- Email: [email protected]
Effects of moisture content and temperature of spaghetti on their mechanical properties
Journal of Food Engineering 59 (2003) 51–60 www.elsevier.com/locate/jfoodeng
Effects of moisture content and temperature of spaghetti on their mechanical properties Bernard Cuq a
a,*
, Francis Goncßalves b, Jean Francßois Mas b, Louis Vareille b, Joel Abecassis a
Laboratoire Technologie des C er eales et des Agro-polym eres, ENSA-INRA Montpellier, 2 place Viala, 34060, Montpellier, Cedex 1, France b Panzani, 4 rue Boileau, Lyon 69006, France Received 1 October 2000; received in revised form 13 September 2001; accepted 22 October 2002
Abstract The mechanical properties of spaghetti were determined as a function of water content at different temperatures (20, 40, 60, or 80 °C). The mechanical properties of pasta were evaluated by an empirical test and characterized by the apparent strength and apparent relaxation coefficients. The fresh spaghetti (at 45% water db) behave as a visco-plastic and soft product. The dry spaghetti (at 10% water db) behave as an elastic and rigid product. An abrupt mechanical transition separates the fresh state and the dry state. This transition depends either from water content and temperature of pasta. A sigmoid model was used to describe this transition as a function of moisture content and/or temperature. The transition of mechanical properties for spaghetti was nearly superimposed with the glass transition temperature of wheat durum semolina. An apparent ‘‘time–temperature–water content’’ superposition showed that an increase in temperature or in water content induces a plasticizing effect on mechanical properties of spaghetti. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pasta; Mechanical properties; Glass transition; Plasticization
1. Introduction For a long time, pasta have been dried at low temperatures (6 60 °C) to reach microbial and physicochemical stability. The development of the high (P 70 °C) and very high (P 90 °C) drying temperatures represented a very important technological progress, by increasing productivity with a significant decrease in drying time (from 17 to 6 h) and by improving the sensorial quality of products (Manser, 1980; Pavan, 1980). In addition to water removal (i.e., ‘‘passive drying’’), high temperature drying may affect the physicochemical properties of pasta which makes it an ‘‘active drying’’ (Abecassis, Chaurand, Matencio, & Feillet, 1989). High temperature drying modifies significantly the texture of pasta (i.e., pasta gets thougher and less sticky after cooking), but the disadvantage is the appearance of a slight red color (Cunin, Handschin, *
Corresponding author. Tel.: +33-04-99-61-2860; fax: +33-04-6752-2094. E-mail address: [email protected] (B. Cuq).
Conde-Petit, & Escher, 1995; Dexter, Matsuo, & Morgan, 1981; Frances & Ollivier, 1985). The physicochemical changes mainly depend on structural modifications of proteins that could interact through hydrophobic interactions and disulfide bonds, and thus form a stable network without starch degradation (Favier, Samson, Aubled, Morel, & Abecassis, 1996; Feillet, Ait-Mouh, Kobrehel, & Autran, 1989). During drying, the mechanical properties of pasta change significantly, the soft product (i.e., fresh pasta) transforms into a rigid product (i.e., dry pasta). The changes in mechanical properties of pasta could induce the formation of internal stress during drying (G€ orlin, 1960). During drying, the mechanical properties of pasta change from plastic behavior (above 39% water db) to elastic behavior (below 23% water db), with an intermediate plasto-elastic behavior (G€ orlin, 1960). Only few published papers describe the mechanical properties of pasta as a function of water content at different temperatures. However, the experimental conditions applied to obtain these data differ significantly from the classical pasta process. Some papers concern the rehydration of
0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00430-2
52
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
dry pasta by storage in rooms at controlled temperature and relative humidity (Andrieu & Stamatopoulos, 1987; Stamatopoulos, 1986), the heating of dry pasta under uncontrolled conditions of RH (Akiyama & Hayakawa, 1994), the semolina processing with a twin screw extruder (Liu, Qi, & Hayakawa, 1997), the equilibration of semolina extrudates at decreasing RH and low temperature (Liu et al., 1997). Obtaining these data would allow to perfectly describe the effect of temperature on the changes in mechanical properties as a function of pasta moisture content. The ‘‘Food Polymer Science’’ approach to the study of the effects of temperature and water content on food properties is increasingly applied by food scientists and technologists (Seow, Cheah, & Chang, 1999). This approach is used particularly to describe the structureproperty relationships, to control the textural properties and to understand the physicochemical properties changes associated with the concepts of plasticization and glass transition (Levine & Slade, 1986, 1990, 1993). Glass transition is considered to involve a marked decrease in molecular mobility affecting the amorphous polymers. Glass transition is associated with a temperature range (known as Tg ) that divides the physical behavior in two states. At high temperatures and/or high water contents, the amorphous materials are in a soft visco-plastic state and termed ‘‘rubbery’’. A decrease in water content or a decrease in temperature above the Tg range induces glass transition in amorphous materials that then behave as solid elastic materials; this state is described as ‘‘glassy’’. On a molecular scale, glass transition corresponds to a change in energy that allows the components to pass from high amplitude motions involving the main macromolecular chains (i.e., the rubbery state) to local molecular motions of low amplitude (i.e., the glassy state) (Ferry, 1980). Tg of most hydrophilic systems is mainly affected by the water content. The objective of our study was to investigate the changes in mechanical properties of fresh pasta as a function of decreasing water content at different temperatures (at 20, 40, 60, and 80 °C). A specific equipment was built to allow the water content of pasta to stabilize at different levels and to control the ambient conditions around the sample during the measurement of mechanical properties.
2. Materials and methods 2.1. Raw material Commercial durum semolina SSSE grade was obtained from Semoulerie de Bellevue (98–99 production, Marseille, France). Semolina was stored before the experiments at 5 °C in 50 l hermetic plastic cans.
2.2. Pasta manufacturing Semolina (800 g) was used to make spaghetti (dry diameter ¼ 1.5 mm) with an experimental minipress (SERCOM, Montpellier, France). The production room was kept at 20 °C. Semolina was hydrated to 49 g water/ 100 g dry matter with deionized water and then mixed for 20 min at 120 rpm. The temperature of the mixer was maintained at 40 °C by a water cooling system. The product was then fed into the auger and extruded under partial vacuum (150 mm Hg). When extrusion conditions were stable, the spaghetti samples were cut and placed directly in the programmed and controlled temperature and relative humidity oven (SECASI, Pessac, France). 2.3. Water content equilibration The fresh pasta were equilibrated at different moisture contents (from 49 to 5–10% db) by drying. That range of water contents were reached by equilibrating the sample at constant temperature (20, 40, 60 or 80 °C) and successive levels of constant relative humidity (from 90% to 10%) after variable time intervals (of 1–96 h). Water contents were determined from duplicates, according to the NF V03-707 method adapted to pasta (i.e., weight measurements after drying at 110 °C for 10 h, milling, and drying at 130 °C for 2 h). 2.4. Mechanical properties Mechanical properties of pasta were determined as a function of temperature and moisture content by en empirical test and using a modified Rheo TA-XT2 Rheometer (Champlan, France). The measurement device of the rheometer was positioned inside a metallic box directly connected to the SECASI drying apparatus. The temperature and relative humidity controlled air was directly sweeped over the rheometer probe and samples. When the equilibrium moisture content of pasta was reached, five samples of pasta were taken out of the SECASI drier, cut into 8 cm length pieces, and placed in a parallel position (with about 5 mm gap between samples) onto the measurement plate of the rheometer (Fig. 1). Mechanical properties were estimated after 10 min equilibration time (to stabilize the atmospheric conditions around the samples) by using an empirical test. A metallic straight probe (50 mm length) was moved perpendicularly to the plate at constant speed (0.1 mm s1 ). These experimental conditions did not allow determine ‘‘true’’ mechanical properties of pasta. The apparent force (N)–deformation (mm) curves were determined at constant deformation rate (0.1 mm s1 ) until 0.3 mm compression. The mechanical properties of pasta were estimated by the apparent strength (i.e., force at 0.3 mm deformation). Standard
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
53
Air
Probe Air
Pasta
Fig. 1. Experimental system used to estimate the mechanical properties of spaghetti at controlled conditions of temperature and relative humidity.
deviation for apparent strength was evaluated as 6.5% from 4–7 measurements. The viscoelastic properties of pasta were evaluated by an apparent force–relaxation test. After sample deformation, the probe was maintained at 0.3 mm displacement for 30 s. From the relaxation curves, an apparent relaxation coefficient (%) was calculated (Eq. (1)). g0 g30 Apparent relaxation coefficient ¼ ð1Þ g0 where g0 is the force (N) recorded initially (at 0.3 mm displacement) and g30 is the force (N) recorded after 30 s relaxation. Standard deviation for apparent relaxation coefficient was evaluated as 13.4% from 4–7 measurements. 2.5. Mathematical calculations Mathematical equations (Eqs. (2)–(7)) have been used to describe the changes in mechanical properties of pasta as a function of temperature and/or moisture content. The model parameters of these equations were calculated to fit experimental data from a nonlinear optimization procedure (Gauss Newton procedure) using the software Microsoft Excel 98. The minimized objectivefunction was the sum of squared residuals (i.e., deviations between experimental and predicted points).
