App118 Determination of Moisture Sorption and Phase Transformation for Instant Coffee Granules
This application note reports a systematic study of moisture sorption behavior and humidity‑induced phase transitions in three instant coffee granule samples using a Dynamic Vapor Sorption (DVS) Resolution system at 25 °C. The authors performed gravimetric sorption–desorption cycles from 0 % to 90 % relative humidity (RH) in 10 % increments, defining equilibrium by a dm/dt stability criterion of 0.002 %/min. Two experimental protocols were used: (1) a standard sorption isotherm measurement and (2) a dedicated phase‑transition determination where samples were dried, ramped to 90 % RH over 18 h, and held for 1 h. Results show distinct differences among the samples: Sample 3, a fine powder with high BET surface area, exhibits the highest total moisture uptake, a larger primary sorption parameter (A ≈ 0.004 mol g⁻¹), and an earlier glass‑transition humidity (~38.6 % p/p₀). Samples 1 and 2, with larger granules and lower surface area, display lower hygroscopicity, higher Tg (~47 % p/p₀), and slower kinetics. Young–Nelson modeling quantifies primary (A) and multilayer (B) sorption sites, revealing a trend of increasing surface affinity from Sample 1 < Sample 2 < Sample 3. All samples deliquesce at 90 % RH, indicating loss of physical stability. The note concludes that higher hygroscopicity (Sample 3) correlates with faster dissolution but poorer shelf‑life stability, whereas Samples 1 and 2 offer better storage stability at the cost of slower dissolution. The document includes isotherm, dm/dt, and phase‑transition plots, as well as a table summarizing pre‑ and post‑analysis observations.
Topics
DVS Resolution System: Principle and Configuration
Overview of the Dynamic Vapor Sorption instrument, microbalance setup, sorbate flow control, and measurement precision.
Experimental Protocols for Moisture Sorption Isotherms and Phase‑Transition Determination
Detailed description of the two measurement methods, humidity ramp steps, equilibrium criteria (dm/dt ≤ 0.002 %/min), temperature control, and timing.
Comparative Moisture Uptake and Kinetics of Three Instant Coffee Samples
Presentation of sorption curves, equilibrium moisture percentages, and kinetic differences among Sample 1, Sample 2, and Sample 3.
Critical Relative Humidity and Glass‑Transition Points
Identification of humidity levels where each sample undergoes a glass transition or deliquescence, with Tg values (~38.6 % for Sample 3, ~47 % for Samples 1/2).
Young–Nelson Modeling of Primary (A) and Multilayer (B) Sorption Parameters
Extraction and interpretation of A and B values for each sample, linking them to surface affinity, hygroscopicity, and sorption mechanisms.
Implications for Product Stability, Shelf Life, and Dissolution Performance
Discussion of how hygroscopicity and phase‑transition behavior affect storage stability, caking risk, and water‑solubility of instant coffee.