CHEESE TECHNOLOGY

Module 6. Role of starter culture in cheese making

Lesson 12
STARTER CULTURES

12.1 Introduction

The use of starter cultures is an essential requirement in the manufacture of most cheeses. A cheese starter may be defined as a milk culture containing selected lactic acid bacteria, usually Lactococcus lactis subsp. lactis, Lc. lactis subsp. cremoris, Lc. lactis subsp. diacetylactis, which convert about 1% (absolute) of lactose in milk almost entirely to lactic acid, along with very little amounts of byproducts such as acetic acid and carbon dioxide. Acid production at an appropriate rate and time are the key steps in the manufacture of good quality cheese.

Milk sours on storage, but if the milk is heated, the souring is delayed because the acid-producing bacteria present in milk are destroyed by heat treatment. The course of deterioration then becomes not a clean, wholesome souring, but a change which may include souring but also often produces taints, protein and fat break down and sometimes even putrefaction. It is, therefore, essential to re-inoculate heated milk with acid producing organisms if clean, sour milk is desired. Starter cultures are intentionally added to cheese milk to initiate and accomplish the desired fermentation. It ensures consistent souring at a controllable rate, gives desired clean lactic flavor and helps to suppress any tendency for taint-producing microorganisms to grow in milk. Acid production affects several aspects of cheese manufacture, e.g.

1. Denaturation and retention of the coagulant in the curd during manufacture and hence the level of residual coagulant in the curd. This influences the rate of proteolysis during ripening and may affect cheese quality.

2. Curd strength, which influences yield.

3. Extent of dissolution of colloidal calcium-phosphate.

4. Gel syneresis, which controls cheese moisture and hence regulates the growth of bacteria in the cheese. Consequently it strongly influences the rate and pattern of ripening and the cheese quality. Acid production, combined with milk heating and stirring of curd-whey mixture causes the casein curd to shrink and expel moisture from the coagulum.

5. The rate of pH decrease determines the extent of dissolution of colloidal calcium phosphate which modifies the susceptibility of the caseins to proteolysis during manufacture and influences the rheological properties of cheese, e.g. textural differences among Emmental, Gouda, Cheddar and Cheshire cheeses.

6. Acidification controls the growth of many species of non-starter bacteria in cheese, especially pathogenic, food poisoning and gas producing microorganisms. In addition to producing acid, many starter bacteria produce probiotics which also restrict or inhibit the growth of non-starter microorganisms.

7. Starter organisms govern flavors and body and texture of cheese.

8. Acid production influences favorably the changes that take place in the curd during cheddaring.

9. Growth of LAB produces the low oxidation-reduction potential (Eh) necessary for production of reduced sulphur compounds such as methanethiol which may contribute to the aroma of Cheddar cheese.

12.2 Requirements of a Good Starter

• It must be vigorous, and produce acid at a quick, consistent and controlled rate.

• It must produce a clean lactic acid flavor and other aroma.

• Must not produce any off flavor, taint, pigment or gas. There must be no appreciable proteolysis or lipolysis.

• Must contain only lactic acid bacteria of desired type.

• It must produce acidity smoothly under the conditions of manufacture.

• Must grow regularly and produce lactic acid at a consistent rate day after day in the cheese milk.

12.3 Type of Cultures

Essentially two types of starter cultures are used: Mesophillic milk an optimum temperature of ~30°C and thermophillic milk an optimum temperature of ~45°C. The choice of culture depends on the cheese being made, e.g. mesophillic cultures are used in the production of Cheddar, Gouda, Edam, Blue and Camembert while thermophillic cultures are used for Swiss and Italian varieties (Table 12.1). This choice is related to the method of manufacture since Swiss and Italian cheeses are cooked at much higher temperature (50-55°C) which the starter bacteria must be capable of withstanding. Growth at 10 and 45°C can be used to distinguish mesophillic from thermophillic cultures while microscopic observations, measurement of the amount and isomer of lactic acid produced and the ability to metabolize citrate can readily distinguish most of the species within these broad categories. Both mesophillic and thermophillic cultures can be further subdivided into mixed (undefined) cultures, in which the number of strains is unknown and defined cultures in which the member of strains is known.

