Starter culture preparation is a critical step in quality control during production. Starter culture bacteria have the ability to influence various processes during incubation by the means of their metabolites.
We have already seen that starter cultures are carefully selected microorganisms deliberately added to milk to initiate fermentation to produce the desired products. They are majorly lactic acid bacteria (though other bacteria types also apply), yeasts, and/or moulds.#Startercultures are carefully selected MO deliberately added to #milk to make desired products. Click To Tweet
You can opt to use these microorganisms either singly or in groups. Those cultures that have only one type of bacteria are called single strain cultures while those that have a mixture of microorganisms are called multiple (mixed) strain cultures.
Starter culture selection depends on the following three determinants:
- The conditions of production
- The availability of different forms of the starter to be used
- One’s knowledge about the starter to be used
Choose the correct starter for a given job and subject it to optimum production conditions. This is because starters are the chief determinants of the quality (and the characteristics) of the fermented milk products.
Bulk starter culture preparation
Milk is the best medium for inoculating starter bacteria due to its unique composition, which enables it to nurture bacteria.
You should blend the bulk milk for starter inoculation to reduce the effects of contaminants that may be present in milk from a single farm. Blending also improves the quality of the bulk.
In short, the milk used for producing the starter should possess the following qualities:
- First grade
- Free from inhibitors/antibiotic
- Able to form a smooth and homogenous coagulum
- Clean flavour and odour
- Free from microorganisms that produce compounds that affect lactic fermentation
- Less than 10 cells per gram of spore-forming bacteria
- Milk whose citric acid content is about 2.2 grams per litre (citric acid affects formation of diacetyl)
- Adequate amounts of minerals, especially manganese and vitamins
- Relatively high solid non-fat (SNF) content
- Milk with relatively low content of free fatty acids
Forms of Starter Cultures
Starter cultures generally come in three forms, namely:
- Frozen starters
- Liquid starters
- Dried starters
It is now a common practice in most dairies to use concentrated starters for production purposes.
These concentrated starters present the following advantages:
- Ease of use at the dairy
- Allows for easy management of the bacteriophage
- Facilitaes production scheduling for a fixed number of production days
- Easy to monitor and control before usage
- High quality and well preserved
- They have very good activity
Despite offering numerous advantages, concentrated cultures still have some disadvantages, namely:
- High quality starters are not suitable for the production of certain fermented milk products
- They need adequate storage space with well-monitored temperatures to preserve quality
- They are delicate and quite expensive to ship because of the equipment and conditions involved. If these are not properly observed, they risk losing activity
- The dairies lose control over which starters are selected for producing certain products
Liquid starter culture preparation
The dairies receive the starter in liquid form from the supplier and has to propagate it for about three cycles before using in the production process.
These starters are very delicate to handle and require professional manipulation to avoid messing with the end product. Phages can easily attack them; therefore, they need extremely hygienic environment for production/handling.
Some specialty fermented dairy products still require the use of liquid starters. Liquid starters are also instrumental in some cases where the concentrated cultures are not readily available.
It offers the dairy an opportunity to be independent of the starter producers and the ability to determine the quality of its own products.
The dairy can obtain a seed freeze-dried starter from a producer for producing bulk liquid starter for its internal use. The operator mixes different freeze-dried strains in the batch and varies the growth temperature to differentiate the culture to suit the production needs.
Starter culture preparation by freezing
The dairy can use a concentrated frozen culture for either bulk culture preparation or directly on the milk for production.
Direct application will save the dairy some production costs, as it eliminates the bulk preparation process and its associated expenses in equipment installation.
Even though it saves the dairy the cost of production, the dairy becomes dependent on the starter manufacturer for its own operations.
Steps for concentrated starter culture preparation:
- Prepare the inoculum
- Prepare the starter media
- Inoculate the fermenter
- Incubate at constant pH and keep neutralized
- Harvest the bacteria cells
- Suspend the harvested bacteria cells in cryo-protectant solution
- Freeze the concentrate
- Package the concentrate aseptically
- Store at low temperatures
These starters are majorly grown in milk treated with proteinase, whey permeate, or whey-based media. Make sure to sterilize these media to eliminate any other kind of bacteria, which may adversely affect the culture bacteria and lower the final product quality.
Sterilization also helps in achieving a constant pH of the media. A constant pH is necessary for the growth of the starter bacteria (resulting into higher yield of the cells).
You can harvest the cells by centrifugal separation of the growth media. To protect these cells against damage that may result from the freezing process, it is advisable to use a cryo-protectant solution, which majorly contains milk solids, glutamic acid, and/or lactose.
Freeze the media by using liquid nitrogen because it acts very fast and inflicts minimum damage to the cells.
You can keep the frozen product for a minimum of six months at -196ºC without quality deterioration. It is preferable to transport these starters using liquid nitrogen to assure quality.
However, if you are to use the product in the short term, you can keep it in a deep freezer that achieves low freezing temperatures of -40°C for up to several months.
Starter culture preparation by drying/lyophilization
Freeze-dried concentrates have been in the market for quite sometime now. They have a technically similar production method as the frozen starters; only that the producer freeze-dries them (lyophilization).
Lyophilized cultures have very high cell count per unit volume of the starter than the frozen culture. They are very active such that only about 10 grams of the culture is sufficient to inoculate up to 1000 liters of milk.
These cultures do not need the extreme storage temperatures. You can store them in the ordinary home deep freezers (or fridges) at a maximum temperature of -20ºC for at least five months.
They are therefore much safer to use and very easy to transport to the dairies from the manufacturer as compared to the liquid cultures.
The use of these starters eliminates the need for sophisticated storage systems at the dairies, which drives down production costs at these dairies. However, adoption of these starters make the dairy dependent on the starter producer for its operational needs.
Different Starter Culture Manufacturers
Chr-Hansen Laboratories (Denmark, USA, France)
Centro Sperimentale del Latte (Italy)
Mauri Foods (Australia, UK)
Microlife Technics (USA)
Scandinavian Dairy Associations (Sweden, Norway, Finland)
Wiesby (West Germany, Denmark)
New Zealand Dairy Research Institute (New Zealand)