- Email: [email protected]
Study of cooking quality of spaghetti dried through microwaves and comparison with hot air dried pasta
Journal of Food Engineering 95 (2009) 453–459
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Study of cooking quality of spaghetti dried through microwaves and comparison with hot air dried pasta Teresa De Pilli *, Roma Giuliani, Antonio Derossi, Carla Severini Department of Food Science, University of Foggia, Via Napoli 25, 71100 Foggia, Italy
a r t i c l e
i n f o
Article history: Received 11 November 2008 Received in revised form 28 May 2009 Accepted 2 June 2009 Available online 6 June 2009 Keywords: Cooking quality Drying Hot air Microwave Spaghetti
a b s t r a c t The aim of this work is to evaluate and compare the effects of microwave and conventional drying (hot air) on the quality characteristics of cooked pasta. Experiments were carried out on pasta type spaghetti. A huge difference was noticed between times necessary to dry samples by hot air and by microwaves, in fact in the first case, the drying time was on the average 204.5 min vs. 61.7 s of microwave treatment (average). The gelatinization degree of samples dried by hot air was faster than that of those dried by microwaves: the medium values of kinetic constant of gelatinization of samples dried by hot air and microwaves were 7.5 and 5.2, respectively. Similar total organic matter values suggest that the cooking quality of samples differently dried was comparable. Moreover, samples dried by microwaves were thicker than pasta dried by hot air (37.8 vs. 27.4). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heating of foods exposed to a microwave field is practically instantaneous, contrasting with the conventional methods of heating whereby heat is transported from the surface to the center 10– 20 times more slowly. Even though the industrial applications of microwaves, mainly in the food industry, have not had the same development like the microwave domestic applications (Schiffmann, 1992), one of the most well succeeded industrial operations was the microwave assisted drying of pasta. Several advantages came out for the first time after the joint work of a pasta processor in Canada (Lipton) and an equipment manufacturer in USA (Cryodry) that was engaged with the task of adapting a microwave stage to an existing dryer operating with a conventional process. Such an experiment is described in a paper by Maurer et al. (1971), where they enumerated a few factors to be considered when evaluating the economy of the microwave process as compared with the conventional one: advantages for the product reflected by the pasta re-hydration or cooking time reduced to half; advantages for the process that is shorter production time, thus reducing the area occupied by 90%, in spite of doubling the final capacity; higher temperatures allowed by the process granting a sharper pasteurization effect on the final product; operational costs appreciably reduced (26% less) in relation to the conventional drying. An industrial drying system for short cut pasta, adapted to be assisted by microwaves (915 MHz), usually consists of three * Corresponding author. Tel.: +39 (0) 881589245; fax: +39 (0) 881581308. E-mail address: [email protected] (T. De Pilli). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.06.005
stages: a hot air pre-drying at 82 °C for 35 min, where the moisture content in pasta drops from 30% to 18% (w.b.); microwaves and hot air (82 °C) for 12 min, where the moisture content reduces from 18% to 13.5–12% (w.b.); and a final equalizing stage without any heat or air movement. The total microwave process takes 1.5 h compared to the 8–12 h conventional process (Schiffmann, 2001). Smith (1979) studied the influence of applying microwaves during drying process, concluding that the combined dielectric plus forced convection heating, introduced after the pasta critical moisture content had been reached, exhibited a synergistic effect over the drying process, that is a higher drying speed as compared to the forced convection drying and microwave drying considered separately. This was possible because the pressure gradient inside the food due to the dielectric heating favors the transport of moisture to the surface, from where it is removed by the hot air. In such a way, the microwaves actuate in the limiting mechanism of the conventional drying the moisture diffusion keeping the rate of drying from being slowed down, thus reducing the overall drying time (Smith, 1979; Svenson, 1987; Datta, 1990; Funebo and Ohlsson, 1998). Berteli and Marsaioli (2005) evaluated the efficiency of air drying of short pasta with the assistance of microwave energy, from 23% to 12% (w.b.) product moisture (without the hot air pre-drying), using at first an adapted microwave domestic oven with a temperature controlled hot air provision system and then transferring the experimental parameters from bench scale to the operation of a continuous pilot scale microwave assisted hot air rotary dryer, at the frequency of 2.45 GHz. The results showed the high efficiency of this treatment, not only as regards the shorter drying time, but also because a final product without fissures was
454
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
obtained. The pilot scale microwave assisted hot air rotary dryer has also shown good results, as the dried product reached a final moisture value around 13.5% (w.b.), a value near the desired moisture range of 12–13% (w.b.), spending only 18–19 min, that means a considerable time saving compared with 6.5 h conventional process. In addition, a remarkable space saving was obtained using this equipment as it required only 10% space compared to the conventional continuous hot air dryer. Many papers studied the rheological characteristics of pasta dried by microwaves, particularly the modification of starch grain and the processing parameters (Berteli and Marsaioli, 2005; Bilbao-Sáinz et al., 2007). Cocci et al. (2008) investigated the cooking kinetics of spaghetti cooked using a traditional method in boiling water and an innovative method by microwave. There are scanty researches about the evaluation of cooking quality of pasta dried by microwaves. The present work is aimed at evaluating the effects of processing variables (power and treatment time) of microwave drying on the cooking quality of pasta type spaghetti. Moreover, a comparison of cooking quality of pasta dried by microwaves (innovative system) and by hot air (conventional system), was evaluated.
2. Materials and methods 2.1. Raw materials Dough: Durum wheat semolina (Divella S.p.A., Rutigliano, Bari, Italy), purchased in the local market and running tap water were used. The chemical characteristics of semolina are the following: moisture % (13.6 ± 0.1), ash % (0.78 ± 0.1), protein % (12.0 ± 0.1); while, a pH of 7.69 ± 0.1, hardness (°F) of 25.1 ± 1.5, total dissolved solids dried at 180 °C mg/L of 645 ± 38.5 and chloride content mg/L of 54.6 ± 0.4 were the values of tap water. 2.2. Samples preparation Dough was manually prepared with semolina and water (2:1 w/ w); the last ingredient was slowly added in order to favor a better imbibition of semolina and obtain an elastic and machinable dough. The dough was extruded using a domestic single screw extruder Imperia A-500 (Torino, Italy). The screw speed was 60 rpm. At the exit of the die (provided with four spherical holes each with a diameter of 1.7 mm), pasta was manually cut with a kitchen knife to obtain spaghetti with a length of about 18 cm. Samples were dried using a climatic room Binder mod. KBF 240 (Tuttlingen, Germany) according to the following processing parameters: – – – –
temperature range 55.9–84.1 °C ± 1; relative humidity of drying air 40%; air flow in the dryer 100 m3/h; drying time range 240–400 min.
A Samsung microwave oven mod. CE 116KT (Samsung Electronic Italia, S.p.A., Cernusco sul Naviglio – Milano, Italy) with a maximum power of 900 W was used to dry samples by microwave. In the hot air dryer, the samples were placed on a wooden support like industrial canes, while in the microwave oven spaghetti were placed on glassy supports. Almost three repetitions of dough were carried out for each condition. 2.3. Moisture % The moisture content was determined according to the AACC methods (2003).
