A Grolsch Tasting Tap Poured Caustic Cleaning Solution. The Fix Is a Valve, Not the Chemical

For years, visitors on the Grolsch brewery tour in Enschede have been handed a small glass of the filtered spring water the beer is brewed with. On Saturday 17 October, nine visitors and two staff members tasted it, noticed a strange, sour taste, and were taken to hospital. Two days later the brewery said what had happened: the line the tasting tap hangs off was in the middle of its scheduled clean, and nothing stopped the cleaning solution reaching the tap.

All eleven have been discharged, and Grolsch says no cleaning solution reached its beer. This article does not add to the reporting of the incident, which the Dutch Labour Inspectorate is still investigating. It explains the chemistry and the plumbing behind it: why breweries clean with caustic soda, how strong that solution is, why it is easy to detect, and how food plants are told to keep it on its own side of the pipework. The lesson travels well beyond one brewery. Every plant that cleans in place has branches like that tasting tap.

What happened at the Grolsch brewery?

A sampling tap that normally pours brewing water poured diluted cleaning solution, because the line behind it was being cleaned and the tap was not locked out. The sequence below is taken from Dutch public broadcaster NOS, regional broadcaster RTV Oost and the English-language outlets NL Times and DutchNews.

Grolsch’s statement, in our translation from the Dutch reported by NOS: “The connection of the sampling tap was not fitted with the technical safety interlock. As a result, cleaning fluid could reach the sampling tap, where (brewing) water is normally tasted.” The company described the liquid as a diluted cleaning agent containing “a small percentage” of caustic soda. To RTV Oost it added that during the regular cleaning process “the production installations and product streams are technically separated from the cleaning cycle in question.”

Read closely, the statement says something useful. The production lines were separated from the cleaning circuit, as they should be. The tasting tap was not. It was a small branch, used a few times a day for visitors, sitting on a line that is cleaned every three weeks, and it was the one outlet the separation did not cover.

Why do breweries clean with caustic soda?

Because caustic removes the organic soil that beer and wort leave behind: sugars, proteins, hop resins and the yeast and bacterial film that grows on them. Acid does a different job, removing mineral scale. A cleaning presentation hosted by the Brewers Association, given by draught specialists from New Glarus Brewing and Verboten Brewing, puts it in two lines: “Caustic – removes organic residue” and “Acid – removes inorganic build up”. The same slide adds, in capitals, “NEVER mix the two together!!!”

Inside a brewery, the routine is called clean-in-place, or CIP. Instead of taking pipes, tanks and heat exchangers apart, the plant pumps a sequence of rinses and cleaning solutions through them along a programmed route. That route is the whole point. Product lines and cleaning lines share steel, and the plant decides, valve by valve, which liquid goes where. For the draught lines that carry beer to a tap, the Brewers Association deck gives the following cycle:

Brewhouse and cellar CIP in a large plant runs hotter and longer than draught-line cleaning, and the exact strength depends on the formulation the plant buys. Grolsch did not say what strength it uses. The draught-line figures are given here for scale, because they are published and because they describe the low end of what a brewery circulates.

How strong is a caustic cleaning solution?

Strong enough that it does not hide. Sodium hydroxide dissolves completely into sodium and hydroxide ions, so its pH follows directly from how much is in the water. A 2% solution holds about 20 grams per litre. Dividing by the molar mass of sodium hydroxide, 40.00 g/mol, gives 0.50 mol of hydroxide per litre, a pOH of about 0.3 and a pH of about 13.7. The table runs the same arithmetic for the strengths in the Brewers Association deck and for that solution diluted by water.

Two notes on that table. It is ideal-solution arithmetic: at the strongest concentrations real activity pulls the measured pH slightly lower, which changes nothing in practice. And the drinking-water range comes from the U.S. EPA’s secondary standards, which list pH 6.5–8.5 and note that high pH gives water a “slippery feel; soda taste”. Each step on the pH scale is a factor of ten. A cleaning solution diluted a thousandfold by rinse water is still a hundred times more alkaline than water at the top of that range.

That is the useful fact for a plant. Caustic is one of the easiest contaminants there is to detect. A pH strip or a handheld meter at an outlet tells you in seconds whether cleaning solution is present, and the rinse endpoint in the Brewers Association deck is exactly that test: flush until the water is pH neutral, or matches the plant’s own water. Conductivity probes on CIP return lines do the same job continuously, because a hydroxide solution conducts far better than rinse water.

How do food plants keep cleaning solution out of product lines?

With a physical break between the two, not with a closed valve. The clearest public statement of the rule is in the Canadian Food Inspection Agency’s guidance on preventing cross connections in dairy plants, which covers lines carrying milk products or drinking water that may be cleaned while still connected to the cleaning circuit. It opens with the reason: “Improper separation of pipelines has been a factor in the outbreak of milkborne illness in the past.” Then it lists what counts as separation: “physical disconnecting of pipelines, double block and bleed valve arrangements, double seat (mix proof) valves, aseptic barriers, or other equally effective systems.”

