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Water sorption, drying and antioxidant properties of dried tomato products
Journal of Food Engineering 52 (2002) 135–141 www.elsevier.com/locate/jfoodeng
Water sorption, drying and antioxidant properties of dried tomato products G. Giovanelli a, B. Zanoni
b,*
, V. Lavelli a, R. Nani
c
a diSTAM – Sezione Tecnologie Alimentari, Universit a di Milano, Via Celoria 2, 20133 Milano, Italy DI.VA.P.R.A. – Settore Microbiologia e Industrie Agrarie, Via Leonardo da Vinci 44, 10095 Grugliasco, Torino, Italy I.V.T.P.A. – Istituto Sperimentale per la Valorizzazione Tecnologica dei Prodotti Agricoli, Via Venezian 26, 20133 Milano, Italy b
c
Received 19 December 2000; accepted 23 April 2001
Abstract This work is focused on some properties of various dried tomato products (tomato pulp, tomato halves and insoluble solids-rich tomato), useful to optimize drying processes. Adsorption and desorption isotherms at 20°C of these products were measured and modelled by the Guggenheim–Anderson–de Boer (GAB) equation. Insoluble solids-rich tomato was the least hygroscopic of all tomato products. Tomato products were air-dried in a pilot plant and the drying kinetics was modelled. The mass transfer equation for drying of thin slabs, modified to include shrinkage of samples during drying, was successfully applied to experimental data. Apparent water diffusivity values ranged from 2:3 109 to 9:1 109 m2 /s as a function of the structure of tomato products. The lycopene and ascorbic content and the antioxidant activity of hydrophilic and lipophilic extracts were measured both on fresh and dried tomato products. Interesting properties of insoluble solids-rich tomato were evidenced: it had the highest lycopene content (ca. 12,000 mg/kg dm) and lipophilic antioxidant activity (ca. 400I50 ; lg dm) of all tomato products. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tomato; Drying; Lycopene; Antioxidant properties
1. Introduction Over the last few years fresh tomato and tomato products, due to their antioxidant activity, have aroused new scientific interest. Tomato components include carotenoids (van den Berg et al., 2000); tomato is the main source of lycopene, which seems to have high beneficial effects on human health (Rao & Agarwal, 1999). Tomato components also include ascorbic acid, flavonoids and other related compounds with an antioxidant activity (Diplock et al., 1998). The nutritional role of tomato has promoted food technology studies, carried out to determine and to prevent oxidative damage, particularly in terms of lycopene degradation, during processing and storage of tomato products (Abushita, Daood, & Biacs, 2000; Anese, Manzocco, Nicoli, & Lerici, 1999; Shi & Le Maguer, 2000).
*
Corresponding author. Tel.: +39-011-670-8705; fax: +39-0116708549. E-mail address: [email protected] (B. Zanoni).
Dried tomato products (i.e., tomato halves, slices, quarters and powders), being commonly dried at high temperatures in the presence of oxygen, show the highest sensitivity to oxidative damage. Air drying of tomato caused a severe oxidative heat damage of product, shown by both a marked loss of ascorbic acid and an increase in the 5-hydroxymethyl-2furfural (HMF) content, resulting in undesirable colour and appearance changes of dried tomatoes (Zanoni, Peri, Nani, & Lavelli, 1999). Conversely, lycopene had a high stability during drying. The lycopene content of tomato halves at 10% final moisture content decreased to a maximum of 10% after drying at 110°C for 4 h and did not change during drying at 80°C for 7 h (Zanoni et al., 1999). The lycopene content of whole tomatoes at 3–4% final moisture content decreased to ca. 4% after drying at 95°C for 6–10 h (Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). Various studies showed a significant oxidative damage to dried tomatoes during storage. After either 6 weeks light exposure in air at room temperature or storage at 6°C in air and in the dark, 30–40% lycopene loss occurred in spray-dried tomato powders at 1.5–2.0%
0260-8774/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 1 ) 0 0 0 9 5 - 4
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moisture content (Anguelova & Warthesen, 2000). A greater lycopene loss (i.e., 60%) occurred after storage in the dark at 45°C for 6 weeks. A similar effect of temperature has been observed in other dried tomato products. Baloch, Khan, and Baloch (1997) found that carotenoid loss was above 50% in tomato powder after 20 days storage in air and in the dark at 40°C. Zanoni et al. (1999) measured a lycopene loss >50% in dried tomato halves after 30 days storage in air and in the dark at 37°C. Oxidative damage can be avoided by optimizing operating conditions for both drying and storage of dried tomato products. Zanoni et al. (1999) proposed low temperatures for short times treatments to optimize tomato drying either by reducing tomato thickness (i.e., producing dried tomato slices, quarters, cubes) or by partially removing water (i.e., producing intermediate moisture tomatoes). Shi et al. (1999) suggested that osmotic and vacuum drying be used to obtain both intermediate moisture and dried tomatoes, while maintaining the antioxidant activity of fresh product. Zanoni, Pagliarini, and Foschino (2000) found an optimal range of conditions for storage of tomato halves under vacuum in the dark; it was represented by residual moisture values between 20% and 40% and 6 18°C storage temperature. An understanding of the characteristics of tomato in terms of hygroscopicity, drying kinetics and water diffusivity is essential for any optimization study. The aim of this work was to determine the water sorption and drying properties of dried tomato products. In this work well-known models were applied to some tomato products, the properties of which are not available in the literature. The effect of drying on the antioxidant activity of these products was also studied.
