QUALITY OF YOGHURT MADE FROM COW'S MILK FORTIFIED WITH WHEY PROTEIN ISOLATE

Fiveyoghurt treatments were made to study the effect of fortifying cow's milk with wheyprotein isolate on the quality of yoghurt, control yoghurt was made by adding 3% non-fat drymilk to cow's milk while the other four treatments were made by fortifying cow's milk with 0.5,1.0, 1.5 and 2.0% whey protein isolate respectively and 3.0% nonfat dry milk to each treatment.All yoghurt treatment was stored in refrigerator for 12 days and was sampled when fresh and at 3,6, 9 and 12 days for chemical, rheological, microbiological analysis and sensory evaluation. Theobtained results indicated that adding whey protein isolate to cow's milk caused a significantincrease of total solids, total protein, ash contents and titratable acidity, while decreased pH valuesand whey syneresis of yoghurt treatments and these effects were proportional to the rate of addingwhey protein isolate. Also, the values of Hardness, adhesiveness, cohesiveness, springiness andgumminess of the resulting yoghurt had increased and this increase was proportional to the rate offortification. Adding whey protein isolate up to 1.5% increased the scores of organolepticproperties and treatment that made with adding 1.5% whey protein isolate was the mostacceptable yoghurt treatments. Total solids, total protein, ash and fat contents of all yoghurttreatments did not change significantly, (P > 0.05) during storage period, while titratable acidityincreased. Whey separation decreased during storage period up to the sixth day of storage periodthen increased up to the end of storage period, while the scores of organoleptic properties werealmost stable up to the ninth day of storage period.