The rehydration properties of powder products serve as a significant indicator in assessing their overall quality. Within the dairy industry, the rehydration process holds particular importance as it presents a critical challenge. Certain powders demonstrate inadequate wettability, resulting in materials floating on the solution's surface, while others disperse slowly, often leading to the formation of lumps. This issue is especially prominent in powders with high protein content. Hence, the ability of a powder to rehydrate effectively is a crucial factor that determines its value and usability in various applications. In this blog post, we introduce the four different techniques that have been identified to study the wettability of the milk powder; immersion wetting, capillary rise or Washburn method, condensation wetting, and spreading wetting or sessile drop method.
Immersion wetting
Immersion wetting is one of the most standardized methods to study the wettability of dairy products. There, a standard amount of powder is gently on the surface of the water, and the time it takes for the powder to be completely submerged is recorded. If the time is less than 60 s, the powder is considered to be wettable. If it is over 120 s, the powder is non-wettable.
Capillary rise
Capillary rise, often also referred to as the Washburn method, has been used to study the wettability of powders in various industries. The typical procedure for dairy products is to pack 2 g of powder into the powder sample holder which is then hung on the hook connected to a very sensitive balance. The container has holes on the bottom, and when immersed in water, the liquid penetrates the powder bed. The mass uptake, i.e. the amount of water absorbed, in 10 minutes is measured and compared between different powders.
Although the measurement is straightforward it has some challenges as well. Powders with a contact angle above 90 degrees cannot generally be measured as the water will not spontaneously rise into the powder. Also, if the powder quickly dissolves into water, the recorded mass uptake is misleading since some of the powder is lost during the measurement.
Condensation wetting
In the condensation wetting experiment, a dry powder is placed in the desiccator with stable relative humidity. The weight of the powder is monitored for at least 96 hours until it reached a constant value. The results are expressed as equilibrated moisture content as a function of water activity to show the water vapor adsorption isotherms.
Sessile drop method
Sessile drop is a standard method to measure contact angles on different types of solid surfaces. A drop of liquid is placed on the surface and the contact angle is measured. To study the wettability of the dairy powder, a relatively large (> 10 ul) is placed on the powder that is compressed into a tablet or packed on a well plate and leveled to create a smooth surface. The contact angle is measured as a function of time. Powders where the contact angle decreases rapidly as a function of time show better wettability than those where the contact angle decreases only slightly even if the measurement time is increased over 100 s.
If you would like to see how Valio, the Finnish dairy company, uses sessile drop measurements in its product development, please watch the short webinar through the link below.
