Spray drying of reconstituted skim milk fermented with Lactobacillus rhamnosus GG: control of glass transition and stickiness

The effects of glass transition and stickiness on the direct spray drying of reconstituted skim milk (RSM) fermented with Lactobacillus rhamnosus GG (LGG) were investigated. The fermented RSM did not spray dry properly due to severe wall depositions; however, it dried well (comparable to the control; RSM with resuspended LGG cells) when skim milk powder (SMP) was added. Adding SMP significantly increased the glass transition and sticky point temperatures of spray-dried powder, ranging from 18.2 to 72.4 °C and 34.5 to 78.5 °C, respectively, in a water activity range of 0–0.33. By adding SMP, droplets quickly shifted from a sticky plastic to a non-sticky glassy state during drying, resulting in reduced wall deposition. Although this spray-dried powder exhibited relatively high moisture sorption and lactose crystallization, the correlations between glass transition, stickiness, and moisture sorption suggested that its storage stability at 25 °C may be on par with the control powder.