Extraction of propolis extract and investigation of its effect on quality and shelf life of unpasteurized Doogh

Doogh is one of the traditional drinks in Iran and yeast is the original reason of its spoilage. In this research, propolis (a product of bee) was used for preventing the yeast growth in unpasteurized doogh. The polymerase chain reaction (PCR) method was used to diagnose the yeast that caused spoilage (Kluyveromyces marxianus). Water, glycerol and ethanol were used as solvents to extract phenolic compounds of propolis. According to extraction efficiency, capability of usage in food products and results of MIC and MBC tests, glycerol extract has been selected for Doogh examinations. The effect of 1.5, 3, 4.5, 6% of glycerol extract on pH, titratable acidity, DPPH, decreased yeast count and sensory properties of unpasteurized Doogh during storage at temperatures of 5 and 25°C. Results revealed that 3% glycerol extract of propolis, decreased yeast during storage at both temperatures of 5 and 25 °C without any negative effect on sensory parameters, for two and more than fourteen days, respectively. Also, total yeasts significantly decreased with increasing the glycerol extract of propolis (P<0.05). Furthermore, the DPPH was not affected during storage at both temperatures. Using the propolis extract in unpasteurized Doogh resulted in the prevention of yeast caused spoilage and specially production of a functional food containing high amounts of antioxidants.