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Glass transition and state diagram for fresh and freeze-dried Chinese gooseberry
Journal of Food Engineering 84 (2008) 307–312 www.elsevier.com/locate/jfoodeng
Glass transition and state diagram for fresh and freeze-dried Chinese gooseberry Haiying Wang, Shaozhi Zhang, Guangming Chen * Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou 310027, China Received 15 October 2006; received in revised form 12 May 2007; accepted 14 May 2007 Available online 26 May 2007
Abstract The glass transition temperature and freezing temperature of Chinese gooseberry, which is also called kiwi fruit, prepared at various water activities at 25 °C were determined by differential scanning calorimetry (DSC), and used to plot the state diagram for the Chinese gooseberry sample. High moisture content (>0.45, dry basis) samples obtained by adding liquid water into freeze-dried samples, were also analyzed. The state diagram was composed by the freezing curve and the glass transition line, which were fitted according to Clausius–Clapeyron model and Gordon–Taylor model, respectively. The maximal-freeze-concentration point was calculated to be at a moisture content of X 0s ¼ 0:153 (g water/g wet basis) and at this point T 0m ¼ 41:8 C, T 0g ¼ 57:1 C. The state diagram could be used to predict the stability during storage and the necessary storing conditions for Chinese gooseberry products. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: State diagram; Glass transition; Freeze-dried; Chinese gooseberry
1. Introduction One of the explanations for the behavior of food materials during processing and storage is based on the food polymer theory (Rahman, 1995). According to this theory, before freezing completely, most products experience glass transition. The glass transition phenomenon happens not at one temperature point but in a temperature range during which the products become non-crystalline solids from a rubbery or leathery state and the products’ stability increase greatly. When the storage temperature is above the glass transition temperature, some molecules will be mobile, leading to undesirable reactions. State diagram assists in predicting food stability during storage as well as selecting a suitable condition of temperature and moisture content for processing (Rahman, 1995; Roos, 1995). A state diagram usually shows freezing curve and glass transition line, and maximal-freeze-concentration
*
Corresponding author. Tel./fax: +86 571 8795 1680. E-mail address: [email protected] (G. Chen).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.05.024
point can be found on it. Differential scanning calorimetry (DSC) is often used in state diagram research. Since real foods are complex multi-component mixtures which have variable compositions even for the same breed, very little data about their state diagrams have been accumulated compared with pure solutions. However, as the importance of state diagram is better recognized, more studies have been carried out for real foods during recent years. State diagrams of processed apple (Aguilera, Cuadros, & del Valle, 1998; Bai, Rahman, Perera, Smith, & Melton, 2001; Sa, Figueiredo, & Sereno, 1999), pineapple (Telis & Sobral, 2001), tomato (Baroni, Sereno, & Hubinger, 2003; Telis & Sobral, 2002), cornstarch (Zhong & Sun, 2005), tuna meat (Rahman, Kasapis, Guizani, & Al-Amri, 2003) and garlic (Rahman, Sablani, Al-Habsi, Al-Maskri, & Al-Belushi, 2005) have been reported. Chinese gooseberry (scientific identification: Actinidia Chinensis, which is also called kiwi fruit) is rich in vitamin C, carbohydrate, fibre, folic acid, pantothenic acid, calcium, protein, magnesium, iron, vitamin B6 and carotene. It is even described as king of the fruits for its high nutritive
308
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312
Nomenclature aw C C1, C2 K K E DHm R2 T 0g T 00g T 000 g Tgm Tgs Tgw TF T 0m
water activity GAB parameter general constants in WLF equation GAB parameter Gordon–Taylor parameter molecular mass ration of water to solids (kw/ks) melting enthalpy (J/g) significance of correlation coefficient maximally freeze-concentration glass transition temperature (°C) characteristic temperature of intersection of glass transition and freezing lines (°C) characteristic glass transition temperature measured by DSC (°C) glass transition temperature of mixture (°C) glass transition temperature of solids (°C) glass transition temperature of water (°C) freezing temperature (°C) end point of freezing (°C)
value. Since the production of Chinese gooseberry is restricted by area and season, long term preservation is attractive (Gerschenson, Rojas, & Marangoni, 2001; Lee, Farid, & Nguang, 2006). The objective of this study is to develop a state diagram for Chinese gooseberry by DSC technique. 2. Materials and methods Fresh Chinese gooseberry was bought from local market. Its moisture content was determined by placing small sample pieces in a drying oven (DHG-9070, Jinghong, China) at 60 °C for 3 h and then at 100 °C for 5 h (Han, 1996). The weights of the samples were recorded each hour until they showed variation less than 0.3% (Sa et al., 1999). The results were given as an average for three samples. In order to prepare samples with different water content, fresh Chinese gooseberry were cut into cylinders, with 3 mm in height and 4 mm in diameter. Such cylinders were then put into a freeze-dryer (Self-made, Weng, Zhou, Chen, Wang, & Xia, 2004). The samples were completely frozen at 60 °C, followed by drying at about 10 Pa, 20 °C for 20 h and then 20 °C for 5 h. All the samples were powdered and further dried in a desiccator over P2O5 for 1–3 weeks for completely dried materials (Roos & Karel, 1991). To get samples with water activity from 0.12 to 0.94, powdered freeze-dried Chinese gooseberry samples were put in DSC pans (4–5 mg) and equilibrated with saturated salt solutions of constant water activities (Table 1) for 24–48 h (Roos & Karel, 1991) and moisture content (dry basis) values were obtained from the weight differences of the samples before and after equilibration.