2. Materials and methods 2.1. Materials Drying trials were carried out using two types of tomato products: (i) commercial tomato pulp and (ii) insoluble solids-rich tomato. Insoluble solids-rich tomato was produced from commercially canned peeled tomatoes. Peeled tomatoes were drained, chopped and mashed into rough pulp using a puree-maker. The rough pulp was centrifuged at 11,000g and 10°C for 30 min, and the separated tomato serum was removed. The insoluble solids-rich fraction was resuspended in a volume of washing solution (i.e., 1% citric acid) corresponding to the amount of serum separated, in order to remove the absorbed soluble solids fraction. The suspension was stirred for 30 min and centrifuged under the same conditions as above to obtain insoluble solids-rich tomato samples.
2.2. Methods 2.2.1. Drying of tomato products Tomato products were dried in a pilot-plant cabinet air dryer designed and built by Thermo Lab (Milan, Italy). Samples were placed onto a perforated stainless steel tray ð40 60 cm2 Þ connected to a balance to measure tomato weight during drying. Two series of drying tests were carried out on tomato pulp. Ca. 800 and 1500 g of drained pulp were placed onto the tray in slabs 15 and 20 mm in thickness, respectively. Drying was carried out at 70°C. Air flow rate was 1.5 m/s in through flow. Tomato pulps were dried to approx. 10% final moisture content. Ca. 500 g of insoluble solids-rich tomato was placed onto the tray in a slab 10 mm in thickness. Samples were dried at 60°C by 1.5 m/s air flow rate in through flow to approx. 7% final moisture content. During drying weight loss, moisture loss and drying rate were calculated by the measured sample weight and by the measured total solids content of fresh samples. At the end of drying products were immediately chilled to 3°C, and the total solids content of dried samples was measured by gravimetry in a vacuum oven at 70°C (Porretta, 1991) to validate the calculated weight losses. The temperature and relative humidity of fresh air were obtained from regional weather reports (Direzione Generale Regione Lombardia, Milan, Italy).
2.2.2. Lycopene content The lycopene content was measured on both fresh and dried tomato samples. Dried tomato samples were rehydrated to about 10% solids content under nitrogen at 4°C for approximately 10 h. Samples (1–5 g) were extracted with tetrahydrofuran (THF) stabilized by adding 0.1% butylated hydroxytoluene (BHT). Tomato samples were homogenized in an Ultraturrax at low speed (for max 60 s) under nitrogen flux using about 30 ml of the extraction solvent in an ice bath to avoid overheating. The homogenized mixture was centrifuged at 11,000g at 5°C for 10 min. The supernatant was collected in a 100 ml flask, and solids were extracted two more times in the same way. The supernatants were added to the flask and the final extract was brought to volume, filtered through a 0:22 lm membrane and immediately injected into an HPLC system. All operations were performed avoiding direct light exposure and using dark glassware. Chromatographic conditions were as follows: Vydac 210 TP C18 column ð25 4:6 mm2 Þ, equipped with a C18 pre-column; isocratic elution with 95:5 methanol: THF (0.1% BHT) at 25°C; UV detection at 454 nm. Lycopene was identified and quantified by a calibration curve built with pure standard compound (Sigma Chemical, Italy).
G. Giovanelli et al. / Journal of Food Engineering 52 (2002) 135–141 Table 1 Water activity values for saturated salt solutions (Greenspan, 1977) Saturated salt solution
Water activity (aw )
MgCl2 NaBr NaNO2 a NaCl KCl BaCl2 a
0.3307 0.5914 0.649 0.7547 0.8511 0.920
a
Reported by Stamp, Linscott, Lomauro, and Labuza (1984).
2.2.3. Ascorbic acid content The ascorbic acid content was measured on both fresh and dried tomato samples. Samples were diluted 1:10 with 0.3% meta-phosphoric acid and homogenized by Ultraturrax. The homogenized mixture was centrifuged at 11,000g at 5°C for 10 min, the supernatant was filtered through a 0:45 lm membrane, and immediately injected into the HPLC system. Chromatographic conditions were as follows: fruit quality column (BioRad, 100 7:8 mm2 ), equipped with a cation H+ precolumn (BioRad, Milan, Italy); isocratic elution with 0:002 N H2 SO4 by 0.7 ml/min at room temperature; injection volume 20 ll; detection by an electrochemical detector set at þ800 mV. Ascorbic acid was identified and quantified by a calibration curve built with a pure standard (BDH, Italy). 2.2.4. Antioxidant activity The antioxidant activity was measured on both fresh and dried tomato samples. The antioxidant activity of hydrophilic and lipophilic extracts of tomato was analysed as described in Lavelli, Peri, and Rizzolo (2000). Briefly, the hydrophilic fraction was extracted by a twostep procedure using 0.1 M phosphate buffer (first step:
137
pH 3.0; second step: pH 7.4), and the antioxidant activity was determined by the xanthine oxidase (XOD, EC 1.1.3.22)/xanthine model system, which produces hydrogen peroxide and superoxide radical. The lipophilic fraction was extracted by THF, and the antioxidant activity was measured by the linoleicacid=CuSO4 model system as a model for lipid peroxidation. For both model systems control reactions were prepared by adding the solvent instead of the extract. The antioxidant activity was calculated as percent of inhibition of the control reaction rate and expressed as I50 , which is the amount of extract (lg of dry matter) that causes 50% inhibition of the model reaction, as interpolated by a dose–response curve. 2.2.5. Water sorption isotherms Adsorption isotherms were determined by a static, gravimetric method using air-tight glass jars containing saturated salt solutions (Table 1). About 5 g of freezedried samples were placed into Petri dishes and then into thermostated jars at 20°C. The equilibrium moisture content was reached within 17 days and measured by the gravimetric method. The desorption isotherm for tomato pulp was determined at 20°C on samples removed from the dryer at different times. Water activity of samples was measured by a dew point hygrometer (Aqualab, Decagon Devices, WA, USA), and the relevant moisture content was measured by the gravimetric method.
