Effect of far-infrared drying on the water state and glass transition temperature in carrots

Journal of Food Engineering 136 (2014) 42–47

Contents lists available at ScienceDirect

Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Effect of far-infrared drying on the water state and glass transition temperature in carrots Congcong Xu a, Yunfei Li a,⇑, Huaning Yu a,b a b

Department of Food Science and Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, PR China State Key Laboratory of Dairy Biotechnology, Technical Centre, Bright Dairy and Food Co., Ltd., Shanghai 200436, PR China

a r t i c l e

i n f o

Article history: Received 18 September 2013 Received in revised form 10 March 2014 Accepted 22 March 2014 Available online 29 March 2014 Keywords: Glass transition temperature Water state Far-infrared drying DSC NMR

a b s t r a c t The effect of far-infrared drying (FID) on the water state and glass transition temperature (Tg) of carrots were assessed by nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). Results showed that, with increasing time, FID caused dramatic changes in the water state in dehydrated carrots. The drastically decreased content of free water in vacuoles was accompanied by sharp increases in the content of immobilized water in the cytoplasm and extracellular space. Subsequently, the content of immobilized water decreased gradually with increasing time. FID elevated the Tg values appreciably by decreasing the content of immobilized water in carrot tissues. In carrots with moisture content <0.39 g/g dry matter, Tg values were increased by 21.36, 35.55 and 40.99 °C with the moisture content decreased by 0.20, 0.25 and 0.30 g/g dry matter at respective infrared power values of 400, 600 and 800 W. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Carrots are root vegetables that are cultivated worldwide. In recent years, possibly due to the appreciable amounts of vitamins (A, B1, B2, B6, B12), b-carotene, fiber content and important minerals contained within carrots, the consumption of carrots has increased steadily. Carrots usually need to be dried before use. Dried carrots are consumed in various ready-to-eat meals such as instant powdered soups, sauces, and seasonings (Doymaz, 2004; Fan et al., 2005; Zielinska and Markowski, 2007). Attributed to the advantages of higher drying rate, shorter drying time, energy-saving and uniform distribution of temperature to give a better-quality product, far-infrared drying (FID) has received considerable attention for drying various agricultural products, such as peaches (Wang and Sheng, 2006), sweet potatoes (Lin et al., 2005) and yams (Lin et al., 2007). Thus, FID could be a favorable way to dry carrots. However, most studies have concentrated on the effect of FID on the quality of carrots, such as color, shrinkage, rehydration ratio and particle density (Baysal et al., 2003), or on the effect of infrared power on drying rate, drying time and energy consumption in carrots (Kocabiyik and Tezer, 2009). The effect of FID on the water state and glass transition temperature (Tg) in carrots has not been investigated. ⇑ Corresponding author. Tel./fax: +86 21 34206918. E-mail address: yfl[email protected] (Y. Li). http://dx.doi.org/10.1016/j.jfoodeng.2014.03.022 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.

A fundamental understanding of the water state in food is crucial for obtaining a product with favorable quality in the food industry (Hills and Remigereau, 1997; Hills and Nott, 1999; Xin et al., 2013). Additionally, the Tg of various amorphous food materials plays an important role in the processing and storage stability of products (Kasapis, 2006). It is well known that, at temperatures below the Tg, the diffusion-controlled deteriorative process in product quality is arrested or not significant within a practical time scale. It has been reported that, if stored below the Tg, the loss rate of L-ascorbic acid of modified osmodehydrofrozen peas was reduced by as much as threefold (Giannakourou and Taoukis, 2003). Hence, foods should be stored at a temperature below their Tg for better preservation. However, due to the high moisture content in plant tissues, the Tg of fruits and vegetables is often lower than the common storage temperature (20 °C). Therefore, elevation of the Tg of fruits and vegetables is necessary. Food materials are considered to be systems of water-plasticized natural polymers, and their Tg is very sensitive to changes in moisture content and temperature. Symaladevi et al. (2009) reported that the Tg of the freeze-dried raspberry was increased by 81.30 °C with the moisture content decreased by 20.8%. Increasing the amount of water loss and solid content is often employed to substantially increase the Tg of the matrix (Goula and Adamopoulos, 2010) and thus enhance the stability of fruits and vegetables. Therefore, FID is a potential method to increase the Tg of carrots by dehydration.

