Freeze-drying vials in a Stay Fresh Mega
A Stay Fresh Mega will freeze-dry material in vials, and for a research bench it’s a capable and inexpensive way to do it. It isn’t a pharmaceutical lyophilizer, and the differences aren’t cosmetic: no stoppering mechanism, no sterile boundary, no validated control system. Here is what the machine can and can’t do with vials, and why.
The constraints below come from two places: the published Stay Fresh specifications, and the physics of sublimation, which doesn’t negotiate.
Why a vial isn’t a tray
Food laid on a tray sits in direct contact with a heated shelf, and conduction is efficient and fairly uniform across the tray. A vial behaves differently. It touches the shelf only at its bottom rim, and most molded and tubing vials have a slightly concave base, so a thin gas gap sits between the shelf and the center of the vial bottom.
Heat reaches the product by three parallel routes: solid conduction through the small contact area at the rim, conduction through the gas in that gap, and radiation from the chamber walls, door and shelf above. Together they make up the vial heat transfer coefficient, Kv. Two consequences matter in practice.
First, the gas conduction term scales with chamber pressure. This is where vial work and food work diverge: pulling the pressure down to protect a delicate formulation also throttles the heat getting into it. Low pressure and slow drying are the same decision, not two.
Second, vials at the edge of a load receive radiation that interior vials don’t. In pharmaceutical practice this edge-vial effect produces roughly 1.5-fold faster drying at the periphery, with the extra radiation reaching as much as 50% of total heat input (Tchessalov et al., Pharmaceutical Research, 2023). The Mega’s acrylic door and its stainless and aluminum interior sit well away from the shelf setpoint, so assume the effect is significant. Load vials as a contiguous block rather than scattered, and ring the block with water-filled dummy vials you don’t care about. Those sacrificial vials absorb the radiation, and the vials you want behave more like center vials.
Headroom: which vials physically fit
Shelf spacing is the first thing to check, and it’s why vial work belongs in a Mega rather than a smaller machine. Stay Fresh shelf spacing runs from 1.2 in on the 7-Shelf, to 1.6 in on the 5-Shelf and the Large, to 2 in (50.8 mm) on the Mega, the most in the range. Against nominal ISO 8362-1 vial heights, allowing roughly 6 mm for a lyophilization stopper seated in the half-inserted position:
| Vial | Nominal height | With half-seated stopper | Fits under 50.8 mm? |
|---|---|---|---|
| 2R | 35 mm | ~41 mm | Yes, comfortably |
| 6R | 40 mm | ~46 mm | Yes |
| 4R / 8R / 10R | 45 mm | ~51 mm | Marginal; measure yours |
| 20R | 55 mm | ~61 mm | No |
| 30R and larger | 60 mm + | ~66 mm + | No |
Shelf spacing is measured shelf to shelf, so the clear height is a little less once you allow for the shelf itself and anything under the vials. Vial dimensions also vary between manufacturers. Measure the clear height in your machine and the stock you actually hold rather than trusting the table. If you need 20R or larger, remove one shelf and run the vials on the shelf below; that roughly doubles the clearance and costs you one level of capacity.
Fill depth is the second geometric constraint, and it’s a process constraint rather than a mechanical one. Resistance to vapor flow rises as the dried layer thickens, so a deep fill dries disproportionately slowly. Standard practice keeps fills at or below 1 cm, with hold times at the final freezing temperature of about 1 hour at 1 cm or less and 2 hours at 1 to 2 cm; fills above 2 cm are considered undesirable (Tchessalov et al., 2023). One centimeter of fill is roughly 1.6 mL in a 2R, 3.1 mL in a 6R and 3.8 mL in a 10R.
Chamber pressure is a recipe setting, not a machine limit
This is the most common misunderstanding about running vials in a food freeze dryer, and getting it wrong wastes either a batch or a week.
The target vacuum figures in Stay Fresh food guidance, such as 400 mTorr on the Mega, 300 mTorr on the Standard and Large, and 550 mTorr as the factory default, are recipe parameters for food. Food is dried fast and warm. A warm shelf drives a high sublimation rate, and a high sublimation rate needs a correspondingly high chamber pressure to carry the vapor away and keep heat flowing into the product. Set a food recipe much below those numbers and the cycle stretches out for no benefit.
Vial work inverts every term in that sentence. You aren’t optimizing throughput, you’re protecting structure, so you run a cool shelf and a low chamber pressure and accept a long cycle. Same machine, different recipe.
What the hardware will do is a separate question. On an empty shelf a Mega typically pulls below 200 mTorr within about 15 minutes. Under load that number rises, because the vapor the product releases is exactly what the pump and the cold trap have to keep up with; the pressure you can hold during primary drying is always higher than the pressure you reach with nothing subliming. Measure where your own loaded chamber settles.
