**1. Introduction**

*Citrus* spp. (Rutaceae) are the most widely produced fruits in the world, with a global production that exceeded 123 million tons in 2013. The list of the most important producing countries is led by China, Brazil, the United States (USA), India, Mexico, and Spain. Fruit production is mainly devoted to juice extraction, but a considerable proportion is traded as fresh entire fruit for direct consumption. Spain is the leading country for exports of fresh produce [1]. From highest to lowest production, the most cultivated citrus species are oranges (*Citrus sinensis* L.), mandarins or tangerines (*Citrus reticulata* Blanco), including clementines (*Citrus clementina* hort. ex Tanaka), satsumas (*Citrus unshiu* Marcow.), and different hybrid mandarins, lemons (*Citrus limon* (L.) Burm. f.) and limes (*Citrus aurantiifolia* (Christm.) Swingle), and grapefruits (*Citrus paradisi* Macfad.). Postharvest handling of

fresh fruit in citrus packing houses is intended to commercialize fruit of maximum quality, increase their postharvest life, and reduce produce losses. In general, postharvest losses can be of physical, physiological, or pathological origin. Physical losses are typically due to rind wounds or bruises caused during harvest, transportation, or postharvest handling in the packing house. These peel injuries are not only important for causing direct losses, but also for being infection sites for economically important postharvest fungal pathogens. Other pathological losses are caused by latent pathogens that infect flowers or young fruit in the grove but develop after harvest. Likewise, some postharvest physiological losses are originated in the field and others are caused by inappropriate handling or storage conditions in the packing house or the marketplace.

Postharvest treatments with conventional synthetic waxes and/or chemical fungicides such as imazalil (IMZ), thiabendazole (TBZ), sodium ortho-phenil phenate (SOPP) or other active ingredients have been used for many years and are still currently used in citrus packing houses to preserve fresh fruit, control postharvest decay, and extend fruit shelf life. Nevertheless, the continuous application of these treatments has arisen important problems for the citrus industry such as health and environmental issues associated with chemical residues or the proliferation of pathogenic resistant strains. Updated regulations from many countries are increasingly restricting the use of agrochemicals and every day more exports markets are demanding fruit with residue levels even lower than those established by official regulations. Due to this situation, research should focus on anticipating a scenario in which conventional chemical fungicides are not available. In such a context, satisfactory postharvest decay control may be accomplished by adopting integrated disease management (IDM) programs based on comprehensive knowledge of pathogen biology and epidemiology and consideration of all preharvest, harvest, and postharvest factors with influence on disease incidence. Among the antifungal postharvest treatments alternative to synthetic chemicals that are investigated worldwide for potential inclusion in IDM programs, in this article we review the development of antifungal edible coatings as a promising novel technology intended to confront two major concerns of citrus postharvest handling, the losses due to physiological problems and the losses due to pathological problems.

#### **2. One Solution for Two Major Citrus Postharvest Problems**

#### *2.1. Physiological Problems*

Citrus fruit are non-climacteric, hence their respiration rate and ethylene production do not exhibit remarkable increase along with changes related to maturity and ripening as in climacteric fruits. However, although they have a relatively long shelf life compared to other tropical and sub-tropical fruits, they may experience important physiological postharvest losses if they are not properly handled and stored. As with other horticultural products, major postharvest losses in citrus are caused by weight loss and physiological disorders.

Water loss of citrus fruit after harvest, although not considered a physiological disorder, is responsible for loss of quality and consumer acceptability, as it results not only in direct quantitative losses (loss of salable weight), but also in losses in appearance (wilting and shriveling) and softening. In addition, most physiological disorders that affect citrus fruit tend to be related to water loss [2]. Some rind disorders that may appear under optimal, non-chilling temperatures include peel pitting, stem-end rind breakdown (SERB), and shriveling and collapse of the stem-end button. Postharvest peel pitting at non-chilling temperatures is a severe disorder that affects fruit from several citrus cultivars worldwide. Although the causes of the disorder are not fully understood, evidence indicates that altered water relations in fruit peel is a major factor contributing to this disorder. In this sense, sudden changes from low to high relative humidity (RH) after harvest induced peel pitting in "Navelina" and "Navelate" oranges [3], "Marsh" grapefruit [4] or "Fallglo" tangerines [5]. Later studies confirmed the link between alterations in the osmotic and turgor potential in the flavedo (the outer pigmented layer of the peel) and albedo (the inner white layer of the peel) with the induction of postharvest peel pitting in citrus fruit [6]. SERB involves the collapse and subsequent darkening of the epidermal tissues

around the stem-end rind of the fruit. The disorder is primarily associated with low RH, particularly during the postharvest period, although preharvest conditions seem to have a critical impact on the susceptibility of fruit to SERB. In some cases, SERB has been reported to be more severe when fruit are harvested from water-stressed trees compared to non-stressed trees or those presenting nutritional imbalances in terms of nitrogen and potassium [7–9]. Shriveling and collapse of the button tissue is an age-related breakdown due to cell weakening and dehydration of mature fruit. The symptoms can vary from discolored, dried out, and extensive collapse of the fruit rind to dehydration or wilting at the stem end where the rind is thinnest [10].

Another group of physiological disorders caused by storage of citrus fruit below sub-optimal temperatures but above the freezing point is chilling injury (CI). CI is characterized by the collapse of discrete areas of the peel that form sunken lesions that tend to coalesce [11]. CI in citrus fruit may appear in various forms depending on species and cultivar and exposure conditions (temperature and duration). For instance, typical symptoms in oranges, mandarins or grapefruits can be browning of the flavedo, appearance of dark sunken areas of collapsed tissue (pitting), or appearance of soft water-soaked areas (watery breakdown), while in lemons can be browning of the albedo or peteca (a special type of rind pitting) [2,10]. In general, although CI symptoms are due to fruit storage below their optimal temperature for relatively long periods, they usually develop upon transfer to higher temperatures.

The high sensitivity of citrus fruit to the induction of physiological disorders in the peel is in many cases triggered by mechanical damage during harvest and postharvest handling. Some of these disorders include "brush burn", "zebra skin", and "oleocellosis", caused by rind abrasion, rough handling, or thorn punctures. Brush burn and zebra skin symptoms usually appear as superficial red/brown staining of the rind and, in the case of the latter, is associated with the position of the segments, whereas oleocellosis might result in dark sunken patches as cells collapse around oil glands [12].
