It can be difficult for entrepreneurs to obtain starter cultures for trials. This article provides contact details of some culture suppliers.
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The isolation of lactic acid bacteria from raw and pasteurized milk is discussed.
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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.
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 In a survey of the causes of slow acid production by cheese starters in the UK (Boyle and Mullan, 2000, unpublished results) found that, some 28 % of respondents attributed slow acid problems to antibiotics.
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This article discusses the background of the change in classification of the lactic streptococci to the genus Lactococcus.
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.
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This article discusses the major functions of starters in dairy fermentations. Recent research on the relative importance of the antimicrobial agents produced by starters is included. The importance of undissociated lactic acid (HLac) is discussed with regard to the inhibition of the growth of Listeria monocyotgenes, Escherichia coli and Staphylococcus aureus.
The author recommends that regulators should require manufacturers of raw milk cheeses to meet a minimum value for HLac that must be achieved prior to product release for retail sale.
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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 15-20 years, the use of starter cell concentrates designated as either Direct Vat Set (DVS) or Direct Vat Inoculation (DVI) cultures are increasing being used, particularly in small plants, to replace bulk starter in cheese manufacture. DVI / DVS cultures for cheese manufacture normally contain defined blends of lactococci and Streptococcus thermophilus. These organisms respond different to salt and temperature and these differences, if not understood, can impact on cheese quality.
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.
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