Structural properties of freeze-dried rice

Journal of Food Engineering 107 (2011) 326–333

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

Structural properties of freeze-dried rice Vasiliki P. Oikonomopoulou a,⇑, Magdalini K. Krokida a, Vaios T. Karathanos b a b

School of Chemical Engineering, National Technical University of Athens, Zografou Campus, 9 Iroon Polytechneiou, Athens 15780, Greece Department of Nutrition, Harokopio University of Athens, Kallithea, 70 El. Venizelou, Athens 17671, Greece

a r t i c l e

i n f o

Article history: Received 3 March 2011 Received in revised form 2 June 2011 Accepted 6 July 2011 Available online 21 July 2011 Keywords: Bulk density Freeze-drying Glass transition temperature Mercury porosimetry Porosity

a b s t r a c t Structural properties, such as porosity, bulk density and true density, of freeze-dried rice, were investigated as affected by process conditions. Rice was boiled at different time periods and freeze-dried under various vacuum conditions. True density of dehydrated rice kernels was measured with a helium stereopycnometer, while bulk density was obtained by measuring their geometric characteristics. Porosity and pore size distribution were also measured with a mercury porosimeter. Simple mathematical models were developed in order to correlate these structural properties with process conditions. Bulk density of freeze-dried rice kernels increased with the applied pressure during freeze-drying, while porosity decreased. In addition, bulk density decreased and porosity increased with increasing boiling time. Moreover, the effect of porosity and water activity on glass transition temperature of dehydrated samples was studied using differential scanning calorimetry (DSC). Glass transition temperature decreased as moisture content and porosity increased, respectively. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Rice is the most important cereal grain with regard to human nutrition and is considered the major staple food for more than half of the world’s population. More than 550 million tons of rice are produced annually around the world. Parboiled rice is popular in some countries. The parboiling process is a hydrothermal treatment (Soponronnarit et al., 2006), including soaking, steaming and drying, that leads to the gelatinization of the starch in the grain. In addition, it causes the enrichment of the grain with thiamine from the outer husk and improves the nutritional importance of rice (Subba Rao and Bhattacharya, 1966). Dehydration processes are widely used for the preservation of foods, since the removal of water minimizes the microbial spoilage and prevents the physical and chemical reactions of the foods’ compounds during storage (Koc et al., 2008; Krokida and MarinosKouris, 2003). Drying procedure comprises of simultaneous heat and mass transfer, which cause significant changes in the physical and chemical composition as well as in the structure of food products depending on the transport mechanisms applied. Therefore, the microstructure and morphology of foods and, as a result, the quality of the final product, are related to the drying method and selected conditions applied (Koc et al., 2008; Rewthong et al., 2011). Among the drying methods that are used in food processing industries, freeze-drying is considered one of the most advanced ⇑ Corresponding author. E-mail addresses: [email protected] (V.P. Oikonomopoulou), mkrok@ central.ntua.gr (M.K. Krokida), [email protected] (V.T. Karathanos). 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.07.009

methods for drying high-value products sensitive to heat, since it prevents undesirable shrinkage and produces materials with high porosity, unchanged nutrition quality, superior taste, aroma, flavors and color retention as well as better rehydration properties (Krokida et al., 1998; Krokida and Maroulis, 1997). Freeze-drying is used for the preservation of sensitive materials and the facilitation of transport and is carried out in two stages; the product is first frozen and then the ice is removed by sublimation directly from the solid to the vapor phase (Krokida et al., 1998; Krokida and Maroulis, 1997). Due to the fact that the method includes low-temperature processing, freeze-dried products are considered superior to those dried under conventional techniques (Ratti, 2001). In addition, the absence of liquid water and the almost complete dehydration minimize the deterioration reactions, such as non-enzymatic browning, protein denaturation, and enzymatic reactions, which cause chemical degradation of the product. The dehydration of rice using freeze-drying has been studied by many authors (Puspitowati and Driscoll, 2007; Rewthong et al., 2011; Yu et al., 2011). Rice is widely used for the production of processed foods, such as puddings, infant foods, puffed grains and breakfast cereals. The porous and spongy structure presents excellent rehydration and as a result, freeze-dried rice is convenient as instant noodles that are ready to eat after adding hot water. In addition, the reduction of the water content helps to the preservation of foods for a long time after packaging and lowers the material’s weight for convenient transport. As a result, this technique could provide a choice of fast food, frozen rice for outdoor usage of food supplies during disasters (Yu et al., 2011).

