Sticky behavior of whey protein isolate and lactose droplets during convective drying

Stickiness of whey protein isolate (WPI) and spray dried lactose droplets was studied at two air temperatures (65±0.5°C, 80±0.5°C), 0.75 m/s air velocity and 2-2.5% relative humidity using an in situ stickiness testing device. A stainless steel probe with 50 mm/min contact/withdrawal speed was used. The moisture and temperature histories were measured through parallel experiments. In each case, the surface of the lactose droplet remained sticky and failed cohesively until the surface was completely surrounded with crystals. The crystal layer remained fragile, fractured upon the probe contact and a thin layer of solution came out to the probe surface even after the moisture (u, dry basis) was lower than 0.2. WPI droplets formed thin and smooth skin immediately after coming in contact with hot air. The tensile strength of this skin increased rapidly and peaked (u = 2.14 at 45°C and u = 1. 47 at 65.7°C) fairly early during drying process. WPI droplet surface became completely non-sticky soon after attaining the peak tensile strength (u =1.32 at 53.4 °C and u= 1.05 at 68.8°C), mainly due to transformation of the outer layer of the skin into glassy material. The skin forming and surface active nature of WPI was exploited to minimize the stickiness of honey during spray drying. Replacement of 5% (w/w) maltodextrin with WPI raised the powder recovery of honey solids from 28% to 80%. Stickiness of the WPI on glass, Teflon and polyurethane surfaces was studied by replacing the contact surface of the probe with these materials. It was found that the stickiness of glass surface was the highest at test temperatures. Teflon surface offered the lowest stickiness at the test temperatures making it suitable materials to minimize solution/particle stickiness through coating