Firstly, it should also be said that you can make cheese without using cultures, but it will lack a complexity of taste.

Starter cultures contain strains of bacteria that develop acidity and promote ripening and therefore flavours.  The culture continues to influence the flavour during its ripening phase too.  There are two types of culture, starter cultures and secondary cultures.  

Starter cultures contain lactic bacteria, and their function is to perform lactic fermentation, to convert the sugar (lactose) in milk into lactic acid.  Selection of a starter culture is based on the style of cheese been made.  Starter culture bacteria are classified into two categories: mesophilic and thermophilic.  Mesophilic bacteria are active at lower temperatures (ideal temp 25°c to 30°c) but become inactive over 40°c.  Thermophilic bacteria are active at higher temperatures (ideal temp 35°c to 45°c) but become inactive over 60°c.  You can use both culture types together as the mesophilic culture will impart its influence in the initial stages of making the cheese and as milk is heated further the thermophilic culture will take over.  If the milk is never heated over 40°c the mesophilic culture will then start ‘working’ again as the cheese cools and is aged. 

Secondary cultures are made up of bacteria, moulds and yeasts.  They are added after starter cultures and before coagulants.  They will have a specific task such as development of a specific rind or the growth of blue veins.  Common secondary cultures include Geotrichumcandidum, Penicillium candidum, Proprionicbacterium, Penicillium roqueforti and Brevibacterium linens. 

Choosing a culture/s to use can seem very confusing as manufacturers around the world will create their own blends of bacteria and give this blend a name or code number and then recipes will quote this code or name.  This is like a recipe asking you to use Griffins ginger nuts rather than just ginger nuts.  You wonder what the difference is between Griffins ginger nuts and just generic ginger nuts?  You must look at the ingredients that have been used to make the two products to see if there is a difference.  The same goes for cheese cultures, you need to look at the ingredients, that is what bacteria have been used to create a particular blend.  In this way blends of cultures are easily decoded.  On my website I list the bacteria that are in each blend that I sell to enable you to make a match with a branded culture listed on a recipe that you may want to try. 

The same blend of cultures can have many different names too.  At first glance it may look like there are hundreds of different cultures available and it can feel overwhelming when in fact it is the same blend of bacteria that has been branded withmany different names.  M244 is in fact the same as Flora Danica.  Also, manufacturers will create a blend of bacteria to make a specific cheese.  This can be replicated very easily by combining cultures to give the same blend. 

A quick word also needs to be said on strengths of cultures.  They are all not the same and you will see this in the amount of culture that is in a tube or packet.  Units are also quoted to help with this, with a ‘unit’ representing 10 litres of milk.  A full 8 unit tube of Flora Danica has enough culture to inoculate 80 litres of milk.  A less than half full 10 unit tube of R704 has enough culture to inoculate 100 litres of milk.  And they are both mesophilic cultures.  So,for ease of simplicity, If a recipe asks for 1tsp of mesophilic culture use 1 tsp of Flora Danica or 1/3rd tsp of R704.  Using excess culture creates more acid which has an influence on flavour.  If you cheese tastes a little sour you could try using a little less culture next time.