Influence of substrate pH value on kefir beverage production in stirred tank reactor

Abstract

Kefirni napitak je fermentirano piće proizvedeno dodavanjem kefirnih zrnaca u vodenu otopinu saharoze. Kefirna zrnca građena su od matriksa, načinjenog od dekstrana, i mikroorganizama, najviše kvasaca, bakterija mliječne kiseline, bifidobakterija te bakterija octene kiseline, odgovornih za provođenje fermentacije. Raspon optimalne pH vrijednosti za svaku je vrstu različit pa će kod određene pH vrijednosti podloge, udio pojedine vrste mikroorganizama varirati zajedno s koncentracijom produkata njihovog metabolizma. U konačnici, pH podloge utječe i na samu dinamiku odvijanja bioprocesa te posljedično i na pokazatelje uspješnosti. U ovom završnom radu uspoređena je dinamika odvijanja proizvodnje kefirnog napitka u bioreaktoru s miješalom kao i njegov sastav tokom cijelog vremena trajanja fermentacije kod dvije različite pH vrijednosti podloge (pH 4 i pH 5). Utvrđeno je da je kod pH podloge 5 ostvareno kraće vrijeme fermentacije, a izvor ugljika je gotovo u potpunosti utrošen. Također, zabilježene su više koncentracije produkata, kao što su etanol, mliječna i octena kiselina, u odnosu na drugi eksperiment.Water kefir is a fermented beverage produced by adding kefir grains to an aqueous sucrose solution. Kefir grains are made of a matrix, made of dextran, and microorganisms, mostly yeasts, bifidobacteria and lactic and acetic acid bacteria, responsible for conducting fermentation. The range of optimal pH value for each species is different, so at a certain pH value of the media, the share of each type of microorganisms will vary together with the concentration of the products of their metabolism. Ultimately, the pH of the media affects the dynamics of the bioprocess and, consequently, the bioprocess efficiency parametres. This bachelor thesis compares the dynamics of water kefir production in a stirred tank bioreactor as well as its composition during the entire fermentation at two different pH values (pH 4 and pH 5) of the media. It was found that at pH 5, a shorter fermentation time was achieved and the carbon source was almost completely consumed. Also, higher concentrations of products like ethanol, lactic and acetic acid were recorded in comparison to the second experiment