Starter Cultures

 Starter cultures are those microorganisms that are used in the production of cultured dairy products such as yogurt and cheese. The natural microflora of the milk is either inefficient, uncontrollable, and unpredictable, or is destroyed altogether by the heat treatments given to the milk. A starter culture can provide particular characteristics in a more controlled and predictable fermentation. The primary function of lactic starters is the production of lactic acid from lactose. Other functions of starter cultures may include the following:

  • flavour, aroma, and alcohol production
  • proteolytic and lipolytic activities
  • inhibition of undesirable organisms

There are two groups of lactic starter cultures:

  1. simple or defined: single strain, or more than one in which the number is known
  2. mixed or compound: more than one strain each providing its own specific characteristics

Starter cultures may be categorized as mesophilic, for example:

  • Lactococcus lactis subsp. cremoris
  • L. delbrueckii subsp. lactis
  • L. lactis subsp. lactis biovar diacetylactis
  • Leuconostoc mesenteroides subsp. cremoris 

or thermophilic:

  • Streptococcus salivarius subsp. thermophilus (S.thermophilus)
  • Lactobacillus delbrueckii subsp. bulgaricus
  • L. delbrueckii subsp. lactis
  • L. casei
  • L. helveticus
  • L. plantarum

Mixtures of mesophilic and thermophilic microorganisms can also be used as in the production of some cheeses.

Please see further details in our cheese technology section.

Bacteriophage

Bacteriophages are viruses that require bacteria host cells for growth and reproduction. Initially, the bacteriophage attaches itself to the bacteria cell wall and injects nuclear substance into the cell. Inside the cell, the nuclear substance produces shells, or phage coats, for the new bacteriophage which are quickly filled with nucleic acid. The bacterial cell ruptures and dies as the new bacteriophage are released. 

Bacteriophages are ubiquitous but generally enter the milk processing plant with the farm milk. They can be inactivated heat treatments of 30 min at 63 to 88° C, or by the use of chemical disinfectants.

Bacteriophages are of most concern in cheese making. They attack and destroy most of the lactic acid bacteria which prevents normal ripening known as slow or dead vat.

Starter Culture Preparation

Commercial manufacturers provide starter cultures in lyophilized (freeze-dryed), frozen or spray-dried forms. The dairy product manufacturers need to inoculate the culture into milk or other suitable substrate. There are a number of steps necessary for the propagation of starter culture ready for production:

  1. Commercial culture
  2. Mother culture - first inoculation; all cultures will originate from this preparation
  3. Intermediate culture - in preparation of larger volumes of prepared starter
  4. Bulk starter culture - this stage is used in dairy product production

Please see further details in our cheese technology section.