What the cold trap actually limits
The real floor is thermodynamic. Vapor leaves the product and condenses on the trap only if the trap is colder than the sublimation front, and the chamber can’t hold a pressure lower than the vapor pressure of the ice already on the trap. Ice vapor pressure is steep and well characterized:
| Ice temperature | Vapor pressure |
|---|---|
| −20 °C | ~776 mTorr |
| −25 °C | ~470 mTorr |
| −30 °C | ~286 mTorr |
| −33 °C | ~200 mTorr |
| −37 °C | ~131 mTorr |
| −40 °C | ~97 mTorr |
The Mega’s trap runs at −35 °F (−37.2 °C), which puts its own vapor pressure at roughly 131 mTorr. The Standard and Large traps run colder, at −40 °F (−40 °C), about 97 mTorr. Those figures, not any control setting, are the honest floor of each machine.
Read the table the other way and it tells you what product temperature each chamber pressure permits. Hold 200 mTorr and the sublimation front can sit as cold as about −33 °C; hold 150 mTorr and it can approach −36 °C. The catch is rate: the driving force for sublimation is the difference between the ice vapor pressure and the chamber pressure, so a front at −33 °C against a 150 mTorr chamber has only about 50 mTorr pushing the vapor out. That’s a slow cycle by design, and it’s exactly the regime vial work wants.
What this means for formulation
For amorphous systems the ceiling on product temperature is the glass transition temperature of the maximally freeze-concentrated solution, Tg′, with the collapse temperature typically a few degrees above it. Exceed it during primary drying and the cake slumps, shrinks from the vial wall or turns glassy instead of porous.
Measured by DSC, sucrose sits at −33.1 ± 0.1 °C and trehalose at −29.4 ± 0.6 °C (Horn and Friess, Frontiers in Chemistry, 2018); an independent determination puts 10% sucrose at −32 °C (NETZSCH application note). Set against the pressures above, both are workable in this machine, with different margins:
- Trehalose at −29.4 °C is comfortable. A chamber around 250 to 286 mTorr already permits a front at −30 °C, with driving force left over.
- Sucrose at −33.1 °C is achievable but tight. You need the chamber near 200 mTorr, patience for a long primary drying, and very little margin for a shelf setting that’s too warm.
- Mannitol and other crystalline systems are straightforward, because a crystalline matrix holds the cake up at product temperatures well above any amorphous Tg′ present.
One consequence runs against intuition: for the most collapse-sensitive sucrose work, the Standard and Large are marginally better placed than the Mega, because their −40 °F trap reaches a lower chamber pressure. You trade capacity for a few degrees of thermal margin. Choose on the formulation, not on the size of the cabinet.
Annealing and crystalline bulking agents
Mannitol is the usual bulking agent, and its behavior is worth understanding before you rely on it. It crystallizes only partially during freezing, around −30 °C, with most of it crystallizing during warming at about −16.5 °C (Horn and Friess, 2018). Partial crystallization is a real risk: an amorphous fraction left behind can crystallize later in storage and destabilize the product.
That’s what annealing is for. Holding the frozen load above Tg′, typically −15 to −10 °C for 3 to 5 hours (Tchessalov et al., 2023), drives crystallization to completion before drying begins. Two cautions: mannitol crystallization generates mechanical force and has been associated with vial breakage, so don’t pack vials rigidly; and where a peptide or protein needs real molecular protection, mannitol is a bulking agent, not a stabilizer. Some amorphous sugar has to remain to vitrify around the molecule. A mixed system needs enough mannitol to crystallize reliably and enough sugar to protect.
Programming the cycle: the preset temperature curve
The standard Stay Fresh process is one automatic cycle with four settings: shelf chill, target vacuum, maximum shelf temperature and final dry hours. That’s enough for food. It isn’t enough for a lyophilization cycle, which needs a freezing hold, an annealing excursion, staged primary drying and a separate secondary drying step, each at its own temperature for its own time.
What makes it possible is the preset temperature curve under Advanced mode. In the user manual’s words: “The Temp row is the target shelf temperature (°F) for each step. The Time row is how many minutes to hold it. There are nine slots.” Shelf temperature can be set from −50 to 180 °F on current machines; older units stop at 150 °F.
Two limitations to know before you build a curve. There’s no per-step vacuum control: target vacuum is a single setting for the whole run, so you pick one pressure and shape the cycle with temperature. And the manual warns that misusing the advanced modes voids the vacuum pump warranty, a fair warning on a pump that isn’t designed to run cold and wet for days.
