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From the Field

What do the values on my phage report mean? When should I be concerned?

Phage reports and their meanings can be confusing to some. What you see in the field is a numerical value of how severe the culture was hit (if at all) by any phage virus in your dairy plant environment. We all know that phage is a virus that attacks the good lactic acid bacteria that are trying to grow by injecting their viral DNA into the lactic acid cell’s DNA, effectively taking it over to produce hundreds of new phage cells. These phage cells attack new lactic acid bacteria at a rate of hundreds to one, effectively killing the lactic acid producing bacteria and their ability to produce lactic acid to lower the pH of your cultured dairy product. Lactic acid production is what lowers the pH of your fluid dairy product to make yogurt, buttermilk, sour cream or cottage cheese. In the case of cottage cheese, we need rapid and controlled growth from the lactic acid bacteria to insure consistent set times and to lower the pH of the skim milk to the cut pH to cut the coagulum so you can make cottage cheese curd.

Here is a brief explanation of what the results mean from the phage report: 

  1. The phage report is communicated as a numerical value from 0.5 to 6 in half point increments, although there is a story behind that number. A 0.50-1.0 level is considered a low level and usually nothing to be concerned about. However, repeated weekly phage hits on that specific strain number means the phage virus is building up on that strain and could cause incubation slowdown in the cheese vat (long set times) that results in future moderate hits, high hits or vat failure. Any hit should be monitored for future hits that may reoccur and progress to the moderate or high level.

  2. A level of 1.5 to 2.5 is considered a moderate or medium phage hit level. This level is concerning as phage are building up on that specific strain and is most likely causing some slower culture growth during incubation in the cheese vat (lengthening set times by 30 to 60 minutes or more), although that depends upon the culture strain affected. More on that later. This level is one in which close attention should be paid to the cheese room or sour cream, buttermilk, or yogurt incubation areas and it is recommended to do extra environmental cleaning and chlorine fogging. It is an indication that continued use of that culture strain most likely would result in further slowdown in the vat (longer set times) and possible vat failure if that strain is not rested for a couple of weeks so it can recover. When you rest a strain and take it out of rotation, you are not exposing it to your plant environment anymore for the resting period. Without exposure, the phage virus cannot infect the host lactic acid bacteria cells anymore on that stain to produce more phage virus cells. Therefore, the buildup against that strain should be less.

  3. A level of 3.0-6.0 means extreme culture inhibition and slowdown during incubation, and could result (or has resulted) in complete vat failure in this or perhaps the next use. A level of 6.0 means no growth at all and total culture death, which means most likely you have lost a vat of skim milk unless you were able to save it with a backup culture and had to dispose of that cheese skim. We try to make changes before phage ever builds up to this stage. This level causes loss of product and money, so we try to take measures to prevent this when the phage starts in the moderately high phase mentioned above.


Now for the story behind the numerical value: The numerical value on the phage report is a weighted average or composite of the individual strains in that culture blend. The species type of each strain, along with the quantity of that kind of bacterial strain in the culture blend, determines the phage value. Important acidifiers might be given a higher composite contribution level depending upon whether those strains are the ones hit by phage in the blend.

For example, let’s say a culture strain blend has five strains in it. All produce lactic acid (which is what lowers the pH, makes the product tart and brings you to cut pH or break pH), but at different speeds or rates of acid production. As cheesemakers know, L. cremoris grows slower than L. lactis. Let’s say this blend has four cremoris strains and one lactis strain. You would think that each strain would contribute 20% to the weighted average of the phage hit on the report if one or more than one individual strains were hit by the phage virus. But that’s not the case. Because the lactis strain is a faster or more important acidifier, it gets a higher weighted average. So let’s suppose in this example it contributes 30% to the value represented on the phage report. The other four cremoris strains then contribute 17.5% to the total value. What you see on the phage report is an average numerical value based on which strains were hit, their weighted average as far as their importance of acid production, and that’s not even considering the quantity of those strains in the blend. Therefore, a phage hit of 1.5, for example, may not look that bad considering the average, but let’s look at that more closely. 

The 1.5 on the phage report may show phage hits on two of the cremoris strains and none on the lactis strain. This is good because the lactis strain is the more important or faster acidifier in the culture blend. Even though two of the cremoris strains were hit that day, the cheesemaker has not noticed any slowdown in the vat and the incubation times have been unaffected. Let’s suppose the two hits on the cremoris strains were moderate at 4.0. The weighted average reported on the phage report comes out a 1.5 because 17.5% of 4 = 0.70 x 2 = 1.4. (rounding up to the next ½ increment gives a 1.5 on the phage report). 

But let’s take another example. You are using the same culture strain with five different cultures in it … four cremoris and one lactis strain. On this day, the phage report shows a value of 2.0. But this time it is different because none of the cremoris strains were hit that day but the lactis strain was hit with a 6.0 value (or showed no growth at all – in other words, phaged out). This 2.0 reported on the phage report is more serious since the lactis strain could be the fastest and main acidifier in the culture blend. The weighted average comes out at 1.80 (rounded off to a 2.0). You think this might not be serious, but in fact it was, and your cheesemaker already started to see slower culture growth in the vat or slower set times. Fortunately, in this case there were four more unaffected strains (all cremoris) in the blend to still reach your cut pH and make cheese with, albeit at a slower acid production rate. 

My colleagues and I watch which strains that are being hit and will notify you when we feel trouble is about to start. That usually means replacing the hit culture strain blend with another strain number and resting the hit strain for 2-4 weeks. However, if trouble starts suddenly, your cheesemakers probably already know when trouble has started with a slow vat, reflected in a slow set time for that day. Don’t be afraid to rest a culture strain and switch to another strain. It is better to be safe than sorry! 

Although no one can prevent phage in their dairy plant, staying on top of them by utilizing the phage report to continually resist their advances is a useful tool in keeping your plant (and culture strains) running smoothly. Preventative measures such as chlorine fogging and proper floors, walls, and drain sanitation can keep your plant trouble free for many weeks or months.