3. Results and discussion 3.1. Water sorption properties Fig. 1 shows the experimental data for water activity (aw ) as a function of the moisture content (ns ). Data were fitted with the Guggenheim–Anderson–de Boer (GAB) equation recommended by the European COST 90 project on water activity (Spiess & Wolf, 1987) to model sorption isotherms ns ¼
Fig. 1. Water sorption isotherms for dried tomato products; symbols represent experimental data (N, adsorption data for tomato pulp at 20°C; r, desorption data for tomato pulp at 20°C; j, adsorption data for insoluble solids-rich tomato at 20°C), continuous lines represent the isotherms modelled by the GAB equation. The dotted line represents the desorption isotherm for tomato halves at 25°C modelled by the GAB equation based on data by Zanoni et al. (1999).
nsm Ckaw ; ð1 kaw Þð1 kaw þ Ckaw Þ
ð1Þ
where ns is the equilibrium moisture content on dry basis; nsm is the monolayer moisture content on dry basis; C is the Guggenheim constant; k is a factor for multilayer molecules with respect to the bulk liquid. The same equation was also used to reprocess sorption experimental data reported for tomato halves by Zanoni et al. (1999). The relevant desorption isotherm at 25°C is shown in Fig. 1. GAB parameters of the isotherms for the dried tomato products tested were calculated by non-linear regression (Table 2). The fitting ability of GAB equations was fundamentally in agreement with the Lewicki (1997)
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Table 2 Estimated parameters of the GAB model Tomato products
nsm (g/g dm)
C
k
Correlation coefficient
Freeze-dried tomato pulp (adsorption at 20°C) Air-dried tomato pulp (desorption at 20°C) Air-dried tomato halves (desorption at 25°C) Freeze-dried insoluble solids-rich tomato (adsorption at 20°C)
0.117 0.087 0.076 0.045
5.86 9.75 5.73 26.83
1.013 0.995 1.021 0.942
0.999 0.999 0.998 0.999
analysis: good description of the isotherms when 5:67 6 C 6 1 and 0:24 < k 6 1. Fig. 1 shows a similar desorption behaviour between air-dried tomato pulp and tomato halves at desorption. Tomato pulp resulted to be more hygroscopic during adsorption than during desorption. Although the opposite (i.e., moisture sorption hysteresis) is usually observed, this phenomenon may be explained by the mildest conditions employed to dry samples for adsorption isotherm evaluation. Freeze-drying prevented thermal degradation and changes in the physical structure of the material. Hence, storage conditions of airdried tomatoes, optimized according to adsorption data obtained from freeze-dried samples, may be inconsistent; dried samples would thus seem to be more stable than they actually are as they would be less sensitive to moisture variations. Insoluble solids-rich tomato was the least hygroscopic of all tomato products. This may depend on removal of soluble solids such as fructose and glucose by centrifugation. As a result, this tomato product requires higher water removal during drying to reduce water activity. It is also more sensitive to moisture variations during storage. 3.2. Drying properties Fig. 2 shows the variation in drying rate as a function of the moisture content, with type of tomato products,
Fig. 2. Drying rate versus moisture content for: N, tomato pulp dried in a slab 20 mm in thickness at 70°C; r, tomato pulp dried in a slab 15 mm in thickness at 70°C; j, insoluble solids-rich tomato dried in a slab 10 mm in thickness at 60°C; , tomato halves 16 mm in thickness dried at 80°C; , tomato halves 16 mm in thickness dried at 110°C.
air temperature and product thickness as variables. The drying kinetics reported by Zanoni et al. (1999) was also included. Drying of all tomato samples moved quickly to a falling rate period, which determined the kinetics of the operation. In order to explain the role of operating conditions on tomato products drying, the drying kinetics was modelled. The experimental conditions applied allowed us to consider all tomato products tested as thin slabs subjected to drying on both sides for a long time in the absence of any significant external resistance. The following equation was then applied to model mass transfer during the falling rate period of drying (Perry & Green, 1984): ns nseq 8 ¼ exp ns0 nseq p2
D p 2t ; L 2
ð2Þ
where ns is the mean moisture on dry basis at time t; nseq and ns0 are the equilibrium and initial moisture contents on dry basis, respectively; L is the thickness of slab and D is the effective diffusivity of water. From Eq. (2) the effective diffusivity can be determined by the slope of the straight line obtained by plotting the experimental data from lnfð ns nseq Þ= ðns0 nseq Þg as a function of t=L2 . Table 3 shows the relevant values for variables in Eq. (2) for data processing. The values for drying air were determined by a psychrometric chart, and the values for nseq were determined by the above GAB equations. All the samples, except for the insoluble solids-rich tomato samples, showed a non-linear relationship on a semi-log plot (Fig. 3). Drying moved to a falling rate period, characterized by a two-stage phenomenon, where the effective water diffusivity of the second stage was bigger than that of the first one. This phenomenon does not usually occur; commonly, diffusivity decreases as drying proceeds (Brennan, 1994). Hawlader, Uddin, Ho, and Teng (1991) found a similar behaviour, when studying the drying kinetics of sliced tomatoes at different air temperatures and flow rates. This was ascribed to the shrinkage of samples during drying and, hence, to a reduction of thickness, resulting in faster water removal. Hawlader et al. (1991) proposed that L in Eq. (2) should be replaced with a modified thickness L0 , related to the moisture content by the following equation:
G. Giovanelli et al. / Journal of Food Engineering 52 (2002) 135–141
Fig. 3. Relationship between lnfðns nseq Þ=ðns0 nseq Þg and t=L2 for: N, tomato pulp in a 20 mm slab; r, tomato pulp in a 15 mm slab; j, insoluble solids-rich tomato; , tomato halves dried at 80°C; , tomato halves dried at 110°C.
