Antioxidant capacity and colour of strawberry jam as influenced by cultivar and storage conditions

ARTICLE IN PRESS LWT 38 (2005) 387–391 www.elsevier.com/locate/lwt Antioxidant capacity and colour of strawberry jam as influenced by cultivar and storage conditions Trude Wicklunda,, Hans J. Rosenfeldb, Berit K. Martinsenc, Margareth W. Sundførb, Per Leac, Tor Bruuna, Rune Blomhoffd, Karin Haffnerb a Department of Chemistry, Biotechnology and Food Science, Agricultural University of Norway, P.O. Box 5003, N-1432Ås, Norway b Department of Plant and Environmental Sciences, Agricultural University of Norway, P.O. Box 5003, N-1432Ås, Norway c Matforsk, Norwegian Food Research Institute, Osloveien 1, N-1430Ås, Norway d Institute for Nutrition Research, Faculty of Medicine, University of Oslo, Blindern, N-0316 Oslo, Norway Received 30 October 2003; received in revised form 3 June 2004; accepted 24 June 2004 Abstract Jam was prepared from five strawberry cultivars; ‘Senga Sengana’, ‘Korona’, ‘Polka’, ‘Honeoye’ and ‘Inga’. The jam was stored at 4 and 20 1C, in darkness and under fluorescent light (950 lux). The quality parameters assessed were colour reflectance at 650 nm, Hunter L
; a
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, anthocyanin pigments and total antioxidant capacity assessed by FRAP-assay. Jam produced of all cultivars stored at 4 1C had significantly better colour qualities and FRAP-values than jam stored at 20 1C. The light conditions during storage did not affect the assessed quality parameters of the product during three months of storage. The cultivars ‘Senga Sengana’, ‘Korona’, ‘Honeoye’ and ‘Polka’ showed the highest a*-values (red colour), but only ‘Korona’, ‘Honeoye’ and ‘Polka’ showed a high total antioxidant capacity, as measured by FRAP and compared to ‘Senga Sengana’ and ‘Inga’. Thus, to achieve a good coloured strawberry jam with high antioxidant capacity, the industry should consider to store the products at 4 1C and to replace ‘Senga Sengana’ with one of the ‘Korona’, ‘Honeoye’ and ‘Polka’ cultivars. r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Strawberry; Jam; Antioxidants; Pigments 1. Introduction During the last years the antioxidants in fruit and vegetables have been emphasized, and especially the colour derived from anthocyanins has been recognized as an important component in reducing the risk of several chronic diseases such as cancer, coronary heart disease, diabetes type 2, hypertension and cataract (Glade, 1999; Lampe, 1999). Reduction in antioxidant activity during processing and storage (Lindley, 1998; Nicoli, Anese, & Parpinel, 1999; Kalt, McDonald, & Donner, 2000) may reduce the health beneficial effects Corresponding author. Tel.: +47-6494-8566; fax: +47-6494-7720. E-mail address: trude.wicklund@ikbm.nlh.no (T. Wicklund). of such food products. Exposure to oxygen, inducing enzyme activity, light and heat may reduce the antioxidative properties, while a high colour strength usually is an indication of a high total antioxidant capacity of a product. An attractive red colour is one of the most important quality characteristics for the strawberry jam processing industry, beside of typical sweet–sour strawberry flavour and convenient jam consistency. Cultivar and degree of ripeness are major factors determining taste and colour of berry jams (Finstad-Bye, 2001; Sundfør Wegner, 2001; Redalen & Haffner, 2002). The basis of anthocyanins is the aglicone anthocyanidin, which binds to carbohydrates to form a more stable structure. The nature of the substitutes, but also 0023-6438/$30.00 r 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2004.06.017 ARTICLE IN PRESS 388 T. Wicklund et al. / LWT 38 (2005) 387–391 the pH and the presence of metal ions, affects the colour. The red colour of fruits and berries belong to the group of anthocyanins, which are phenolic compounds and containing at least one phenolic ring structure (Francis, 1985). The colour of the product depends on these natural pigments and their degradation products. Colour stability of red fruit products is affected by temperature, pH, oxygen, sugar content, ascorbic acid and metals (Withy, Nguyen, Wrolstad, & Heatherbell, 1993). The degradation of pigments results in discolouration of the product. During processing the pigments can be hydrolysed, and degraded to anthocyanidin and sugar. The anthocyanidins are unstable when exposed to light and are more easily oxidized than the anthocyanins, and consequently more susceptible to browning reactions (Herrmann, 1972). The major pigments in strawberries are pelargonidin3-glucoside and cyanidin-3-glucoside. The pelargonidin3-monoglucoside provides a bright red colour while the cyanidin-3-monoglucoside is