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Wort Souring is the process of Mixed Fermentation where lactic acid bacteria, usually Lactobacillus (although there are also Pediococcus cultures available that work well for this technique), is given a "head start" as it's pitched before the yeast so that it will be able to produce significant amounts of lactic acid before the Saccharomyces completes the main fermentation. This is a broadly defined technique and there are many variations on souring wort. These include souring in the primary fermenter, souring in a secondary vessel, or souring in the boil kettle itself (kettle souring). There are also various methods of inoculating the wort with Lactobacillus. Finally, the brewer has the option of pasteurizing the wort by heating it to kill the Lactobacillus before adding the yeast for the main fermentation.
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https://beerandbrewing.com/become-a-kettle-sour-expert/" onclick="window.open(this.href);return false;" rel="nofollowKettle souring is a technique that allows brewers to rapidly sour unfermented wort in just a matter of days, often taking only a mere 24 hours. It could not be more simple - after creating wort and conducting a brief boil for sanitation, the wort is cooled to 75 - 95 degrees F (24 - 35 C) and a cultured strain (or blend of strains) of pure lactobacillus is introduced. Once introduced, the lactobacillus will go to work and begin consuming sugars in the wort transforming them into lactic acid, providing the tart flavor we know and love. Given some time, usually 1 to 3 days, the souring will be complete.
Our controlled infection will let Lacto do its job, which is primarily creating lactic acid, but with the chemistry of acids and alcohols plus the nuanced differences between strains of Lacto, you can end up with low but noticeable levels of succinate, formate, and other chemicals, which can lead to interesting esters that come across as geranium, woody flavors, raspberry, and even sweet cream.