Author
Cheese Lab
Every commercial cheesemaking dairy has a quiet fear. The vat is filling, the culture has been pitched, and at the forty-minute mark the pH is wrong. Not dropping. The starter has stalled. If the dairy is large and the vat is a tonne of milk, the cost of that vat is measured in thousands of euros.
In almost every case, the answer is bacteriophages.
What a phage is
A bacteriophage is a virus that infects bacteria. It does nothing to humans or cheese directly — it cannot cross species boundaries — but it attacks lactic acid bacteria with almost surgical specificity. A phage that infects Lactococcus lactis will latch onto surface receptors on the cell, inject its genetic material, hijack the cell's machinery to produce hundreds of new phage copies, and lyse the cell open when it is done. Multiply that by a billion cells over a few hours and the starter population collapses.
Phages are everywhere in dairies. They ride in on raw milk, on air currents, on whey dripping from a filter, on the boots of a worker walking between rooms. A dairy that has produced cheese for a decade has built up an ambient phage community perfectly matched to the starters it has been using. This is why phages are the number-one source of starter failure in industrial cheesemaking.
How the industry actually copes
- Phage-resistant strains. Modern commercial starters are screened against libraries of known phages. Resistant mutants are isolated, characterised, and blended.
- Rotation schedules. Rather than use the same starter for every vat, dairies rotate through three or four different starter blends on different days. If a phage gets a foothold against Monday's blend, Tuesday's blend is different enough to shake it off.
- CRISPR systems. LAB carry their own CRISPR arrays that acquire phage-derived sequences as memory and use them to recognise future infections. This natural defence — the same system now famous for gene editing — first drew serious attention in dairy.
- Hygiene hurdles. Strict cleaning, whey separation from the milk line, compartmentalised rooms, positive air pressure in culture rooms.
Why a home cheesemaker should care
Phage failure on a home scale is rare but not unheard of. If a hobbyist's cheese consistently fails to acidify on the same culture — especially after a long period of making the same style — phages are worth suspecting before ingredients or equipment. The fix, almost always, is to switch to a different starter blend for a few batches. The phages that attack one blend usually cannot touch another.
Bacteriophages and the Industrial Problem Behind Starter Rotation covers the mechanisms and history in detail.
Most cheesemaking failures get blamed on technique. A surprising number of them are an invisible arms race playing out in the vat.