Post-acidification of set yoghurt always occurs during cold storage causing short shelf life thus consumer unacceptability.The short storage life of particular yoghurt affects the consumption level as well. This study concentrated on the inhibitory effect ofpreservatives on post-acidification of yoghurt. The set yogurt mixture was produced and inoculated with 2% starter culture containingStreptococcus thermophilus and Lactobacillus bulgaricus. After the required inoculation; ε-polylysine (as natural preservative) andpotassium sorbate (as chemical preservative) were added separately at different concentrations. Titratable acidity, pH and firmness ofyogurts during cold storage for 20 days were examined. Evaluation of sensory properties was done for the best concentrations of eachpreservative. The pH values and titratable acidity were significantly lower (<0.05) in yoghurts with added preservatives than those inblank yoghurt. The results showed that ε-polylysine and potassium sorbate used at 0.005% (w/v) and 0.1% (w/v), respectively gavethe best (optimum) inhibition of post-acidification during refrigerated storage for 20 days. The sensory evaluation revealed that0.005% (w/v) ε-polylysine was the most preferred preservative to consumers with respect to taste, colour, mouth feel, appearance andoverall acceptability. Hence, these results support the conclusion that there is an inhibitory effect of potassium sorbate and ε-polylysine on post-acidification of set yoghurt during cold storage while the later was most preferred by the panelists.
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