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Fig. 4. Comparison between experimental data (symbols) and predicted data (lines) on drying kinetics for: N, tomato pulp in a 20 mm slab;, tomato pulp in a 15 mm slab; r, insoluble solids-rich tomato; , tomato halves dried at 80°C; , tomato halves dried at 110°C.
Table 3 Characteristics of tomato products and air drying for experimental kinetic data processing Sample
L (mm)
ns0 (g/g dm)
Fresh air temperature (°C)
Fresh air relative humidity (%)
Drying air temperature (°C)
Drying air relative humidity (%)
nseq (g/g dm)
Tomato pulp Tomato pulp Tomato halves Tomato halves Insoluble solidsrich tomato
15 20 16 16 10
10.36 12.51 16.37 17.52 8.09
16 13 13 13 20
15 20 80 80 55
70 70 80 110 60
0.8 1 2.8 0.9 6.5
0.006 0.008 0.011 0.004 0.031
Table 4 Kinetic characteristics of tomato products Sample
n index
Effective water diffusivity, D (m2 /s)
Correlation coefficient, r
Tomato pulp dried in a slab 15 mm in thickness at 70°C Tomato pulp dried in a slab 20 mm in thickness at 70°C Insoluble solids-rich tomato dried in a slab 10 mm in thickness at 60°C Tomato halves 16 mm in thickness dried at 110°C Tomato halves 16 mm in thickness dried at 80°C
0.08 0.14 0
9:14 109 7:77 109 5:48 109
0.99 0.99 0.99
0.14 0.14
4:01 109 2:26 109
0.99 0.99
L0 ¼ L
mt m0
n ;
ð3Þ
where L is the thickness of slab; m0 and mt are the masses of sample at time zero and at drying time t, respectively; n is an index optimized to obtain a straight line for the experimental data from lnfð ns nseq Þ= ðns0 nseq Þg as a function of t=L02 . This equation was successfully applied to our experimental data, and the model was able to predict the drying kinetics by a single value for effective water diffusivity (Fig. 4). Data for n index, water diffusivity and the relevant correlation coefficient for straight lines are reported in Table 4.
In terms of drying properties, tomato products were distinguished as a function of their structure. The highest values for water diffusivity were determined for tomato pulps as a result of their liquid state and absence of skin, whereas the lowest values were determined for tomato halves. The effective water diffusivity values for tomato pulps were in agreement with those for products with similar structure such as fruit purees, as it can be derived by Mittal (1999) review. The values for tomato halves were in agreement with those of Hawlader et al. (1991) for slide tomatoes dried under similar operating conditions. The insoluble solids-rich tomato samples were placed between pulp and tomato halves; their value for water diffusivity was higher than that of tomato halves,
400 60 nd – 1900 70 1140 70 – 830 50 2170 160 11167 150 625 20 4620 350 – 3300 100 1108 14 – nd, not detectable.
850 50 2186 13 12202 24 94.4 0.1 92.6 0.1 89.7 0.4 Tomato halves Tomato pulp Insoluble solidsrich tomato
Dried
Lycopene content (mg/kg dm) Antioxidant activity of hydrophilic extract (I50 , lg dm) Ascorbic acid content (mg/kg dm) Antioxidant activity of lipophilic extract (I50 , lg dm) Lycopene content (mg/kg dm)
Fresh Samples
Table 5 Antioxidant properties of tomato products
Table 5 shows the antioxidant properties of the different types of fresh and dried tomato products. Experimental data for tomato halves air-dried at 80°C by Lavelli, Hippeli, Peri, and Elstner (1999) were included. Experimental data provided a general survey on the beneficial effect of tomato products. This effect is not exclusively related to either a single component or the antioxidant effectiveness in a single model system. Data on fresh products showed peculiar properties of the insoluble solids-rich tomato samples, which had a much higher lycopene content than tomato halves (ca. 14 times higher) and tomato pulp (ca. six times higher). This resulted in a higher antioxidant activity of the lipophilic extract, which was ca. five times higher than that of tomato halves and ca. two times higher than that of tomato pulp. Since data confirmed that lycopene and lipophilic antioxidant activity had a high stability during air-drying (Zanoni et al., 1999) for all types of products tested, insoluble solids-rich dried tomato can be an interesting product as a food ingredient, useful for its antioxidant and colouring properties. A different behaviour was observed for the ascorbic acid content and the antioxidant activity of the hydrophilic extract. Data confirmed that ascorbic acid was very sensitive to oxidative heat damage. Tomato pulp has been found to have a lower ascorbic acid content than tomato halves, due to the damage occurring during processing (Giovanelli et al., 2000). A considerable loss of ascorbic acid has been observed after air-drying of both tomato halves and tomato pulp (Zanoni et al., 1999). Ascorbic acid was not determined on the insoluble solids-rich tomato, since it was removed together with the serum. Data on the antioxidant activity of the hydrophilic extract showed a more complex behaviour. Values for fresh product reflected the differences in ascorbic acid values between samples, but this phenomenon did not occur after air-drying of tomato pulp. Although no ascorbic acid was present in dried tomato pulp, the relevant antioxidant activity was not significantly different from that of fresh samples. We were not able to explain this phenomenon clearly; it may be assumed that other hydrophilic compounds with an antioxidant activity became available during drying of tomato pulp. Some authors (Giovanelli et al., 2000; Stewart et al., 2000) noted that heat treatments increased the level of free flavonols, which have a considerable antioxidant activity.
Antioxidant activity of lipophilic extract (I50 , lg dm)
3.3. Antioxidant properties
1900 100 920 30 378 38
Ascorbic acid content (mg/kg dm)
Antioxidant activity of hydrophilic extract (I50 , lg dm)
although the drying air temperature was much lower (i.e., 60°C compared to 80°C and 110°C). Table 4 also shows the well-known effects of both thickness reduction (see tomato pulps) and temperature increase (see tomato halves) on the increase of water diffusivity.
