Applications Of Leuconostoc Mesenteroides Z₂₅ In Improving The Malolactic Fermentation Of Cider

Cider-making as a technology is important in the fine processing of apple fruits. Malolactic fermentation (MLF) started by microorganisms is a key step for manufacturing the good quality products of cider. However, little attention was paid to appling selective lactic acid bacteria (LAB) to improve the final quality of cider products through MLF. Therefore, the objectives of this present study were: (1) to deal with the abilities of 4 Leuconostoc mesenteroides strains plus Oenoncoccus oeni Z0 as a control strain to start MLF in cider-making; (2) to investigate factors that influence the cell growth of these tested strains in cider-based environment; (3) to monitor the change of organic acids, especially the reduction of malic acid and increase of lactic acid due to MLF after involvement of L. mesenteroides Z₂₅ in fermented cider; and (4) to evaluate the advantages of a strain Z₂₅-added technology in making cider products based on the aromatic formation of final cider products. The following are main results.1. Malolactic activity, sensory analysis, softness index and acetate formation were used to reflect the contribution of each tested strain to the aroma of cider. Among the tested lactic bacteria, strain Z0 as control presented a malolactic activity of 115.00U. It produced 309.97mg/L acetic acid during MLF. The softness index of the cider formed by strain Z0 was 3.78. Strain Z₂₅ produced a malolactic activity of 122.82U following MLF. This strain formed 336.51mg/L acetic acid due to its MLF, and gave the softness index of 4.01 to final cider. Taking all data from each strain into account, we think strain Z₂₅ has the best show in involving the MLF of cider, and should be chosen as a candidate for following studies.2. SO2, ethanol concentration, fermentation temperature and pH are several factors affecting the cell growth of LAB in cider which is a complex system. Strain Z₂₅ tolerated severe cider-making conditions, and grew better at pH 3.2, with a SO2 concentration of 50mg/L and 10% (v/v) ethanol. Moreover, its cell growth was independent from the inoculation size. Strain Z₂₅ is able to adapt to cider-making environment very well, and should be promising as a malolactic fermentation strain for cider processing. 3. The optimal parameters for alcohol fermentation started by yeast strain Y13 were fermentation temperature at 20℃, 6% inoculum, initial sugar concentration of 25°Bx, and initial pH of 3.3. The importance of all these factors affecting cider quality in order was inoculums, followed by initial sugar concentration, initial pH, and fermentation temperature. Concentrated apple juice, if necessary, should be diluted to 14°Bx for cider making. Sugar should be supplemented to cider during the first fermentation phase in twice every each day but not exceed over 20°Bx. In this case, the ethanol level of the fermented cider could be elevated, favoring cider fermentation.4. The optimal parameters for MLF started by strain Z₂₅ after ending alcohol fermentation were fermentation temperature at 20℃, 6% inoculum, 10% ethanol concentration (v/v) and malic acid of 4g/L in fermented cider. The level of malic acid decreased from 4.00g/L to 0.25g/L, whereas that of lactic acid grew from 0.99g/L to 3.50g/L during a 12-day MLF in the presence of strain Z₂₅.5. Finally, The aromatic formation in MLF-producing cider, compared to non-MLF cider, was enhanced in the presence of strain Z₂₅. Especially, the levels of the total esters and total alcohols were much larger than that without MLF. Among such esters and alcohols,the contents of isobutyl alcohol , isoamyl alcohol ,ethyl lactate ,ethyl acetate,caproic acid ethylester were 9.12 folds,2.95 folds,39.61 folds,1.21 folds,2.26 folds more than that without MLF, respectively. It was the subtle change of these substances that made the cider being much better and fruitier. The improvement of these alcohols and esters were proved to correspond to good sensory taste of the final cider products. Obviously, use of strain Z₂₅ as adjunct culture would be promising in improving the final flavor and quality of cider products due to the starting of malolactic fermentation.