A pain in the food industry’s neck: Stickiness and Caking

Several factors can lead to stickiness in food powders, both during production and storage. The primary cause of stickiness is moisture. Properties like viscosity, surface tension, and solubility make water a catalyst for stickiness and caking.

Water naturally exists in the atmosphere, and its presence during production is inevitable. As relative humidity increases, moisture can adsorb onto the powder’s surface or be absorbed by the bulk solids. Caking can happen when humidity levels reach a certain saturation point. In some industries, like tea and whey powder production, caking can occur even when relative humidity is as low as 33%. Temperature also plays a role—warmer air can hold more moisture, further impacting the powder.

When moisture adsorbs onto powder surfaces, it can form mono/multi-layered structures or capillary condensate. These changes alter the powder’s surface micro-roughness, leading to liquid bridges. The formation of these bridges increases cohesiveness between particles, contributing to caking.

Solids typically exist in either crystalline or amorphous phases. Crystalline solids have molecules arranged in a specific lattice structure, while amorphous solids lack this pattern. In the food industry, many solids, like sugar, chocolate mousse, mayonnaise, and ketchup, are amorphous. Their physical state and molecular mobility are affected by temperature and composition. These factors influence the stability and processability of food powders. Key attributes for handling and processing amorphous solids include phase transition, handling stability, and processability.

Predicting powder stickiness and caking is crucial for preventing adhesion between particles. However, each product reacts differently to varying temperature and humidity. This makes predicting stickiness even more complex. To address this challenge, we employ specialized equipment and expertise.

We use two main methods:

1. Water Vapor Adsorption Isotherms: We measure isotherms at different temperatures and humidity levels (0-95%). Using the Guggenheim, Anderson, and de Boer (GAB) model, we can determine the stickiness point from these measurements, though this process can be complex.
2. Powder Rheometry: This method measures stickiness directly by monitoring adhesive forces and friction in real time, considering temperature and humidity changes. Our modified powder rheometer allows for more accurate stickiness measurements, providing a preferred alternative to vapor adsorption.

Our collaboration with Anton Paar in Graz, Austria, has led to advancements in using the powder rheometer for stickiness assessment. A joint publication from Delft University of Technology, Anton Paar, and Delft Solids Solutions compares this new approach to conventional water vapor sorption measurements.

The state of amorphous solids depends on molecular movement, which is influenced by temperature. Amorphous solids can exist in a glassy (frozen) state or a rubbery (viscous) state. The viscosity of these solids is affected by both temperature and moisture content. Elastic forces in response to shear, like compression or extension, cause viscosity in solids.

The sticky point temperature is the temperature at which powder particles adhere to each other or to equipment surfaces. Stickiness can be cohesive (similar surfaces attract) or adhesive (dissimilar surfaces attract). It is adhesive stickiness that causes powders to stick to machinery and packaging materials. At this temperature, the powder has its lowest flowability.

Stickiness involves the formation of liquid bridges between particles. These bridges can form during production, with substances like melted fats or strong sugar solutions. When these liquid bridges transform into crystal bridges during drying (such as spray drying), the powder enters the caking stage.

Compressibility measures how the volume of a sample changes when pressure is applied. In food powders, the combination of moisture, temperature, and external pressure triggers caking. Compressibility typically happens during storage. When powders are stacked, they can form lumps or cemented materials at the bottom. Time also plays a role—caking develops after prolonged storage, such as in hoppers or bulk bags.

We recently worked with a cinnamon powder producer in Indonesia who was designing a new storage facility. The company needed to ensure their cinnamon powder would flow properly within their silo. Using the modified powder rheometer, we investigated the conditions under which the cinnamon powder became sticky, particularly given the climate differences between Indonesia and Western Europe.

After identifying the non-sticky conditions, we performed ring shear tests in a controlled climate chamber. This helped determine the optimal silo design, including hopper half-angle and discharge opening. The results also accounted for wall friction and bulk density, ensuring a suitable solution for their storage needs.