Production and quality assessment of a set-style yogurt fortified with low concentrations of Thymus vulgaris L phenolic extracts

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Abstract
The aim of this work was to evaluate the effect of Thymus vulgaris L phenolic extracts (TVPE) on the growth of Streptococcus thermophilus and Lactobacil- lus delbrueckii subsp. bulgaricus, and the impact of their addition in small amounts (2-8%) to a set-style yogurt with regard to variations in physico- chemical, bacteria count and organoleptic properties during 21 days storage at 4°C. The phenolic components were extracted from the crushed aerial parts of the test plant by cold maceration in hydromethanolic solution (methanol: water, 80:20 v/v). The plant polyphenolic profile was determined by HPLC. The survival ability of yogurt starter cultures in the presence of TVPE was evaluated through the determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The total phenolic content (TPC) was 20.2 ± 0.57 μg GAE / mg extract. The amount of flavonoid compounds was 39.83 μg EQ/mg extract. As the quantitative measurements revealed, apigenin and fisetin (8.17, 6.83, μg GAE /mg extract respectively) emerged as major phenolic compounds from thyme aerial parts. The MIC and MBC against the growth of both lactic bacteria were obtained at 60% of TVPE concentration. A significant (p< 0.01) changes in pH, trable acidity and apparent viscosity were observed in all experimental yogurts. The best scores of sensory evaluation were recorded in samples added with 2% and 4% of TVPE as well as the control. Based on collected data, 2% and 4% of TVPE can be incorporated into yogurt without risk of deterioration in physicochemical quality or inhibition of lactic bacteria.