Browsing by Subject "diasetyyli"
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(2013)Diacetyl and acetoin are typical butter flavours, which are also present in other fermented dairy products such as sour milk. Diacetyl and acetoin are mainly produced from their precursor ?-acetolactate (ALA) by lactic acid bacteria as a product of their metabolism. In the literature review the formation of diacetyl and acetoin were examined along with the typical techniques to analyse diacetyl. The aim of the experimental work was to develop and validate a method for the determination of diacetyl and acetoin in sour milk by gas-chromatographic technique coupled with flame ionisation detector (GC-FID). In this study a suitable column and gas-chromatographic parameters to analyse diacetyl and acetoin were obtained and two sample preparation techniques were tested. In addition, the reactivity of diacetyl and acetoin and their interactions with the sample matrix were examined. Finally the method was validated. In the complete method, the proteins of the sample were precipitated with acetone and the sample was centrifuged. ALA along with protein-bound diacetyl and acetoin were extracted from the supernatant by solid-phase extraction (SPE), followed by the GC-FID-analysis of free diacetyl and acetoin in the SPE-permeate. GC-FID-analysis was performed using a polar ZB-FFAP-column, split-ratio 30:1 and inlet and detector temperatures 200 ?C and 250 ?C, respectively. Diacetyl and acetoin are reactive and easily evaporated, which complicated their analysis. ALA was mainly decomposed to acetoin in the hot injector. This finding resulted in extraction of ALA from the sample by SPE. In the sour milk matrix, diacetyl and acetoin were believed to appear in three different forms: free and strongly or weakly bound to proteins. The compounds were easily transformed from one form to another. Possibly, also cyclopentanone, the internal standard, was partly bound to the sample matrix. The reactivity of the compounds prevented the determination of total diacetyl and acetoin. Therefore, the method was suitable only for the determination of free diacetyl and acetoin, which, in fact, constitute the buttery aroma in sour milk. Sour milk contained more free acetoin than diacetyl. The method was found to be selective and sensitive for the determination of diacetyl and acetoin. However, the reactivity of the compounds impaired the repeatability of the results. Further research is needed to discover factors affecting the binding of diacetyl and acetoin to the sample matrix. Furthermore, different internal standards and solvents ought to be tested in place of cyclopentanone and acetoin.
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(2017)In dairy industry starter cultues are used to acidify milk and, in addition, produce aroma compounds, such as diacetyl. Diacetyl have buttery aroma and it is a desired flavor compound in products like butter, buttermilk and cottage cheese. Diacetyl can be metabolised from pyruvate which is a common intermediate of carbohydrate and citrate catabolisms. The metabolism from pyruvate to lactic acid or citrate depends on the strains and their growth conditions. Lactococcus lactis ssp. lactis biov. diacetylactis is an exceptional lactic acid bacteria type, which can use citrate as a sole energy source. The citrate metabolism enzymes and the citrate uptake to the cell, have a key role, when diacetyl production is tried to increase in LABs with their growth conditions. New starter cultures of lactic acid bacteria are needed to solve for acidification problems caused by bacteriophages. Dairy industry has developed control methods for bacteriophages like starter rotation. For starter rotation there should be more than one starter culture, which has same properties in acidification and aroma compounds production but different bacteriophage insensitivity. The aim of this Master’s thesis was to find a strain, which can be used as an aroma producer starter culture in milk based products. Requirements for the strain were a diacetyl production of 120 ppm or more and different phage insensitivity than the strain in use. Diacetyl productions of 132 strains were determined with a colorimetric method, which is based on Voges-Proskauer reaction. The strains were grown in 1.1% citrate containing milk medium in aerobic conditions. Based on absorbance measurements 23 selected strains’ diacetyl production levels in same growth conditions were determinded with headspace gas chromatography–mass spectrometry. Two of the 23 strains produced more diacetyl than the strain in use, but they had the same phage insensitivity. Because of similarity to the features of the used strain, it was decided to find out growth conditions, which affect the diacetyl production by the strain L. diacetylactis H63, when growing in bioreactors. Factors for two experimental designs were chosen based on literature. Partial least square (PLS) regression method was used to analyse, which of the chosen parameters affects the diacetyl and acetoin production of the strain H63. Copper concentration, pH and the redox-potential of the growth medium, when samples were taken to determinations, explain the diacetyl and acetoin production of the strain H63 based on the model, which PLS method created. Also the initial pH of the growth correlates with diacetyl production. Result of this research is that by using same growth conditions it was not possible to find a strain from selected collection with the same diacetyl production capabilities but different phage insensitivity as the strain in use.
Now showing items 1-2 of 2