Relationship between pH and Medium Dissolved Solids in Terms of Growth and Metabolism of Lactobacilli andSaccharomyces cerevisiaeduring Ethanol Production

Abstract

The specific growth rates of four species of lactobacilli decreased linearly with increases in the concentration of dissolved solids (sugars) in liquid growth medium. This was most likely due to the osmotic stress exerted by the sugars on the bacteria. The reduction in growth rates corresponded to decreased lactic acid production. Medium pH was another factor studied. As the medium pH decreased from 5.5 to 4.0, there was a reduction in the specific growth rate of lactobacilli and a corresponding decrease in the lactic acid produced. In contrast, medium pH did not have any significant effect on the specific growth rate of yeast at any particular concentration of dissolved solids in the medium. However, medium pH had a significant (P < 0.001) effect on ethanol production. A medium pH of 5.5 resulted in maximal ethanol production in all media with different concentrations of dissolved solids. When the data were analyzed as a 4 (pH levels) by 4 (concentrations of dissolved solids) factorial experiment, there was no synergistic effect (P > 0.2923) observed between pH of the medium and concentration of dissolved solids of the medium in reducing bacterial growth and metabolism. The data suggest that reduction of initial medium pH to 4.0 for the control of lactobacilli during ethanol production is not a good practice as there is a reduction (P < 0.001) in the ethanol produced by the yeast at pH 4.0. Setting the mash (medium) with >30% (wt/vol) dissolved solids at a pH of 5.0 to 5.5 will minimize the effects of bacterial contamination and maximize ethanol production by yeast.

Characterization, identification and comparative evaluation of bioethanol tolerance and production capacity of isolated yeast strains from fermented date palm sap (Toddy)

“…All the strains showed positive growth at pH 4, 5 and 6, while no growth was seen at pH lower than 4 and higher than 6. It has been reported by Narendranath and Power (2005) that yeasts generally have an optimum pH between 4.0 and 6.0 although they can grow in a wide range of pH 2.5 to 8.5.…”

Assessment of intrinsic physico-chemical fermentation conditions (pH and total titratable acidity) on liberated reducing sugar by bacteria and bacteria-yeast coupled fermentation of selected lignocellulosic biomass to ethanol

“…At lower reducing sugar concentrations, the amount of ethanol produced are at minimal values and vice-versa because the reducing sugars are liberated sugars required for bioconversion into metabolic end products such as ethanol (Tahir et al, 2010). pH ranging from 4.0-6.0 are understood to be the most favourable for optimal bioethanol production and this values are observed during the progress of fermentation by Narendranath and Power (2005) and Fakruddin et al (2012) to be responsible for optimum production of bioethanol. A decreasing trend in pH during fermentation and an increase in total titratable acidity is a consequential phenomenon resulting from the metabolic activities of microorganisms during fermentation as this demonstrates the progress of the process Wahab et al (2005).…”

Control of Lactobacillus plantarum Contamination in Bioethanol Fermentation by Adding Plantaricins

“…Firstly, Lactobacillus can compete against S. cerevisiae for saccharides and other micronutrients in fermentation broth (Narendranath and Power, 2005). Secondly, lactate generated by Lactobacillus can decrease the fermentation pH value, thus inhibiting yeast biomass and bioethanol yield (Watanabe et al, 2008;Katakura et al, 2011).…”

“…However, industrial-scale bioethanol fermentation is frequently stressed by bacterial contaminants (Muthaiyan et al, 2011). Bacterial contamination can inhibit the growth of S. cerevisiae and result in decreased bioethanol yield, then eventually lead to economic losses (Thomas et al, 2001;Narendranath and Power, 2005). Lactobacillus is the major bacteria contaminant in bioethanol fermentation because of its rapid proliferation and tolerance to ethanol and low pH (Narendranath et al, 1997).…”