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Drying kinetics and quality parameters of pumpkin slices dehydrated using different methods
Journal of Food Engineering 94 (2009) 14–20
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Drying kinetics and quality parameters of pumpkin slices dehydrated using different methods Agnieszka Nawirska a,*, Adam Figiel b, Alicja Z. Kucharska a, Anna Sokół-Łe˛towska a, Anita Biesiada c a
Department of Fruit, Vegetables and Grain Technology, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland c Department of Horticulture, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland b
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 6 February 2009 Accepted 19 February 2009 Available online 10 March 2009 Keywords: Pumpkin Drying Shrinkage Density Colour Carotenoids
a b s t r a c t Drying of pumpkin slices was carried out using convective, vacuum-microwave, vacuum and freeze drying methods. The aim of our study was to determine the drying shrinkage and bulk density, as well as to measure the colour and carotenoid content for the slices of 12 pumpkin cultivars. Another major objective was to establish the moisture content, specific density and cutting strength of the slices before dried, as well as the kinetics of their dehydration by the convective and vacuum-microwave methods. The application of the vacuum-microwave method has approximately tenfold shortened the time of pumpkin slice drying as compared to the convective method. In the hot air stream, the slices of the species Cucurbita pepo were dehydrated faster than those of the species Cucurbita maxima. When use was made of the vacuum-microwave process, the slices of C. maxima dried at a faster rate. Each of the drying methods used was more effective than the convective method. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In Poland two pumpkin species are grown for fruit crops harvested in the phase of physiological maturity: Cucurbita maxima and Cucurbita pepo. The nutritional value of pumpkin fruits is high but varies from one species or cultivar to another. Thus, in the fresh mass of the fruit, total carotenoid content, a major contributory factor in the high nutritional value of pumpkins, ranges from 2 to 10 mg/100 g, the content of vitamins C and E accounting for 9– 10 mg/100 g and 1.03–1.06 mg/100 g, respectively (Terazowa et al., 2001; Murkovic et al., 2002; Kunachowicz et al., 2005). Pumpkin fruit is also a valuable source of other vitamins, e.g., B6, K, thiamine, and riboflavin, as well as minerals, e.g., potassium, phosphorus, magnesium, iron and selenium (USDA National Nutrient Database, 2004). Pumpkin flesh is a delicious and fully appreciated additive in a diversity of products for children and adults. Pumpkin fruits are being processed to obtain juice, pomace, pickles and dried products. One of the oldest methods for the preservation of food is drying, which consists in removing water from the product in order to provide microbiological safety (Malthlouthi, 2001), and the most popular drying method includes convection. In this method the drying agent supplies heat to the material and removes moisture (in the form of water vapour) from the material at the same time. The * Corresponding author. Tel.: +48 71 32 05 187; fax: +48 71 32 05 477. E-mail address: [email protected] (A. Nawirska). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.02.025
method itself is a low-cost one, but has the disadvantage of entailing a time-consuming process. During contact with oxygen that is present in the air, the product becomes exposed to high temperature for a long time, and such exposure reduces the content of some valuable components which readily undergo oxidation at elevated temperature. Another drawback of the convective method is the concomitant substantial shrinkage (Lozano et al., 1983). In the vacuum method the contact between the material being dried and oxygen is limited. Owing to the reduced pressure, effective drying can be achieved at low temperature. However, shrinkage is comparatively high and the necessity of using a vacuum system raises the cost of the drying process. Drying by sublimation (freeze drying) guarantees an excellent quality of the product. Colour, flavour, chemical composition and shape undergo only slight changes. The method, however, involves very high costs and is time-consuming. It requires pre-freezing and long-lasting storage of the raw material in the drying chamber at reduced pressure, even though the increased temperature of the heating plate accelerates the sublimation process. Vacuum-microwave drying is of a very short duration. In this method, microwaves penetrate into the interior of the product being dried, and cause the water to boil at a comparatively low temperature, owing to the reduced pressure in the drying chamber. It is due to the water boiling inside the material that the relative difference in the pressure between the interior and the environment of the material provides an extremely quick water vapour removal and produces favourable conditions for the occurrence of the
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
puffing phenomenon, which considerably limits the drying shrinkage effect (Sham et al., 2001). Regretfully, the method is rather costly and requires strict control, as at the final stage of drying an abrupt rise in the temperature of the material is likely to occur (Drouzas and Schubert, 1996). The potentiality for achieving a high-quality product of an attractive texture has directed the attention of many investigators to the applicability of the vacuummicrowave method inter alia to the drying of cranberries (Sunjka et al., 2004), strawberries (Krulis et al., 2005), tomatoes (Durance and Wang, 2002), carrots (Lin et al., 1998), bananas (Maskan, 2000) and garlic (Figiel, 2006). Industrial applications of vacuummicrowave drying are becoming increasingly frequent (Attiyate, 1979; http://www.paula.com.pl). Pumpkin slices are generally dried using the convective method (Sojak, 1999), in some instances with osmotic drying as a prior step (Garcia et al., 2007), which is also applied as a pretreatment stage to vacuum drying (ArévaloPinedo and Murr, 2007). The literature also contains references to the drying of pumpkin slices by the microwave method (Alibas, 2007), but as yet no reports are available on vacuum-microwave dehydrated pumpkin slices. VM method, when properly applied, can be used for achieving a high-quality product. Thus the necessity of basic quality parameters determination of VM dehydrated pumpkin samples of different cultivars seems to be important. The aim of our study was to determine the drying shrinkage and bulk density, as well as to measure the colour and carotenoid content for the slices of 12 pumpkin cultivars subjected to convective, vacuum-microwave, vacuum and freeze drying. Another major objective was to establish the moisture content, specific density and cutting strength of the slices before dried, as well as the kinetics for their dehydration by the convective and vacuum-microwave methods.
