Introduction
Agglomeration of food powders can be defined as a size enlargement process in which the starting material is fine particles of powder joined or bound with one another. This produces an aggregate porous structure much larger in size than the original material, such that the primary particles can still be identified and resolved (Mukherjee and Bhattacharya 2006; Jinapong et al., 2008; Hapgood and Khanmohammadi, 2009).
The reason for forming clusters, particulates, granulates etc. is to turn a powdery, non-flowing and often poorly soluble particle into a more free-flowing granule which usually has better solubility and improved wettability.
In general terms, the clusters are held together with bonds formed by adding a binder used for agglomeration. In the food industry, agglomeration is used for instant products such as coffee, powdered milk, whey proteins, granola and snacks, or cocoa that disperses and/or dissolves quickly in liquids like water or milk.
There have been many recent advances in granulation theory because of better knowledge about the physics of powder mixing and how technology plays its part. Essential to this theory is that wetting and spreading of the fluid through the powder particles is a prerequisite for good granulation.
We now look at the key processes used in agglomeration.
Wet Agglomeration
The process of wet agglomeration is one where a liquid binder is introduced into the powder formulation so that there is binding and the development of adhesion forces which occurs between the dry particles being agglomerated. An appropriate level of mixing is needed to coat the particles evenly and encourage or promote growth of the aggregate to its desired size. Drying is required to stabilize the agglomerates after they have been formed.
The key step in wet agglomeration is the first which is wetting itself. Wetting begins the adhesion process between particles. Following wetting comes nucleation. This is the part of the process where native particles are introduced to each other. Liquid bridges form and along with capillary forces bind the particles together.
Coalescence occurs as part of the growth phase. These small groups of particles come together to create larger particles until clusters are formed of the desired size.
Consolidation occurs as the agglomerates increase in density and strength through drying and through collisions with other particles. Mixing also causes some particles to break apart and erode away. This is why we find smaller particles and fines in packaging of say granola because during the process attrition undoes the work of the agglomerator.
The correct particle size is achieved through a mix of erosion and growth. Both parts of the process must be balanced and countered to achieve the right product type.
The final step is to stabilize the particulate through drying. The agglomerated particles are dried to less than 5% water content. This is cooled to be low their glass transition temperature.
The wet agglomeration is sub-divided into two main categories based on the method of agitation: mechanical mixing and pneumatic mixing.
Pneumatic Mixing Systems
Fluidized bed Agglomeration
Fluidized bed agglomeration or fluidized bed granulation as it is also known as, is one of the most suitable processes for creating clusters and agglomerates from food powders. These are specifically ones with high porosity and good mechanical resistance for handling and packaging (Turchiuli et al., 2005). The process has the benefit of using a single piece of equipment, the fluidized bed. The powders to be granulated are sprayed with a binder solution onto the fluidized bed of dry ingredients. The size of the cluster is based on the amount of binder added and the time spent by the cluster in the fluidized bed. Small powder particles are stuck together whilst moving in an atmosphere of heated air and steam. The small particles, ‘agglomerate’ into larger particles (or clusters), making them flow freely.
The other critical equipment is a pneumatic conveyor for moving the newly formed and dried clusters away from the bed.
The mechanism of agglomerate formation depends on the feed formulation and applied conditions (Schubert, 1993). Turchiuli and others (2005) stated that agglomerates, obtained in a fluidized bed agglomeration process, can be produced with different shapes, structures, and therefore end‐use properties depending on the type of binder used, its amount and the way it is introduced to the equipment. This process generally works best by spraying the binding liquid onto a fluidized bed of particles, where upon wetting, the particles are bound together by liquid bridges. Depending on the type of wetting liquid used, the liquid bridges will solidify with dry heating to form a solid bridges (Jinapong et al., 2008).
In the drying process, residual moisture in the capillaries of the cluster and on the surface evaporates away. What is left behind are hollow spaces in the cluster whilst the new structure is solidified throughout using the added and now hardening binder. The fluidized bed is a low kinetic energy device so very porous structures are generated with lots of internal capillaries. The usual size range of the agglomerate is from 100 micrometers to up to 5 millimeters.