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During freeze-drying, the ice sublimation causes significant changes in the shape and volume of the food products. The structure of freeze-dried foods is strongly affected by the size of ice crystals formed during freezing, depending on the freezing rate and temperature (Rhim et al., 2011). In addition, depending on the process conditions, the ice crystals that sublime create pores or gaps with variable characteristics, thus, it seems very interesting to investigate the effect of freeze-drying process conditions on the structural properties of the products (Krokida et al., 1998; Krokida and Maroulis, 1997). Structural properties, like density and porosity, characterize the texture and quality of dehydrated products by controlling the taste and appearance. Information on porous formation in foods during processing is needed for process design, influencing a wide variety of other properties, such as mechanical properties, thermal conductivity, thermal diffusivity and mass diffusion (Koc et al., 2008; Rahman, 2001). The effect of drying conditions on the structural properties of freeze-dried agricultural plants was studied (Krokida et al., 1998). Rahman and Sablani (2003) examined the variation of porosity of freeze-dried abalone depending on the changes of freeze-drying temperature. Karathanos et al. (1996a,b) investigated the structural collapse of agricultural plants during freezedrying. Rahman et al. (2002) studied the pores and physicochemical characteristics of dried tuna produced by different methods of drying and completely different pore size distribution curves were observed for each method. In addition, pore formation in foods during processing results in significant changes on transport properties, such as thermal conductivity, thermal diffusivity and mass diffusion coefficient (Karathanos and Saravacos, 1993; Rahman, 2001) as well as on mechanical properties. The factors that affect the mechanical properties will affect the measurement of the glass transition temperature (Tg) which is used to characterize the textural changes of food materials (Ross et al., 2002). Phase and state transitions of foods are important in characterizing their quality and in designing processes (Sablani et al., 2009). Glass transition is a second order, time–temperature dependent transition where an amorphous material (or an amorphous region within a semicrystalline material) changes from a hard and brittle phase into a molten, rubbery-like phase. Many authors used the theory of glass transition to explain the shrinkage, collapse and cracking during drying (Cnossen et al., 2001; Karathanos et al., 1993, 1996a,b; Krokida et al., 1998; Rahman, 2001; Sablani et al., 2008). If the temperature of the products is above the glass transition temperature, the viscosity will be lowered and the material will shrink or collapse. Thermal transitions of rice kernels have been measured by methods, such as Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) (Normand and Marshall, 1989; Perdon et al., 2000; Sun et al., 2002). The objective of the present research was to determine the effect of process conditions on the structural properties of freezedried rice. Freeze-drying was performed under controlled drying conditions, regulating pressure during drying. The effect of water activity and process parameters on glass transition temperature was also studied using DSC. Simple mathematical models were developed according to the experimental data, in order to predict the values of porosity, bulk density and total cumulative volume correlated with freeze-drying conditions.

2. Materials and methods Parboiled rice was chosen as raw material, obtained by Ev.Ge. Pistiolas (Agrino) S.A. (Agrinion, Greece).

