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Title: Preservation and utilization of malolactic fermenting lactic acid bacteria
Author: Zhao, G.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2005
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Malolactic fermentation (MLF) is an important process in the wine production. MLF results from the metabolism of certain lactic acid bacteria (LAB) and consists in the conversion of malate to lactate and CO2. Except deacidification, MLF can improve the quality and microbiological stability of the wines. The aim of this project was to investigate the preservation and utilization of the LAB with particular reference to the MLF. In order to measure the effect of various preservation methods and their productivity, an assay of cell vitality was developed. This measured the capacity to overcome and recover from freezing and freeze-drying. It was shown that this method was easily used and reliable. The effect of cultural conditions on the cryotolerance and vitality of the LAB was investigated, including: (1) the growth phase, (2) the growth temperature, (3) the medium pH, (4) composition of the medium, and (5) preincubation conditions. The optimal cultural conditions to obtain higher vitality after freezing varied with the species of LAB. When the pH of culture medium was controlled at pH 5 the LAB attained the highest vitality after freezing. When L. plantarum was preincubated in 5g/l yeast extract solution at 25°C for 1 hour, the survival rate of L. plantarum greatly increased, from 5.2% to 46.5%. The conditions of freeze-drying of the LAB were investigated. It was found that 4% yeast extract was the most effective protectant for L. plantarum and L. brevis and 5% glutamate were the best protectant for O. oeni. When the LAB was frozen quickly at -65°C, the vitality obtained was higher than those frozen slowly at -20°C after freeze-drying. Another factor to be considered important was ethanol tolerance when the freeze-dried malolactic bacteria were used in wines. Among the suspension media tested, 5% glutamate and 10% sucrose were the best for freeze-dried L. brevis and O. oeni respectively to obtain high vitality in high ethanol solutions. These studies showed that there were no consistent underlying processes that could be easily identified and that preservation was a species specific, multifactorial process. The MLF was then investigated further by studying the effect of wine components on the batch MLF of L. brevis and O. oeni using a defined synthetic wine. This uniquely allowed a systematic study of the MLF in high alcohol environments. Alcohol tolerance was dependent on temperature and important fermentation intermediates such as citrate, pyruvate and malate. Malolactic fermentations were inhibited when glucose concentration was 2 g/1 to 6 g/1. The inhibition to MLF of O. oeni caused by glucose was relieved when fructose was present. Nutritional status was also an important factor that affected the MLF, when the synthetic wine did not contain added yeast extract, malic acid degradation of L brevis and O. oeni was low (6.1% and 54.3% respectively). Rapid and continuous malolactic fermentation was achieved in the membrane bioreactor (MBR) with high cell density of O. oeni (greater than 108CFU/ml). More than 95% degradation of malic acid in the synthetic wine was obtained at 0.48 1/h of flow rate and 10.4 h residence time. High ethanol concentration of wine was main factor that caused the loss in malic acid degrading activity of O. oeni in the MBR. The poor nutritional condition of wine was not the main factor causing loss in the stability of malic acid degrading activity of O. oeni. The shear stress had little influence on the malic acid degradation of O. oeni under the conditions investigated. Ethanol stress adaptation could improve the stability of malic acid degrading activity of O. oeni in the MBR.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available