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:
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.