2.4. Experimental design Two factorial designs, relating to conventional (A) and microwave drying (B), at two variables and five levels obtained by Central Composite Design (CCD) are reported in Table 1 (Box et al., 1978). They were used to evaluate both the single influence of each processing variable and their possible interactions on quality cooking of pasta. Drying temperature (factorial design relating to conventional drying), power percentage of microwave oven (factorial design relating to microwave drying) and moisture content of pasta were the considered independent variables. Eleven trials with different combinations of the process variable values for each factorial design were obtained using the following equation:
ntot ¼ n0 þ nc þ n ¼ 4 þ 4 þ 3 ¼ 11 where n0 = 2n (n is the number of variables), nc is the number of central point and n* is the number of star point. The 11 combinations obtained for each Central Composite Design are reported in Table 2. As regard to microwave drying, in the same table the microwave power expressed as amount of irradiation absorbed by pasta per time unit, i.e. Watt irradiation times (s)/sample amount (g) is reported. The values of moisture pasta were determined through drying curves obtained by taking samples at regular intervals. The values of drying temperature and power percentages were chosen to simulate a drying treatment at low, medium and high temperatures, while the values of pasta moisture were chosen through preliminary tests. During drying process, the samples were taken at definite times, which corresponded at the values of pasta moisture reported in the two factorial plans. These drying times were extrapolated from drying curves and experimentally verified. Tables 3 and 4 show the values of percentage error between treatment times of samples dried by hot air and microwaves, extrapolated from drying curves, and those of the experimental ones. The maximum percentage error was less than 10%, so it is reasonable to consider the drying curve obtained for both the types of treatments reliable. All experiments were performed in triplicate. 2.5. Analyses 2.5.1. Cooking method 100 g of Spaghetti (1.7 mm thickness, 18 cm length) was cooked in 1 L of boiling tap water without salt. All cooking tests were carried out at room temperature and replicated two times. 2.5.2. Optimum cooking time determination Determination of cooking time was performed by means of the Braibanti technique (Dalbon, 1983); this widely used technique (Riva et al., 1991; Edwards et al., 1993) defines the optimal cooking time as the time required to observe the disappearance of the white uncooked core in a small pasta sample manually squeezed between two thin plexiglass plates. 2.5.3. Starch gelatinization kinetic In order to determine the starch gelatinization kinetic, 100 g of spaghetti were cooked, as previously reported; 4 g of sample were taken every minute until complete cooking time (6 min), cooled and analyzed. For the determination of gelatinization degree the chemical method proposed by Wootton and Chaudhry (1980) and modified by Dalla Rosa et al. (1989) was used. The degree of gelatinization was expressed as the ratio between gelatinized starch and total starch in cooked pasta. It was calculated from the colorimetric
455
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459 Table 1 Central Composite Designs of pasta samples dried by hot air (A) or microwaves (B). Levels
Moisture of dried pasta (%)
Drying temperature (°C)*
Power (%)**
+2 +1 0 1 2
14.8 14 12 10 9.2
84.1 80 70 60 55.9
88.3 [794.5 W] 80 [720 W] 60 [540 W] 40 [360 W] 31.7 [285.5 W]
* **
Central Composite Design A: moisture of dried pasta and drying temperature (hot air drying system). Central Composite Design B: moisture of dried pasta and power % (microwaves drying system).
Table 2 Factorial plans of pasta samples dried by hot air (A) or microwaves (B). Samples
Moisture of dried pasta (%)
Drying temperatures (°C)*
Power (%)**
Absorbed Irradiation (Watt seconds/g)1
1 2 3 4 5 6 7 8 9 10 11
10 14 10 14 12 12 9.2 14.8 12 12 12
60 60 80 80 55.9 84.1 70 70 70 70 70
40 [360 W] 40 [360 W] 80 [720 W] 80 [720 W] 31.7 [285.5 W] 88.3 [794.5 W] 60 [540 W] 60 [540 W] 60 [540 W] 60 [540 W] 60 [540 W]
1314 1188 1908 1584 1056 1827 1566 1296 1350 1350 1350
* ** 1
Factorial plan A: moisture of dried pasta and drying temperature (hot air drying system). Factorial plan B: moisture of dried pasta and power % (microwaves drying system). Watt * irradiation times (s)/sample amount (g)
Table 3 Validation of treatment time relating to samples dried by hot air. Drying temperature (°C)
Pasta moisture (%)
Theoretical drying time (min)
Experimental drying time (min)
Error (%)
55.9 60 60 70 70 70 80 80 84.1
12.0 ± 0.1 14.0 ± 0.2 10.0 ± 1 14.8 ± 0.8 12.0 ± 0.7 9.2 ± 0.4 14.0 ± 0.2 10.0 ± 0.7 12.0 ± 0.6
335 247 382 131 208 278 104 160 135
330 ± 2 240 ± 4 360 ± 17 140 ± 14 200 ± 30 300 ± 10 110 ± 20 170 ± 7 132 ± 6
1.5 2.9 5.9 6.6 3.9 0.3 6.5 6.1 2.2
Table 4 Validation of treatment time relating to samples dried by microwaves. Power (%)
Pasta moisture (%)
Theoretical drying time (s)
Experimental drying time (s)
Error (%)
30 40 40 60 60 60 80 80 90
12.0 ± 0.2 14.0 ± 0.1 10.0 ± 0.7 14.8 ± 0.2 12.0 ± 0.1 9.2 ± 0.6 14.0 ± 0.2 10.0 ± 0.1 12.0 ± 0.3
95 70 84 46 54 64 37 47 35
100 ± 2 70 ± 3 90 ± 4 42 ± 2 54 ± 1 68 ± 4 37 ± 1 47 ± 1 38 ± 2
5.1 0 4.6 9.1 0 6.1 0 0 8.4
measurement of starch–iodine complex formed in aqueous suspension of sample before and after complete starch gelatinization. Each cooked pasta sample was divided into two subsamples (A and B) 2 g of each. Sample A was homogenized with deionised water (100 ml) for 10 min using a laboratory blender (Waring Commercial, Torrington, CT, USA) and eventually centrifuged at 3600 rpm (ALC mod. 4239R, Milano, Italy). Liquid phase of 1 ml was diluted with 10 ml of distilled water and treated with iodine solution (0.1 ml) (prepared with 4 g potassium iodide and 1 g iodine in 100 ml of distilled water). Sample B was prepared as sample A
and subsequently thermally treated at 135 °C (in autoclave) for 1 h in order to reach complete gelatinization (AACC, 1995). Absorbance at 600 nm was measured with a Beckman spectrophotometer mod. DU 640 (Beckman Instruments, California, USA). The degree of gelatinization (% gel) was calculated from the ratio:
% Gel ¼ AA =AB 100 where AA and AB are the absorbances of the iodine complexes prepared from the aqueous suspension before and after thermal gelatinization.