2. Materials and methods 2.1. Materials In experiment conducted in 2007 there was estimated the effect of variety on quality of pumpkin chips. Slices of pumpkin flash obtained from 12 cultivars belonged to two species C. maxima (1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melo_ nowa Zółta’, and 6 – ‘Uchiki Kiuri’ cvs) and C. pepo (7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs) were tested. Both the species were grown in the Experimental Plant of Piastów belonging to the Department of Horticulture, Wroclaw University of Environmental and Life Sciences. Four weeks old transplants produced in greenhouse were planted on the plots fertilized with nitrogen (200 kg N ha1) in second decade of May and fruits were harvested in half of September. 2.2. Preparation of samples Slices of pumpkin flesh (3.5 mm thick and 18 mm in diameter) were prepared with the aid of a cutter (Gastrotech, Kraków, Poland) and a steel-made blanking tool, which was cylindrical in shape and pointed on one of the sides. A small rubber piston moving in the cylinder interior enabled a simultaneous removal of five samples without damaging them. 2.3. Specific density measurement The specific density of the sample was determined as the ratio of the mass of five randomly chosen slices to their volume. The slices were weighed using Metler Toledo scales (USA) with an accuracy of 0.001 g. The volume of the slices was determined after having
15
formed a column of them. The height and diameter of the column were measured by means of a slide caliper with an accuracy of 0.01 mm. The measurement was conducted in three replications. 2.4. Cutting strength test Prior to the drying process, the pumpkin slices were made subject to a cutting strength test using an Instron 5544 strength testing machine (Instron, High Wycombe, UK) equipped with a tensometric head of a measuring range of 2 kN. The slices (laid horizontally) were cut with the aid of a QTS-25 SB cutting attachment at the speed of 2 mm/s. The average value of the maximal cutting force Ftmax was determined from 10 replications. 2.5. Drying The pumpkin slices were dried by the following four methods: convective (C), vacuum (V), vacuum-microwave (VM) and freeze drying (F). The process was discontinued after the moisture content fell below 0.1 kg/kg db. The kinetics for the C and VM drying processes were determined from the mass loss in samples of a known initial moisture. The samples were weighed in defined time intervals. Taking into account the differences in the initial moisture between particular pumpkin varieties, the variations in the moisture ratio MR occurring at drying time s were established in terms of the formula:
MR ¼
MðsÞ M e M0 Me
ð1Þ
where M(s) denotes moisture content after drying time s; Ve stands for equilibrium moisture content, and M0 is initial moisture. The equilibrium moisture content Me was determined at the final stage of drying as an asymptotic value of the function fit to the experimental points using Table Curve 2D Windows v2.03. Knowing the moisture ratio (MR) values, it is possible to compare the kinetics of drying for raw materials differing in initial moisture. Drying via the convective method (C) was conducted with the aid of a drier designed and constructed by the staff of the Institute of Agricultural Engineering, Wrocław. The drier enabled simultaneous dehydration of six samples placed in baskets at the ends of vertical air pipes. The samples were laid in a sieve and formed a single layer. Air stream velocity and temperature amounted to 1 m/s and 60 °C, respectively. Vacuum drying (V) of the pumpkin samples, which were arranged in a single layer on Petri dishes, was performed in an SPT-200 drier (ZEAMiL Horyzont, Kraków, Poland) at 60 °C. VM drying was carried out in an SM-200 drier (Plazmatronika, Wrocław, Poland). Samples of a 60 g mass were placed in a cylinder rotating at a speed of 6 rev min1. The pressure in the cylinder varied from 4 to 6 kPa. Microwave power amounted to 480 W. In the freeze drying (F) method use was made of an OE-950 drier (Hungary). The temperature and pressure in the closed drying chamber were 60 °C and 65 Pa, respectively. The samples were arranged in a single layer on the heating plate of a temperature of +30 °C, which accelerated the sublimation process. 2.6. Measurement of drying shrinkage and bulk density of dried material Drying shrinkage (S) and bulk density (qb) were determined using a measuring cylinder of 27 mm diameter. The pumpkin samples were placed in the cylinder, which was gently shaken. Measurements were conducted when the samples occupied the smallest possible volume. Drying shrinkage was calculated in terms of the following formula:
16
V0 Vs V0
1
80
where V0 is volume of fresh slices, and Vs is volume of dried slices. Bulk density was the quotient of the mass of the dried sample and its volume determined in the measuring cylinder. 2.7. Total carotenoid content The method consists in the extraction of carotenoids from the sample tested, using 80% acetone, and in their determination in the extract by the colorimetric method at the wavelengths of 470, 646 and 663 nm according to Rumin´ska et al. (1990). Carotenoid content was calculated as follows (the result being given in mg/ 100 g):
Kn ¼
90
ð2Þ
1000A470 3:27C a 104C b 229
ð3Þ
where
Cutting Force F tmax (N)
S¼
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
2
2
70
3 1
60
4
7
6
5
50
6
40
7
11
30
8
4
5 8
9
3
10
20 9
12
10 0 80
82
84
86
88
90
92
94
11 12
10
96
Moisture content M wb (%) Fig. 2. Effect of moisture content (Mwb) on the maximal force (Ftmax) required for the cutting of the pumpkin slices, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – _ ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
C a ¼ 12:71A663 2:81A646 C b ¼ 20:13A646 5:03A663 A470, A646, and A663 being absorbance values at wavelengths 470, 646, and 663 nm, respectively. 2.8. Pumpkin slices colour assessment The colour of the pumpkin slices was measured with ColorQuest XE (HunterLab). Milled slices were placed in a glass cuvette, and the colour was recorded using CIE L*a*b* 10°/D65 colour spaces, where L* indicates lightness, a* is the +a redness, and b* is the +yellowness.
comparison to their moisture (which reached 95%). The cultivars of the species C. maxima were characterized by lower densities (ranging from 0.99 to 1.07 kg/m3) as compared to the cultivars of the species C. pepo (varying between 1.06 and 1.12 kg/m3). It has been observed that the increase in the moisture content (Mwb) of the pumpkin slices is paralleled by the decrease in the force required for their cutting, Ftmax. The lower values of the maximal cutting force (Ftmax) can be attributed to a less compact texture of the pumpkin flesh. 3.2. Drying kinetics
3. Results
The decrease in the reduced moisture of the samples, MR, at time s of drying performed by the convective method (Fig. 3) was described using the semi-empirical Page’s equation:
3.1. Density, moisture content and cutting resistance of the raw material
MR ¼ ek1 s
Fig. 1 includes points that represent the 12 cultivars examined. The horizontal coordinates of the points refer to moisture content, Mwb, and the vertical ones indicate specific density, qs. The relation in Fig. 1 clearly shows that the increase in the moisture of the pumpkin is concomitant with the increase in its specific density. However, the Ambar sample, which had the lowest moisture content (81.4%), displayed a relatively high density (1.005 kg m3); the cultivars Bambino and Karowita exhibited low specific densities in
ð4Þ
Table 1 compiles the values of the drying constant k1. Higher k1 values are indicative of a higher drying rate. It was found that in the majority of instances the pumpkin varieties of the species C. pepo dried at a faster rate than did the varieties of the species C.
