Propagating Starter Cultures

[Sailor Con Queso]

Can you just freeze a mixed culture batch of whey to use as "seed" for a Mother culture? For example, I just made a Stilton with both starter cultures and Penicillium r. What if I take some of that whey and freeze it into cubes. When I need that culture mix just throw a couple of cubes into some skim milk and let it sit until it hits the pH target.

It seems to me that when you are using a mother culture, that it is critical that you know the correct pH target at rennet so that you can adjust ripening times. This goes beyond "add 1/8 tsp Meso culture and let sit for 30 minutes". The time is going to depend on both the concentration of bacteria and their activity level. I know some pH targets, but my spreadsheet is WAY too full of holes. In looking at my data, an average at rennet seems to be around 6.5. I would only feel very comfortable adjusting times for cheddar, and a few others.

[linuxboy]

Freezing whey would work, too, but from a procedure and quality control standpoint, harder to do. How would you ensure the whey had no contamination? How would you account for the variation in viability and concentration? What about the ratio of starter strains to p roqueforti?

For home production, freezing whey should work well, I'm just not sure you would have consistent, and repeatable results. Cheese should still come out OK if you hit the pH targets and if the whey is not infected.

[FRANCOIS]

You should be setting rennet at a fixed delta pH, not a specific pH like 6.5 since your starting pH can vary wildly.   A lot of cheeses use a .1 to .2 delta pH

[krops13]

You should be setting rennet at a fixed delta pH, not a specific pH like 6.5 since your starting pH can vary wildly.   A lot of cheeses use a .1 to .2 delta pH

what does this mean?

[DeejayDebi]

instead of a fix number like 6.5 there should be a range of numbers like 6.5 to 6.6.

[krops13]

how would you know when to use differant ph values?

[linuxboy]

Delta means change. So instead of a fixed pH, you wait for the pH to drop .1-.2 before renetting.

[Michelle]

Krops13: "Ok sooooooooo when it gets to the ph it likes (im using just meso right now) it will start to reproduce".

Actually it is the opposite.  It's the bacteria that cause the drop in pH, not the other way round.

Most bacteria (including your starter) prefer to grow at a neutral pH.  As the bacteria grow they metabolise the milk sugars into lactic acid, dropping the pH.  As the pH drops, the milk becomes more acidic, and this is inhibitory to the bacteria.  So when the pH gets low enough the bacteria actually stop growing.

In the diary industry where they want to grow the starter bacteria concentrations to very high levels they will actually add alkali to the starter vats to raise the pH when it gets too low.  The bacteria then reproduce more and the acidity drops again.  They will dose alkali several times until they get to the concentration of starter bacteria they need.

I guess this is not something most home cheese makers will do but just thought you might be interested!

Cheers,
Michelle

[linuxboy]

I remembered this thread and wanted to make one more comment. Michelle is absolutely right, pH is raised, usually with ammonium hydroxide, to produce DVI culture that has high enough CFUs per gram. At home or in your small lab, if you want to try and bump up the CFU count, you can try using milk with a higher casein content. This is because casein is a buffer and will absorb the acid produced.

A good balance between pourability and casein content is 11% protein. Meaning use skim milk that should have something like 3-5% protein, and add nonfat dry milk to bring it up to 11%, then UP the mixture by boiling it, let it cool to target temp, and inoculate. You should be able to get about double the CFUs as with regular milk. And remember to chill the whole thing as soon as you reach target pH, per the targets I mentioned earlier for thermo and meso, to prevent the acid from killing the viable bacteria.