In supplement manufacturing, particularly during the depositing step where gummy or soft-chew shapes are formed, the physical properties of the syrup are critically important to achieving clean, consistent shapes. Viscosity and surface tension are the two properties most often cited, but their relative importance depends on the stage of deposition, and a third property, yield stress, decides whether the shape holds its form once it lands.
Viscosity: The Dominant Factor
Viscosity is the most critical physical property for clean shape formation. It governs how the syrup flows into molds and fills the cavity. Key aspects include:
- Flow control: A syrup with too low viscosity will run or seep into gaps, causing irregular shapes or flashing (thin edges). Too high viscosity may not fill the mold completely, leading to gaps or air pockets.
- Shear-thinning behavior: Many supplement syrups exhibit non-Newtonian flow, meaning their viscosity decreases under shear (e.g., during pumping or deposition) and recovers once deposited. This recovery is essential for the syrup to "snap back" into shape without spreading.
- Temperature sensitivity: Viscosity decreases rapidly with temperature. Maintaining a consistent deposition temperature (typically around 60-70°C for gelatin-based syrups) ensures repeatable filling and shape retention.
Without proper viscosity control, even the best molds will produce defective shapes.
Surface Tension: The Secondary Critical Property
Surface tension influences how the syrup interacts with the mold surface and itself. While less dominant than viscosity, it's essential for clean edge definition:
- Wetting behavior: High surface tension causes the syrup to bead up and resist spreading, which can lead to poor mold filling and domed tops. Low surface tension helps the syrup wet the mold evenly, but if too low, it can creep up the sides of the mold cavity, causing "feathered" edges.
- Droplet formation: During drop-wise deposition, surface tension drives the syrup to pinch off cleanly from the nozzle. Stringing (long tails) is a viscoelastic effect rather than a surface tension one: an elastic syrup stretches into a filament instead of breaking cleanly. That is why the deposit temperature matters for tailing, and why tailing gets worse as the syrup cools and becomes more elastic.
- Release properties: Once the syrup has set, demolding depends on how strongly the gel adheres to the mold, not on the liquid's surface tension. Non-stick coatings and release agents handle that stage; surface tension's role is limited to the moment of deposition.
Yield Stress: What Keeps the Shape From Slumping
Viscosity describes how the syrup flows while the depositor is pushing it. The property that keeps the deposited shape from spreading is yield stress, the minimum force required to make the syrup flow at all. A syrup with enough yield stress stays put in the mold cavity, holding its edges and corners until it sets. With too little yield stress, the syrup levels out, rounds its edges, and loses fine detail, even when its viscosity at the nozzle measured correctly.
Gelling-agent type and level, solids content, and pH all move yield stress, so it is tuned together with viscosity rather than as an afterthought. Shear recovery is the other half of the picture. After the shear of pumping and deposition, the syrup must rebuild its structure fast enough to resist slumping before it sets, and that recovery time can be measured directly with oscillatory rheology. A clean shape is the result of three properties working together: viscosity for filling, yield stress for holding, surface tension for edge definition.
Which Is Most Critical?
For the deposition step itself, viscosity is the most critical property because it directly controls bulk flow and mold filling. Holding the shape once it lands comes from yield stress, which is why formulators treat the two together. Surface tension becomes critical when dealing with fine details, sharp corners, or non-stick mold surfaces, but it typically plays a supporting role.
In practice, manufacturers adjust these properties through formulation (e.g., using gelling agents, humectants, or emulsifiers) and process parameters (temperature, deposition speed) to achieve the desired balance. A well-optimized system ensures that the syrup flows easily into the mold, holds its shape immediately after deposition, and releases cleanly without flash or defects.