Owing to the porous character and large surface structure, the agglomerate is moistened significantly better. The granule is much more soluble and can also be pressed more effectively into other shapes.
The main benefits are the production of homogeneous granules and it is a much gentler process. Dust formation is minimised in this process which means there is less product loss. The unit is much safer to use. The binder can also be sprayed into the particles which means a much more uniform spread.
The issues with a fluidized bed granulator/agglomerator are that a fluid bed is much more labour intensive and time consuming to set up and to clean. At least two operators are needed when using this equipment. In many cases it is difficult to achieve a measure of reproducibility.
Spray Drying
Spray drying involves spraying a liquid raw material as fine droplets into heated air which causes the droplets to dry into fine particles. To agglomerate fully, the dried particles which are collected from the dry air outlet are re-introduced at the point where the particles are partly dried and still relatively sticky. These collide and create porous agglomerates. Spray drying agglomeration produces irregularly shaped but high porous particles with excellent instant properties. It is the standard process for producing instantized coffee or milk.
Steam-Jet Agglomeration of Food Powders
A typical continuous process for partly cooking fine dry particles to generate the necessary adhesion. The process has been used for decades in the food industry.
The level of agglomeration is controlled by particle size distribution in the raw materials, gas and steam flow conditions and the adhesion forces between the particles. After the steam section the particles are exposed to warm air flowing upwards and countercurrent to the particles, which solidifies the liquid bridges formed between the particles.
Mechanical Mixing of Food Powders
The mechanical agglomerators do not rely on pneumatic systems but on plenty of mechanical energy to create the granulation needed.
Pan or disk agglomerators are not often used for food powders. These rely on the rotation of a disk or bowl to agitate the powder as it is sprayed with water. It creates particulates which have a much higher density than those formed in fluidized beds. This type of granulation is also expensive nowadays because it relies on much more labour and time than other technologies.
Drum agglomerators – rely on a drum to agitate the powder as liquid is added using a spray nozzle along the drum. It is continuous. the agglomerates are spherical because of drum rotation. The benefits are that it will agglomerate powders with a wide size distribution and has much lower energy needs that in fluidized bed agglomeration. Drum agglomerators handle very large capacities but this is of little benefit in the food industry, The disadvantage is the broad particle size distribution.
The Mixer Agglomerator: Mixer agglomerators agitate the food powder with the use of a blade inside a bowl rather like a cement mixer. The geometry of such systems varies widely. The blade can be oriented in all directions, vertically, horizontally or obliquely. The mixer too can be highly variable which will use a three-bladed version.
The High Shear Mixture Agglomerator involves dry powder mixing for about 2 to 5 minutes. The liquid binder is added and mixed in for 1 or 2 minutes. Wet massing of particulates occurs. The material is then wet sieved to remove fines and overly large particulates. The food product is dried and usually dry sieved to achieve fractions of uniform size.
Shear is variable whilst the wetting solution is sprayed over the bulk of the powder as it is mixing. The benefits are that it works with large particle size distribution, and also for good distribution of viscous liquids. Generally, short processing times are used. A smaller amount of liquid binder is needed compared to the fluidized bed agglomerator. It also allows for highly cohesive materials to be granulated and is excellent for binders including maple syrup or honey.
Equipment is straightforward and commonly applied. The agglomerates are relatively relatively dense and spherical agglomerates.
The issues are down to any increase in temperature because of mechanical heating which will degrade any ingredients which are thermally labile. Excessive addition of liquid binder causes over wetting of granules and produces lumps that may be too large and require reducing in size. In all steps of the process there is loss of materials during the various steps in processing. In some cases up to 35% can be lost because there are numerous steps involved. There are also multiple processing steps and control can also be difficult and cumbersome.
Freeze Granulation Technology
The Swedish Ceramic Institute developed this technology. A food powder is sprayed into liquid nitrogen and dried by sublimation. The main advantages are its ball shape, it produced free flowing granules with optimal homogeneity, good granule density. The granules are solid with no cavities and there is generally no material waste.
