Non-thermal plasma and ultrasound-assisted open lactic acid fermentation of distillery stillage
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Stillage is the main by-product of bioethanol production and the cost of its treatment significantly affects the economy of bioethanol production. A process of thermal sterilization before lactic acid fermentation (LAF) is energy demanding and is causing deterioration of valuable compounds in stillage. In this study, ultrasound (UT) and plasma (PT) treatments were used for microbial inactivation, and a significant reduction in the number of viable microorganisms in the stillage after PT and UT was observed. After application of treatment, LAF by Lactobacillus rhamnosus ATCC 7469 was initiated. The concentration of LA is used to quantify the efficiency of the stillage revalorization. The highest LA productivity of 1.21 g/Lh and yield of 0.82 g/g were obtained after PT, while UT of 10 min provided productivity of 1.02 g/Lh and LA yield of 0.69 g/g. The results were benchmarked against closed LAF. Around 20% better revalorization of stillage by PT was achieved when compared with conventional sterilization. In addition, an excellent L (+) LA stereoselectivity of 95.5% was attained after PT. From the aspect of energy efficiency, that of PT was three times lower than UT and almost ten times lower than thermal sterilization, but it is the most expensive due to the high consumption of gas which could reduce application of closed Ar atmosphere on larger scales. This way, a simpler and energy efficient process for LA production on stillage was accomplished by “open” fermentation.
KeywordsBiorefinery Pretreatments Microbial inactivation Sterilization Lactic acid bacteria
The authors want to acknowledge Milica Carević, PhD and Prof. Dejan Bezbradica, PhD, Faculty of Technology and Metallurgy, University of Belgrade, for their help in HPLC analysis of samples and Stevan Jovanović, ing., Insitute of Physics Belgrade, University of Belgrade, for his help in calculation of capital costs.
This work was supported by Serbian Ministry of Education, Science and Technological Development, project number TR 31017, project no. I-1/2018 of Scientific and Technological Collaboration of Republic of Serbia and PR China and III 43007.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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