Issues Surrounding the Handling of Dairy Food Powders

Introduction 

We are confronted with an  ever-increasing demand for convenience food products. These are foods that are designed now in helping us maintain a healthy lifestyle. One of those food types happens to be food powders, and in this article we focus especially on the handling of dairy food powders.

Dry powders hold a particular significance for their ease of use and applicability in the food industry. Dairy powders are no different. In fact, they are a good case study for highlighting the benefits the industry of dry powder manufacturing. However,  quality deterioration during processing and storage, especially dairy powders, can lower their functional performance and so reduce commercial value. In particular, dairy powders are prone to caking, stickiness, and browning during processing and storage, which adversely affects their reconstitutional properties. Similarly, variations in flow behavior of food powders due to intrinsic and extrinsic factors leads to product and energy losses and emissions to the atmosphere. These factors reduce commercial value, consumer satisfaction, and sustainability of food powder production systems. This article emphasises the importance of rheological measurements on food powders which can be effective tools to understand all those powder issues. It includes predicting caking, cohesion, and stickiness.

Studying flow properties of powders at varying relative humidity, time, and temperature of storage can help us understand how manufacturing and storage practices can be improved to minimize product losses and caking and improve flowability. The flow function coefficient obtained at different consolidation stresses using powder shear rheology can be used for the optimal design of powder hoppers and silos, while powder flow characteristics under aerated conditions are useful to design powder conveyors. Additionally, these measurements can help in finding the ideal packaging solutions and manufacturing conditions.

Overall, effective utilization of powder rheology data is not only useful in designing optimum products and processes but also helpful in predicting product performance at the consumer level. This in turn makes the whole process more sustainable by reducing energy and material losses during manufacturing and storage.

Why do we create food powders in the first place?

Food powders are dry for starters. By removing moisture, we reduce the overall volume considerably. It makes transportation from one place to another much more straightforward and cheaper, and also means the volume managed is smaller.

For the food processor, that means straightforward use in food production systems like beverage make-up, production of snacks, confectionary. Blending is easier too. The shelf-life and nutritional value of the ingredient is prolonged.

For the customers that implies food materials they can handle much more easily, they need less storage space and there is a general level of convenience in that. You just need to add water! Overall, we have added commercial value to these food materials.

Dry Powder Manufacturing And Sustainability

Sustainability is a bit of a buzz word at the moment and any lecturer or researcher seems to have to add the word to their presentation if they want anyone to sit-up and listen. I’m not going to lie because it’s a term that resonates.

Sustainability means finding a balance between environmental, economic and social factors. None more so that in food production systems. It’s about making effective use of a number of resources so that food manufacturing is economic enough to make the food we buy affordable. It’s also socially acceptable and doesn’t cause harm to us or the environment.

When you think about a food powder production system, we consider three aspects:

1. Minimising material loss during production especially those losses during drying.
2. Increasing energy efficiency because drying itself is one of the most energy inefficient processes we have.
3. Reducing product quality losses during processing, transportation and storage. That also implies loss of commercial value too.

If these issues are not addressed then you will experience increased production downtime, loss of energy out of the system and into the environment, and a higher energy intake due to non-flowability of powders. That also means losses in commercial value too.

Energy Consumption In the Manufacture of Dairy Products

Powder drying is an expensive and high energy consuming process and the drying of dairy powders is no different (Ladha-Sabur et al., 2019). The tabulated end use energy (MJ/ kg product) has been estimated in terms of fuel and electricity consumption for comparison for dairy powders.  It’s noted that milk powder is the most energy intensive when it comes to fuel use at 14.99 MJ/kg (1.23 KJ/kg for electricity use), then whey powder (9.87 KJ/Kg – fuel use but only 0.14 KJ/kg for electricity use) followed by casein and lactose production (4.12 KJ/Kg- fuel; 0.92 KJ/Kg -electricity),  cheese (3.94 KJ/Kg- fuel; 1.1 KJ/Kg – electricity). Ice cream uses a lot of electricity – 2.89 kJ/Kg but only 1.63 KJ of fuel/kg of product.

