Module 2. Classification of starters
Lesson 2
STARTERS CULTURES AND THEIR CLASSIFICATION
A starter culture is a microbial culture which actually performs fermentation. Starter preparations assist the beginning of the fermentation process in preparation of various foods and fermented drinks. A number of bacterial and other microbial strains have been used either in single or in combination for producing the desired effect in the finished product.
2.2 Definitions
· Starter cultures are bacterial or fungal strains either pure or mixed, used to initiate a fermentation process.
· Starter culture means selected strains of food-grade microorganisms of known and stable metabolic activities and that is used to produce fermented foods of desirable appearance, body, texture and flavour
· Starter culture means the microorganisms that are selected based on their ability to produce lactic acid for curd production and a low pH to prevent spoilage, produce metabolites that give desirable flavours: or produce enzymes that ripen the dairy product.
2.3 Role/Functions of Starter Cultures
The primary function of lactic starters is the production of lactic acid from lactose. In addition to lactic acid production the starter cultures are also useful in different ways as stated below.
Table 2.1 Functions of starter cultures
|
Function |
Result |
|
Acid production |
Gel formation Expulsion (syneresis) of whey for texturing Preservation of milk Helps in the development of flavour |
|
Flavour |
Formation of flavour compounds like diacetyl and acetaldehyde |
|
Preservation |
Lowering of pH and redox potential Production of lactic acid Production of antibiotics Production of H2O2 Production of acetate |
|
Gas formation |
Eye formation in certain cheeses Production of open texture Ex. blue veined cheese |
|
Stabilizer formation |
Development of body and viscosity Ex. Polysaccharide materials |
|
Lactose utilization |
Reduces the development of gas and off flavours Suitable for lactose intolerant people |
|
Lowering of redox potential |
Helps in preservation Helps in development of flavour |
|
Proteolysis and lipolysis |
Helpful in the ripening/maturation of cheeses |
|
Miscellaneous compounds |
Production of alcohol in kefir and kumis |
2.4 Classification /Taxonomic Groups as Per Bergey's ManualIntroduction
Starter cultures are
generally classified based on their ability to utilize the lactose as shown in
Fig. 2.
Fig. 2.1 Classification of starter cultures
2.4.1 Bacteria
2.4.1.1 Genus lactococcus
Bergey’s Manual of Systematic Bacteriology (1986), combined all the mesophilic lactic acid bacteria (LAB) with Lactococcus lactis to form a single species as they possess
1. Identical isoprenoid quinines and the enzyme β-phosphotase
2. Indistinguishable lactic dehydrogenase
3. Identical percentage of guanine and cytosine.
4. High DNA homology
The only properties that distinguish them are plasmid controlled.
|
Lactococcus lactis subsp lactis |
Acid producer but non-flavour producer |
|
Lactococcus lactis subsp cremoris |
Acid producer but non-flavour producer |
|
Lactococcus lactis subsp lactis biovar diacetylactis |
Both acid & flavour producer |
All the above organisms are mesophilic in nature and their optimum growth temperature is between 25-30°C. All are homofermentative organisms
2.4.1.2 Genus streptococcus
The members of the Streptococcus are Gram-positive organisms that usually form pairs or chains. In 1937, Sherman separated the genus according to physiological and growth characteristics, especially with regards to temperature limitations on growth. Four general groups designated by Sherman are (1) pyogenic, (2) viridans, (3) enterococcus, and (4) lactic. This categorization has become somewhat obsolete as relationships between species have been shown to overlap.
The only species used as starter culture is Streptococcus salivarius subsp thermophilus. This is a yoghurt culture, which is thermophilic in nature with optimum growth temperature of 38-42°C. All are homofermentative organisms.
2.4.1.3 Genus leuconostoc
All are heterofermentative organisms capable of producing lactic acid, CO2 and aromatic compounds (ethanol and acetic acid) from glucose. These organisms are normally used along with lactic acid bacteria (LAB) in multiple or mixed strain cheese starter cultures, which produces flavour compounds.
Leuconostoc cremoris
Leuconostoc citrovorum
Leuconostoc dextranicum
2.4.1.4 Genus lactobacillus
Lactobacillus delbruekii subsp bulgaricus is used for the preparation of yoghurt along with Streptococcus salivarius subsp thermophilus. These two organisms exhibit a symbiotic relationship.
Lactobacillus acidophilus is a probiotic culture, used
for preparation of acidophilus milk and other probiotic
milk products like Bifighurt, Bioghurt,
etc. The members of lactobacillus are classified based on fermentation
of glucose into 3 groups as shown in Figure 2.2.
Fig. 2.2 Classification of lactobacillus based on glucose fermentation
2.4.1.5 Genus bifid bacterium
Found in the gut of infants, intestines of man, various animals and honeybees. These organisms are generally used in preparation of therapeutic fermented milk products in combination with yoghurt, acidophilus milk or yakult starter cultures.
Eg: Bioghurt, Biograde, Bifighurt, Cultura ‘AB, Yakult, Miru-Miru.
