Microbiology Of Starter Cultures: Types Of Bacteria

 

A starter culture is a bacteria culture that you can use to manufacture fermented products such as yoghurt, kefir, cheese, salami and butter among many other cultured dairy and non-dairy products.

You inoculate ("seed") the starter culture into the milk/dairy product and allow it to grow/multiply under controlled conditions. This controlled environment will allow the bacteria to multiply and impart the characteristic features of the resultant cultured dairy product such as acidity (or pH), aroma, consistency, and flavor.

Bacteria break down lactic acid in the dairy product, resulting into increased acidity (or low pH). The low pH imparts a preservative effect to the product and improves its nutritive and digestive quality.

What is microbiology?

Microbiology is the study of microscopic organisms such as bacteria, fungi, algae, and the infectious agents at the borderline of life such as viruses. The study encompasses microbiological characteristics such as form, structure, reproduction, physiology, classification, and metabolism.

Microbiology further looks at:

  • their distribution in nature,
  • relationships with each other and other living organisms, and
  • their effect on plants and animals.

For the sake of this brief history of microbiology, we will look at microorganisms related to the food industry.

History of Microbiology

Microbiology is a very old subject. The first person to postulate the existence of microorganisms was Aristotle in 4 B. C. He suggested that living organisms are made up of cells.

It was only until 13th century when people realized that ground pieces of glass provided a greater magnifying power. They were able to see tiny objects that they could otherwise not see through their naked eyes.

Following these developments, Roger Bacon postulated that invisible living creatures cased diseases.

In 1530, Fracastoro of Verona coined the term syphilis to describe an outbreak that ravaged Europe in the 1400's when the returning French soldiers spread the disease.

He called the disease agent 'seminaria morbi' (living germs) that spread 'contagium vivum' (via contact with an individual with the germ).

In 1658, Athanasius Kircher defined the invisible organisms found in decaying bodies, meat, milk, and secretions as worms.

In 1665, Robert Hooke made a powerful compound microscope that he used to confirm Aristotle's postulate that living beings comprise repeating units of cells. That same year, an Italian named Francisco Redi confirmed that maggots are the larval stage of flies.

Antonie van Leeuwenhoek made powerful lenses in 1676 that could magnify up to 3000µm (*300) - 3mm size.

In 1765, Lazzaro Spallanzani boiled meat in hay infusion and covered the broth in an air-tight container. Bacteria could not grow for a longer time.

Later in 1810, Nicholas Appert discovered that bacteria could not grow in foods in air-tight cans. His method of preservation became popularly known as appertization and later as canning.

In 1861, Louis Pasteur confirmed that air contains microorganisms when he cultured cotton wool that he had used to filter air. Louis also discovered the widely popular pasteurization method of food preservation.

Here is the timeline infographic showing a brief history of microbiology.

history of microbiology

Whenever bacteria are mentioned, a bad thing is almost always implied. Bacteria are synonymous with diseases, poisoning, and even death.

As studies on microorganisms continued, the scientists discovered that not all bacteria were harmful. There are also very many beneficial bacteria.

Some of the benefits of microorganisms include:

  1. Production of antibiotics to treat diseases affecting man, animals, and plants. Bacteria can also act as biological pesticides in organic farming.
  2. Probiotics are used to create longevity products such as yogurts and other fermented products.
  3. Bacteria can produce enzymes used in food production/processing. Some cleaning products and dyes rely on bacterial enzymes.
  4. In the food processing industry, bacteria are used to make vinegar from alcohol.
  5. Bacteria play a vital role in maintaining a balanced environmental ecosystem. For instance, nitrogen fixing bacteria help plants harvest nitrogen from the air to improve productivity.

Differentiating D, L, and DL Cultures

Different dairy products have different qualities and distinctive characteristics, which are dependent on the type of culture you use to make that specific product. The cultures may contain a pure strain of bacteria (single strain) or multiple strain type (with many species of bacteria; each strain has its specific role to play in the mixture).

Some starter culture bacteria only ferment lactose into lactic acid. Such strains include Streptococcus lactis, Streptococcus cremoris, and Streptococcus thermophilus. They are majorly used to make acidified dairy products. Other strains such as Streptococcus diacetylactis and Leuconostoc citrovorum produce flavor and aroma as well.