2.1. Freeze-drying Rice was stored at room temperature and dark conditions before the experimental procedure and was boiled in excess de-ionized water at 100 °C for different time durations, ranging from 4 to 24 min. Then, it was frozen at 30 °C for 72 h in a biomedical freezer (SANYO, MDF-236, Osaka, Japan), and before the dehydration procedure, rice kernels were placed in a container and tempered for 1 h in liquid N2. The materials were then dehydrated for 24 h using a laboratory freeze-dryer (Leybold-Heraeus GT 2A, Koln, Germany). Freeze-drying was performed under various vacuum conditions, ranging from 4 Pa to 125 Pa (0.04 mbar to 1.25 mbar) absolute pressure. The vacuum was reduced by leaking air through one of the pressure release valves. Two replicates for each drying condition were performed. Dehydrated products were stored in a desiccator above dry silica gel, in order to prevent any increase in absorbed moisture, until required for experiments. 2.2. Measurement of bulk density, true density and porosity Bulk and true density, porosity and pore size distribution of dried rice samples were measured in order to study the pore formation during the drying process. The mass of the dried materials was measured using an electronic balance with an accuracy of 104 g. The true volume of the samples was estimated using a stereopycnometer (Quantachrome multipycnometer MVP-1, Boynton Beach, FL, USA) with an accuracy of 0.001 cm3, utilizing helium gas. The stereopycnometer was operated at room temperature. Before each measurement the samples were ground to powder to remove most of the internal pores. Three replicates of each sample were used. The true density (kg/m3) is the ratio of the mass of dry solids to the volume of dry solids and is expressed by the equation:

ms Vs

qts ¼

ð1Þ

where qts (kg/m3) is the true density, ms (kg) the mass of dry solids and Vs (m3) the volume of dry solids. The total (bulk) volume was obtained by measuring the actual geometric characteristics of the dried rice materials, using a digital Vernier caliper, with an accuracy of 0.001 cm. The results were the average of 15 replicates. Measurements of diameter and height of the rice kernels were calculated as the average of four measurements performed at different parts of each sample. Each rice kernel is considered to consist of a cylindrical part and two hemispheres. The total volume of rice kernels is estimated using the equation:

Vt ¼

p  d2  ðh  dÞ 4 4

þ

3

p

 3 d 2

ð2Þ

where Vt (m3) is the total volume of each rice kernel, d (m) the diameter and h (m) the height of the cylindrical part of the rice kernel. The bulk density was determined using the equation:

qbs ¼

ms Vt

ð3Þ

where qbs (kg/m3) is the bulk density and ms (kg) the mass of dry solids. The porosity was estimated using the equation:

e¼1

qbs qts

ð4Þ

Porosity and pore size distribution of representative samples were also measured using a mercury porosimeter (Porosimeter 2000, Fisons Instruments, Milano, Italy). The pressure range of the porosimeter was 0 to 2000 bar.

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2.3. Moisture sorption

X¼

Representative samples of dehydrated products were first stored in a desiccator at room temperature, for three weeks prior to the beginning of the experiment. The samples were, then, placed in chambers and equilibrated over saturated salts solutions (LiCl, MgCl2, Mg(NO3)2, NaCl, KNO3) of constant water activities environments, ranging from 0.11 to 0.94, at room temperature. Equilibrium was reached, when three consecutive weight measurements did not differ more than 0.001 g. The required time for equilibrium was about three weeks. The moisture content of each sample was determined using the oven method at 70 °C under vacuum, until constant weight. 2.4. Measurement of glass transition temperature Glass transition temperature (Tg) was estimated using a differential scanning calorimeter (Perkin Elmer DSC 6, CT, USA) with Pyris operation software. 5–10 mg from each sample, were sealed in aluminum pans and were scanned from 0 °C to 100 °C at a rate of 5 °C/min. A sealed, empty aluminum pan was used as a reference. All measurements were done in triplicate and the mean value is presented. 2.5. Mathematical modeling Several mathematical models were developed in order to predict the values of porosity and bulk density correlated with freeze-drying conditions and boiling time for rice kernels. The simplest and most appropriate power model has been selected according to the equation:

e ¼ ko 

 mo  no P t  exp Po to

ð5Þ

where ko, mo, no are parameters dependent on the material, e is the porosity, P (mbar) is the pressure, t (min) is the boiling time, Po (mbar) and to (min) are the corresponding values at reference conditions. Specifically, Po is the pressure at 20 °C equal to 0.80 mbar and to is the average value of boiling time equal to 14 min. In addition, mathematical models were developed to predict total cumulative volume of rice kernels, measured using a mercury porosimeter, correlated with freeze-drying conditions and boiling time. The most appropriate model that has been used is described by the equation:

