Lactic acid bacteria are able to ferment citrate, which is present in many raw materials used for food fermentations such as fruit, vegetables, and milk. Citrate can be fermented by a limited amount of mesophilic lactic acid bacteria [(84,85) for a review]. Its degradation results in acetate, diacetyl, acetoin, 2,3-butanediol, and CO2. Besides the formation of the flavor component diacetyl, which is a crucial flavor attribute in buttermilk, ripened cream butter, sour cream, and quarg, the formation of CO2 is responsible for eye formation in cheese.
In the production of Swiss-type of cheeses, such as Gruyere, Emmental, and Maas-dammer, propionibacteria are used to achieve the characteristic sweet and nutty flavor notes as well as the large eyes. Various metabolic pathways have been described for the utilization of lactate as energy source and aspartate as electron acceptor (86). The eye-formation in Swiss-type cheese can be controlled by a combination of aspartase-weak propionibacteria and facultative heterofermentative lactobacilli (87).
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