Modelling the stability of lycopene-rich by-products of tomato processing

Food Chemistry 125 (2011) 529–535

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Modelling the stability of lycopene-rich by-products of tomato processing Vera Lavelli ⇑, Maria Claudia Torresani DISTAM, Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Università degli Studi di Milano, via Celoria 2, 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 3 May 2010 Received in revised form 27 July 2010 Accepted 8 September 2010

Keywords: Tomato By-product Water activity Colour Lycopene

a b s t r a c t Tomato by-products were produced by puree manufacturing from heat-stabilised fruits and raw fruits to simulate both conventional and innovative processing technologies. By-products were freeze-dried, ground and stored in five relative humidity environments in the range 11–75%, for 4 months at 30 °C. The aims were: (a) to investigate the effect of heating applied during tomato processing on by-product hygroscopicity and stability, (b) to find out the optimal water activity (aw) range for by-product stability. Hygroscopicity was studied by applying the Guggenheim–Anderson–de Boer (GAB) model. By-product stability was studied by evaluating the kinetics of lycopene, b-carotene, rutin and chlorogenic acid degradation and the changes in Hunter’s colourimetric parameters during storage. By-products obtained from heat-stabilised fruit and raw fruits had the same hygroscopicity, with an average estimated nsm value of 0.080 ± 0.013 kg water/kg dry solids, corresponding to the mean aw of 0.44 and the confidence interval of 0.31 < aw < 0.51 (on the 95% probability level). During storage, in both by-products the rate constant for lycopene degradation was maximum at the aw level of 0.17 (half-life time was 38 d); it then decreased by more than threefold with increasing the aw level up to 0.75 (halflife time was 138 d). b-Carotene degradation rates had the same order of magnitude as those of lycopene and decreased with increasing the aw level in the heat-treated by-product. However, b-carotene degradation was accelerated at aw levels P0.56 in the by-product obtained from raw fruits, suggesting the involvement of lipoxygenase. Chlorogenic acid and rutin were more stable than the carotenoids and showed an opposite dependence of their stability on the aw level, being significantly degraded only at the highest aw level. The degradation of these phenolics was higher in the by-product obtained from raw fruits, indicating the likely involvement of polyphenol oxidase. The colour difference DE represented the sum of different degradation processes, indicating that for maximum stability, i.e. minimal DE variation, a dehydration level corresponding to 0.22 6 aw 6 0.56, has to be achieved and then maintained by preventing moisture exchanges with both environment and other food components. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Industrial processing of tomato generates a considerable amount of waste, consisting of peel, seeds and a part of the pulp, which are known as tomato pomace. These residues contain valuable nutritional compounds, mainly fibres (59% d.w.), proteins (19% d.w.) and antioxidants. In fact a variety of epidemiological trials have suggested that the higher intake of lycopene-containing foods such as tomato products or blood lycopene concentrations are associated with decreased cardiovascular disease and prostate cancer risk (Reboul et al., 2005). Most of the lycopene content of tomato is associated with the water insoluble fraction. In fact, tomato skin has 2.5 times higher lycopene level than the pulp, with significant amounts of phenolics and ascorbic acid (George, Kaur, Khurdiya, & Kapoor, 2004). Hence, tomato pomace has the potential of being

⇑ Corresponding author. Tel.: +39 2 50319172; fax: +39 2 50316632. E-mail address: [email protected] (V. Lavelli). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.09.044

recovered and used for food applications. A recovery strategy would provide various benefits. Firstly, since these residues accumulate in large amounts during the tomato ripening season, they cause pollution problems. Secondly, producers could find advantages from decreasing disposal costs of waste and the opportunity of extra-income. Finally, the consumers would take advantage of some valuable compounds that are present in the whole tomato, which could be reintroduced into food. A conventional method for tomato processing generates tomato pomace at three stages: tomatoes are first chopped and heated to 60–65 °C (cold break) or 90–95 °C (hot break). The resulting pulp is then passed through a pulper (producing tomato pomace 1) and a finisher (producing tomato pomace 2). Tomato pomace (1 and 2) is then extracted by a turbopress for further juice extraction (producing tomato pomace 3). The tomato juice obtained is concentrated in continuous evaporators up to the desired dry residue concentration and sterilised. Residues collected at different stages of tomato processing such as after the pulper, the finisher, and the turbopress do not differ greatly in composition (Del Valle, Camara,