V cum;t ¼ k1 

 m1   P t  exp n1  Po to

ð6Þ

where k1 (m3/kg), m1, n1 are parameters dependent on the material and Vcum,t (m3/kg) is the total cumulative volume. Many equations have been proposed in the literature to describe the sorption data of several food products. Among them, the Guggenheim–Anderson–de Boer (GAB) model is considered to be the most versatile sorption model and has been successfully used by many authors to model sorption isotherms of grains and cereals. Siripatrawan and Jantawat (2006) proposed that GAB model produces the optimal fit to the experimental sorption data of rice crackers throughout the entire range of water activity, while the same results were found for raw and parboiled paddy, brown rice and bran (Reddy and Chakraverty, 2004) and rough rice (San Marin et al., 2001). GAB equation, also, appears the most suitable model for the prediction of the moisture sorption isotherms of other grains, such as amaranth grains (Pagano and Mascheroni, 2005) and quinoa grains (Tolaba et al., 2004). The GAB model, applied to the sorption isotherms of freezedried rice, is expressed as:

X m  C  K  aw ð1  K  aw Þ  ð1  K  aw þ C  K  aw Þ

ð7Þ

where X (kg water/kg dry solids) is the moisture content of the material on a dry basis, aw is the water activity, Xm is the moisture content corresponding to an adsorbed monolayer, C and K are constants related to the temperature effect. 2.6. Scanning Electron Microscopy Scanning Electron Microscopy (SEM) was used to visualize the microstructure of rice kernels. Freeze-dried rice kernels were cut with a blade at a thickness of 1–2 mm and coated with gold. Samples were sputtered with gold particles, creating a layer with 15 nm thickness, using a SC7620 Mini Sputter Coater (Quorum Technologies, West Sussex, UK), in order to make the surface reflect the electron beam. The specimens were then photographed using an electron microscope (Quanta 200 FEI (2004), OR, USA) operated at 20 kV. 2.7. Statistical analysis Regression analysis was used to estimate the models’ parameters, as described by Maroulis et al. (1988). The influence of process conditions on the structural characteristics of freeze-dried rice kernels was also, analyzed using analysis of variance (ANOVA). The analyses were performed using Statistica software (Statistica Release 7, Statsoft Inc., Tulsa, OK, USA). Significant differences were considered when p < 0.05. 3. Results and discussion 3.1. Bulk density, true density, porosity The true density of freeze-dried rice kernels was determined using the Eq. (1) and it was taken as a constant equal to the density of the solid material. The value of true density was estimated equal to 1504 with a standard deviation of ±33 kg/m3. The bulk density of freeze-dried rice kernels, as measured experimentally, was found to be a strong function of freeze-drying conditions and boiling time. The corresponding results are presented in Fig. 1, where solid lines represent the predicted values of bulk density. Regression analysis showed that bulk density of freeze-dried materials decreased significantly (p < 0.001) as the pressure was decreased from 1.25 to 0.04 mbar. In addition, materials boiled for longer time showed significant (p < 0.001) lower values of bulk density compared to those boiled for shorter time. As it can be seen in Fig. 2, the porosity of freeze-dried materials, measured by helium stereopycnometer, was significantly affected by boiling time and freeze-drying conditions. At low pressures, the porosity was the highest noticed and decreased as the pressure increased. Similar results were found for various products in the past. Krokida et al. (1998) observed the same trend for freeze-dried bananas, apples, carrots and potatoes, while Rahman and Sablani (2003) for freeze-dried abalone. Furthermore, the porosity was significantly affected by boiling time, presenting higher values while elongating the boiling procedure. Freeze-dried rice boiled for the shortest period showed the lowest values of porosity and the highest values of bulk density. As boiling time increases there will be a much higher water uptake reaching about 2.5 times the mass of dried rice. Therefore, a bulk volume increase will take place. During subsequent freezing and freeze-drying the water which sublimes creates pores. The amount of pores (porosity) was related to the water uptake and it was higher when the water uptake was increased. Consequently, if no collapse would happen due to the extension of boiling time, the

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Table 1 Parameter estimation for porosity and total cumulative volume of freeze-dried rice.

