Abstract:
Sensory quality and physicochemical properties are very important to determine yoghurt quality. Sensory attributes of yoghurt can be affected by starter cultures and pasteurization temperature in yoghurt manufacturing process. This study was aimed to investigate effect of three commercial starter cultures namely YoFlex Mild 1.0 and YF-L904 (thermophilic yoghurt cultures containing strains of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophiles) and RST-743(Lactococcus lactic subsp. lactic Streptococcus thermophiles) combined with different pasteurization temperature and cold storage on physicochemical properties and descriptive sensory quality of yoghurt made from bovine milk. Accordingly fresh bovine milk with pH of 6.7 was used to prepare yoghurt in the presence of these commercial starter cultures YoFlex Mild 1.0, YF-L904 and RST-743 under the conditions of two pasteurization temperatures (T1=85 and T2= 95 ). The physicochemical properties including pH, titratable acidity (TA), syneresis, protein, fat content, dry matter (DM), viscosity and Non-Fat Solid (NFS) and descriptive sensory properties including appearance, aroma, taste and texture were evaluated after 24h and seven day cold storage labeled Day0 & Day7 storage time. General Linear Model (GLM) by multivariate analysis was performed to analyze mean result and significances level were determined at P < 0.05. The results showed that pH value decrease as both pasteurization temperatures and storage time increases whereas; the titriable acidity was in reverse. But both pH and titriable acidity was significantly different (P < 0.05) for all samples. Protein and fat content were slightly decreases as both pasteurization temperature and storage time increase except in yoghurt samples prepared by RST-743 at pasteurization temperature of 95 (T2) in which very slight increment shown for protein. However, the change was not significantly different (P>0.05) in the protein and fat content of all yoghurt, samples. Syneresis decrease as pasteurization temperature increase but slightly increase as storage time increases whereas; Viscosity had shown opposite. The descriptive sensory analyses were also performed for four sensory properties appearance, aroma, taste and texture. The appearance with attributes watery, shiny, milky and creamy scores ranged from 11.19 1.31 for creamy to 2.12 1.46 for watery in YF at the same storage time Day0 and pasteurization temperature of T1= 85 and T2= 95 respectivel. Watery properties of all the yoghurt samples were increases as storage time increase but decrease as pasteurization temperature increases. Among the aroma attributes buttery, pungent, smoky and salty; buttery scores maximum mean value (8.45 2.18) for Yf yoghurt at T2 and day0. The mean scores of all attributes of aroma were not significantly different (p>0.05) for all effects. In similar sweetness, sour, salty, and bitter tastes were in order of higher to lower mean value result with slight increasing as both storage time and pasteurization temperature increases. But significant change was not observed (p>0.05). From texture attributes smoothness, slimy, stick, grainy and firmness evaluated by panelists, smoothness and firmness presented higher mean scores and all texture attributes did not differ significantly among prepared yoghurt (P > 0.05).