Implication of water state on glass transition temperature in hot air-dried carrot slices

•Water state and Tg of hot air-dried carrot dramatically changed.•When free water dominated in tissues, the Tg approached a constant.•Decreased relaxation behavior of immobilized water increased Tg.•Tg had a negatively linear relationship with the T22 of immobilized water.•The deteriorated microstructure could explain these changes.

This study assessed and correlated the water state and the glass transition temperature (Tg) of carrots treated by hot air drying at 50, 60, and 70 °C. Results showed that within carrots with high moisture content (Xw, 4.15 ≥ Xw ≥ 1.01 g water/g dry matter), free water in vacuoles dominated in tissues and the Tg remained practically constant (−48.4 ± 1.04 °C). And when the Xw of carrots was ≤0.41 g water/g dry matter, the Tg was increased by 6.79 and 22.7 °C with the relaxation time (T22) of the immobilized water in the cytoplasm/extracellular space decreasing by 12.4 and 11.8 m, respectively. Moreover, correlation equations indicated that the Tg had a negatively linear relationship with the T22 in dried carrots. These results suggested that the Tg were elevated appreciably by reducing the relaxation behavior of immobilized water in hot air-dried carrots. The more pronounced variation of the microstructure (e.g., vacuole disruption, cell wall dissociation, and turgidity loss) of tissues could be responsible for these results.