Fresh milk production only uses 1.8 KJ/kg of fuel and 0.93 KJ/Kg of electricity in its manufacture which is at the bottom end of the scale along with butter (0.65 KJ/kg; electricity, and 0.85 KJ/kg of fuel), and cream (1 KJ/Kg electricity; and 0.18 KJ/kg of fuel). Drying milk to make powders means it is roughly 7 or 8 times more energy expensive which is a huge disparity. Drying is highly energy intensive and that’s always been a known fact of life for manufacturers.

Common Dairy Powders

There are a number of common dairy powders. Skimmed milk powder is very popular. It has a high lactose content and protein but very low fat content. Its shelf-life is more than 12 months. Compare this to whole milk powder which has all the fat retained. Naturally, it is more prone to lipid oxidation even at low water activity. Its shelf-life is more like 9 months!

Whey powder is prepared directly from liquid whey following cheese making and is lactose rich. Typically, it is highly nutritious because of the variety of whey proteins in its mix. It’s very popular in sports nutrition foods. The proteins are concentrated further using ultrafiltration and then dried to make whey protein concentrate (WPC) and whey protein isolate (WPI).

When the mineral and solid components are removed using ultrafiltration and even microfiltration, we end up with milk protein concentrate which is rich in casein and whey proteins. These are both milk proteins. The permeate collected from ultrafiltration can be dried to generate a milk permeate powder which is lactose and mineral rich. This powder behaves differently from others because it is highly sticky because of the lactose content. The higher the lactose content the stickier it gets. It also depends on the type of lactose found in the powder too.

Dairy Powder Manufacturing

Dairy powders are prepared using 4 key unique processes:

1. Milk standardization
2. Vacuum evaporation… an energy intense process whose energy efficiency is improved by the addition of different stages. You can use the steam from one stage to heat subsequent stages which helps recycle energy within the system.
3. Filtration using microfiltration, ultrafiltration and nanofiltration. various concentrates are produced. The pumping costs here are usually expensive particularly as transmembrane pressure difference rise and permeate flow-rates fall away from both concentration and fouling. Milk as a liquid anyway is much more viscous that we might imagine and increasingly becomes so as it is concentrated.
4. Drying – not very efficient and one of the most wasteful of all the processes.

Minimising Mass And Energy Losses

Drying as mentioned earlier is a process that is one of the worst for energy efficiency. A lot of engineering research effort has and is still being directed to minimising energy and mass losses. Most drying is via spray drying.

The milk concentrate is warmed to 45ºC where it is mixed with very hot  air from a radiator to between 180 to 200ºC. The hot air comes into contact  with the liquid product coming from a high pressure pump that sprays it into a closed chamber using a nozzle. The nozzle is effectively an atomizer that turns the liquid into very small particles. Atomization is a very energy intensive process. In that process, the hot air enters the chamber and mixes with the  liquid droplets from the atomiser in the atomising zone. Water is transferred from the milk droplet. By the time it reaches the bottom of the spray drying chamber that powder particle is almost dry. The wet air is removed from the spray drying chamber to a cyclone where the fine particles are removed and then onto another cyclone  where even finer particles are removed. The air is removed using an exhaust fan and then to atmosphere. This air has risen in temperature to 70ºC and it is lost to the atmosphere.  If this air is containing small powder particles i.e. fines then these are also lost to the atmosphere. As a process, spray drying is just not cost effective enough because mass and low-grade heat is lost. Add onto this a certain degree of environmental pollution of lost particulates and hot air.

The ideal situation is to obtain a cold, particulate-free air exhaust. It’s then a question of heat and particle recovery. One way is to infuse the incoming cold liquid milk with this fines-laded air which both entraps these fines but also helps to warm up the incoming milk. The heat efficiency is improved in the manufacturing process by this simple step. So to improve sustainability, make the drying step more effective using renewable heat and reusing energy initially supplied during drying.

Atomization In Spray Drying of Liquid Dairy Powders

Atomisation is an energy intensive process. Air is a very poor conductor of thermal energy. because of this, the energy supplied is not properly used for getting the milk dried hence the loss of energy into the atmosphere.

To prevent fines entering the atmosphere, we also use bag houses which are connected to the air outlets from the spraying chamber. These bag houses retain very small particles. These are reused by mixing with the feed which redries them in the drying chamber.