The sentence that matters most for the Grolsch case is the next one: “The installation of any number of segregating valves (set of valves with no break to atmosphere) does not constitute a physical break”. Two closed valves in a row are still a connection. A valve can leak past its seat, be opened by mistake or be left in the wrong position, and nothing in the line shows it.

The guidance asks for two further things. First, interlocks: “Use micro-switches or other sensors on valves used for the double block and bleed to signal that the valves are properly positioned for CIP cleaning”, and “The valves move to the fail-safe blocking position with the bleed line open if air pressure or electrical power is removed from the valve solenoids.” That is the kind of technical interlock Grolsch says was missing at its tasting tap. Second, a walk-down: “Look at the actual piping and valves in the establishment to verify that the schematic is accurate and that in actuality, no cross connections exist.”

Grolsch’s fix after the incident, permanently disconnecting the tap, is the first and simplest item on that list.

Which outlets does a CIP plan forget?

The small ones. A cleaning programme is usually drawn around the equipment that makes product: tanks, transfer lines, heat exchangers, fillers. The branches that sit on those lines for other reasons are easier to miss, because nobody thinks of them as product outlets. A short list worth walking with a pH strip in hand:

• Sampling and tasting taps on water, wort and beer lines, used by quality staff or, as at Grolsch, by visitors.
• Hose stations and utility drops that share a header with a line in the cleaning route.
• Drain and vent points that are meant to show leakage. A bleed port that is plugged or piped back into a product line stops being a bleed.
• Temporary connections: hoses and spool pieces installed for one job and left in place.
• Draught towers in a taproom, where the same lines that pour beer are filled with caustic every two weeks.

For each, the question is the same as for a production line: during a cleaning cycle, what physically stops cleaning solution from coming out of this outlet? If the answer is “someone remembers not to open it”, the outlet is not separated. A sign on the tap, a lockout on the handle or a valve sensor tied into the CIP controller each turns memory into a barrier, and a physical disconnection removes the question.

What this means on a specification

A plant buying caustic for CIP is specifying a solution, a procedure and a set of barriers together. The table pairs each with what to write down and where the published basis comes from.

On the make-up row: 50% sodium hydroxide is the common bulk strength because it ships and stores as a liquid, and plants dilute it at the point of use. Lower stock strengths such as 25% or 5% cost more per pound of hydroxide but involve less dilution. Whatever the stock, the dilution should be written down and the strength at use measured, because the pH table above only describes the solution you actually made. Mixing caustic with water releases heat; add the caustic to the water, not the reverse, and follow the safety data sheet for the product in hand.

About supply. This article reports a brewery incident as published by Dutch and English-language news outlets and the brewery’s own statements, and explains general cleaning chemistry. It makes no claim about ours: it does not say what cleaning chemical Grolsch used or who supplied it, and it does not say anything about the source of the sodium or potassium hydroxide we list. Prices and pack sizes are on the product pages.

Common questions

What happened at the Grolsch brewery tour?

On 17 October 2026, nine visitors and two staff tasted water from a sampling tap during a tour in Enschede and fell ill. Grolsch said the line feeding the tap was being cleaned at the time, the tap’s connection had no technical safety interlock, and the liquid was diluted cleaning solution with a small percentage of caustic soda. All eleven were discharged.

Why do breweries use caustic soda?

Caustic soda, or sodium hydroxide, removes the organic residue beer leaves in pipes and tanks: sugars, proteins, hop resins and microbial film. Acid cleaners remove mineral scale instead. Both are pumped through the equipment in a clean-in-place cycle, with water rinses between them. The two are never mixed.

What strength of caustic is used to clean beer lines?

A draught-cleaning presentation hosted by the Brewers Association gives 2–3% sodium or potassium hydroxide at 80°F to 110°F, with at least 15 minutes of recirculation or 20 minutes of static soak. Brewhouse and cellar CIP may run hotter and longer, depending on the formulation.

How do you know caustic has been rinsed out?

Measure it. Rinse until the water at the outlet is pH neutral, matching the plant’s own water. On our arithmetic, a 2% sodium hydroxide solution is about pH 13.7 and still about pH 10.7 after a thousandfold dilution, well above the 6.5–8.5 range for drinking water, so a pH strip or meter shows it clearly.

What is a double block and bleed valve?

Two blocking valves in series with a drain open to the atmosphere between them. If either valve leaks, the liquid runs out of the drain instead of crossing into the other line. Canadian food-plant guidance accepts it as a physical break between cleaning solution and product, with position sensors and a fail-safe closed position.

Sodium hydroxide or potassium hydroxide for line cleaning?

Both are used. The Brewers Association deck lists either at 2–3%. The two do the same job on organic soil at those strengths. The choice usually follows the cleaning formulation a plant buys; whichever is used, its safety data sheet sets handling.

What we stock

Sodium Hydroxide 50% Membrane Grade (Caustic Soda, Lye)

The usual bulk strength, diluted at the point of use.

Sodium Hydroxide 25% Technical Grade

Half the stock strength, less dilution per batch.

Sodium Hydroxide 5% Technical Grade

A dilute stock for small systems and taproom lines.

Potassium Hydroxide (KOH) Technical Grade

The other hydroxide named for line cleaning.

Key numbers and sources