Citrate metabolism and aroma compound production in lactic acid bacteria

Abstract

The main activity of lactic acid bacteria (LAB) during fermentation is the catabolism of sugars present in food, producing lactic acid by homoor heterofermentative pathways. In addition, these microorganisms also have the capability to metabolize other substrates, such as citrate. Citrate is present in fruit juices, milk and vegetables and is also added as a preservative to foods. Citrate fermentation by LAB leads to the production of 4-carbon compounds, mainly diacetyl, acetoin and butanediol, which have aromatic properties. One of these compounds, diacetyl is responsible for the buttery aroma of dairy products such as butter, acid cream and cottage cheese. In addition, it is an important component of the flavour of different kinds of chesses and yoghurt. Moreover, the CO2 produced as a consequence of citrate metabolism contributes to the formation of "eyes" (holes) in Gouda, Danbo and other cheeses. Thus, the utilization of citrate in milk by LAB has a very positive effect on the quality of the end products. Therefore, the interest of the dairy industry in controlling citrate utilization by LAB has promoted research into the proteins and effectors controlling its metabolic pathway. In this chapter we summarize the current knowledge of citrate utilization by LAB. The transport of citrate and its metabolism to pyruvate, as well as further conversion to aroma compounds, is described and, the differences in the co-metabolism of citrate with glucose between homo or heterofermentative bacteria is discussed. In addition, the molecular mechanisms controlling expression of genes responsible for transport and conversion of citrate into pyruvate are presented, as are their correlation with the physiological function of citrate metabolism. To date, two different models of regulation have been described which are unique to LAB. In Lactococcus lactis, a specific transcriptional activation of the promoters controlling the cit operons takes place at low pH to provide an adaptative response to acidic stress. In Weissella paramesenteroides, the CitI transcriptional regulator functions as a citrate-activated switch allowing the cell to optimize the generation of metabolic energy. CitI, its operators and citrate transport and metabolic operons are highly conserved in several LAB. Therefore, this mechanism of sensing and response to citrate appears to have been conserved and propogated during the evolution of LAB