Flavor Compounds Produced from Carbohydrate Fermentation by LAB

Flavor Compounds Produced from Carbohydrate Fermentation by LAB 

The primary substrates of LAB in fermented foods are the sugars of raw materials, either present (e.g., lactose in milk glucose, fructose and sucrose in plant material (Daeschel et al. 1987) or 

Citrate is present in milk, fruit, and vegetables. It can be co-metabolized with sugars by citrateutilizing LAB. Citrate utilization results in an excess of pyruvate, which is thus converted to diacetyl (2,3-butanedione), acetoin (2-hydroxy-3-butanone), and 2,3-butanediol to equilibrate the redox balance of cellular metabolism (Collins 1972 Bartowsky and Henschke 2004). Some LAB can also synthesize 2,3-pentanedione from pyruvate and threonine (Ott et al. 2000). Diacetyl and 2,3-pentanedione are associated with a buttery aroma, which positively contributes to the flavor of a range of fermented dairy products such as butter (MalUa et al. 2008), yogurt (Routray and Mishra 2011), and cheese (Curioni and Bosset 2002). Diacetyl also contributes to wine style, while it is responsible for flavor defects in beer. Diacetyl is widely produced by LAB, including species of the Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, Pediococcus, and Oenococcus genera. 

LAB are able to produce many important aroma cort iounds from amino acid (AA) catabolism (Table 19.1). LAB possess a powerful proteolytic system, in relation to their multiple auxotrophy for AA, which has been reviewed in detail (Fox and Wallace 1997 Christensen et al. 1999 Smit et al. 2005 Steele et al. 2013). LAB hydrolyze proteins into peptides and amino acids, which impact the taste of foods. Large hydrophobic peptides are associated with (undesirable) bitter taste, while peptides and A A contribute to the basic taste of cheese. The hydrolysis of large hydrophobic peptides can be accelerated by the release of intracellular peptidases from lysed LAB cells in cheese paste, in which they remain active during the ripening period, thus decreasing cheese bitterness (McSweeney and Sousa 2000). 

In conclusion, the conversion of AA by LAB generates a variety of aroma compounds, including branched-chain acids (cheesy) and aldehydes (malty), aromatic alcohols and aldehydes (floral), and volatile sulfur compounds (garlic, cabbage, boiled potato) (Table 19.1). The expression of LAB enzymes in situ is influenced by a number of factors detailed in Section 19.5. 

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