Atomization – a very efficient way is to break up the milk droplets effectively.

Three atomizer types are available. The pressure nozzle which operates as 3.5MPa which is very high. The atomizer is one described as a filament extension atomizer. This is a new atomizer which uses extention forces to produce fine droplets – a very energy efficient system.

The pneumatic nozzle whose air pressure is 0.3 MPa with an air mass rate of 0.5 to 0.6 m3/kg and the rotary wheel.

Losses In Dairy Powder Functionality

 Any introduction of heat will produce losses in protein functionality and no more so than with dairy powders. The issue relates to simple denaturation.

Beta-lactoglobulin becomes less soluble than alpha-lactalbumin with spray drying. Alpha-lactalbumin is generally more stable than beta-lactoglobulin (Ferreira et al., 2001). It is thought to be due to the binding of calcium which improves its thermal stability (Permyakov & Berliner, 2000). The alpha-lactalbumin also aggregates with beta-lactoglobulin rather than with itself. The losses in functionality can be up to 40% for lactoglobulin. Ensuring the temperature parameters of the spray dryer i.e. inlet and outlet temperatures, are optimised will help to reduce loss of functionality.

Minimizing Product Quality Losses By Reducing Stickiness

To reduce loss of product quality means examining powder stickiness zones. (Giangrancesco et al. (2010). One of the major issues in powder technology is a tendency for stickiness. Powders will happily stick to each other and to equipment surfaces. In hopper design, there is a bottom to the powder chamber and you can see thee are stocky zones for pwder on the surfaces of the equipment. You need to identify those sticky zones and then use a device to dislodge these powders. They then do not stick to the surface for a longer time than necessary.  If they stick to that surface for a longer time  then there is a hazard of a fire explosion. We want to avoid such situations.

In a study by Fletcher et al., (2006), they looked at deposition  versus elevation within the dryer for different initial particle velocities. They plotted number of particles sticking to the spray dryer wall versus the height of the spray drier (m). There is a concentration of particles sticking more at different heights than in other parts. One of the worst sites is the inlet pipe, the middle of the main chamber and then finally at the outlet pipe. There is little if any sticking at the walls in the lower and upper regions of the spray drying chamber. There is the obvious situation of trying to prevent sticking in these three zones. There are devices that actually prevent sticking to these particular parts of the chamber and piping. These include hammers, mechanical hammers and sonic arms.

From a chemical perspective, its useful to know the different states associated with powder stickiness. Lactose is a sticky material. You can predict the change in the glass transition temperature (Centigrade) versus the water content (Roos, 2002).

If fresh lactose is present, in a liquid form or in an anhydrous form such as beta-anhydrite, then it is very sticky on the equipment surface. It is also extremely hygroscopic and will pick up water far too easy. It will take moisture from the atmosphere where it then cakes and clogs up the equipment and fill the pipes. This is to be avoided.

Roos (2002) examined this phenomenon developing a phase-state diagram for milk powder and for skim milk powder. It shows the stickability of the powder versus the temperature of operation. If the powder is produced below the line, then the powder is not that stocky but  if the moisture content rises and the temperature is above the line for the glassy states versus liquid state, then the powder is very sticky. This is a sticky zone or caking zone. We need to avoid this situation. It means getting the moisture content of the powder as low as feasible and operating in a region of low temperature.  Then it does not stick to the equipment surfaces including any storage bins such as silos and hoppers.

Flowability Issues During Manufacturing

Thee are core flow issues flow of powders in in silos and hoppers  due to stickiness. Powder is stored to a certain height in a silo.   The issues are primarily:-

1. Arching of bulk mass – If it stops flowing from the bottom which is usually the conical section, then  it starts forming an arch. This arching effect does not allow powder to move further down the silo. That in turn causes clogging of the powder.
2. Funnel flow – you may have a stagnant zone near the comical section. In both of these, and because of the stagnant zone, the powder in the middle flows fine but not flowing as effectively from the sides. This is a poor situation too!
3. rat-holing – here the stagnant zone is a little bit taller compared to the final flow. This produces a throw out channel from the top to the bottom. There will then be a big stagnant zone on the side.
4. Flooding of excess product – powder sticks to the sides and then as soon as powder flows from the top, iot goes through very freely throughout the bottom. Its like free flowing. It’s not a desirable situation either. in other words, powder is incoming, its free flowing from the bottom and theres a stagnant zone there for a long time.
5. Segregation of particles based upon the particle size of decomposition – you do not want inconsistent quality within your batch, even.