T0m Tw Xm X 0s X 00s X 000 s X 0s Xs Xw b kw s sg
maximally freeze-concentration freezing temperature (°C) freezing point of water (°C) moisture content at fully occupied active sorption sites with one molecule of water (g/g dry basis) solid content at the maximally freeze-concentration (g solids/g wet basis) solids mass fraction at T00g (kg solids/kg sample) solids mass fraction at Tg000 (kg solids/kg sample) initial mass fraction of solids in equation (g solids/g wet basis) mass fraction of solids (g solids/g wet basis) mass fraction of water (g water/g basis) molar freezing point constant of water (1860 kg K/kg mol) molecular mass of water time constants for crystallization at Tgi time constants for crystallization at T
Table 1 Water activity of saturated salt solutions at 25 °C (Sa et al., 1999; Roos, 1987) Saturated salt solution
a25 w
LiCl CH3COOK MgCl2 6H2O K2CO3 Mg(NO3)2 6H2O NaNO2 NaCl KCl KNO3
0.12 0.23 0.33 0.44 0.52 0.61 0.75 0.85 0.94
To get samples with water activity higher than 0.94, water was added directly by micro-syringe into the freeze-dried powder in small aluminum DSC pans and then the pans were sealed and put in a dry desiccator at 4 °C for 24 h (Telis & Sobral, 2001). The weight gains after equilibration were measured and used to calculate the water contents of the samples. The relationship between water activity and moisture content (dry basis) is correlated with the Guggenheim– Anderson–de Boer (GAB) model (Eqn. (1)). Xw ¼
X m CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
ð1Þ
where Xw is the moisture content in dry basis; Xm is the moisture content at fully occupied active sorption sites with one molecule of water, which is secure moisture content for high quality preservation of freeze-dried food; C and K are the GAB parameters associated with the enthalpies of monolayer and multilayer, respectively.
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312
T gm ¼
X s ðT gs Þ þ kX w ðT gw Þ X s þ kX w
ð2Þ
where Xs and Xw are the mass fraction of solids and water (wet basis), respectively; Tgm, Tgs and Tgw are the glass transition temperature of mixture, solids and water, respectively. Tgw = 135.0 °C (Sa et al., 1999); k is the Gordon– Taylor parameter. 3. Result and discussion
1.0
Equlibrium moisture content (g water/g dry basis)
The transition temperatures of Chinese gooseberry samples at different moisture contents were measured with differential scanning calorimetry (DSC-Q100, TA, USA). The instrument was calibrated for heat flow and temperature using distilled water (melting point 0.0 °C, DHm = 333 J/ g) and indium (melting point 156.6 °C, DHm = 28.44 J/g) (Chinese Standard GB/T 13464-92). An empty aluminum pan was used as reference. Samples were sealed in aluminum pans (1.75 mm height; 6.67 mm internal diameter) and nitrogen was used as carrier gas (50 mL/min). Liquid nitrogen was used for sample cooling with a cooling rate of 10 °C/min. All the scans were taken at the same heating rate (10 °C/min) between 120 and 25 °C. Annealing was performed holding the samples at the temperature point T 0m 3.0 °C for 30 min (Bai et al., 2001). All the results were given as an average for three samples. The glass transition temperature of foods with low water activities is modeled by Gordon–Taylor equation (Eqn. (2)) (Bai et al., 2001; Sa et al., 1999; Telis & Sobral, 2001).
309
0.8
0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Water activity Fig. 1. Sorption isotherm of Chinese gooseberry (j experimental data; — GAB model).
Table 2 Model fitting for sorption experimental data Models
Freeze-dried apple (Sa et al., 1999)
GAB model Xm 0.112a C 2.093a K 0.985a R2GAB 0.994a
0.112b 1.955b 0.987b 0.997b
Freeze-dried pineapple (Telis & Sobral, 2001)
Freeze-dried Chinese gooseberry
7.190a 0.012a 0.815a 0.991a
0.459 0.265 0.827 0.987
7.916b 0.010b 0.821b 0.985b
a
Parameters from the references (Sa et al., 1999; Telis & Sobral, 2001). Parameters modeled with the data from the references (Sa et al., 1999; Telis & Sobral, 2001). b
The moisture content measured for fresh Chinese gooseberry was (0.84 ± 0.01) (g water/g wet basis), which was close to the reference data 0.8341 or 0.8382 (g water/g wet basis). The sorption experimental data were fitted with the GAB model. Sorption isotherm is shown in Fig. 1. On the whole, the GAB model well fits the experimental data. The empirical parameters and correlation coefficients calculated by non-linear regression are listed in Table 2. Since Chinese gooseberry is also a fruit with large content of fructose and small content of polymer, its sorption isotherm presents a shape of ‘J’, similar with those obtained by Telis (Telis & Sobral, 2001). Freezing points and glass transitions of samples at different moisture contents were determined from heat flow curves with professional software (Universal Analysis, TA, USA), as shown in Figs. 2 and 3. Here, the onset temperature (Tgi) of the glass transition region is defined as the intersection of the first and the second tangent and considered as the glass transition temperature. The mid-point (Tgp) is defined as the inflexion of the curve part between the first and the third tangent. The end point (Tgp) is defined as the intersection of the second and third tangent. 1
Ref.: http://www.mcgill.ca/cine/resources/data/miao/. Ref.: http://www.food-allergens.de/symposium-vol1(1)/data/kiwi/ kiwi-composition.htm. 2
Fig. 2. Freezing point determination on DSC thermogram (Moisture content 0.504 (g water/g wet basis)).