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& Torija, 2006). An innovative plant for fruit puree processing, operating the pulping/finishing steps on raw fruits at room temperature, thus producing an unheated pomace, has been developed (Germini, Paschke, & Marchelli, 2007; Lavelli, Pompei, & Casadei, 2008; Sanchez-Moreno, Plaza, de Ancos, & Cano, 2004). Alternative strategies have been proposed for the exploitation of tomato pomace. Firstly, tomato skins and seeds need to be separated for their further utilisation. To this aim, a pilot-scale flotation-cum-sedimentation system has been developed (Kaur, Sogi, Garg, & Bawa, 2005). Tomato seeds can be processed for oil extraction and protein recovery (Sogi, Bathia, Garg, & Bawa, 2005). Tomato skin is considered as a useful source of lycopene. Besides serving as a micronutrient with important health benefits, lycopene is an excellent natural food colourant. Extraction procedures of lycopene from tomato waste by employing enzyme (Choudhari & Ananthanarayan, 2007), solvents (Kaur, Wani, Oberoi, & Sogi, 2008) and supercritical CO2 (Nobre, Palavra, Pessoa, & Mendes, 2009) have been described. An alternative approach is the use of whole tomato skin powder. Enrichment of tomato paste with tomato peel is an interesting option for increasing carotenoid intake (Reboul et al., 2005). Tomato skin powder was incorporated into refined oils for carotenoid solubilisation in view of upgrading low quality oils (Benakmoum, Abbeddou, Ammouche, Kefalas, & Gerasopoulos, 2008). The skin powder also provides proteins, cellulose and pectins, thus representing a good candidate to be used to modulate water sorption and rheological properties of food. Indeed the use of skin powders in the formulation of ketchup, improves its textural properties (Farahnaky, Arahnaky, Abbasi, Jamalian, & Mesbahi, 2008). The direct addition of tomato peel to dry fermented sausages has been proposed to obtain a new product enriched in lycopene with good textural properties (Calvo, Garcia, & Selgas, 2008). Knowledge on the stability of tomato pomace obtained from the conventional and innovative processes is lacking. The fresh pomace has a high moisture content that makes it susceptible to microbial proliferation. For that reason, pomace is preserved by drying (Kaur et al., 2008). On the other hand, oxidation and browning reactions are the major causes of degradation of dried and intermediate moisture foods (Brennan, 1994; Lavelli, Zanoni, & Zaniboni, 2007; Rahman, 1995). As a result, knowledge on the rate of carotenoid and phenolic degradation and colour changes in dry and intermediate moisture tomato by-products, would be helpful in order to plan potential food uses of these products. In this study, both raw and heat-treated tomato fruits were processed into puree and the residues were collected, separated from seeds and then freeze-dried. The stability of the by-product powders was studied by evaluating the kinetics of colour changes and carotenoid and phenolic degradation during storage at 30 °C as a function of aw. The aims were: (a) to investigate the effect of heating applied during tomato processing on by-product stability, and (b) to find out the optimal aw range for by-product stability.

2. Materials and methods 2.1. Tomato samples Mature tomatoes (10 kg, var. Ikram) were washed with tap water and cut using a K 3000 Braun Multisystem apparatus (Braun, Kronberg, Germany). The resulting pieces were separated in two equal parts. One of them was heated for 3 h at 100 °C at atmospheric pressure in an open boiler with moderate agitation, passed through a screw extractor (model 9008 Reber, Luzzara, Italia) and the heat-treated pomace was recovered. The remaining half-batch was directly passed through the screw extractor, and the unheated pomace was recovered. Both unheated and heat-treated pomace

