In the early part of my career I was employed at various times as a bulk starter culture maker, cheese maker and laboratory technician. I learnt very early the dread words, “the starter is going slow it must be a phage attack” I can remember many times as a lab tech getting ready to go home and being brought whey samples by the cheese factory manager saying “start a Pearce test on these please”, starting the test and then either he or me coming back 6 hours later to read the results.

So what are bacteriophages and how can we prevent them having a serious impact on cheese making?

Bacteriophages are a virus that attacks a specific strain of bacteria. Their mode of action is to inject their DNA into a living cell. The phage then replicates itself and the cell then bursts releasing a number of phage, these then attack many other cells. The result is that the phage multiplies much more rapidly than the bacteria being attacked. The resultant impact on cheese making is that acid production capacity is greatly reduced. This can result in no acid production at all (a dead vat) or poor quality low acid cheese.

Prevention

1. Hygiene, phage can only grow when they have a food source, therefore it is essential to regularly hose whey off the floor and remove spilt curd. Regular sanitising of the floor also will help.
2. Limit personnel access from raw milk areas to pasteurised milk. Phage will be present in raw milk but are destroyed by pasteurisation. Do not allow tanker drivers into cheese making areas. Ensure footbaths from raw to pasteurised areas are cleaned and refilled with sanitising solution regularly.
3. If making bulk culture limit entry of personnel to the bulk culture area. Maintain this room in an immaculate condition.
4. Starter selection

• Bulk Culture; ensure cultures used in a rotation are not phage related. Have a rotation of either 2 or 3 cultures or groups of cultures that have different phage relationships.
• Conduct activity tests daily to check if phage is building up, activity tests can detect low levels of phage enabling changes to be made before the acid production is slowed in the vat.
• Direct Vat cultures, 
  • Undefined cultures, these mixes have a large range of cultures that may vary slightly from batch to batch, because of the large number if 1 strain is attacked by a phage there is plenty of acid production still available from the other strains.
  • Defined cultures. These contain a smaller number of selected strains which makes them more susceptible to a phage attack; it is recommended that different strains are used in rotation.