456
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
2.5.4. Total organic matter (TOM) Total organic matter (TOM) is the amount of organic matter released from the cooked pasta during exhaustive rinsing. The method is based on washing the drained pasta with 500 ml of water at room temperature to remove the substance coating the surface of pasta cooked. An aliquot of the washing water is evaporated at 80 °C. The organic matter in the residue is determined by titration with ferrous ammonium sulfate in an excess of potassium dichromate. The chemical method is from D’Egidio et al. (1976, 1990). The results are expressed as grams of starch obtained from 100 g of pasta, which are as follows:
TOM ðg ¼ 100 g pastaÞ ¼ ½ðB SÞ 20=B Fd 0:00347 where B is ferrous ammonium sulfate used for the blank (ml); S is ferrous ammonium sulfate used for the sample (ml); Fd is the dilution factor; and 0.00347 is the factor calculated for the transformation of glucose into starch, correcting for the incomplete oxidation of starch (97.25%). 2.5.5. Textural characteristics Textural determinations were carried out by a TA.HDi 500 Texture Analyser (Stable Micro Systems, Surrey, UK) equipped with 50 N load cell. Measurements were carried out on a cooked sample dipped in cool water soon after cooking in order to stop the cooking process. For each sample, two strands of spaghetti were placed on a rectangular support (11 9 1 cm) and broke through a probe in the shape of blunt wedge (the dimension of flat was 3 30 mm). Analyses were carried out at room temperature and the probe was moved with a constant speed of 0.3 mm/s. In order to evaluate the firmness, the probe penetrated the thirteen percentage of spaghetti diameter and after that it came back to the starting position. Afterwards, the probe was penetrated in the sample until breaking point. The firmness results were expressed as ratio between the maximum cutting force (N) required to totally cut the spaghetti and the maximum cutting force (N) required to cut the thirteen percentage of spaghetti diameter. 2.5.6. Statistical analysis Data were submitted to statistical analysis using Statsoft 6.0 (Tulsa, OK, USA) software. The analysis was carried out in two steps. The first step involved a stepwise regression to identify the relevant variables; the second step used a multiple regression (Standard Least Square Fitting) to fit a second order mathematical model, according to the following polynomial equation:
Y ¼ B0 þ
X
B i vi þ
X
Bii v2i þ
X
mined considering the angular coefficient of regression curves relating to straight-line of drying curves. Test T (for independent variables) was used to compare the treatment times, the kinetic constants of gelatinization, the total organic matter, and the firmness of samples dried by conventional and microwave drying. 3. Results and discussion Fig. 1 shows treatment times of samples dried by hot air as a function of drying temperature and pasta moisture. The values of treatment time were included between 90 and 360 min. The drying temperature was the variable that had a big and negative effect on the treatment time. In particular the shortest treatment times were obtained at the highest values of drying temperature. As expected, in order to obtain samples more dried, longest processing times were required, nevertheless, the pasta moisture had a non-linear effect on processing time. Fig. 2 shows treatment times of samples dried by microwaves as a function of power percentages and pasta moisture. The values of treatment time ranged among 22 and 104 s. The power percentage was the variable that had a big and negative effect on the treatment time. In particular the shortest treatment times were obtained at the highest values of power percentages. As well as pasta dried by hot air, the samples most dried required a long treatment time and the pasta moisture had a non-linear effect on processing time. From the comparison of processing times used for dried samples by hot air or microwaves a huge difference emerged: in fact time necessary to dry samples by hot air was 204.5 min vs. 61.7 s of microwave treatment (Table 5). The optimal cooking times, determined by observation of disappearance of the white uncooked core in pasta samples, were 10 min for samples dried by hot air and 15 min for those dried by microwaves. The increase of cooking time of pasta dried by microwaves could be attributed to the delay of starch swelling and, subsequently, a longer time for gelatinization was required. During pasta cooking, water migrates from outside to inner parts causing starch gelatinization and protein denaturation. The extension of cooking time is related to the increase of water absorption time from pasta. It should be noticed that when spaghetti are dried they behave as a rigid, elastic material (Cheng et al., 2005). With the water migration and structural changes such as starch gelatini-
Bij vi vj
where y is the dependent variable (gelatinization degree, total organic matter (TOM), and firmness), B0 is a constant value, vi and vj are the independent variables (drying temperature, power percentage of microwave oven, and pasta moisture) in coded values and Bi, Bii and Bij are the model regression coefficients. This model allowed the effects of the linear (vi), quadratic (vi2) and combined (vivj) terms of the independent variables to be assessed on the dependent variable. The fitness of mathematical model to the experimental data was evaluated by means of correlation coefficient (r) and p-level. Iso-response surfaces were developed to describe both individual and interactive effects of the independent variables of drying processing on gelatinization degree, total organic matter (TOM), and firmness. Standard deviations, variation coefficient and kinetic constants were calculated by Excel for Office XP (Microsoft Corporation) software. The kinetic constants of starch gelatinization were deter-
Fig. 1. Treatment time as a function of drying temperature and pasta moisture of samples dried by hot air.
457
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
Fig. 3. Effect of drying temperature and moisture of pasta on the elasticity of samples dried by hot air. Fig. 2. Treatment time as a function of power percentages and pasta moisture of samples dried by microwaves.
zation and protein denaturation they behave like a viscoelastic material (Sozer and Dalgic, 2007). Fig. 3 shows the effect of drying temperature and pasta moisture on firmness values of samples dried by hot air. It can be observed that firmness of cooked spaghetti increases at the highest values of drying temperature and the lowest values of pasta moisture (i.e. long treatment times). An opposite behaviour was obtained at the highest values of drying temperature and pasta moisture. Several advantages of high temperature drying (HTD), such as better firmness, less cooking loss, reduced stickiness, increased resistance to cracking, and shorter drying time, have been reported in literature. Wyland (1981) showed that the increase of drying temperature (from 40 to 80 °C) improved spaghetti firmness values, and decreased cooking loss and cooked weight values. Braibanti (1980) suggested that during HTD, the gluten was partially coagulated: this gluten structure retained the starch longer during cooking, leading to less starch loss (Fang and Khan, 1996). Cubadda (1987) hypothesized that there is a physical competition between the swelling of starch and the interaction of denatured proteins. If the starch swells faster than interaction of proteins, the gluten network will be weak and the cooked pasta will become sticky and less elastic. In samples of pasta with a high value of moisture content and submitted to short drying process, the interaction of denatured proteins was scanty, so during cooking process the swelling and gelatinization of starch prevailed against formation and denaturation of gluten that caused a reduced firmness of pasta samples. Samples dried by microwaves showed small changes in the firmness values (Fig. 4). In particular, the only variable that had a significant and positive effect was power percentage. Altan and Mascan (2005) observed from light microscope that
Fig. 4. Effect of power percentages and moisture of pasta on the elasticity of samples dried by microwaves.
when microwave was the only drying system, the structure of starch or gluten changed interestingly: it is possible to assume that these changes occur already at low power percentages. In support of this hypothesis are results of T test which show a significant difference between firmness of samples dried by hot air or microwave (Table 5). In particular samples dried by microwaves showed a greater firmness.
Table 5 T test of independent variables of samples dried by hot air (HA) and microwave (M). Independent variable
Average HA
Average M
T Value
Treatment time HA (min) vs. treatment time M (s) Firmness HA vs. firmness M Kinetic constant gelatinization HA vs. kinetic constant gelatinization M TOM HA vs. TOM M
204.5 ± 85.7 27.4 ± 9 7.5 ± 0.5 1.9 ± 0.2
61.7 ± 23.0 37.8 ± 2.1 5.2 ± 0.7 1.7 ± 0.4
5.3** 3.7* 19.3** 1.2a
a
Not significant differences level <95%. Significant differences at the 95% level. ** Significant differences at the 99% level. *
458
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
To better understand the gelatinization degree of samples dried by two systems, gelatinization curves were built and kinetic constants were calculated. Comparing the medium values of gelatinization kinetic constants resulted that samples dried by hot air gelatinized faster than microwaved ones (Table 5). These data are in agreement with the results of Bilbao-Sáinz et al. (2007). They observed a high gelatinization enthalpy for starch samples treated with microwaves which indicated that more energy was required for the gelatinization of these starch granules compared to the energy required for the gelatinization of conductive heated samples. These differences could be related to fast heating rates. Two possible explanations exist on increasing of gelatinization enthalpy that occurs when starch samples are rapidly heated: the starch granules are annealing when exposed to such high temperatures (Blanshard, 1987) or they are extensively hydrated when the heating occurs quickly. If the crystalline regions are rising in size, an increase in the energy to disorder the system is required. Overall, the more ordered nature of starch granules post annealing restricts water penetration and consequently gelatinization temperatures are elevated (Tester et al., 2001). Donovan (1979), Evans and Haisman (1982) have suggested that the enthalpy associated with gelatinization is the result of multiple thermal processes occurring within the same time frame. In essence, the gelatinization endotherm represents the difference between the endothermic energy associated with the transition (melting of the crystallites and granule swelling) and the exothermic energy associated with the transition (hydration of starch molecules). It seems likely that the extent of hydration of starch granules is greater when processing at rapid heating rates, since water diffusivity is higher at elevated temperature values. This being the case the starch solutions would exhibit a greater endothermic response because of a more complete hydration; the exothermic portion of the gelatinization process would be smaller, allowing more of the endothermic events to be recorded. In Figs. 5 and 6 pasta moisture and drying temperature or power percentage on gelatinization kinetic constants are shown. The operating variables that had the greatest effect on gelatinization speed were the drying temperature and power percentage. In particular, gelatinization speed increased for values of drying temperatures and power percentages until 70 °C or 70%, respectively, while decreased for higher values of these variables. Drying condition that used the highest values of drying temperature and power percentages could favor the formation of starch–lipid complexes (Altan and Mascan (2005)) that reduce the amylose, which reacts with iodine in order to form starch–iodine blue complexes.
Fig. 5. Effect of drying temperature and moisture of pasta on the kinetic constant of gelatinization of samples dried by hot air.