43

C. Xu et al. / Journal of Food Engineering 136 (2014) 42–47

We investigated the effect of FID on changes in the water state in carrot tissues as well as the effectiveness of FID on the increase in the Tg of carrots. Furthermore, the relationship between the Tg and different water states in carrots was explored using correlation analysis and path analysis to assess the effect of the water state on the Tg in carrots. 2. Materials and methods 2.1. Materials The cultivar of the carrots used in this study was grown in Shanghai province and harvested in May. On the basis of uniformity and absence of damage, fresh carrots (Daucus carota L.) were purchased from a local market and stored at 4 °C. Raw carrots were peeled and cut with a sharp knife into slices with thickness 5.0 ± 1 mm (diameter, 30.5 ± 1.6 mm). Samples of sliced carrots were placed uniformly on a wire mesh tray in a thin layer. Drying experiments were carried out at the infrared power of 400, 600 and 800 W (3 W) at an air velocity of 1.0 ms1. Carrot samples were dried for 10 h and some samples were taken out every hour for the determination of indicators. 2.2. Calculation of moisture content and water loss To estimate the moisture content, sliced carrot samples (initial mass, W1) were dried in an oven at 105 °C to attain a constant mass, W2. Moisture content (Xws, g/g dry matter; Xw, g/g total) and solid content (Xs, g/g total) were calculated using Eqs. (1) and (2), respectively.

X ws ¼

W1  W2 ; W2

Xw ¼

W1  W2 W1

Xs ¼ 1  Xw

ð1Þ ð2Þ

The water loss (WL, g water/g initial dry matter) of carrot samples is expressed according to Eq. (3) (Lowithun and Charoenrein, 2009).

WL ¼

ðM 0  m0 Þ  ðM  mÞ m0

2.4. DSC A differential scanning calorimeter (Pyris Diamond; Perkin–Elmer, Boston, MA, USA) was employed to measure the thermal transitions in carrot samples obtained from FID treatment. Temperature and melting enthalpy (DHm) calibrations of the instrument were undertaken with distilled water (melting point = 0.0 °C, DHm = 333.88 J/g) and indium (melting point = 156.60 °C, DHm = 28.45 J/g). An empty sealed aluminum pan was used as a reference. Nitrogen gas at a flow rate of 50 mL/min was employed as the purge gas to avoid water condensation around the sample during the runs. Samples (10–15 mg) from each treatment were encapsulated into hermetically sealed aluminum pans and then loaded onto the equipment at room temperature. According to the method proposed by Shi et al. (2012), the Tg of samples was assayed with some modifications. Briefly, samples were cooled from the load temperature (30 °C) to 20 °C and equilibrated at 20 °C for 5 min. After equilibration, samples were cooled sequentially to 100 °C at 20 °C/min, and then equilibrated at 100 °C for 5 min. Finally, samples were scanned from 100 to 50 °C at 10 °C/min to determine the Tg. The TA universal analysis software provided with the DSC instrument was employed to assess the onset (Tgo), mid-point (Tgm) and end (Tge) of the glass transition. The mid-point of the transition was taken to be the Tg. Two thermograms are presented to show the glass transition: one for samples containing un-freezable water (Fig. 1a) and another one for samples containing freezable water (Fig. 1b). Measurements were made in triplicate and a mean value taken. The Gordon–Taylor model is considered to be a reliable predictor of the Tg as a function of moisture content. The Gordon–Taylor empirical equation is:

Tg ¼

X s ðT gs Þ þ kX w ðT gw Þ X s þ kX w

ð4Þ

where Xs and Xw (g/g total) are the corresponding contents of solid matrix and water; Tg, Tgs and Tgw are the Tg of the samples, solid matrix and water; and k is the Gordon–Taylor parameter, respectively. The Tg, Xs and Xw were detected in the study, and the Tgw was 133.88 °C. The k and Tgs were estimated with non-linear regression analysis using Matlab R2013a (MathWorks).

ð3Þ

where M0 (g) is the initial mass of fresh carrots before drying treatment, M (g) is the mass of carrots after drying treatment, m0 (g) is the dry mass of fresh carrots, and m (g) is the dry mass of carrots after drying treatment. Data were reported as the mean value of three replicates for each treatment.

