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By Dr. Katrina Brockbank and Jamie Clayton
|Figure 1: The contrasting effect of moisture on the basic flowability energy (BFE) of flour and MCC can be attributed to the relative impact of different mechanisms that define flow behavior in either case.|
For many dry foodstuffs the ingress of even small quantities of water can compromise product performance. Granular foods — such as instant coffee, sugars and gravy granules — can all change, with the uptake of water, from relatively free-flowing, easily manageable powders to a far less desirable clumped or even solid mass.
Likewise, in manufacture, inadequately controlled moisture levels can compromise process-stream flow characteristics and other properties, leading to blockages, stoppages and reduced efficiency.
Knowing when — and to what extent — to control moisture depends in the first instance on understanding how a powder responds to humidity. Recently completed experimental work addresses the question and includes data for mannitol, sorbitol, flour and microcrystalline cellulose (MCC) that illustrate the breadth of behavior exhibited.
Results also show how dynamic, shear and bulk-powder characterization, in combination, provide the understanding needed to effectively manage humidity’s potential impact.
Humidity and the resultant uptake of moisture can have a transformative, often unpredictable, effect on powder behavior and processing efficiency because of its influence on a broad range of behaviors, including flowability and compressibility. In particular, storing powders at unsuitable humidity levels is the primary cause of caking, adverse not only to processing, but also end-use performance and product value.
Food-powder processors must understand the extent to which a powder will take up moisture and, crucially, how this affects processing characteristics in various production environments. Managers are then in a position to assess what, if any, moisture control strategies are required.
Focus on powder flow
|Figure 2: Using basic principles of dynamic powder testing, the torque and force acting on a blade rotating through a powder are measured and used to derive dynamic powder properties.|
Powder-flow properties are some of the most important when it comes to processing behavior, influencing movement but also defining performance in important unit operations such as blending, die-filling and fluidization. Powder flowability depends on many variables. Multiple mechanisms contribute to overall behavior. That’s why predicting flow performance, and by extension humidity’s impact, is challenging. Most important — and this might seem counterintuitive — these diverse effects mean that certain moisture levels may in practice be beneficial to powder processing, a finding often overlooked.
Powder behavior, including its unique flowability characteristics, is defined by parameters relating to both “environmental” variables, such as humidity and air content, and the constituent particle’s physical attributes. Flow depends on how easily particles can move relative to one another. Flow is affected by mechanisms such as: friction; mechanical interlocking; electrostatic interaction; and cohesion. The relative magnitude and impact of these competing mechanisms is specific to each individual powder, leading to widely diverse behaviors.
To illustrate this point consider flour (see figure 1). Tests show that as moisture content increases there is an initial increase in Basic Flowability Energy (BFE), a dynamic property that quantifies the ease with which a powder will move, as might be expected. This can be attributed to liquid bridging, a mechanism often responsible for moisture’s detrimental impact on flow behavior. When a liquid bridges the gap between particles it increases the adhesive forces between them and inhibits particle motion. However, beyond a certain concentration, BFE starts to decrease. Moisture lubricates the particles, reducing inter-particulate forces and increasing flowability.
Conversely, powders such as microcrystalline cellulose (MCC), a food additive widely used as a thickener, exhibit electrostatic charge between particles at low humidity. For MCC, increasing moisture levels initially decreases BFE, as moisture has an earthing effect on the charge, grounding the sample and reducing inter-particulate forces. Here, liquid bridging begins to dominate at a higher humidity percentage, above 50% RH, at which point BFE begins to increase once more.
This simple example illustrates how moisture affects just one of the dynamic powder properties used to quantify flow behavior. However, water’s effect cannot be described using a single value. Instead, a holistic approach generates a range of properties directly related to process performance. Multi-faceted powder characterization delivers comprehensive insight into powder behavior.
Multi-faceted powder characterization
Traditionally, powder characterization relies on bulk-property measurements for information on density, permeability and compressibility and shear testing, which defines behavior as moderate to high shear. Shear analysis was developed in the 1960s to support hopper-design methodology and is useful for characterizing consolidated powders and their transition from a static state into a dynamic regime. More recently, however, powder processors are turning to dynamic powder measurement for further insight into powder behavior, most especially how powders flow and perform in low-stress and aerated environments.
|Figure 3: Percentage change in Basic Flowability Energy with respect to humidity highlights distinct differences in the flow characteristics of mannitol and sorbitol.|
Dynamic characterization measures the torque and axial force acting on a rotating blade as it passes through a powder sample. This generates flow-energy values that directly correlate with in-process powder behavior. With dynamic measurements it is possible to simulate and directly test the effects of various operating environments by, for example, measuring the properties of the powder in the consolidated, conditioned, aerated or even fluidized state.
Such testing produces dynamic parameters such as BFE, Specific Energy (SE) and Aerated Energy (AE) that directly correlate to in-process behavior to support a transition from traditional “trial and error” operation to a more knowledge-led approach. Measuring dynamic characteristics allows precise and reliable quantification of powder properties and provides data that can be used to increase efficiency.
