, it is possible to create a huge amount of turbulence in the vessel. It is possible to run the variable speed agitator in reverse only with large shaped paddles, allowing lumps of ingredients to be broken down by pushing them with the back of the paddles onto the side of the vessel. Ribbon blenders are also suitable for most kinds of mixing, as long as the ingredients have a strong structure.

Simultaneous heating and mixing: Some producers need the ability to pre-slurry starches, milk powders and mixes using a homogeniser/emulsifier. This provides very quick high shear mixing and distribution of powders into liquid. The disadvantages are that it can damage the wall of the starch granule which when heated, starts to swell and will burst before becoming fully hydrated. The solution to this problem is to use a combination of a multi-directional agitator and heating system such as Jet Cook which will cause no mechanical damage to the starch granules. Further, the simultaneous heating and mixing is beneficial in both the hydration and distribution of starch/hydrocolloids/powders within a mixing vessel. This also reduces two processing stages into one. This process can be used in both batch and continuous processing.

Ingredients/powders that don’t traditionally mix well: Powders such as CMC, guar and xanthum are products that typically do not mix well with waters and have to be pre-slurried prior to being added to a mix for thickening. The dispersion of powder into a liquid can be a challenging process throughout the food and drinks industry, especially with unskilled operators.

By using a non-mechanical powder entrainment system such as In-line Jet Cook, powders can be pulled by a partial vacuum into a liquid processing flow running in excess of 1,000 metres per second. The powders are instantly distributed into this high-speed processing flow which creates seven trillion droplets of water a second while also being heated, the result being a finished product without any fish eyes or agglomerations. Systems such as this can pull in excess of 500 kilos of powder in less than five minutes. The alternative to this single operation would be multi-stage process involving separate mixing and heating equipment. 

Powder entrainment: Adding powders directly into a process vessel can result in these ingredients adhering to support pillars and getting caught on the grill, agitator blades and the sides of the vessel, and are therefore not mixed in the product. When the cooking process starts, the steam from the product can gelatinise/hydrate the powders that have not been mixed into the liquid phase on parts of the vessel, and they will eventually fall into the mix as lumps.  

This is particularly a problem with hydrocolloids and milk powders and the lumps they form are not easily broken down. The solution would be to entrain the powders via a powder entrainment hopper into the liquid phase, so that no powders are put into the main processing vessel but are instead added as re-circulation slurry. A vertical conical screw blender can offer a good solution to handle mixed powder with different densities – it also controls the speed of addition which can be important in some recipes. The downside for this type of powder addition mechanism are the cleaning and maintenance requirements.

Agitation in holding vessels: This is an area that is constantly over-looked. Once ingredients are mixed, cooked and in suspension they are usually ready to be transferred to the holding/depositing vessel prior to being deposited. The difference in viscosity of the finished product, along with the volume and size of the particulates, depict the type/style of agitation required. A product such as minestrone soup has a low viscosity and a high percentage of particulates with different sizes. This can be a depositing challenge if filling into small bowls or pouches, as achieving the correct distribution of liquid and particles is critical for both nutritional declaration and customer satisfaction. An inclined agitator with large paddles or horizontal type variable speed agitator would be the perfect solution for low viscosity soup or sauces with particulates that need to be kept in suspension. 

Stuart Rigby is product & process manager at DC Norris.

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