Determination of aroma compounds and exopolysaccharides formation by lactic acid bacteri isolated from traditional yogurts

Thesis (Master)--Izmir Institute of Technology, Biotechnology, Izmir, 2007Includes bibliographical references (leaves: 86-94)Text in English; Abstract: Turkish and Englishxii, 99 leavesYogurt, consumed widely around the world, is a fermented milk product as a result of the lactic acid fermentation by addition of starter cultures containing Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. The most important criteria for yogurt production is the selection of starter cultures since each culture affects the end-product quality differently. Our natural flora and traditional flavor have been continously changing due to the introduction of imported commercial starter cultures. Because of the necessity to preserve our natural starter cultures and to increase the availability of them for industrial use, these cultures must be isolated from artisanal yogurts, genetically characterized and investigated regarding their technological properties. Aims of our study were to determine the technological and organoleptic properties of starter cultures previously isolated and genetically characterized and to assess the feasibility of near-infrared (NIR) spectroscopy for determining the yogurt quality.Total of 20 different yogurt samples were produced using combinations of 5 Streptococcus thermophilus and 4 Lactobacillus delbrueckii ssp. bulgaricus strains. Yogurt samples were stored at 4 °C for 21 days. Chemical, physical, microbial and organoleptic analyses for samples were conducted at predetermined days.As a conclusion, it was found that 4 Streptococcus thermophilus and 4 Lactobacillus delbrueckii ssp. bulgaricus yogurt isolates have potential to be used in dairy industry regarding their high technological and organoleptic characteristics. In addition, NIR spectroscopy technique could be used successfully on yogurt samples to predict total solids, fat, pH, syneresis, and microbial counts of yogurts based on the calibration models (Genetic Algoritm)