Powder handling expertise is central to the success of food manufacturers who produce a truly diverse range of products. From powdered drinks and soups to granulated and tableted sauce and recipe mixes, the food processing industry deals with ingredients that vary widely in terms of their composition and properties.
Ensuring that powders flow efficiently is crucial whether the aim is to deliver a free-flowing sachet of hot chocolate, achieve high performance in a spice blending facility, or produce sauce granules of reliable consistency.
The moisture level in a powdered foodstuff is widely recognised as having an important impact on behaviour. In some unit operations, water is used intentionally, to promote vital processes such as granulation, for example, but in many instances, the uncontrolled uptake of water compromises the value and performance of an ingredient.
Even relatively low moisture levels can cause caking and/or rapidly transform a free-flowing material in to one that is much more difficult to handle, resulting in significant processing problems. On the other hand, there are mechanisms by which water can improve the flow properties of powders, by lubricating inter-particle movement, for example, or by increasing conductivity thereby discharging the electrostatic charge within a sample.
Problems due to moisture can often be prevented or solved through the application of a drying step or by controlling storage conditions. However, such strategies are often energy-intensive and/or costly, making it essential to apply sufficient, but not excessive control. Understanding the extent to which a powder takes up moisture when exposed to humidity and, more importantly, how this moisture alters processing characteristics, supports this goal.
When it comes to choosing powder testing techniques for process-related investigations, it is imperative to apply methods that are not only reliable and reproducible but which also generate data that correlate with performance in the operating environment.
Powders are multiphase systems, made up of particles, typically unquantified amounts of air, and in many instances low levels of liquid too. As a result, many different parameters influence powder behaviour, from the size and shape of the particles present, to the consolidation state of the overall sample.
From a practical perspective, this complexity makes it infeasible to reliably predict powder behaviour using mathematical models, and processors therefore rely heavily on powder testing methods. A variety of techniques are routinely applied, many of which seek to describe the intricacies of powder behaviour with just a single number. Increasingly though, there is recognition that a multifaceted analytical approach, based on measurement of a diverse range of parameters, is far more productive.
Traditional testing strategies include those based on bulk property measurement, most specifically bulk density, and shear testing. The application of automated modern technology has enhanced the reproducibility of these techniques and so they retain an important place in today’s multifaceted powder testing toolkit.
Bulk properties such as density, permeability and compressibility provide valuable insight into powder behaviour, while shear properties are particularly useful for understanding the behaviour of powders under moderate to high stress, and for hopper design. However, modern techniques such as dynamic powder measurement have steadily risen to prominence over recent years by demonstrating proven applicability, and these bring new capabilities.
A powder rheometer, for instance, measures the torque and axial force acting on a rotating blade as it moves through a sample of powder to generate values of flow energy. This sensitive technique can be applied to powders that are consolidated, conditioned, aerated, and even fluidised, to directly assess the impact of air and closely simulate the operating environment. In combination with bulk and shear parameters, dynamic powder properties provide powerful insight into powder.
THE MOISTURE IN A POWDERED FOODSTUFF IS WIDELY AS HAVING AN IMPORTANT IMPACT ON BEHAVIOUR
Impact Of Humidity
A study was conducted to investigate the effect of humidity on two excipients widely used by food processors: microcrystalline cellulose (MCC) and lactose.
In the first step, the powders were allowed to equilibrate in environments of varying relative humidity to assess the levels of moisture adsorbed/absorbed. The results indicate that MCC takes up an order of magnitude (four to nine percent) more water than the hydrophobic lactose (0.3 to 0.7 percent) under conditions of equivalent humidity.
These results are interesting in their own right, but the more important question for processors is: How do these differences in moisture uptake alter powder behaviour? To answer this question, the dynamic, bulk and shear properties of the two powders were measured using a powder rheometer.
Lactose & Moisture
In the tests carried out with lactose, dynamic and bulk measurements provided the most interesting insight into behaviour. Shear stress, in contrast remained relatively constant with moisture content, providing little differentiation between the samples.
Turning first to dynamic data, as moisture content increases the basic flowability energy (BFE) of the lactose falls suggesting that with this material, the presence of water may lubricate inter-particle interactions.
BFE is a dynamic parameter measured as the rheometer blade passes downwards through the powder sample, exerting a compacting motion. It therefore tends to reflect how easily the powder will flow under forcing conditions, when extruded, for example, or forced into a semi-filled die.
Specific energy (SE), in contrast, is measured as the blade completes an upward traverse which applies a gentle, lifting action. The results therefore tend to correlate more closely with unconfined flow behaviour, how the powder will flow from an open vessel for example.