samples were separately dispersed in 1% citric acid (1:1 w/w), to allow seed separation by flotation, and to keep the pH below 4.0 (Muratore, Rizzo, Licciardello, & Maccarone, 2008). Dispersions were then centrifuged (6000 rpm, 12 min, at room temperature) and the supernatants were eliminated. Insoluble residues obtained from unheated and heat-treated pomace samples were spread on stainless steel trays to a thickness of about 1 cm and frozen in a chamber at 45 °C. After 24 h, they were transferred to a precooled freeze-drier. Freeze-drying was performed at 45 °C for 8 h, 20 °C for 24 h, 0 °C for 24 h, and 10 °C for 10 h. The chamber pressure was maintained at 0.3 mbar throughout the drying process. A Lyoflex Edwards (Crawley, UK) freeze-drier was used. 2.2. Chemicals Rutin (quercetin 3-O-rutinoside) and chlorogenic acid were purchased from Extrasynthese (Lyon, France). All other chemicals were purchased from Sigma–Aldrich (Milan, Italy). 2.3. Methods 2.3.1. Storage study Freeze-dried tomato by-products were ground into powder with a model K 3000 Braun Multisystem blender and sieved (800 lm). The powders were first weighed into Petri dishes (6 cm diameter, 5.5 g of product in each dish). The dishes were then placed into air-tight plastic boxes on wire-mesh racks situated above saturated salt solutions. The boxes were stored for 131 d at 30 °C in a thermostated cabinet. To create different relative humidity environments the following saturated salt solutions were used: LiCl (aw at 30 °C = 0.113 ± 0.002), CH3COOK (aw at 30 °C = 0.216 ± 0.005), MgCl2 (aw at 30 °C = 0.324 ± 0.002), NaBr (aw at 30 °C = 0.560 ± 0.004), NaCl (aw at 30 °C = 0.7509 ± 0.0011). Duplicate boxes were incubated for each aw level. 2.3.2. pH and acidity Freeze-dried tomato by-products were diluted with water (0.5 g of powder in 20 ml, in duplicate). The pH was determined with a model 62 pH meter (Radiometer, Copenhagen, Denmark). Titratable acidity was determined by titration with 0.1 M NaOH to pH 8.1. Results were expressed as grams of citric acid per 100 g of dry product. 2.3.3. Moisture content and aw Moisture content of tomato by-products after freeze-drying and after equilibration with the various relative humidity environments was determined using a vacuum oven at 70 °C and 50 Torr for 18 h. The aw of tomato by-products was determined every 2 d up to 12 d of incubation and then every 6 d up for 131 d. The aw of samples and saturated salt solutions was measured at the beginning and at the end of incubation. A dew point hygrometer (Aqualab, Decagon Devices, WA, USA) was used. Duplicate determinations were made for each sample. 2.3.4. Modelling of sorption isotherm The Guggenheim–Anderson–de Boer (GAB) equation was applied to model experimental data for ns as a function of aw. The GAB model is expressed as indicated in Eq. (1):

ns ¼ nsm Ckaw =½ð1  kaw Þð1  kaw þ Ckaw Þ

ð1Þ

where ns is the equilibrium moisture content on dry basis (kg water/kg dry solids); nsm is the monolayer moisture content on dry basis; C and k are related to the temperature effect (Brennan, 1994).

V. Lavelli, M.C. Torresani / Food Chemistry 125 (2011) 529–535

2.3.5. Colour Colour evaluation of samples stored in various relative humidity environments was performed in triplicate at 0, 6, 13, 21, 29, 53, 69, 78 and 131 d of incubation. Colour was measured with a Chroma meter II (Konica Minolta, Osaka, Japan), which provides the Hunter L*, a*, and b* coordinates, representing: lightness and darkness (L*), redness (+a*), greenness (a*), yellowness (+b*), and blueness (b*). The chromameter was calibrated with a white standard. The Petri dishes containing the tomato samples were covered with a glass cover and the head of the colourimeter was put directly on top of the glass to take colour measurements. To study the total variation in colour over time, colour difference, namely DE was calculated, as indicated in Eq. (2): 



DE ¼ ½ða  a0 Þ2 þ ðb  b0 Þ2 þ ðL  L0 Þ2 1=2

ð2Þ

where a*0, b*0, and L*0 are the values of the colourimetric parameters of the sample at the beginning of storage and a*, b*, and L* are the colourimetric parameters at a given time (Barrerio, Milano, & Sandoval, 1997). 2.3.6. HPLC equipment The HPLC equipment consisted of a model 600 HPLC pump coupled with a Waters model 2996 photodiode array detector, operated by Empower software (Waters, Vimodrone, Italy). 2.3.7. Carotenoids Tomato by-products were extracted in two-steps according to Khachik, Beecher, and Whittaker (1986) by using tetrahydrofuran (THF) stabilised by the addition of 0.1% butylated hydroxytoluene (2,6-di-tert-butyl-p-cresol, BHT). Aliquots of tomato by-products of 0.125 g dry weight were added to 10 ml of stabilised THF. The mixture was vortexed for 1 min and centrifuged (12000g at 5 °C for 10 min). The supernatant was recovered into a 25-ml flask. Residual solids were added again with 10 ml of stabilised THF. The mixture was vortexed for 1 min, kept under magnetic stirring for 30 min and then centrifuged (12000 g at 5 °C for 10 min). The second clarified THF extract was combined with the first one into the 25-ml flask and brought up to 25 ml with stabilised THF. Extractions were carried out in triplicate on samples at the beginning of incubation and in duplicate on samples stored in different relative humidity environments for 7, 49 and 131 d. Carotenoid content was analysed by HPLC as described previously (Lavelli, Peri, & Rizzolo, 2000). In brief, a Vydac 201TP54 C-18 column (250 mm  4.6 mm), equipped with a C-18 precolumn, was used. Chromatographic separation was performed with methanol/stabilised THF (95:5) as an eluent under isocratic conditions, 1.0 ml/min flow rate, at room temperature. UV–Vis detector was set at 454 nm. Lycopene and b-carotene were quantified from calibration curves built with pure standards, and expressed as milligrams per kilogram of dry product. 2.3.8. Phenolics Tomato by-products were extracted with methanol. Aliquots of tomato by-products of 0.25 g dry weight were added to 10 ml of methanol. The mixtures were vortexed for 1 min, kept under magnetic stirring for 30 min and then centrifuged (12000g at 5 °C for 10 min). Extractions were carried out in triplicate on samples at the beginning of incubation and in duplicate on samples stored in different relative humidity environments for 7, 49 and 131 d. The phenolic contents of methanolic extracts were analysed according to the HPLC procedure by Tomás-Barberán et al. (2001) with modifications. A 250  4.6 mm i.d, 5 l, Symmetry reverse phase C-18 column (Waters, Vimodrone, Italy) equipped with a Symmetry C-18 precolumn was used. Formic acid (5%) was added to both methanol and water before preparing the following mobile

