Development of stickiness of whey protein isolate and lactose droplets during convective drying

The stickiness development of droplets of whey protein isolate (WPI), lactose and their mixture solutions was determined using an in situ stickiness testing device at 24, 65 and 80 °C. Stainless steel, Teflon, glass and polyurethane probes were used. At room temperature, the presence of 0.5-1% (w/w) WPI greatly lowered the observed tensile strength of water and lactose solutions due to surface adsorption and led to a weakening of the cohesive strength. At elevated temperatures, lactose droplets remained sticky showing cohesive failure until the surface was completely covered with a thin crystal layer. WPI droplets formed a thin, smooth skin immediately on coming in contact with drying air. This surface became non-sticky early in the course of drying due to the transformation of the surface to a glassy state. The skin forming and surface active nature of WPI was exploited to minimize the stickiness of honey in a pilot scale spray drying trial. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80% in a pilot scale drying test. At elevated temperature the magnitude of stickiness on probe materials was in the order of glass > stainless steel > polyurethane > Teflon. The Teflon surface offered the lowest stickiness both at low and high temperatures making it a suitable material to minimize stickiness through surface coating.