3. Results 3.1. Mechanical properties of pasta The mechanical properties of spaghetti were characterized by an empirical test during drying. Pasta were firstly characterized by apparent ‘‘force–deformation’’ curves measured under a constant deformation rate. Typical curves for spaghetti at 80 °C with water contents ranging from 27.1% to 9.4% db are presented in Fig. 2.
Force (N)
80
9.4% 60 12.4% 40
20 20.3% 27.1% 0 0
10
20
30
40
Time (sec)
Fig. 2. Influence of pasta water content (% db) on the mechanical properties of spaghetti at 80 °C.
These curves are characterized by initial linear increases in force as a function of time (or deformation) that can be described by linear relationships irrespective of the water content. Even if a true calculation of stress–strain relationships is not possible, we could consider that spaghetti behaves as an elastic material under very small deformations. The decrease in water content of pasta induces a strengthening effect (i.e., shift of ‘‘force–deformation’’ curves to high forces). During the drying, spaghetti are transformed in a more rigid material. Apparent strength of spaghetti were measured as a function of water content at different temperatures (20– 80 °C). Fig. 3 shows experimental data obtained at 40 °C. The moist spaghetti are characterized by nearly constant low values of apparent strength (close to 2 N) regardless of the temperature. Moist spaghetti behave as a soft material. During the drying, the decrease in water content (from 44% to 10% db) produces a large increase in apparent strength (from 2 to 75 N). We observed an abrupt transition from low to high values of apparent strength at a critical level of moisture content that depends on the temperature. The critical levels of moisture
54
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
60
80 - 40˚C -
- 40˚C 50
60 40 30
40
20
20
10 0
0 0
10
20
30
40
50
Fig. 3. Experimental values ðÞ and calculated curves for the changes in apparent strength of spaghetti during the drying (at 40 °C). The calculated curves were obtained using the simple model ( ) or the generalized model (––).
content were 30% at 20 °C, 22% at 40 °C, 18% at 60 °C and 14% at 80 °C. The relationships between apparent strength and moisture content are characterized by a sigmoid shape, regardless of the temperature (between 20 and 80 °C). The dried spaghetti are characterized by nearly constant high values of apparent strength (close to 75 N) regardless of the temperature. Dried spaghetti behave as a rigid material. The visco-elastic properties of spaghetti were evaluated by relaxation tests. Fig. 2 shows typical relaxation curves measured at 80 °C and different water contents. Relaxation curves are characterized by a rapid decrease in force during short relaxation times, that tends to a constant value. These relaxation curves are usually observed for visco-elastic solids. Apparent relaxation coefficients were calculated from the relaxation curves (Eq. (1)) as a function of moisture content at different temperatures (20–80 °C). The data obtained at 40 °C are presented on Fig. 4 as a typical example. The moist spaghetti (30% moisture content) are characterized by nearly constant high values of apparent relaxation coefficient (close to 50%) regardless of the temperature. Moist spaghetti behave as a visco-plastic material. During the drying, the decrease in water content produces a large decrease in apparent relaxation coefficients. We observed an abrupt transition from high to low values of apparent relaxation coefficients at a critical level of moisture content. The relationships between apparent relaxation coefficient and moisture content are characterized by a sigmoid shape, regardless of the temperature (between 20 and 80 °C). The dried spaghetti are characterized by nearly constant low values of apparent relaxation coefficient (close to 1–3%) regardless of the temperature. Dried spaghetti behave as a solid elastic material.
0
10
20
30
40
50
Fig. 4. Experimental values ðÞ and calculated curves for the changes in apparent relaxation coefficient of spaghetti during the drying (at 40 °C). The calculated curves were obtained using the simple model ( ) or the generalized model (––).
It should be noticed that relatively low values of apparent relaxation coefficients were observed at 20, 40 and 60 °C for the moist pasta at 40–45% db. These low values could be associated to the lack of formation of apparent rigid zone onto the pasta surface during water removal, especially at low temperature or to a partial relaxation behavior of product during the slow deformation tests. 3.2. Simple model The changes in mechanical properties (i.e., apparent strength and relaxation coefficient) as a function of water content at constant temperature can be described by using the logistic model (Eq. (2)) adapted to describe sigmoid curves. The measured apparent relaxation coefficients are modeled without taking into consideration the low values observed for the moist pasta at 20, 40 and 60 °C. Y ðW Þ ¼
YH YL þ YL i 1 þ exp W W A
ð2Þ
where Y ðW Þ is the calculated mechanical property, W is the water content (% db), YH is the calculated maximum value of property, YL is the calculated minimum value of property, Wi is the water content (% db) at the inflection point of the sigmoid curve (as schematized in Fig. 5), and A is an empirical constant (% db) which characterizes the spread of the transition from low to high values of mechanical property. The calculated values of model parameters (YH , YL , Wi , and A) are presented in Table 1. The logistic model fitted well experimental data, regardless the temperature (Table 1). High values of regressions coefficient have been found for apparent strength (R2 ¼ 0:939–0.983) and apparent relaxation coefficient (R2 ¼ 0:957–0.988).
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
5
40 35
4
55
where DW is the transition spread of mechanical properties (% db), Wonset and Wendpoint are the water content at the onset and end-point of the transition, respectively. Increasing temperature (from 20 to 80 °C) lead to a decreased in transition spread (from 18% to 3.5% db).
30 3 25 2 20 1
15 10
0 0
20
40
60
80
100
Fig. 5. Influence of the temperature on the critical water content ð Þ and on the spread ðdÞ of the transition in apparent strength during the drying of spaghetti.