Fig. 6. Effect of power percentages and moisture of pasta on the kinetic constant of gelatinization of samples dried by microwaves.
The cooking quality was evaluated through determination of total organic matter (TOM). In Fig. 7 the TOM values are reported as a function of drying temperature and pasta moisture. The highest values of this parameter and, consequently, the poor cooking quality of pasta, were obtained at the lowest drying temperature (50 °C) and the highest value of pasta moisture, i.e. short treatment time. These data are in agreement with the results of Ibrahim (1982): an increase in temperature from 60 to 80 °C resulted in a progressive decrease in cooking loss and a progressive increase in cooked spaghetti firmness. This also agreed with the findings of Grant (1989) that high temperature drying (HTD) (72 °C) decreased spaghetti cooking loss. The reduction of cooking loss may be due to the partially coagulated gluten structure formed during HTD, leading to less starch loss (Braibanti, 1980). So, the lowest temperature and the short treatment times caused a lack of positive effect of gluten denaturation favoring the release of organic matter during cooking. For samples dried through microwaves, the increase of power percentages led to a considerable reduction of TOM values (Fig. 8). The comparison of TOM average values of samples dried
Fig. 7. Effect of drying temperature and moisture of pasta on the total organic matter (TOM) of samples dried by hot air.
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
Fig. 8. Effect of power percentages and moisture of pasta on the total organic matter (TOM) of samples dried by microwaves.
by hot air and microwaves did not show a significant difference (Table 5). 4. Conclusion The treatments carried out at intermediate power percentages resulted in the better processing conditions in microwave drying system since high values of firmness and starch gelatinization kinetic were obtained. Moreover, the increase of power percentages in microwave drying did not cause a considerable decrease of TOM values in cooked spaghetti. This suggests that it is possible to obtain a good product without using high power percentages that could cause an irregular distribution of water inside the product with a consequent formation of very dry or wet zones and some tensions inside the product. Pasta dried by microwaves had a higher firmness and showed a gelatinization degree less than that dried by hot air. This comported an improvement of cooking resistance of pasta and an increase of cooking time. The TOM values of samples dried by different systems were comparable. References Altan, A., Mascan, M., 2005. Microwave assisted drying of short-cut (ditalini) macaroni: drying characteristics and effect of drying processes on starch properties. Food Research International 38, 787–796. American Association of Cereal Chemists, 1995. Approved Methods of the AACC, ninth ed. Method 76-11, The Association, St. Paul, MN. American Association of Cereal Chemists, 2003. Approved Methods of the AACC, 10th ed. Method 44-01, The Association, St. Paul, MN. Berteli, M.N., Marsaioli, A.J., 2005. Evaluation of short cut pasta air dehydration assisted by microwaves as compared to the conventional drying process. Journal of Food Engineering 68, 175–183. Bilbao-Sáinz, C., Butler, M., Weaver, T., Bent, J., 2007. Wheat starch gelatinization under microwave irradiation and conduction heating. Carbohydrate Polymers 69, 224–232.
459
Blanshard, J.M.V., 1987. Starch granule structure and function: a physicochemical approach. In: Galliard, T. (Ed.), Starch: Properties and Potential. John Wiley and Sons for Society of Chemical Industry, Chishester. pp. 16–54. Box, G.E.P., Hunter, W.G., Hunter, J.S., 1978. Statistics for Experimenters. An Introduction to Design, Data Analysis and Model Building. Jon Wiley and Sons, New York. Braibanti & C.S.p.A., 1980. New developments in pasta drying technology. Journal of Macaroni 61 (12), 48. Cheng, Y., Shimizu, N., Kimura, T., 2005. The viscoelastic properties of soybean curd (tofu) as affected by soymilk concentration and type of coagulant. International Journal Food Science Technology 40, 385–390. Cocci, E., Sacchetti, G., Vallicelli, M., Angioloni, A., Dalla Rosa, M., 2008. Spaghetti cooking by microwave oven: cooking kinetics and product quality. Journal of Food Engineering 85, 537–546. Cubadda, R., 1987. Quality aspect of durum wheat and pasta: interaction with production technology. Tecnica Molitoria 38, 1–6. Dalbon, G., 1983. Fattori che influiscono sulle caratteristiche di cottura delle paste alimentari e possibilità di migliorare la qualità con opportune tecnologie. Tecnica Molitoria 34, 553–563. Dalla Rosa, M., Lerici, C.R., Cencic, L., Pinnavaia, G., 1989. Sul grado di gelatinizzazione dell’amido in alimenti diversi. Tecnica Molitoria 40, 692–699. Datta, A.K., 1990. Heat and mass transfer in the microwave processing of food. Chemical Engineering Progress 86, 47–53. D’Egidio, M.G., Sgrulletta, D., Mariani, G., Galterio, G., De Stefanis, E., Fortini, S., 1976. Metodo per la misura della collosità e della qualità nelle paste alimentari. Tecnica Molitoria 27, 89–93. D’Egidio, M.G., Mariani, B.M., Nardi, S., Novaro, P., Cubadda, R., 1990. Chemical and technological variables and their relationships: a predictive equation for pasta cooking quality. Cereal Chemistry 67, 275–281. Donovan, J.W., 1979. Phase transitions of the starch–water system. Biopolymers 18, 263–275. Edwards, N.M., Izydorczyk, J.E., Dexter, J.E., Biliaderis, C.G., 1993. Cooked pasta texture: comparison of dynamic viscoelastic properties to instrumental assessment of firmness. Cereal Chemistry 70, 122–126. Evans, I.D., Haisman, D.R., 1982. The effect of solutes on the gelatinisation temperature-range of potato starch. Starch 34, 224–231. Fang, K., Khan, K., 1996. Pasta containing regrinds: effect of high temperature drying on product quality. Cereal Chemistry 73 (3), 317–322. Funebo, T., Ohlsson, T., 1998. Microwave assisted air dehydration of apple and mushroom. Journal of Food Engineering 38, 353–367. Grant, L.A., 1989. The effects of various pasta conditions and manipulated starch damage levels on pasta stickiness. Ph.D. dissertation, Department of Cereal Science, North Dakota State University, Fargo. Ibrahim, R.H.R., 1982. High temperature drying of pasta: effect on quality and biochemical components. Ph.D. dissertation, North Dakota State University, Fargo. Maurer, R.L., Tremblay, M.R., Chadwick, E.A., 1971. Microwave of pasta: improves product, reduces cost and production time. Food Technology 25, 32–37. Riva, M., Piazza, L., Schiraldi, A., 1991. Starch gelatinization in pasta cooking: differential flux calorimetry investigation. Cereal Chemistry 68, 622–627. Schiffmann, R.F., 1992. Microwave processing in the U.S. food industry. Food Technology 56, 50–52. Schiffmann, R.F., 2001. Microwave processes for the food industry. In: Datta, A.K., Anantheswaran, R.C. (Eds.), Handbook of Microwave Technology for Food Applications. Marcel Dekker, New York. pp. 299–337. Smith, F.J., 1979. Microwave–hot air drying of pasta, onions and bacon. Microwave Energy Applications Newsletter 12 (6), 6–12. Sozer, N., Dalgic, A.C., 2007. Modelling of rheological characteristics of various spaghetti types. European Food Research Technology 225, 183–190. Svenson, G., 1987. Microwave systems save time, energy. Prepared Foods 156, 86– 90. Tester, R.F., Debon, S.J.J., Qi, X., Sommerville, M.D., Yousuf, R., Yusuph, M., 2001. Amylopectin crystallization in starch. In: Barsby, T.L., Donald, A.M., Frazier, P.J. (Eds.), Starch: Advances in Structure and Function. The Royal Society of Chemistry, Cambridge. Wootton, M., Chaudhry, M.A., 1980. Gelatinization and in vitro digestibility of starch in baked products. Journal of Food Science 45, 1783–1784. Wyland, A.R., 1981. Influence of drying temperature and farina blending on spaghetti quality. M.S. thesis, Department of Cereal Science, North Dakota State University, Fargo.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Study of cooking quality of spaghetti dried through microwaves and comparison with hot air dried pasta Teresa De Pilli *, Roma Giuliani, Antonio Derossi, Carla Severini Department of Food Science, University of Foggia, Via Napoli 25, 71100 Foggia, Italy
