edible coatings are a new promising approach for controlling the quality and extending the shelf-life of fresh agricultural products (baldwin et al., 2011). edible coatings may control moisture transfer protecting products from deterioration and improving textural quality, help to retain volatile components, reduce the internal oxygen partial pressure decreasing a metabolism of fresh products (falguera v et al., 2011). moreover, active edible coatings may have antimicrobial, antioxidant, antibrowning and texture enhancing properties (rojas-grau et. al., 2009).
edible coatings are based on completely biodegradable and biocompatible polymers from natural sources such as polysaccharides, proteins, and lipids (vargas et al., 2008) therefore they respond to consumer demand for more natural and environmentally friendly approaches of product quality control. due to all the mentioned benefits edible coatings attract much current scientific and practical interest worldwide.
to be applicable on fresh agricultural products, edible coatings have to answer a long list of various requirements. the necessary requirements include (1) good adhesion to a food surface, (2) normal gas permeability and (3) an acceptable color, taste, flavor and texture.
in addition to be valuable edible coatings are expected (4) to improve appearance and mechanical properties of agricultural products, (5) to retard moisture migration, (6) to protect agricultural products from physiological and/or microbiological damages. single coating material is often unable to satisfy all these requirements.
therefore, layer-by-layer (lbl) approach could be very helpful. layer-by-layer method is based on the alternate deposition of various polymers to produce ultrathin layers on surface. physicochemical properties of the created films can be controlled by changing the deposition conditions and by altering the nature of polymers themselves. the technique has applications range from optical devices to biomaterials. despite lbl approach has great potential to control properties and functionality of edible coatings until recently it was almost unexplored in the field of food quality and shelf life extending.
layer by layer coatings on fresh agricultural products
in our group we have demonstrated a practice of lbl approach for improvement of quality and shelf-life extending of fresh agricultural products (figure 1).
for instance, natural polysaccharides a polyanion alginate and a polycation chitosan, were implemented on fresh-cut melon to form lbl alginate-chitosan edible coating (poverenov, danino et al., 2013).
the quality of melons coated by lbl coating was compared with quality of melons coated by single-layer alginate or chitosan coatings and with the non-coated control fruits (figure 2 and 3). the lbl coating was found to possess beneficial properties of both ingredients, providing coated melons with good firmness, gas exchange ability and shelf life extension.
figure 2. fresh cut melons coated by: alginate coating, layer-by-layer alginate-chitosan coating,
chitosan coating and non-coated after 10 days of storage at 7 oc.
figure 3. firmness and oxygen content of the fresh cut melons coated by alginate coating,
layer-by-layer alginate-chitosan coating, chitosan coating and non-coated.
additional study performed in our laboratory the antimicrobial effect of external antimicrobial essential oil citral was compared with an effect of adding of second chitosan antimicrobial layer (poverenov, cohen et al., 2013).
figure 4. chitosan coating and non-coated after 10 days of storage at 7 oc.
it was found that the antimicrobial activity of chitosan is much more significant and can be controlled by chitosan concentration (figure 4).
we have also investigated an effect of combination of other coating materials, for instance gelatine and chitosan. gelatin is a natural edible protein that may provide good adhesion to food surface and can be an internal layer. on the other hand, chitosan has unique antimicrobial activity but has no good adhesion properties. therefore it can serve as a protective external layer. the effect of layer by layer, gelatine-chitosan, edible coating on postharvest quality of fresh cut melons was studied in our laboratory. we have found that gelatin-chitosan coated fruits benefit significant shelf life extending. while a normal storage period of fresh-cut melons is 3-5 days at 7oc, upon treatment with lbl coating we succeed to increase storage of melons to 10-12 days where melons keep their appropriate quality.
layer by layer edible coating approach allows combining of beneficial properties of different coating materials. in our studies we have demonstrate the combination of active polysaccharides with well adhesive and texture enhancing polysaccharides and combination of antimicrobial polysaccharide with well adhesive protein. we have successfully coated fresh cut melons utilizing lbl formulations based on natural edible polymers. our coatings have demonstrated excellent capabilities of product safety and quality improvement. the lbl method allows precise control of coating physical and mechanical properties and the coating activity. the method is cheap, simple, based on natural ingredients and, the most important, can significantly improve the postharvest quality and safety of fresh agricultural products. the applications of lbl edible coating approach on various additional fruits and vegetables are under investigation in our laboratory.
these results are part of ms. rehyt cohen and mrs. shani danino final project thesis works in the department of food quality and safety in agricultural research organization, the volcani center. the research leading to these results was funded by the chief scientist of the israeli ministry of agriculture and rural development and by the european union seventh framework programme (fp7/2007-2013) under grant agreement n. 289719 (project quafety).
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poverenov e, danino s, horev b, granit r, vinokur y, rodov v (2013).
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a) comparison of antimicrobial abilities of external essential oil additive, citral, with antimicrobial activity of the external chitosan layer.
b) increase of antimicrobial activity with increase of chitosan concentration.
*department of food quality and safety, agricultural research organization, the volcani center, bet-dagan.
(published in israel agriculture, 2014)