Five tricky powders and tips for handling them

1.      Be gentle with infant formula!

When mixing powders, you always need to find a balance between applying force and minimizing breakage of sensitive ingredients. In the case of infant formula, about 90% of the ingredients by weight consist of milk powder or derivatives from milk. These are fragile ingredients so you need to be gentle with them. A double shaft paddle mixer is an example of a mixer that gives very little breakage at the same time as offering high flexibility to mix different recipes.

What about when conveying powder products such as infant formula, skim milk powder or whey protein concentrate around a plant? Here too, breakage of the fragile particles or segregation of a powder mix can occur if you are not careful.

Screw conveyors or vibrating conveyors are suitable for transporting powders over very short distances but over longer distances, pneumatic conveyors need to be used. These can be either based on suction using a vacuum system or pressure by blowing air into a tube to convey the product from A to B.

Here we can distinguish between moving powders in a lean or a dense phase depending on the concentration of powder in relation to air. In terms of air velocity, in the lean phase, most of the products are conveyed at air speeds of between 15 and 35 m/s depending on the specific products. The lean phase is used for 95% of powder products because the majority of powders are not sensitive to breakage during conveying.

In contrast, in the dense phase, the air velocity is between 2 and 10 m/s to create less damage or segregation of the powder product. Here are typical applications where a dense phase conveying system is required: downstream of a spray dryer, downstream of a powder mixer and upstream of a packing machine.

The choice of a dense phase can be critical to preserving the homogeneity of an infant formula after mixing. At Tetra Pak, we have real expertise on the complexities of dense phase technology. Expertise is needed because the slower the air velocity, the greater the risk of clogging the tubes. And don’t forget that inside the pipes where powders are travelling, there are bends, valves and turbulence.

2.      How to add tiny amounts to a big batch

Today’s food producers create products that cater to emerging global trends such as wellness and nutrition. The increasing demand for products with added health benefits by enriching the recipe with proteins, minerals, vitamins or probiotics is one reason why modern powder mixing has become so complex.

Sometimes a customer wants to add a very small amount of a particular ingredient to a powder mix. The dose could be a few grams (see photo) in a batch of one tonne and should be blended evenly in the whole batch. This could be trace elements added to enrich an infant formula or sports supplement based on whey protein. As an example, iron could be supplemented with 45 mg/kg or 0.45% of the total mix.

If a particular ingredient is less than 0.5% of the recipe, our recommendation might be to do a premix where the minor ingredient is mixed with a major ingredient such as the base powder or a carrier such as lactose or maltodextrin. In this way, the premix can be added to the main mix at a dosage level of 2% or more. This allows more homogeneous mixing than if the minor ingredient was dosed separately. The premixing is made in a small mixer suitable for the purpose.

3.      What makes it hard to mix 3 in 1 blends?

“3 in 1” is the term for a mixture typically containing dried milk or creamer, coffee and sugar. You pour a spoonful into a cup and then just add hot water. This convenience product can sometimes contain about 50% sugar. What’s important to know about crystal sugar is its high density of about 0.9 kg/l compared to 0.5 kg/l for milk powder. This extra weight has to be taken into account when loading a mixer. For example, with a double shaft paddle mixer of 1,000 litres capacity, the volume of mix needs to be kept below 50% of the filling capacity, otherwise the forces and torque required to move the crystal sugar can overload the motor.

Everybody in the powder handling industry knows that powder density is one of the key characteristics to take into account when mixing, conveying or using any mechanical equipment to handle powder. However, not many know that the key factor to consider is not just the density, but the friction. Friction depends not just on the density but on the shape, hardness and surface energy of the powder particles.

An interesting result happened in an experiment at our Powder Development Centre in France. A double shaft paddle mixer was loaded with 700 kg of crystal sugar and the load on the paddles was measured. When 300 kg of milk powder was added afterwards, the load actually dropped by half despite the extra weight in the mixer. The milk powder partially coated the sugar particles and thereby reduced the friction on the paddles of the mixer.  This illustrates why you have to look at the whole recipe being processed to determine the amount of load. In this way, you can maximize the amount of powder that can be loaded into a mixer for 3 in 1 blends to gain a higher production capacity.

Whenever possible, carry out mixing tests at pilot plant scale to define the most efficient mixing time and load capacity for each recipe.

4.      DHA and ARA stick around!

DHA stands for docosahexaenoic acid and ARA stands for arachidonic acid. Both are long-chain fatty acids and they are sometimes added as a supplement to infant formula.

DHA and ARA powders are well known in powder handling for their high degree of stickiness, but there are many kinds with different properties. Most types of DHA and ARA tend to attach to the sides of the mixer more than other ingredients. This can be challenging as some of these sticky supplements may be lost when the mixer is cleaned and they may not be mixed sufficiently with the rest of the infant formula.   

Our powder handling team recently developed an option for powder mixers based on air jet technology. By directing jets of air onto the sides of the mixer and applying a short blast of air at the time of discharge, powders stuck to the sides become airborne and go back into the mix. This technique is called “flushing” and can be used to reduce the build-up of sticky ingredients inside a mixer and to reduce product losses. Indeed, the new Tetra Pak® Air Jet Cleaning system for Powder has proven to be efficient at removing some kinds of DHA and ARA deposits.

5.      Getting oat flour to flow isn’t easy

Oat-based drinks are becoming ever more popular after the success of the original oat drink Oatly. The oat flour used to make these drinks can be troublesome to pour into a mixer as it is not free-flowing – far from it!

A customer making oat drinks could be tipping flour from a hopper into a high shear mixer in a powder-to-liquid process. Oat flour in particular tends to flow poorly and get stuck in the hopper. Simple ways to avoid this are to introduce a device such as a vibrating bottom or side, or else flush the system with compressed air. 

At Tetra Pak, we have detailed knowhow about hopper design and the connecting pipework. Hoppers often have to be custom-made for a particular type of powder so they have the right angle on the sides plus the right accessories such as a vibration device or air flushing system.

Advice is available

What is clear from all these examples is that a knowledge of powder characteristics is crucial to selecting the right process and equipment not only for mixing, but also for hopper design and pneumatic conveying. No matter how tricky the ingredient is, there are ways to handle it and expert advice is at hand.