1
1
2 3
Specific Density ρ s (g / cm3)
1 1.12
9
8
1.1
11
2 3
10
12
4
1.08
5
5 7
1.06
6 3
1.04 1.02
7 8
4
11
0.98
12 0.96 80
82
84
5 6
0.4
7 8
0.2
9
10
6
1
4 0.6
9
2
1
Moisture ratio MR
0.8 1.14
86
88
90
92
94
96
Moisture Mwb (%) Fig. 1. Specific density (qs) of pumpkin slices vs moisture content (Mwb), 1 – _ ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6– ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
10
0 0
60
120
Time τ (min)
180
240
11 12
Fig. 3. Kinetics of drying by the convective method, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – _ ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
17
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
Table 1 Values of the parameters k1, k2, a and b of the functions describing the kinetics of drying by the convective and vacuum-microwave methods. R2 = coefficient of determination. Cultivar
Convective method
Vacuum-microwave method
k1 s
1 – Amazonka 2 – Ambar 3 – Bambino 4 – Karowita _ 5 – Melonowa Zółta 6 – Uchiki Kuri 7 – Danka 8 – Junona 9 – Miranda 10 – Pyza 11 – Warszawska Makaronowa 12 – Jet F1
MR ¼ b ek2 s
MR ¼ 1 a s
MR ¼ e
2
k1
R
0.033 0.036 0.054 0.049 0.045 0.038 0.056 0.068 0.072 0.069 0.063 0.058
0.9996 0.9996 0.9941 0.9967 0.9962 0.9995 0.9953 0.9977 0.9972 0.9981 0.9976 0.9961
maxima when they were exposed to the hot air stream. Taking into account the physical and mechanical properties of the raw material, it becomes clear that a higher initial moisture content and a less compact texture of the pumpkin slices account for a higher efficiency of drying by the convective method. Drying of pumpkin slices by the VM method (Fig. 4) took an approximately tenfold shorter time as compared to the convective method (Fig. 3). The time of pumpkin slices drying by the convective method approached 240 min and was similar to the time of drying 5 20 40 mm C. maxima slices (Alibas, 2007), as well as C. moschata slices of a 4 mm thickness and a surface area of 20– 25 cm2 (Garcia et al., 2007), at 50 °C, with the aid of the same method. In the latter case, the inclusion of osmotic dehydration as a pretreatment step into the convective method has reduced the drying time by about 1 h. The kinetics of pumpkin slices drying by the VM method has been described with a linear function (Eq. (6)) for the drying time up to the sixth minute of the process, and by an exponential function (semi-empirical Page’s Eq. (7)) for the remaining drying time (Pabis and Jaros, 2002).
MR ¼ 1 a s
ð5Þ
M R ¼ b ek2 s
ð6Þ
1
1
2
a
R
b
k2
R2
0.104 0.107 0.098 0.099 0.096 0.096 0.099 0.089 0.087 0.091 0.101 0.105
0.9995 0.9984 0.9983 0.9981 0.9996 0.9992 0.9997 0.9982 0.9996 0.9984 0.9994 0.9995
2.54 3.59 1.69 1.90 2.26 2.08 2.65 1.50 1.51 1.59 2.30 2.15
0.314 0.380 0.229 0.248 0.274 0.258 0.306 0.188 0.187 0.204 0.289 0.286
0.997 0.9941 0.9907 0.9845 0.9879 0.9847 0.9880 0.9922 0.9928 0.9948 0.9938 0.9916
The values of the parameters a, b and k2 are compiled in Table 1. The higher values of the parameter a are indicative of a higher drying rate in the initial period, while the higher values of k2 indicate a higher drying rate in the subsequent period. When use was made of the VM method, the slices of the C. maxima pumpkin species were drying at a faster rate than the slices of the C. pepo pumpkin species, in contrast to the convection method. In the VM method, the lower initial moisture content, as well as the lower density and a more compact texture of the pumpkin slices, were conducive to the rise in the efficiency of drying. 3.3. Drying shrinkage As can be seen from the bars in Fig. 5, drying shrinkage is generally higher in the slices of the pumpkin species C. pepo than in those of the species C. maxima; the highest resistance to shrinkage being observed in the cultivars Amazonka, Ambar and Uchiki Kuri. The slices dehydrated by the F method developed minimal drying shrinkage irrespective of the pumpkin cultivar. In the case of the species C. maxima, the largest shrinkage was observed in the samples dried by the V method. The samples dehydrated by the VM or by the C method displayed a minor drying shrinkage. An exception was the sample of the pumpkin variety Karowita dried by the VM method, where the slices experienced a greater shrinkage as compared to those dried by the other methods. In the case of the C. pepo species, it has been observed that the values of drying
2
0.6
3
1.00
4
0.90
6 0.4
7 8
0.2
0 0
6
12
Time τ (min)
18
24
0.70 0.60 0.50 0.40 0.30
9
0.20
10
0.10
11 12
Fig. 4. Kinetics of drying by the vacuum-microwave method, 1 – ‘Amazonka’, 2 – _ ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
VM V C F
0.80
5 Shrinkage S
Moisture ratio MR
0.8
0.00 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 5. Shrinkage (S) of slices cut from the pumpkin cultivars examined, dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa _ Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
shrinkage followed a decreasing trend when the methods VM, V, and C were used consecutively. This trend fails to apply to the Miranda cv., whose slices dehydrated by the V method developed a slightly greater shrinkage than did the slices dried by the VM method. The varieties Ambar, Amazonka, and Uchiki Kuri were found to be the most resistant to shrinkage, regardless of the drying method applied. They were characterized by the lowest density and moisture content (Fig. 1), as well as by a high cutting strength (Fig. 2). 3.4. Bulk density of the dried material The highest bulk density was determined in the dried slices of the pumpkin cultivars Ambar, Amazonka, and Uchiki Kuri (Fig. 6), the same that displayed the highest resistance to drying shrinkage (Fig. 5). This finding is attributable to the low moisture of the raw material from which the slices have been obtained (Fig. 1), and consequently to the high dry solids content. The comparatively high bulk density values of the C. pepo slices are to be attributed to the remarkable drying shrinkage rather than the dry solids content, which was low, taking into account the high moisture value. The low bulk density of the slices of the Karowita, and Bambino cultivars resulted from the high moisture of the raw material and from the relatively high drying shrinkage. Pumpkin slices dehydrated using the F method showed visibly the lowest
300 VM V C F
3
Bulk density (kg/m )
250
200
150
100
50
0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar
Carotenoid (mg/100g)
Fig. 6. Bulk density of slices cut from the pumpkin cultivars examined, dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa _ Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
F C V VM
bulk density, which is due to the slight volume change associated with the application of this drying method. 3.5. Carotenoids Fig. 7 includes the values of total carotenoid content for the pumpkin varieties examined. Thus, the overall amount of carotenoids in the C. maxima samples was approximately eight times as high as that in the C. pepo samples. A significant difference was observed in the carotenoid content between the samples of the 12 pumpkin cultivars dehydrated by the four drying methods. The highest average carotenoid content in the dried material from the two pumpkin species was determined with the freeze drying (F), then vacuum-microwave (VM) and the lowest with the convective method (C). Among the varieties arising from the two species, the highest carotenoid content was measured in Karowita (C. maxima) and Danka (C. pepo), and the lowest in Ambar (C. maxima) and Pyza (C. pepo). 3.6. Colour The value of the parameter L*, which indicates the lightness of milled pumpkin slices, varied from 56.91 to 86.11 (Fig. 8). Samples obtained from the species C. maxima were darker (lower value of L*), which seems to be attributable to the higher carotenoid content. The use of the VM method brought about some darkening of the samples, apparently caused by an inadequate choice of the drying parameters, which produced local intensive changing in colour. Alibas (2007), using microwave, air and combined microwaveair drying for the dehydration of C. maxima slices, attained darker dried materials with L* values ranging from 22.80 to 27.91. Regardless of the drying method used, the lightest dried materials were obtained from the Ambar cv. (C. maxima), which, in comparison to the other cultivars examined, contained many carbohydrates (approximately 6%) (Biesiada et al., 2006). Among the varieties C. maxima, carotenoid content was the least in Ambar cv. In C. pepo species there were no cultivar which was the best among all drying methods but Junona cv. samples obtained by methods vacuum (V) and convective (C), were the lightest. To obtain the best colour of dried pumpkin slices, parameters a*, b* and L should gain high values. Dried materials obtained from the species C. maxima displayed remarkably higher redness values (a* = 7.61–22.35) as compared to the species C. pepo (a* = 1.76–12.30), but the a* parameter val-
C F V VM
90 80
Lightness Parameter (L*)
18
70 60 50 40 30 20 10 0
1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 7. Carotenoid content of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 8. Lightness parameter (L*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
Redness Parameter (a*)
of C. pepo which were dried using the VM method. From our investigations it is clear that convective method of drying was the worst.