The freezing point (TF) is taken as the temperature at endothermic peak (Fig. 2). The end point of freezing (T 0m ) is taken as the initial point of ice melting at endothermic peak. (Rahman, 2006; Rahman et al., 2005). The samples with different water content behaved differently during DSC tests. For samples with moisture content less than 0.19 (g water/g wet basis), since much of the water were linked to the solid matrix, only the phenomenon of
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Fig. 3. Glass transition determination on DSC thermogram (Moisture content 0.175 (g water/g wet basis)).
glass transition could be observed. Tgi decreased from 23.2 °C to 60.8 °C when the moisture content increased from 0.0 to 0.19 (g water/g wet basis) (Table 3). When the moisture content was in the range from 0.32 to 0.40 (g water/g wet basis), an exothermic recrystallisation (devitrification) peak of unfrozen water was observed as shown in Fig. 4. The devitrification could be eliminated by annealing the sample (Fig. 5). The freezing temperature at the maximal-freeze-concentration point, T 0m , was 41.8°C (Table 4). When the moisture content was higher than 0.50 (g water/g wet basis), the glass transition phenomenon disappeared and only the melting peak could be found (Table 4).
Table 3 Glass transition temperature of Chinese gooseberry when there is no formation of ice Solid content Xs (g solid/g wet basis)
Tgi/°C
Tgm/°C
Tge/°C
0.81 0.83 0.85 0.87 0.93 0.95 1.00
60.8 56.6 60.6 45.2 25.5 24.4 23.2
55.8 53.4 55.8 40.9 18.7 19.8 35.8
52.2 49.1 52.7 37.3 14.3 13.7 45.7
Fig. 5. DSC thermogram for annealed sample (Moisture content 0.333 (g water/g wet basis)).
Table 4 Glass transition temperature and freezing point of Chinese gooseberry when there is freezable water 0 Solid T gi ¼ T 000 g = C Tgm/°C Tge/°C T m = C TF/°C DHsample/ content kJ/kg Xs (g solid/ g wet basis)
0.20 0.30 0.53 0.65 0.67 0.69
– – 61.1 59.8 48.1 60.5
– – 55.2 54.9 41.6 55.0
– – 51.6 52.0 39.1 51.3
29.6 33.2 39.6 33.6 41.3 41.8
2.2 2.7 7.7 19.0 21.1 19.7
172.2 170.6 102.4 34.5 33.3 40.1
With the DSC data, the state diagram of Chinese gooseberry flesh could be drawn as shown in Fig. 6. The glass transition data were fitted with Gordon–Taylor equation using non-linear regression method and the values of Tgs, Tgw and k are listed in Table 5. Clausius–Clapeyron equation (Eqn. (3)) well represents the decreased freezing temperature with the increasing solid
40 20
A
D
Temperature (oC)
0 -20 //
-40
/
Tm=-41.8
o
Tg =-49.2 C /
/
B
G
o
Tg=-57.1 C -60
///
F
o
Tg =-60.8 C
C
-80 -100 -120 -140 0.0
//'
//
Xs =0.835 Xs =0.871
E
/
0.2
0.4
0.6
0.8Xs =0.847 1.0
Solids (fraction) Fig. 4. DSC thermogram for non-annealed sample (Moisture content 0.338 (g water/g dry basis)).
Fig. 6. State diagram of Chinese gooseberry sample (AB0 : freezing curve; DE: glass transition line; F: glass transition point of maximally freezeconcentration).
