Introduction: Vacuum deaeration gives trapped air a path out of thick cream, and viscosity sets how fast that path actually works.
Freshly made cream often looks perfect at the surface and shows pinholes a day later. The usual explanation is that the vacuum "sucked the bubbles out," which makes the process sound far more decisive than it is. Air leaves a thick emulsion slowly, unevenly, and only when several conditions line up. Understanding what vacuum actually changes — and where the physics runs out of room — is what separates a realistic expectation from a disappointing batch. That same understanding makes it easier to read equipment descriptions of deaeration and judge what a vacuum system can reasonably deliver on a cream, lotion, or ointment line.
Viscosity is resistance to flow, and creams sit at the high end of the scale. Water is close to 1 mPa·s at room temperature, thin lotions run in the hundreds to low thousands, and semi-solid creams and ointments can measure in the tens of thousands or beyond. A cream can be thousands of times harder to push than water. For an air bubble, that resistance is the entire problem: a bubble rises only because it is lighter than the material around it, and the cream has to flow out of the way for it to move upward. In a thin liquid that takes seconds. In a thick cream it can take hours, or it simply never happens. Size makes the situation worse. Mixing does not create neat round bubbles — it folds air into films and clusters that break into small pockets. A smaller bubble has less buoyancy and more drag for its volume, so its rise speed falls sharply. Air parked near the tank wall, behind the agitator, or in the discharge line barely moves at all. That is why a batch can look smooth at the top while the bottom still carries pinholes, and why a cream that looked clean in the kettle can show surface defects after filling. The practical consequence is that during high-viscosity cream homogenizing, the vacuum is not doing the lifting. It changes conditions so that a bubble can expand, then relies on the material moving enough for that bubble to reach a surface where it can break.
A trapped bubble sits in balance: the pressure of the gas inside it matches the pressure of the cream and the headspace around it. Vacuum deaeration attacks that balance from the outside. When headspace pressure drops, the bubble responds, and four effects act together.
Read together, those four points explain why deaeration behaves like a process step rather than a switch. The vacuum level sets how much a bubble can grow, and the hold time sets how far that grown bubble can travel. A machine can reach a deep vacuum very quickly and still leave air in the batch if the material stops moving the moment the gauge settles.
Viscosity is not a fixed number, and that is the first limit. Cream firms up as it cools, so a batch that is deaerated warm moves more freely than the same batch deaerated after cooling. Production schedules add a second constraint: the time a batch can sit under vacuum competes with the rest of the cycle, and holding longer is not always possible. Where a heating jacket is available, warming the product before deaeration lowers viscosity for the duration of the step, which makes bubble expansion more productive. Formula matters too — a stiffer, more structured cream resists bubble movement more than a soft lotion at the same nominal viscosity. Distance to a free surface is the second limit. Only material near the surface releases gas quickly; everything below it has to be brought upward first. Agitation and scraped-surface designs renew that surface, and slow sweeping motion keeps material at the vessel wall from sitting in place. That is a transport job. The removal itself still comes from the pressure difference, which is why strong mixing alone does not degas a batch, and why a still batch under vacuum degasses slowly no matter how deep the vacuum goes. What happens afterwards is the third limit. Transfer lines, pumps, and filling heads can pull air back into a smooth batch, and a homogenizer is a separate tool with a separate job. Its rotor-stator zone breaks droplets and disperses phases, and that same high-shear zone can drag air in. Running homogenization after deaeration can undo much of the work, so the sequence of steps inside the batch matters as much as the vacuum setting. That is the honest ceiling on the process: vacuum deaeration reduces entrained air under suitable conditions, while zero bubbles is a different target that no single step delivers on its own. Results track the formula, the viscosity at the moment of deaeration, the vacuum condition, and the operating method. Within that frame, fixed vacuum emulsifying machines such as the PROMAKE range — offered for creams, lotions, ointments, and similar high-viscosity materials, with optional heating jackets and control configurations — include a vacuum deaeration function that works as part of the batch sequence rather than as a standalone promise.
Vacuum deaeration is best understood as a pressure tool, not a vacuum cleaner. Lowering headspace pressure lets trapped gas expand, and a larger bubble has a better chance of rising through a cream that is still moving. Viscosity decides how much of that chance is real, and hold time decides whether the bubble ever reaches a surface. The realistic expectation is a meaningful reduction in visible and entrapped air when the formula, temperature, vacuum level, and mixing state are working together — not a batch that is provably bubble-free at every depth. Reading a machine's deaeration function with that in mind keeps expectations aligned with what the physics can deliver.
A: No. Vacuum deaeration reduces entrained air under suitable conditions, but a thick cream holds small bubbles in place through viscous resistance, and some remain near walls, behind the agitator, or deep in the batch. The realistic outcome is fewer and smaller visible voids, not a mathematically bubble-free product.
A: A bubble rises only when the surrounding material flows out of its way, and a cream resists that flow thousands of times more than water does. Small bubbles created during mixing have even less buoyancy for their volume, so they may stay suspended for hours or never travel to the surface at all.
A: Viscosity at the moment of deaeration, product temperature, the vacuum level reached, how long the vacuum is held, whether the batch keeps moving, and how much air enters later during transfer and filling. The order of homogenizing and deaeration steps also changes the result.
Bioprocessing Equipment - ASME