1200 50 3950 290 –
G. Giovanelli et al. / Journal of Food Engineering 52 (2002) 135–141
Moisture content (%)
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Water sorption, drying and antioxidant properties of dried tomato products G. Giovanelli a, B. Zanoni
b,*
, V. Lavelli a, R. Nani
c
a diSTAM – Sezione Tecnologie Alimentari, Universit a di Milano, Via Celoria 2, 20133 Milano, Italy DI.VA.P.R.A. – Settore Microbiologia e Industrie Agrarie, Via Leonardo da Vinci 44, 10095 Grugliasco, Torino, Italy I.V.T.P.A. – Istituto Sperimentale per la Valorizzazione Tecnologica dei Prodotti Agricoli, Via Venezian 26, 20133 Milano, Italy b
c
Received 19 December 2000; accepted 23 April 2001
Abstract This work is focused on some properties of various dried tomato products (tomato pulp, tomato halves and insoluble solids-rich tomato), useful to optimize drying processes. Adsorption and desorption isotherms at 20°C of these products were measured and modelled by the Guggenheim–Anderson–de Boer (GAB) equation. Insoluble solids-rich tomato was the least hygroscopic of all tomato products. Tomato products were air-dried in a pilot plant and the drying kinetics was modelled. The mass transfer equation for drying of thin slabs, modified to include shrinkage of samples during drying, was successfully applied to experimental data. Apparent water diffusivity values ranged from 2:3 109 to 9:1 109 m2 /s as a function of the structure of tomato products. The lycopene and ascorbic content and the antioxidant activity of hydrophilic and lipophilic extracts were measured both on fresh and dried tomato products. Interesting properties of insoluble solids-rich tomato were evidenced: it had the highest lycopene content (ca. 12,000 mg/kg dm) and lipophilic antioxidant activity (ca. 400I50 ; lg dm) of all tomato products. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Tomato; Drying; Lycopene; Antioxidant properties
1. Introduction Over the last few years fresh tomato and tomato products, due to their antioxidant activity, have aroused new scientific interest. Tomato components include carotenoids (van den Berg et al., 2000); tomato is the main source of lycopene, which seems to have high beneficial effects on human health (Rao & Agarwal, 1999). Tomato components also include ascorbic acid, flavonoids and other related compounds with an antioxidant activity (Diplock et al., 1998). The nutritional role of tomato has promoted food technology studies, carried out to determine and to prevent oxidative damage, particularly in terms of lycopene degradation, during processing and storage of tomato products (Abushita, Daood, & Biacs, 2000; Anese, Manzocco, Nicoli, & Lerici, 1999; Shi & Le Maguer, 2000).
*
Corresponding author. Tel.: +39-011-670-8705; fax: +39-0116708549. E-mail address: [email protected] (B. Zanoni).
Dried tomato products (i.e., tomato halves, slices, quarters and powders), being commonly dried at high temperatures in the presence of oxygen, show the highest sensitivity to oxidative damage. Air drying of tomato caused a severe oxidative heat damage of product, shown by both a marked loss of ascorbic acid and an increase in the 5-hydroxymethyl-2furfural (HMF) content, resulting in undesirable colour and appearance changes of dried tomatoes (Zanoni, Peri, Nani, & Lavelli, 1999). Conversely, lycopene had a high stability during drying. The lycopene content of tomato halves at 10% final moisture content decreased to a maximum of 10% after drying at 110°C for 4 h and did not change during drying at 80°C for 7 h (Zanoni et al., 1999). The lycopene content of whole tomatoes at 3–4% final moisture content decreased to ca. 4% after drying at 95°C for 6–10 h (Shi, Le Maguer, Kakuda, Liptay, & Niekamp, 1999). Various studies showed a significant oxidative damage to dried tomatoes during storage. After either 6 weeks light exposure in air at room temperature or storage at 6°C in air and in the dark, 30–40% lycopene loss occurred in spray-dried tomato powders at 1.5–2.0%
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moisture content (Anguelova & Warthesen, 2000). A greater lycopene loss (i.e., 60%) occurred after storage in the dark at 45°C for 6 weeks. A similar effect of temperature has been observed in other dried tomato products. Baloch, Khan, and Baloch (1997) found that carotenoid loss was above 50% in tomato powder after 20 days storage in air and in the dark at 40°C. Zanoni et al. (1999) measured a lycopene loss >50% in dried tomato halves after 30 days storage in air and in the dark at 37°C. Oxidative damage can be avoided by optimizing operating conditions for both drying and storage of dried tomato products. Zanoni et al. (1999) proposed low temperatures for short times treatments to optimize tomato drying either by reducing tomato thickness (i.e., producing dried tomato slices, quarters, cubes) or by partially removing water (i.e., producing intermediate moisture tomatoes). Shi et al. (1999) suggested that osmotic and vacuum drying be used to obtain both intermediate moisture and dried tomatoes, while maintaining the antioxidant activity of fresh product. Zanoni, Pagliarini, and Foschino (2000) found an optimal range of conditions for storage of tomato halves under vacuum in the dark; it was represented by residual moisture values between 20% and 40% and 6 18°C storage temperature. An understanding of the characteristics of tomato in terms of hygroscopicity, drying kinetics and water diffusivity is essential for any optimization study. The aim of this work was to determine the water sorption and drying properties of dried tomato products. In this work well-known models were applied to some tomato products, the properties of which are not available in the literature. The effect of drying on the antioxidant activity of these products was also studied.
