Improving Oenococcus oeni to overcome challenges of wine malolactic fermentation

•Physicochemical challenges often limit the success of MLF in wine.•Improving the main effector, Oenococcus oeni, benefits wine processing and quality.•Strain selection from indigenous isolates or following mutagenesis has had success.•Recombinant methods can be more precise but are limited by efficiency or regulation.•Genome shuffling and directed evolution are nonrecombinant methods of much promise.

Oenococcus oeni is crucial for winemaking, bringing stabilization, deacidification, and sensory impacts through malolactic fermentation (MLF) to most wine styles. The poor nutritional make-up of wine together with typically low processing temperatures and pH and high ethanol content and sulfur dioxide (SO2) hinder O. oeni growth and activity. Production delays and interventions with starter cultures and nutritional supplements have significant cost and quality implications; thus, optimization of O. oeni has long been a priority. A range of optimization strategies, some guided by detailed characterization of O. oeni, have been exploited. Varying degrees of success have been seen with classical strain selection, mutagenesis, gene recombination, genome shuffling, and, most recently, directed evolution (DE). The merits, limitations, and future prospects of each are discussed.