Volatiles retention during the spray drying of coffee and maltodextrin

The quality of spray-dried coffee is often compromised by the loss of aroma components. The influences of feed solids content (20%-50%) and drier outlet temperature (70-120ºC) on the retention of selected volatiles in coffee and maltodextrin solutions were investigated. Volatiles were analysed before and after spray drying using headspace solid phase micro-extraction, followed by gas chromatography-mass spectrometry. It was found that higher feed solid contents gave markedly higher retentions (~80% retention for 50% solids compared to ~20% retention for 20% solids) at all drying temperatures and for both materials (in agreement with Thijssen’s selective diffusion theory). For maltodextrin, volatile retention was relatively insensitive to temperature, but for coffee it was noticeably worse at 120ºC. This may be explained with reference to the particle outlet temperature and its glass transition temperature. An exceptional result was found with 2-Methylbutanal which produced a “retention” of more than 100% for coffee (but not maltodextrin) when spray drying at an outlet temperature of 120ºC. It is likely that this compound is formed during spray drying by a Strecker degradation reaction.