531

phase: water/methanol (95:5, v/v) (A); water/methanol (88:12, v/v) (B); water/methanol (20:80, v/v) (C); and methanol (D). The following gradient elution was used: 0–5 min, 100% A; 5–10 min linear gradient to reach 100% B; 10–13 min 100% B; 13–35 min linear gradient to reach 75% B and 25% C; 35–50 min linear gradient to reach 50% B and 50% C; 50–52 min linear gradient to reach 100% C; 52–57 min 100% C; 57–60 min 100% D. Injection volume was 20 ll. Flow rate was 1 ml/min. Standards of chlorogenic acid and rutin were used to identify peaks by retention times and UV–Vis spectra, and to build calibration curves for quantification, and concentrations were expressed as milligrams per kilogramme of dry product. 2.3.9. Statistical analysis of data Experimental data were processed by one-way ANOVA using the least significant difference (LSD) as a multiple range test, and by linear and non-linear regression analyses using Statgraphics 5.1 (STCC Inc.; Rockville, MD). 3. Results and discussion 3.1. Moisture content, pH, acidity and sorption characteristics of tomato by-products Moisture contents of by-products obtained from processing of both raw and heat-treated tomatoes were 92.7 ± 0.1 and 90 ± 0.2, respectively. High moisture contents, ranging from 66% to 93%, were also found in tomato pomace samples recovered from different industrial processes, hence these products are unstable (Del Valle et al., 2006). In this study, tomato by-products had the same pH of 3.52 ± 0.07, which is below the critical threshold for the growth of pathogenic microorganisms. Acidity was 8.7 ± 0.3 g citric acid/ 100 g d.w. By-product hygroscopicity was studied in the aw range between 0.11 and 0.75 under adsorption conditions, at 30 °C. By-products had the same hygroscopic behaviour irrespectively of heating applied during tomato processing. The samples stored at relative humidity levels P32% reached the equilibrium aw after 6 d of incubation, whereas those stored at a relative humidity of 22% reached the equilibrium aw after 12 d of incubation. The samples stored at 11% relative humidity did not reach the equilibrium aw level of 0.11, most likely due to their high hygroscopicity. These samples reached an aw level of 0.17, which remained unchanged after prolonging incubation up to 131 d. As the first step, data for aw as a function of ns were fitted with the GAB equation for each tomato by-product. The GAB parameters did not show significant differences between by-products obtained from processing either raw or heat-treated tomatoes (data not shown). Therefore in the second step, data were processed together (R2 was 98.6%) and the average values for the GAB parameters were obtained (Fig. 1). The values of nsm, C, and k are calculated by regression analysis of Eq. (1). The estimated value of nsm was 0.080 ± 0.013 kg water/kg dry solids, corresponding to the mean aw of 0.44 and the confidence interval of 0.31 < aw < 0.51 (on the 95% probability level). C and k were 2.2 ± 0.5 and 0.915 ± 0.05, respectively. Iglesias and Chirife (1982) found a nsm value of 0.060 kg water/kg dry solids for tomato under absorption conditions in the aw range 0.1–0.8, at 27 °C. Our result does not match this nsm value, which however is within the 95% confidence level found in this study. The observed difference is most likely attributable to the different composition of tomato by-products considered in this study with respect to the tomato pulp considered by Iglesias and Chirife (1982).