2.5. Statistical analyses Statistical analyses were undertaken using one-way analysis of variance (ANOVA) followed by the Duncan test at a confidence level of 95%. Data are expressed as the mean ± standard deviation (SD). Pearson’s correlation analysis was conducted by combining the data from all treatments to address the relationship between

2.3. NMR

(mW)

-8 End

Tg

-10 -12

Heat FlowEndo Up

A NMI 20-Analyst system (22.6 MHz; Shanghai Niumag Corporation, Shanghai, China) was employed to monitor the water state in carrot samples. The ideal center frequency and pulse width were found automatically by the system to achieve the optimal conditions for NMR. Approximately 400 mg of stripped carrot samples was placed inside glass tubes (outer diameter, 10 mm) to avoid exceeding the active region of the radiofrequency coil. Carr–Purcell–Meiboom–Gill (CPMG) pulse sequences were used to measure the spin–spin relaxation time (T2). Optimal pulse parameters in the CPMG test were: 90° pulse width = 16.5 ls; 180° pulse width = 33.0 ls; receiver frequency = 23 MHz; recycle time = 3500 ms, repeated scanning number = 8; echo count = 10,000; echo time = 105 ls. Data were reported as the mean value of three duplicate NMR measurements (total of six runs) for each treatment.

a

Onset M

-14 -16

Tg

b

End

-18 Onset

-20 -100

-80

-60

-40

-20

0

20

40

Temperature (oC) Fig. 1. DSC thermogram showing the glass transition temperature of carrots dehydrated at an infrared power of 400 W for 8 h (a) and 3 h (b).

C. Xu et al. / Journal of Food Engineering 136 (2014) 42–47

the selected indicators. Regression analysis and path analysis were carried out to show the linear relationship and different effects of the indicators. Data were analyzed using SAS v9.1 (SAS Institute, Cary, NC, USA). The Gordon–Taylor parameters, k (Gordon–Taylor parameter) and Tgs (Tg of the solid matrix) were estimated with non-linear regression analysis using Matlab R2013a (MathWorks).

3. Results and discussion 3.1. Moisture content and water loss of carrots Values of Xws and WL of different dried samples were shown in Fig. 2. From an initial moisture content of 9.26 g/g dry matter, values of Xws in dried carrots decreased gradually by 9.09, 9.15 and 9.17 g/g dry matter at the respective infrared power of 400, 600 and 800 W (3 W) for 10 h. The amount of WL increased continuously by 8.56, 9.11 and 9.61 g water/g initial dry matter at 3 W, respectively. These results indicated that total moisture content in carrots decreased significantly during FID.

3.2. Water state as measured by NMR 3.2.1. Water at different states NMR is a non-destructive test and can be used to monitor the water state in biological samples such as cells and tissues (Rucin´ska-Sobkowiak et al., 2012). The spin–spin relaxation time (T2) and relative area (M) were determined using NMR to investigate the effect of FID on the state of water in carrot tissues. According to the analysis of T2 inversion spectra, the water state within raw fruits and vegetables can usually be categorized into ‘‘bound water’’, ‘‘immobilized water’’ and ‘‘free water’’, which correspond to different cell compartments: cell walls, cytoplasm and extracellular space and vacuoles (Shao and Li, 2011; Xin et al., 2013). The population groups of the longest relaxation time (T23), intermediate relaxation time (T22) and shortest relaxation time (T21) correspond to free water, immobilized water and bound water in tissues, respectively. The relative areas of M0, M23, M22 and M21 are employed to express the relative content of total water, free water, immobilized water and bound water in tissues, respectively. As shown in Fig. 3, the initial distribution of T2 relaxation data in raw carrot samples demonstrated the range of the three population groups: T21 (0.4–1 ms), T22 (5–40 ms) and T23 (50–850 ms). The T22 and T23 were the two main population groups in carrots, and the T21 was negligible in the initial distribution. Therefore, only the two main population groups were analyzed further.

