Microbiological stability of continuous systems with immobilized yeast cells

Abstract

An extremely important condition for successful performances of continuous fermentation in reactor systems with immobilized yeast cells is their microbiological stability. This stability depends on several different factors and the most important among them are the following: purity of the immobilized yeast, its concentration and viability, temperature of the main and secondary fermentation, general sanitary conditions, as well as the mode of support regeneration. Some of the most significant agents that cause microbiological contamination are certain bacteria and yeast strains that multiply well at low temperatures and bind firmly to surface of the support or immobilized yeast. Laboratory tests showed that microbiological contamination was more expressed in bioreactors for continuous main fermentation than in bioreactors for secondary fermentation. This can be explained by the following facts. Ethanol, higher alcohols, esters, diacet- yl and other volatile components disappear in the course of worth fermentation and also some changes occur in pH value of the medium, redox potential and relation between oxygen and carbon dioxide. Presence of these inhibitory factors, which prevent growth of the majority of micro-organisms, results in much lower spectrum of microbial contaminants in the process of continuous secondary fermentation. It has been pointed out that the least dangerous of bacterial flora is Pantoea agglomerans because it does not tolerate restricting conditions in bioreactor and it can easily be rinsed from the column. From among the bacteria of lactic acid fermentation, the most often contaminants are the heterofermentative kinds. Lactobacillus bre- vis, Lactobacillus linden and Pediococcus damnosus. These kinds are more tolerant to the conditions in reactor but they bind to the support and/of immobilized yeast with great difficulty and, after some time, they are rinsed from the coloumn. The most dangerous contaminants of the continuous systems are "wild" yeasts, both those that express similar metabolic activity to brewers yeast (Saccharomyces cerevisiae ex Diastaticus) and those that can assimilate ethanol (Candida crusei).

Postizanje mikrobiološke stabilnosti je veoma važan parametar uspešnog izvođenja kontinualnih fermentacija sa imobilisanim ćelijama kvasca. Po- stoji nekoliko faktora koji mogu značajno uticati na održavanje mikrobiološke stabilnosti kao što su, čistoća kvasca koji se imobiliše, njegova koncentacija i fiziološka aktivnost, temperatura u bioreaktoru, op- šta higijena i adekvatna regeneracija nosača. Da bi se ispitala mogućnost rasta i razmnožavanja mikroorganizama koji se i kod tradicionalnog procesa javljaju kao najčešći uzročnici kvarenja piva, u laboratorijskim uslovima je namerno izazivana kontaminacija bioreaktora za glavno i naknadno vrenje. Bakterije kojima su kontaminirani bioreaktori su vrste Obesumbacterium proteus, Pantoea agglomerans, Lactobacillus brevis, Lactobacillus hndneri, Pediococcus damnosus dok su od "divljih " kvasaca za kontaminaciju korišćene vrste Saccharomyces cerevisiae (ex diastaticus ) i Candida krusei. Rezultati istraživanja pokazuju da su bioreaktori za naknadno vrenje piva manje osetljivi prema mikrobiološkoj kontaminaciji od reaktora za izvođenje glavnog vrenja. Naime, utvrđeno je da je sladovina u odnosu na mlado pivo zbog prisustva velikog broja hranljivih materija bolji supstrat za razvoj neželjene mikroflore. Međutim, dobijeni rezultati govore da ovi mikroorganizmi bez obzira na različite mogućnosti za njihovo preživljavanje u bioreaktorima za glavno i naknadno vrenje, svojim metabolitičkim aktivnostima ne mogu značajno uticati na izmenu senzornih svojstava piva.