C F V VM
25
20
19
4. Conclusions
15
10
5
0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 9. Redness parameter (a*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
Table 2 Coefficients of correlation between carotenoid content and colour parameters for pumpkin slices (Cucurbita maxima and Cucurbita pepo) dried using different methods (significance level = 0.05).
L* a* b* Carotene
L*
a*
1.00 0.63 0.06 0.62
1.00 0.67 0.92
b*
Car
The flesh of the pumpkin with increased moisture content was generally characterized by higher specific density and lower cutting strength. We also show that, the application of the vacuummicrowave method has approximately tenfold shortened the time of pumpkin slice drying as compared to the convective method. In the hot air stream, the slices of the species C. pepo were dehydrated faster than those of species C. maxima. However, when use made of the vacuum-microwave process, the slices of C. maxima dried at faster rate. In the majority of instances, the dried slices of the C. pepo species displayed a higher drying shrinkage than did those of the C. maxima species. The highest bulk density was measured in the slices of the cultivars Ambar, Amazonka, and Uchiki Kuri, which belong to the species C. maxima. Considered from the viewpoint of colour and carotene content, each of the drying methods used was more effective than the convective method. When use was made of vacuum-microwave method, the dried products had more attractive colour. Acknowledgment This work was supported by the Polish Ministry of Science and Higher Education under Grant Nr N 31008932/3913.
1.00 0.62
1.00
References
50
C F V VM
Yellowness Parameter (b*)
45 40 35 30 25 20 15 10 5 0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 10. Yellowness parameter (b*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
ues of the C. maxima Ambar cv. were half as high (Fig. 9). Among the varieties of C. pepo, the dried slices of Danka exhibited the highest values of the parameter a*, which is attributable to the high content of carotenoids. In all of the samples the parameter a* was very well correlated with the carotenoid content (R = 0.92) (Table 2). Among the varieties of C. maxima, the values of the yellowness parameter b* ranged from 50.3 in the slices of Amazonka dried by the V method to 29.54 in the slices of Bambino dehydrated by the VM method (Fig. 10). Slightly lower values were measured in the varieties of C. pepo. The highest average values of yellowness were determined in the slices of C. maxima dehydrated by the F method, whereas the lowest ones were obtained for the slice samples
Alibas, I., 2007. Microwave, air and combined microwave-air-drying parameters of pumpkin slices. LWT 40, 1445–1451. Arévalo-Pinedo, A., Murr, F.E.X., 2007. Influence of pre-treatments on the drying kinetics during vacuum drying of carrot and pumpkin. Journal of Food Engineering 80, 152–156. Attiyate, Y., 1979. Microwave vacuum drying: first industrial application. Food Engineering 51 (2), 78–79. Biesiada, A., Kucharska, A., Sokół-Łe˛towska, A., 2006. Plonowanie i wartos´ci _ _ odzywcze wybranych odmian uzytkowych Cucurbita pepo L. oraz Cucurbita maxima Duch.. Folia Horticulturae (Suppl. 1), 66–69. Drouzas, A.E., Schubert, H., 1996. Microwave application in vacuum drying of fruits. Journal of Food Engineering 28, 203–209. Durance, T.D., Wang, J.H., 2002. Energy consumption, density, and rehydration rate of vacuum-microwave and hot-air convection-dehydrated tomatoes. Journal of Food Science 67 (6), 2212–2216. Figiel, A., 2006. Drying kinetics and drying shrinkage of garlic subjected to vacuummicrowave dehydration. Acta Agrophysica 7 (1), 49–58. Garcia, C.C., Mauro, M.A., Kiura, M., 2007. Kinetics of osmotic dehydration and airdrying of pumpkins (Cucurbita moschata). Journal of Food Engineering 82, 284– 291. Krulis, M., Kuhnert, S., Leiker, M., Rohm, H., 2005. Influence of energy input and initial moisture on physical properties of microwave-vacuum dried strawberries. European Food Research Technology 221, 803–808. Kunachowicz, H., Nadolna, I., Przygoda, B., Iwanowicz, K., 2005. Tabele składu i _ _ ´ ci. Wydawnictwo Lekarskie PZWL, Warszawa. wartos´ci odzywczej zywnos Lin, T.M., Durance, T.D., Scaman, C.H., 1998. Characterization of vacuum microwave, air and freeze dried carrot slices. Food Research International 31 (2), 111–117. Lozano, J.E., Rotstein, E., Urbicain, M.J., 1983. Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents. Journal of Food Science 48, 1497– 1502. Malthlouthi, M., 2001. Water content, water activity, water structure and the stability of foodstuffs. Food Control 12, 409–417. Maskan, M., 2000. Microwave/air and microwave finish drying of banana. Journal of Food Engineering 44, 71–78. Murkovic, M., Hillderbrandt, A., Winkler, J., Pfannhauser, W., 2002. Variability of vitamin E in pumpkin seeds (Cucurbita pepo). Zeitschrift für Lebensmitteluntersuchung und -Forschung 4, 275–278. Pabis, S., Jaros, M., 2002. The first period convection drying of vegetables and the effect of shape-dependent shrinkage. Biosystems Engineering 81 (2), 201–211. Rumin´ska, A., Suchorska, K., We˛glarz, Z., 1990. Ros´liny lecznicze i specjalne cz. I Wiadomos´ci ogólne. Wydawnictwo SGGW AR, Warszawa. Sham, P.W.Y., Scaman, C.H., Durance, T.D., 2001. Texture of vacuum microwave dehydrated apple chips as affected by calcium pretreatment, vacuum level, and apple variety. Journal of Food Science 66 (9), 1341–1347.
