CHEESE TECHNOLOGY

Module 6. Role of starter culture in cheesemaking

Lesson 13
PROBLEMS ASSOCIATED WITH CHEESE STARTERS

13.1 Introduction

Starter trouble has been known since the beginning of cheese technology, but it has only become acute since pasteurization of milk has been practiced for cheesemaking. This is not because pasteurization ‘weakens’ the milk, but because bulk raw milk is a starter itself; some lactococci will always grow and sour the milk. In pasteurized milk, if for any reason the starter fails, there are too few lactococci left in the milk to permit souring. As already emphasized in the previous lesson, a good starter must produce lactic acid at a vigorous and steady rate. When it no longer does so, the starter is said to be slow or defective.

13.2 Defects in Starter

Sharp acid taste: It is due to over-incubation (due to higher temperature and longer time) of milk and increased rate of inoculum.

Bitter taste: This may come from milk.

Cheesy flavor: This may be due to growth of undesirable bacteria.

Flat flavor: This is caused by less citrate in milk, less number of citrate fermenting organisms or unfavorable incubation conditions for the growth of citrate fermenting organisms.

Uncoagulated starter: This is due to too low/high incubation temperature of starter or poor activity of mother culture or due to the presence of bacteriophage and antibiotics.

Gassiness: This is due to improperly pasteurized milk, post-pasteurization contamination of milk, unsterilized transfer equipment and/or contaminated mother culture.

13.3 Bacteriophage

Phages are viruses or ultra-microscopic organisms, specifically parasitic on bacteria. Bacteriophage causes lysis or destruction of bacteria and can be transmitted from one culture to another. Lysis of bacteria by phages involves the attachment of the phage particles to the cell wall, which is then digested to permit entry and multiplication of the phage in the bacteria.

Preventive measures against phage attack:

• There are two sterilization methods of combating phage in cheese premises: the use of heat or chemicals. Neither method is fully effective unless the plant surfaces and equipment are thoroughly clean.

• Contamination by droplets of moisture containing phage should be avoided in the vicinity of cheese or starter rooms (curd and whey separation).

• Cracked floors should be repaired.

• Hypochlorite solution should be sprayed/applied on the interfaces of the equipment (20-50 ppm for overnight or 200-300 ppm for immediate use of the equipment).

• Starter cultures should be used in rotation.

• Phage-resistant and/or mixed-strain starters can be used.

13.4 Causes of Slow Starters

1. Spontaneous loss of viability inherent in the starter culture.

2. Incompetence in the handling of starter

• Allowing contamination

• Too infrequent sub-culturing

• Use of unsuitable media

• Culturing at inappropriate temperature.

3. Inhibitory causes in the milk itself

• Abnormal milk

• Silage milk

• Seasonal factors

• Conditions in milk adverse to the growth of organism e.g. changes in chemical composition

• Inhibitory bacteria in milk

• Inhibitory substances in milk

• Excessive leucocytes

• Presence of antibiotics

• Presence of preservatives

4. Deviation from standard cheesemaking procedure

5. Bacteriophage action

13.5 Causes of Starter Failures

13.5.1 Intrinsic factors

a) Physiological starter: LAB produces lactic acid at the rate of 10% of their weight. This rapid production of acid, though advantageous in the manufacture, is a marked disadvantage to culture itself.

b) Genetic instability: LAB spontaneously produces mutants which are unable to utilize lactose and become deficient in protease activity. Consequently, they do not grow properly in milk.

13.5.2 Extrinsic factors

a) Variations in processing conditions e.g. temperature, salting rate, etc.

b) Variations in milk composition e.g. due to mastitis, mineral content, period of lactation, etc.

c) Variations in levels of natural and added inhibitors e.g. bacteriophage, antibiotics, detergents and sanitizers etc.

d) Mastitis:

• Changes the chemical composition of milk: decrease in lactose, casein, calcium and acidity

• Changes concentrations of some enzymes, vitamins and bacterial growth factors

• Increases the number of bacteria

• Produces substances toxic to starter organisms

e) Colostrum - Milk drawn up to 24 h after calving does not promote growth.

f) Late lactation - Changes the chemical composition of milk in terms of decreased lactose and increased chlorides.

g) Excessive aeration - LAB grow and multiply on energy derived from the breakdown of milk sugars. This is an anaerobic process. Also, some bacteria are provided with a mechanism which can convert the oxygen of the air to H₂O₂ which in turn is strongly toxic to LAB. Aeration reduces the CO₂ which stimulates growth of many contaminants.

h) Inhibitory bacteria: A large member of microorganisms can produce chemical substances which are inhibitory.

13. 6 Natural Inhibitory Substances in Milk

13.6.1 Antibiotics in milk

Widespread use of penicillin and other antibiotics may lead to the animals, under treatment, excreting antibiotics in their milk to be a cause of starter inactivity. Antibiotics may also be produced by the starter organisms themselves, by one or more strains in a mixed starter culture or by contaminating microorganisms.

13.6.2 Inhibitions by rancid milk

Raw milk rapidly turns rancid, unless cooled quickly because of the presence of an abnormal concentration of lipase. Such rancid milk may be inhibitory to LAB and other microorganisms. Fatty acids (C₈-C₁₂) exert specific effect on the growth of microorganisms.

13.6.3 Lactenins

Lactenins (L1 and L2) are present as natural inhibitory substances in milk. They are inactive under anaerobic conditions and in the presence of -SH compounds. The presence of both fractions is necessary for maximum inhibition, and even then the effect is bacteriostatic rather than bactericidal. Lactenins are destroyed by heating at 74°C/20 min.