Effects of temperature and concentration on mechanism and kinetics of thermally induced deposition from coffee extracts

Marek Kroslak, Massimo Morbidelli, Jan Sefcik

Research output: Contribution to journalArticle

20 Downloads (Pure)

Abstract

Production of soluble (instant) coffee powders typically involves extraction of roasted coffee by water followed by evaporation in order to concentrate extracts before spray or freeze drying to produce dry coffee powder. In the course of evaporation, deposition of dissolved material from coffee extracts is a major cause of fouling at the heat exchange surfaces of evaporators. Therefore, in order to improve the design and optimization of evaporation processes of coffee extracts, better understanding of the deposition mechanism and kinetics is needed. In this study, optical waveguide lightmode spectroscopy (OWLS) was used to monitor the initial formation of nanometer scale deposits on surfaces exposed to coffee extracts. OWLS measurements were complemented by light scattering from extract solutions, gravimetry of macroscopic deposits, and scanning electron microscopy imaging of deposited layers. Primary molecular-scale layers of about 1 mg m^−2 were rapidly formed in the first stage of deposition, even at ambient temperature, followed by the secondary deposition with kinetics strongly dependent on temperature. Secondary deposition rates were low and largely independent of the extract concentration at ambient temperature, but became strongly dependent on the extract concentration at elevated temperatures. In particular, activation energies for the deposition between 25◦C and 70◦C were much higher for the original extract (13.3 mass %, solids) than for diluted extracts (up to 1.3 mass %, solids). Furthermore, heating of the original extracts above 60◦C resulted in rapid aggregation of suspended macromolecules into large clusters, while only gradual aggregation was observed in diluted extracts.
Original languageEnglish
Pages (from-to)1755–1766
Number of pages12
JournalChemical Papers
Volume68
Issue number12
Early online date4 Nov 2014
DOIs
Publication statusPublished - 1 Dec 2014

Keywords

  • coffee extracts
  • deposition
  • aggregation
  • fouling
  • evaporation
  • heat exchange

Fingerprint Dive into the research topics of 'Effects of temperature and concentration on mechanism and kinetics of thermally induced deposition from coffee extracts'. Together they form a unique fingerprint.

  • Cite this