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_ Sojak, M., 1999. Modeling of the kinetics of pumpkin dehydration. Inzynieria Rolnicza 2 (8), 87–94 (in Polish). Sunjka, P.S., Rennie, T.J., Beaudry, C., Raghavan, G.S.V., 2004. Microwave-convective and microwave-vacuum drying of cranberries: a comparative study. Drying Technology 22 (5), 1217–1231.
Terazowa, Y., Ito, K., Yoshida, K., 2001. Changes in carbohydrate composition in pumpkin (kabocha) during fruit growth. Journal of the Japanese Society Horticultural Science 70, 656–658. USDA National Nutrient Database for Standard Reference, 2004. Nutritional Value of Pumpkin and Winter Squash. Release 17.
Contents lists available at ScienceDirect
Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Drying kinetics and quality parameters of pumpkin slices dehydrated using different methods Agnieszka Nawirska a,*, Adam Figiel b, Alicja Z. Kucharska a, Anna Sokół-Łe˛towska a, Anita Biesiada c a
Department of Fruit, Vegetables and Grain Technology, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland c Department of Horticulture, Wrocław University of Environmental and Life Sciences, ul. C.K. Norwida 25, 50-375 Wrocław, Poland b
a r t i c l e
i n f o
Article history: Received 3 November 2008 Received in revised form 6 February 2009 Accepted 19 February 2009 Available online 10 March 2009 Keywords: Pumpkin Drying Shrinkage Density Colour Carotenoids
a b s t r a c t Drying of pumpkin slices was carried out using convective, vacuum-microwave, vacuum and freeze drying methods. The aim of our study was to determine the drying shrinkage and bulk density, as well as to measure the colour and carotenoid content for the slices of 12 pumpkin cultivars. Another major objective was to establish the moisture content, specific density and cutting strength of the slices before dried, as well as the kinetics of their dehydration by the convective and vacuum-microwave methods. The application of the vacuum-microwave method has approximately tenfold shortened the time of pumpkin slice drying as compared to the convective method. In the hot air stream, the slices of the species Cucurbita pepo were dehydrated faster than those of the species Cucurbita maxima. When use was made of the vacuum-microwave process, the slices of C. maxima dried at a faster rate. Each of the drying methods used was more effective than the convective method. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In Poland two pumpkin species are grown for fruit crops harvested in the phase of physiological maturity: Cucurbita maxima and Cucurbita pepo. The nutritional value of pumpkin fruits is high but varies from one species or cultivar to another. Thus, in the fresh mass of the fruit, total carotenoid content, a major contributory factor in the high nutritional value of pumpkins, ranges from 2 to 10 mg/100 g, the content of vitamins C and E accounting for 9– 10 mg/100 g and 1.03–1.06 mg/100 g, respectively (Terazowa et al., 2001; Murkovic et al., 2002; Kunachowicz et al., 2005). Pumpkin fruit is also a valuable source of other vitamins, e.g., B6, K, thiamine, and riboflavin, as well as minerals, e.g., potassium, phosphorus, magnesium, iron and selenium (USDA National Nutrient Database, 2004). Pumpkin flesh is a delicious and fully appreciated additive in a diversity of products for children and adults. Pumpkin fruits are being processed to obtain juice, pomace, pickles and dried products. One of the oldest methods for the preservation of food is drying, which consists in removing water from the product in order to provide microbiological safety (Malthlouthi, 2001), and the most popular drying method includes convection. In this method the drying agent supplies heat to the material and removes moisture (in the form of water vapour) from the material at the same time. The * Corresponding author. Tel.: +48 71 32 05 187; fax: +48 71 32 05 477. E-mail address: [email protected] (A. Nawirska). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.02.025
method itself is a low-cost one, but has the disadvantage of entailing a time-consuming process. During contact with oxygen that is present in the air, the product becomes exposed to high temperature for a long time, and such exposure reduces the content of some valuable components which readily undergo oxidation at elevated temperature. Another drawback of the convective method is the concomitant substantial shrinkage (Lozano et al., 1983). In the vacuum method the contact between the material being dried and oxygen is limited. Owing to the reduced pressure, effective drying can be achieved at low temperature. However, shrinkage is comparatively high and the necessity of using a vacuum system raises the cost of the drying process. Drying by sublimation (freeze drying) guarantees an excellent quality of the product. Colour, flavour, chemical composition and shape undergo only slight changes. The method, however, involves very high costs and is time-consuming. It requires pre-freezing and long-lasting storage of the raw material in the drying chamber at reduced pressure, even though the increased temperature of the heating plate accelerates the sublimation process. Vacuum-microwave drying is of a very short duration. In this method, microwaves penetrate into the interior of the product being dried, and cause the water to boil at a comparatively low temperature, owing to the reduced pressure in the drying chamber. It is due to the water boiling inside the material that the relative difference in the pressure between the interior and the environment of the material provides an extremely quick water vapour removal and produces favourable conditions for the occurrence of the
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
puffing phenomenon, which considerably limits the drying shrinkage effect (Sham et al., 2001). Regretfully, the method is rather costly and requires strict control, as at the final stage of drying an abrupt rise in the temperature of the material is likely to occur (Drouzas and Schubert, 1996). The potentiality for achieving a high-quality product of an attractive texture has directed the attention of many investigators to the applicability of the vacuummicrowave method inter alia to the drying of cranberries (Sunjka et al., 2004), strawberries (Krulis et al., 2005), tomatoes (Durance and Wang, 2002), carrots (Lin et al., 1998), bananas (Maskan, 2000) and garlic (Figiel, 2006). Industrial applications of vacuummicrowave drying are becoming increasingly frequent (Attiyate, 1979; http://www.paula.com.pl). Pumpkin slices are generally dried using the convective method (Sojak, 1999), in some instances with osmotic drying as a prior step (Garcia et al., 2007), which is also applied as a pretreatment stage to vacuum drying (ArévaloPinedo and Murr, 2007). The literature also contains references to the drying of pumpkin slices by the microwave method (Alibas, 2007), but as yet no reports are available on vacuum-microwave dehydrated pumpkin slices. VM method, when properly applied, can be used for achieving a high-quality product. Thus the necessity of basic quality parameters determination of VM dehydrated pumpkin samples of different cultivars seems to be important. The aim of our study was to determine the drying shrinkage and bulk density, as well as to measure the colour and carotenoid content for the slices of 12 pumpkin cultivars subjected to convective, vacuum-microwave, vacuum and freeze drying. Another major objective was to establish the moisture content, specific density and cutting strength of the slices before dried, as well as the kinetics for their dehydration by the convective and vacuum-microwave methods.