a r t i c l e
i n f o
Article history: Received 11 November 2008 Received in revised form 28 May 2009 Accepted 2 June 2009 Available online 6 June 2009 Keywords: Cooking quality Drying Hot air Microwave Spaghetti
a b s t r a c t The aim of this work is to evaluate and compare the effects of microwave and conventional drying (hot air) on the quality characteristics of cooked pasta. Experiments were carried out on pasta type spaghetti. A huge difference was noticed between times necessary to dry samples by hot air and by microwaves, in fact in the first case, the drying time was on the average 204.5 min vs. 61.7 s of microwave treatment (average). The gelatinization degree of samples dried by hot air was faster than that of those dried by microwaves: the medium values of kinetic constant of gelatinization of samples dried by hot air and microwaves were 7.5 and 5.2, respectively. Similar total organic matter values suggest that the cooking quality of samples differently dried was comparable. Moreover, samples dried by microwaves were thicker than pasta dried by hot air (37.8 vs. 27.4). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heating of foods exposed to a microwave field is practically instantaneous, contrasting with the conventional methods of heating whereby heat is transported from the surface to the center 10– 20 times more slowly. Even though the industrial applications of microwaves, mainly in the food industry, have not had the same development like the microwave domestic applications (Schiffmann, 1992), one of the most well succeeded industrial operations was the microwave assisted drying of pasta. Several advantages came out for the first time after the joint work of a pasta processor in Canada (Lipton) and an equipment manufacturer in USA (Cryodry) that was engaged with the task of adapting a microwave stage to an existing dryer operating with a conventional process. Such an experiment is described in a paper by Maurer et al. (1971), where they enumerated a few factors to be considered when evaluating the economy of the microwave process as compared with the conventional one: advantages for the product reflected by the pasta re-hydration or cooking time reduced to half; advantages for the process that is shorter production time, thus reducing the area occupied by 90%, in spite of doubling the final capacity; higher temperatures allowed by the process granting a sharper pasteurization effect on the final product; operational costs appreciably reduced (26% less) in relation to the conventional drying. An industrial drying system for short cut pasta, adapted to be assisted by microwaves (915 MHz), usually consists of three * Corresponding author. Tel.: +39 (0) 881589245; fax: +39 (0) 881581308. E-mail address: [email protected] (T. De Pilli). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.06.005
stages: a hot air pre-drying at 82 °C for 35 min, where the moisture content in pasta drops from 30% to 18% (w.b.); microwaves and hot air (82 °C) for 12 min, where the moisture content reduces from 18% to 13.5–12% (w.b.); and a final equalizing stage without any heat or air movement. The total microwave process takes 1.5 h compared to the 8–12 h conventional process (Schiffmann, 2001). Smith (1979) studied the influence of applying microwaves during drying process, concluding that the combined dielectric plus forced convection heating, introduced after the pasta critical moisture content had been reached, exhibited a synergistic effect over the drying process, that is a higher drying speed as compared to the forced convection drying and microwave drying considered separately. This was possible because the pressure gradient inside the food due to the dielectric heating favors the transport of moisture to the surface, from where it is removed by the hot air. In such a way, the microwaves actuate in the limiting mechanism of the conventional drying the moisture diffusion keeping the rate of drying from being slowed down, thus reducing the overall drying time (Smith, 1979; Svenson, 1987; Datta, 1990; Funebo and Ohlsson, 1998). Berteli and Marsaioli (2005) evaluated the efficiency of air drying of short pasta with the assistance of microwave energy, from 23% to 12% (w.b.) product moisture (without the hot air pre-drying), using at first an adapted microwave domestic oven with a temperature controlled hot air provision system and then transferring the experimental parameters from bench scale to the operation of a continuous pilot scale microwave assisted hot air rotary dryer, at the frequency of 2.45 GHz. The results showed the high efficiency of this treatment, not only as regards the shorter drying time, but also because a final product without fissures was
454
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
obtained. The pilot scale microwave assisted hot air rotary dryer has also shown good results, as the dried product reached a final moisture value around 13.5% (w.b.), a value near the desired moisture range of 12–13% (w.b.), spending only 18–19 min, that means a considerable time saving compared with 6.5 h conventional process. In addition, a remarkable space saving was obtained using this equipment as it required only 10% space compared to the conventional continuous hot air dryer. Many papers studied the rheological characteristics of pasta dried by microwaves, particularly the modification of starch grain and the processing parameters (Berteli and Marsaioli, 2005; Bilbao-Sáinz et al., 2007). Cocci et al. (2008) investigated the cooking kinetics of spaghetti cooked using a traditional method in boiling water and an innovative method by microwave. There are scanty researches about the evaluation of cooking quality of pasta dried by microwaves. The present work is aimed at evaluating the effects of processing variables (power and treatment time) of microwave drying on the cooking quality of pasta type spaghetti. Moreover, a comparison of cooking quality of pasta dried by microwaves (innovative system) and by hot air (conventional system), was evaluated.
2. Materials and methods 2.1. Raw materials Dough: Durum wheat semolina (Divella S.p.A., Rutigliano, Bari, Italy), purchased in the local market and running tap water were used. The chemical characteristics of semolina are the following: moisture % (13.6 ± 0.1), ash % (0.78 ± 0.1), protein % (12.0 ± 0.1); while, a pH of 7.69 ± 0.1, hardness (°F) of 25.1 ± 1.5, total dissolved solids dried at 180 °C mg/L of 645 ± 38.5 and chloride content mg/L of 54.6 ± 0.4 were the values of tap water. 2.2. Samples preparation Dough was manually prepared with semolina and water (2:1 w/ w); the last ingredient was slowly added in order to favor a better imbibition of semolina and obtain an elastic and machinable dough. The dough was extruded using a domestic single screw extruder Imperia A-500 (Torino, Italy). The screw speed was 60 rpm. At the exit of the die (provided with four spherical holes each with a diameter of 1.7 mm), pasta was manually cut with a kitchen knife to obtain spaghetti with a length of about 18 cm. Samples were dried using a climatic room Binder mod. KBF 240 (Tuttlingen, Germany) according to the following processing parameters: – – – –
temperature range 55.9–84.1 °C ± 1; relative humidity of drying air 40%; air flow in the dryer 100 m3/h; drying time range 240–400 min.
A Samsung microwave oven mod. CE 116KT (Samsung Electronic Italia, S.p.A., Cernusco sul Naviglio – Milano, Italy) with a maximum power of 900 W was used to dry samples by microwave. In the hot air dryer, the samples were placed on a wooden support like industrial canes, while in the microwave oven spaghetti were placed on glassy supports. Almost three repetitions of dough were carried out for each condition. 2.3. Moisture % The moisture content was determined according to the AACC methods (2003).
