Chapter 18
Edible Coatings for Enhancing Quality and Health Benefits of Berry Fruits Yanyun Zhao* Department of Food Science & Technology, 100 Wiegand Hall, Oregon State University, Corvallis, OR 97331-6602 *
[email protected] Edible coatings may be used on the surface of fresh berry fruits to modify the internal atmosphere, decrease the transpiration loss, and delay the ripening during postharvest storage and handling. Meanwhile, certain coating materials, such as chitosan has strong antifungal ability against Botrytis cinerea and Rhizopus sp., the two main fungi causing the decay of berry fruits. Therefore, edible coating technology, when designed and applied correctly, is an effective means to control decay and extend shelf-life of some berry fruits. In addition, edible coatings may provide an excellent vehicle to further enhance health benefit of berry fruits where the lack of some important nutraceuticals, such as vitamin E and calcium may be compensated by incorporating them into the coatings. This chapter briefly discusses the concepts and means of applying edible coatings on the surface of berry fruits and gives a review of the research conducted in the author’s laboratory in the development of edible coatings and their demonstrations on a broad range of berry fruits, including strawberries, raspberries, hardy kiwifruits, and blueberries.
Edible Coatings and Their Application on Berry Fruits Edible coatings are the thin layers of edible materials formed on the surface of food by casting, dipping, spraying, or brushing. They offer a selective barrier against the transmission of gases, water vapor, and solutes while also providing © 2010 American Chemical Society
mechanical protection (Figure 1). By regulating the transfer of moisture, oxygen, carbon dioxide, aroma and flavor compounds in a food system, edible coatings have demonstrated the capability of improving food quality and prolonging shelflife of fresh fruits and vegetables, including berry fruits. Edible coatings may also be used to advantage on processed fruits and vegetables for improving structural integrity of frozen fruits and vegetables and preventing moisture absorption and oxidation of dried fruits or vegetables (1). In addition, edible coatings can provide active functions by carrying functional ingredients into the coating matrix, such as antioxidants, antimicrobials, nutrients and flavors to further enhance food stability, quality, functionality and safety (2–4). Berry fruits, such as strawberries and raspberries are highly perishable and have high physiological postharvest activities. As a consequence, they have short ripening and senescent periods that make marketing of fresh fruits a challenge. Their storage life is often terminated by fungal infection (5). The most prevalent method of maintaining quality and controlling decay is rapid cooling after harvest and storage at low temperatures for controlling moisture loss, postharvest respiration, and microbial growth (5). Edible coatings applied on the surface of berry fruits would provide an additional barrier for water and gas exchange with the environment. In addition, it can help delay decay incidence when using the coating material with anti-fungal property.
Criteria for Developing Successful Coating The success of applying an edible coating for extending shelf-life and enhancing quality of berry fruits depends on several factors (4), including - Barrier properties of the coatings to moisture, oxygen, and carbon dioxide; - Surface characteristics of the coatings, including coverage, adherence, glossiness, and coating wettability; - Sensory properties of the coating materials, such as color, taste, etc.; and - Antimicrobial and antioxidant properties of coating materials. Since the major quality deteriorates involved in fresh berry fruits are via mass transfer phenomena, including moisture adsorption, oxygen invasion, flavor loss, and mold growth, permeation, absorption and diffusion to water, oxygen and carbon dioxide and prevention of mold growth are among the most important functional properties for edible coatings applied on berry fruits. These criteria in turn depend on the chemical composition and structure of the coating-forming polymers, the characteristics of the fruits, and the storage conditions.