As typical examples, some calculated curves are plotted in Figs. 3 and 4. The model parameters related to the mechanical properties on either side of the transition (YH and YL ) are not greatly affected by changes in temperature (Table 1). On either side of the transition, the mechanical properties of spaghetti do not depend on temperature. The critical water content at the inflection point (Wi ) of the transition in mechanical properties is also affected by changes in temperature (Table 1). The critical water content is reduced when the temperature increases. The transition spread of mechanical properties (i.e., the water content range from the onset to the endpoint of the transition in mechanical properties) was determined by using the intercepts of derivative functions. The transition spread (DW ) was then found to be proportional to the model parameter ‘‘A’’ (Eq. (3)). DW ¼ Wonset Wendpoint ¼ 4A
ð3Þ
3.3. Generalized model To take simultaneously into consideration the effects of temperature and water content on the changes in mechanical properties of pasta, the experimental data were described using a generalized model. The generalized model takes simultaneously into consideration the changes in mechanical properties as function of water content (Eq. (4)) and the effect of temperature on the model parameters (Eq. (5)). Y ðW ; T Þ ¼
YH ðT Þ YL ðT Þ þ YL ðT Þ i ðT Þ 1 þ exp W W AðT Þ
ð4Þ
where Y ðW ; T Þ is the calculated mechanical property as a function of water content and temperature. The parameters of Eq. (4) (YH ðT Þ, YL ðT Þ, Wi ðT Þ and AðT Þ) are related to the temperature using exponential functions (Eq. (5)). yðT Þ ¼ y0 exp ð kT Þ
ð5Þ
where y0 and k are the parameters of the exponential equations. We have supposed that the effect of temperature on the minimum (YL ðT Þ) and maximum (YH ðT Þ) calculated values of mechanical properties are the same. The calculated values of model parameters are presented in Table 2. The generalized model fitted well (R2 ¼ 0:957) the changes in mechanical properties of spaghetti as a function of water content and temperature. For instance, the calculated curves at 40 °C are plotted in Figs. 3 and 4. It should be noted that the calculated curves using the generalized model (Eqs. (4)
Table 1 Model parameters and correlation coefficients calculated using the simple models to fit the experimental changes in apparent strength and relaxation coefficients as a function of moisture content (Eq. (2)) Temperature (°C)
R2
Model parameters YH
Apparent strength 20 40 60 80
73.3 76.8 72.5 76.2
Apparent relaxation coefficient 20 40 60 80
61.2% 50.8% 57.4% 59.7%
N N N N
YL
Wi (% db)
A (% db)
2.70 N 2.49 N 2.29 N 10.2 N
30.7 21.6 17.8 14.0
4.47 3.42 2.80 1.99
0.971 0.980 0.983 0.939
2.89% 2.50% 1.12% 1.87%
26.8 19.4 15.5 12.8
2.21 1.36 1.82 0.87
0.984 0.957 0.988 0.972
56
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
Table 2 Model parameters and correlation coefficients calculated using the generalized models to fit the experimental changes in apparent strength and relaxation coefficients as a function of moisture content (Eq. (4)) and temperature (Eq. (5)) Model coefficients (Eq. (5))
Mechanical properties Apparent strength
Apparent relaxation coefficient
Minimum calculated value of mechanical property (YL ) y0 k (103 °C1 )
4.3 N )1.1
2.09% )0.1
Maximum calculated value of mechanical property (YH ) y0 k (103 °C1 )
70.4 N )1.1
56.5% )0.1
Critical water content at the inflection point (Wi ) y0 (% db) k (102 °C1 )
37.9 1.24
32.7 1.2
Transition spread ðAÞ Y0 (% db) k (103 °C1 )
4.55 7.1
3.13 1.45
Regression coefficient R2
0.957
0.947
and (5)) are relatively close to those that were obtained using the simple models (Eq. (2)) at the different temperatures. The dependence of the generalized model with temperature was estimated by the calculated values of parameter k (Table 2). The minimum and maximum calculated values of mechanical properties (YL and YH ) are associated to very low values of k. These parameters are thus not significantly affected by the temperature (as previously assumed from the results in Table 1). On the other hand, high values of k were calculated for the critical water content at the inflection point (Wi ) and for
the transition spread (A). An increase in temperature induces a significant decrease in the critical water content at the inflection point (Wi ) and transition spread (A) (Fig. 5). It could be noticed in Fig. 6 that the dependence of critical water content (Wi ) with temperature is similar when using the generalized model (Eqs. (4) and (5)) or the simple model (Eq. (2)). On the other hand, the dependence of transition spread (Wi ) with temperature seems to be slightly under-estimated by using generalized model (Fig. 5). The generalized model is thus not fully adapted to describe the effect of temperature on the transition spread.
80 70 60
Apparent strength (N)
50 40 30 20 10 0
0 10
90 20
70 30
Water content (%db)
50 40
30 10
Temperature (˚C)
Fig. 6. Three-dimensional plot of apparent strength for pasta as a function of temperature and moisture content.
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
3.4. Three-dimensional plot The generalized model (Eqs. (4) and (5)) and the optimized model parameters (Table 2) were used to construct a three-dimensional display of mechanical properties, as that shown in Fig. 6 for the apparent strength. The three-dimensional plot enables the description of the combined effects of the temperature and water content on the mechanical properties of pasta, and more particularly around the transition. 3.5. Reciprocal model A close up examination of the three-dimensional plot (presented in Fig. 6) allows to note that the transition of mechanical properties from soft to rigid pasta can also be observed during a temperature decrease, at constant moisture content (Fig. 6). Indeed, a decrease in temperature induces a similar abrupt transition in mechanical properties, from a low mechanical strength to a high mechanical strength, at constant moisture content. This description permits to define critical values of temperature that are associated with the onset-point, the inflection point, and the end-point of the transition expected during the cooling of pasta. Description of the mechanical properties of spaghetti could be thus performed using a reciprocal model that takes into consideration the effect of temperature (Eq. (6)) and then the effect of moisture content on the model parameters (Eq. (7)). YH ðW Þ YL ðW Þ þ YL ðW Þ; Y ðT ; W Þ ¼ i ðW Þ 1 þ exp T T AðW Þ
ð6Þ
57
where Y ðT ; W Þ is the calculated mechanical property as a function of temperature and water content. The parameters of Eq. (4) (YH ðW Þ, YL ðW Þ, Ti ðW Þ and AðW Þ) are related to the water content using exponential functions as those previously used for the generalized model (Eq. (7)). yðW Þ ¼ y0 exp ð kW Þ ð7Þ The calculated values of model parameters are presented in Table 3. The reciprocal models fitted well (R2 ¼ 0:943–0.953) the changes in mechanical properties as a function of temperature and moisture content. The dependence of the reciprocal model with moisture content was estimated by the calculated values of parameter k (Table 3). The minimum and maximum calculated values of mechanical properties (YL and YH ) are not affected by the water content (k ¼ 0). On the other hand, high values of k were calculated for the critical temperature at the inflection point (Ti ) and for the transition spread ðAÞ. An increase in moisture content induces a significant decrease in the critical temperature at the inflection point (Wi ) and transition spread (A).
4. Discussion The use of an empirical test to determine the mechanical properties of spaghetti allowed to directly measure the changes in apparent strength and relaxation coefficient during the drying of fresh pasta, at different temperatures (20–80 °C). However, it should be noted that the tested drying conditions (i.e., equilibration at successive decreasing levels of RH) largely differ from the classical pasta drying conditions in which water
Table 3 Model parameters and correlation coefficients calculated using the reciprocal models to fit the experimental changes in apparent strength and relaxation coefficients as a function of temperature (Eq. (6)) and moisture content (Eq. (7)) Model coefficients (Eq. (7))
Mechanical properties Apparent strength
Apparent relaxation coefficient
Minimum calculated value of mechanical property (YL ) y0 k
4.65 N 0
2.4% 2.7 103 (% db1 )
Maximum calculated value of mechanical property (YH ) y0 k
74 N 0
60.3% 2.7 103 (% db1 )
Critical temperature at the inflection point (Wi ) Y0 k
296 °C 9.02 102 (% db1 )
293 °C 10.1 102 (% db1 )
Transition spread ðAÞ y0 k
31.9 °C 4.46 102 (% db1 )
18.2 °C 5.71 102 (% db1 )
Regression coefficient R2
0.943
0.953
58
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
contents are not equilibrated inside pasta during the drying. In addition, we could note that the classical pasta drying conditions require variable conditions of temperature and relative humidity during the drying (Frances & Ollivier, 1985; Abecassis et al., 1989). A dramatic change in the mechanical behavior of spaghetti (apparent strength and relaxation coefficients) is produced by a decrease of moisture content. Fresh pasta behaves as a visco-plastic and soft product. Dry pasta behaves as an elastic and rigid product. During the drying, an abrupt mechanical transition separates fresh and dry states. Our experimental conditions have been defined in order to study large changes in mechanical properties, from fresh to dry pasta. These conditions appeared to be not enough sensitive to demonstrate the effects of water content or temperature on the mechanical properties on either side of the transition (Table 1). It should be noticed that Stamatopoulos (1986) showed a significant decrease in the Young modulus (from 1.3 to 0.3 MPa) for fresh pasta when the temperature increased from 20 to 70 °C. The abrupt mechanical transition (from fresh to dry pasta) was described by a simple mathematical model that takes into consideration its sigmoid shape. The location of the transition shifted towards lower values when temperature and water content increased. Comparable similar transitions of mechanical properties for pasta have been observed by G€ orlin (1960) and by Stamatopoulos (1986). They have shown a transition from a plastic behavior (fresh pasta) to an elastic behavior (dry pasta). However, Stamatopoulos (1986) postulated that the location (i.e., water content) of the transition was not affected by the temperature (between 20 and 70 °C), even if their data may have demonstrated a slight effect of temperature. The effects of temperature and moisture content on the mechanical properties of pasta could be interpreted using the concepts of plasticization and glass transition. The effect of water content on the critical temperatures (Tonset , Tinflection , and Tendpoint ) at the transition of mechanical properties (apparent strength) of pasta was thus compared with the glass transition temperatures of wheat durum semolina determined by modulated DSC (Cuq & Verniere Icard, 2001) (Fig. 7). The transition of mechanical properties was superimposed on the glass transition of durum wheat semolina. The glass transition temperature determined by DSC is classically located at lower temperatures than the transition temperature determined from mechanical properties (Cocero & Kokini, 1991; Kalichevski, Blanshard, & Tobarczuk, 1993). The moist pasta are in a rubbery state above the glass transition. The changes in mechanical properties of pasta during the drying are relatively low until the transition where an abrupt increase in mechanical strength is observed. Below the Tg , the dry pasta is in a glassy state. The transition of the mechanical properties
Temperature (˚C) 125
100
75
50
Tonset Tinflection
25
Tendpoint 0 0
10
20
30
40
50
Water content (g / 100 g dm) Fig. 7. Comparison between the critical temperatures (Tonset , Tinflection , and Tendpoint ) at the transition in mechanical properties of pasta during drying, and the glass transition temperature Tg ðrÞ for wheat durum semolina measured by DSC (from Cuq & Verniere Icard, 2001).