2.4. Experimental design Two factorial designs, relating to conventional (A) and microwave drying (B), at two variables and five levels obtained by Central Composite Design (CCD) are reported in Table 1 (Box et al., 1978). They were used to evaluate both the single influence of each processing variable and their possible interactions on quality cooking of pasta. Drying temperature (factorial design relating to conventional drying), power percentage of microwave oven (factorial design relating to microwave drying) and moisture content of pasta were the considered independent variables. Eleven trials with different combinations of the process variable values for each factorial design were obtained using the following equation:
ntot ¼ n0 þ nc þ n ¼ 4 þ 4 þ 3 ¼ 11 where n0 = 2n (n is the number of variables), nc is the number of central point and n* is the number of star point. The 11 combinations obtained for each Central Composite Design are reported in Table 2. As regard to microwave drying, in the same table the microwave power expressed as amount of irradiation absorbed by pasta per time unit, i.e. Watt irradiation times (s)/sample amount (g) is reported. The values of moisture pasta were determined through drying curves obtained by taking samples at regular intervals. The values of drying temperature and power percentages were chosen to simulate a drying treatment at low, medium and high temperatures, while the values of pasta moisture were chosen through preliminary tests. During drying process, the samples were taken at definite times, which corresponded at the values of pasta moisture reported in the two factorial plans. These drying times were extrapolated from drying curves and experimentally verified. Tables 3 and 4 show the values of percentage error between treatment times of samples dried by hot air and microwaves, extrapolated from drying curves, and those of the experimental ones. The maximum percentage error was less than 10%, so it is reasonable to consider the drying curve obtained for both the types of treatments reliable. All experiments were performed in triplicate. 2.5. Analyses 2.5.1. Cooking method 100 g of Spaghetti (1.7 mm thickness, 18 cm length) was cooked in 1 L of boiling tap water without salt. All cooking tests were carried out at room temperature and replicated two times. 2.5.2. Optimum cooking time determination Determination of cooking time was performed by means of the Braibanti technique (Dalbon, 1983); this widely used technique (Riva et al., 1991; Edwards et al., 1993) defines the optimal cooking time as the time required to observe the disappearance of the white uncooked core in a small pasta sample manually squeezed between two thin plexiglass plates. 2.5.3. Starch gelatinization kinetic In order to determine the starch gelatinization kinetic, 100 g of spaghetti were cooked, as previously reported; 4 g of sample were taken every minute until complete cooking time (6 min), cooled and analyzed. For the determination of gelatinization degree the chemical method proposed by Wootton and Chaudhry (1980) and modified by Dalla Rosa et al. (1989) was used. The degree of gelatinization was expressed as the ratio between gelatinized starch and total starch in cooked pasta. It was calculated from the colorimetric
455
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459 Table 1 Central Composite Designs of pasta samples dried by hot air (A) or microwaves (B). Levels
Moisture of dried pasta (%)
Drying temperature (°C)*
Power (%)**
+2 +1 0 1 2
14.8 14 12 10 9.2
84.1 80 70 60 55.9
88.3 [794.5 W] 80 [720 W] 60 [540 W] 40 [360 W] 31.7 [285.5 W]
* **
Central Composite Design A: moisture of dried pasta and drying temperature (hot air drying system). Central Composite Design B: moisture of dried pasta and power % (microwaves drying system).
Table 2 Factorial plans of pasta samples dried by hot air (A) or microwaves (B). Samples
Moisture of dried pasta (%)
Drying temperatures (°C)*
Power (%)**
Absorbed Irradiation (Watt seconds/g)1
1 2 3 4 5 6 7 8 9 10 11
10 14 10 14 12 12 9.2 14.8 12 12 12
60 60 80 80 55.9 84.1 70 70 70 70 70
40 [360 W] 40 [360 W] 80 [720 W] 80 [720 W] 31.7 [285.5 W] 88.3 [794.5 W] 60 [540 W] 60 [540 W] 60 [540 W] 60 [540 W] 60 [540 W]
1314 1188 1908 1584 1056 1827 1566 1296 1350 1350 1350
* ** 1
Factorial plan A: moisture of dried pasta and drying temperature (hot air drying system). Factorial plan B: moisture of dried pasta and power % (microwaves drying system). Watt * irradiation times (s)/sample amount (g)
Table 3 Validation of treatment time relating to samples dried by hot air. Drying temperature (°C)
Pasta moisture (%)
Theoretical drying time (min)
Experimental drying time (min)
Error (%)
55.9 60 60 70 70 70 80 80 84.1
12.0 ± 0.1 14.0 ± 0.2 10.0 ± 1 14.8 ± 0.8 12.0 ± 0.7 9.2 ± 0.4 14.0 ± 0.2 10.0 ± 0.7 12.0 ± 0.6
335 247 382 131 208 278 104 160 135
330 ± 2 240 ± 4 360 ± 17 140 ± 14 200 ± 30 300 ± 10 110 ± 20 170 ± 7 132 ± 6
1.5 2.9 5.9 6.6 3.9 0.3 6.5 6.1 2.2
Table 4 Validation of treatment time relating to samples dried by microwaves. Power (%)
Pasta moisture (%)
Theoretical drying time (s)
Experimental drying time (s)
Error (%)
30 40 40 60 60 60 80 80 90
12.0 ± 0.2 14.0 ± 0.1 10.0 ± 0.7 14.8 ± 0.2 12.0 ± 0.1 9.2 ± 0.6 14.0 ± 0.2 10.0 ± 0.1 12.0 ± 0.3
95 70 84 46 54 64 37 47 35
100 ± 2 70 ± 3 90 ± 4 42 ± 2 54 ± 1 68 ± 4 37 ± 1 47 ± 1 38 ± 2
5.1 0 4.6 9.1 0 6.1 0 0 8.4
measurement of starch–iodine complex formed in aqueous suspension of sample before and after complete starch gelatinization. Each cooked pasta sample was divided into two subsamples (A and B) 2 g of each. Sample A was homogenized with deionised water (100 ml) for 10 min using a laboratory blender (Waring Commercial, Torrington, CT, USA) and eventually centrifuged at 3600 rpm (ALC mod. 4239R, Milano, Italy). Liquid phase of 1 ml was diluted with 10 ml of distilled water and treated with iodine solution (0.1 ml) (prepared with 4 g potassium iodide and 1 g iodine in 100 ml of distilled water). Sample B was prepared as sample A
and subsequently thermally treated at 135 °C (in autoclave) for 1 h in order to reach complete gelatinization (AACC, 1995). Absorbance at 600 nm was measured with a Beckman spectrophotometer mod. DU 640 (Beckman Instruments, California, USA). The degree of gelatinization (% gel) was calculated from the ratio:
% Gel ¼ AA =AB 100 where AA and AB are the absorbances of the iodine complexes prepared from the aqueous suspension before and after thermal gelatinization.