600 500

Amplitude

44

T23

400 300 200 100

T21

T22

0 0.1

1.0

10.0

100.0

1,000.0 10,000.0

Relaxation time (ms) Fig. 3. Representative distribution of transverse relaxation times (T21, T22 and T23) of raw carrots. The T21 (0.4–1 ms) corresponding to bound water was negligible.

3.2.2. Free water in vacuoles The changes in T23 reflected the variation of free water in vacuoles. As shown in Fig. 4A, with increasing time, the T23 values decreased gradually from 683.87 ms in raw carrots to 0 ms in all samples dehydrated for 10 h. This feature was accompanied by a continuous reduction in M23 values from 732.31 to 0 (Fig. 4B). These trends concurred with the report which stated the T2 peak value corresponding to the free water of microwave-dehydrated stem lettuces was reduced by 717.19 ms compared with untreated samples (Wang et al., 2012). As can be seen from Fig. 4B, there was a sharp decrease in M23 values at respective times of 5, 4 and 3 h for 3 W, indicating that a large amount of free water was lost at those times. In addition, values of M0 decreased gradually in accordance with decreases in Xws during the dehydration of carrots (Fig. 4C). These findings suggested that FID caused a dramatic loss of most mobile water in vacuoles and, finally, that all of the free water in carrots was removed.

3.2.3. Immobilized water in the cytoplasm and extracellular space The T22 represented the immobilized water and, because of the various changes of nutritional ingredients in the cytoplasm and extracellular space during FID dehydration, changes in the T22 were more complicated compared with those of the T23 (Fig. 5A). The T22 values among the three treatments all increased at 1 h. These increases may have been due to the shift of free water to immobilized water resulting from the increase in the concentration of carbohydrates (mainly glucose, fructose and sucrose) and the degradation of nutritional ingredients in the cytoplasm. Subsequently,

Fig. 2. Changes of (A) moisture content (Xws) and (B) water loss (WL) in carrots during far–infrared drying.

C. Xu et al. / Journal of Food Engineering 136 (2014) 42–47

45

the T22 values decreased continuously within 3, 2, and 1 h at 3 W due to the shift of immobilized water to water more tightly bound to the polysaccharides in the cell wall (e.g., pectin). Li et al. (2012) also showed that immobilized water can be transformed into bound water during drying at 40, 50 and 60 °C. After a short-term decrease, a sharp increase in the T22 values was observed at the respective times of 5, 4 and 3 h under 3 W treatments, during which the amount of free water decreased sharply. Hence, the sharp increase could have been associated with a sharp shift from free water to immobilized water. This process was followed by gradual decreases in the T22 values. Under conditions in which most of the free water was lost, immobilized water could become more mobile and undergo a considerable amount of loss. Simultaneously, some immobilized water could also shift to more tightly bound water in tissues. In the present study, the M22 values at the beginning were not very large because of the sharply increased values of M22 at 5, 4, and 3 h under 3 W (Fig. 5B). Subsequently, the M22 values decreased from the highest values of 254.12, 233.34 and 244.97 to the values of 56.56, 42.36 and 36.51 in samples treated at 3 W, respectively. Together with the results of T22, we suggested that FID caused dramatic changes in the water state in dehydrated carrots with increasing time. Free water in vacuoles could shift to immobilized water in the cytoplasm and extracellular space of carrots during FID. With increasing time, the immobilized water was reduced.

3.3. The Tg as measured by DSC

Fig. 4. Changes of relaxation time (T2) and the relative area (M) in carrots during the far–infrared drying. (A) T23, (B) M23 and (C) M0.

3.3.1. The Tg at high moisture content For samples (raw carrots and carrots treated at 400 W for 1 h) with moisture content (Xws P 5.21, g/g dry matter), the Tg appeared to be practically negligible in comparison with the latent heat of ice melting (not shown in Table 1). Drying times shorter than 4, 3 and 2 h at 3 W resulted in samples with high moisture content (Xws P 0.77, g/g dry matter). In samples with high moisture content (0.77 6 Xws 6 5.21, g/g dry matter), Tg values were independent of the initial concentration and remained practically constant. This finding may have been because at high moisture content, the cells of samples represent a dilute solution system with low viscosity. In this system, the effect of the viscosity of cell solutions is quite small. In our case, the glass transition for samples with high moisture content started from 53.78 °C and ended at 44.41 °C (Table 1). The mean value of Tg (total number of measurements) was estimated to be 49.42 °C. Similar results were obtained by Bai et al. (2001) when testing the Tg of apple slices with different moisture content.

