Browsing by Subject "varahapate"

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.