of pasta may be thus explained by the glass transition of the hydrophilic amorphous components of wheat semolina. The model used to describe the effect of water content and temperature on mechanical properties of pasta (Eqs. (2), (4)–(7)) was derived from a model proposed by Peleg (1993, 1994a, 1994b) to characterize the variations of mechanical properties of wheat products around the glass transition. For instance, the glass transition of wheat grain at ambient temperature was characterized by its location (19% water) and spread of transition (4% water) (Peleg, 1993). These values are relatively close to the location (27–31% water) and the spread (2.2–4.5% water) determined in the present study for the transition in mechanical properties of pasta at 20 °C (Table 1). On the other hand, the effect of water content on the location of the glass transition was characterized by Peleg (1993, 1994b) for wheat gluten (k ¼ 0:127) and starch (k ¼ 0:11). These values are in a same order of magnitude than those found in the present study for the transition in pasta mechanical properties (Table 3). The effect of water content on the spread of the glass transition was characterized for the wheat gliadins (k ¼ 0:06). These values are relatively close to those (k ¼ 0:045–0.057) found in the present study for the transition in pasta mechanical properties (Table 3). The characteristics of the sigmoid curve proposed to describe the transition of mechanical properties of pasta during drying are thus consistent with bibliographic data on the glass transition behavior of wheat products. Glass transition is generally considered to involve a marked decrease in molecular mobility affecting the amorphous polymers (Ferry, 1980). The description of mechanical properties of pasta through the concept of
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
59
2
1.5
1
0.5
0 0
5
10
15
20
Fig. 8. Comparison between the master-curve constructed from the Cole–Cole simulation and the ‘‘time–temperature–water content’’ superposition curve constructed from the ‘‘force–relaxation’’ data.
glass transition lets implies the assumption that the molecular mobility of the main components in pasta mainly depends on the plasticization rate. The molecular mobility is enhanced by an increase in temperature (i.e., thermal plasticization) or water content (i.e., water plasticization). Our results seemed to be in agreement with this theory on molecular mobility as shown the calculated master-curve derived from a Cole–Cole equation (Tschoegl, 1989). The parameters of the Cole– Cole equation (Eq. (8)) were empirically adapted to an apparent master-curve from preliminary calculations. Sglassy Srubbery logðstrengthÞ ¼ log þ Srubbery ð8Þ 1 þ ðt=t0 Þc where t is the time (s), t0 is the time at the inflection point (s), Sglassy is the apparent strength at the glassy state (N), Srubbery is the apparent strength at the rubbery state (N). We have carried out an apparent ‘‘time–temperature– water content’’ superposition. All of the experimental force–time curves were plotted on log–log scales. The force–time curves (determined irrespective of the temperature and the pasta water content) could be superimposed (one behind the other) on the apparent master-curve by using a shifting factor whose value depends on temperature and water content (Fig. 8). However, the available data did not allow us to built a continuous apparent master-curve. The fact of being able to nearly adjust the whole experimental force–relaxation data on the apparent master-curve (regardless of the temperature and of pasta water content) indicates that the mechanical properties of pasta are controlled by a phenomenon based on molecular mobility, whose in-
tensity depends on the plasticization rate. An increase in temperature or in water content could thus induce a similar effect on the molecular mobility components and on the mechanical properties of pasta.
References Abecassis, J., Chaurand, M., Matencio, F., & Feillet, P. (1989). Einfluss des Wassergehaltes der Teigwaren bei der Hochtemperaturtrocknung. Getreide Mehl und Brot, 43, 58–62. Akiyama, T., & Hayakawa, K. I. (1994). Tensile fracture stress of a pasta product at temperatures from 293 to 343 K. Lebensmittel Wissenschaft und Technology, 27, 93–94. Andrieu, J., & Stamatopoulos, A. (1987). Durum wheat pasta drying kinetics. Lebensmittel Wissenschaft und Technology, 19, 448–456. Cocero, A. M., & Kokini, J. L. (1991). The study of the glass transition of glutenin using small amplitude oscillatory rheological measurements and differential scanning calorimetry. Journal of Rheology, 35, 257–270. Cunin, C., Handschin, S., Conde-Petit, B., & Escher, F. (1995). Development of microstructure of pasta during processing and cooking. In E. Windhab & B. Wolf (Eds.) Food rheology and structure. Proceedings of 1st symposium on food rheology and structure, Zurich. Cuq, B., & Verniere Icard, C. (2001). Characterization of glass transition of durum wheat semolina using modulated differential scanning calorimetry. Journal of Cereal Science, 33, 213–221. Dexter, J. E., Matsuo, R. R., & Morgan, B. C. (1981). High temperature drying effect on spaghetti properties. International Journal of Food Science, 46, 1741–1746. Favier, J., Samson, M. F., Aubled, C., Morel, M. H., & Abecassis, J. (1996). Suivi de la cinetique thermique dÕinsolubilisation des proteines de la p^ate alimentaire par chromatographie dÕexclusiondiffusion haute performance. Sciences des Aliments, 16, 573–591. Feillet, P., Ait-Mouh, O., Kobrehel, K., & Autran, J. C. (1989). The role of low molecular weight glutenins in the determination of
60
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
cooking quality of pasta products: an overview. Cereal Chemistry, 66, 26–30. Ferry, J. D. (1980). Viscoelastic properties of polymers. New York, NY: John Wiley and Sons. Frances, Y., & Ollivier, J. L. (1985). LÕutilisation des tres hautes temperatures pour le sechage des p^ates alimentaires. Industries Agricoles et Alimentaires, 5, 471–475. G€ orlin, P. (1960). Verh€ utung von schwindungsrissen bei der makkaronitrocknung. Getreide and Mehl, 4, 39–43. Kalichevski, M. T., Blanshard, J. M., & Tobarczuk, P. F. (1993). Effect of water content and sugars on the glass transition of casein and sodium caseinate. International Journal of Food Science and Technology, 28, 139–151. Levine, L., & Slade, H. (1986). A polymer physico-chemical approach to the study of commercial starch hydrolysis products. Carbohydrate Polymers, 6, 213–244. Levine, L., & Slade, H. (1990). Influence of the glassy and rubbery state on the thermal, mechanical, and structural properties of doughs and baked products. In H. Faridi & J. M. Faubion (Eds.), Dough rheology and baked product texture (pp. 157–330). Reinhold, NY: van Nostrand. Levine, L., & Slade, H. (1993). The glassy state phenomenon in food molecules. In J. M. V. Blanshard & P. J. Lillford (Eds.), The glassy state in foods (pp. 35–102). Nottingham: Nottingham University Press.
Liu, H., Qi, J., & Hayakawa, K. (1997). Rheological properties including tensile fracture stress of semolina extrudates influenced by moisture content. Journal of Food Science, 62, 813– 820. Manser, J. (1980). Sechage des p^ates alimentaires a tres haute temperature. Diagramme B€uhler, 69, 11–12. Pavan, G. (1980). High temperature drying improves pasta quality. Food Engineering International, 2, 37–39. Peleg, M. (1993). Mapping the stiffness–temperature–moisture relationship of solid biomaterials at and around their glass transition. Rheological Acta, 32, 575–580. Peleg, M. (1994a). A model of mechanical changes in biomaterials at and around their glass transition. Biotechnology Progress, 10, 385– 388. Peleg, M. (1994b). Mathematical characterization and graphical presentation of the stiffness–temperature–moisture relationship of gliadin. Biotechnology Progress, 10, 652–654. Seow, C. C., Cheah, P. B., & Chang, Y. P. (1999). Antiplasticization by water in reduced-moisture food systems. Journal of Food Science, 64, 576–581. Stamatopoulos, A. (1986) Contribution a lÕetude theorique et experimentale du sechage des p^ates alimentaires. Ph.D. thesis, University of Montpellier. Tschoegl, N. W. (1989). The phenomenological theory of linear viscoelastic behavior. New York, NY: Springer-Verlag.
