Abstract
Bacterial contamination is one of the major challenges faced by yeast fermentation industries as the contaminating microorganisms produce lactic acid and acetic acid, which reduces the viability of yeast, and hence fermentation yields. The primary bacterial contaminants of yeast fermentations are lactic acid bacteria (LAB). This study aims to identify potential natural antibacterial fractions from raw and pretreated lignocellulosic biomasses found in Abu Dhabi, UAE, in terms of LAB inhibition capacity, allowing growth of the yeast. The analysis was carried out using plating technique. Pretreatment liquid of the mangrove stem Avicennia marina hydrothermally pretreated at 210 °C exhibited the widest inhibition zone with an average diameter of 14.5 mm, followed by the pretreatment liquid of mangrove leaf pretreated at 190 °C, Salicornia bigelovii pretreated at 202 °C and rachis of date palm Phoenix dactylifera pretreated at 200 °C. The compounds responsible for the antibacterial activity will be characterized in further study.
Highlights
Bacterial contamination is of significant concern for industrial yeast fermentation (Bischoff et al 2007) as it decreases the available levels of carbohydrate and nutrients (Muthaiyan et al 2011; Nwobi et al 2015), leading to lower product yields and production of unwanted organic acids that may inhibit yeast growth, e.g. lactic acid (De Oliva-Neto and Yokoya 1998; Makanjuola et al 1992; Leja and Broda 2009; Muthaiyan et al 2011)
Materials and methods For the antibacterial analysis, the samples used were the hydrothermal pretreatment liquid of various plant biomasses and water extracts of certain plant biomass which are as follows: date palm leaflets Pretreated at 200 °C; date palm leaflets pretreated in seawater at 200 and 210 °C; date palm rachis pretreated at 180, 190, 200 and 210 °C; mangrove stem A. marina pretreated at 190, 200 and 210 °C; mangrove leaf A. marina pretreated at 190, 200 and 210 °C; S. bigelovii pretreated at 118,160 and 200 °C; A. marina stem water extract; S. sinus persicus water extract; halophyte stem S. iranshahari water extract; halophyte Leaf S. iranshahari water extract; Jasmine Hedges water extract; and Bermuda grass water extract
No growth of lactic acid bacteria (LAB) and yeast was observed when inoculated with pretreatment liquid of date palm rachis at 210 °C, A.marina stem at 190 °C, A.marina leaf treated at 200 and 210 °C, indicating that they contain compounds that are inhibitory to both LAB and yeast
Summary
Bacterial contamination is of significant concern for industrial yeast fermentation (Bischoff et al 2007) as it decreases the available levels of carbohydrate and nutrients (Muthaiyan et al 2011; Nwobi et al 2015), leading to lower product yields and production of unwanted organic acids that may inhibit yeast growth, e.g. lactic acid (De Oliva-Neto and Yokoya 1998; Makanjuola et al 1992; Leja and Broda 2009; Muthaiyan et al 2011). Among the bacterial contaminants in industrial yeast fermentation, lactic acid bacteria (LAB) are the most prominent (Essia Ngang et al 1990). In yeast fermentation, non-biological inhibitory agents have been implemented as contamination control agents. Antiseptics such as hydrogen peroxide, potassium metabisulfite (Chang et al 1997), and 3,4,4-trichlorocarbanilide (De Oliva-Neto and Yokoya, 1998) have been shown to inhibit and control bacterial contamination in ethanol fermentations on a laboratory scale. Full scale and pilot bioethanol plants employ acid washing and use antibiotics on a large scale to prevent contamination by lactic acid bacteria (De Oliva-Neto and Yokoya, 1998). Penicillin is added over a concentration of 1.5 mg/l, whereas virginiamycin is added in a range from 0.5 to 64 mg/l depending on the type of contaminant
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