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312 Table 5 Gordon–Taylor model fitting for experimental data Parameters
Heat pump dried apple (Bai et al., 2001)
Freeze-dried pineapple (Telis & Sobral, 2001)
Freeze-dried Chinese gooseberry
Gordon–Taylor model 41.3 Tgs (°C) Tgw (°C) 134.8 k 3.59 R2 –
57.75 135.15 0.21 0.954
23.2 135 5.72 0.972
Results T 0g ð CÞ T 0m ð CÞ
51.6 52.0
57.2 41.8
57.8 50.3
content in the range from 0.2 to 0.7 (g solids/g wet basis) with R2 = 0.928. b 1 X 0s D¼ ln ð3Þ kw 1 X 0s þ EX 0s where D is the freezing point depression (Tw TF); TF is the freezing point of the sample (°C); Tw is the freezing point of water (°C); b is the molar freezing point constant of water (1860 kg K /kg mol); kw is the molecular mass of water; X 0s is the initial solids mass fraction (kg solids/ kg sample); and E is the molecular mass ration of water to solids (kw/ks), E = 0.101. In Fig. 6, line AB0 represents freezing temperature curve and line DE represents glass transition temperature Tgi. Several characteristic points B0 , G, F and C are clearly defined by (Rahman et al., 2005). Point B0 is the maximal-freeze-concentration point, at which the freezing temperature T 0m ¼ 41:8 C and the water content (1 X 0s Þ ¼ 0:153 (g water/g wet basis). This part of water is considered as unfreezable (Rahman et al., 2005). Point F is the maximal-freeze-concentration glass transition point, its temperature T 0g ¼ 57:2 C and its solid content is the same as X0 s at point B0 ; point G (T 00g ¼ 49:2 C and X 00s ¼ 0:871 (g water/g wet basis)) is the intersection of line AB0 and line DE, which is different from the experimental value (Tgi = 45.2 °C and Xs = 0.871 (g water/g wet basis)) in Table 3. This difference is due to the discrepancy of the fitted curves from the experimental data. Point C represents the glass transition of high water content samples measured by DSC with annealing, its temperature 000 (T 000 g =60.8 °C and X s =0.835 (g water/g wet basis)). There are numbers of factors which can influence food stability (Rahman, 2006). Among them temperature is prominent. Low temperature cannot only slow down the adverse reactions of microbe, enzyme, respiration and other factors, but also suppress the growth of ice crystals. Big ice crystals may destroy the micro-structures of food product and lead to the decline of food quality. With understanding of the state diagram, the best storing condition for products could be proposed. For example, for Chinese gooseberry dried to water content of 0.1 (g water/ g wet basis), it had better be stored below its glass transition point, 38.3 °C. For products that have to be stored
311
above the glass transition temperature, Williams–Landel– Ferry (WLF) equation (Roos, 1995) could be employed to estimate their shelf life. WLF equation is given as follows: log
s C 1 ðT T g Þ ¼ sg C 2 þ ðT T g Þ
ð4Þ
where sg and s are time constants for crystallization at Tg and T, respectively; C1 and C2are general constants. According to Sun (Sun, 1997), for the system which has a freezing point much higher than its glass transition point, C1 = 20 and C2 = 155. The storage period under glass state, sg, is estimated as follows: since the growth rate of ice crystal under glass state is estimated as 1 mm per 103 years (Hua, Li, & Liu, 1999), it will take 20–30 years for a trivial crystal to grow big enough to destroy the Chinese gooseberry cells whose typical diameters are about 20– 30 lm, therefore sg equals 20–30 years. As an example, let’s consider dried Chinese gooseberry product with water content Xw = 0.1 (g water/g wet basis), its glass transition temperature can be found from the diagram to be 38.3 °C. According to (Eq. (4)), the shelf life of this product at 5 °C can then be calculated to be 3.2 and 5.0 days. It is easy to make similar estimation for products with other water contents and stored at other temperatures. 4. Conclusions Chinese gooseberry is one of the most popular and nutritive fruits that is worthy for storage. The state diagram of freeze-dried Chinese gooseberry was drawn according to DSC data in this paper. It is composed of freezing line and glass transition line. The glass transition temperature for the maximal-freeze-concentrated condition, T 0g , was found to be 57.2 °C. T 0m , the freezing temperature for the maximal-freeze-concentrated condition was found to be 41.8 °C. The unfreezable water fraction was found to be 0.153 (g water/g wet basis). The state diagram obtained here may be helpful in developing better products made of Chinese gooseberry. Acknowledgements The authors acknowledge the financial supports from National Natural Science Foundation of China (No. 50076039) and National Foundation for Scholars Returning from Abroad. References Aguilera, J. M., Cuadros, T. R., & del Valle, J. M. (1998). Differential scanning calorimetry of low-moisture apple products. Carbohydrate Polymers, 37, 79–86. 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 Research International, 34, 89–95. Baroni, A. F., Sereno, A. M., & Hubinger, M. D. (2003). Thermal transitions of osmotically dehydrated tomato by modulated
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temperature differential scanning calorimetry. Thermochimica Acta, 395, 237–249. GB/T 13464-92. (1992). Thermal analysis test methods for thermal stability of materials. Beijing: Standards Press of China. Gerschenson, L. N., Rojas, A. M., & Marangoni, A. G. (2001). Effects of processing on kiwi fruit dynamic rheological behavior and tissue structure. Food Research International, 34, 1–6. Han, Y. S. (1996). Food chemistry experimental guidebook. Beijing: China Agriculture Press, pp. 1–20. Hua, Z. Z., Li, Y. F., & Liu, B. L. (1999). Principles and equipments of food freezing and refrigerated storage. Beijing, China: CMP Press, p. 129. Lee, K. T., Farid, M., & Nguang, S. K. (2006). The mathematical modelling of the rehydration characteristics of fruits. Journal of Food Engineering, 72, 16–23. Rahman, M. S. (1995). Food properties handbook. Boca Raton, Florida: CRC Press, Inc, pp. 70–93. Rahman, M. S. (2006). State diagram of foods: its potential use in food processing and product stability. Trends in Food Science and Technology, 17, 129–141. Rahman, M. S., Kasapis, S., Guizani, N., & Al-Amri, O. (2003). State diagram of tuna meat: freezing curve and glass transition. Journal of Food Engineering, 57, 321–326. Rahman, M. S., Sablani, S. S., Al-Habsi, N., Al-Maskri, S., & Al-Belushi, R. (2005). State diagram of freeze-dried garlic powder by differential scanning calorimetry and cooling curve methods. Journal of Food Science, 70(2), E135–E141.
