Effect of edible wheat gluten-based films and coatings on refrigerated strawberry (Fragaria ananassa) quality
Introduction
The potential of edible coatings to maintain the quality and extend shelf life of fresh products and to reduce the amount of disposable non-biodegradable packaging materials are being widely studied (Rose, 1992, Park, 1999, Amarante and Banks, 2001). Edible coatings can provide an alternative to extend the postharvest life of fresh fruits and vegetables and can also result in the same effect as modified atmosphere storage where the internal gas composition is adjusted (Park, 1999). Traditionally, films and coatings have been used to reduce water loss, but new film materials and edible coatings formulated with a wider range of permeability characteristics facilitate achieving a “modified atmosphere” effect in fresh fruits (Smith et al., 1987). Edible films and coatings are generally produced using biological materials such as proteins, lipids and polysaccharides. Films made of polysaccharides or proteins usually have suitable mechanical and gas barrier properties but show high permeability to moisture and poor water vapor barrier properties. In contrast, films composed of lipids (waxes or other lipids) exhibit good water vapor barrier properties but show poor mechanical resistance and high oxygen permeability. When such ingredients are combined, they could physically and/or chemically interact and may result in films or coatings with improved properties (Diab et al., 2001). Surface coatings can decrease fruit peel permeance, modify the internal atmosphere, reduce water loss, and depress fruit respiration rate (Bai et al., 2003, Banks et al., 1993).
Several researchers have studied the application of coatings to vegetables such as tomato, cucumber and red peppers (El Gaouth et al., 1991b, El Gaouth et al., 1992) and fruits like bananas (Banks, 1984), apples (Banks et al., 1997), mangoes (Dhalla and Hanson, 1988), pears (Amarante, 1998), citrus fruits (Hagenmaier and Baker, 1993), avocado (Bender et al., 1993), strawberries (García et al., 1998a, García et al., 1998b) and guavas (Oliveira and Cereda, 1999). Wheat gluten coating reduced moisture loss from sharon fruit and cherry tomatoes (Tanada-Palmu et al., 2000).
Highly perishable berries and tropical fruits are products most likely to benefit economically from coatings. Strawberries are a soft fruit with high respiration and softening rates, making the availability of high quality strawberries challenging. Due to its high metabolism, strawberries must be kept at 4–5 °C, which can extend its high quality for 6 or 7 days. The use of coating or edible film envolving strawberries can also be an alternative to improve their shelf life (Santos, 1997).
The objectives of this work were to study the ability of gluten-based coatings and films to extend the shelf life of strawberries stored at the refrigerator after commercialization, in comparison with non-coated fruit, and to study the influence of different types of coatings and films on the quality attributes of strawberries, such as weight loss, firmness retention, surface color development, titratable acidity, soluble solids, moisture, reducing sugar content and sensory evaluation.
Section snippets
Materials
Vital wheat gluten was obtained from Rhodia (Campinas, Brazil). The reactants used to prepare the film were ethanol and ammonium hydroxide purchased from Synth (São Paulo, Brazil) and glycerol from Merck (Darmstadt, Germany). The fatty acids, stearic and palmitic acids were obtained from Vetec (São Paulo, Brazil) and beeswax from Chemco (São Paulo, Brazil). The PVC film was a product of Reds 2000 (São Paulo, Brazil) and organic strawberries (Fragaria ananassa cv ‘Oso Grande’) were purchased
Results and discussion
All the coatings adhered well to the strawberry surfaces. The gluten and composite coatings and films were transparent, while the bilayer coating was opaque and white. All of the strawberries shrank during 16 days of storage, especially the control fruit compared with the other treatments, with the exception of the strawberries coated with the bilayer coating, which almost maintained their original size. The strawberries covered with the gluten pack or PVC film maintained good visual quality up
Conclusions
Wheat gluten coatings and films extended the shelf life of strawberries and retarded the senescence process compared with strawberries used as a control treatment. The addition of lipids to the gluten coatings showed a beneficial effect on firmness retention and reduced weight loss of the strawberries. The fruit with the gluten film also had higher firmness retention compared to the control fruit. The pouch film and gluten pack reduced the number of infected fruit in comparison with the control
Acknowledgment
We thank FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for the scholarship conceded to P.S. Tanada-Palmu.
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