Comparison of Viscoelastic Properties of Set and Stirred Yogurts Made from High Pressure and Thermally Treated Milks

Abstract

Viscoelastic properties of set and stirred yogurts made from raw (control), high pressure (300 and 400 MPa, 20 min, 4°C), and heat treated (80°C, 30 min) milks were investigated using a using a computer controlled rotational viscometer under oscillatory testing program, including three operating modes: a strain sweep, frequency sweep, and an isothermal time sweep. Linear, exponential, power-law, and Weltmann models were used to assess and describe the strain, frequency, and time dependent viscoelastic properties of yogurts. Their significance was analyzed using Duncan’s multiple range tests. The results indicated that the set yogurts had larger moduli (storage moduli G’ and loss moduli G”) and lower phase shift (tan δ) than stirred yogurts. High pressure treatment increased storage moduli G’ of both stirred and set yogurts significantly (p < 0.05). The influence on loss modulus was relatively lower. Unlike yogurts made from heat treated and raw milks, the ones made from pressure treated milks resulted in significantly different gel, frequency, and strain dependent properties for both types of yogurts. However, time dependent properties were not affected by pressure treatment.