Roos, Y. H., & Karel, M. (1991). Plasticizing effect of water on thermal behavior and crystallization of amorphous food models. Journal of Food Science, 56, 38–43. Roos, Y. H. (1995). Characterization of food polymers using state diagrams. Journal of Food Engineering, 24, 339–360. Roos, Y. H. (1987). Effect of moisture on the thermal behavior of strawberries studied using differential scanning calorimetry. Journal of Food Science, 52, 146–149. Sa, M. M., Figueiredo, A. M., & Sereno, A. M. (1999). Glass transitions and state diagrams for fresh and processed apple. Thermochimica Acta, 329, 31–38. Sun, W. Q. (1997). Glassy state and seed storage stability: the WLF kinetics of seed viability loss at T > Tg and the plasticization effect of water on storage stability. Annals of Botany, 79, 291– 297. Telis, V. R. N., & Sobral, P. J. A. (2001). Glass transitions and state diagram for freeze-dried pineapple. Lebensm-Wiss u-Technol, 34, 199–205. Telis, V. R. N., & Sobral, P. J. A. (2002). Glass transitions for freeze-dried and air-dried tomato. Food Research International, 35, 435–443. Weng, Y., Zhou, X. L., Chen, G. M., Wang, Q., & Xia, P. (2004). Development of low temperature freeze-drying equipment. Cryogenic Technology, 3, 12–16 (in Chinese). Zhong, Z. K., & Sun, X. S. (2005). Thermal characterization and phase behavior of cornstarch studied by differential scanning calorimetry. Journal of Food Engineering, 69, 453–459.
Glass transition and state diagram for fresh and freeze-dried Chinese gooseberry Haiying Wang, Shaozhi Zhang, Guangming Chen * Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou 310027, China Received 15 October 2006; received in revised form 12 May 2007; accepted 14 May 2007 Available online 26 May 2007
Abstract The glass transition temperature and freezing temperature of Chinese gooseberry, which is also called kiwi fruit, prepared at various water activities at 25 °C were determined by differential scanning calorimetry (DSC), and used to plot the state diagram for the Chinese gooseberry sample. High moisture content (>0.45, dry basis) samples obtained by adding liquid water into freeze-dried samples, were also analyzed. The state diagram was composed by the freezing curve and the glass transition line, which were fitted according to Clausius–Clapeyron model and Gordon–Taylor model, respectively. The maximal-freeze-concentration point was calculated to be at a moisture content of X 0s ¼ 0:153 (g water/g wet basis) and at this point T 0m ¼ 41:8 C, T 0g ¼ 57:1 C. The state diagram could be used to predict the stability during storage and the necessary storing conditions for Chinese gooseberry products. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: State diagram; Glass transition; Freeze-dried; Chinese gooseberry
1. Introduction One of the explanations for the behavior of food materials during processing and storage is based on the food polymer theory (Rahman, 1995). According to this theory, before freezing completely, most products experience glass transition. The glass transition phenomenon happens not at one temperature point but in a temperature range during which the products become non-crystalline solids from a rubbery or leathery state and the products’ stability increase greatly. When the storage temperature is above the glass transition temperature, some molecules will be mobile, leading to undesirable reactions. State diagram assists in predicting food stability during storage as well as selecting a suitable condition of temperature and moisture content for processing (Rahman, 1995; Roos, 1995). A state diagram usually shows freezing curve and glass transition line, and maximal-freeze-concentration
*
Corresponding author. Tel./fax: +86 571 8795 1680. E-mail address: [email protected] (G. Chen).
0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.05.024
point can be found on it. Differential scanning calorimetry (DSC) is often used in state diagram research. Since real foods are complex multi-component mixtures which have variable compositions even for the same breed, very little data about their state diagrams have been accumulated compared with pure solutions. However, as the importance of state diagram is better recognized, more studies have been carried out for real foods during recent years. State diagrams of processed apple (Aguilera, Cuadros, & del Valle, 1998; Bai, Rahman, Perera, Smith, & Melton, 2001; Sa, Figueiredo, & Sereno, 1999), pineapple (Telis & Sobral, 2001), tomato (Baroni, Sereno, & Hubinger, 2003; Telis & Sobral, 2002), cornstarch (Zhong & Sun, 2005), tuna meat (Rahman, Kasapis, Guizani, & Al-Amri, 2003) and garlic (Rahman, Sablani, Al-Habsi, Al-Maskri, & Al-Belushi, 2005) have been reported. Chinese gooseberry (scientific identification: Actinidia Chinensis, which is also called kiwi fruit) is rich in vitamin C, carbohydrate, fibre, folic acid, pantothenic acid, calcium, protein, magnesium, iron, vitamin B6 and carotene. It is even described as king of the fruits for its high nutritive
308
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312
Nomenclature aw C C1, C2 K K E DHm R2 T 0g T 00g T 000 g Tgm Tgs Tgw TF T 0m