Fig. 5. Changes in relaxation time (T2) and the relative area (M) in carrots during the far–infrared drying. (A) T22 and (B) M22.

46

C. Xu et al. / Journal of Food Engineering 136 (2014) 42–47

Table 1 The glass transition temperature (°C) of dried carrots with moisture content P0.77 g/ g dry mattera. Time (h)

Xws

Tgo

Tg

Tge

400 W 0 1 2 3 4

9.26 5.21 3.81 1.30 0.79

N N 54.97 ± 4.21 55.10 ± 0.24 53.83 ± 0.61

N N 51.96 ± 0.21 49.86 ± 2.41 50.10 ± 1.35

N N 46.74 ± 1.49 44.49 ± 2.56 44.04 ± 2.31

600 W 0 1 2 3

9.26 3.98 2.50 0.77

N 54.14 ± 1.59 52.32 ± 0.25 51.33 ± 2.02

N 51.52 ± 0.95 47.55 ± 0.64 46.22 ± 0.98

N 46.23 ± 0.14 42.55 ± 1.23 42.52 ± 0.19

800 W 0 1 2

9.26 2.86 1.66

N 56.23 ± 0.72 52.32 ± 3.42

N 51.25 ± 0.08 46.91 ± 1.20

N 46.16 ± 0.38 42.55 ± 2.31

400, 600, and 800 W: treatment under different far–infrared power; Xws: moisture content, g/g dry matter; Tgo: onset glass transition temperature; Tg: glass transition temperature; Tge: end glass transition temperature; Tm: onset melting temperature; N = not detected. a Data are the mean ± standard deviation (SD). Mean values for all measurements: Tgo = 53.78 ± 1.67 °C; Tg = 49.42 ± 2.23 °C; Tge = 44.41 ± 1.79 °C.

3.3.2. The Tg at low moisture content Drying times longer than 5, 4 and 3 h at 3 W resulted in samples with moisture content 60.39 g/g dry matter (Table 2). Given the NMR results, practically all of the free water was removed at those times, and immobilized water dominated in tissues. The Tg values increased obviously with decreasing content of immobilized water during dehydration. With moisture content decreased by 0.20, 0.25 and 0.30 g/g dry matter, the values of the onset temperature of glass transition (Tgo) for samples treated at 3 W increased by

Table 2 The glass transition temperature (°C) of dried carrots with moisture content 60.39 g/g dry mattera. Time (h)

Xws

Tgo

Tg

Tge

400 W 5 6 7 8 9 10

0.37 0.30 0.22 0.20 0.18 0.17

64.47 ± 3.12 56.36 ± 0.61 49.97 ± 1.45 48.88 ± 0.78 46.19 ± 1.48 43.72 ± 3.44

59.85 ± 4.329 51.10 ± 0.99 44.65 ± 1.32 43.14 ± 0.81 40.31 ± 1.26 38.49 ± 3.48

55.33 ± 1.5 45.67 ± 2.32 40.02 ± 2.56 37.36 ± 2.11 35.21 ± 0.61 33.14 ± 1.44

600 W 4 5 6 7 8 9 10

0.35 0.31 0.18 0.16 0.13 0.12 0.11

60.33 ± 4.23 55.22 ± 3.13 46.33 ± 1.43 40.34 ± 0.64 38.34 ± 3.10 31.23 ± 1.72 27.15 ± 0.52

58.6 ± 3.06 52.11 ± 3.88 39.16 ± 1.33 34.01 ± 1.33 30.01 ± 2.33 26.60 ± 1.07 23.05 ± 0.72

57.71 ± 3.11 48.22 ± 3.88 33.85 ± 4.13 29.75 ± 2.01 22.27 ± 2.39 22.19 ± 0.01 19.36 ± 2.34

800 W 3 4 5 6 7 8 9 10

0.39 0.26 0.16 0.14 0.11 0.10 0.10 0.09

63.47 ± 4.77 53.23 ± 0.24 42.55 ± 1.46 40.86 ± 0.35 30.98 ± 2.63 28.82 ± 3.35 27.10 ± 0.59 26.67 ± 1.93

