2.2. Determination and modeling of the thermal transitions using
DSC
A differential scanning calorimetry (DSC, Q200, TA Instruments,
USA) was used to measure the glass transition and freezing point
of the L. edodes mushrooms at different water contents, following
the method described by Xiao et al. [31] with slight modifications.
Liquid nitrogen was used for sample cooling before the
experiments. The DSC was calibrated for heat flow and temperature
using distilled water (melting point 0.0 ◦C, Hm = 334 kJ/kg)
and indium (melting point 156.5 ◦C, Hm = 28.5 kJ/kg). The samples
(about 5 ∼ 8 mg) were enclosed in hermetically sealed aluminum
pans and loaded onto the equipment at room temperature. An
empty sealed aluminum pan was used as a reference in each analysis,
and nitrogen gas at a flow rate of 50 mL/min was used as the
purge gas to avoid water condensation around the L. edodes sample.
2.2.1. Glass transition of samples containing unfreezable water
The L. edodes samples (containing unfreezable water) were
sealed in aluminum pans and cooled from ambient temperature to
−85 ◦C at 5 ◦C/min, held for 10 min and heated at 5 ◦C/min to 60 ◦C.
The L. edodes samples were then scanned from 60 ◦C to −85 ◦C at
5 ◦C/min and reheated at 5 ◦C/min up to 60 ◦C. A double scanning
program of the samples was used in this method to eliminate the
enthalpy relaxation of the amorphous samples, which appears in
the first scan. Through the second scanning, a clear baseline shift of
the glass transition was observed [26,29]. TA Instruments Universal
analysis software was used to analyze the onset, mid and end points
of the glass transition. The experiments were performed in triplicate
to determine the glass transition temperature at each water
content/water activity.