Water Sorption, Glass Transition, and Microstructures of Refractance Window– and Freeze-Dried Mango (Philippine “Carabao” Var.) Powder

Abstract

Water sorption isotherms, glass transition, and microstructures of Refractance Window (RW)– and freeze-dried Philippine “Carabao” mango powders were investigated. Water sorption isotherms were developed by the isopiestic method, while thermal transition of the powders, at various water activities (a _w  = 0.11–0.86), was determined using differential scanning calorimetry (DSC). The sorption isotherms of RW- and freeze-dried (FD) mango powders exhibited a type III sigmoidal curve, showing higher and lower adsorption capacities above and below 0.5 a _w , respectively. A significant difference (p < 0.05) in water content of RW- and freeze-dried mango powders for equivalent water activities was obtained above 0.5 a _w . The onset glass transition temperature (T _gi ) of RW- and freeze-dried mango powder solids decreased as the water content increased. There were no significant differences (p ≥ 0.05) in T _gi of RW- and freeze-dried mango powder solids at constant water activities, except for a _w  = 0.86. Microscopic examination of mango powders indicated that freeze-dried mango powders exhibited greater surface area and porosity in comparison to RW-dried mango powders.

Keywords:

• Freeze drying
• Glass transition temperature
• Mango powder
• Refractance window drying
• Scanning electron microscopy
• Water adsorption

ACKNOWLEDGMENTS

This study was supported by the Ford Foundation International Fellowship Program (IFP)-Philippines and IFP/Institute of International Education, New York. We thank the Philippine Center for Postharvest Development and Mechanization (PhilMech) for granting study leave to author Ofero Caparino. Special thanks to Richard E. Magoon and Karin M. Bolland (MCD Technologies, Inc., Tacoma, WA) for allowing the use of their RW drying facilities and for their assistance with the experiments; Engr. Frank Younce for technical assistance on the operation of the freeze dryer; and Dr. Valerie Lynch-Holm for helping with SEM imaging.

Notes

RW: Refractance Window-dried mango powder; FD: Freeze dried mango powder.

^a Present study.

^b Rangel–Marrón et al.^[28]

^c Falade and Aworth.^[23]

^d Talla et al.^[45]

^e Syamaladevi et al.^[10]

^f Jakubczyk et al.^[27]

NA - Not available.

M _g - Monolayer moisture content.

^{(a–s); (1–23)}Different superscript letters and numbers represent statistical significant differences of glass transition temperatures of RW- and freeze-dried mango powders at various water activities (a _w  = 0.113–0.86).