2. Materials and methods 2.1. Materials Drying trials were carried out using two types of tomato products: (i) commercial tomato pulp and (ii) insoluble solids-rich tomato. Insoluble solids-rich tomato was produced from commercially canned peeled tomatoes. Peeled tomatoes were drained, chopped and mashed into rough pulp using a puree-maker. The rough pulp was centrifuged at 11,000g and 10°C for 30 min, and the separated tomato serum was removed. The insoluble solids-rich fraction was resuspended in a volume of washing solution (i.e., 1% citric acid) corresponding to the amount of serum separated, in order to remove the absorbed soluble solids fraction. The suspension was stirred for 30 min and centrifuged under the same conditions as above to obtain insoluble solids-rich tomato samples.
2.2. Methods 2.2.1. Drying of tomato products Tomato products were dried in a pilot-plant cabinet air dryer designed and built by Thermo Lab (Milan, Italy). Samples were placed onto a perforated stainless steel tray ð40 60 cm2 Þ connected to a balance to measure tomato weight during drying. Two series of drying tests were carried out on tomato pulp. Ca. 800 and 1500 g of drained pulp were placed onto the tray in slabs 15 and 20 mm in thickness, respectively. Drying was carried out at 70°C. Air flow rate was 1.5 m/s in through flow. Tomato pulps were dried to approx. 10% final moisture content. Ca. 500 g of insoluble solids-rich tomato was placed onto the tray in a slab 10 mm in thickness. Samples were dried at 60°C by 1.5 m/s air flow rate in through flow to approx. 7% final moisture content. During drying weight loss, moisture loss and drying rate were calculated by the measured sample weight and by the measured total solids content of fresh samples. At the end of drying products were immediately chilled to 3°C, and the total solids content of dried samples was measured by gravimetry in a vacuum oven at 70°C (Porretta, 1991) to validate the calculated weight losses. The temperature and relative humidity of fresh air were obtained from regional weather reports (Direzione Generale Regione Lombardia, Milan, Italy).
2.2.2. Lycopene content The lycopene content was measured on both fresh and dried tomato samples. Dried tomato samples were rehydrated to about 10% solids content under nitrogen at 4°C for approximately 10 h. Samples (1–5 g) were extracted with tetrahydrofuran (THF) stabilized by adding 0.1% butylated hydroxytoluene (BHT). Tomato samples were homogenized in an Ultraturrax at low speed (for max 60 s) under nitrogen flux using about 30 ml of the extraction solvent in an ice bath to avoid overheating. The homogenized mixture was centrifuged at 11,000g at 5°C for 10 min. The supernatant was collected in a 100 ml flask, and solids were extracted two more times in the same way. The supernatants were added to the flask and the final extract was brought to volume, filtered through a 0:22 lm membrane and immediately injected into an HPLC system. All operations were performed avoiding direct light exposure and using dark glassware. Chromatographic conditions were as follows: Vydac 210 TP C18 column ð25 4:6 mm2 Þ, equipped with a C18 pre-column; isocratic elution with 95:5 methanol: THF (0.1% BHT) at 25°C; UV detection at 454 nm. Lycopene was identified and quantified by a calibration curve built with pure standard compound (Sigma Chemical, Italy).
G. Giovanelli et al. / Journal of Food Engineering 52 (2002) 135–141 Table 1 Water activity values for saturated salt solutions (Greenspan, 1977) Saturated salt solution
Water activity (aw )
MgCl2 NaBr NaNO2 a NaCl KCl BaCl2 a
0.3307 0.5914 0.649 0.7547 0.8511 0.920
a
Reported by Stamp, Linscott, Lomauro, and Labuza (1984).
2.2.3. Ascorbic acid content The ascorbic acid content was measured on both fresh and dried tomato samples. Samples were diluted 1:10 with 0.3% meta-phosphoric acid and homogenized by Ultraturrax. The homogenized mixture was centrifuged at 11,000g at 5°C for 10 min, the supernatant was filtered through a 0:45 lm membrane, and immediately injected into the HPLC system. Chromatographic conditions were as follows: fruit quality column (BioRad, 100 7:8 mm2 ), equipped with a cation H+ precolumn (BioRad, Milan, Italy); isocratic elution with 0:002 N H2 SO4 by 0.7 ml/min at room temperature; injection volume 20 ll; detection by an electrochemical detector set at þ800 mV. Ascorbic acid was identified and quantified by a calibration curve built with a pure standard (BDH, Italy). 2.2.4. Antioxidant activity The antioxidant activity was measured on both fresh and dried tomato samples. The antioxidant activity of hydrophilic and lipophilic extracts of tomato was analysed as described in Lavelli, Peri, and Rizzolo (2000). Briefly, the hydrophilic fraction was extracted by a twostep procedure using 0.1 M phosphate buffer (first step:
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pH 3.0; second step: pH 7.4), and the antioxidant activity was determined by the xanthine oxidase (XOD, EC 1.1.3.22)/xanthine model system, which produces hydrogen peroxide and superoxide radical. The lipophilic fraction was extracted by THF, and the antioxidant activity was measured by the linoleicacid=CuSO4 model system as a model for lipid peroxidation. For both model systems control reactions were prepared by adding the solvent instead of the extract. The antioxidant activity was calculated as percent of inhibition of the control reaction rate and expressed as I50 , which is the amount of extract (lg of dry matter) that causes 50% inhibition of the model reaction, as interpolated by a dose–response curve. 2.2.5. Water sorption isotherms Adsorption isotherms were determined by a static, gravimetric method using air-tight glass jars containing saturated salt solutions (Table 1). About 5 g of freezedried samples were placed into Petri dishes and then into thermostated jars at 20°C. The equilibrium moisture content was reached within 17 days and measured by the gravimetric method. The desorption isotherm for tomato pulp was determined at 20°C on samples removed from the dryer at different times. Water activity of samples was measured by a dew point hygrometer (Aqualab, Decagon Devices, WA, USA), and the relevant moisture content was measured by the gravimetric method.
