Tunnel vs. Flash Pasteurization: What Your Drink Actually Needs

Tunnel and flash pasteurization both kill the microbes that would spoil your drink on the shelf. They get there in different orders, at different temperatures, over different lengths of time, and those differences show up in the can. Here's how each one works, where each one earns its keep, and how to figure out what a botanical, juice-forward, or low-ABV drink really needs.

What pasteurization is measuring: the PU

The industry keeps score in pasteurization units. 1 PU equals the lethal effect of holding a product at 60 °C (140 °F) for 1 minute.

The scale climbs fast. Above 60 °C, PU pile up exponentially, so a few extra degrees can do the work of many extra minutes. That's why two very different processes can land on the same PU number.

Your PU target comes from the product itself: its pH, alcohol, sugar, carbonation, and the organisms most likely to cause trouble. A beer, a seltzer, and a juice-forward mocktail won't share a number. A responsible co-packer sets the target with your formulator for each product and then verifies it.

How tunnel and flash pasteurization work

Tunnel pasteurization

You fill and seal the can first. Then the sealed cans ride a conveyor through a long chamber where zones of water spray warm them up, hold them at pasteurizing temperature, and cool them back down.

The big advantage is timing. Heat hits the liquid, the headspace, the inside of the can, and the underside of the lid all at once, after the package is closed. Nothing can get back in after treatment.

The trade-off is time under heat. A tunnel runs at more moderate temperatures, but the product spends much longer warm than it would in a flash system, and the center of a can lags behind the wall on the way up and the way down.

Flash pasteurization (HTST)

Flash, or high-temperature short-time, treats the liquid before it's packaged. The product runs through a heat exchanger, gets heated to a higher temperature, holds for a short stretch, and gets cooled before it reaches the filler.

The advantage is a short heat exposure. The catch is everything downstream: the treated liquid still travels through lines, a buffer tank, and the filler before the package is sealed. That fill has to be kept clean enough that you don't reintroduce what you just killed.

Which one is harder on delicate drinks?

It depends on the PU target, and that's the honest answer. Heat can flatten fresh top notes, pull volatile aromatics, shift color, and push juice toward a cooked character. Both methods apply heat, so both can do it.

Flash has a real edge on exposure time, which is why it's popular with fragile juice and botanical products. A tunnel's longer warm period is its main flavor risk.

A tunnel set to a sensible PU target is often kind to a well-built formula, though. The damage usually comes from over-processing: a target set higher than the product needs, a formula that didn't account for heat, or oxygen left in the liquid going into the tunnel. Heat speeds up oxidation, so a high-DO product will stale faster once it's warm.

Those are all things you can control before the first can gets sealed.

Set the PU target as low as your product can safely support, then build the formula so it doesn't need a high one.

What your drink actually needs

Run your product through these questions before you pick a process or a co-packer:

• What's the pH? Acidity is one of your strongest safety hurdles. If a non-alcoholic drink sits above pH 4.6, talk to a process authority first, because low-acid beverages face stricter FDA rules and usually a different kind of thermal process altogether.
• How much alcohol is in it? Alcohol helps hold spoilage organisms back. Low-ABV and non-alcoholic drinks lose that help, so they lean harder on pH, pasteurization, and clean handling.
• Is there sugar or juice? Fermentable sugar plus a surviving yeast cell can mean refermentation in a sealed can, and swollen cans. Juice adds sugar and a wider range of microbes.
• Is it carbonated? In a tunnel, pressure inside the can climbs as it heats. Your CO2 volumes and fill level have to fit within what the can and lid can handle.
• How fragile are the aromatics? Citrus peel, fresh herbs, and delicate florals tend to show heat first. Your formulator can often pick heat-stable versions or adjust dosing to account for the process.
• How much oxygen is in the liquid? Lower dissolved oxygen going in means less oxidation during heating and on the shelf.
• How long does it need to last? Your distributor's shelf-life expectation shapes how much margin you need to build in.

Dialing in PU without cooking the flavor

This is a formulation job and a process job at the same time. Here's the loop that works:

1. Build the hurdles into the recipe. Your formulator sets pH, acid, and sugar levels with the process in mind, so the product doesn't depend on heat alone for stability.
2. Set the PU target together. The formulator, the co-packer's QC team, and a process authority where one is needed agree on a target with a real safety margin and no extra.
3. Get oxygen out first. Deoxygenating the liquid and controlling headspace before the tunnel protects flavor during heating.
4. Verify the PU delivered. A setting on a control panel is a starting point. Measure what the product actually received and adjust from there.
5. Trend shelf stability. Track pH and dissolved oxygen on retained samples over time. Drift tells you something's changing before a customer does.
6. Taste it blind. Compare pasteurized samples against unpasteurized controls in a controlled sensory setting, then refine and lock the formula.

For a deeper look at the other side of the decision, read pasteurization vs. cold fill.

How we'd handle it at Tennessee Hills

We run a tunnel pasteurizer in our 35,000 sq ft plant in Bristol, TN. It lets us make drinks shelf-stable without preservatives, with the heat applied after every can is sealed. We don't offer flash pasteurization, so if your product truly needs it, we'll tell you straight.

For most canned RTDs, seltzers, functional drinks, and botanicals, the tunnel does the job well when the PU target and formula are built for it. That's where our process starts:

• Formulation. We connect you with vetted third-party formulators who write a production-ready formula covering ingredients, processing steps including pasteurization, and testing thresholds.
• PU verification in-house. Our college-grade lab and trained QC staff verify pasteurization units, along with pH, ABV, Brix, dissolved CO2, and dissolved oxygen in ppb.
• Oxygen control. We deoxygenate liquid before it's canned, and nitrogen microdosing is available for still drinks and can pressure.
• Shelf-life trending. We track pH and DO over time on your product with remote shelf-life trending.
• Sensory approval. You taste and approve in our purpose-built sensory deprivation room, and we encourage you to be on-site for your first run.
• Traceability. Test results, ingredient lots, and date codes tie back to your lot numbers in our NetSuite manufacturing system.

Bring your formula, or the idea for one. Request a quote or book a facility tour, and we'll walk through your PU target with you on the production floor.