WFL Publisher Science and Technology Meri-Rastilantie 3 B, FI-00980 Helsinki, Finland e-mail:
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Journal of Food, Agriculture & Environment Vol.6 (3&4) : - . 2008
www.world-food.net
Quality changes during storage of fresh-cut or intact Swiss chard leafy vegetables A. Ferrante 1*, L. Incrocci 1
2
and G. Serra
3
Dept. Produzione Vegetale, University of Milan, Via Celoria, 2 CAP 20133 , Milan, Italy. 2Dept. Biologia delle Piante Agrarie, University of Pisa, Pisa, Italy. 3Scuola Superiore S. Anna – Pisa, Italy. * e-mail:
[email protected]
Received 8 January 2008, accepted 25 March 2008.
Abstract Fresh-cut leafy vegetables have been gaining importance in the human diet both for their nutraceutical proprieties and low calories content. The preparation procedures for fresh-cut production and the storage system may lead to severe leaf damages that may affect leaf colour and induce shortened shelf life. The aim of this work was to study the leaf pigment changes in cut and whole Swiss chard leaves under dark or light storage conditions. The effect of treatments was evaluated by chlorophyll, carotenoid and anthocyanin determinations. The leaf membrane integrity was estimated by electrolyte leakage. All measurements were performed until 12 days of storage. The chlorophyll reduction was statistically significant in cut Swiss chard leaves stored in darkness. Total carotenoids did not significantly decline. Anthocyanin content strongly declined in cut leaves in both dark and light storage conditions. Leaf electrolyte leakage increased after 8 days of storage and higher leakage was observed in Swiss chard stored in light conditions without any difference between cut or whole leaves. Results suggest that Swiss chard can be stored until 8 days without any visible changes. Dark storage seemed to preserve quality losses and reduce membrane permeability. Key words: Anthocyanins, carotenoids, chlorophyll, electrolytes, membrane permeability.
Introduction In the recent years the intensification of human life activities and the development of advanced systems in food processing led the producers and consumers to new offers and demands. The lifestyle of people has been changing and the human nutrition has been orienting towards the ready-to-eat products such as pre-cooked and minimal processed foods 1, 2. Leafy vegetables play an important role in the human diet to contrast the high caloric foods, rich in lipids and sugars. The leafy vegetables quality may be divided in internal and external quality. The internal quality is represented by vitamins, antioxidants, minerals and functional elements (such as carotenoids, polyphenols, etc.). The external quality may be sub-divided in visible and non-visible components 3. The latter is referred to absence of human pathogens, pesticide residuals or other chemical products used in agriculture 4. Bland washes with tap water can be able to reduce the native microbial population, chemical residues and improve product quality. However, the application of modified atmosphere packaging can sufficiently reduce the undesirable microbial growth and catabolic processes that lead to quality losses 5. Another important component of the external quality, that is identified with the visual appearance (colour, defects and mechanical damages) of produce, is necessarily required to induce the consumers to buy the products 6. The colour changes are the most common postharvest disorder that may compromise the economic value of the produces. Many factors are involved before and after harvesting. In pre-harvest the causes of colour loss are attributed to diseases, lack of mineral nutrients, growing 132
conditions, species etc. During the postharvest stage the colour change may be due to no optimal storage temperatures, mechanical damages, enzymatic disorders (browning), hormone unbalances or presence of ethylene. The most part of metabolic processes that induce quality losses are temperature dependent, therefore a good storage facilities may improve the postharvest life of minimally processed vegetables. The aim of this research work was focused on the effect of cut operations and light or dark storage conditions on leaf pigments and membrane stability in Swiss chard leafy vegetables. Materials and Methods Plant material: Swiss chard (Beta vulgaris L.) was grown in floating system with the following nutrient solution (concentrations are expressed in mM) 13 N-NO3, 1.5 P, 8 K, 3.5 Ca, 1.7 Mg, 9.5 Na, 8.0 C, 2.7 S, 0.04 Fe and Hoagland’s concentration for micronutrients. The leafy vegetable was harvested at commercial stage. Storage and treatments: After the harvesting, the Swiss chard was washed with distilled water and slightly dried. Prior storage the Swiss chard was cut and packed in sealed plastic bags (highdensity polyethylene film) for storage simulation. Cut and whole Swiss chard leaves were stored at 5°C in darkness or with 150 µmol m-2 s-1 light intensity and 12 h photoperiod. The storage was prolonged for 12 days. Each plastic bag was filled with 400 g of Swiss chard and the bag capacity was 500 ml. Five bags were
Journal of Food, Agriculture & Environment, Vol.6 (3&4), July-October 2008
opened at each sampling time, with a total of 25 bags for each treatment. Chlorophyll, carotenoids and anthocyanins determination: Total chlorophyll content and total carotenoids were extracted using methanol 99.9% as solvent. Samples were kept in dark cold room at 4°C for 24 hours. Extracts absorbance readings were performed at 665.2 and 652.4 nm for chlorophyll pigments and 470 nm for total carotenoids. Chlorophyll and total carotenoids were calculated by Lichtenthaler’s formula 7. Visual appearance was determined by daily observation of chopped leafy vegetables during storage. Samples of the frozen tissue (100 mg) were ground in pre-chilled mortar and extracted into methanolic HCl (1%). Samples were incubated overnight at 4°C in darkness. The concentration of cyanidin-3-glucoside equivalents (29600 ε extinction coefficient) determined spectrophotometrically at 535 nm. Electrolyte leakage determination: Electrolyte leakage of Swiss chard leaves was determined by measuring the electrical conductivity of distilled water (20 ml) in which 3 leaf discs were incubated for 2 h at room temperature. Electrolyte leakage was expressed as the percentage of total electrolytes of the same discs after a freeze-thaw cycle. Statistical analysis: The data were subjected to one-way analysis of variance and differences among treatments were analyzed by LSD post-test (P
