Starter Cultures and Fermented Milk Products

Module 3. Propagation of starter cultures

Lesson 3
PURPOSE OF PROPAGATION, TRADITIONAL METHODS-ADVANTAGES AND LIMITATIONS

3.1 Introduction

Starter propagation is the most important operation of the quality control unit of dairy plant as the quality of starter is having a direct bearing on the quality of finished product. The main aim of propagation is to maintain pure cultures and activate cultures without any loss of viability . The culture organisms are preserved in small quantities known as‘stock cultures’. Fermentation process of any cultured dairy product relies on the ‘purity’ and ‘activity’ of the starter culture.

An active starter must have the following characteristics

1. Must be active

2. Must contain maximum number of viable organisms

3. Must be free from contaminants

To obtain the above qualities of culture

1. The inoculum (Inoculation) is carried out under aseptic conditions

2. Growth is initiated in a sterile medium

3.2 Traditional Method

The traditional method is also known as simple microbiological technique. In this method the starter are propagated in the laboratory itself taking the stock cultures which are in in-active state or dormant state. Commercial manufacturers provide starter cultures in lyophilized (freeze-dried), frozen or spray-dried forms. These cultures cannot be added directly in the milk to initiate fermentation process because they are in an in-active state. They need to be brought in to active form and also scale up to prepare bulk quantities.

The dairy product manufacturers need to inoculate the culture into milk or other suitable substrate. Number of steps are to be followed during propagation of starter culture for ready to use. They can be seen from the following:

I. Stock culture: can be obtained from

1.Research establishments

a.WDCM - The World Directory of collection of cultures of microorganisms.

Situated at the National Institute of Genetics, Shizuoka, Japan

http:// wdcm.nig.ac.jp

b. NCDC - National Collection of dairy Culture- NDRI ,Karnal.

c. ATCC -American Type Culture Collection

P.O Box 1549,Manassa, VA 20108, USA

Web: http:// www.atcc.org.

d.ECCO - European Culture Collections Organization

e.NCFB - National Collection of Food Bacteria-UK

f.NCIM - National Collection of Industrial Microorganisms- Pune

g.NIZO - Netherlands Dairy Research Institute

2. Educational Colleges

3. Cultural stock Organizations

4. Commercial manufacturers

a. Chr.Hansen laboratoriam-Harsholm, Denmark. Specialized in DVS (Direct vat Set) cultures and ready set cultures. In India- ESDEE chemocrats, 46 White Hall, 143 A, Kranti Marg, Mumbai-400 036

b. Wisby laboratoriam - Tonder , denmark.Specialized in DIP (Direct-in-product) cultures for direct inoculation of milk.In India- Food & Pharma specialities, D-22, Nizamuddin East, New Delhi

c. Gist-brocades- Delft, TheNeteherlands- In Australia & USA

II.Mother culture - (first inoculation): allcultures will originate from this preparation.

III.Intermediate culture (feeder) –is used in preparation of larger volumes of prepared starter.

IV.Bulk starter culture - this stage is used in dairy product production.

3.2.1 Scale up system of propagation

Formula :

Stock and mother cultures are propagated in laboratory and feeder and bulk cultures in the starter room of the dairy plant.

Fig. 3.1 Starter propagation steps

Different stages of starter propagation are

1. Selection of milk

2. Treatment of milk

3. Inoculation

4. Incubation

5. Cooling of starters

3.2.2 Selection of milk

Milk is a good medium for the propagation of starters because it gives better and milder flavour,better viscosity. The milk meant for the starter propagation should be of high quality.

• Milk of first grade should be used
• Milk of abnormal quality i.e. mastitis milk or colostrums milk or late lactation milk should not be used
• Milk should be free from antibiotic residues
• Milk should be free from bacteriophage
• Milk should not contain residues of detergents and sanitizers
• Milks should have a low bacterial load
• Milk should have high SNF content
• Milk should have clean flavour and odour

Milk of above characteristics is difficult to obtain and hence the NFDM (SMP) is used for the propagation of starters as it avoids daily variations. Flavour and odour defects are easy to detect if skim milk is used for the starter propagation.Skim milk with a total solids content of 10-11 % is desirable for the propagation of starters.

3.2.3 Treatment of milk

Milk is usually subjected to heat treatment before using for the inoculation of starters.

The purpose of heat treatment is

• To inactivate harmful organisms and bacteriophage in milk
• To inactivate the natural germicidal properties of milk such as immunoglobulin, LP system, lactoferrin and lysozyme.
• To reduce the oxidation reduction potential of milk by driving out the dissolved oxygen
• To denature the proteins to make available the nitrogenous compounds for the bacteria and also formation of SH compounds which in small concentration act as stimulants for the growth of starters.
• To improve the viscosity of the finished culture due to denaturation of whey proteins by improving their water retention properties to minimize the wheying off.

A heat treatment of milk to 90°C for 1 hour or boiling/steaming for 30 min is usually preferred. Autoclaving at 121°C for 15 min is also followed but this may result in the formation of curd with soft and sloppy body and texture. It may also give rise to scorched flavour making the curd difficult to judge. Tyndalization of reconstituted skim milk is the best.

Tyndalization

a.First day the reconstituted skim milk is autoclaved at 110°C under 10 lbs pressure for 10 minutes. The milk samples are kept at room temperature so that the spores which escaped the heat treatment may geminate and cause curdling of milk if they are present in large number.

b.On the second day the samples are examined for any curdling. The curdled samples are discarded and the other samples are steamed for 30 minutes.

c.After steaming the milk samples are kept at room temperature.

d.On the third day the samples are again examined for any curdling. The curdled samples are discarded and the other samples are again steamed for 30 minutes.

e.The samples so sterilized are either kept at room temperature or in refrigerator till they are used.

3.2.4 Inoculation

Inoculation is preferred with sterilized equipment. Inoculation has to be carried out in separate rooms with UV lamp designated for this purpose or by using Laminar flow system. Freeze dried ampoule is sterilized with alcohol and liquid culture tubes are shown to flames.

The amounts of inoculums depend on the activity of starter, temperature of incubation and time of incubation. Normally 0.5% to 2.0% is used subject to variations depending upon the situation. Possibilities of external contamination are minimized as far as possible by working quickly.

3.2.5 Incubation

Incubation temperature depends on the amount of inoculum and on the type of starter culture. For the mesophilic organisms the temperature is 21-22°C for 14-16 hours and for the thermophilic organisms it is 41-43°C for 3 -4 hours. Change in temperature may affect the composition of starters in mixed population.

3.2.6 Cooling

After incubation the culture is cooled to stop further development. Refrigeration appears to give appropriatecooling effect.

The drawbacks in traditional method of propagation is

1. Time consuming

2. Requires skilled operators

3. May lead to contamination by bacteriophage.