3. Results and discussion 3.1. Water sorption properties Fig. 1 shows the experimental data for water activity (aw ) as a function of the moisture content (ns ). Data were fitted with the Guggenheim–Anderson–de Boer (GAB) equation recommended by the European COST 90 project on water activity (Spiess & Wolf, 1987) to model sorption isotherms ns ¼
Fig. 1. Water sorption isotherms for dried tomato products; symbols represent experimental data (N, adsorption data for tomato pulp at 20°C; r, desorption data for tomato pulp at 20°C; j, adsorption data for insoluble solids-rich tomato at 20°C), continuous lines represent the isotherms modelled by the GAB equation. The dotted line represents the desorption isotherm for tomato halves at 25°C modelled by the GAB equation based on data by Zanoni et al. (1999).
nsm Ckaw ; ð1 kaw Þð1 kaw þ Ckaw Þ
ð1Þ
where ns is the equilibrium moisture content on dry basis; nsm is the monolayer moisture content on dry basis; C is the Guggenheim constant; k is a factor for multilayer molecules with respect to the bulk liquid. The same equation was also used to reprocess sorption experimental data reported for tomato halves by Zanoni et al. (1999). The relevant desorption isotherm at 25°C is shown in Fig. 1. GAB parameters of the isotherms for the dried tomato products tested were calculated by non-linear regression (Table 2). The fitting ability of GAB equations was fundamentally in agreement with the Lewicki (1997)
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Table 2 Estimated parameters of the GAB model Tomato products
nsm (g/g dm)
C
k
Correlation coefficient
Freeze-dried tomato pulp (adsorption at 20°C) Air-dried tomato pulp (desorption at 20°C) Air-dried tomato halves (desorption at 25°C) Freeze-dried insoluble solids-rich tomato (adsorption at 20°C)
0.117 0.087 0.076 0.045
5.86 9.75 5.73 26.83
1.013 0.995 1.021 0.942
0.999 0.999 0.998 0.999
analysis: good description of the isotherms when 5:67 6 C 6 1 and 0:24 < k 6 1. Fig. 1 shows a similar desorption behaviour between air-dried tomato pulp and tomato halves at desorption. Tomato pulp resulted to be more hygroscopic during adsorption than during desorption. Although the opposite (i.e., moisture sorption hysteresis) is usually observed, this phenomenon may be explained by the mildest conditions employed to dry samples for adsorption isotherm evaluation. Freeze-drying prevented thermal degradation and changes in the physical structure of the material. Hence, storage conditions of airdried tomatoes, optimized according to adsorption data obtained from freeze-dried samples, may be inconsistent; dried samples would thus seem to be more stable than they actually are as they would be less sensitive to moisture variations. Insoluble solids-rich tomato was the least hygroscopic of all tomato products. This may depend on removal of soluble solids such as fructose and glucose by centrifugation. As a result, this tomato product requires higher water removal during drying to reduce water activity. It is also more sensitive to moisture variations during storage. 3.2. Drying properties Fig. 2 shows the variation in drying rate as a function of the moisture content, with type of tomato products,
Fig. 2. Drying rate versus moisture content for: N, tomato pulp dried in a slab 20 mm in thickness at 70°C; r, tomato pulp dried in a slab 15 mm in thickness at 70°C; j, insoluble solids-rich tomato dried in a slab 10 mm in thickness at 60°C; , tomato halves 16 mm in thickness dried at 80°C; , tomato halves 16 mm in thickness dried at 110°C.
air temperature and product thickness as variables. The drying kinetics reported by Zanoni et al. (1999) was also included. Drying of all tomato samples moved quickly to a falling rate period, which determined the kinetics of the operation. In order to explain the role of operating conditions on tomato products drying, the drying kinetics was modelled. The experimental conditions applied allowed us to consider all tomato products tested as thin slabs subjected to drying on both sides for a long time in the absence of any significant external resistance. The following equation was then applied to model mass transfer during the falling rate period of drying (Perry & Green, 1984): ns nseq 8 ¼ exp ns0 nseq p2
D p 2t ; L 2
ð2Þ
where ns is the mean moisture on dry basis at time t; nseq and ns0 are the equilibrium and initial moisture contents on dry basis, respectively; L is the thickness of slab and D is the effective diffusivity of water. From Eq. (2) the effective diffusivity can be determined by the slope of the straight line obtained by plotting the experimental data from lnfð ns nseq Þ= ðns0 nseq Þg as a function of t=L2 . Table 3 shows the relevant values for variables in Eq. (2) for data processing. The values for drying air were determined by a psychrometric chart, and the values for nseq were determined by the above GAB equations. All the samples, except for the insoluble solids-rich tomato samples, showed a non-linear relationship on a semi-log plot (Fig. 3). Drying moved to a falling rate period, characterized by a two-stage phenomenon, where the effective water diffusivity of the second stage was bigger than that of the first one. This phenomenon does not usually occur; commonly, diffusivity decreases as drying proceeds (Brennan, 1994). Hawlader, Uddin, Ho, and Teng (1991) found a similar behaviour, when studying the drying kinetics of sliced tomatoes at different air temperatures and flow rates. This was ascribed to the shrinkage of samples during drying and, hence, to a reduction of thickness, resulting in faster water removal. Hawlader et al. (1991) proposed that L in Eq. (2) should be replaced with a modified thickness L0 , related to the moisture content by the following equation:
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Fig. 3. Relationship between lnfðns nseq Þ=ðns0 nseq Þg and t=L2 for: N, tomato pulp in a 20 mm slab; r, tomato pulp in a 15 mm slab; j, insoluble solids-rich tomato; , tomato halves dried at 80°C; , tomato halves dried at 110°C.
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Fig. 4. Comparison between experimental data (symbols) and predicted data (lines) on drying kinetics for: N, tomato pulp in a 20 mm slab;, tomato pulp in a 15 mm slab; r, insoluble solids-rich tomato; , tomato halves dried at 80°C; , tomato halves dried at 110°C.
