3.1.4. Sulphur Doxide

Sulphur dioxide (SO2) is another compound well known for its bactericidal action and plays an essential role in the growth of LAB and development of MLF [32]. This component is found in wine with variable concentrations according to the winemaking conditions and the yeas<sup>t</sup> strain responsible for alcoholic fermentation. SO2 is purposely added to wines to inhibit the growth of undesirable microorganisms and for its antioxidant e ffect. Sulphur dioxide in its free form, as well as in its bound form with aldehydes and ketones, is a potent inhibitor of many microbes, including LAB. Three liberated forms of SO2 are present in wine: molecular SO2, bisulphite (HSO3 -), and sulphite (SO32-). Molecular SO2 is e ffective as a bacterial preservative [39], and a well-known synergistic e ffect is the impact of pH on the level of molecular SO2. The lethal level of molecular SO2 for most wine LAB is low (0.3 mg/L), but it is possible that certain selected wine LAB strains could have a better resistance to molecular SO2. Depending on the pH of the juice/wine, the amount of molecular SO2 is between 1% and 7% of the free SO2 content. The molecular SO2 increases with a decrease in pH and an increase in temperature and/or alcohol.

For MLF to be successful, the values of these chemical parameters described above must correspond to those that allow the bacterial cultures to function successfully. A favorable level of any of these components may compensate for an unfavorable level of one or several of the others. It is important to remember these factors function synergistically, i.e., their actions together have a greater total e ffect than the sum of their individual actions.

#### *3.2. Lesser-Known Factors that A*ff*ect Malolactic Fermentation*

A number of lesser known, but equally important, factors can influence the course of MLF and are outlined below.

#### 3.2.1. Yeast Strain Selection

It has been known for some time that certain yeasts selected to conduct the alcoholic fermentation (AF) interact better with certain wine LAB for the successful achievement of MLF. Under specific conditions, certain yeas<sup>t</sup> strains may produce high concentrations of SO2, which has a negative influence on the growth and survival of the wine LAB. Similarly, yeas<sup>t</sup> strains that exhibit an inordinate need for nutrients could exhaust the medium to such an extent that no reserve nutrients are available

for the bacteria. Implementing a specific nutrition strategy for a particular yeas<sup>t</sup> in the early stages of AF can largely surmount this [40–43] and avoid the production of certain unwanted metabolites or toxins derived from yeas<sup>t</sup> stress. More recently, other bacterial growth inhibiters derived from yeas<sup>t</sup> metabolism have been reported, such as medium-chain fatty acids [44] and yeas<sup>t</sup> peptides (between 5 and 10 kDa) [45,46]. More recently, Liu et al. [30] reported certain peptides being stimulating for *O. oeni*. These effects depend on the nature and the level of fatty acids in the wine or the size of the yeas<sup>t</sup> peptides, and can be exacerbated by low pH. On the other hand, the contact with the yeas<sup>t</sup> lees has a very stimulating effect on MLF. The autolysis process releases amino acids and vitamins, and the must become richer in nutrients for the LAB. There may also be a detoxifying effect by yeas<sup>t</sup> polysaccharides, as they may adsorb inhibitory compounds or complex them. Fumi et al. [10] reported yeas<sup>t</sup> strains compatible with *O. oeni* starter cultures being also compatible with a *Lb. plantarum* starter culture strain.
