New biological tools to control and secure malolactic fermentation in high pH wines

Abstract

Originally, the role of the malolactic fermentation (MLF) was simply to improve the microbial stability of wine via biological deacidification. However, there is an accumulation of evidence to support the fact that lactic acid bacteria (LAB) also contribute positively to the taste and aroma of wine. Many different LAB enter into grape juice and wine from the surface of grape berries, cluster stems, vine leaves, soil and winery equipment. Due to the highly selective environment of juices and wine, only a few types of LAB are able to grow. Wine pH is most selective, and at pH below 3.5 generally only strains of Oenococcus oeni can survive and express malolactic activity, while wines with pH above 3.5 can contain various species of Pediococcus, as well as strains of Lactobacillus. The trend toward harvesting higher maturity grapes has resulted in the processing of higher pH musts and the production of wines containing increased levels of alcohol. These conditions favor the growth of indigenous bacteria and often O. oeni does not prevail at the end of alcoholic fermentation. More Lactobacillus sp. predominate and are often responsible for spontaneous MLF (du Toit et al. 2011). Some L. plantarum strains can tolerate the high alcohol concentrations and SO2 levels normally encountered in wine. Due to their very complex and diverse metabolism a range of compositional changes can be induced, which may affect the quality of the final product positively or negatively. A recent isolate have shown most interesting results, not only for its capacity to induce MLF after direct inoculation in freeze-dried form, but also for their positive contribution to the wine aroma. Co-inoculation (inoculation of selected wine LAB 24 hours after the yeast) can ensure the early implantation and dominance of the selected strain, the early onset and completion of MLF, and can possibly prevent the appearance of the spoilage yeast and bacteria. Applying an important L. plantarum inoculum with high malolactic activity assures an immediate dominance, as well as predictable and complete MLF in short time and allows an early stabilization of the wine. Since it degrades hexose sugars by the homo-fermentative pathway, which poses no risk of acetic acid production from the residual sugars that may be present in high pH wines, it is an interesting alternative to control MLF in high pH wines.