2. Materials and methods 2.1. Materials In experiment conducted in 2007 there was estimated the effect of variety on quality of pumpkin chips. Slices of pumpkin flash obtained from 12 cultivars belonged to two species C. maxima (1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melo_ nowa Zółta’, and 6 – ‘Uchiki Kiuri’ cvs) and C. pepo (7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs) were tested. Both the species were grown in the Experimental Plant of Piastów belonging to the Department of Horticulture, Wroclaw University of Environmental and Life Sciences. Four weeks old transplants produced in greenhouse were planted on the plots fertilized with nitrogen (200 kg N ha1) in second decade of May and fruits were harvested in half of September. 2.2. Preparation of samples Slices of pumpkin flesh (3.5 mm thick and 18 mm in diameter) were prepared with the aid of a cutter (Gastrotech, Kraków, Poland) and a steel-made blanking tool, which was cylindrical in shape and pointed on one of the sides. A small rubber piston moving in the cylinder interior enabled a simultaneous removal of five samples without damaging them. 2.3. Specific density measurement The specific density of the sample was determined as the ratio of the mass of five randomly chosen slices to their volume. The slices were weighed using Metler Toledo scales (USA) with an accuracy of 0.001 g. The volume of the slices was determined after having
15
formed a column of them. The height and diameter of the column were measured by means of a slide caliper with an accuracy of 0.01 mm. The measurement was conducted in three replications. 2.4. Cutting strength test Prior to the drying process, the pumpkin slices were made subject to a cutting strength test using an Instron 5544 strength testing machine (Instron, High Wycombe, UK) equipped with a tensometric head of a measuring range of 2 kN. The slices (laid horizontally) were cut with the aid of a QTS-25 SB cutting attachment at the speed of 2 mm/s. The average value of the maximal cutting force Ftmax was determined from 10 replications. 2.5. Drying The pumpkin slices were dried by the following four methods: convective (C), vacuum (V), vacuum-microwave (VM) and freeze drying (F). The process was discontinued after the moisture content fell below 0.1 kg/kg db. The kinetics for the C and VM drying processes were determined from the mass loss in samples of a known initial moisture. The samples were weighed in defined time intervals. Taking into account the differences in the initial moisture between particular pumpkin varieties, the variations in the moisture ratio MR occurring at drying time s were established in terms of the formula:
MR ¼
MðsÞ M e M0 Me
ð1Þ
where M(s) denotes moisture content after drying time s; Ve stands for equilibrium moisture content, and M0 is initial moisture. The equilibrium moisture content Me was determined at the final stage of drying as an asymptotic value of the function fit to the experimental points using Table Curve 2D Windows v2.03. Knowing the moisture ratio (MR) values, it is possible to compare the kinetics of drying for raw materials differing in initial moisture. Drying via the convective method (C) was conducted with the aid of a drier designed and constructed by the staff of the Institute of Agricultural Engineering, Wrocław. The drier enabled simultaneous dehydration of six samples placed in baskets at the ends of vertical air pipes. The samples were laid in a sieve and formed a single layer. Air stream velocity and temperature amounted to 1 m/s and 60 °C, respectively. Vacuum drying (V) of the pumpkin samples, which were arranged in a single layer on Petri dishes, was performed in an SPT-200 drier (ZEAMiL Horyzont, Kraków, Poland) at 60 °C. VM drying was carried out in an SM-200 drier (Plazmatronika, Wrocław, Poland). Samples of a 60 g mass were placed in a cylinder rotating at a speed of 6 rev min1. The pressure in the cylinder varied from 4 to 6 kPa. Microwave power amounted to 480 W. In the freeze drying (F) method use was made of an OE-950 drier (Hungary). The temperature and pressure in the closed drying chamber were 60 °C and 65 Pa, respectively. The samples were arranged in a single layer on the heating plate of a temperature of +30 °C, which accelerated the sublimation process. 2.6. Measurement of drying shrinkage and bulk density of dried material Drying shrinkage (S) and bulk density (qb) were determined using a measuring cylinder of 27 mm diameter. The pumpkin samples were placed in the cylinder, which was gently shaken. Measurements were conducted when the samples occupied the smallest possible volume. Drying shrinkage was calculated in terms of the following formula:
16
V0 Vs V0
1
80
where V0 is volume of fresh slices, and Vs is volume of dried slices. Bulk density was the quotient of the mass of the dried sample and its volume determined in the measuring cylinder. 2.7. Total carotenoid content The method consists in the extraction of carotenoids from the sample tested, using 80% acetone, and in their determination in the extract by the colorimetric method at the wavelengths of 470, 646 and 663 nm according to Rumin´ska et al. (1990). Carotenoid content was calculated as follows (the result being given in mg/ 100 g):
Kn ¼
90
ð2Þ
1000A470 3:27C a 104C b 229
ð3Þ
where
Cutting Force F tmax (N)
S¼
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
2
2
70
3 1
60
4
7
6
5
50
6
40
7
11
30
8
4
5 8
9
3
10
20 9
12
10 0 80
82
84
86
88
90
92
94
11 12
10
96
Moisture content M wb (%) Fig. 2. Effect of moisture content (Mwb) on the maximal force (Ftmax) required for the cutting of the pumpkin slices, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – _ ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
C a ¼ 12:71A663 2:81A646 C b ¼ 20:13A646 5:03A663 A470, A646, and A663 being absorbance values at wavelengths 470, 646, and 663 nm, respectively. 2.8. Pumpkin slices colour assessment The colour of the pumpkin slices was measured with ColorQuest XE (HunterLab). Milled slices were placed in a glass cuvette, and the colour was recorded using CIE L*a*b* 10°/D65 colour spaces, where L* indicates lightness, a* is the +a redness, and b* is the +yellowness.
