Stickiness during drying of amorphous skin-forming solutions using a probe tack test

This paper develops the methodology used to track the development of stickiness for droplets of amorphous skin-forming solutions during drying, by means of the probe tack test. Three material solutions were selected for study: maltodextrin DE5, whey protein isolate and gum arabic, each at two concentrations between 20 and 40 wt%. Two air temperatures were used for drying, 40 and 78 °C. Three modes of failure were observed, cohesive, adhesive and cohesive–adhesive corresponding respectively to failure within the viscous bulk of the solution, at the adhesive interface, or a combination which was often associated with spot tack where partial adhesion remains at the probe surface. Stickiness was recorded as a peak tensile strength and this varied with drying. Early in drying, the peak tensile strengths were low and reproducibility was relatively good with a COV ± 3.4%. Later in drying, the peak tensile strength reached a maximum and reproducibility diminished with a COV ± 7.2%, due to both the changing failure mode associated with spot tack and uncertainty in the contact area between the probe and the sample surface. Subsequently, the surface rapidly became dry and no adhesion was observed. All three material solutions had this profile but the location of the maximum tensile strength and point of non-adhesion varied with drying temperature, initial solution concentration and material type. Importantly, all solutions reached the point of non-adhesion when the bulk moisture contents were still relatively high, ∼1 kg kg−1. This means materials such as maltodextrin DE5, whey protein isolate and gum arabic offer distinct advantages in food coating systems because they are sticky over a small range of bulk moisture contents and the surfaces are easily dried below the point of non-adhesion thus minimising the probability of unwanted inter-particle agglomeration. The probe tack test is, therefore, a useful tool for formulation and drying process design.