900 P=0.04 mbar P=0.13 mbar Calculated values

ko

mo

no

R2

0.208 ± 0.001 0.000

0.017 ± 0.002 0.000

0.232 ± 0.006 0.000

0.988

p-Value

k1 (106 m3/kg)

m1

n1

R2

444.810 ± 21.606 0.000

0.037 ± 0.012 0.002

0.507 ± 0.039 0.000

0.985

3

Bulk density (kg/m )

Porosity

P=1.25 mbar

800

Total cumulative volume

700

p-Value

600 900

500

P=0.04 mbar P=0.13 mbar P=1.25 mbar

700

3

Cumulative Volume (x10-6 m /kg)

800

400 0

5

10

15

20

25

Boiling time (min) Fig. 1. Correlation of bulk density with boiling time for various applied pressures during freeze-drying (1 mbar = 100 Pa).

0.70

0.65

600 500 400 300 200 100 0

0.60

0.001

0.01

0.1

1

10

100

Porosity

Pore Radius ( m) 0.55

Fig. 3. Volume of mercury intruded measured with mercury porosimetry versus pore radius of representative freeze-dried rice samples (t = 12 min), for various applied pressures during freeze-drying (1 mbar = 100 Pa).

P=0.04 mbar

0.50

P=0.13 mbar P=1.25 mbar Calculated values

0.45

1100 1000

t=4 min

0.35 0

5

10

15

20

25

Boiling time (min) Fig. 2. Correlation of porosity with boiling time for various applied pressures during freeze-drying (1 mbar = 100 Pa).

porosity would be higher. The increase of porosity while increasing boiling time, varied from 40% to 50%, taking as a reference the shortest boiling time, depending on the pressure applied. The results of parameter estimation of the mathematical model for porosity and bulk density measured by helium stereopycnometer, are summarized in Table 1.

Cumulative Volume (x10-6 m 3/kg)

0.40

Pore size distribution curves of representative freeze-dried rice samples, are shown in Figs. 3 and 4. The singularity point at which the concave curve turns to convex, presents the average pore radius. The average pore radius for all samples was in the range of 7.8 to 9.8 lm. Mercury porosimetry is considered to be very useful for the verification of the microstructure of dehydrated food products (Karathanos et al., 1996a,b; Rahman et al., 2002). The porosity of freeze-dried rice boiled for 12 min and dried at 0.04 mbar, measured with mercury porosimetry, was determined equal to 0.537.

t=20 min

800 700 600 500 400 300 200 100 0 0.001

3.2. Pore size distribution

t=12 min

900

0.01

0.1

1

10

100

Pore Radius ( m) Fig. 4. Volume of mercury intruded measured with mercury porosimetry versus pore radius of representative freeze-dried rice samples (P = 0.04 mbar), for various boiling periods.

Using the helium stereopycnometer the porosity was measured equal to 0.583. The difference of the measurement between the porosity measured by the mercury porosimeter and the porosity obtained from the helium stereopycnometer data was recorded for all the materials examined. This trend is attributed to the fact that very large pores cannot be measured by mercury porosimeter, since mercury easily penetrates into them before any pressure is

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1100

P=0.04 mbar P=0.13 mbar P=1.25 mbar Calculated values

900

(x10-6 m 3/kg)

Total cumulative volume

1000

800

700

600

500

400 0

5

10

15

20

25

Boiling time (min) Fig. 5. Total cumulative volume measured by mercury porosimeter versus boiling time for various applied pressures during freeze-drying (1 mbar = 100 Pa).

applied. Furthermore, mercury cannot penetrate into pores smaller than 2 nm, while helium gas is much more permeable to small pores. The variation of the total cumulative volume, measured by mercury porosimeter, with the freeze-drying conditions and boiling time is presented in Fig. 5. The predicted values are presented with the solid lines. It is obvious that the increment of the boiling period and the reduction of the applied pressure during freeze-drying led to the rise of the total cumulative volume.