Effects of moisture content and temperature of spaghetti on their mechanical properties Bernard Cuq a
a,*
, Francis Goncßalves b, Jean Francßois Mas b, Louis Vareille b, Joel Abecassis a
Laboratoire Technologie des C er eales et des Agro-polym eres, ENSA-INRA Montpellier, 2 place Viala, 34060, Montpellier, Cedex 1, France b Panzani, 4 rue Boileau, Lyon 69006, France Received 1 October 2000; received in revised form 13 September 2001; accepted 22 October 2002
Abstract The mechanical properties of spaghetti were determined as a function of water content at different temperatures (20, 40, 60, or 80 °C). The mechanical properties of pasta were evaluated by an empirical test and characterized by the apparent strength and apparent relaxation coefficients. The fresh spaghetti (at 45% water db) behave as a visco-plastic and soft product. The dry spaghetti (at 10% water db) behave as an elastic and rigid product. An abrupt mechanical transition separates the fresh state and the dry state. This transition depends either from water content and temperature of pasta. A sigmoid model was used to describe this transition as a function of moisture content and/or temperature. The transition of mechanical properties for spaghetti was nearly superimposed with the glass transition temperature of wheat durum semolina. An apparent ‘‘time–temperature–water content’’ superposition showed that an increase in temperature or in water content induces a plasticizing effect on mechanical properties of spaghetti. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pasta; Mechanical properties; Glass transition; Plasticization
1. Introduction For a long time, pasta have been dried at low temperatures (6 60 °C) to reach microbial and physicochemical stability. The development of the high (P 70 °C) and very high (P 90 °C) drying temperatures represented a very important technological progress, by increasing productivity with a significant decrease in drying time (from 17 to 6 h) and by improving the sensorial quality of products (Manser, 1980; Pavan, 1980). In addition to water removal (i.e., ‘‘passive drying’’), high temperature drying may affect the physicochemical properties of pasta which makes it an ‘‘active drying’’ (Abecassis, Chaurand, Matencio, & Feillet, 1989). High temperature drying modifies significantly the texture of pasta (i.e., pasta gets thougher and less sticky after cooking), but the disadvantage is the appearance of a slight red color (Cunin, Handschin, *
Corresponding author. Tel.: +33-04-99-61-2860; fax: +33-04-6752-2094. E-mail address: [email protected] (B. Cuq).
Conde-Petit, & Escher, 1995; Dexter, Matsuo, & Morgan, 1981; Frances & Ollivier, 1985). The physicochemical changes mainly depend on structural modifications of proteins that could interact through hydrophobic interactions and disulfide bonds, and thus form a stable network without starch degradation (Favier, Samson, Aubled, Morel, & Abecassis, 1996; Feillet, Ait-Mouh, Kobrehel, & Autran, 1989). During drying, the mechanical properties of pasta change significantly, the soft product (i.e., fresh pasta) transforms into a rigid product (i.e., dry pasta). The changes in mechanical properties of pasta could induce the formation of internal stress during drying (G€ orlin, 1960). During drying, the mechanical properties of pasta change from plastic behavior (above 39% water db) to elastic behavior (below 23% water db), with an intermediate plasto-elastic behavior (G€ orlin, 1960). Only few published papers describe the mechanical properties of pasta as a function of water content at different temperatures. However, the experimental conditions applied to obtain these data differ significantly from the classical pasta process. Some papers concern the rehydration of
0260-8774/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0260-8774(02)00430-2
52
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
dry pasta by storage in rooms at controlled temperature and relative humidity (Andrieu & Stamatopoulos, 1987; Stamatopoulos, 1986), the heating of dry pasta under uncontrolled conditions of RH (Akiyama & Hayakawa, 1994), the semolina processing with a twin screw extruder (Liu, Qi, & Hayakawa, 1997), the equilibration of semolina extrudates at decreasing RH and low temperature (Liu et al., 1997). Obtaining these data would allow to perfectly describe the effect of temperature on the changes in mechanical properties as a function of pasta moisture content. The ‘‘Food Polymer Science’’ approach to the study of the effects of temperature and water content on food properties is increasingly applied by food scientists and technologists (Seow, Cheah, & Chang, 1999). This approach is used particularly to describe the structureproperty relationships, to control the textural properties and to understand the physicochemical properties changes associated with the concepts of plasticization and glass transition (Levine & Slade, 1986, 1990, 1993). Glass transition is considered to involve a marked decrease in molecular mobility affecting the amorphous polymers. Glass transition is associated with a temperature range (known as Tg ) that divides the physical behavior in two states. At high temperatures and/or high water contents, the amorphous materials are in a soft visco-plastic state and termed ‘‘rubbery’’. A decrease in water content or a decrease in temperature above the Tg range induces glass transition in amorphous materials that then behave as solid elastic materials; this state is described as ‘‘glassy’’. On a molecular scale, glass transition corresponds to a change in energy that allows the components to pass from high amplitude motions involving the main macromolecular chains (i.e., the rubbery state) to local molecular motions of low amplitude (i.e., the glassy state) (Ferry, 1980). Tg of most hydrophilic systems is mainly affected by the water content. The objective of our study was to investigate the changes in mechanical properties of fresh pasta as a function of decreasing water content at different temperatures (at 20, 40, 60, and 80 °C). A specific equipment was built to allow the water content of pasta to stabilize at different levels and to control the ambient conditions around the sample during the measurement of mechanical properties.
2. Materials and methods 2.1. Raw material Commercial durum semolina SSSE grade was obtained from Semoulerie de Bellevue (98–99 production, Marseille, France). Semolina was stored before the experiments at 5 °C in 50 l hermetic plastic cans.
2.2. Pasta manufacturing Semolina (800 g) was used to make spaghetti (dry diameter ¼ 1.5 mm) with an experimental minipress (SERCOM, Montpellier, France). The production room was kept at 20 °C. Semolina was hydrated to 49 g water/ 100 g dry matter with deionized water and then mixed for 20 min at 120 rpm. The temperature of the mixer was maintained at 40 °C by a water cooling system. The product was then fed into the auger and extruded under partial vacuum (150 mm Hg). When extrusion conditions were stable, the spaghetti samples were cut and placed directly in the programmed and controlled temperature and relative humidity oven (SECASI, Pessac, France). 2.3. Water content equilibration The fresh pasta were equilibrated at different moisture contents (from 49 to 5–10% db) by drying. That range of water contents were reached by equilibrating the sample at constant temperature (20, 40, 60 or 80 °C) and successive levels of constant relative humidity (from 90% to 10%) after variable time intervals (of 1–96 h). Water contents were determined from duplicates, according to the NF V03-707 method adapted to pasta (i.e., weight measurements after drying at 110 °C for 10 h, milling, and drying at 130 °C for 2 h). 2.4. Mechanical properties Mechanical properties of pasta were determined as a function of temperature and moisture content by en empirical test and using a modified Rheo TA-XT2 Rheometer (Champlan, France). The measurement device of the rheometer was positioned inside a metallic box directly connected to the SECASI drying apparatus. The temperature and relative humidity controlled air was directly sweeped over the rheometer probe and samples. When the equilibrium moisture content of pasta was reached, five samples of pasta were taken out of the SECASI drier, cut into 8 cm length pieces, and placed in a parallel position (with about 5 mm gap between samples) onto the measurement plate of the rheometer (Fig. 1). Mechanical properties were estimated after 10 min equilibration time (to stabilize the atmospheric conditions around the samples) by using an empirical test. A metallic straight probe (50 mm length) was moved perpendicularly to the plate at constant speed (0.1 mm s1 ). These experimental conditions did not allow determine ‘‘true’’ mechanical properties of pasta. The apparent force (N)–deformation (mm) curves were determined at constant deformation rate (0.1 mm s1 ) until 0.3 mm compression. The mechanical properties of pasta were estimated by the apparent strength (i.e., force at 0.3 mm deformation). Standard
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
53
Air
Probe Air
Pasta
Fig. 1. Experimental system used to estimate the mechanical properties of spaghetti at controlled conditions of temperature and relative humidity.
deviation for apparent strength was evaluated as 6.5% from 4–7 measurements. The viscoelastic properties of pasta were evaluated by an apparent force–relaxation test. After sample deformation, the probe was maintained at 0.3 mm displacement for 30 s. From the relaxation curves, an apparent relaxation coefficient (%) was calculated (Eq. (1)). g0 g30 Apparent relaxation coefficient ¼ ð1Þ g0 where g0 is the force (N) recorded initially (at 0.3 mm displacement) and g30 is the force (N) recorded after 30 s relaxation. Standard deviation for apparent relaxation coefficient was evaluated as 13.4% from 4–7 measurements. 2.5. Mathematical calculations Mathematical equations (Eqs. (2)–(7)) have been used to describe the changes in mechanical properties of pasta as a function of temperature and/or moisture content. The model parameters of these equations were calculated to fit experimental data from a nonlinear optimization procedure (Gauss Newton procedure) using the software Microsoft Excel 98. The minimized objectivefunction was the sum of squared residuals (i.e., deviations between experimental and predicted points).