BFE is the baseline dynamic measure and indicates as to how the powder will flow under forcing conditions, such as in feeders or extruders. It is measured as the instrument blade rotates downwards through the conditioned sample, forcing the powder against the base of the test vessel. Conversely, SE is measured as the blade traverses upward through an unconfined sample and indicates how powder flows in the absence of applied stress. Such may occur on the process line when, for example, a powder is being poured from one location to another.
AE methodology is identical to that for BFE except that measurements are made as air is pumped up through the sample at a controlled velocity. This provides a direct measure of the powder’s response to air, extremely valuable for fluidization and pneumatic conveying applications. More generally it also yields one of the most sensitive measures of the strength of cohesive bonds between particles.
|Figure 4: Measurements of permeability, percentage change in pressure drop, as a function of humidity show that mannitol undergoes a significant change in permeability while sorbitol remains relatively unaffected by the presence of moisture.|
The Freeman Technology FT4 Powder Rheometer is a universal powder tester that measures dynamic, bulk and shear powder properties. This enables multi-faceted testing and gives the user the ability to systematically test, for example, the effect of changing humidity levels on relevant powder properties. How all three sets of properties vary as a function of moisture uptake provides important insight into what humidity level is best suited to all elements of the production process and the necessity or otherwise of control. Such testing therefore directly supports process optimization and the appropriate application of drying strategies.
Humidity’s impact on mannitol and sorbitol
The unpredictable effect of moisture on food ingredients is well illustrated by comparing the effects of changing humidity on the artificial sweeteners mannitol and sorbitol. In an experimental study, the dynamic, shear and bulk powder properties of these powders were measured using a powder rheometer to fully capture humidity’s impact on both materials. Mannitol and sorbitol find widespread medical and commercial application as sugar substitutes. Despite their identical chemical formula — they are isomers of the same compound — the results show that they display quite distinct and contrary responses to moisture.
When moisture is introduced to hydrophobic mannitol it acts as an intra-particular lubricant, reducing BFE. As illustrated in Figure 3 this downward trend continues until around 50-60% relative humidity. At this point liquid bridging is established, leading to more inhibited flowability and a rapid increase in BFE.
Sorbitol on the other hand, has the converse response, exhibiting an increase in BFE at low RH percentages, followed by a decrease of around 30% to 40%. Sorbitol initially adsorbs water at the surface, creating strong liquid/solution bridges between particles. Beyond 40% RH, however, the material’s high solubility means that any additional moisture will have a lubricating effect, eventually reducing the sample to a “wet mass” phase that at high humidity takes on a slurried form.
|Figure 5: Shear test data show different trends in behavior for mannitol and sorbitol, relative to dynamic characterisation, suggesting that the process environment might be extremely important in determining the impact of humidity.|
This contrast in response is also evident when the powders are subject to permeability testing. Here the mannitol shows a large change in permeability as RH is increased from the baseline state to around 30 – 40%. The pressure drop across the sample test bed increases sharply, indicating a significant reduction in permeability. However, beyond this point, pressure drop falls back to the baseline condition, and then lower, showing that permeability steadily increases at higher levels of RH.
Sorbitol, in contrast, exhibits a relatively consistent permeability as humidity increases, with only modest changes in measured pressure drop.
These responses can once again be related to the relative hygroscopicity of the powders, with the hydrophobic mannitol experiencing significantly greater variation in permeability than the hydrophilic sorbitol. Above an RH of between 60% – 65% both systems will experience surface saturation and their relative permeability performance then become increasing similar.
Shear testing was also undertaken for both powders. For many materials, shear data are less sensitive to the impact of any changes induced by humidity — as shown in Figure 5 by the results for mannitol. Sorbitol, however, shows quite extreme behavior at higher levels of humidity and, in contrast to the trends observed during dynamic and permeability testing, shows a steady increase in shear stress, suggesting poorer flow properties as humidity increases.
These results illustrate how the processing environment can have a strong influence on the behavior exhibited by sorbitol, shear data being more indicative of performance under moderate to high shear conditions, such as in a hopper, while dynamic parameters and permeability more closely reflect in-process behavior across a more diverse range of unit operations. This underlines the importance of measuring powders using different techniques and, most especially, interpreting results within the context of how the powders will be processed and used.
To fully optimize powder processing it is essential to comprehensively characterize powders using a variety of techniques. This enables not only precise measurement of the effects of humidity but also simulation of the conditions to which the powders will be subjected.
Dr. Katrina Brockbank is powder technologist and Jamie Clayton is operations manager with Freeman Technology. Freeman Technology’s business is the design, development and application of instrumentation for powder characterization, to solve powder processing problems. The FT4 Powder Rheometer® combines three test methodologies in a single instrument providing dynamic, shear and bulk property measurement.