Edible Coating Materials for Berry Fruits Biopolymers, such as proteins, polysaccharides, lipids, and resins are the common coating forming materials that can be used alone or in combinations. The physical and chemical characteristics of the biopolymers greatly influence the functionality of resulting coatings. Selection of coating materials is generally 282
based on their water solubility, hydrophilic and hydrophobic nature, easy formation of coatings, and sensory property. The properties of some common coating materials and their applications on fresh fruits and vegetables have been discussed in great details in the literatures (4, 6), and are summarized below. - Polysaccharides, such as cellulose, hydroxypropylcellulose (HPC), hydroxy propyl methyl cellulose (HPMC), starch, carrageenan, alginate, gum, chitosan. Overall, these materials have relatively high water permeability, and effective O2 and CO2 barriers at RH < 70 %. - Proteins, including corn zein, soy protein, whey protein, collagen, gelatin, and casein. They are more permeable to water vapors, and have effective oxygen barriers at low RH. - Lipids, such as beewaxes, fatty acids, and shellac. These materials have good water vapor barriers, but the coatings from these materials are usually too brittle and unstable when subjecting to different storage conditions, thus need to combine with polysaccharides or proteins for achieving desirable coating functionality. The author’s laboratory has examined several of the above coating materials for their applications on berry fruits with a special interest in chitosan. Chitosan, a linear polymer of 2-amino-2-deoxy-β-D-glucan, is a deacetylated form of chitin, a naturally occurring cationic biopolymer (7, 8). It occurs as the shell component of crustaceans (crab and shrimp), as the skeletal substance of invertebrates, and as the cell wall constituent of fungi and insect (9). Chitosan has been one of the most promising coating materials for fresh produce because of its excellent film-forming property, broad antimicrobial activity, and compatibility with other substances, such as vitamins, minerals, and antimicrobial agents (4). Chitosanbased coatings have shown the effectiveness in delaying ripening and decreasing respiration rate of fruits and vegetables, and reducing weight loss, color wilting and fungal infection in bell pepper, cucumber, and tomatoes (10–12). Another very attractive function of chitosan is its broad anti-fungi property (13–15), by inducing a plant-defense enzyme, chitinase in plant tissues, which degrades fungal cell walls (14). In addition, chitosan-based coatings can carry high concentrations of vitamins and minerals for increasing the content of these nutrients in the fresh and frozen fruits without altering its anti-fungal and moisture barrier functionality (2, 3).
Examples of Edible Coating on Berry Fruits Studied in the Author’s Lab The following sections briefly reports the studies conducted in the author’s laboratory by using edible coatings, especially chitosan based coatings for extending shelf-life, enhancing quality and nutraceutical benefit of berry fruits during post-harvest storage.
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Figure 1. Edible coating with selective and active functions.
Figure 2. Effect of chitosan-based coatings on incidence of decay of fresh berry fruits stored at 2.0 °C and 88 % RH: (a) Strawberry (Puget Reliance); (c) Red raspberry (Tullmeen). GC = Gluconal® CAL, VE = dl-alpha-tocopheryl acetate. Reproduced with permission from reference (2). Copyright 2004.
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Table I. Calcium and vitamin E content in fruits coated with chitosan-based coatings containing calcium and vitamin E during fresh and frozen storage Calcium content x 102 (g kg-1)
Vitamin E content x 102 (g kg-1)
Control
Coated+
Control
Strawberry (Puget Reliance)
23.63±1.62a (4.73%)
36.57±1.62b (7.31%)
0.34±0.09a (4.57%)
2.45±0.09b (32.69%)
Raspberry (Tullmeen)
22.03±2.92a (4.41%)
57.74±2.92b (11.55%)
1.15±0.13a (15.36%)
7.66±0.13b (102.13%)
Frozen
Control
Coated
Control
Coated
Strawberry (Totem)
21.24±0.85a
33.61±0.85b
0.40±0.05a
(4.27%)
(6.72%)
(5.37%)
2.99±0.05b (39.85%)
Fresh
Coated++
+ Chitosan coating containing 5% Gluconal® CAL ++ Chitosan coating containing 0.2% dl-α-tocopheryl acetate a Mean ± standand deviation of each treatment. Means with different superscript with each row indicate significant differences at p