water activity GAB parameter general constants in WLF equation GAB parameter Gordon–Taylor parameter molecular mass ration of water to solids (kw/ks) melting enthalpy (J/g) significance of correlation coefficient maximally freeze-concentration glass transition temperature (°C) characteristic temperature of intersection of glass transition and freezing lines (°C) characteristic glass transition temperature measured by DSC (°C) glass transition temperature of mixture (°C) glass transition temperature of solids (°C) glass transition temperature of water (°C) freezing temperature (°C) end point of freezing (°C)
value. Since the production of Chinese gooseberry is restricted by area and season, long term preservation is attractive (Gerschenson, Rojas, & Marangoni, 2001; Lee, Farid, & Nguang, 2006). The objective of this study is to develop a state diagram for Chinese gooseberry by DSC technique. 2. Materials and methods Fresh Chinese gooseberry was bought from local market. Its moisture content was determined by placing small sample pieces in a drying oven (DHG-9070, Jinghong, China) at 60 °C for 3 h and then at 100 °C for 5 h (Han, 1996). The weights of the samples were recorded each hour until they showed variation less than 0.3% (Sa et al., 1999). The results were given as an average for three samples. In order to prepare samples with different water content, fresh Chinese gooseberry were cut into cylinders, with 3 mm in height and 4 mm in diameter. Such cylinders were then put into a freeze-dryer (Self-made, Weng, Zhou, Chen, Wang, & Xia, 2004). The samples were completely frozen at 60 °C, followed by drying at about 10 Pa, 20 °C for 20 h and then 20 °C for 5 h. All the samples were powdered and further dried in a desiccator over P2O5 for 1–3 weeks for completely dried materials (Roos & Karel, 1991). To get samples with water activity from 0.12 to 0.94, powdered freeze-dried Chinese gooseberry samples were put in DSC pans (4–5 mg) and equilibrated with saturated salt solutions of constant water activities (Table 1) for 24–48 h (Roos & Karel, 1991) and moisture content (dry basis) values were obtained from the weight differences of the samples before and after equilibration.
T0m Tw Xm X 0s X 00s X 000 s X 0s Xs Xw b kw s sg
maximally freeze-concentration freezing temperature (°C) freezing point of water (°C) moisture content at fully occupied active sorption sites with one molecule of water (g/g dry basis) solid content at the maximally freeze-concentration (g solids/g wet basis) solids mass fraction at T00g (kg solids/kg sample) solids mass fraction at Tg000 (kg solids/kg sample) initial mass fraction of solids in equation (g solids/g wet basis) mass fraction of solids (g solids/g wet basis) mass fraction of water (g water/g basis) molar freezing point constant of water (1860 kg K/kg mol) molecular mass of water time constants for crystallization at Tgi time constants for crystallization at T
Table 1 Water activity of saturated salt solutions at 25 °C (Sa et al., 1999; Roos, 1987) Saturated salt solution
a25 w
LiCl CH3COOK MgCl2 6H2O K2CO3 Mg(NO3)2 6H2O NaNO2 NaCl KCl KNO3
0.12 0.23 0.33 0.44 0.52 0.61 0.75 0.85 0.94
To get samples with water activity higher than 0.94, water was added directly by micro-syringe into the freeze-dried powder in small aluminum DSC pans and then the pans were sealed and put in a dry desiccator at 4 °C for 24 h (Telis & Sobral, 2001). The weight gains after equilibration were measured and used to calculate the water contents of the samples. The relationship between water activity and moisture content (dry basis) is correlated with the Guggenheim– Anderson–de Boer (GAB) model (Eqn. (1)). Xw ¼
X m CKaw ð1 Kaw Þð1 Kaw þ CKaw Þ
ð1Þ
where Xw is the moisture content in dry basis; Xm is the moisture content at fully occupied active sorption sites with one molecule of water, which is secure moisture content for high quality preservation of freeze-dried food; C and K are the GAB parameters associated with the enthalpies of monolayer and multilayer, respectively.
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312
T gm ¼
X s ðT gs Þ þ kX w ðT gw Þ X s þ kX w
ð2Þ
where Xs and Xw are the mass fraction of solids and water (wet basis), respectively; Tgm, Tgs and Tgw are the glass transition temperature of mixture, solids and water, respectively. Tgw = 135.0 °C (Sa et al., 1999); k is the Gordon– Taylor parameter. 3. Result and discussion
1.0
Equlibrium moisture content (g water/g dry basis)
The transition temperatures of Chinese gooseberry samples at different moisture contents were measured with differential scanning calorimetry (DSC-Q100, TA, USA). The instrument was calibrated for heat flow and temperature using distilled water (melting point 0.0 °C, DHm = 333 J/ g) and indium (melting point 156.6 °C, DHm = 28.44 J/g) (Chinese Standard GB/T 13464-92). An empty aluminum pan was used as reference. Samples were sealed in aluminum pans (1.75 mm height; 6.67 mm internal diameter) and nitrogen was used as carrier gas (50 mL/min). Liquid nitrogen was used for sample cooling with a cooling rate of 10 °C/min. All the scans were taken at the same heating rate (10 °C/min) between 120 and 25 °C. Annealing was performed holding the samples at the temperature point T 0m 3.0 °C for 30 min (Bai et al., 2001). All the results were given as an average for three samples. The glass transition temperature of foods with low water activities is modeled by Gordon–Taylor equation (Eqn. (2)) (Bai et al., 2001; Sa et al., 1999; Telis & Sobral, 2001).
309
0.8
0.6
0.4
0.2
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Water activity Fig. 1. Sorption isotherm of Chinese gooseberry (j experimental data; — GAB model).