The factor which causes these stagnant zones or cohesive and lumping tendency  is due to interparticle interactions. These are caused from:-

• liquid bridging – lactose particles stick together because of a liquid bridge due to moisture due to the plasticisation caused by water .
• Van Der Waals forces – avoid by choosing appropriate relative humidity and storage temperature.
• Stickiness
• Caking

Predicting Flowability issues

Using rheological measurements to predict powder behaviour. the flowability of dairy powders is measured by assessing whether the powder is flowable or cohesive. based on their cohesion or flowable behaviour, we can choose an appropriate rheology testing method.

For storage and processing, we’ve already mentioned stickiness, caking, clumping and gravitational based lumping tendency. we want to avoid all these conditions. These tests should be close enough to the actual conditions that powder particles are undergoing during storage and transport. Finally, the physicochemical properties are noted such as particle size, powder composition (lactose content in particular), density and moisture content as these affect particle flow behaviour. The density of the powder refers to the interstitial air between the particles versus amount of powder – the level of air between the particles as well as in the particle. 

Rheological Methods

The traditional method was the Ring Shear Test where you use this shear testing device to separate equipment where you measure the force needed to fill your material

Flow Cell test – superseded the ring shear test. Air is fluidizing your powder particles in a bulk powder bag. After fluidizing you use a spindle to check the viscosity of the powder. You can do other tests as well as using this flow cell. The latest method is to use a shear cell method. It uses a cup and spindle which is attached to a rheometer. Here we applied different ‘console’  edition it stretches ad look for the shear failure point of the powder.   Then we can also use the flow cell, we can also use the valve friction test where we can get the valve friction angle which is used for the hopper design.

Depending on the cohesivity of the free flowing tendency of the powder, we will decide on the best method. It’s a toss up between a flow cell or a shear cell. It also depends on the load type of the load on the powder, whether it is related to packaging of the powder or whether it is related to holding powder in the big silo, depending on the product load we determine the type of rheological method for powder testing.

The Shear Cell

For shear cell analysis, we apply consolidation stresses and based upon those consolidation stresses do analysis to obtain the flow function coefficient.   Look at the example of the MPI85 which is milk protein isolate 85%. Here we apply a preshear stress to get the equilibriated powder conditions. Then we apply the shearing force to fail your powder.  you apply three shearing forces to get a corresponding normal stress (kPa) otherwise called a consolidation stress,  and of shear stress (kPa)at which it will fail  versus time (s). The peak point will thus give us the shear stress where it will fail based upon these three points, and based upon the preshear points we get the Mohr circle which gives the distribution  of the consolidation stress and shear stress (its also called the Mohr-Coulomb analysis which defines the yield locus) and also gives you two values:-

The Principle Consolidation Stress (σ1) and the unconfined yield strength  (σc) of the measured powder.

The ratio of these parameters is the ‘flow function coefficient’ ie. σ₁/σ_c

Flowability is quantified by the ffc. If the ffc is greater than 10 or between 4 and 10, the powder is free-flowing. If the ffc is between a ratio of 2 and 4, it is cohesive. If the ffc is between 1 and 2 it is very cohesive and if it is less than 1 then it is non-flowing.

From the flow cell perspective, you can use the pressure drop method to find out how much flow-rate of the air is needed to fluidize the powder so that it does not stick to the surfaces of the  pipelines. Similarly, you can do a Deaeration Time Analysis where you measure the air-retention capacity of the powders. You apply air to fluidize your powder and then you let the air go out and then see how much time it takes for the powder to leave the air to go out from the system. This particular test is helpful for packaging of powders.

Retention of Product Quality During Storage

Retaining product quality of a dairy powder during storage is critical. During storage, water activity and the relative humidity of the surrounding air is important as is the temperature of storage. These three   things will determine whether your powder is stable or not (Pisecky 2012).

