Malolactic fermentation (the conversion of malate to lactate) has been recognised as a desired process in wine and cider making and is being progressively developed. It not only concerns the conversion of malate but also involves flavour-related biotransformations catalysed by malolactic fermentation bacteria. This review considers strategies to improve the reliability and performance of malolactic fermentation and documents the development of cell propagation systems and maturation processes, especially in relation to the physiology and biochemistry of malolactic fermentation bacteria. This includes the use of starter cultures and high cell concentrations for propagation and biotransformation, the types of bioreactors and their operational modes, especially those associated with the process of malolactic fermentation. Oenococcus oeni, the predominant organism associated with malolactic fermentation, is an acidophilic bacterium and is able to grow in wine at pH 3.5 or lower in the presence of ethanol and sulphite. As malolactic fermentation takes place in a highly alcoholic (up to 13% v/v) and highly acidic (pH 3.5 or lower) environment, slow growth and poor yields are frequently encountered when starter cultures are used. As a result, it requires several weeks or even months in such conditions to achieve full maturation. The malolactic fermentation process is also affected by temperature, malate concentration, nutrient composition and cell concentration. With an improved understanding of malolactic fermentation and the use of high cell concentrations, appropriate bioreactor designs and various operational modes, process innovation involving the separation of cell propagation from the maturation process now looks feasible. The review assesses the performance of malolactic fermentation systems and the relative benefits of high-cell-concentration biotransformation systems (free cells, immobilised cells or membrane bioreactor) to achieve malolactic fermentation with high productivity. (c) 2006 Society of Chemical Industry.