About D, L, and DL cultures

Starter cultures that contain the three stains of Streptococcus lactis, Streptococcus cremoris and Streptococcus diacetylactis to produce both acid and flavor in the dairy product are classified as D cultures (D is for the diacetylactis).

On the other hand, if you opt to use Leuconostoc citrovorum to produce the aroma and flavor in the dairy product, you will end up with an L culture (L is for the leuconostoc).

A combination of both D and L cultures produces a DL culture, which has the qualities of all the bacteria used.

  • D culture: - contains Streptococcus lactis, Streptococcus cremoris and Streptococcus diacetylactis
  • L culture: - contains Streptococcus lactis, Streptococcus cremoris and Leuconostoc citrovorum
  • DL culture: - contains Streptococcus lactis, Streptococcus cremoris, Streptococcus diacetylactis and Leuconostoc citrovorum

List of bacteria present in the starter culture

Bacterial strain

What it does

Used to make

Propionic bacterium shermanii

Flavor/aroma, eye formation

Emmental cheese

Lactobacillus bulgaricus

Acidity, aroma/flavor

Kefir, yoghurt

Lactobacillus lactis,

Flavor/aroma

Cheese

Lactobacillus helveticus

Flavor/aroma

Cheese

Lactobacillus acidophilus

Acidity

Acidophilus milk, cultured milk

Streptococcus thermophilus

Acidity

Yoghurt, cheddar and emmenthal cheese

Streptococcus diacetylactis

Acidity, flavor/aroma

Butter, cultured cream, cultured milk

Streptococcus lactis, Streptococcus cremoris

Acidity

Cheese, butter, cultured cream, cultured milk

Leuconostoc citrovorum, Leuconostoc dextranicum

Flavor/aroma

Cheese, butter, cultured cream, cultured milk

Streptococcus durans, Streptococcus faecalis

Acidity, flavor/aroma

Cheddar cheese, Italian soft cheese

Symbiotic Relations of Starter Culture Bacteria

Some Streptococcus diacetylactis bacteria produce concentrated acid in the product that they do not need the help of Streptococcus lactis/Streptococcus cremoris in acidifying the culture. The bacterial strains are combined because they have a mutual benefit in the culture.

For instance, Leuconostoc citrovorum needs the nutrients (metabolites) produced by Streptococcus lactis/Streptococcus cremoris for its growth. It means that if you eliminate these, the Streptococcus citrovorum will not grow properly in the culture.

The slow growth in the acid-less environment will affect aroma production. Consequently, the quality of the final product will not be consistent. You will fail to achieve a similar product as the one you produce in the presence of acid-producing bacteria.

Bacterial tolerance towards temperature, pH, or salt concentration in the medium affects the products you produce. Control the process to produce the results you want.

The purpose of mixing the strains is to impart the symbiotic advantages to the culture. This will reduce competition among the bacteria in the starter culture. The bacteria characteristics are to complement one another in the process of product formation.

The table below lists some of the tolerance levels for the culture bacteria

Bacterial strain

Optimum temp (°C)

Max % salt tolerance

% acid formation

Ferments citric acid (- no; + yes)

Str. lactis

About 30

4-6.5

0.8-1.0

Str. cremoris

25-30

4

0.8-1.0

Str. diacetylactis

About 30

4-6.5

0.8-1.0

+

Str.thermophilus

40-45

2

0.8-1.0

Leuc. citrovorum

20-25

small

+

Lb. helveticus

40-45

2

2.5-3.0

Lb. lactis

40-45

2

1.5-2.0

Lb. bulgaricus

40-50

2

1.5-2.0

Lb. acidophilus

35-40

1.5-2.0

Where to get starter culture to make fermented dairy products

Various laboratories that specialize in manufacturing starters make special cultures for the production of the various dairy products. These starters can be obtained by special orders form the manufacturers or their vendors. These cultures have been specially mixed to produce the desired effect on the processed dairy products.

 

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Food Scientist | Interested in Data Science for Quality Management | Learning python | Agribusiness consultant with special interest in food processing and quality assurance. | Solve this if you can - if a ship had 26 goats and 10 sheep onboard, how old is the ship's captain?