Pore Percentage (%)

(a) 50 40

t=12 min t=20 min

30 20 10 0

Pore Radius ( m)

3.3. Moisture sorption isotherms The experimental adsorption results and the predicted moisture sorption isotherms of freeze-dried rice boiled for 4 min, for different pressures during freeze-drying are presented in Fig. 8. Sorption isotherms obtained, follow the characteristic sigmoid profile (type II), which corresponds to multilayer adsorption of rice and are in good agreement with previous studies (Albena and Nikolay, 2004; Basunia and Abe, 1999; Sablani et al., 2009). As shown in Fig. 8, equilibrium moisture content increased with the increment of water activity. The reduction of moisture content causes the decrement of food moisture’s vapor pressure, lower than unity. The characteristic sigmoid shape of water sorption isotherms is attributed to this phenomenon (Ramesh, 2003). The increase of water content causes increased segmental mobility of chains in amorphous regions of glassy and partially crystalline polymers (Rowland, 1980). The knowledge of the equilibrium moisture content of rice kernels plays an important role in food processing engineering and would also help to the specification of the storage conditions of the products. The estimated constants of the GAB model are presented in Table 2. The values of the monolayer moisture content Xm, represent a measure of the sorption possibility of the food product. Below Xm, water molecules are unfreezable and strongly bounded via hydrogen bonds and their participation in

40 t=12 min

40

t=20 min

30

Pore Percentage (%)

Pore Percentage (%)

(b)

The results of parameter estimation of the mathematical model for total cumulative volume, measured with mercury porosimeter, are summarized in Table 1. Fig. 6 presents the pore size distribution of representative freeze-dried rice samples, as measured with mercury porosimeter, for various boiling periods. Fig. 6a and b presents the pore size distribution of rice kernels for 0.13 and 1.25 mbar of absolute freeze-drying pressure, respectively. The percentage of large pores increased with the increment of the boiling period for various applied pressures. As the boiling procedure is elongated, rice will absorb more water, resulting in expansion of the material and production of larger water-containing regions and larger ice crystals. Consequently, the sublimation of ice will create larger pores or gaps inside the kernel. In addition, Fig. 7 shows the pore size distribution of freeze-dried rice samples as a function of the applied pressure during freeze-drying. The percentage of large pores is higher at the lower applied pressure, where the porosity is higher. The higher applied pressure during freeze-drying leads to the reduction of viscosity of food materials which causes acceleration of shrinkage (Krokida et al., 1998). As a result, the porosity and pore size of rice kernels will be lower at higher pressures. These results strengthen the observation obtained from helium stereopycnometer data, which indicate the high porosity with the decrement of the applied pressure.

20 10 0

P=0.04 mbar P=0.13 mbar

30

P=1.25 mbar

20 10 0

Pore Radius ( m)

Pore Radius ( m) Fig. 6. Pore size distribution of representative freeze-dried rice samples, for various applied pressures during freeze-drying (a) 0.13 mbar and (b) 1.25 mbar. (1 mbar = 100 Pa).

Fig. 7. Pore size distribution of representative freeze-dried rice samples (t = 12 min), for various applied pressures during freeze-drying (1 mbar = 100 Pa).

V.P. Oikonomopoulou et al. / Journal of Food Engineering 107 (2011) 326–333

Fig. 8. Sorption isotherms of freeze dried rice boiled for 4 min, for various applied pressures during freeze-drying (1 mbar = 100 Pa).

Table 2 Estimated values of coefficients (Xm, K, C) of GAB model for adsorption of freeze-dried rice samples. P (mbar)

Xm

K

C

R2

0.04 p-Value 0.13 p-Value 1.25 p-Value

9.628 ± 1.374 0.000 13.620 ± 2.106 0.000 11.839 ± 2.192 0.001

0.598 ± 0.057 0.000 0.460 ± 0.064 0.000 0.522 ± 0.073 0.000

10.219 ± 3.433 0.000 7.380 ± 1.290 0.000 7.325 ± 1.993 0.008

0.992

331

Fig. 9. Glass transition temperature versus moisture content for various applied pressures during freeze-drying (4 min boiling time) (1 mbar = 100 Pa).