comparison to their moisture (which reached 95%). The cultivars of the species C. maxima were characterized by lower densities (ranging from 0.99 to 1.07 kg/m3) as compared to the cultivars of the species C. pepo (varying between 1.06 and 1.12 kg/m3). It has been observed that the increase in the moisture content (Mwb) of the pumpkin slices is paralleled by the decrease in the force required for their cutting, Ftmax. The lower values of the maximal cutting force (Ftmax) can be attributed to a less compact texture of the pumpkin flesh. 3.2. Drying kinetics
3. Results
The decrease in the reduced moisture of the samples, MR, at time s of drying performed by the convective method (Fig. 3) was described using the semi-empirical Page’s equation:
3.1. Density, moisture content and cutting resistance of the raw material
MR ¼ ek1 s
Fig. 1 includes points that represent the 12 cultivars examined. The horizontal coordinates of the points refer to moisture content, Mwb, and the vertical ones indicate specific density, qs. The relation in Fig. 1 clearly shows that the increase in the moisture of the pumpkin is concomitant with the increase in its specific density. However, the Ambar sample, which had the lowest moisture content (81.4%), displayed a relatively high density (1.005 kg m3); the cultivars Bambino and Karowita exhibited low specific densities in
ð4Þ
Table 1 compiles the values of the drying constant k1. Higher k1 values are indicative of a higher drying rate. It was found that in the majority of instances the pumpkin varieties of the species C. pepo dried at a faster rate than did the varieties of the species C.
1
1
2 3
Specific Density ρ s (g / cm3)
1 1.12
9
8
1.1
11
2 3
10
12
4
1.08
5
5 7
1.06
6 3
1.04 1.02
7 8
4
11
0.98
12 0.96 80
82
84
5 6
0.4
7 8
0.2
9
10
6
1
4 0.6
9
2
1
Moisture ratio MR
0.8 1.14
86
88
90
92
94
96
Moisture Mwb (%) Fig. 1. Specific density (qs) of pumpkin slices vs moisture content (Mwb), 1 – _ ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6– ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
10
0 0
60
120
Time τ (min)
180
240
11 12
Fig. 3. Kinetics of drying by the convective method, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – _ ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
17
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
Table 1 Values of the parameters k1, k2, a and b of the functions describing the kinetics of drying by the convective and vacuum-microwave methods. R2 = coefficient of determination. Cultivar
Convective method
Vacuum-microwave method
k1 s
1 – Amazonka 2 – Ambar 3 – Bambino 4 – Karowita _ 5 – Melonowa Zółta 6 – Uchiki Kuri 7 – Danka 8 – Junona 9 – Miranda 10 – Pyza 11 – Warszawska Makaronowa 12 – Jet F1
MR ¼ b ek2 s
MR ¼ 1 a s
MR ¼ e
2
k1
R
0.033 0.036 0.054 0.049 0.045 0.038 0.056 0.068 0.072 0.069 0.063 0.058
0.9996 0.9996 0.9941 0.9967 0.9962 0.9995 0.9953 0.9977 0.9972 0.9981 0.9976 0.9961
maxima when they were exposed to the hot air stream. Taking into account the physical and mechanical properties of the raw material, it becomes clear that a higher initial moisture content and a less compact texture of the pumpkin slices account for a higher efficiency of drying by the convective method. Drying of pumpkin slices by the VM method (Fig. 4) took an approximately tenfold shorter time as compared to the convective method (Fig. 3). The time of pumpkin slices drying by the convective method approached 240 min and was similar to the time of drying 5 20 40 mm C. maxima slices (Alibas, 2007), as well as C. moschata slices of a 4 mm thickness and a surface area of 20– 25 cm2 (Garcia et al., 2007), at 50 °C, with the aid of the same method. In the latter case, the inclusion of osmotic dehydration as a pretreatment step into the convective method has reduced the drying time by about 1 h. The kinetics of pumpkin slices drying by the VM method has been described with a linear function (Eq. (6)) for the drying time up to the sixth minute of the process, and by an exponential function (semi-empirical Page’s Eq. (7)) for the remaining drying time (Pabis and Jaros, 2002).
MR ¼ 1 a s
ð5Þ
M R ¼ b ek2 s
ð6Þ
1
1
2
a
R
b
k2
R2
0.104 0.107 0.098 0.099 0.096 0.096 0.099 0.089 0.087 0.091 0.101 0.105
0.9995 0.9984 0.9983 0.9981 0.9996 0.9992 0.9997 0.9982 0.9996 0.9984 0.9994 0.9995
2.54 3.59 1.69 1.90 2.26 2.08 2.65 1.50 1.51 1.59 2.30 2.15
0.314 0.380 0.229 0.248 0.274 0.258 0.306 0.188 0.187 0.204 0.289 0.286
0.997 0.9941 0.9907 0.9845 0.9879 0.9847 0.9880 0.9922 0.9928 0.9948 0.9938 0.9916
The values of the parameters a, b and k2 are compiled in Table 1. The higher values of the parameter a are indicative of a higher drying rate in the initial period, while the higher values of k2 indicate a higher drying rate in the subsequent period. When use was made of the VM method, the slices of the C. maxima pumpkin species were drying at a faster rate than the slices of the C. pepo pumpkin species, in contrast to the convection method. In the VM method, the lower initial moisture content, as well as the lower density and a more compact texture of the pumpkin slices, were conducive to the rise in the efficiency of drying. 3.3. Drying shrinkage As can be seen from the bars in Fig. 5, drying shrinkage is generally higher in the slices of the pumpkin species C. pepo than in those of the species C. maxima; the highest resistance to shrinkage being observed in the cultivars Amazonka, Ambar and Uchiki Kuri. The slices dehydrated by the F method developed minimal drying shrinkage irrespective of the pumpkin cultivar. In the case of the species C. maxima, the largest shrinkage was observed in the samples dried by the V method. The samples dehydrated by the VM or by the C method displayed a minor drying shrinkage. An exception was the sample of the pumpkin variety Karowita dried by the VM method, where the slices experienced a greater shrinkage as compared to those dried by the other methods. In the case of the C. pepo species, it has been observed that the values of drying
2
0.6
3
1.00
4
0.90
6 0.4
7 8
0.2
0 0
6
12
Time τ (min)
18
24
0.70 0.60 0.50 0.40 0.30
9
0.20
10
0.10
11 12
Fig. 4. Kinetics of drying by the vacuum-microwave method, 1 – ‘Amazonka’, 2 – _ ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
VM V C F
0.80
5 Shrinkage S
Moisture ratio MR
0.8
0.00 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 5. Shrinkage (S) of slices cut from the pumpkin cultivars examined, dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa _ Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