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strongly bound to specific sites on the food solids (Brennan, 1994). Though nsm is reputed to be a valuable index to predict food oxidative stability, a direct evaluation of quality indices variation as a function of aw is of importance to validate this hypothesis.

ns (kg water/kg dry solids)

0.25 0.2 0.15

3.2. Modelling of carotenoid degradation

0.1 0.05 0 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80

aw Fig. 1. Water sorption properties of tomato by-products: (d), experimental data for freeze-dried tomato samples equilibrated over saturated salt solutions at 30 °C; (—), adsorption isotherm obtained by fitting experimental data with GAB model. Results obtained from by-products produced by processing both raw tomatoes and heattreated tomatoes were averaged. Error bars represent SD.

The identification of nsm is of importance since the maximum stability of a lipid-containing food generally occurs at the monomolecular moisture content, in which a monolayer of water is

Table 1 Antioxidant contents (average ± SD) of tomato by-products after freeze-drying.

From heat-treated tomato

116 ± 7 6490 ± 20 26 ± 2 121 ± 2

106 ± 7 7390 ± 70 17 ± 1 97 ± 1

ln (C/Co) lycopene

b-Carotene Lycopene Rutin Chlorogenic acid

By-product From raw tomato

0

0

-0.5

-0.5

-1

-1

-1.5

-1.5

ln (C/Co) rutin

Antioxidant (mg/kg d.w.)

The contents of lycopene and b-carotene in tomato by-products are shown in Table 1. The all-trans isomers of these carotenoids were detected, which are the most predominant isomers in fresh tomato and its derivatives (Shi & Le Maguer, 2000). With respect to the whole tomato, tomato pomace contains a higher amount of lycopene and a lower amount of b-carotene, since the distribution of these carotenoids in the fruit is different. In fact, lycopene content is higher in the outer pericarp whereas b-carotene content is higher in the locule (Shi & Le Maguer, 2000). The by-product obtained from heat-treated tomatoes had a slightly lower b-carotene content and a higher lycopene content than that obtained from raw tomatoes. It is well known that lycopene has a high thermal stability and that thermal treatments enhance its extraction from the matrix (Kaur et al., 2008; Shi & Le Maguer, 2000). On a wet weight basis, the lycopene contents of by-products considered in this study were 474 (raw tomato by-product) and 739 mg/kg (heat-treated tomato by-product). These values have the same order of magnitude as those reported by Choudhari and Ananthanarayan (2007) for tomato waste obtained on both lab-scale and industrial-scale, using an optimised enzyme-aided procedure. As expected, the lycopene content in tomato by-product was 2–3 higher than that of whole tomato, due to lycopene association to the insoluble fibre fraction. Carotenoids are stable in tomato juice stored at room temperature up to 180 d (Ordóñez-Santos, Vázquez-Odériz, ArbonesMaciñeira, & Romero-Rodríguez, 2009). On the contrary, carotenoids

-2 -2.5

-2.5

-3

-3

-3.5

-3.5 -4

-4 0

50

100 time (d)

150

0

0

-0.5

-0.5

0

50

0

50

100 time (d)

150

-1

-1

ln (C/Co) rutin

ln (C/Co) lycopene

-2

-1.5 -2 -2.5

-1.5 -2 -2.5

-3

-3

-3.5

-3.5

-4 0

50

100 time (d)

150

-4 100

150

time (d)

Fig. 2. Time course of the degradation of lycopene and rutin in tomato by-products obtained from raw fruit processing (upper) and heat-treated fruit processing (lower) during storage at aw levels of 0.17 (j), 0.32 (d) and 0. 75 (N), at 30 °C. Data were fitted to first-order kinetics. Error bars represent SD.

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V. Lavelli, M.C. Torresani / Food Chemistry 125 (2011) 529–535 Table 2 Rate constants (average ± SE) for lycopene and b-carotene degradation in tomato byproducts stored at different aw levels at 30 °C, as calculated by assuming first-order kinetics (lnC = lnC0 + kt). aw

Colourimetric parameter

By-product From raw tomato

0.17 0.22 0.32 0.56 0.75

Table 4 Hunter colourimetric parameters (average ± SD) for tomato by-products after freezedrying.

