The Invisible Enemy: Bacteriophage in Cheese and Yoghurt Making.

If you’ve ever had a batch of cheese or yoghurt that just wouldn’t acidify properly — the pH stalls, the curd never sets right, the yoghurt stays thin and sour-smelling rather than thick and tangy — there’s a good chance you’ve met bacteriophage, even if you didn’t know it at the time.

What Phage Actually Is

Bacteriophages (“phages” for short) are viruses that infect bacteria — specifically, in the dairy world, the lactic acid bacteria (LAB) in your starter culture. A phage attaches to a bacterial cell, injects its genetic material, and hijacks the cell to produce hundreds of new phage particles in minutes. The host bacterium is destroyed in the process. Because phages are highly strain-specific, an infection doesn’t kill all your bacteria — it wipes out the particular strain(s) it targets, which is often enough to derail fermentation entirely.

The result: slow or inconsistent acidification, and — at commercial scale — real financial losses from failed batches.

Phages don’t come from nowhere. They live and multiply in liquid — milk, whey, rinse water — and they’re shed continuously by the very starter cultures you’re using. Over time, in any dairy that reuses the same environment and the same strains, phage populations build up in the plant itself. This is why the problem tends to get worse the longer a facility uses an identical culture, not better.

Why This Matters Whether You’re Making 10L or 10,000L

It’s tempting to think of phage as a “big factory” problem, but it affects small producers just as much — arguably more, since artisan operations often:

• Reuse the same bowls, cloths, and moulds batch after batch without proper sanitation
• Use the same single culture or culture blend for months or years because it’s “the one that gives the right flavour”
• Save back whey or use raw milk, both of which can harbour phage picked up from the environment

A single-strain artisan operation that never rotates cultures is, from a phage’s perspective, an easy and stable target. This is often the hidden explanation behind a run of “off” batches that a cheesemaker attributes to milk quality or weather, when it’s actually a build-up of phage in the environment.

The Core Defence: Culture Rotation

The dairy industry’s primary tool against phage isn’t eliminating it (that’s essentially impossible) — it’s staying one step ahead of it through culture rotation.

The logic: phages are specific to particular bacterial strains. If you use Culture A continuously, phages that attack Culture A will build up in your plant. If you then switch to Culture B — genetically unrelated, with a different phage sensitivity profile — those phages have nothing to infect, and their population collapses. Rotate back to a third, unrelated Culture C before switching to A again, and you keep any single phage population from ever getting a sustained foothold.

A well-designed rotation typically uses two to five cultures with different, non-overlapping phage sensitivities, chosen so they still deliver a similar flavour and texture profile in the final product — the customer shouldn’t notice the switch, but the phages should.

Where CHR Hansen Fits In

The reason why Finest Kind stocks CHR Hansen cultures (now part of Novonesis, following its 2023 merger with Novozymes) is because the company is one  of the major global suppliers of dairy starter cultures, and phage protection has long been part of their in depth research:

• Phage-resistant and phage-robust strains. Hansen breeds and selects strains carrying natural phage-defence mechanisms — restriction-modification systems, abortive infection systems, and CRISPR-based defences — that make certain strains inherently harder for common dairy phages to infect.
• DVS (Direct Vat Set) format. Hansen’s DVS cultures are added directly to the vat rather than being propagated in-house through a bulk starter tank. This matters for phage control because in-house propagation is itself a major phage risk point — every time you grow up a bulk starter in an open vessel, you give any phage present a chance to multiply alongside your bacteria. DVS skips that propagation stage entirely.

Practical Steps: Artisan Scale

If you’re a small or farmhouse producer using a single culture (or a Hansen DVS sachet) week in, week out, here’s what actually helps:

1. Get at least 2–3 alternate cultures, chosen to be genetically unrelated to your main one but similar in flavour outcome. Contact Finest Kind for advise on which cultures are phage opposites.
2. Rotate on a schedule, even a simple one (e.g., alternate weekly, or after every few batches). You don’t need an industrial program — consistency of switching matters more than complexity.
3. Tighten sanitation, especially around anything that touches whey: cloths, moulds, draining tables, and any equipment reused batch-to-batch. Hot water and a proper sanitiser cycle do far more against phage than most artisan operations realise, since phages are destroyed by heat and standard dairy sanitisers. For example, rinse with luke warm water, wash with sunlight soap in hot water, rinse well in clean water, sanitise with sodium hypochlorite ( Jik), rinse well and air dry or use paper towel.
4. Don’t reuse whey as a starter or in animal feed near your production area without caution — it’s one of the most concentrated sources of phage in any dairy.
5. Watch for the pattern, not just a single bad batch: if slow acidification recurs specifically with one culture and clears up when you switch, that’s a strong phage signature — not a milk or technique problem.

Practical Steps: Commercial/Processing Scale

For larger operations already running formal HACCP and quality systems, phage management becomes more structured:

1. Formal rotation programs — typically 3–5 cultures per product line, supplied and validated by your supplier specifically for phage diversity, not just flavour. Contact Finest Kind for the phage rotation schedule.
2. Environmental controls — separating whey handling and bulk starter propagation (if used) from the main production area, with dedicated drains and air handling where feasible, such as positive air flow.
3. Validated sanitation protocols — peracetic acid or alkaline detergent cycles specifically verified against phage survival, not just general bacterial load. Ask your sanitation supplier for advise.
4. Testing  —  it is possible to test and identify which phage types are circulating in a given plant and adjust rotation recommendations accordingly; it’s worth doing this rather than guessing.

Which Finest Kind / Chr. Hansen Cultures Need Rotation

Need rotation (primary acid producers, used continuously):

• Flora Danica / CHN-22, R704, R708 (mesophilic, cheese)
• STM5 (thermophilic, Emmentaler)

These do the main acidifying work batch after batch, so phage builds up against them fastest.

Lower priority for rotation:

• YC-X11, YF-L811, ABT-5 (yoghurt) — single-use, direct-to-vat, no bulk propagation stage, so phage builds up much more slowly
• Kefir — a diverse mix of bacteria and yeast, so one phage can’t wipe out the whole culture
• LH-B02 (flavour adjunct) a secondary culture, not the main acidifiers, so a phage hit here doesn’t fail the batch

The Bottom Line

Phage isn’t a sign you’re doing something wrong — it’s simply a permanent feature of working with lactic acid bacteria, at any scale. The producers who suffer least are the ones who assume phage is already present and build rotation and sanitation into their routine as standard practice, rather than the ones who wait for a mysteriously “dead” vat to go looking for an explanation.

If you’re currently running a single culture without a rotation plan, that’s usually the highest-value first change to make — before investing in fancier monitoring or equipment.