The Influence of Relative Humidity on the Properties of Flour

Powder behavior is influenced by various environmental conditions, such as temperature and humidity. The same powder sample can function uniquely at different locations for the same application. Thus, knowing the powder properties and characteristics is important for both process design and quality control.

Anton Paar offers a versatile toolbox for the characterization of powders, including adsorption analyzers, to investigate water adsorption of samples as well as their accessible surface area. In addition, laser diffraction methods measure particle and aggregate sizes, and powder rheological equipment enables the characterization of their flow- and mechanical behavior.
In this application report, the influence of relative humidity on wheat flour was investigated. Flour samples were stored in an environmental chamber for various time periods and characterized afterwards by the previously mentioned techniques.

Surface Area and Vapor Sorption

All granular materials possess surface area which interact with the surrounding atmosphere. This interaction can be measured using vacuum-volumetric or gravimetric techniques that measure the amount of gas or vapor (sorbate) sorbed onto the solid surface of the sorbent (flour) at a known absolute temperature, pressure or relative humidity.
The plot of the adsorbed gas or vapor amount versus pressure at a constant temperature is known as the sorption isotherm and is used to determine the surface area when applying a form of the well-known BET (Brunauer-Emmett-Teller) equation:

where W is the weight of the sorbate adsorbed at relative pressure of P/P0, and Wm is the weight of the sorbate constituting a monolayer of surface coverage. The C term (BET C constant), indicates the magnitude of sorbent/sorbate interaction.
Similarly, water vapor sorption experiments can be monitored gravimetrically to know how much and how fast water was sorbed into the sorbent at a fixed temperature and relative humidity setting. This information is important to determine the storage and stability for many common items such as dry foods or pharmaceuticals.

Particle Size Measurements

An important aspect which impacts the behavior and processing of dry foods such as flour is its particle size. In fact, moisture can induce both the formation of starch agglomerates, causing the formation of a coarse phase, and the swelling of the single starch granules in the fine phase. The presence of large agglomerates changes the flow behavior, while the swelling of primary starch granules leads to an increased “stickiness” of the material. This can strongly impede flow through silos or feeders and cause serious problems. For this reason, we investigated the effect of moisture on particle size to inform on flour’s behavior during manufacturing, packaging and storage.

Powder-Rheological Measurements

The Warren Spring cohesion measurement method is applied to measure powder cohesion, especially for very cohesive powders such as flour or calcium carbonate. The measurement is based on the work of Geldart and Abdullah (3), by using a torsional device called the Warren-Spring-Bradford tester. The powder is studied in a consolidated state, which ensures a homogenized powder bed. The results of Warren Spring measurements can be used to analyze the flowability of cohesive powders. Further information about the Warren Spring method can be found in the application report “Methods for Powder and Granular Media Characterization with the Anton Paar Powder Cell”.

Here, SWS is the Warren Spring cohesion value, M is the torque, RO and RI are the outer and inner diameters of the Warren Spring geometry. This gives a comprehensive single value for the flowability of the powder in a compressed state.

The compression itself is characterized by compressibility measurements, whereby an increasing normal force is applied onto the powder and its compressive response measured. This gives an indication on the behavior of the powder when experiencing load, like at the bottom of a container or silo.

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This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.

For more information on this source, please visit Quantachrome Instruments.

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