Table 2 Model fitting for sorption experimental data Models
Freeze-dried apple (Sa et al., 1999)
GAB model Xm 0.112a C 2.093a K 0.985a R2GAB 0.994a
0.112b 1.955b 0.987b 0.997b
Freeze-dried pineapple (Telis & Sobral, 2001)
Freeze-dried Chinese gooseberry
7.190a 0.012a 0.815a 0.991a
0.459 0.265 0.827 0.987
7.916b 0.010b 0.821b 0.985b
a
Parameters from the references (Sa et al., 1999; Telis & Sobral, 2001). Parameters modeled with the data from the references (Sa et al., 1999; Telis & Sobral, 2001). b
The moisture content measured for fresh Chinese gooseberry was (0.84 ± 0.01) (g water/g wet basis), which was close to the reference data 0.8341 or 0.8382 (g water/g wet basis). The sorption experimental data were fitted with the GAB model. Sorption isotherm is shown in Fig. 1. On the whole, the GAB model well fits the experimental data. The empirical parameters and correlation coefficients calculated by non-linear regression are listed in Table 2. Since Chinese gooseberry is also a fruit with large content of fructose and small content of polymer, its sorption isotherm presents a shape of ‘J’, similar with those obtained by Telis (Telis & Sobral, 2001). Freezing points and glass transitions of samples at different moisture contents were determined from heat flow curves with professional software (Universal Analysis, TA, USA), as shown in Figs. 2 and 3. Here, the onset temperature (Tgi) of the glass transition region is defined as the intersection of the first and the second tangent and considered as the glass transition temperature. The mid-point (Tgp) is defined as the inflexion of the curve part between the first and the third tangent. The end point (Tgp) is defined as the intersection of the second and third tangent. 1
Ref.: http://www.mcgill.ca/cine/resources/data/miao/. Ref.: http://www.food-allergens.de/symposium-vol1(1)/data/kiwi/ kiwi-composition.htm. 2
Fig. 2. Freezing point determination on DSC thermogram (Moisture content 0.504 (g water/g wet basis)).
The freezing point (TF) is taken as the temperature at endothermic peak (Fig. 2). The end point of freezing (T 0m ) is taken as the initial point of ice melting at endothermic peak. (Rahman, 2006; Rahman et al., 2005). The samples with different water content behaved differently during DSC tests. For samples with moisture content less than 0.19 (g water/g wet basis), since much of the water were linked to the solid matrix, only the phenomenon of
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H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312
Fig. 3. Glass transition determination on DSC thermogram (Moisture content 0.175 (g water/g wet basis)).
glass transition could be observed. Tgi decreased from 23.2 °C to 60.8 °C when the moisture content increased from 0.0 to 0.19 (g water/g wet basis) (Table 3). When the moisture content was in the range from 0.32 to 0.40 (g water/g wet basis), an exothermic recrystallisation (devitrification) peak of unfrozen water was observed as shown in Fig. 4. The devitrification could be eliminated by annealing the sample (Fig. 5). The freezing temperature at the maximal-freeze-concentration point, T 0m , was 41.8°C (Table 4). When the moisture content was higher than 0.50 (g water/g wet basis), the glass transition phenomenon disappeared and only the melting peak could be found (Table 4).
Table 3 Glass transition temperature of Chinese gooseberry when there is no formation of ice Solid content Xs (g solid/g wet basis)
Tgi/°C
Tgm/°C
Tge/°C
0.81 0.83 0.85 0.87 0.93 0.95 1.00
60.8 56.6 60.6 45.2 25.5 24.4 23.2
55.8 53.4 55.8 40.9 18.7 19.8 35.8
52.2 49.1 52.7 37.3 14.3 13.7 45.7
Fig. 5. DSC thermogram for annealed sample (Moisture content 0.333 (g water/g wet basis)).
Table 4 Glass transition temperature and freezing point of Chinese gooseberry when there is freezable water 0 Solid T gi ¼ T 000 g = C Tgm/°C Tge/°C T m = C TF/°C DHsample/ content kJ/kg Xs (g solid/ g wet basis)
0.20 0.30 0.53 0.65 0.67 0.69
– – 61.1 59.8 48.1 60.5
– – 55.2 54.9 41.6 55.0
– – 51.6 52.0 39.1 51.3
29.6 33.2 39.6 33.6 41.3 41.8
2.2 2.7 7.7 19.0 21.1 19.7
172.2 170.6 102.4 34.5 33.3 40.1
With the DSC data, the state diagram of Chinese gooseberry flesh could be drawn as shown in Fig. 6. The glass transition data were fitted with Gordon–Taylor equation using non-linear regression method and the values of Tgs, Tgw and k are listed in Table 5. Clausius–Clapeyron equation (Eqn. (3)) well represents the decreased freezing temperature with the increasing solid
40 20
A
D
Temperature (oC)
0 -20 //
-40
/
Tm=-41.8
o
Tg =-49.2 C /
/
B
G
o
Tg=-57.1 C -60
///
F
o
Tg =-60.8 C
C
-80 -100 -120 -140 0.0
//'
//
Xs =0.835 Xs =0.871
E
/
0.2
0.4
0.6
0.8Xs =0.847 1.0
Solids (fraction) Fig. 4. DSC thermogram for non-annealed sample (Moisture content 0.338 (g water/g dry basis)).