purple. The monomeric anthocyanins are red at pH 1.0, while they are colourless at pH 4.5 (Wrolstad, 1976). Generally temperature is the most important factor for degradation of the colour pigments. The total amount of pigments in strawberries is also important for the stability of the colour of the produce. Anthocyanins may react with ascorbic acid, resulting in degradation of both components (Francis, 1985). The jam recipe, processing procedures, jar type, storage conditions and duration are important factors for the jam quality. Continuous processing in stainlesssteel pans is used in modern jam processing. The temperature during processing is often about 90 1C, and the jam is held at this temperature for 3–5 min, to achieve proper heating of the entire fruit. After boiling the jam is held in a reservoir to ensure that it is of the right consistency when filled, to obtain a good distribution of particles as well as appropriate gel strength. Traditionally jam is stored at room temperature in glass jars in warehouses and stores. Low temperature is generally not regarded as necessary to prevent degradation, as the jam during processing is added both preservatives and sugar, and the pH of the produce is usually low. The shelf life of jam is normally 6–12 months. Studies on the degradation of pigments in jam stored at different temperatures show that degradation increases at higher temperatures (Garcı́a-Viguera et al., 1998). The optimal storage temperature for jam is believed to be 4 1C (Ochoa, Kesseler, Vullioud, & Lozano, 1999). Exposure to oxygen is harmful for the quality of jam (Blom & Enersen, 1983). The objective of the experiments was to study how colour and antioxidant capacity of strawberry jam from different cultivars were influenced by light and temperature during storage. 2. Materials and methods 2.1. Short description of cultivars The raw material for strawberry jam was derived from five cultivars grown at agricultural experimental stations in the southeastern part of Norway (591400 N). ‘Senga Sengana’: Older German cultivar, with dark red coloured and aromatic berries, still preferred by the Norwegian processing industry. ‘Korona’: Dutch cultivar, mainly produced for fresh consumption in Norway, difficult to harvest without calyx for industry use. ‘Polka’: Dutch cultivar, relatively dark coloured and tasty berries, can possibly replace ‘Senga Sengana’ for industry use. ‘Honeoye’: Early maturing American cultivar, good taste and aroma, suitability for jam industry is unknown. ‘Inga’: Norwegian cultivar, recently released, large berries with good storage ability, suitability for jam industry is unknown. The berries of ‘Senga Sengana’, ‘Korona’, ‘Honeoye’ and ‘Polka’ were picked at optimum ripening degree for industrial use. Berries of the cultivar ‘Inga’ were picked slightly unripe. The berries were frozen and stored at 20 1C for two months before processing. 2.2. Jam processing and storage conditions The jam was processed on the pilot plant jam processing equipment (FlowTech AS, Skanderborg, Denmark) at The Norwegian Food Research Institute. The equipment consists of two stainless-steel pans with steam heating, and a semi automatic filling system based on weight. The frozen strawberries (4 kg) were kept at room temperature for 45 min before they were filled into the pan with the water (400 ml). The temperature was raised to 10 1C before sugar (4.7 kg) was added. Strawberries and sugar were mixed before heating to 80 1C. Pectin solution (LM 101) (60 g in 700 ml water) was then added and the temperature raised to 90 1C and held there for 3 min before cooling down to 80 1C. The preserving agents, sodium-benzoate and potassium-sorbat (3+4 g) and citric acid (140 g) were added and the material was stirred for 10 min before cooling to 60 1C and filling in transparent glass jars. Further cooling was done at 15 1C for 2 h before refrigeration. The jam processing was done in duplicates. The jam was kept refrigerated (4 1C) for one week, and then moved to the different storage conditions (Table 1). The jam was stored for three months at these conditions. ARTICLE IN PRESS T. Wicklund et al. / LWT 38 (2005) 387–391 Table 1 Storage conditions for the samples Sample no. Light (lux) Temperature (1C) 1 2 3 4 0 (dark) 0 (dark) 950 950 20 4 20 4 2.3. Analysis of the jam 2.3.1. Pigments Total monomeric anthocyanins were determined by the pH-differential method as described by Wrolstad (1976). The pigments were extracted from the fruit or jam by acidified methanol (0.01 mol/l HCl in methanol). An average of 4 g homogenized jam or fruit was diluted to 25 ml in acidified methanol (0.84 ml concentrated HCl was added to 1 l methanol). The extract was kept cool overnight, and filtrated (Schleicher & Schuell filter 520 12; folded) the next day. Samples were diluted with buffers directly in disposable cells (1 cm path length) and absorbance read at 520 nm on a HP 8542 diode array spectrophotometer. Pigment content was calculated as pelargonidin-3glucoside, with molar absorbance 22400 and molecule weight 433.2 g/mol. As the jam only contains 40% berries in the end product, the results from the measurements were adjusted to 100% berries (multiplying by 2.5). 