# Comprehensive Assessment of the Dynamics of Banana Chilling Injury by Advanced Optical Techniques
## 1 Introduction  
After harvest, fresh fruit and vegetables are highly physiologically active and perishable products. Continued metabolic processes such as transpiration or respiration may significantly affect their quality, thus, shortening shelf life. Practically used since long, low, nonfreezing storage temperatures effectively slow down physiological activity, retain quality, and prolong shelf life, although in a produce-specific way [1, 2]. Particularly subtropical and/or tropical fruits and vegetables are very sensitive to these low temperatures (<10 \({}^{\circ}\)C). Exposed to such chilling stress during storage, sorting, transportation, or shipping, they may suffer serious injuries. The physiological disorders attributed to chilling injury (CI) comprise damages at various metabolic levels, such as membrane integrity, reactive oxygen species or transcriptome [3, 4, 5]. The manifestation of CI depends on the specific sensitivity of products or cultivars to low temperatures and on intensity, duration, and/or intermittency of exposure to chilling temperatures. It is also a function of the physiological status and the maturity stage of the produce. Symptoms of chilling injury, however, typically become severely visible only in retail after removal from chilled conditions and negatively affect produce quality and marketability [4, 6, 7]. Various postharvest approaches (chemical treatments, produce-dependent time-temperature management, short-time heat treatments such as hot water dips, hot air or hot vapor treatments) were suggested to reduce and/or alleviate the symptoms of CI [8; 9]. Among the economically most important tropical fruit, green-ripe bananas are very chilling-sensitive and develop all CI-associated symptoms when stored below a threshold temperature of approx. 13 \({}^{\circ}\)C [10; 11; 12]. However, in banana, chilling susceptibility highly depends on cultivar [8; 13], maturity stage [14; 15] and ripeness status (i.e., whether ethylene-treated or not; [14; 16]) of the fruit. Optimal conditions for storage and transport of green-ripe bananas are 13-14 \({}^{\circ}\)C and 90 to 95% relative humidity [17], and 15-20 \({}^{\circ}\)C for ripening [18]. Although storage at temperatures below the above threshold reduces mass losses, respiration, and ethylene biosynthesis of fruit, it also induces CI. Frequent unique visible symptoms (smoky peel surface discoloration, dark-brown streaked subepidermal tissue discoloration) indicate that CI is mainly but not exclusively a peel disorder [10; 19]. In severe cases, the peel can turn dark brown or black [20; 21], and only then, the flesh becomes brown, drastically reduces its characteristic fruit flavor [22] and develops off-flavor [23]. In any case, chilling-injured green-ripe fruit are more sensitive to mechanical damage during handling [24] and, in addition, may fail to ripen [20; 25]. The latter is, however, valid only if artificial ripening has not been previously induced by application of ethylene [26]. On the other hand, moderate CI may develop after one hour at 10 \({}^{\circ}\)C or 72 h at 12.8 \({}^{\circ}\)C [24]. Irrespective of green- or full-ripe fruit, CI symptoms may not become apparent before 18 to 24 h after actual damage has occurred [18; 24]. During ripening at non-chilling temperatures, the typical color changes from green to yellow result from the breakdown of chlorophyll (Chl), which then unmasks the plastidine carotenoids [26; 27; 28] without or with only very minor additional synthesis of the latter pigments [25; 26; 29]. In contrast to some earlier findings [14], some recent reports indicated that chilling conditions did not inhibit but tended to accelerate chlorophyll degradation in green-ripe fruit [16; 26]. However, chilling-induced brown and/or black discolorations do not depend on either of these pigments but result from enzymatic oxidation of various phenolics by the concerted action of phenylalanine ammonia lyase (PAL) and polyphenol oxidase (PPO) in combination with non-enzymatic reactions [13; 30]. [PERSON] et al.