Classification of Lactic Acid Cultures

Classification of lactic cultures, is confusing, because many LAB have been renamed. Table 7.1 lists the old and new Latin names for some common lactic cultures.

It is helpful to categorize lactic cultures according to general technological and growth characteristics. From that perspective, LAB are grouped by four criteria, namely:

  • Principal metabolites (end products of fermentation)
  • Optimum growth temperatures: meso- versus thermophilic
  • Starter composition:
    • Pure defined strains
    • Mixed defined strains
    • Pure (single) strains
  • Forms of inoculation

(I) Principal metabolites: homo- versus heterofermentative

Homofermentative means that lactic acid is the principal metabolite without production of gas (CO2) and flavour compounds.

Heterofermentative means that lactic acid is the principal end product of fermentation but technologically significant amounts of one or more of the following metabolites are also produced.

  • Carbon dioxide (CO2 ) which causes the small gas holes in Havarti, Gouda and other cheeses. Gasiness in most cheese varieties is a defect.
  • Short chain fatty acids such as acetic acid and propionic
  • Acetaldehyde, a principal component of yoghurt flavour
  • Diacetyl, a principal flavour note in sour cream, butter milk, Dutch cheese and Havarti cheese
  • Ethyl alcohol

(II) Optimum growth temperatures: meso- versus thermophilic

Mesophilic cultures as the name implies prefer medium range temperatures, rather than cold temperatures (psychrophilic) or hot temperatures (thermophilic).

  • Optimum growth range for mesophyllic cultures is 30 - 35C.
  • Acid production is slow or absent at temperatures less than 20C.
  • Growth is inhibited at temperatures greater than 39C.
  • Generally any cheese which does not require high temperatures to dry the curd will utilize mesophilic cultures. These include Cheddar, soft ripened cheese, most fresh cheese, and most washed cheese.
  • Include both homo- and heterofermentative cultues 

Thermophilic cultures are defined by their ability to grow at temperatures above 40C. With respect to cheese making their important characteristics are:

  • Optimum growth in the range of 39-50C
  • Survive 55C or higher
  • Minimum growth temperature is about 20C below which cell counts decrease rapidly, so, bulk thermophilic cultures should not be stored at temperatures <20C.
  • Thermophilic starters are normally mixtures of cocci and rod cultures which at the time of inoculation are about equal in numbers. That is, the initial inoculum is 50% cocci and 50% rods.
  • Rod/cocci blends grow together in a relationship referred to as 'mutualism' where the overall growth rate and acid production is faster than either culture on its own. The rods produce amino acids and peptides which stimulate the growth of cocci, and the cocci produce formic acid which is required by rods.
  • The balance between the rods and cocci can be controlled by temperature and pH
    • The cocci prefer higher temperatures (optimum about 46C) than the rods (optimum about 39C).
    • The rods are more acid tolerant than the cocci, so, normally the cocci develop the initial acidity and out grow the rods. But, as the acidity increases the rods begin to grow faster than the cocci.
  • Some thermophilic rod cultures have the ability to ferment galactose as well as glucose which is desirable in some cheese, especially Mozzarella.
  • Although yoghurt cultures which include both rod and cocci, produce acetaldehyde which is the principal component of the characteristic yoghurt flavour, none of the thermophilic LAB are considered heterofermentative 

(III) Starter composition:

  • Pure defined cultures are single strain cultures selected from natural mixed populations for specific properties such as proteolytic characteristics or resistance to phage (bacterial viruses).
    • May be rotated to avoid phage infection
    • Have the advantages of uniform rate of acid development and uniform flavour profiles
  • A mixed defined culture is a blend of single strain cultures.
    • May be rotated to avoid phage infection
    • Has the advantages of uniform rate of acid development and uniform flavour profiles
  • Mixed cultures are nonspecific blends of cultures, some what like a natural eco system
    • Normally have complex systems of phage resistance
    • Mixed mesophilic starters are still common, but thermophilic starters are usually mixed defined cultures.
    • Disadvantage is nonuniform rates of acid development from vat to vat and nonuniform flavour profiles.

(IV) Forms of Inoculation

Cultures can be carried and prepared for cheese milk inoculation in one of three general formats:

  • Traditional starters which need several scale up transfers. This system requires some microbiological facilities and expertise and is only feasible for very large plants or perhaps for smaller plants which use mixed strain cultures.
  • Bulk set culture. In this system, the culture supplier does all the purification and transfer work, and delivers a bulk set culture which is used to inoculate a bulk culture, which in turn is used to inoculate the cheese milk. Bulk cultures are the norm in medium to large plants because the cost savings are significant.
  • Direct to the vat cultures require no scale up at the cheese plant. Concentrated cultures ready to inoculate the cheese milk are supplied directly by the culture supplier. 

Table 7.1: Some lactic acid bacteria commonly used in cheese making.

 

Old Name

New Name

Comments

Mesophilic Cultures

Streptococcus cremoris

Streptococcus lactis

Lactococcus lactis ssp cremoris

Lactococcus lactis ssp lactis

  • As a mixed blend these two form the most common mesophilic and homofermentative culture.
  • Used for many low temperature varieties; fresh cheese, Cheddar, American varieties etc.

Leuconostoc citrovorum

Leuconostoc lactis

Leuconostoc mesenteroides spp cremoris

Leuconostoc lactis

  • Heterofermentative cultures; ferment citrate; produce both CO2 and diacetyl.
  • Often mixed with L. lactis ssp cremoris / lactis for traditional butter and butter milk.
  • May be used for cheese with small holes.

Streptococcus diacetylactis

Lactococcus lactis ssp lactis biovar diacetylactis

  • Hetero culture; ferments citrate; produces both CO2 and diacetyl
  • Mixed with homofermentative lactococci for cheese with small holes

Thermophilic Cultures

Streptococcus thermophilus

Lactobacillus helveticus

Streptococcus thermophilus

Lactobacillus helveticus

  • Commonly used coccus/rod blend for high temperature varieties, Swiss and Italian
  • L. helveticus galactose positive, used to reduce browning in Moz, and to promote proteolysis in Cheddar

Lactobacillus bulgaricus

Lactobacillus delbrueckii ssp bulgaricus

  • Commonly blended with S. salivarius. ssp thermophilus for yoghurt
  • Alternative to L. helveticus in high temperature cheese

Lactobacillus lactis

Lactobacillus delbrueckii ssp lactis

  • Alternative to L. helveticus and L. bulgaricus where low acid is preferred as in mild and probiotic  yoghurts