Extrusion Agglomeration of Food Powders
Initially, the powder is dry mixed to obtain an homogenous dispersion of all ingredients. The powder mix to be agglomerated is mixed with liquid, additives and dispersants. The powder is first wetted by mixing with between 3 and 20% water which promotes plasticity and adhesion forces. It is compressed as a wetted powder mixture inside the barrel using the displacement of a piston inside a mold. the mixture is then forced through die holes of a relatively small diameter. The pressure needed to compress the mixture is created by the small open surface of the die holes and friction between powder and the walls of the die holes. The product is dried and broken down to the desired particle size. It may also be cut as the agglomerated strands leave the die. Any rounding off is conducted in spheronizer before further drying.
Extruded food powders are very dense. It is used for minerals and highly dense hygroscopic foods which benefit from a reduced surface are and for those products which are subject to oxidation.
The process is not applied in the same way as other forms of extrusion which relies on dough cooking and then passing through a die.
Dry Agglomeration Of Food Powders
Dry agglomeration is a method by which the clusters or particles are formed without using a binder or indeed any liquid including water. It is almost the same type of process as a tableting machine.
the most common process is roller compaction. In this technology, powders are forced between two rolls that compresses the powders into dense sticks or sheets. The sheets or sticks are ground into granules.
The granules formed depend on the material properties of the powder. Food particles with crystalline structure deform in a plastic manner under pressure. Amorphous materials deform in a viscoelastic manner.
Roller compaction is often applied to individual ingredients rather than a finished food powder
The benefits of roller compaction are the absence of the binding solution, the ability to generate dust-free powders, the production of a consistent granule size and bulk density and also as a suitable method for moisture and heat-sensitive materials with no drying required.
The downside is that the final product is often not as soluble as it could be because it has a higher density and lower porosity that the original material There is often a lot of fines which have to be re-worked. Lots of specialized equipment is needed especially for large batch sizes.
Agglomerated food powders made by this method include sucrose.
Agglomeration Examples
Agglomerated particles disperse far easier than food powders in both hot and cold dilutions, and can carry minor components such as vitamins or flavourings. We use both ‘batch’ and ‘continuous’ agglomeration processes to make powders for products such as custard, vending ingredients, sports nutrition and artificial sweetener applications, to name but a few.
As an example, the agglomeration of custard from a basic powder into a cluster will remove the need for the consumer to ‘paste’ the product in cold milk before heating and thereby significantly reduces the creation of lumps, thus fulfilling ever home cooks dream – no more lumpy custard.
Different types of agglomerate are now possible with diffest particle shapes, structures and end-use properties. This depends in large part on the binder used, the amount and the way it is introduced into the agglomerator.
Milk powders are often agglomerated to improve their wettability and level of dissolving power. Agglomeration is usually achieved with a binder such as maltodextrin as an aqueous binder. In one example the optimum binder concentration was 10% w/v maltodextrin (Jinapong et al., 2008). This produced the largest particle size of 260 microns with good flowability and low cohesiveness. the wettability was 42 seconds but the researchers acknolwledge the dispersibility could be better.
As a test material, zein is a good model. In this study physical (size, shape) and end-use properties (wettability, friability, flowability) of zein particles agglomerated with maltodextrin introduced as a liquid solution (A) or as a powder (B) were compared. (A) type agglomerates were larger, more irregular and more fragile than (B) type ones. In case (A) the use of less concentrated binder solutions was found advantageous for the properties of agglomerates. In case (B), better agglomerates properties were obtained using binder particles smaller than the solid ones and limiting the duration of the operation by spraying limited amount of water (Turchiuli et al., 2005).
In the pharmaceutical industry, hydrophobic powders often have to be agglomerated in the proces of producing good dispersability or in tablet making. many drug molecules are hydrophobic too.
References
, . (2006). Characterization of agglomeration process as a function of moisture content using a model food powder. J Text. Stud. 37 pp. 35– 49 (Article)
(1993). Instantization of powdered food products. Int. Chem. Eng. 33 pp. 28–45.
, , , . (2005). Fluidised bed agglomeration: agglomerates shape and end‐use properties. Powder Technol. 157 pp. 168–75 (Article).
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