2.3.2. Colour Instrumental colour measurements of the jams, placed in a measuring chamber, were conducted with a Hunter Lab colour measurement system (LabScan XE, Reston, VA, USA). The instrument operates between 400 and 700 nm (10 nm intervals). The instrument was calibrated with a standard white and a standard black reflective plate. Both reflectance values and L*, a* and b*-values were used for evaluating jam colour as described by Haffner, Finstad, Rosenfeld, and Skrede (2003). 2.3.3. Antioxidant capacity—FRAP A modification (Halvorsen et al., 2002) of the Ferric Reducing Ability of Plasma (FRAP) assay of Benzie and Strain (1996) was used to assess the total antioxidant capacity. For analysis of antioxidants, the samples were homogenized in a food processor. Approximately 3 g homogenate was dissolved in 30 ml methanol. The samples were mixed, and sonicated at 0 1C for 15 min. Three samples of 1.5 ml were centrifuged at 12500 rpm for 2 min at 4 1C. The concentration of total antioxidants was measured in triplicates of the supernatant. The total antioxidant capacity (FRAP-value) is given as 389 mmol 100 g1 of the sample, adjusted to 100% berries (multiplying by 2.5). A Technicon RA 1000 system (Technicon Instruments Corporation, New York, USA) was used for the measurements of absorption changes that appear when the TPTZ–Fe3+ complex reduces to the TPTZ–Fe2+ form in the presence of antioxidants. An intense blue colour with absorption maximum at 593 nm develops. The measurements were performed at 600 nm. An aqueous solution of 1000 mmol/l FeSO4  7H2O was used for calibration of the instrument. 2.4. Data analysis Principal component analysis (PCA) (Unscrambler, Windows version 7.5, 1999, Camo a/s, Trondheim, Norway) was performed on mean values of 20 samples and 6 variables. The PCA is a multilinear modelling method given an interpretable overview of the main information in a multidimensional data table. The information carried by the original variables is projected onto a smaller number of principal components (PC), i.e. linear functions of the original variables. All variables were mean centred and scaled to unit variance prior to analysis. The calibration model was validated by full scale cross validation. To detect statistical significance, ANOVA and Tukey’s test (SAS-system) were applied. 3. Results and discussion The most important information, visualized by means of the PCA-biplot (Fig. 1), was the difference between effects of the two storage temperatures, explaining 59% of the total variation in principal component 1. Samples stored at 4 1C had a lower L*-value, higher a* and b* values, a higher reflectance at 650 nm (dark red colour), and a higher content of anthocyanins and total antioxidant capacity than samples stored at 20 1C, confirming the results of Garcı́a-Viguera et al. (1999) and Norwegian experiments with raspberry jam. To achieve a good colour in berry jam, the industry product should be stored at chilling temperature, 4 1C (Haffner et al., 2003). Principal component 2 explained 24% of the total variation, expressing the differences between the cultivars, but with no special overall preference for any cultivar concerning the measured variables. The most distinct variable expressing the differences between the cultivars was the L-value. A high positive correlation between the a*-value and anthocyanin content was detected (r ¼ 0:95), but no correlation between FRAPvalues and the other variables was found. The ANOVA (Table 2) confirmed the PCA overview, showing significant differences between the storage ARTICLE IN PRESS 390 T. Wicklund et al. / LWT 38 (2005) 387–391 temperatures and between the cultivars. The values of fresh produced jam were kept out of the statistical treatment of the data. The differences between dark and light storage were non-significant, which confirmed the findings of Garcı́a-Viguera, Zafrilla, Romero, Abellan, Artes, and Tomas (1999) and Viberg, Ekstrom, Fredlund, Oste, and Sjoholm (1997). The latter performed research on black currant jam, showing no changes in colour or antioxidant capacity to occur when stored in darkness or light over a period of 3 months. All cultivars analysed were considered to be suitable for jam production, mainly