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It can be difficult for entrepreneurs to obtain starter cultures for trials. This article provides contact details of some culture suppliers.

The isolation of lactic acid bacteria from raw and pasteurized milk is discussed.

Starter bacteria in yoghurt

This article discusses the origins and role of starters in dairy fermentations, the ecology of starter bacteria, the classification of starter bacteria,  the types of starter culture used and concludes with some observations on artisanal cultures. The author has  provided a broader perspective on the use of starter cultures in food fermentations in the Encyclopedia of Food Microbiology. The chapter can be downloaded from Elsevier Ltd.

Ecology of starter bacteria

So where did modern starter cultures come from? Most starters in use to today have originated from lactic acid bacteria originally present as part of the contaminating microflora of milk. These bacteria have probably originated from vegetation in the case of lactococci (Sandine et al., 1972) or the intestinal tract in the case of Bifidobacterium spp., enterococci and Lactobacillus acidophilus.

Growth and acid production by starter cultures may be inhibited by bacterial viruses, bacteriophages,  or added substances including antibiotics, sterilant and detergent residues, or  free fatty acids produced by or as a result of the growth of microorganisms, and natural often called indigenous antimicrobial proteins.

Antibiotics

Milk should not contain antibiotic residues.  Milk production in the UK is regulated by the Dairy Products (Hygiene) Regulations 1995.  These regulations include the standards for raw milk.  Prior to 1990 milk was deemed to be contaminated if an antibiotic concentration of > 0.01 international units (iu) /ml was present, the standard has now been increased to 0.006 iu/ml.  Manufacturers buying milk from producers impose stringent financial penalties on farmers producing contaminated milk and have procedures to exclude this from the food chain.  Despite legislation and financial penalties, there is evidence to suggest that residues occasionally still cause problems.  In a survey of the causes of slow acid production by cheese starters in the UK (Boyle and Mullan, 2000, unpublished results) found that, some 28 % of respondents attributed slow acid problems to antibiotics. 

Antibiotics gain entry to milk because of mastitis treatment; mastitis means inflammation of the udder.  Although this term includes all inflammatory conditions of the udder, it isdefined here as a bacterial infection of the udder.  The common causative organisms of mastitis in the UK are Str. agalactiae, Str. dysgalactiae, coagulase-negative staphylococci and Staphylococcus aureus. 

The antibiotics used in veterinary medicine belong to six major groups:-

Aminoglycosides e.g. gentamicin
Penicillins and cephalosporins (ß-Lactams) e.g. cloxacillin
Macrolides-e.g. erythromycin
Quinolones and fluroquinolones
Sulphonamides e.g. trimethoprim
Tetracyclines e.g. tetracycline

The lactic group of the genus Streptococcus originally included the species Str. lactis and Str. cremoris and a subspecies of Str. lactis, Str. lactis subsp. diacetylactis (Deibel and Seeley, 1974). However, even in the 1970s workers were suggesting that Str. lactis strains might be variants of Str. diacetylactis that were unable to ferment citric acid, since citrate permease – negative strains of Str. diacetylactis had been described (Lawrence, Thomas and Terzaghi, 1976).

Bacteria in this group were designated as the lactic streptococci. The designation 'lactic' was used by Sherman (1937) for mainly historical reasons, including the use of the term by Lister (1878) to describe a bacterium that we now know as Lc. lactis subsp. lactis.

The major functions of starters in dairy fermentations are shown in table 1. Further information is available in the section on starters.

What are starter concentrates?

Traditionally 'bulk starter' in liquid form was used to inoculate the milk used in the manufacture of cheese, yoghurt, buttermilk and other fermented products. Over the past 10-15 years, the use of starter cell concentrates designated as either Direct Vat Set (DVS) or Direct Vat Inoculation (DVI) cultures have increasing being used, particularly in small plants, to replace bulk starter in cheese manufacture. Note that the terms DVI and DVS are used interchangeably although particular culture suppliers will tend to use only one term.

In addition to these high activity cell concentrates, lower activity commercial cell concentrates have been used for many years to inoculate milk for bulk starter manufacture, and in the manufacture of 'long set products' that require extended incubation. This contribution while mainly concerned with DVS or DVI cultures will also comment on concentrates use in bulk starter manufacture.

 

Identification of species with a biochemical key

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