Table 3 Characteristics of tomato products and air drying for experimental kinetic data processing Sample
L (mm)
ns0 (g/g dm)
Fresh air temperature (°C)
Fresh air relative humidity (%)
Drying air temperature (°C)
Drying air relative humidity (%)
nseq (g/g dm)
Tomato pulp Tomato pulp Tomato halves Tomato halves Insoluble solidsrich tomato
15 20 16 16 10
10.36 12.51 16.37 17.52 8.09
16 13 13 13 20
15 20 80 80 55
70 70 80 110 60
0.8 1 2.8 0.9 6.5
0.006 0.008 0.011 0.004 0.031
Table 4 Kinetic characteristics of tomato products Sample
n index
Effective water diffusivity, D (m2 /s)
Correlation coefficient, r
Tomato pulp dried in a slab 15 mm in thickness at 70°C Tomato pulp dried in a slab 20 mm in thickness at 70°C Insoluble solids-rich tomato dried in a slab 10 mm in thickness at 60°C Tomato halves 16 mm in thickness dried at 110°C Tomato halves 16 mm in thickness dried at 80°C
0.08 0.14 0
9:14 109 7:77 109 5:48 109
0.99 0.99 0.99
0.14 0.14
4:01 109 2:26 109
0.99 0.99
L0 ¼ L
mt m0
n ;
ð3Þ
where L is the thickness of slab; m0 and mt are the masses of sample at time zero and at drying time t, respectively; n is an index optimized to obtain a straight line for the experimental data from lnfð ns nseq Þ= ðns0 nseq Þg as a function of t=L02 . This equation was successfully applied to our experimental data, and the model was able to predict the drying kinetics by a single value for effective water diffusivity (Fig. 4). Data for n index, water diffusivity and the relevant correlation coefficient for straight lines are reported in Table 4.
In terms of drying properties, tomato products were distinguished as a function of their structure. The highest values for water diffusivity were determined for tomato pulps as a result of their liquid state and absence of skin, whereas the lowest values were determined for tomato halves. The effective water diffusivity values for tomato pulps were in agreement with those for products with similar structure such as fruit purees, as it can be derived by Mittal (1999) review. The values for tomato halves were in agreement with those of Hawlader et al. (1991) for slide tomatoes dried under similar operating conditions. The insoluble solids-rich tomato samples were placed between pulp and tomato halves; their value for water diffusivity was higher than that of tomato halves,
400 60 nd – 1900 70 1140 70 – 830 50 2170 160 11167 150 625 20 4620 350 – 3300 100 1108 14 – nd, not detectable.
850 50 2186 13 12202 24 94.4 0.1 92.6 0.1 89.7 0.4 Tomato halves Tomato pulp Insoluble solidsrich tomato
Dried
Lycopene content (mg/kg dm) Antioxidant activity of hydrophilic extract (I50 , lg dm) Ascorbic acid content (mg/kg dm) Antioxidant activity of lipophilic extract (I50 , lg dm) Lycopene content (mg/kg dm)
Fresh Samples
Table 5 Antioxidant properties of tomato products
Table 5 shows the antioxidant properties of the different types of fresh and dried tomato products. Experimental data for tomato halves air-dried at 80°C by Lavelli, Hippeli, Peri, and Elstner (1999) were included. Experimental data provided a general survey on the beneficial effect of tomato products. This effect is not exclusively related to either a single component or the antioxidant effectiveness in a single model system. Data on fresh products showed peculiar properties of the insoluble solids-rich tomato samples, which had a much higher lycopene content than tomato halves (ca. 14 times higher) and tomato pulp (ca. six times higher). This resulted in a higher antioxidant activity of the lipophilic extract, which was ca. five times higher than that of tomato halves and ca. two times higher than that of tomato pulp. Since data confirmed that lycopene and lipophilic antioxidant activity had a high stability during air-drying (Zanoni et al., 1999) for all types of products tested, insoluble solids-rich dried tomato can be an interesting product as a food ingredient, useful for its antioxidant and colouring properties. A different behaviour was observed for the ascorbic acid content and the antioxidant activity of the hydrophilic extract. Data confirmed that ascorbic acid was very sensitive to oxidative heat damage. Tomato pulp has been found to have a lower ascorbic acid content than tomato halves, due to the damage occurring during processing (Giovanelli et al., 2000). A considerable loss of ascorbic acid has been observed after air-drying of both tomato halves and tomato pulp (Zanoni et al., 1999). Ascorbic acid was not determined on the insoluble solids-rich tomato, since it was removed together with the serum. Data on the antioxidant activity of the hydrophilic extract showed a more complex behaviour. Values for fresh product reflected the differences in ascorbic acid values between samples, but this phenomenon did not occur after air-drying of tomato pulp. Although no ascorbic acid was present in dried tomato pulp, the relevant antioxidant activity was not significantly different from that of fresh samples. We were not able to explain this phenomenon clearly; it may be assumed that other hydrophilic compounds with an antioxidant activity became available during drying of tomato pulp. Some authors (Giovanelli et al., 2000; Stewart et al., 2000) noted that heat treatments increased the level of free flavonols, which have a considerable antioxidant activity.
Antioxidant activity of lipophilic extract (I50 , lg dm)
3.3. Antioxidant properties
1900 100 920 30 378 38
Ascorbic acid content (mg/kg dm)
Antioxidant activity of hydrophilic extract (I50 , lg dm)
although the drying air temperature was much lower (i.e., 60°C compared to 80°C and 110°C). Table 4 also shows the well-known effects of both thickness reduction (see tomato pulps) and temperature increase (see tomato halves) on the increase of water diffusivity.
1200 50 3950 290 –
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Moisture content (%)
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