(a)

0.997 0.994

any deteriorative reactions is limited; so the prediction of the monolayer moisture content is very crucial (Kaymak-Ertekin and Gedik, 2004). Above Xm, the relationship between moisture content and water activity is linear, which corresponds to the multilayer adsorption and the filling of micropores with water molecules. After the linear part, the trend is asymptotic and in this area, water molecules are free to participate in biochemical reactions (Ertugay and Certel, 2000). As a result, moisture sorption isotherms can be used for the prediction of the shelf life of dried foods.

(b) 3.4. Glass transition temperature Fig. 9 illustrates the effect of moisture content on glass transition temperature, for a representative boiling period. It is clear that the values of Tg decreased while increasing the water activity for all the materials, regardless the process conditions. This behavior is attributed to the plasticization effect of water on the amorphous parts of the material, due to the low glass transition temperature of water close to 135 °C (Bhandari and Howes, 1999; Champion et al., 2000; Johari et al., 1987). The same trend was recorded for all samples regardless the boiling periods. An increase in moisture content leads to an increase in free volume and a decrease in local viscosity. As a result, the segmental mobility of the molecular chain of starch in the amorphous region would increase, resulting in a decrease of Tg (Levine and Slade, 1989). The Tg of rice kernels varied from 62.1 °C to 21.1 °C with the increment of the moisture content. Similar results with the estimated values of glass transition temperature, in the range of 60 °C to 20 °C, were observed in the past by many authors (Perdon et al., 2000; Sun et al., 2002). Rice is largely composed of starch and regarded as a hydroscopic material.

Fig. 10. Correlation of glass transition temperature with porosity for two different water activities: (a) 0.75 and (b) 0.94.

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According to Sun et al. (2002), the Tg of rice is due to the Tg of starch. The knowledge of glass transition temperature is very important because above the Tg, molecular mobility is higher and causes acceleration of the reaction rates (Hsu and Heldman, 2005) while below Tg the food is rigid and stable. Porosity of foods influences the quality and texture, and the measurement of glass transition temperature is used to characterize the textural changes of food materials and therefore, their quality. The correlation of glass transition temperature with porosity and boiling time for various water activities is shown in Figs. 10 and 11, respectively. Glass transition temperature shifted towards lower values while porosity increased, for various water activities. The glass transition temperature appears to be sensitive to the network morphology of the dried samples and the high levels of intercellular spaces can lead to the reduction on the values of Tg. For a constant pressure, the glass transition temperature was not greatly influenced by boiling time.

(a)

3.5. Scanning electron microscopy

(b) The alterations of porosity as a function of boiling period and freeze-drying conditions are visualized with SEM photographs and presented in Fig. 12. Scanning microscope was operated at 500 magnification. As it can be seen, the increase of boiling time leads to noticeable changes of microstructure of cooked rice kernels. At higher boiling periods both porosity and average pore size seemed to increase. Porosity depends on the water uptake and is higher at longer boiling times where water uptake is higher. In addition, as far as freeze-drying pressure is concerned, when the absolute pressure in the freeze-drying chamber is higher, then the porosity is lower compared to the low process pressures. 4. Conclusions

Fig. 11. Correlation of glass transition temperature with boiling time for two different water activities: (a) 0.75 and (b) 0.94 (1 mbar = 100 Pa).

Freeze-drying conditions and boiling time affected significantly the bulk density and porosity of dried rice kernels. The bulk density decreased and the porosity increased with decreasing the chamber pressure during freeze-drying. In addition, rice samples boiled for 4 min presented the lowest value of porosity and the highest value of bulk density while those boiled for 24 min showed the highest value of porosity and the lowest value of bulk density, respectively.

P=0.04 mbar

P=0.13 mbar

P=1.25 mbar

t=4 min

t=4 min

t=4 min

t=20 min

t=20 min

t=20 min

Fig. 12. Microstructure of freeze-dried rice as a function of boiling period and applied pressure during freeze-drying (1 mbar = 100 Pa).

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