Pectinatus and Megasphaera in packaged beer

Bacteria that need no oxygen

Most brewery hygiene thinking grew up around organisms that tolerate some oxygen. Pectinatus and Megasphaera do not. They are strict anaerobes, meaning oxygen harms them. As brewers got better at keeping dissolved oxygen low in packaged beer, these bacteria found a more comfortable home.

Both stain Gram negative. Pectinatus forms curved rods that move with a characteristic wriggle. Megasphaera forms large cocci, often in pairs or short chains.

What they do to beer

The signs are hard to miss:

• Pectinatus produces propionic and acetic acids plus hydrogen sulphide and other sulphur compounds. The beer smells of rotten eggs and turns hazy.
• Megasphaera produces butyric acid, which smells of rancid butter or baby sick, plus caproic acid and hydrogen sulphide.

These off flavours are strong enough that a drinker usually rejects the beer on the first sniff. A spoiled pack can also gush or swell as gas builds. Our guide to oxygen pick up in packaging explains the low oxygen conditions they exploit.

Which beers are most at risk

Pectinatus and Megasphaera favour beers with:

• Lower alcohol, commonly below about 5% ABV.
• Higher pH, roughly above 4.2.
• Very low dissolved oxygen.
• Residual sugars and no pasteurisation or sterile filtration.

That describes many session lagers and lower alcohol beers sent out unpasteurised. Non alcoholic and low alcohol beers, with little ethanol to slow growth, carry even higher risk. Warm storage in Indian distribution speeds growth: both organisms grow well in the range of about 25 to 35 °C.

Where they hide

Pectinatus and Megasphaera are not usually found in raw materials. They live in biofilms in the packaging area: filler bowls, filler valves, conveyor lubricants, drains, crowners and can seamers. Biofilms protect them from oxygen, which is why they can survive in an area that looks open to air.

Drains and the floor under the filler are frequent sources. Splashing from cleaning those areas can carry them onto the filler. Packaging hygiene matters more here than brewhouse hygiene.

How to detect them

Because they are strict anaerobes, standard aerobic plates miss them completely. Use:

• NBB medium or another beer spoiler medium, incubated anaerobically.
• Membrane filtration of large volumes of packaged beer.
• Forcing tests: holding packaged beer at around 30 °C for one to two weeks and checking for haze, gas or smell.
• PCR, which is useful because these organisms grow slowly and are hard to culture.

Swabbing filler parts and plating them anaerobically helps find the reservoir before it reaches beer. Our guide to PCR testing covers the faster route.

How to protect packaged beer

The three lines of defence are hygiene, process and stability:

• Hygiene: clean filler parts daily, take apart what can be dismantled, keep drains clean and avoid spraying drains near open packaging.
• Process: pasteurisation or sterile filtration removes the risk for higher risk beers.
• Stability: cool storage and short shelf life limit growth if a few cells get through.

Flash pasteurisation and tunnel pasteurisation both kill these organisms at normal brewery settings. Our guide to sterile filtration covers the alternative.

Low alcohol and non alcoholic beer

The growth of low and no alcohol beer makes these organisms more relevant. A beer at 0.5% ABV has almost none of the ethanol that slows bacterial growth. It may also carry more residual sugar, especially if it was made by arrested or limited fermentation. That combination is close to ideal for Pectinatus and Megasphaera, as well as many other spoilers.

Most producers of non alcoholic beer pasteurise or sterile filter for this reason. Relying on hygiene alone is a much bigger gamble than with a 5% lager.

If you plan to launch a low alcohol beer, build the microbiological controls into the plan from the start. Decide on pasteurisation or sterile filtration, set a forcing test as part of release and agree storage conditions with distributors. Retrofitting these after a spoiled batch is more expensive than planning them first. Our guide to pasteurisation units explains how heat treatment is measured.

Planning for the risk

A brewery packaging unpasteurised low alcohol beer for distribution across hot regions should treat these organisms as a real risk. That means anaerobic plating of packaged beer, swabbing the filler and a forcing test as part of release.

Deciding which beers need pasteurisation, which can rely on hygiene alone and how to test both is the kind of trade off we work through on the advanced course.

Your first step: put three packs from your next batch in a warm cupboard at around 30 °C for two weeks and smell them at the end.

Common questions

What does Pectinatus smell like in beer?

Rotten eggs, from hydrogen sulphide, along with sour notes from propionic and acetic acids.

What does Megasphaera do to beer?

It produces butyric acid, which smells like rancid butter or baby sick, plus hydrogen sulphide.

Which beers are most at risk from Pectinatus and Megasphaera?

Unpasteurised beers with low alcohol, higher pH and very low oxygen, especially when stored warm.