From heat-treated tomato

Lycopene k ± SE (d1)

b-Carotene k ± SE (d1)

Lycopene k ± SE (d1)

b-Carotene k ± SE (d1)

0.017 ± 0.0012 0.010 ± 0.0005 0.007 ± 0.0005 0.006 ± 0.0004 0.005 ± 0.0008

0.016 ± 0.0017 0.015 ± 0.0020 0.015 ± 0.0020 0.022 ± 0.0007 0.017 ± 0.0006

0.019 ± 0.0012 0.010 ± 0.0015 0.009 ± 0.0011 0.005 ± 0.0008 0.005 ± 0.0008

0.013 ± 0.0020 0.014 ± 0.0030 0.012 ± 0.0006 0.009 ± 0.0003 0.010 ± 0.0006

Absolute values for the correlation coefficients were >0.94, P < 0.01.

are unstable in low-moisture tomato products. It has been hypothesised that the predominant all-trans isomers reversibly isomerises to more oxidizable cis isomers; in parallel both cis and trans isomers autooxidise forming volatile fragments (Anguelova & Warthersen, 2000). Accordingly in this study, during storage of tomato by-products at various aw levels at 30 °C, lycopene and b-carotene contents progressively decreased. Cis-isomers did not accumulate, this means that autoxidation of either the all-trans form or cis isomer intermediates was the major pathway for degradation. In this study, carotenoid degradation was modelled by assuming first-order kinetics, as already reported (Sharma & Le Maguer, 1996). The behaviour of lycopene is provided as an example in Fig. 2. The rate constants for carotenoid degradation are shown in Table 2. Regarding lycopene, the same dependence of the degradation rate from the aw level was observed for the by-products obtained from raw and heat-treated tomatoes: the rate constant was maximum at the aw level of 0.17 (half-life 38 d); it then decreased by more than threefold with increasing the aw level up to 0.75 (halflife 138 d). The fact that lycopene degradation was the same irrespectively of the heating applied, indicated that this carotenoid was not involved in enzymatic oxidation by lipoxygenase, either as a substrate or as an indirect co-oxidated substrate. Consistently with this result, a previous study has demonstrated that lycopene is resistant to oxidation by lipoxygenase (Cabibel & Nicolas, 1991). In a previous study, lycopene was encapsulated with polymeric carriers to enhance its stability and to control its release when ingested in the body (Chiu et al., 2007). Freeze-dried encapsulated lycopene (aw not specified) has been found to degrade during storage, with a first-order rate constant of 0.021 d1 at 30 °C (this datum was extrapolated from the Arrhenius equation provided by Chiu et al. (2007)). Interestingly, this result is consistent with the degradation rate found in this study for the tomato by-product stored at the lowest aw. In order to compare the extent of lycopene degradation observed in the present study with the results of other literature studies, the percent retention of lycopene was calculated after

a* b* L*

By-product From raw tomato

From heat-treated tomato

24.9 ± 0.5 26.3 ± 0.6 61.9 ± 0.5

23.8 ± 0.5 30.9 ± 0.4 59.3 ± 0.5

1 month of storage, from the degradation rate constants reported in Table 2. Lycopene retention after 1 month of storage at 30 °C was 56% in the by-products stored at the most unfavourable aw level of 0.17, whereas it increased up to about 84% in the products stored at 0.32 6 aw 6 0.75. Shi and Le Maguer (2000) found that residual lycopene in tomato powder (aw not specified), after 30 d of storage at 20 °C was 46% under air and 90% under nitrogen. Davoodi, Vijayanand, Kulkarni, and Ramana (2007) have found 83% of residual lycopene in dried tomato stored at 20 °C, whereas the retention percentage increases to 96% after pre-treatment with K2S2O and CaCl2 before drying. This study pointed out the remarkable effect of aw on lycopene degradation rate. It may therefore be hypothesised that maximum lycopene stability could be achieved by combining both optimal pre-treatment and encapsulation process with optimal storage aw. Regarding b-carotene, the degradation rates in tomato byproducts had the same order of magnitude as those of lycopene. For b-carotene, the degradation rate was dependent on heating applied during fruit processing. In the by-product obtained from raw tomatoes, an increase in b-carotene degradation was observed at aw levels P0.56, with respect to the by-product obtained from heat-treated tomatoes. This result suggests the involvement of lipoxygenase, whose threshold aw for catalytic activity is 0.5 (Rahman, 1995). In fact it has been demonstrated that b-carotene is co-oxidated during the lipoxygenase-catalysed reaction, while lycopene is resistant (Cabibel & Nicolas, 1991). 3.3. Modelling of phenolic degradation The contents of chlorogenic acid and rutin in tomato by-products (Table 1) were low with respect to the amounts present in the whole fruit (Slimestad & Verheul, 2009). In this study, a part of phenolics was probably extracted in tomato puree, and another part could have been lost during seed separation by flotation, which is a necessary step for tomato skin utilisation. In addition, the heat-treated tomato by-product had a lower amount of phenolics with respect to the raw tomato by-product, indicating that intense heating was another reason for phenolic loss. During storage of tomato by-products at various aw levels at 30 °C, chlorogenic acid and rutin were more stable than the carotenoids and showed an opposite dependence of their stability on the aw level. In fact, chlorogenic acid and rutin contents decreased significantly only at the highest aw level. The degradation of these

