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Mursik is prepared from cows' milk fermented in ash-treated gourds. Blood may be added to fresh milk before fermentation, or to already fermented milk. The milk (or blood-milk mixture) is heated to boiling point then cooled to ambient temperature, after which it is left to undergo spontaneous fermentation for three to five days, through the action of lactic acid bacteria, yeast and mould species. Lactobacillus plantarum was found to be the most dominant of the lactic acid bacteria involved in the production of mursik. Other lactic acid bacteria isolated from mursik are Leuconostoc mesenteroides and Enterococcus faecium. Saccharomyces sp. and Geotrichum candidum have also been isolated from mursik
The use of charcoal from specific trees has long been used in the traditional production of fermented milk products in Kenya. Among the common tree species used by farmers for milk treatment in production of mursik are Senna didymobotrya, Lippia kituiensis, Prunus africana and Olea europaea ssp. africana.
A gourd is smeared inside with special charcoal called "osek"; from this, gray lines can be seen when pouring the thick, sour milk. In preparing the gourd, the Kalenjin women, for instance, make a brush (sosiot) from a branch of cycad tree used to clean the inside. Brushes made from this tree are hardy and may last for up to two years before replacement.
Charcoal "osek", formed from the smouldering embers of branches from the Ite or Itet tree (peanut butter cassia, scientifically known as Senna didymobotrya), is used as a milk preservative. Women use the embers to coat the inside of the cleaned gourd. The charcoal has various effects. It lines the inside of the gourd, reducing its porosity rendering it airtight. The smoke from the embers also has a preservative effect which prevents undesired bacterial multiplication that causes spoilage, while allowing natural souring. The charcoal smoke imparts a special flavour to the milk, and a bluish colour which is of high aesthetic value to the consumer. Having prepared the gourd, women pasteurize the milk by boiling. The pasteurized milk is left to cool before pouring into the gourd. Finally the gourd is corked to render it airtight, making it possible for the milk to be preserved for up to a month.
There are various flavours of mursik, depending on how it is prepared and what quality of milk is used.
Many tree species have been adjudged suitable for the purpose of imparting the preservative and aromatic effect to milk. There are many causes of unusual milk flavour, including the effect of plain gourd walls, which impart a bittersweet tang, and the plainness of white ripened milk, the role of itet is paramount. Several trees are good for the purpose. One characteristic is common though: high tannin content in the bark of the tree concerned. The popular ones include sertwet (acacia), simotwet, and in 'shamba', wattle. Senetwet is by far the most commonly used, on account of nearly universal availability, although sertwet is preferred by the purists.
Mursik can be prepared from a full gourd of milk corked all at once. Another method of preparing it is by pouring in a pint every three days or so. The fermented milk provides the culture for the new milk, and seems to accelerate its ripening. After the gourd is full, it is corked for a while, to achieve a varied consistency of proper sour milk, and results in a clear, sharp (almost bitter in some cases) liquid in which white globules of butter float, shaken well. Another type is the fast fermenting, even type, which gives a white, porridge like consistency.
- Mathara, Julius Maina (1999). "Studies on lactic acid producing microflora in mursik and kule naoto, traditional fermented milks from Nandi and Masai communities in Kenya". University of Nairobi digital repository. University of Nairobi. Retrieved 30 November 2013.
- Mureithi, William; Christoffel Den Biggelaar, Edward W. Wesakania, Kuria Kamau and Catherine Gatundu (2000). "Management of trees used in mursik (fermented milk) production in Trans-Nzoia District, Kenya". Journal of Ethnobiology. Management of trees used in mursik (fermented milk) production in Trans-Nzoia District, Kenya 20 (1): 75–91. Retrieved 30 November 2013.
- Ngule, C.M.; T Anthoney Swamy and Obey Jackie K (2013). "Phytochemical and bioactivity evaluation of Senna didymobotrya Fresen Irwin used by the Nandi community in Kenya". International Journal of Bioassays 2 (7): 1037–1043. Retrieved 30 November 2013.