STARTER CULTURES AND FERMENTED MILK PRODUCTS

Module 5. Quality and activity of starters

Lesson 8
ACTIVITY AND PURITY TEST AND STANDARDS FOR STARTER CULTURES

8.1 Introduction

The starter cultures, for the production of a fermented milk product, are chosen based on certain properties. The ideal properties that a good starter should possess should be assessed regularly. The quality control tests include organoleptic, chemical and microbiological tests.

8.2 Properties of Ideal Starters

1. A good starter should have the ability to produce lactic acid at vigorous and steady rate.

2. A starter should be pure without any contaminants

3. It should be able to grow rapidly in suitable organic substances

4. It can be easily cultivable in large quantities

5. It should be able to maintain physiological constancy

6. It should be able to produce necessary enzymes readily and profusely in order to bring about the desired chemical changes

7. It should have the ability to carry out transformation under comparatively simple and workable modifications of environmental conditions

8.3 Activity and Purity Tests of Starter Cultures

Activity and purity of starter cultures are judged by organoleptic, chemical and microbial tests such as

1. Organoleptic tests

2. Chemical tests

3. Microbial tests

4. Purity tests

5. Activity rating tests

8.3.1 Organoleptic tests

Organoleptic tests require high degree of training and experience. These tests measure the qualities i.e appearance, flavour and consistency

8.3.1.1 Body

The body should be like firm custard. The texture should be such that the curd be able to retain its shape during gentle tapping of culture and on pronounced tapping it should break clearly and smoothly.

8.3.1.2 Texture

It should not be lumpy and should not have whey pockets and should be smooth and viscous after thorough agitation.

8.3.1.3 Taste

The taste should be clean and acidic, without bitterness and saltiness.

8.3.1.4 Flavour

Should have typical aroma; be free from off flavours and free from aroma like malty, bitter, rancid, unclean, yeasty, fruity and putrid etc.

8.3.2 Chemical tests

Various chemical parameters used for assessing the activity of starters include

• Titratable acidity
• Volatile fatty acids
• Tests for diacetyl and acetyl methyl carbinol
• Tests for acetaldehyde

8.3.3 Microbiological methods

The microbiological methods used for determining the activity and purity of starter microorganism are

• Plate count test

• Direct microscopic count test: The ratio between rods and cocci can be obtained

• Selective plating techniques

• Tests for the contaminants like coliforms, yeasts and molds (both should be absent in 1 ml of culture). Starter must be free from foreign bacteria, yeasts and molds.

Depending upon the type and number of contaminants, the starter bacteria could be edged out in the competition for essential nutrients. Use of poor quality milk powder or improper heat treatment of the starter media are two possible reasons.

8.3.4 Purity tests

The purity of the starters is evaluated by using microscopic or chemical tests

8.3.4.1 Microscopic examination

This is performed by Grams staining or Newman’s stain. Gram -ve bacteria, spore formers, yeasts and molds and staphylococci etc should be absent. The lactic bacteria should appear as Gram +ve cocci or thin rods.

8.3.4.2 Catalase test

Add few drops of 3% hydrogen peroxide to starter culture. Presence of effervescence indicates possible contamination of the given culture. Lactic acid bacteria are negative for catalase and thus catalase positive test indicates contamination of starters

8.3.5 Activity rating

The starters are evaluated for their activity using different methods

8.3.5.1 Titratable acidity test

The cultures lose their activity due to acid injury if they are allowed to over ripen. A developed acidity in the range of 0.7 to 0.85% LA is optimal.

8.3.5.2 Horral elliker’s test

It is used mainly for mesophilic type of cultures. Prepare and sterilize the reconstituted milk. With sterile pipettes, transfer 0.3 ml of starter culture to be tested into each tube with 10 ml portions of this milk. Place tubes of milk in a water bath and adjust temperature to 37.7°C. Incubate inoculated tubes at 37.7°C for 3 1/2 hours. At end of incubation period, the entire contents of each tube are titrated with N/10 sodium hydroxide and phenolphthalein indicator. A titration value of 0.4 %LA or higher indicates that the culture should be well suited for cheese making. A titration value of 0.30% to 0.35 % LA indicates a slow culture. Starter developing acidity of less than 0.30 % LA invariably graded as inactive as it produces little or no acid during cheese manufacture.

8.3.5.3 Whitehead-cox activity test

Whitehead and Cox have recommended an activity test simulating some basic conditions of starter development in cheddar cheese and have used their method extensively for determining how well suited cultures are for this purpose

• Take 1,000 ml pasteurized milk and adjust to a temperature of 30°C.
• Add 10 ml of culture, to be tested.
• Ripen for one hour or the usual ripening time used, in any specific factory.
• Add 2 ml of rennet.
• Cut the curd when set. The curd is cut with a spatula or knife.
• Heat the curd to the usual cooking temperature, ranging from 38-39°C, or higher, whatever is commonly used.
• Hold the curd at the final cooking temperature for about 1/2 hour, or a little longer.
• Drain the whey and test its acidity. About 25 minutes later, test the whey which has drained from the curd and dump the remaining whey from the container. This is assumed to be equivalent to packing.
• Now test the acidity of whey which has drained from the curd, and this is the final activity test value.

8.3.5.4 Creatine test

The creatine test makes it possible to quickly secure general information on the comparative amounts of acetyl methyl carbinol plus diacetyl. It is a common method used for indicating diacetyl content in flavour producing mesophilic cultures (dahi, lassi, buttermilk, cultured cream, cottage cheese etc.).The cultures form a pink coloured complex with creatine.

8.3.5.5 Dye reduction test

An excellent culture reduces the resazurin dye or methylene blue dye in 35 min and a fairly good culture takes 50 to 60 min.