As the water activity falls, various reactions begin to stop completely. Bacterial growth and mould growth are inhibited when the water activity is below 0.6. Below a water activity of 0.4, then the oxidation of any fat actually starts to rise. This has to be avoided. So there is a point where you have oxidation at a minimum but where stickiness, caking, lactose displacement and collapse are also at a minimum. So you need to be below that point so that your powder does not have a tendency to stick or form lumps and then cause problems with reconstitution behaviour.

In terms of particle size, you want to make sure your particle size is within the ideal particle size range, where you have very good instant aeration properties of the food powders.

Flavour Issues During Processing

Flavours are always generated during processing. In some instances this is highly desirable as in roasting potatoes or a chicken but with dairy powders, flavour changes become highly problematic. With most dairy foods, a bland and clean flavour is highly desirable because of the way it is used in subsequent product development and formulation work. Milk powder which contains lactose in particular has a slightly sweeter taste. It all stems back to the type of heating especially pasteurisation that is used in the first instance to stabilize milk before it is sold or used for further processing.

UHT is a much more heat-severe process than HTST. In the USA, HTST is a much more prevalent process where as UHT dominates in Europe. When lactose is present there is a greater likelihood of Maillard browning with proteins. Lactose itself can isomerize into other sugars which may turn into galactose and those degradation products linked to glucose. Lactose reacts with lysine to form lactulosyl-lysine-R. This latter derivative can continue to form melanoidins (Fox, 2009). 

Sustainability During Storage

Another important aspect is to avoid spoilage of the powder through poor packaging. That means preventing moisture loss or ingress coupled to minimising lipid and fat oxidation over the shelf-life of the dairy powder. Stopping oxygen damage to the dairy powder is paramount because oxygen causes so much damage. Changes in the nutritional value of the fat and protein occur. Another way to overcome this is to create packaging materials which have barriers to oxygen and moisture. Multi-layered packaging material is very useful here e.g. metalised polyesters, biodegradeable food materials with a number of  layers, some impervious to moisture or which stop moisture migration and oxygen migration into the material during the storage. So innovative packaging very important during product storage.

Summary

The possible solutions of increasing sustainability in dairy powder production systems are the following:-

1. Reducing material losses during storage, transportation and production
2. Reusing energy used during spray drying especially when it comes using some of the thermal energy lost during drying
3. Retaining quality during storage and transportation using innovative technologies. This includes retaining powder mass during drying itself by returning it to the incoming liquid milk or using a Filament Extension atomizer device.
4. Increasing the use of sustainable packaging.

References

Carter, B., Patel, H., Barbano, D. M., & Drake, M. (2018). The effect of spray drying on the difference in flavor and functional properties of liquid and dried whey proteins, milk proteins, and micellar casein concentrates. J. Dairy Sci., 101(5), pp. 3900-3909.

Ferreira, I. M. P. L. V. O., Mendes, E., & Ferreira, M. A. (2001). HPLC/UV Analysis of protein in dairy products using a hydrophobic interaction chromatographic column. Analytical Science, 17(4), pp. 499-501.

Fletcher, D. F., Guo, B., Harvie, D. J. E., Langrish, T. A. G., Nijdam, J. J., & Williams, J. (2006). What is important in the simulation of spray dryer performance and how do current CFD models perform? Applied Mathematical Modelling, 30(11), pp. 1281-1292 (Article).

Fox, P. F. (2009). Lactose: Chemistry and properties. Advanced Dairy Chemistry: Volume 3: Lactose, Water, Salts and Minor Constituents, 1-15.

Gianfrancesco, A., Turchiuli, C., Flick, D., & Dumoulin, E. (2010). CFD modeling and simulation of maltodextrin solutions spray drying to control stickiness. Food and Bioprocess Technology, 3(6), pp. 946-955 (Article)

Ladha-Sabur et al., (2019) Trends in Food Science & Technology. 86 pp. 270-280

Permyakov, E. A., & Berliner, L. J. (2000). α-lactalbumin: Structure and function. FEBS Letters, 473(3), pp. 269-274

Pisecky (2012) Handbook of Milk Powder Manufacture. GEA Process Engineering Series.

Roos, Y. H. (2002) Importance of glass transition and water activity to spray drying and stability of dairy powders. Le Lait, 82(4), 475-484.