3. Results 3.1. Mechanical properties of pasta The mechanical properties of spaghetti were characterized by an empirical test during drying. Pasta were firstly characterized by apparent ‘‘force–deformation’’ curves measured under a constant deformation rate. Typical curves for spaghetti at 80 °C with water contents ranging from 27.1% to 9.4% db are presented in Fig. 2.
Force (N)
80
9.4% 60 12.4% 40
20 20.3% 27.1% 0 0
10
20
30
40
Time (sec)
Fig. 2. Influence of pasta water content (% db) on the mechanical properties of spaghetti at 80 °C.
These curves are characterized by initial linear increases in force as a function of time (or deformation) that can be described by linear relationships irrespective of the water content. Even if a true calculation of stress–strain relationships is not possible, we could consider that spaghetti behaves as an elastic material under very small deformations. The decrease in water content of pasta induces a strengthening effect (i.e., shift of ‘‘force–deformation’’ curves to high forces). During the drying, spaghetti are transformed in a more rigid material. Apparent strength of spaghetti were measured as a function of water content at different temperatures (20– 80 °C). Fig. 3 shows experimental data obtained at 40 °C. The moist spaghetti are characterized by nearly constant low values of apparent strength (close to 2 N) regardless of the temperature. Moist spaghetti behave as a soft material. During the drying, the decrease in water content (from 44% to 10% db) produces a large increase in apparent strength (from 2 to 75 N). We observed an abrupt transition from low to high values of apparent strength at a critical level of moisture content that depends on the temperature. The critical levels of moisture
54
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
60
80 - 40˚C -
- 40˚C 50
60 40 30
40
20
20
10 0
0 0
10
20
30
40
50
Fig. 3. Experimental values ðÞ and calculated curves for the changes in apparent strength of spaghetti during the drying (at 40 °C). The calculated curves were obtained using the simple model ( ) or the generalized model (––).
content were 30% at 20 °C, 22% at 40 °C, 18% at 60 °C and 14% at 80 °C. The relationships between apparent strength and moisture content are characterized by a sigmoid shape, regardless of the temperature (between 20 and 80 °C). The dried spaghetti are characterized by nearly constant high values of apparent strength (close to 75 N) regardless of the temperature. Dried spaghetti behave as a rigid material. The visco-elastic properties of spaghetti were evaluated by relaxation tests. Fig. 2 shows typical relaxation curves measured at 80 °C and different water contents. Relaxation curves are characterized by a rapid decrease in force during short relaxation times, that tends to a constant value. These relaxation curves are usually observed for visco-elastic solids. Apparent relaxation coefficients were calculated from the relaxation curves (Eq. (1)) as a function of moisture content at different temperatures (20–80 °C). The data obtained at 40 °C are presented on Fig. 4 as a typical example. The moist spaghetti (30% moisture content) are characterized by nearly constant high values of apparent relaxation coefficient (close to 50%) regardless of the temperature. Moist spaghetti behave as a visco-plastic material. During the drying, the decrease in water content produces a large decrease in apparent relaxation coefficients. We observed an abrupt transition from high to low values of apparent relaxation coefficients at a critical level of moisture content. The relationships between apparent relaxation coefficient and moisture content are characterized by a sigmoid shape, regardless of the temperature (between 20 and 80 °C). The dried spaghetti are characterized by nearly constant low values of apparent relaxation coefficient (close to 1–3%) regardless of the temperature. Dried spaghetti behave as a solid elastic material.
0
10
20
30
40
50
Fig. 4. Experimental values ðÞ and calculated curves for the changes in apparent relaxation coefficient of spaghetti during the drying (at 40 °C). The calculated curves were obtained using the simple model ( ) or the generalized model (––).
It should be noticed that relatively low values of apparent relaxation coefficients were observed at 20, 40 and 60 °C for the moist pasta at 40–45% db. These low values could be associated to the lack of formation of apparent rigid zone onto the pasta surface during water removal, especially at low temperature or to a partial relaxation behavior of product during the slow deformation tests. 3.2. Simple model The changes in mechanical properties (i.e., apparent strength and relaxation coefficient) as a function of water content at constant temperature can be described by using the logistic model (Eq. (2)) adapted to describe sigmoid curves. The measured apparent relaxation coefficients are modeled without taking into consideration the low values observed for the moist pasta at 20, 40 and 60 °C. Y ðW Þ ¼
YH YL þ YL i 1 þ exp W W A
ð2Þ
where Y ðW Þ is the calculated mechanical property, W is the water content (% db), YH is the calculated maximum value of property, YL is the calculated minimum value of property, Wi is the water content (% db) at the inflection point of the sigmoid curve (as schematized in Fig. 5), and A is an empirical constant (% db) which characterizes the spread of the transition from low to high values of mechanical property. The calculated values of model parameters (YH , YL , Wi , and A) are presented in Table 1. The logistic model fitted well experimental data, regardless the temperature (Table 1). High values of regressions coefficient have been found for apparent strength (R2 ¼ 0:939–0.983) and apparent relaxation coefficient (R2 ¼ 0:957–0.988).
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
5
40 35
4
55
where DW is the transition spread of mechanical properties (% db), Wonset and Wendpoint are the water content at the onset and end-point of the transition, respectively. Increasing temperature (from 20 to 80 °C) lead to a decreased in transition spread (from 18% to 3.5% db).
30 3 25 2 20 1
15 10
0 0
20
40
60
80
100
Fig. 5. Influence of the temperature on the critical water content ð Þ and on the spread ðdÞ of the transition in apparent strength during the drying of spaghetti.
