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.
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.
Modern starter cultures have developed from the practice of retaining small quantities of whey or cream from the successful manufacture of a fermented product on a previous day and using this as the inoculum or starter for the preceding day’s production. This practice has been called various names but the term 'back-slopping' is used widely particularly in fermented sausage manufacture.
Classification of starter bacteria
The bacteria used in the manufacture of fermented dairy products are generally lactic acid bacteria (LAB); however, Propionibacterium shermanii and Bifidobacterium spp. which are not lactic acid bacteria, although Bifidobacterium species do produce lactic acid, are also used. In addition, other bacteria including Brevibacterium linens, responsible for the flavour of Limburger cheese; and moulds (Penicillium species) are used in the manufacture of Camembert, Roquefort and Stilton cheeses.
There are currently sixteen genera in the LAB. Species from the Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus and Tetragenococcus genera are important in food fermentations and have recently been reviewed by the author for the Encylopedia of Food Microbiology, 2nd Edition. This section will review important properties of major genera used in dairy fermentations (Enterococcus, Lactobacillus, Lactococcus, Leuconostoc and Streptococcus).
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.
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.