shrinkage followed a decreasing trend when the methods VM, V, and C were used consecutively. This trend fails to apply to the Miranda cv., whose slices dehydrated by the V method developed a slightly greater shrinkage than did the slices dried by the VM method. The varieties Ambar, Amazonka, and Uchiki Kuri were found to be the most resistant to shrinkage, regardless of the drying method applied. They were characterized by the lowest density and moisture content (Fig. 1), as well as by a high cutting strength (Fig. 2). 3.4. Bulk density of the dried material The highest bulk density was determined in the dried slices of the pumpkin cultivars Ambar, Amazonka, and Uchiki Kuri (Fig. 6), the same that displayed the highest resistance to drying shrinkage (Fig. 5). This finding is attributable to the low moisture of the raw material from which the slices have been obtained (Fig. 1), and consequently to the high dry solids content. The comparatively high bulk density values of the C. pepo slices are to be attributed to the remarkable drying shrinkage rather than the dry solids content, which was low, taking into account the high moisture value. The low bulk density of the slices of the Karowita, and Bambino cultivars resulted from the high moisture of the raw material and from the relatively high drying shrinkage. Pumpkin slices dehydrated using the F method showed visibly the lowest
300 VM V C F
3
Bulk density (kg/m )
250
200
150
100
50
0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar
Carotenoid (mg/100g)
Fig. 6. Bulk density of slices cut from the pumpkin cultivars examined, dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – ‘Melonowa _ Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
F C V VM
bulk density, which is due to the slight volume change associated with the application of this drying method. 3.5. Carotenoids Fig. 7 includes the values of total carotenoid content for the pumpkin varieties examined. Thus, the overall amount of carotenoids in the C. maxima samples was approximately eight times as high as that in the C. pepo samples. A significant difference was observed in the carotenoid content between the samples of the 12 pumpkin cultivars dehydrated by the four drying methods. The highest average carotenoid content in the dried material from the two pumpkin species was determined with the freeze drying (F), then vacuum-microwave (VM) and the lowest with the convective method (C). Among the varieties arising from the two species, the highest carotenoid content was measured in Karowita (C. maxima) and Danka (C. pepo), and the lowest in Ambar (C. maxima) and Pyza (C. pepo). 3.6. Colour The value of the parameter L*, which indicates the lightness of milled pumpkin slices, varied from 56.91 to 86.11 (Fig. 8). Samples obtained from the species C. maxima were darker (lower value of L*), which seems to be attributable to the higher carotenoid content. The use of the VM method brought about some darkening of the samples, apparently caused by an inadequate choice of the drying parameters, which produced local intensive changing in colour. Alibas (2007), using microwave, air and combined microwaveair drying for the dehydration of C. maxima slices, attained darker dried materials with L* values ranging from 22.80 to 27.91. Regardless of the drying method used, the lightest dried materials were obtained from the Ambar cv. (C. maxima), which, in comparison to the other cultivars examined, contained many carbohydrates (approximately 6%) (Biesiada et al., 2006). Among the varieties C. maxima, carotenoid content was the least in Ambar cv. In C. pepo species there were no cultivar which was the best among all drying methods but Junona cv. samples obtained by methods vacuum (V) and convective (C), were the lightest. To obtain the best colour of dried pumpkin slices, parameters a*, b* and L should gain high values. Dried materials obtained from the species C. maxima displayed remarkably higher redness values (a* = 7.61–22.35) as compared to the species C. pepo (a* = 1.76–12.30), but the a* parameter val-
C F V VM
90 80
Lightness Parameter (L*)
18
70 60 50 40 30 20 10 0
1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 7. Carotenoid content of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 8. Lightness parameter (L*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
A. Nawirska et al. / Journal of Food Engineering 94 (2009) 14–20
Redness Parameter (a*)
of C. pepo which were dried using the VM method. From our investigations it is clear that convective method of drying was the worst.
C F V VM
25
20
19
4. Conclusions
15
10
5
0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 9. Redness parameter (a*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
Table 2 Coefficients of correlation between carotenoid content and colour parameters for pumpkin slices (Cucurbita maxima and Cucurbita pepo) dried using different methods (significance level = 0.05).
L* a* b* Carotene
L*
a*
1.00 0.63 0.06 0.62
1.00 0.67 0.92
b*
Car
The flesh of the pumpkin with increased moisture content was generally characterized by higher specific density and lower cutting strength. We also show that, the application of the vacuummicrowave method has approximately tenfold shortened the time of pumpkin slice drying as compared to the convective method. In the hot air stream, the slices of the species C. pepo were dehydrated faster than those of species C. maxima. However, when use made of the vacuum-microwave process, the slices of C. maxima dried at faster rate. In the majority of instances, the dried slices of the C. pepo species displayed a higher drying shrinkage than did those of the C. maxima species. The highest bulk density was measured in the slices of the cultivars Ambar, Amazonka, and Uchiki Kuri, which belong to the species C. maxima. Considered from the viewpoint of colour and carotene content, each of the drying methods used was more effective than the convective method. When use was made of vacuum-microwave method, the dried products had more attractive colour. Acknowledgment This work was supported by the Polish Ministry of Science and Higher Education under Grant Nr N 31008932/3913.
1.00 0.62
1.00
References
50
C F V VM
Yellowness Parameter (b*)
45 40 35 30 25 20 15 10 5 0 1
2
3
4
5
6
7
8
9
10
11
12
Cultivar Fig. 10. Yellowness parameter (b*) of Cucurbita maxima and Cucurbita pepo slices dehydrated by the vacuum-microwave (VM), vacuum (V), convective (C) and freeze drying (F) methods, 1 – ‘Amazonka’, 2 – ‘Ambar’, 3 – ‘Bambino’, 4 – ‘Karowita’, 5 – _ ‘Melonowa Zółta’, 6 – ‘Uchiki Kiuri’, 7 – ‘Danka’, 8 – ‘Junona’, 9 – ‘Miranda’, 10 – ‘Pyza’, 11 – ‘Makaronowa Warszawska’, and 12 – ‘Jet F1’ cvs.
ues of the C. maxima Ambar cv. were half as high (Fig. 9). Among the varieties of C. pepo, the dried slices of Danka exhibited the highest values of the parameter a*, which is attributable to the high content of carotenoids. In all of the samples the parameter a* was very well correlated with the carotenoid content (R = 0.92) (Table 2). Among the varieties of C. maxima, the values of the yellowness parameter b* ranged from 50.3 in the slices of Amazonka dried by the V method to 29.54 in the slices of Bambino dehydrated by the VM method (Fig. 10). Slightly lower values were measured in the varieties of C. pepo. The highest average values of yellowness were determined in the slices of C. maxima dehydrated by the F method, whereas the lowest ones were obtained for the slice samples
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