Optimisation of foam-mat freeze-drying conditions for blueberry powder and evaluation of powder properties

The aim of this study was to develop blueberry powder via foam-mat freeze-drying (FMFD) and to investigate the foam and powder properties. The spray-drying (SD) method was also carried out as a comparison to foam-mat freeze-drying. Foam-mat freeze-drying of blueberry juice was conducted using constant temperature (-55 0C) and vacuum pressure (0.04 mbar) for 24 h, while spray-drying was tested using two feed flow rates, 180 (SD 180) and 360 (SD 360) mL h-1, and other conditions of spray-drying were kept constant. Foam properties were compared amongst foam made with ratio 2.8 of maltodextrin/whey protein isolate (M3W1), trehalose/β-lactoglobulin (T3BL1), and trehalose/bovine serum albumin (T3A1). The M3W1 foam was found less stable than those with T3BL1 and T3A1 since the M3W1 gave high foam density. FMFD powder made with M3W1 had higher moisture content (3.5%), longer rehydration time (90 s), and lower bulk density (0.32 g cm-3) over the T3BL1 and T3A1. The FMFD powders made with trehalose/pure proteins generated pores and ordered structures, while the M3W1 powder had broken glass-like structures. The M3W1 reconstituted powder gave high a*/redness which was probably attributed to high total monomeric anthocyanins (TMA) content (8.5 mg Cyn3Gl g-1 solids). The MD/WPI as matrices had reduced losses in the total phenolic content (TPC) and individual anthocyanin than T3A1 or T3BL1. The TPC and TMA retention of M3W1 reconstituted powder were recorded as 73 and 95%, respectively. FMFD and SD powders made with MD/WPI 0.4-3.2 were also compared. The FMFD had a higher yield, lower moisture content, longer rehydration time and lower bulk density compared to those of spray-dried (SD) powders. The FMFD samples were purple powders and flake-like shaped of particles, while SD samples were bright pink powders. The SD powder particles were smooth, spherical and smaller sized than the FMFD powder. The TMA of FMFD powders showed higher levels than those of SD samples. Delphinidin-3-glucosiede (Del3Gl), Cyanidin-3-glucoside (Cyn3Gl), and Malvidin-3-glucoside (Mal3Gl) retentions were greater, in the order: FMFD > SD.