Table 3 Rate constants (average ± SE) for chlorogenic acid and rutin degradation in tomato by-products stored at different aw levels at 30 °C, as calculated by assuming first-order kinetics (lnC = lnC0 + kt). aw

By-product From raw tomato

0.17–0.56 0.75

From heat-treated tomato

Rutin k ± SE (d1)

Chlorogenic acid k ± SE (d1)

Rutin k ± SE (d1)

Chlorogenic acid k ± SE (d1)

n.s. 0.0066 ± 0.0013

n.s. 0.0028 ± 0.0003

n.s. 0.0017 ± 0.0007

n.s. 0.0008 ± 0.0002

Absolute values for the correlation coefficients were >0.93, P < 0.01. n.s. = not significant.

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phenolics was analysed by assuming first-order kinetics (Fig. 2 and Table 3). The first-order rate constants for both rutin and chlorogenic acid degradation were higher in the by-product obtained from raw tomatoes than in that obtained from the heat-treated tomatoes. This result indicates the likely involvement of polyphenol oxidase in phenolic degradation.

control aw 0.17 aw 0.22 aw 0.32

aw 0.56 aw 0.75

3.4. Modelling of colour changes The Hunter colourimetric parameters of freeze-dried tomato by-products are shown in Table 4. The by-product obtained from processing of heat-treated tomatoes had a different colour with respect to that obtained from raw tomatoes, as demonstrated by higher b* value and lower a* and L* values. Changes in the colourimetric parameters are in general fitted with zero or first-order kinetics (Barrerio et al., 1997). In this study during storage of the by-products at different aw levels, changes in the colourimetric parameters were best fitted by zero-order kinetics (Fig. 3). The rate constants for L*, a* and b* variations are reported in Table 5. Both tomato by-products showed similar trends for the variation of the colourimetric parameters. The lightness index L* increased at the lowest aw levels, namely 0.17 and 0.22, most likely due to lycopene degradation. At the aw levels 0.32 and 0.56, L* did not change significantly. At the highest aw level of 0.75 L* decreased, especially in the by-product obtained

a*

60 50 40 30

40

20

35

35

18

30

30

25

25

20

50

100

15

8

10

6

5

5

150

50

150

0

50

60 50 40 30 100

150

100

0

150

0

50

time (d)

100

150

100 time (d)

150

time (d)

40

40

20

35

35

18

30

30

25

25

16 14

b*

a*

70

L*

100 time (d)

80

50

4 2

0 0

90

10

10

time (d)

0

12

20

15

0 0

16 14

20

12 ΔE

L*

70

40

ΔE

80

from raw tomato processing, probably due to browning related to phenolic oxidation, as discussed in the previous paragraph. The redness index a* decreased during storage, with minimal variation in the aw interval between 0.22 and 0.56. On the contrary, the yellowness index b* increased especially at the aw levels where lycopene degradation was maximum. In the by-product obtained from heat-treated tomatoes, the variations of the colourimetric parameters were in general lower with respect to those obtained from raw tomatoes, however the initial colour was also different.

b*

90

Fig. 4. Pictures of freeze-dried by-products produced from raw tomato processing (upper) and heat-treated tomato processing (lower), taken at time 0 (control) and after 131 d of storage at aw levels between 0.17 and 0. 75, at 30 °C.

20

10

15

15

8

10

10

6

5

5

4 2

0

0 0

time (d)

50

100

150

0

50

100

150

0 0

time (d)

time (d)

50

Fig. 3. Time course of the variations of the colourimetric parameters L*, a* and b* and of DE in tomato by-products obtained from raw fruit processing (upper) and heattreated fruit processing (lower) during storage at aw levels of 0.17 (j), 0.32 (d) and 0. 75 (N), at 30 °C. Data for L*, a* and b* were fitted to zero-order kinetics. Error bars represent SD.