Here, the trend in SE data is contrary to that of the BFE data: SE increases with increasing moisture content. This is an interesting behaviour and highlights an important, industrially relevant issue, which is that powder flow behaviour is strongly influenced by the processing environment.
The presence of moisture is highly likely to produce liquid bridges in the lactose that would tend to increase the adhesivity of the system. This would rationalise the observed trend in SE. However, the BFE data suggest that under forcing conditions this effect is more than offset by a competing lubricating mechanism that makes inter-particle movement easier. Under compacting conditions the net impact of the moisture is therefore beneficial.
Aerated energy (AE) values are measured using the same methodology as for BFE, but with air flowing upwards through the sample at a controlled velocity. The AE values for lactose exhibit a similar trend to the BFE data suggesting that in this environment, too, increasing moisture content reduces cohesivity within the sample.
Evaluating bulk properties, the permeability data for lactose is perhaps most revealing. The steady increase in pressure drop observed during testing indicates that the powder becomes less permeable to air as moisture content increases. This supports the view that liquid bridges form within the system that inhibit the passage of air. In contrast, both compressibility and bulk density change very little as a function of moisture content.
The insensitivity of bulk density to moisture content, with changes of only two to three percent across the range of experimental conditions studies assessed, is particularly noteworthy because it suggests that in this instance bulk density/packing changes are not responsible for the observed trends in flow characteristics (as quantified by the dynamic test data). For this powder, testing methods based on bulk density might therefore fail to detect the process relevant changes in behaviour induced by moisture.
NOT ALL MOISTURE IS BAD WHEN IT COMES TO POWDER FLOW BEHAVIOUR
MCC's Effect On The Mix
As with the lactose, the most insightful data for MCC is are the dynamic and bulk property measurements with shear analysis once more providing little differentiation.
The basic flowability energy (BFE) and AE curves for MCC, although quite different, echo one another in terms of exhibiting a minimum flow energy. Both, therefore, indicate that moisture improves the flow properties of MCC up to a certain point, beyond which flowability reduces.
An additional observation made during these experiments was that desiccated MCC in particular, had a tendency to coat the inner wall of the storage vessel prior to testing, suggesting a tendency towards electrostatic charging. This provides vital insight in to why the powder might display the flowability characteristics it does. If the high BFE value for drier samples arises from electrostatic interaction between the particles then increasing moisture level may reduce BFE by discharging the sample.
The steady increase of BFE above a certain level of moisture is a more commonly observed pattern, although contrary to the effect observed with the lactose. It is attributable to the material adsorbing sufficient moisture to begin to agglomerate due to increased adhesion and capillary forces between particles.
Large particles, or agglomerates, can present significant resistance to the kind of compacting flow pattern applied in BFE testing, and therefore are often associated with high BFE values when compared with finer, more cohesive powders whose structures contain more void spaces.
During aeration testing, the air separates particles, in general causing a reduction in flow energy. Here though there are two competing mechanisms: discharge of the sample and agglomeration.
With an aerated sample the effect of electrostatics tends to be relatively small because of the separation caused by the air, while agglomeration can have a marked impact, leading as it does to the formation of agglomerates with higher mass, larger size and increased adhesive forces. In this case, the agglomeration mechanism dominates the aerated system and AE values rise relatively rapidly as moisture content increases.
This formation of agglomerates results in large void spaces within the powder bed, a trend reflected in the steady increase in the MCC permeability data. Beds with large particles and substantial voidage, although difficult to fluidise, present relatively low resistance to air flow and therefore tend to be highly permeable.
The compressibility of the MCC, on the other hand, and indeed bulk density, change very little as moisture content increases, suggesting that, as with the lactose, packing behaviour is not an important factor with respect to the changes induced by humidity.
One important note to make about the MCC is that it exhibits these quite dramatic changes in behaviour over conditions that are industrially relevant, across a 25-50 percent relative humidity range that could easily represent the ambient environment. This suggests that MCC could readily exhibit variable, not easily predicted behaviour within an industrial setting.
The study shows how powders can exhibit a very different response to moisture. Some ingredients, such as the MCC, readily take-up water when exposed to conditions of high humidity, while others, as exemplified by the hydrophobic lactose, do not. However, even a small increase in moisture content can be sufficient to substantially change powder properties and influence processing performance, as the data for lactose illustrate.
The results dispel the view that all moisture is bad when it comes to powder flow behaviour with both MCC and lactose showing improved flow properties with the inclusion of certain levels of moisture. More importantly, however, they demonstrate the values of using a multifaceted approach to reliable quantify the impact of humidity.
In this work, dynamic and bulk properties provided far more sensitive differentiation than shear analysis and, in combination, generated a secure basis for operational and design decisions relating to the control of moisture within the processing environment.
[This article originally appeared in the January 2013 issue of APFI.]