Comparative study of denaturation of whey protein isolate (WPI) in convective air drying and isothermal heat treatment processes

• Denaturation of WPI was higher in convective drying (CD) than in isothermal heat treatment (IHT) ⩽65 °C.
• The stress due to dehydration denatured significant (p < 0.05) amount of WPI during CD ⩽65 °C.
• Both the dehydration and thermal stresses were dominant during CD of WPI ⩾65 °C.
• Denaturation of WPI was higher in IHT than in CD at >65 °C.
• Denaturation sensitivity of whey proteins during CD followed β-Lg > α-Lac > BSA.

The extent and nature of denaturation of whey protein isolate (WPI) in convective air drying environments was measured and analysed using single droplet drying. A custom-built, single droplet drying instrument was used for this purpose. Single droplets having 5 ± 0.1 μl volume (initial droplet diameter 1.5 ± 0.1 mm) containing 10% (w/v) WPI were dried at air temperatures of 45, 65 and 80 °C for 600 s at constant air velocity of 0.5 m/s. The extent and nature of denaturation of WPI in isothermal heat treatment processes was measured at 65 and 80 °C for 600 s and compared with those obtained from convective air drying. The extent of denaturation of WPI in a high hydrostatic pressure environment (600 MPa for 600 s) was also determined. The results showed that at the end of 600 s of convective drying at 65 °C the denaturation of WPI was 68.3%, while it was only 10.8% during isothermal heat treatment at the same medium temperature. When the medium temperature was maintained at 80 °C, the denaturation loss of WPI was 90.0% and 68.7% during isothermal heat treatment and convective drying, respectively. The bovine serum albumin (BSA) fraction of WPI was found to be more stable in the convective drying conditions than β-lactoglobulin and α-lactalbumin, especially at longer drying times. The extent of denaturation of WPI in convective air drying (65 and 80 °C) and isotheral heat treatment (80 °C) for 600 s was found to be higher than its denaturation in a high hydrostatic pressure environment at ambient temperature (600 MPa for 600 s).