Table 12.1 Starter cultures used in major cheese varieties

12.3.1 Mixed strain mesophillic cultures

Undefined or mixed-strain mesophillic cultures are composed mainly of Lc. lactis subsp. cremoris, although occasionally they also contain the closely related Lc. lactis subsp. lactis. Some mesophillic mixed cultures also contain a lactococcus (Lc. lactis subsp. diacetylactis) which metabolizes citrate (Cit⁺) to CO₂ and flavor compounds. Thus, many mesophillic cultures are comprised of Cit^־ and Cit⁺ lactococci. Depending on the nature of the flavor producers, mesophillic mixed cultures are classified as: L-type, containing leucouostoc spp; D- type, containing Cit⁺ lactococci (diacetylactis), DL- type containing both, and O-type containing no flavor producers. The flavor producers use citrate as energy source. The acid and flavor producers in mesophillic cultures comprise about 90% and 10% of the microflora respectively. They are called mixed cultures not only because they contain different bacterial species but also because they contain different strains of the same species.

Mixed starters consist of two or more strains or species and so may be more variable in behavior. These are safer to use because if one strain becomes phaged, others can usually continue to work. However, it is difficult to maintain a constant mixed starter because one strain becomes dominant after a few transfers. Therefore, each strain should be cultured separately, and mixed immediately before addition to the vat.

12.3.2 Defined strain mesophillic cultures

These are mainly pure cultures of Lc. lactis subsp. cremoris. The important differences between these cultures and traditional mixed cultures are that the number of strains is known and they do not contain flavor producers.

Single strain starter is a pure culture usually consisting of either Lc. lactis subsp. lactis or Lc. lactis subsp. cremoris. This type of culture has the advantage that if found satisfactory in vigor and flavor, it can give a steady acid production and thereby a predictable quality fermented dairy product. However, there is serious disadvantage as well with this type of starter. During its application, if it gets attacked by phage or fails by any other reason the quality of the product can be adversely affected.

Other bacterial species are sometimes incorporated into a dairy starter cultures and these are as follows:

1. S. faecium: for manufacture of modified Cheddar cheese.

2. Brevibacterium linens: Imparts distinctive, reddish orange color to the rind of Brick and Limburger cheese.

3. Propionibacterium freudenreichii subsp. shermanii, for Swiss cheese; produces large gas holes in cheese.

Molds like Penicillium camemberti, P. caseiocolum and P. candidum and the blue mold P. roquefortii, which grows internally in the cheese are used in Blue cheeses like Roquefort, Blue stilton, Danish blue, Gorgonzola and Mycella.

Most cultures are used in dairy industry either singly, in pairs or in a mixture, thus, giving the industry the opportunity to manufacture different types of fermented dairy products. In theory, a single strain starter consists of one type of organism but in practice, it is rarely used. However, single strains can be paired to safeguard against bacteriophage attack, intolerance of salt, or cooking temperature and variation in the quality of the product. Multiple strain starter cultures consist of known member of single strains, so that the starter can be used for an extended period of time during cheese making season. A mixed strain starter is a combination of Lc. lactis subsp. lactis, Lc. lactis subsp. cremoris and the gas and aroma producing mesophillic LAB (Lc. lactis subsp. diacetylactis, Lc. lactis subsp. cremoris and/or Leuc. dextranicum).

12.4 Factors Affecting Activity of Starter Culture

• Temperature

• pH

• Strain compatibility

• Growth medium inhibitors

• Bacteriophage

• Incubation period

• Heat treatment of milk

• Degree of aeration (aeration, agitation and surface culture not favorable)

• CO₂ (minimum concentration is essential)

• Storage conditions

12.5 Desired Characteristics

Good starter should show a smooth curd. At the completion of the incubation, there should be no separation of the whey from the curd. After the cooled starter has been stirred or shaken, it should have the consistency of rich cream. It should be glossy in appearance and not dull or chalky. The starter should show no curd particles or lumps, and when poured from the container it should not show a ropy consistency. The flavor should be pronounced, yet delicate. Neither a flat flavor nor a sharp acid taste is desired.

Suggested Readings

Banks, J.M. (1998). The Technology of Dairy Products, 2nd Ed., R. Early (Ed.) Chapman and Hall, Blackie Academic and Professional, London.