60.84 ± 5.12 50.41 ± 0.49 37.16 ± 1.99 33.01 ± 0.04 26.10 ± 5.66 22.94 ± 1.02 20.13 ± 0.41 19.85 ± 1.60

58.04 ± 1.09 47.42 ± 2.46 34.95 ± 0.48 26.75 ± 0.35 21.22 ± 1.69 16.06 ± 1.32 13.46 ± 0.36 13.03 ± 0.74

400, 600, and 800 W: treatment under different far-infrared power; Xws: moisture content, g/g dry matter; Tgo: onset glass transition temperature; Tg: glass transition temperature; Tge: end glass transition temperature. a Data are the mean ± standard deviation (SD). All samples had the same initial moisture content.

20.75, 33.18, and 36.80 °C, respectively; and the Tg values of samples were elevated by 21.36, 35.55 and 40.99 °C, respectively. Similar trends have been reported in several studies on various dried fruits and vegetables, such as kiwifruit, tomatoes and Agaricus bisporus (Moraga et al., 2006; Goula et al., 2008; Shi et al., 2012). One main reason for the elevation in the Tg with decreased moisture content is the plasticization effect of water on the amorphous constituents of the matrix (Rahman et al., 2005). Additionally, fruits and vegetables can usually be regarded to be binary mixtures of solids and water. The Gordon–Taylor model is a reliable predictor of the Tg as a function of moisture content. In the present study, it was fitted well by the experimental data with the following parameters: Tgs = 3.14 °C and k = 2.34 (R2 = 0.992). The calculated value of k = 2.34 for carrots was in the range reported for garlic (Rahman et al., 2005) and A. bisporus (Shi et al., 2012), but slightly different than for that for the Chinese gooseberry (Wang et al., 2008) and raspberry (Symaladevi et al., 2009). The difference in the k value could result from differences in the chemical compositions of various fruits and vegetables (Goula et al., 2008). As can be seen, the plasticization effect of the immobilized water on Tg was significant in FID–treated carrots. 3.3.3. Relationship between the water state and Tg in carrots The results of the correlation analysis were shown in Table 3 and show that the Tg was correlated negatively with the relaxation times of T22 and T23 (p < 0.01) and the relative areas of M22 (p < 0.05), M23 (p < 0.05) and M0 (p < 0.01) in carrots. These findings were consistent with the report by Xin et al. (2013), who stated that the Tg of osmotically treated broccoli was correlated negatively with the relaxation times of the T22 and the relative proportion of the T23 population. In addition, a regression equation (Tg = 22.15–1.37 T22 (R2 = 0.90; p < 0.05)) and an equation from path analysis (Tg = 0.864 T22 (R2 = 0.90; p < 0.05)) were obtained in the present study. These equations showed that the Tg had a linear relationship with the T22 and that changes in the Tg were affected appreciably by immobilized water. Compared with the effect of immobilized water, the effect of free water was not as appreciable. In addition, compared with the correlation between the relative areas and Tg, the correlation between the relaxation times and Tg was higher. This finding may be because that the relaxation behavior has a direct relationship with water mobility in different parts of the cell as well as the interaction between water and other cell components (Gussoni et al., 2007; Guizani et al., 2010). However, the relative areas reflect the relative content of water at different states. These results indicated that the relaxation behavior correlated with the Tg, and that the water state exerted a significant impact on the Tg of FID–treated carrots. It is well known that water is a ‘‘mobility enhancer’’ in that its low molecular weight greatly enhances the mobility of macromolecules by increasing the free volume and reducing the local viscosity in food materials. NMR results showed that at high moisture content, the most mobile water (free water in vacuoles) dominated in tissues. Under these conditions, the matrix was a dilute solution

Table 3 Correlation coefficient between the water state and glass transition temperature in carrots. Coefficient

T22

T23

M22

M23

M0

Tg

0.864**

0.655**

0.384*

0.408*

0.722**

Xws: moisture content, g/g dry matter; T22 and T23: relaxation times reflecting immobilized water and free water, respectively; M0, M22 and M23: relative areas reflecting the relative content of total water, immobilized water and free water, respectively. * Correlation is significant at the 0.05 level. ** Correlation is significant at the 0.01 level.