due to an acceptable jam colour. ‘Senga Sengana’, still preferred by the Norwegian jam industry, showed high a*-values (red colour), but not significant different from ‘Korona’, ‘Honeoye’ and ‘Polka’. In contrast to high a*-values, ‘Senga Sengana’ showed the lowest FRAP-values. ‘Polka’-jam had an overall good quality with acceptable red PC2 L* l Poka-D20 0.5 Polka-L20 Honeoye-D20 Senga-D20 Inga-D20 Korona-D20 Honeoye-L20 Senga-L20 Korona-L20 Inga-L20 0 -0.5 Anthocyanine b* 650 nm Polka-L4 Honeoye-D4 Polka-D4 Senga-L4 Senga-D4 FRAP Korona-D4 Inga-L4Inga-D4 Honeoye-L4 a* 60 50 mg /100g 1.0 colour, and despite of lower anthocyanin content, relatively high FRAP-values were measured. ‘Korona’jam showed the highest pigment and FRAP-values, combined with the lowest L* and b*-measurements (Table 2). Thus there was an overall low correlation between colour measurements and FRAP in strawberry jam, the coefficient of correlation ranging from 0.22 to 0.42. This lack of correlation between FRAP and colour values may be caused by the antioxidant capacity of colourless fenolic compounds, as described by Jiratanan and Liu (2004). The anthocyanin content was highly influenced by the different storage conditions (Fig. 2). The FRAP-values of ‘Senga Sengana’, ‘Korona’ and ‘Polka’ were more stable during three months storage, while ‘Honeoye’ and ‘Inga’ showed a decrease during storage (Fig. 3). Taking into consideration the values of the measured variables in freshly produced jam compared to jam stored for three months, all variables decreased significantly Anthocyanin 40 30 20 10 Korona-L4 0 -1.0 PC1 -1.0 -0.5 0 0.5 Senga Korona Polka Honeoye Inga 1.0 Fig. 1. Principal component analysis of strawberry jam stored at 4 and 20 1C in light (L) and dark (D). The samples and measured variables (in bold letters) are shown on the same plot. Fig. 2. Anthocyanin content in strawberry jam at the start of the experiment and after three months of storage at 4 and 20 1C in dark and in light. —Fresh Produced, —Stored Light 4 1C, —Stored Dark 4 1C, —Stored Light 20 1C, —Stored Dark 20 1C. Table 2 Mean values and level of significance of main effects of cultivar, dark/light and storage temperature, after three months of storage Main effect L*-value a*-value b*-value Reflectance 650 nm Antho-cyanins mg/100 g berries FRAP value mmol/100 g berries ‘Senga Sengana’ ‘Korona’ ‘Honeoye’ ‘Polka’ ‘Inga’ 14.1ab 13.0b 14.4ab 16.5a 14.2ab 25.0a 24.4a 24.3a 24.1a 23.0b 16.8ab 15.8b 17.0ab 17.7a 16.3ab 8.7ab 8.0b 8.5ab 9.2a 7.8b 21.4ab 22.9a 21.5ab 17.1c 18.7b 7.5c 11.4a 9.8b 10.7ab 8.4c Cultivar * *** * ** *** *** Light Dark 13.9 14.9 24.2 24.2 16.4 17.0 8.3 8.6 21.0 19.6 9.7 9.4 Light/dark ns ns ns ns ns ns 4 1C 20 1C 14.2 14.7 27.30 21.10 17.8 15.6 9.2 7.7 29.4 11.2 9.8 9.2 Temperature *** *** *** *** *** **
Pp0:05;

Pp0:01;


Pp0:001; ns=non significant. ARTICLE IN PRESS T. Wicklund et al. / LWT 38 (2005) 387–391 20 FRAP-value 18 mmol/100g 16 ns 14 ns 12 a b ab bb ns 10 8 a a bbb 6 4 2 0 Senga Korona Polka Honeoye Inga Fig. 3. FRAP-values of strawberry jam, freshly produced and after three months of storage at 4 and 20 1C, in dark and in fluorescent light. Bars with the same letter are not significantly different (Po0:05), ns=non-significant. —Fresh Produced, —Stored Light 4 1C, — Stored Dark 4 1C, —Stored Light 20 1C, —Stored Dark 20 1C. (Po0:001). The anthocyanin content showed the highest decrease (Fig. 2). Light during storage influenced the cultivars only to a small extent while temperature was the most important factor for decrease in anthocyanin value. Temperature and light during storage did not influence FRAP-value for ‘Polka’, while a significant decrease in anthocyanin content was observed. Despite of being picked slightly unripe, the cultivar ‘Inga’ did not differ in L0 -, a*-, b*-values, anthocyanins and FRAP-values from the other cultivars. In conclusion, jam stored at 4 1C had a higher content of anthocyanins and total antioxidant capacity than samples stored at 20 1C, while there were no significant differences between dark and light storage. The cultivars ‘Senga Sengana’, ‘Korona’, ‘Honeoye’ and ‘Polka’ showed the highest a*-values (red colour), but only ‘Korona’, ‘Honeoye’ and ‘Polka’ showed a high total antioxidant capacity. Thus, to achieve a good coloured strawberry jam with high antioxidant capacity, the industry should consider to store the products at 4 1C and to prefer ‘Korona’, ‘Honeoye’ or ‘Polka’ cultivars. References Benzie, I. F. F., & Strain, J. J. (1996). The ferric reducing ability of plasma (FRAP) as a measure of ‘‘Antioxidant power: the FRAP assay. Analytical Biochemistry, 239(1), 70–76. Blom, H., & Enersen, G. (1983). 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