Bifid bacterium bifidum, Bifidobacterium longum, Bifidobacterium infantis, Bifidobacterium breve, etc.
The optimum growth temperature is 37°C- 41°C. Anaerobic conditions are essential for optimum growth. Milk fermented with bifidobacteria has a distinctive vinegar taste due to the production of acetate plus lactate from the metabolism of carbohydrates.
2.4.1.6 Genus propionibacterium
Propionibacterium freudenreichii and Propionibacterium shermanii are used in swiss cheese. It has the ability to produce large gas holes in the cheese during ripening / maturation period. P. jensenii, P. thoenii and P. acidipropionici are other organisms present in these genera.
2.4.1.7 Genus brevebacterium
Brevebacterium linens is used as starter culture in preparation of bacterial surface ripened cheeses. It imparts distinctive, reddish orange colour to the rind of (or formation of smear on) Brick and Limburger cheese or Camembert cheese.
2.4.2 Molds
Moulds are used for the manufacture of some semi soft cheese varieties and in some fermented milk products. Moulds enhance the flavour and modify slightly the body and texture of curd.
2.4.2.1 White mold
It is used in manufacture of surface mould ripened cheeses like Camembert and Brie cheeses.
Eg: Penicillium camemberti, Penicillium caseicolum, Penicillium candidum
2.4.2.2 Blue mold
It is used in manufacture of internal mould ripened cheeses like Roquefort, Blue Stilton, Danish blue, Gorgonzola and mycella cheeses.
Eg: Penicillium roquefortii
2.4.2.3 Other molds
Mucor rasmusen – used in Norway for the manufacture of ripened skim milk cheese.
Asperigillus oryzae – used in Japan for the manufacture of Soya milk cheese.
Geotricum candidum– used in the manufacture of Villi a cultured product of Finland. The mould grows on the surface of the milk to form the white velvety layer.
2.4.3 Yeasts
Yeasts are used in the manufacture of Kefir and Kumiss
2.4.3.1 Kefir grains
Kefir grains consist of a mixture of different microorganisms such as Candida kefir,
Kluyeromyces marxianus, Saccharomyces kefir, Torulopsis kefir.
2.4.3.2 Kumiss
The important starter microflora of kumiss include Torulopsis spp.
Kluyeromyces marxianus var lactis, Saccharomyces cervisiae
2.5 Types of Starters
Starters are grouped under different categories based on composition of microflora, growth temperature, type of products, flavour production and type of fermentation into the following categories
2.5.1 Based on the composition of micro flora/ organisms
a. Single: Always used as a single organism in the preparation of dahi or cheese. The only problem is there will be sudden failure of starter due to bacteriophage attack which leads to heavy loss to the industry.
b. Paired compatible strain: Two strains of cultures having complementary activities in know proportion are used. This will reduce chances of culture failures. . In case of bacteriophage attack, only one type of organism will be affected and the other organism will carry out the fermentation without any problem.
c. Mixed Strain: More than two organisms which may have different characteristics like, acid production, flavour production, slime production etc. in unknown proportion are used.
d. Multiple mixedstrain: More than two strains in known proportion are used. The quality and behaviour of these strains is predictable.
2.5.2 Based on the growth temperature
Based on the growth temperature organisms can be divided into mesophilic and thermophilic.
Mesophilic starter cultures: The optimum growth temperature of these cultures is 30°C and they have a growth temperature range of 22- 40°C. The mesophilic starter cultures generally contain the organisms of Lactococci.
Ex. Dahi cultures : Lactococcus spp.
Cheddar cheese: Lactococcus lactis subsp lactis, Lactococcus lactis subsp cremoris Lactococcus lactis subsp lactis biovar diacetylactis, Leuconostoc mesenteroides subsp cremoris
Thermophilic starter cultures: The optimum growth temperature of these cultures is 40°C and they have a growth temperature range of 32- 45°C.
Ex: Streptococcus thermophilus
Lactobacillus delbrueckii subsp bulgaricus
Lb. delbrueckii subsp lactis
Lb. Casei
Lb. Helveticus
Lb. plantarum
2.5.3 Product for which used
Yoghurt: Streptococcus thermophilus
Lactobacillus delbrueckii subsp bulgaricus
Swiss cheese: Streptococcus salivarius subsp thermophilus
Lactobacillus delbrueckii subsp bulgaricus
Lactobacillus helveticus
Lactobacillus casei
2.5.4 Based on the flavour production
The starters are grouped into B, D, BD and N type based on their ability of flavour production
B (L) type: Leuconostocs as flavour producer (old name is Betacocccus)
D type: L. lactis subsp lactis biovar diacetylactis
BD (LD) type: Mixer of both of the above cultures
N or O type: Absence of flavour producing organism
2.5.5 Based on the type of fermentation
The starters are classified as homo or hetero fermenter based on end products resulting from glucose metabolism.
Homo fermentative cultures: eg. Lactococcus lactis subsp lactis
Hetero fermentative cultures eg. Leuconostoc dextranicum