(150b) Sticky point determination in dairy powders using the particle gun technique

Stickiness, adhesion and caking of dairy powder particles in chambers, cyclones and ducting surfaces are common and have the potential to cause severe problems in spray drying operations. Stickiness has been shown to be a surface phenomenon related to the glass transition temperature (Tg) of the main sugar component, lactose. Dairy powder stickiness problems are currently being approached through implementing sticky curves using the T-Tg concept to identify sticky and non-sticky conditions.

The particle gun method developed in prior work was used to investigate the sticky point of skim milk powder (SMP) under various testing conditions. The particle gun fires particles at a collection plate and is designed to mimic conditions encountered in spray drying operations. The effects of altering key variables relating to the particle gun rig operation were quantified. Variables investigated included ambient air relative humidity (RH) and temperature, powder water activity (a_w), air velocity and angle of impact. From these experiments the error inherent in the technique was determined.

Ambient RH conditions influenced the point at which significant amounts of powder started to stick to the target plate (the ?sticky point?) as determined by the particle gun, with SMP becoming stickier at a higher ambient %RH over the range 40-70%RH. This sticky point occurs at a constant temperature above the glass transition temperature of lactose regardless of the temperature and RH conditions of the air in the particle gun that are used to create the difference. We have called this temperature difference (T-Tg)_critical. SMP a_w did not significantly affect the sticky point obtained, confirming stickiness is indeed a surface phenomenon. Air velocity through the particle gun was varied from 10-30 m.s^-1 and was found to have a small influence on the sticky point, shifting the sticky point higher at the higher air velocities. The angle of impact was varied between 29-90^o with the effect of reducing the rate of stickiness development at a smaller angle of impact; however the (T-Tg)_critical value was not significantly altered.