Volatiles retention during the sub-atmospheric spray freeze drying of coffee and maltodextrin

Sub-atmospheric fluidised bed freeze drying is a potential method of producing high quality dried beverages (such as coffee) more cheaply than using conventional freeze drying methods. Equipment at Loughborough has been further modified to enable fluidised bed freeze drying to be carried out at -30ºC to enable materials with low collapse temperatures to be freeze dried. Two base materials were tested: instant coffee and maltodextrin, which were both doped with a representative selection of volatile compounds found in ground coffee. Even at these low temperatures it was found that significant drying had occurred after 2 hours of fluidisation at 0.1 bara. Coffee and maltodextrin (both initially 20% solids content) had dried to moisture contents of 15% (wb) and 10.5% (wb) respectively. Volatiles were analyzed before and after drying using headspace solid phase micro-extraction, followed by gas chromatography-mass spectrometry. Slightly better volatile retention was found with maltodextrin (average ~60%) compared with coffee (average ~50%). Retentions were comparable to those obtained by conventional freeze drying. It was found from spray freezing experiments with maltodextrin that approximately half the volatile loss from spray freeze drying occurs in the spray freezing step.