Here is a defensible nine-slot starting curve for a mannitol-containing formulation in vials, with target vacuum set to 200 to 300 mTorr for the run. It is a starting point, not a validated recipe.
| Slot | Shelf °F | Minutes | Purpose |
|---|---|---|---|
| 1 | −40 | 120 | Freeze and hold to full solidification |
| 2 | +10 | 240 | Anneal (−12 °C) to complete mannitol crystallization |
| 3 | −40 | 90 | Re-cool before drying |
| 4 | −20 | 240 | Early primary drying, conservative |
| 5 | −10 | 360 | Primary drying |
| 6 | 0 | 360 | Primary drying |
| 7 | +10 | 360 | Late primary drying |
| 8 | +25 | 240 | Transition to secondary drying |
| 9 | +70 | 480 | Secondary drying, removes bound water |
That totals about 41 hours. For a sucrose-dominant formulation, drop slots 4 to 7 by 10 to 20 °F and lengthen them, and set the vacuum nearer 200 mTorr: you’re buying margin against a −33 °C collapse temperature and paying for it in time.
The single most important thing about this table: these are shelf temperatures, not product temperatures. During primary drying, sublimation is endothermic and the product runs well below the shelf beneath it. A shelf at −10 °F (−23 °C) can hold a product front near −33 °C at low chamber pressure. The gap closes as sublimation finishes, which is why product temperature rising to meet the shelf is the classic sign that primary drying is complete.
The only way to see it is to instrument the batch. Put a temperature probe in a center vial: the Large and Mega take the 2-pin version, in standard, needle or pad types, and the 3-pin fits the Standard. A probe changes ice nucleation in the vial it sits in, so that vial dries slightly ahead of its neighbors; read it as an indicator, not a measurement of the whole batch. Raise shelf temperatures in small steps across successive runs rather than within one. Every 1 °C of product temperature is worth roughly 13% off primary drying time (Horn and Friess, 2018), which is the incentive to push, and exactly why overshooting is expensive when it collapses a batch you can’t repeat.
Backfilling with nitrogen, and the stoppering problem
A pharmaceutical lyophilizer seats the stoppers inside the chamber, under vacuum or a dry nitrogen backfill, by collapsing the shelf stack hydraulically. The vial is closed before it meets room air. The Mega has no such mechanism, and for vial work that’s the single biggest difference.
What you can control is the air the cakes meet. Break the vacuum with dry nitrogen through the vacuum release valve rather than with room air. Feed regulated low-pressure nitrogen to the valve, open it slowly, and let the chamber come up to atmosphere under nitrogen. The cakes then sit in a dry, inert atmosphere instead of a humid one while you work. Open the valve gradually whatever you’re backfilling with; letting gas in fast can disturb the load, and a dried cake is fragile.
The remaining exposure is capping. Reduce it:
- Lay out the crimper, caps and seals before the door opens, and cap each vial as you remove it rather than unloading the whole batch first.
- Work in the lowest-humidity space available, or cap inside a nitrogen-purged glove bag for material that warrants it.
- Accept that the headspace ends up as nitrogen at atmospheric pressure, not an inert gas at reduced pressure, and store accordingly.
Verifying the result
Weighing the load to constant mass tells you drying has stopped progressing. It doesn’t tell you residual moisture. For that the reference method is Karl Fischer titration, and if your work carries any downstream stability claim, that’s the measurement that matters. Inspect the cakes too: shrinkage from the vial wall, a glassy or shiny look, or a slumped cake all mean the product went above its collapse temperature during primary drying.
What this machine is not
The Mega has no clean-in-place or steam-in-place, no sterile boundary, no validated control system, no audit trail meeting 21 CFR Part 11, and no in-chamber stoppering. It isn’t suitable for making sterile product for human use.
It is suitable for research and development where those controls aren’t required: drying research peptides and reagents, preserving buffers and media, archiving samples, and developing a cycle cheaply before committing time on instruments that cost an order of magnitude more. For that work, the Mega’s 2 in shelf spacing, six shelves, nine-step programmable curve and 10.2 CFM pump make a reasonable platform at a price a department budget can absorb. See the lab page for choosing between chambers, and spare Mega trays if you’ll alternate vial runs with bulk work.
References
- Tchessalov S, et al. Practical Advice on Scientific Design of Freeze-Drying Process: 2023 Update. Pharmaceutical Research 2023;40(10):2433–2455. link.springer.com
- Horn J, Friess W. Detection of Collapse and Crystallization of Saccharide, Protein, and Mannitol Formulations by Optical Fibers in Lyophilization. Frontiers in Chemistry 2018;6:4. frontiersin.org
- NETZSCH Analyzing & Testing. The Role of DSC in Lyophilisation Processes. analyzing-testing.netzsch.com
- Stay Fresh Mega Freeze Dryer User Manual, model 12H11560US, Advanced modes. Mega user manual (PDF)
This guide describes general process principles and the published capabilities of Stay Fresh equipment. It is not a validated protocol. Cycle parameters must be developed and verified for each formulation.