Raspberry fresh fruit quality as affected by pectin- and alginate-based edible coatings enriched with essential oils
Introduction
Red raspberries (Rubus idaeus L.) are of high economic importance and widely consumed in fresh, frozen, or in processed forms, such as jams and juices. In addition to their attractive color and flavor, raspberries contain a unique phytochemical profile. Specifically they are rich in ellagitannins and anthocyanins, which distinguishes them from other berries and fruits (Rao and Snyder, 2010).
The postharvest life of berries is generally determined by their susceptibility to water loss, softening, mechanical injuries, and especially to the presence of postharvest pathogens (Tezotto-Uliana et al., 2014). Various studies have proposed strategies to control postharvest pathogens, while preserving the quality of this fruit, such as modified atmospheres, forced-air cooling or other cooling processes, heat shock, osmotic treatments, irradiation, and edible coatings (Velickova et al., 2013).
Edible coatings have been of increasing interest because of their capacity to reduce respiration and transpiration rates, while increase storage periods and the retention of berry firmness (Azevedo et al., 2014, Tezotto-Uliana et al., 2014, Velickova et al., 2013, Vu et al., 2011). Edible coatings also provide good mechanical properties, are non-toxic and non-polluting, and can be applied at low cost.
The incorporation of antimicrobial agents, such as essential oils or their compounds, into edible coatings can enhance the functionality of coatings in protecting food from microbial spoilage and thus extending their postharvest life and quality (Antunes et al., 2012). Because the chemical composition of plant-derived products, such as essential oils, are highly variable with season and cultural practices, the utilization of sole compounds instead of naturally produced essential oil mixtures is a better approach to obtain an edible coating with constant characteristics (Azevedo et al., 2014, Miguel, 2010).
Eugenol, the main constituent of the essential oil isolated from clove flower buds [Syzygium aromaticum (L.) (Merrill & Perry)] has antioxidant, antimicrobial, antinociceptive, and antiviral activities (Cortés-Rojas et al., 2014). Citral is a mixture of two stereoisomeric monoterpenes aldehydes: the E-isomer specifically referred as geranial or citral A (40–62%), and the Z-isomer (25–38%) known as neral or citral B. This isomer mixture can be isolated from the essential oil of Cymbopogon citratus (lemongrass) and Litsea cubeba. Citral is used in traditional medicine as antispasmodic, analgesic, anti-inflammatory, antipyretic, diuretic, and sedative. Citral also possesses antimicrobial activity and insecticidal property (Maswal and Dar, 2014).
Polysaccharide edible coatings have good coating-forming and low oxygen permeability properties, as well as, the capacity to decrease the respiration rate of fresh-cut products (Campos et al., 2011). Sodium alginate (AL) is a polysaccharide derived from brown sea algae (Phaeophyceae) and is a linear unbranched polymer containing mannuronic and guluronic acids (Rojas-Graü et al., 2008). Pectin (PE) based edible coatings are extracted from apple waste or citrus peel and are homopolymeric linear chain of galacturonic acid units (Oms-Oliu et al., 2008).
The objective of this research was to study the effects of edible coatings based on AL or PE with Cit and/or Eug incorporated, on the storage ability and fruit quality of fresh red raspberries. Knowledge of how essential oils influence storage ability could prolong the postharvest lifespan of fresh raspberry, which could increase the market window and have positive economic implications.
Section snippets
Edible coatings formulations
Pectin (PE) and sodium alginate (AL) (Sigma–Aldrich Chemic, Steinhein, Germany) were the biopolymers used for coating formulations and calcium chloride (Sigma–Aldrich Chemic, Steinhein, Germany) was used to induce cross linking reaction (Olivas et al., 2007). The essential oils components, citral (Cit) and eugenol (Eug), were purchased from Sigma–Aldrich Chemic, Steinhein, Germany.
The coating forming solutions based on AL or PE were formulated as described by Rojas-Graü et al. (2007). Ascorbic
General quality parameters
Fruit color is an important indicator of fruit ripeness and is used by consumers to make conclusions on the ripeness and freshness of the raspberry fruit (Mikulic-petkovsek et al., 2015, Krüger et al., 2011). The lightness, color L* parameter, which indicates darkening of the fruits, generally did not change until 14 days storage in any AL treatment (Table 2). °Hue color parameter decreased mainly from 0 to 7 days in all treatments indicating an increase in redness and advanced ripening (
Conclusions
Fresh raspberry can be stored for at least 7 days at 0.5 °C, when edible coatings of AL and PE are applied at 1–2% concentrations and enriched with Eug and Cit at MIC concentrations (0.1 and 0.15%, respectively). Fruit quality declined by 14 days of storage and led to reduced consumer acceptance. Raspberries immersed in water (control + water) performed worse than fruit stored without any immersion or wash (control), so if hygienic conditions can be provided pre-, during and post-harvest, coating
Acknowledgment
This research was supported by FCT, Project: PTDC/AGR-ALI/121085/2010—INNCOAT-Innovative edible coatings to improve storage of small fruit and fresh-cut.
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