3.1.2. Ethanol

Ethanol is known for its bactericidal properties and it is the main yeas<sup>t</sup> metabolite produced during alcoholic fermentation. It can play an integral role in the ability of wine LAB to survive in wine and induce the malolactic fermentation. It is difficult to specify the concentrations which will completely prevent LAB development. Radler [2], Peynaud and Domercq [3], and Henick-Kling [4] reported an increasing inhibition above 5% (*v*/*v*). Wibowo et al. [29] stated in their review that the ability of LAB to survive and grow in wine decreases as the alcohol concentration increases above 10% (*v*/*v*). Henick-Kling [6] indicated a strong impact of temperature on the toxicity of ethanol. A temperature of 25 ◦C and above, combined with alcohol levels above 14.5% (*v*/*v*) can inhibit bacterial growth and the malolactic fermentation. However, the ethanol tolerance is very strain dependent. Information in the literature is contradictory regarding the alcohol sensitivity for the various species of wine LAB. Davis et al. [5] reported strains of *Lactobacillus* and *Pediococcus* being in general more tolerant to high ethanol concentrations than *O. oeni*. From the observation of Wibowo et al. [29], most *Lactobacillus spp*. can tolerate about 15% (*v*/*v*). Britz and Tracey [37] acknowledged that all *O. oeni* strains are able to survive and grow at 10% (*v*/*v*) ethanol at pH 4.7. Studying the combined effects of pH, temperature, ethanol, and malate concentrations on *L. plantarum* and *O. oeni*. Guerzoni et al. [36] sugges<sup>t</sup> *L. plantarum* being more competitive in early steps of alcoholic fermentation. However, more severe conditions, e.g., ethanol concentrations higher than 6% (*v*/*v*), favor *O. oeni*. Most robust commercial *O. oeni* strains, which are produced with a specific process for pre-adaptation (MBR™ process) to different wine conditions, show good survival and good malolactic activity up to 16% (*v*/*v*) alcohol, depending on other environmental factors even higher. With regard to commercial *L. plantarum* starter culture preparations for the induction of MLF in wine, there are two different approaches: Pre-inoculation: Prahl et al. [7] proposed inoculating must before alcoholic fermentation using a direct inoculum of a freeze-dried facultative hetero-fermentative *L. plantarum* starter culture. Inoculation before the

wine yeas<sup>t</sup> addition was recommended due to the sensitivity of the described *L. plantarum* strain towards alcohol. Contrastingly, a patent filed in 2004 [9] on "Alcohol-tolerant malolactic strains for the maturation of wines with average or high pH" relates to LAB strains of the genera *Lactobacillus* and *Pediococcus* displaying a good alcohol tolerance and the capability to induce a complete MLF upon direct inoculation into a wine with an alcohol content of 10% (*v*/*v*) or more and an average high pH level. More recently, more alcohol resistant *Lactobacillus* starter culture had been released, which can tolerate up to 15% (*v*/*v*) [10].