As typical examples, some calculated curves are plotted in Figs. 3 and 4. The model parameters related to the mechanical properties on either side of the transition (YH and YL ) are not greatly affected by changes in temperature (Table 1). On either side of the transition, the mechanical properties of spaghetti do not depend on temperature. The critical water content at the inflection point (Wi ) of the transition in mechanical properties is also affected by changes in temperature (Table 1). The critical water content is reduced when the temperature increases. The transition spread of mechanical properties (i.e., the water content range from the onset to the endpoint of the transition in mechanical properties) was determined by using the intercepts of derivative functions. The transition spread (DW ) was then found to be proportional to the model parameter ‘‘A’’ (Eq. (3)). DW ¼ Wonset Wendpoint ¼ 4A
ð3Þ
3.3. Generalized model To take simultaneously into consideration the effects of temperature and water content on the changes in mechanical properties of pasta, the experimental data were described using a generalized model. The generalized model takes simultaneously into consideration the changes in mechanical properties as function of water content (Eq. (4)) and the effect of temperature on the model parameters (Eq. (5)). Y ðW ; T Þ ¼
YH ðT Þ YL ðT Þ þ YL ðT Þ i ðT Þ 1 þ exp W W AðT Þ
ð4Þ
where Y ðW ; T Þ is the calculated mechanical property as a function of water content and temperature. The parameters of Eq. (4) (YH ðT Þ, YL ðT Þ, Wi ðT Þ and AðT Þ) are related to the temperature using exponential functions (Eq. (5)). yðT Þ ¼ y0 exp ð kT Þ
ð5Þ
where y0 and k are the parameters of the exponential equations. We have supposed that the effect of temperature on the minimum (YL ðT Þ) and maximum (YH ðT Þ) calculated values of mechanical properties are the same. The calculated values of model parameters are presented in Table 2. The generalized model fitted well (R2 ¼ 0:957) the changes in mechanical properties of spaghetti as a function of water content and temperature. For instance, the calculated curves at 40 °C are plotted in Figs. 3 and 4. It should be noted that the calculated curves using the generalized model (Eqs. (4)
Table 1 Model parameters and correlation coefficients calculated using the simple models to fit the experimental changes in apparent strength and relaxation coefficients as a function of moisture content (Eq. (2)) Temperature (°C)
R2
Model parameters YH
Apparent strength 20 40 60 80
73.3 76.8 72.5 76.2
Apparent relaxation coefficient 20 40 60 80
61.2% 50.8% 57.4% 59.7%
N N N N
YL
Wi (% db)
A (% db)
2.70 N 2.49 N 2.29 N 10.2 N
30.7 21.6 17.8 14.0
4.47 3.42 2.80 1.99
0.971 0.980 0.983 0.939
2.89% 2.50% 1.12% 1.87%
26.8 19.4 15.5 12.8
2.21 1.36 1.82 0.87
0.984 0.957 0.988 0.972
56
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
Table 2 Model parameters and correlation coefficients calculated using the generalized models to fit the experimental changes in apparent strength and relaxation coefficients as a function of moisture content (Eq. (4)) and temperature (Eq. (5)) Model coefficients (Eq. (5))
Mechanical properties Apparent strength
Apparent relaxation coefficient
Minimum calculated value of mechanical property (YL ) y0 k (103 °C1 )
4.3 N )1.1
2.09% )0.1
Maximum calculated value of mechanical property (YH ) y0 k (103 °C1 )
70.4 N )1.1
56.5% )0.1
Critical water content at the inflection point (Wi ) y0 (% db) k (102 °C1 )
37.9 1.24
32.7 1.2
Transition spread ðAÞ Y0 (% db) k (103 °C1 )
4.55 7.1
3.13 1.45
Regression coefficient R2
0.957
0.947
and (5)) are relatively close to those that were obtained using the simple models (Eq. (2)) at the different temperatures. The dependence of the generalized model with temperature was estimated by the calculated values of parameter k (Table 2). The minimum and maximum calculated values of mechanical properties (YL and YH ) are associated to very low values of k. These parameters are thus not significantly affected by the temperature (as previously assumed from the results in Table 1). On the other hand, high values of k were calculated for the critical water content at the inflection point (Wi ) and for
the transition spread (A). An increase in temperature induces a significant decrease in the critical water content at the inflection point (Wi ) and transition spread (A) (Fig. 5). It could be noticed in Fig. 6 that the dependence of critical water content (Wi ) with temperature is similar when using the generalized model (Eqs. (4) and (5)) or the simple model (Eq. (2)). On the other hand, the dependence of transition spread (Wi ) with temperature seems to be slightly under-estimated by using generalized model (Fig. 5). The generalized model is thus not fully adapted to describe the effect of temperature on the transition spread.
80 70 60
Apparent strength (N)
50 40 30 20 10 0
0 10
90 20
70 30
Water content (%db)
50 40
30 10
Temperature (˚C)
Fig. 6. Three-dimensional plot of apparent strength for pasta as a function of temperature and moisture content.
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
3.4. Three-dimensional plot The generalized model (Eqs. (4) and (5)) and the optimized model parameters (Table 2) were used to construct a three-dimensional display of mechanical properties, as that shown in Fig. 6 for the apparent strength. The three-dimensional plot enables the description of the combined effects of the temperature and water content on the mechanical properties of pasta, and more particularly around the transition. 3.5. Reciprocal model A close up examination of the three-dimensional plot (presented in Fig. 6) allows to note that the transition of mechanical properties from soft to rigid pasta can also be observed during a temperature decrease, at constant moisture content (Fig. 6). Indeed, a decrease in temperature induces a similar abrupt transition in mechanical properties, from a low mechanical strength to a high mechanical strength, at constant moisture content. This description permits to define critical values of temperature that are associated with the onset-point, the inflection point, and the end-point of the transition expected during the cooling of pasta. Description of the mechanical properties of spaghetti could be thus performed using a reciprocal model that takes into consideration the effect of temperature (Eq. (6)) and then the effect of moisture content on the model parameters (Eq. (7)). YH ðW Þ YL ðW Þ þ YL ðW Þ; Y ðT ; W Þ ¼ i ðW Þ 1 þ exp T T AðW Þ
ð6Þ
57
where Y ðT ; W Þ is the calculated mechanical property as a function of temperature and water content. The parameters of Eq. (4) (YH ðW Þ, YL ðW Þ, Ti ðW Þ and AðW Þ) are related to the water content using exponential functions as those previously used for the generalized model (Eq. (7)). yðW Þ ¼ y0 exp ð kW Þ ð7Þ The calculated values of model parameters are presented in Table 3. The reciprocal models fitted well (R2 ¼ 0:943–0.953) the changes in mechanical properties as a function of temperature and moisture content. The dependence of the reciprocal model with moisture content was estimated by the calculated values of parameter k (Table 3). The minimum and maximum calculated values of mechanical properties (YL and YH ) are not affected by the water content (k ¼ 0). On the other hand, high values of k were calculated for the critical temperature at the inflection point (Ti ) and for the transition spread ðAÞ. An increase in moisture content induces a significant decrease in the critical temperature at the inflection point (Wi ) and transition spread (A).
4. Discussion The use of an empirical test to determine the mechanical properties of spaghetti allowed to directly measure the changes in apparent strength and relaxation coefficient during the drying of fresh pasta, at different temperatures (20–80 °C). However, it should be noted that the tested drying conditions (i.e., equilibration at successive decreasing levels of RH) largely differ from the classical pasta drying conditions in which water
Table 3 Model parameters and correlation coefficients calculated using the reciprocal models to fit the experimental changes in apparent strength and relaxation coefficients as a function of temperature (Eq. (6)) and moisture content (Eq. (7)) Model coefficients (Eq. (7))
Mechanical properties Apparent strength
Apparent relaxation coefficient
Minimum calculated value of mechanical property (YL ) y0 k
4.65 N 0
2.4% 2.7 103 (% db1 )
Maximum calculated value of mechanical property (YH ) y0 k
74 N 0
60.3% 2.7 103 (% db1 )
Critical temperature at the inflection point (Wi ) Y0 k
296 °C 9.02 102 (% db1 )
293 °C 10.1 102 (% db1 )
Transition spread ðAÞ y0 k
31.9 °C 4.46 102 (% db1 )
18.2 °C 5.71 102 (% db1 )
Regression coefficient R2
0.943
0.953
58
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
contents are not equilibrated inside pasta during the drying. In addition, we could note that the classical pasta drying conditions require variable conditions of temperature and relative humidity during the drying (Frances & Ollivier, 1985; Abecassis et al., 1989). A dramatic change in the mechanical behavior of spaghetti (apparent strength and relaxation coefficients) is produced by a decrease of moisture content. Fresh pasta behaves as a visco-plastic and soft product. Dry pasta behaves as an elastic and rigid product. During the drying, an abrupt mechanical transition separates fresh and dry states. Our experimental conditions have been defined in order to study large changes in mechanical properties, from fresh to dry pasta. These conditions appeared to be not enough sensitive to demonstrate the effects of water content or temperature on the mechanical properties on either side of the transition (Table 1). It should be noticed that Stamatopoulos (1986) showed a significant decrease in the Young modulus (from 1.3 to 0.3 MPa) for fresh pasta when the temperature increased from 20 to 70 °C. The abrupt mechanical transition (from fresh to dry pasta) was described by a simple mathematical model that takes into consideration its sigmoid shape. The location of the transition shifted towards lower values when temperature and water content increased. Comparable similar transitions of mechanical properties for pasta have been observed by G€ orlin (1960) and by Stamatopoulos (1986). They have shown a transition from a plastic behavior (fresh pasta) to an elastic behavior (dry pasta). However, Stamatopoulos (1986) postulated that the location (i.e., water content) of the transition was not affected by the temperature (between 20 and 70 °C), even if their data may have demonstrated a slight effect of temperature. The effects of temperature and moisture content on the mechanical properties of pasta could be interpreted using the concepts of plasticization and glass transition. The effect of water content on the critical temperatures (Tonset , Tinflection , and Tendpoint ) at the transition of mechanical properties (apparent strength) of pasta was thus compared with the glass transition temperatures of wheat durum semolina determined by modulated DSC (Cuq & Verniere Icard, 2001) (Fig. 7). The transition of mechanical properties was superimposed on the glass transition of durum wheat semolina. The glass transition temperature determined by DSC is classically located at lower temperatures than the transition temperature determined from mechanical properties (Cocero & Kokini, 1991; Kalichevski, Blanshard, & Tobarczuk, 1993). The moist pasta are in a rubbery state above the glass transition. The changes in mechanical properties of pasta during the drying are relatively low until the transition where an abrupt increase in mechanical strength is observed. Below the Tg , the dry pasta is in a glassy state. The transition of the mechanical properties
Temperature (˚C) 125
100
75
50
Tonset Tinflection
25
Tendpoint 0 0
10
20
30
40
50
Water content (g / 100 g dm) Fig. 7. Comparison between the critical temperatures (Tonset , Tinflection , and Tendpoint ) at the transition in mechanical properties of pasta during drying, and the glass transition temperature Tg ðrÞ for wheat durum semolina measured by DSC (from Cuq & Verniere Icard, 2001).