C. Xu et al. / Journal of Food Engineering 136 (2014) 42–47

References

0 -10 -20

Tg

-30 -40 -50 -60 -70 0.70

47

0.75

0.80

0.85

0.90

0.95

1.00

Xs (g/g total) Fig. 6. Variation of the glass transition temperature in dehydrated carrots with low moisture content (Xw 6 0.39, g/g dry matter).

system with low viscosity and so the Tg was nearly independent of moisture content. During dehydration initiated by FID, free water in vacuoles was mostly removed and could shift to the less mobile water (immobilized water in the cytoplasm and extracellular space). Then, the immobilized water was also removed gradually. Thus, the decreased content of the mobility enhancer of cell tissue may cause considerable accumulation of carbohydrates in the space between the cell wall and plasmalemma, the plasmolyzed cytoplasm, and concentrated vacuoles (Hatakeyama et al., 2010). These features were followed by the decreased free volume and increased local viscosity of the matrix, which could decrease the mobility of the matrix. Collectively, the Tg of samples was elevated. The curves in Fig. 6 indicated intuitively that the Tg increased with increasing solid content in samples containing only immobilized water. Another reason for the increased Tg may be the shift from immobilized water to bound water, which has a fast proton chemical exchange effect with hydroxyl protons on the polysaccharides in the rigid cell wall (e.g., pectin). Pectin possesses three types of hydrophilic groups in its molecular structure that can reduce the dynamic mobility of surrounding water and increase local viscosity (Einhorn-Stoll et al., 2012). Therefore, we suggest that the water state exerts a significant impact on the Tg of FID–treated carrots, and that the Tg increases appreciably according to the decreased content of immobilized water.

4. Conclusions A combination of DSC and NMR was applied to investigate the effect of FID on the state of water and Tg in carrots. FID caused dramatic changes in the water state in dehydrated carrot tissues. The water state exerted a significant impact on the Tg of FID–dehydrated carrots. Besides, FID enhanced the Tg appreciably according to the decreased content of immobilized water in the cytoplasm and extracellular space, which could be due to the decreased mobility of the matrix in cells.

Acknowledgment This work was supported by a grant from the National Natural Science Foundation of China (Grant number 31271909).