Table 5 Rate constants (average ± SE) for colourimetric parameters’ degradation in tomato by-products stored at different aw levels at 30 °C, as calculated by assuming zero-order kinetics (C = C0 + kt). aw

By-product From raw tomato

0.17 0.22 0.32 0.56 0.75

From heat-treated tomato

L* k ± SE (UC  d1)

a* k ± SE (UC  d1)

b* k ± SE (UC  d1)

L* k ± SE (UC  d1)

a* k ± SE (UC  d1)

b* k ± SE (UC  d1)

0.055 ± 0.007 0.019 ± 0.005 n.s. n.s. 0.053 ± 0.005

0.089 ± 0.006 0.058 ± 0.005 0.063 ± 0.005 0.069 ± 0.005 0.097 ± 0.007

0.059 ± 0.005 0.064 ± 0.004 0.052 ± 0.005 0.034 ± 0.008 n.s.

0.049 ± 0.005 0.026 ± 0.004 n.s. n.s. 0.027 ± 0.003

0.067 ± 0.004 0.049 ± 0.004 0.045 ± 0.005 0.056 ± 0.004 0.066 ± 0.004

0.032 ± 0.006 0.033 ± 0.006 0.033 ± 0.005 0.020 ± 0.008 n.s.

Absolute values for the correlation coefficients were >0.90, P < 0.01. n.s. = not significant.

V. Lavelli, M.C. Torresani / Food Chemistry 125 (2011) 529–535

The total colour difference, namely DE, takes into account the variations of all the colourimetric parameters over time with respect to their corresponding values at time 0. In both by-products the minimum DE variation, i.e. maximum stability occurred in the aw interval from 0.22 to 0.56. The pictures of the by-products show that below this range product discolouration occurred, whereas above it browning was evident (Fig. 4). 4. Conclusions The most promising uses for high-lycopene powders, such as tomato by-product powders, are as food colouring and antioxidant agents. To this regard, this study demonstrated that, besides well known factors affecting the oxidation rate of lycopene-rich products, such as light and oxygen exposure, control of aw also has a remarkable effect. For maximum stability, i.e. minimal DE variation, a dehydration level corresponding to 0.22 6 aw 6 0.56, has to be achieved and then maintained by preventing moisture exchanges with both environment and other food components. The observed aw range for maximum by-product stability was close to the monomolecular moisture content (aw = 0.44), thus resembling the stability trend for dehydrated foods containing unsaturated lipids. In tomato byproducts the nature of the degradation phenomena was different according to the aw level. Below the stability interval, the main degradation was lycopene oxidation, whereas above it degradation involved polyphenol oxidation. The tomato by-product obtained from processing the raw fruit simulating an innovative process, had a similar stability to that obtained from the heat-treated fruit, but it was less stable at aw P 0.56. References Anguelova, T., & Warthersen, J. (2000). Lycopene stability in tomato powders. Journal of Food Science, 65, 67–70. Barrerio, J. A., Milano, M., & Sandoval, A. J. (1997). Kinetics of colour change of double concentrated tomato paste during thermal treatment. Journal of Food Engineering, 33, 359–371. Benakmoum, A., Abbeddou, S., Ammouche, A., Kefalas, P., & Gerasopoulos, P. D. (2008). Valorisation of low quality edible oil with tomato peel waste. Food Chemistry, 110, 684–690. Brennan, J. G. (1994). Water activity and food quality. In G. Campbell-Platt (Ed.), Food dehydration. A dictionary and guide (pp. 129–131). Oxford: ButterworthHeinemann Ltd.. Cabibel, M., & Nicolas, J. (1991). Lipoxygenase from tomato fruit (Lycopersicon esculentum). Partial purification, some properties, and in vitro cooxidation of some carotenoid pigments. Sciences des Aliments, 11, 277–290. Calvo, M. M., Garcia, M. L., & Selgas, M. D. (2008). Dry fermented sausages enriched with lycopene from tomato peel. Meat Science, 80, 167–172. Chiu, Y. T., Chiu, C. P., Chien, J. T., Ho, G. H., Yang, J., & Chen, B. H. (2007). Encapsulation of lycopene extract from tomato pulp waste with gelatin and poly(c-glutamic acid) as carrier. Journal of Agricultural and Food Chemistry, 55, 5123–5130. Choudhari, S. M., & Ananthanarayan, L. (2007). Enzyme aided extraction of lycopene from tomato tissues. Food Chemistry, 102, 77–81. Davoodi, M. G., Vijayanand, P., Kulkarni, S. G., & Ramana, K. V. R. (2007). Effect of different pre-treatments and dehydration methods on quality characteristics

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