Slimy Packs With No Leak: What Grows in Gel Made With Well Water

The complaint arrived with a photograph: a pack opened after three weeks in a customer’s fridge, gel intact, no leak, but a string of clear slime stretched between the film walls and a smell somewhere between a swimming pool and a drains. The batch number traced back to a Thursday run. So did nineteen other complaints that month, all from the same fill line, none of them leaks.

This is a microbiological failure, not a polymer failure, and it is more common than the industry admits because nobody tests for it. A gel pack is 99.5% water. If that water is not potable at the fill head, the pack is a growth medium in a sealed bag.

Where the organisms come from

Four sources, in rough order of frequency.

Well water or borehole water. Iron-related bacteria, Pseudomonas and various slime-forming species live in boreholes and distribution lines. They are harmless to most people and completely capable of turning a gel pack cloudy in two weeks. A plant using its own well to save on municipal supply is the classic case.

Stored water. A 5,000 litre buffer tank held at 25 °C is an incubator. Chlorine residual disappears within a day in warm water, and the tank develops a biofilm on the walls that sheds continuously into the line.

Residual gel in the system. Gel left in a hose, a pump or a filler head over a weekend is a perfect substrate: water, polymer, and a large surface area. The next batch inoculates itself.

Dirty ice or condensate. In plants that use flake ice to pre-chill water, the ice machine is often the least-cleaned piece of equipment in the building.

Why the polymer makes it worse

Superabsorbent gel is not a biocide and does not inhibit growth. It also traps organisms against the film, holds water at the surface, and creates a thin layer with a huge interface area. The slightly alkaline pH of a typical gel, 6.5 to 7.5, is close to ideal for many environmental species.

Some cheap SAP grades carry higher residual monomer and process residues, which can act as a nutrient source. That is not the primary cause, but it shortens the time to visible slime from six weeks to two.

What to test, and what the numbers mean

You do not need a microbiology lab to catch this. A dip slide, available from any water-testing supplier for a few dollars, gives you a total viable count in 48 hours at 30 °C.

• Test the water at the fill head, not at the inlet. The line is where the biofilm lives.
• Test the finished gel by diluting 1 g in 9 ml of sterile saline and plating.
• Under 100 CFU/ml in the water is a working target for a sealed pack with a 12-month life.
• Over 1,000 CFU/ml, expect complaints within a month at 25 °C storage.
• Any visible slime at three weeks means the count was probably above 10⁵ at fill.

Also check chlorine residual at the fill head. If it is zero, the water has no residual protection through the line, and everything downstream of the treatment point is a growth surface.

The fix, in order of cost

The cheapest change is temperature. Fill at 20–25 °C rather than 30 °C and store filled packs below 25 °C. Every 10 °C roughly halves the growth rate, and many slime complaints disappear with nothing else changed.

Next is water treatment. A 5 micron prefilter, a carbon block and a UV unit at the fill head costs less than a thousand dollars and handles most borehole water. Add chlorine at 1–2 ppm with a contact time of 20 minutes, then confirm the residual at the head.

Then hygiene. Tear down the filler every Friday, flush with 1% caustic at 60 °C, rinse, and sanitise with peracetic acid. Gel residue that is allowed to sit over a weekend is the single most common source of a persistent problem.

Finally, formulation. A food-grade biocide at 0.05–0.1% is legal in many markets for non-food-contact packs and buys you a lot of margin. Check the local rules before adding it, and declare it on the documentation.

The case that changed a plant’s mind

A packer in Spain blamed its SAP supplier for six months of slime complaints and switched grades twice. The problem persisted. A dip slide taken at the filler head showed 4 × 10⁴ CFU/ml. The borehole was fine at the pump; the 3,000 litre tank had a biofilm on the inner wall that had been building since the tank was installed. Cleaning the tank and adding UV solved it in a week, at a cost of about EUR 700.

Put a dip slide in the weekly routine next to the pH check. It costs less than one rejected pallet. Food-grade SAP powder with a low residual monomer content removes one nutrient source, but it cannot fix water that should not be going into a pack in the first place.