Fig. 6. State diagram of Chinese gooseberry sample (AB0 : freezing curve; DE: glass transition line; F: glass transition point of maximally freezeconcentration).
H. Wang et al. / Journal of Food Engineering 84 (2008) 307–312 Table 5 Gordon–Taylor model fitting for experimental data Parameters
Heat pump dried apple (Bai et al., 2001)
Freeze-dried pineapple (Telis & Sobral, 2001)
Freeze-dried Chinese gooseberry
Gordon–Taylor model 41.3 Tgs (°C) Tgw (°C) 134.8 k 3.59 R2 –
57.75 135.15 0.21 0.954
23.2 135 5.72 0.972
Results T 0g ð CÞ T 0m ð CÞ
51.6 52.0
57.2 41.8
57.8 50.3
content in the range from 0.2 to 0.7 (g solids/g wet basis) with R2 = 0.928. b 1 X 0s D¼ ln ð3Þ kw 1 X 0s þ EX 0s where D is the freezing point depression (Tw TF); TF is the freezing point of the sample (°C); Tw is the freezing point of water (°C); b is the molar freezing point constant of water (1860 kg K /kg mol); kw is the molecular mass of water; X 0s is the initial solids mass fraction (kg solids/ kg sample); and E is the molecular mass ration of water to solids (kw/ks), E = 0.101. In Fig. 6, line AB0 represents freezing temperature curve and line DE represents glass transition temperature Tgi. Several characteristic points B0 , G, F and C are clearly defined by (Rahman et al., 2005). Point B0 is the maximal-freeze-concentration point, at which the freezing temperature T 0m ¼ 41:8 C and the water content (1 X 0s Þ ¼ 0:153 (g water/g wet basis). This part of water is considered as unfreezable (Rahman et al., 2005). Point F is the maximal-freeze-concentration glass transition point, its temperature T 0g ¼ 57:2 C and its solid content is the same as X0 s at point B0 ; point G (T 00g ¼ 49:2 C and X 00s ¼ 0:871 (g water/g wet basis)) is the intersection of line AB0 and line DE, which is different from the experimental value (Tgi = 45.2 °C and Xs = 0.871 (g water/g wet basis)) in Table 3. This difference is due to the discrepancy of the fitted curves from the experimental data. Point C represents the glass transition of high water content samples measured by DSC with annealing, its temperature 000 (T 000 g =60.8 °C and X s =0.835 (g water/g wet basis)). There are numbers of factors which can influence food stability (Rahman, 2006). Among them temperature is prominent. Low temperature cannot only slow down the adverse reactions of microbe, enzyme, respiration and other factors, but also suppress the growth of ice crystals. Big ice crystals may destroy the micro-structures of food product and lead to the decline of food quality. With understanding of the state diagram, the best storing condition for products could be proposed. For example, for Chinese gooseberry dried to water content of 0.1 (g water/ g wet basis), it had better be stored below its glass transition point, 38.3 °C. For products that have to be stored
311
above the glass transition temperature, Williams–Landel– Ferry (WLF) equation (Roos, 1995) could be employed to estimate their shelf life. WLF equation is given as follows: log
s C 1 ðT T g Þ ¼ sg C 2 þ ðT T g Þ
ð4Þ
where sg and s are time constants for crystallization at Tg and T, respectively; C1 and C2are general constants. According to Sun (Sun, 1997), for the system which has a freezing point much higher than its glass transition point, C1 = 20 and C2 = 155. The storage period under glass state, sg, is estimated as follows: since the growth rate of ice crystal under glass state is estimated as 1 mm per 103 years (Hua, Li, & Liu, 1999), it will take 20–30 years for a trivial crystal to grow big enough to destroy the Chinese gooseberry cells whose typical diameters are about 20– 30 lm, therefore sg equals 20–30 years. As an example, let’s consider dried Chinese gooseberry product with water content Xw = 0.1 (g water/g wet basis), its glass transition temperature can be found from the diagram to be 38.3 °C. According to (Eq. (4)), the shelf life of this product at 5 °C can then be calculated to be 3.2 and 5.0 days. It is easy to make similar estimation for products with other water contents and stored at other temperatures. 4. Conclusions Chinese gooseberry is one of the most popular and nutritive fruits that is worthy for storage. The state diagram of freeze-dried Chinese gooseberry was drawn according to DSC data in this paper. It is composed of freezing line and glass transition line. The glass transition temperature for the maximal-freeze-concentrated condition, T 0g , was found to be 57.2 °C. T 0m , the freezing temperature for the maximal-freeze-concentrated condition was found to be 41.8 °C. The unfreezable water fraction was found to be 0.153 (g water/g wet basis). The state diagram obtained here may be helpful in developing better products made of Chinese gooseberry. Acknowledgements The authors acknowledge the financial supports from National Natural Science Foundation of China (No. 50076039) and National Foundation for Scholars Returning from Abroad. References Aguilera, J. M., Cuadros, T. R., & del Valle, J. M. (1998). Differential scanning calorimetry of low-moisture apple products. Carbohydrate Polymers, 37, 79–86. 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 Research International, 34, 89–95. Baroni, A. F., Sereno, A. M., & Hubinger, M. D. (2003). Thermal transitions of osmotically dehydrated tomato by modulated
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