456
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
2.5.4. Total organic matter (TOM) Total organic matter (TOM) is the amount of organic matter released from the cooked pasta during exhaustive rinsing. The method is based on washing the drained pasta with 500 ml of water at room temperature to remove the substance coating the surface of pasta cooked. An aliquot of the washing water is evaporated at 80 °C. The organic matter in the residue is determined by titration with ferrous ammonium sulfate in an excess of potassium dichromate. The chemical method is from D’Egidio et al. (1976, 1990). The results are expressed as grams of starch obtained from 100 g of pasta, which are as follows:
TOM ðg ¼ 100 g pastaÞ ¼ ½ðB SÞ 20=B Fd 0:00347 where B is ferrous ammonium sulfate used for the blank (ml); S is ferrous ammonium sulfate used for the sample (ml); Fd is the dilution factor; and 0.00347 is the factor calculated for the transformation of glucose into starch, correcting for the incomplete oxidation of starch (97.25%). 2.5.5. Textural characteristics Textural determinations were carried out by a TA.HDi 500 Texture Analyser (Stable Micro Systems, Surrey, UK) equipped with 50 N load cell. Measurements were carried out on a cooked sample dipped in cool water soon after cooking in order to stop the cooking process. For each sample, two strands of spaghetti were placed on a rectangular support (11 9 1 cm) and broke through a probe in the shape of blunt wedge (the dimension of flat was 3 30 mm). Analyses were carried out at room temperature and the probe was moved with a constant speed of 0.3 mm/s. In order to evaluate the firmness, the probe penetrated the thirteen percentage of spaghetti diameter and after that it came back to the starting position. Afterwards, the probe was penetrated in the sample until breaking point. The firmness results were expressed as ratio between the maximum cutting force (N) required to totally cut the spaghetti and the maximum cutting force (N) required to cut the thirteen percentage of spaghetti diameter. 2.5.6. Statistical analysis Data were submitted to statistical analysis using Statsoft 6.0 (Tulsa, OK, USA) software. The analysis was carried out in two steps. The first step involved a stepwise regression to identify the relevant variables; the second step used a multiple regression (Standard Least Square Fitting) to fit a second order mathematical model, according to the following polynomial equation:
Y ¼ B0 þ
X
B i vi þ
X
Bii v2i þ
X
mined considering the angular coefficient of regression curves relating to straight-line of drying curves. Test T (for independent variables) was used to compare the treatment times, the kinetic constants of gelatinization, the total organic matter, and the firmness of samples dried by conventional and microwave drying. 3. Results and discussion Fig. 1 shows treatment times of samples dried by hot air as a function of drying temperature and pasta moisture. The values of treatment time were included between 90 and 360 min. The drying temperature was the variable that had a big and negative effect on the treatment time. In particular the shortest treatment times were obtained at the highest values of drying temperature. As expected, in order to obtain samples more dried, longest processing times were required, nevertheless, the pasta moisture had a non-linear effect on processing time. Fig. 2 shows treatment times of samples dried by microwaves as a function of power percentages and pasta moisture. The values of treatment time ranged among 22 and 104 s. The power percentage was the variable that had a big and negative effect on the treatment time. In particular the shortest treatment times were obtained at the highest values of power percentages. As well as pasta dried by hot air, the samples most dried required a long treatment time and the pasta moisture had a non-linear effect on processing time. From the comparison of processing times used for dried samples by hot air or microwaves a huge difference emerged: in fact time necessary to dry samples by hot air was 204.5 min vs. 61.7 s of microwave treatment (Table 5). The optimal cooking times, determined by observation of disappearance of the white uncooked core in pasta samples, were 10 min for samples dried by hot air and 15 min for those dried by microwaves. The increase of cooking time of pasta dried by microwaves could be attributed to the delay of starch swelling and, subsequently, a longer time for gelatinization was required. During pasta cooking, water migrates from outside to inner parts causing starch gelatinization and protein denaturation. The extension of cooking time is related to the increase of water absorption time from pasta. It should be noticed that when spaghetti are dried they behave as a rigid, elastic material (Cheng et al., 2005). With the water migration and structural changes such as starch gelatini-
Bij vi vj
where y is the dependent variable (gelatinization degree, total organic matter (TOM), and firmness), B0 is a constant value, vi and vj are the independent variables (drying temperature, power percentage of microwave oven, and pasta moisture) in coded values and Bi, Bii and Bij are the model regression coefficients. This model allowed the effects of the linear (vi), quadratic (vi2) and combined (vivj) terms of the independent variables to be assessed on the dependent variable. The fitness of mathematical model to the experimental data was evaluated by means of correlation coefficient (r) and p-level. Iso-response surfaces were developed to describe both individual and interactive effects of the independent variables of drying processing on gelatinization degree, total organic matter (TOM), and firmness. Standard deviations, variation coefficient and kinetic constants were calculated by Excel for Office XP (Microsoft Corporation) software. The kinetic constants of starch gelatinization were deter-
Fig. 1. Treatment time as a function of drying temperature and pasta moisture of samples dried by hot air.
457
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
Fig. 3. Effect of drying temperature and moisture of pasta on the elasticity of samples dried by hot air. Fig. 2. Treatment time as a function of power percentages and pasta moisture of samples dried by microwaves.
zation and protein denaturation they behave like a viscoelastic material (Sozer and Dalgic, 2007). Fig. 3 shows the effect of drying temperature and pasta moisture on firmness values of samples dried by hot air. It can be observed that firmness of cooked spaghetti increases at the highest values of drying temperature and the lowest values of pasta moisture (i.e. long treatment times). An opposite behaviour was obtained at the highest values of drying temperature and pasta moisture. Several advantages of high temperature drying (HTD), such as better firmness, less cooking loss, reduced stickiness, increased resistance to cracking, and shorter drying time, have been reported in literature. Wyland (1981) showed that the increase of drying temperature (from 40 to 80 °C) improved spaghetti firmness values, and decreased cooking loss and cooked weight values. Braibanti (1980) suggested that during HTD, the gluten was partially coagulated: this gluten structure retained the starch longer during cooking, leading to less starch loss (Fang and Khan, 1996). Cubadda (1987) hypothesized that there is a physical competition between the swelling of starch and the interaction of denatured proteins. If the starch swells faster than interaction of proteins, the gluten network will be weak and the cooked pasta will become sticky and less elastic. In samples of pasta with a high value of moisture content and submitted to short drying process, the interaction of denatured proteins was scanty, so during cooking process the swelling and gelatinization of starch prevailed against formation and denaturation of gluten that caused a reduced firmness of pasta samples. Samples dried by microwaves showed small changes in the firmness values (Fig. 4). In particular, the only variable that had a significant and positive effect was power percentage. Altan and Mascan (2005) observed from light microscope that
Fig. 4. Effect of power percentages and moisture of pasta on the elasticity of samples dried by microwaves.
when microwave was the only drying system, the structure of starch or gluten changed interestingly: it is possible to assume that these changes occur already at low power percentages. In support of this hypothesis are results of T test which show a significant difference between firmness of samples dried by hot air or microwave (Table 5). In particular samples dried by microwaves showed a greater firmness.
Table 5 T test of independent variables of samples dried by hot air (HA) and microwave (M). Independent variable
Average HA
Average M
T Value
Treatment time HA (min) vs. treatment time M (s) Firmness HA vs. firmness M Kinetic constant gelatinization HA vs. kinetic constant gelatinization M TOM HA vs. TOM M
204.5 ± 85.7 27.4 ± 9 7.5 ± 0.5 1.9 ± 0.2
61.7 ± 23.0 37.8 ± 2.1 5.2 ± 0.7 1.7 ± 0.4
5.3** 3.7* 19.3** 1.2a
a
Not significant differences level <95%. Significant differences at the 95% level. ** Significant differences at the 99% level. *
458
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
To better understand the gelatinization degree of samples dried by two systems, gelatinization curves were built and kinetic constants were calculated. Comparing the medium values of gelatinization kinetic constants resulted that samples dried by hot air gelatinized faster than microwaved ones (Table 5). These data are in agreement with the results of Bilbao-Sáinz et al. (2007). They observed a high gelatinization enthalpy for starch samples treated with microwaves which indicated that more energy was required for the gelatinization of these starch granules compared to the energy required for the gelatinization of conductive heated samples. These differences could be related to fast heating rates. Two possible explanations exist on increasing of gelatinization enthalpy that occurs when starch samples are rapidly heated: the starch granules are annealing when exposed to such high temperatures (Blanshard, 1987) or they are extensively hydrated when the heating occurs quickly. If the crystalline regions are rising in size, an increase in the energy to disorder the system is required. Overall, the more ordered nature of starch granules post annealing restricts water penetration and consequently gelatinization temperatures are elevated (Tester et al., 2001). Donovan (1979), Evans and Haisman (1982) have suggested that the enthalpy associated with gelatinization is the result of multiple thermal processes occurring within the same time frame. In essence, the gelatinization endotherm represents the difference between the endothermic energy associated with the transition (melting of the crystallites and granule swelling) and the exothermic energy associated with the transition (hydration of starch molecules). It seems likely that the extent of hydration of starch granules is greater when processing at rapid heating rates, since water diffusivity is higher at elevated temperature values. This being the case the starch solutions would exhibit a greater endothermic response because of a more complete hydration; the exothermic portion of the gelatinization process would be smaller, allowing more of the endothermic events to be recorded. In Figs. 5 and 6 pasta moisture and drying temperature or power percentage on gelatinization kinetic constants are shown. The operating variables that had the greatest effect on gelatinization speed were the drying temperature and power percentage. In particular, gelatinization speed increased for values of drying temperatures and power percentages until 70 °C or 70%, respectively, while decreased for higher values of these variables. Drying condition that used the highest values of drying temperature and power percentages could favor the formation of starch–lipid complexes (Altan and Mascan (2005)) that reduce the amylose, which reacts with iodine in order to form starch–iodine blue complexes.
Fig. 5. Effect of drying temperature and moisture of pasta on the kinetic constant of gelatinization of samples dried by hot air.
Fig. 6. Effect of power percentages and moisture of pasta on the kinetic constant of gelatinization of samples dried by microwaves.
