Metabolic engineering of lactic acid bacteria for the improvement of fermented dairy products

The lactic acid bacteria Lactococcus lactis and Streptococcus thermophilus play a crucial role in the manufacture of dairy products such as cheese, buttermilk and yogurt. In this contribution, different metabolic engineering strategies are described aimed at improvement of the quality of the fermented dairy product. By increased production of diacetyl or acetaldehyde, the particular flavour of respectively buttermilk and yogurt are improved. The health benefits of dairy products are improved by complete conversion of lactose and galactose to, for instance, the sweetener alanine or by induced high-level vitamin-production. The structural properties of fermented products is increased by the production of exopolysaccharides with desired properties. L. lactis is used as starter bacterium in the manufacture of many different cheese types, such as Gouda and Cheddar cheese, and for butter and buttermilk production. It can grow rapidly on milk, converting lactose homofermentively to lactic acid. As nitrogen source it hydrolyzes the milk protein casein by way of an extracellular proteinase. This process of proteolysis continues in ready-made cheese during the ripening process resulting in amino acid-release and subsequent flavour-formation. In addition, some L. lactis strains are able to convert the citric acid in milk, resulting in gas (CO2) formation and production of the butter flavour, diacetyl [1]. Some L. lactis strains produce relevant amounts of exopolysaccharides resulting in high viscosity of the end-product. This process occurs in some Scandinavian dairy products, such as viili and longfil.