of pasta may be thus explained by the glass transition of the hydrophilic amorphous components of wheat semolina. The model used to describe the effect of water content and temperature on mechanical properties of pasta (Eqs. (2), (4)–(7)) was derived from a model proposed by Peleg (1993, 1994a, 1994b) to characterize the variations of mechanical properties of wheat products around the glass transition. For instance, the glass transition of wheat grain at ambient temperature was characterized by its location (19% water) and spread of transition (4% water) (Peleg, 1993). These values are relatively close to the location (27–31% water) and the spread (2.2–4.5% water) determined in the present study for the transition in mechanical properties of pasta at 20 °C (Table 1). On the other hand, the effect of water content on the location of the glass transition was characterized by Peleg (1993, 1994b) for wheat gluten (k ¼ 0:127) and starch (k ¼ 0:11). These values are in a same order of magnitude than those found in the present study for the transition in pasta mechanical properties (Table 3). The effect of water content on the spread of the glass transition was characterized for the wheat gliadins (k ¼ 0:06). These values are relatively close to those (k ¼ 0:045–0.057) found in the present study for the transition in pasta mechanical properties (Table 3). The characteristics of the sigmoid curve proposed to describe the transition of mechanical properties of pasta during drying are thus consistent with bibliographic data on the glass transition behavior of wheat products. Glass transition is generally considered to involve a marked decrease in molecular mobility affecting the amorphous polymers (Ferry, 1980). The description of mechanical properties of pasta through the concept of
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
59
2
1.5
1
0.5
0 0
5
10
15
20
Fig. 8. Comparison between the master-curve constructed from the Cole–Cole simulation and the ‘‘time–temperature–water content’’ superposition curve constructed from the ‘‘force–relaxation’’ data.
glass transition lets implies the assumption that the molecular mobility of the main components in pasta mainly depends on the plasticization rate. The molecular mobility is enhanced by an increase in temperature (i.e., thermal plasticization) or water content (i.e., water plasticization). Our results seemed to be in agreement with this theory on molecular mobility as shown the calculated master-curve derived from a Cole–Cole equation (Tschoegl, 1989). The parameters of the Cole– Cole equation (Eq. (8)) were empirically adapted to an apparent master-curve from preliminary calculations. Sglassy Srubbery logðstrengthÞ ¼ log þ Srubbery ð8Þ 1 þ ðt=t0 Þc where t is the time (s), t0 is the time at the inflection point (s), Sglassy is the apparent strength at the glassy state (N), Srubbery is the apparent strength at the rubbery state (N). We have carried out an apparent ‘‘time–temperature– water content’’ superposition. All of the experimental force–time curves were plotted on log–log scales. The force–time curves (determined irrespective of the temperature and the pasta water content) could be superimposed (one behind the other) on the apparent master-curve by using a shifting factor whose value depends on temperature and water content (Fig. 8). However, the available data did not allow us to built a continuous apparent master-curve. The fact of being able to nearly adjust the whole experimental force–relaxation data on the apparent master-curve (regardless of the temperature and of pasta water content) indicates that the mechanical properties of pasta are controlled by a phenomenon based on molecular mobility, whose in-
tensity depends on the plasticization rate. An increase in temperature or in water content could thus induce a similar effect on the molecular mobility components and on the mechanical properties of pasta.
References Abecassis, J., Chaurand, M., Matencio, F., & Feillet, P. (1989). Einfluss des Wassergehaltes der Teigwaren bei der Hochtemperaturtrocknung. Getreide Mehl und Brot, 43, 58–62. Akiyama, T., & Hayakawa, K. I. (1994). Tensile fracture stress of a pasta product at temperatures from 293 to 343 K. Lebensmittel Wissenschaft und Technology, 27, 93–94. Andrieu, J., & Stamatopoulos, A. (1987). Durum wheat pasta drying kinetics. Lebensmittel Wissenschaft und Technology, 19, 448–456. Cocero, A. M., & Kokini, J. L. (1991). The study of the glass transition of glutenin using small amplitude oscillatory rheological measurements and differential scanning calorimetry. Journal of Rheology, 35, 257–270. Cunin, C., Handschin, S., Conde-Petit, B., & Escher, F. (1995). Development of microstructure of pasta during processing and cooking. In E. Windhab & B. Wolf (Eds.) Food rheology and structure. Proceedings of 1st symposium on food rheology and structure, Zurich. Cuq, B., & Verniere Icard, C. (2001). Characterization of glass transition of durum wheat semolina using modulated differential scanning calorimetry. Journal of Cereal Science, 33, 213–221. Dexter, J. E., Matsuo, R. R., & Morgan, B. C. (1981). High temperature drying effect on spaghetti properties. International Journal of Food Science, 46, 1741–1746. Favier, J., Samson, M. F., Aubled, C., Morel, M. H., & Abecassis, J. (1996). Suivi de la cinetique thermique dÕinsolubilisation des proteines de la p^ate alimentaire par chromatographie dÕexclusiondiffusion haute performance. Sciences des Aliments, 16, 573–591. Feillet, P., Ait-Mouh, O., Kobrehel, K., & Autran, J. C. (1989). The role of low molecular weight glutenins in the determination of
60
B. Cuq et al. / Journal of Food Engineering 59 (2003) 51–60
cooking quality of pasta products: an overview. Cereal Chemistry, 66, 26–30. Ferry, J. D. (1980). Viscoelastic properties of polymers. New York, NY: John Wiley and Sons. Frances, Y., & Ollivier, J. L. (1985). LÕutilisation des tres hautes temperatures pour le sechage des p^ates alimentaires. Industries Agricoles et Alimentaires, 5, 471–475. G€ orlin, P. (1960). Verh€ utung von schwindungsrissen bei der makkaronitrocknung. Getreide and Mehl, 4, 39–43. Kalichevski, M. T., Blanshard, J. M., & Tobarczuk, P. F. (1993). Effect of water content and sugars on the glass transition of casein and sodium caseinate. International Journal of Food Science and Technology, 28, 139–151. Levine, L., & Slade, H. (1986). A polymer physico-chemical approach to the study of commercial starch hydrolysis products. Carbohydrate Polymers, 6, 213–244. Levine, L., & Slade, H. (1990). Influence of the glassy and rubbery state on the thermal, mechanical, and structural properties of doughs and baked products. In H. Faridi & J. M. Faubion (Eds.), Dough rheology and baked product texture (pp. 157–330). Reinhold, NY: van Nostrand. Levine, L., & Slade, H. (1993). The glassy state phenomenon in food molecules. In J. M. V. Blanshard & P. J. Lillford (Eds.), The glassy state in foods (pp. 35–102). Nottingham: Nottingham University Press.
Liu, H., Qi, J., & Hayakawa, K. (1997). Rheological properties including tensile fracture stress of semolina extrudates influenced by moisture content. Journal of Food Science, 62, 813– 820. Manser, J. (1980). Sechage des p^ates alimentaires a tres haute temperature. Diagramme B€uhler, 69, 11–12. Pavan, G. (1980). High temperature drying improves pasta quality. Food Engineering International, 2, 37–39. Peleg, M. (1993). Mapping the stiffness–temperature–moisture relationship of solid biomaterials at and around their glass transition. Rheological Acta, 32, 575–580. Peleg, M. (1994a). A model of mechanical changes in biomaterials at and around their glass transition. Biotechnology Progress, 10, 385– 388. Peleg, M. (1994b). Mathematical characterization and graphical presentation of the stiffness–temperature–moisture relationship of gliadin. Biotechnology Progress, 10, 652–654. Seow, C. C., Cheah, P. B., & Chang, Y. P. (1999). Antiplasticization by water in reduced-moisture food systems. Journal of Food Science, 64, 576–581. Stamatopoulos, A. (1986) Contribution a lÕetude theorique et experimentale du sechage des p^ates alimentaires. Ph.D. thesis, University of Montpellier. Tschoegl, N. W. (1989). The phenomenological theory of linear viscoelastic behavior. New York, NY: Springer-Verlag.