Bai, Y., Rahman, M.S., Perera, C.O., Smith, B., Melton, L.D., 2001. State diagram of apple slices: glass transition and freezing curves. Food Res. Int. 34 (2), 89–95. Baysal, T., Icier, F., Ersus, S., Yıldız, H., 2003. Effects of microwave and infrared drying on the quality of carrot and garlic. Eur. Food Res. Technol. 218 (1), 68–73. Doymaz, I., 2004. Convective air drying characteristics of thin layer carrots. J. Food Eng. 61, 359–364. Einhorn-Stoll, U., Hatakeyama, H., Hatakeyama, T., 2012. Influence of pectin modification on water binding properties. Food Hydrocolloids 27 (2), 494–502. Fan, L.P., Zhang, M., Mujumdar, A.S., 2005. Vacuum frying of carrot chips. Drying Technol. 23, 645–656. Giannakourou, M.C., Taoukis, P.S., 2003. Stability of dehydrofrozen green peas pretreated with nonconventional osmotic agents. J. Food Sci. 68 (6), 2002–2010. Goula, A.M., Adamopoulos, K.G., 2010. Kinetic models of b-carotene degradation during air drying of carrots. Drying Technol. 28 (6), 752–761. Goula, A.M., Karapantsios, T.D., Achilias, D.S., Adamopoulos, K.G., 2008. Water sorption isotherms and glass transition temperature of spray dried tomato pulp. J. Food Eng. 85 (1), 73–83. Guizani, N., Al-Saidi, G.S., Rahman, M.S., Bornaz, S., Al-Alawi, A.A., 2010. State diagram of dates: glass transition, freezing curve and maximal-freezeconcentration condition. J. Food Eng. 99 (1), 92–97. Gussoni, M., Greco, F., Vezzoli, A., Paleari, M.A., Moretti, V.M., Lanza, B., Zetta, L., 2007. Osmotic and aging effects in caviar oocytes throughout water and lipid changes assessed by 1H NMR T1 and T2 relaxation and MRI. Magn. Reson. Imaging 25 (1), 117–128. Hatakeyama, T., Tanaka, M., Hatakeyama, H., 2010. Thermal properties of freezing bound water restrained by polysaccharides. J. Biomater. Sci. Polym. Ed. 21 (14), 1865–1875. Hills, B.P., Nott, K.P., 1999. NMR studies of water compartmentation in carrot parenchyma tissue during drying and freezing. Appl. Magn. Reson. 17 (4), 521– 535. Hills, B.P., Remigereau, B., 1997. NMR studies of changes in subcellular water compartmentation in parenchyma apple tissue during drying and freezing. Int. J. Food Sci. Technol. 32 (1), 51–61. Kasapis, S., 2006. Definition and application of the network glass transition temperature. Food Hydrocolloids 20, 218–228. Kocabiyik, H., Tezer, D., 2009. Drying of carrot slices using infrared radiation. Int. J. Food Sci. Technol. 44 (5), 953–959. Li, X., Ma, L.Z., Tao, Y., Kong, B.H., Li, P.J., 2012. Low field-NMR in measuring water mobility and distribution in beef granules during drying process. Adv. Mater.Res. 550, 3406–3410. Lin, Y.P., Lee, T.Y., Tsen, J.H., King, V., 2007. Dehydration of yam slices using FIRassisted freeze drying. J. Food Eng. 79 (4), 1295–1301. Lin, Y.P., Tsen, J.H., King, V., 2005. Effects of far-infrared radiation on the freezedrying of sweet potato. J. Food Eng. 68 (2), 249–255. Lowithun, N., Charoenrein, S., 2009. Influence of osmodehydrofreezing with different sugars on the quality of frozen rambutan. Int. J. Food Sci. Technol. 44 (11), 2183–2188. Moraga, G., Martínez-Navarrete, N., Chiralt, A., 2006. Water sorption isotherms and phase transitions in kiwifruit. J. Food Eng. 72 (2), 147–156. Rahman, M.S., Sablani, S.S., Al-Habsi, N., Al-Maskri, S., Al-Belushi, R., 2005. Statediagram of freeze-dried garlic powder by differential scanning calorimetry and cooling curve methods. J. Food Sci. 70 (2), E135–E141. Rucin´ska-Sobkowiak, R., Nowaczyk, G., Krzesłowska, M., Rabe˛da, I., Jurga, S., 2012. Water status and water diffusion transport in lupine roots exposed to lead. Environ. Exp. Bot.. Shao, X.L., Li, Y.F., 2011. Application of low-field NMR to analyze water characteristics and predict unfrozen water in blanched sweet corn. Food Bioprocess Technol., 1–7. Shi, Q.L., Wang, X.H., Zhao, Y., Fang, Z., 2012. Glass transition and state diagram for freeze-dried Agaricus bisporus. J. Food Eng. 111 (4), 667–674. Symaladevi, R.M., Sablani, S.S., Tang, J., Powers, J., Swanson, B.G., 2009. State diagram and water adsorption isotherm of raspberry (Rubus idaeus). J. Food Eng. 91 (3), 460–467. Wang, H., Zhang, S., Chen, G., 2008. Glass transition and state diagram for fresh and freeze-dried Chinese gooseberry. J. Food Eng. 84 (2), 307–312. Wang, J., Sheng, K., 2006. Far-infrared and microwave drying of peach. LWT-Food Sci. Technol. 39 (3), 247–255. Wang, Y., Zhang, M., Mujumdar, A.S., Mothibe, K.J., 2012. Experimental investigation and mechanism analysis on microwave freeze drying of stem lettuce cubes in a circular conduit. Drying Technol. 30 (11–12), 1377–1386. Xin, Y., Zhang, M., Adhikari, B., 2013. Effect of trehalose and ultrasound-assisted osmotic dehydration on the state of water and glass transition temperature of broccoli (Brassica oleracea l. var. botrytis l.). J. Food Eng. 119 (3), 640–647. Zielinska, M., Markowski, M., 2007. Drying behavior of carrots dried in a spoutfluidized bed dryer. Drying Technol. 25, 261–270.