The cooking quality was evaluated through determination of total organic matter (TOM). In Fig. 7 the TOM values are reported as a function of drying temperature and pasta moisture. The highest values of this parameter and, consequently, the poor cooking quality of pasta, were obtained at the lowest drying temperature (50 °C) and the highest value of pasta moisture, i.e. short treatment time. These data are in agreement with the results of Ibrahim (1982): an increase in temperature from 60 to 80 °C resulted in a progressive decrease in cooking loss and a progressive increase in cooked spaghetti firmness. This also agreed with the findings of Grant (1989) that high temperature drying (HTD) (72 °C) decreased spaghetti cooking loss. The reduction of cooking loss may be due to the partially coagulated gluten structure formed during HTD, leading to less starch loss (Braibanti, 1980). So, the lowest temperature and the short treatment times caused a lack of positive effect of gluten denaturation favoring the release of organic matter during cooking. For samples dried through microwaves, the increase of power percentages led to a considerable reduction of TOM values (Fig. 8). The comparison of TOM average values of samples dried
Fig. 7. Effect of drying temperature and moisture of pasta on the total organic matter (TOM) of samples dried by hot air.
T. De Pilli et al. / Journal of Food Engineering 95 (2009) 453–459
Fig. 8. Effect of power percentages and moisture of pasta on the total organic matter (TOM) of samples dried by microwaves.
by hot air and microwaves did not show a significant difference (Table 5). 4. Conclusion The treatments carried out at intermediate power percentages resulted in the better processing conditions in microwave drying system since high values of firmness and starch gelatinization kinetic were obtained. Moreover, the increase of power percentages in microwave drying did not cause a considerable decrease of TOM values in cooked spaghetti. This suggests that it is possible to obtain a good product without using high power percentages that could cause an irregular distribution of water inside the product with a consequent formation of very dry or wet zones and some tensions inside the product. Pasta dried by microwaves had a higher firmness and showed a gelatinization degree less than that dried by hot air. This comported an improvement of cooking resistance of pasta and an increase of cooking time. The TOM values of samples dried by different systems were comparable. References Altan, A., Mascan, M., 2005. Microwave assisted drying of short-cut (ditalini) macaroni: drying characteristics and effect of drying processes on starch properties. Food Research International 38, 787–796. American Association of Cereal Chemists, 1995. Approved Methods of the AACC, ninth ed. Method 76-11, The Association, St. Paul, MN. American Association of Cereal Chemists, 2003. Approved Methods of the AACC, 10th ed. Method 44-01, The Association, St. Paul, MN. Berteli, M.N., Marsaioli, A.J., 2005. Evaluation of short cut pasta air dehydration assisted by microwaves as compared to the conventional drying process. Journal of Food Engineering 68, 175–183. Bilbao-Sáinz, C., Butler, M., Weaver, T., Bent, J., 2007. Wheat starch gelatinization under microwave irradiation and conduction heating. Carbohydrate Polymers 69, 224–232.
459
Blanshard, J.M.V., 1987. Starch granule structure and function: a physicochemical approach. In: Galliard, T. (Ed.), Starch: Properties and Potential. John Wiley and Sons for Society of Chemical Industry, Chishester. pp. 16–54. Box, G.E.P., Hunter, W.G., Hunter, J.S., 1978. Statistics for Experimenters. An Introduction to Design, Data Analysis and Model Building. Jon Wiley and Sons, New York. Braibanti & C.S.p.A., 1980. New developments in pasta drying technology. Journal of Macaroni 61 (12), 48. Cheng, Y., Shimizu, N., Kimura, T., 2005. The viscoelastic properties of soybean curd (tofu) as affected by soymilk concentration and type of coagulant. International Journal Food Science Technology 40, 385–390. Cocci, E., Sacchetti, G., Vallicelli, M., Angioloni, A., Dalla Rosa, M., 2008. Spaghetti cooking by microwave oven: cooking kinetics and product quality. Journal of Food Engineering 85, 537–546. Cubadda, R., 1987. Quality aspect of durum wheat and pasta: interaction with production technology. Tecnica Molitoria 38, 1–6. Dalbon, G., 1983. Fattori che influiscono sulle caratteristiche di cottura delle paste alimentari e possibilità di migliorare la qualità con opportune tecnologie. Tecnica Molitoria 34, 553–563. Dalla Rosa, M., Lerici, C.R., Cencic, L., Pinnavaia, G., 1989. Sul grado di gelatinizzazione dell’amido in alimenti diversi. Tecnica Molitoria 40, 692–699. Datta, A.K., 1990. Heat and mass transfer in the microwave processing of food. Chemical Engineering Progress 86, 47–53. D’Egidio, M.G., Sgrulletta, D., Mariani, G., Galterio, G., De Stefanis, E., Fortini, S., 1976. Metodo per la misura della collosità e della qualità nelle paste alimentari. Tecnica Molitoria 27, 89–93. D’Egidio, M.G., Mariani, B.M., Nardi, S., Novaro, P., Cubadda, R., 1990. Chemical and technological variables and their relationships: a predictive equation for pasta cooking quality. Cereal Chemistry 67, 275–281. Donovan, J.W., 1979. Phase transitions of the starch–water system. Biopolymers 18, 263–275. Edwards, N.M., Izydorczyk, J.E., Dexter, J.E., Biliaderis, C.G., 1993. Cooked pasta texture: comparison of dynamic viscoelastic properties to instrumental assessment of firmness. Cereal Chemistry 70, 122–126. Evans, I.D., Haisman, D.R., 1982. The effect of solutes on the gelatinisation temperature-range of potato starch. Starch 34, 224–231. Fang, K., Khan, K., 1996. Pasta containing regrinds: effect of high temperature drying on product quality. Cereal Chemistry 73 (3), 317–322. Funebo, T., Ohlsson, T., 1998. Microwave assisted air dehydration of apple and mushroom. Journal of Food Engineering 38, 353–367. Grant, L.A., 1989. The effects of various pasta conditions and manipulated starch damage levels on pasta stickiness. Ph.D. dissertation, Department of Cereal Science, North Dakota State University, Fargo. Ibrahim, R.H.R., 1982. High temperature drying of pasta: effect on quality and biochemical components. Ph.D. dissertation, North Dakota State University, Fargo. Maurer, R.L., Tremblay, M.R., Chadwick, E.A., 1971. Microwave of pasta: improves product, reduces cost and production time. Food Technology 25, 32–37. Riva, M., Piazza, L., Schiraldi, A., 1991. Starch gelatinization in pasta cooking: differential flux calorimetry investigation. Cereal Chemistry 68, 622–627. Schiffmann, R.F., 1992. Microwave processing in the U.S. food industry. Food Technology 56, 50–52. Schiffmann, R.F., 2001. Microwave processes for the food industry. In: Datta, A.K., Anantheswaran, R.C. (Eds.), Handbook of Microwave Technology for Food Applications. Marcel Dekker, New York. pp. 299–337. Smith, F.J., 1979. Microwave–hot air drying of pasta, onions and bacon. Microwave Energy Applications Newsletter 12 (6), 6–12. Sozer, N., Dalgic, A.C., 2007. Modelling of rheological characteristics of various spaghetti types. European Food Research Technology 225, 183–190. Svenson, G., 1987. Microwave systems save time, energy. Prepared Foods 156, 86– 90. Tester, R.F., Debon, S.J.J., Qi, X., Sommerville, M.D., Yousuf, R., Yusuph, M., 2001. Amylopectin crystallization in starch. In: Barsby, T.L., Donald, A.M., Frazier, P.J. (Eds.), Starch: Advances in Structure and Function. The Royal Society of Chemistry, Cambridge. Wootton, M., Chaudhry, M.A., 1980. Gelatinization and in vitro digestibility of starch in baked products. Journal of Food Science 45, 1783–1784. Wyland, A.R., 1981. Influence of drying temperature and farina blending on spaghetti quality. M.S. thesis, Department of Cereal Science, North Dakota State University, Fargo.