Drying and Denaturation Kinetics of Whey Protein Isolate (WPI) During Convective Air Drying Process

Abstract

The denaturation and drying kinetics of whey protein isolate (WPI) in a convective drying (CD) environment was measured using single droplet drying experiments. The moisture content and temperature histories during drying of WPI droplets were predicted using reaction kinetics–based models. The denaturation kinetics of WPI in the CD process was predicted using first-order reaction kinetics considering the denaturation rate constant to be moisture content and temperature dependent. Single droplets of WPI (10% [w/v], 2.0 ± 0.1 mm initial diameter) were used throughout these experiments. The drying experiments were carried out at two temperatures (65 and 80°C) at a constant air velocity (0.5 m/s) for 600 s. The extent and nature of the denaturation of WPI during the CD was compared with those in isothermal heat treatments (IHT) at the same medium temperatures. The denaturation of WPI was 68.31% in convective air drying at 65°C and 600 s and it was 10.79% in the IHT at the same temperature and time. The stress due to dehydration and the exposure time were found to be responsible for the denaturation of WPI in the CD process and long exposure time was found to be responsible for its denaturation in the IHT process. At the media temperature of 80°C, the denaturation loss of WPI was 90.00 and 68.73% in IHT and CD processes, respectively. Both the thermal (moist heat) and dehydration stresses were found to be responsible for denaturation of WPI during CD process and very high thermal stress was found to be responsible for denaturation of WPI during the IHT. There was good agreement between the experimental and reaction engineering approach (REA)-predicted moisture content and temperature histories. The experimental moisture content and temperature histories were followed by the respective REA predictions within 6.5% (R ² = 0.995) and 3% (R ² = 0.981) errors, respectively. The denaturation kinetics of WPI during CD was predicted well (R ² = 0.95 – 0.98; average error = 6.5 ± 0.5%) by a first-order reaction kinetics model.

Keywords:

• Denaturation kinetics
• Drying kinetics
• Isothermal heat treatment
• Reaction engineering (REA) approach
• Single droplet drying
• Whey protein isolate

ACKNOWLEDGMENTS

The first author thanks the Australian government for providing him with an Endeavour Postgraduate Scholarship. The authors thank Bruce Armstrong for technical help during the experiments. The Fonterra Cooperative (New Zealand) is acknowledged for the donation of whey protein isolate (WPI). This work was also partially supported by the Australian government's Collaborative Research Network (CRN) initiative.