Physiological Impacts of Fruit Ripening and Storage Conditions on Aroma Volatile Formation in Apple and Strawberry Fruit: A Review

After a brief description of the ‘‘history of research’’ of aroma volatiles of apple and strawberry fruit, possible reasons for the reduced production of these important quality attributes by particular preand postharvest procedures are given. Among the possible physiological factors in association with reduced aroma volatile production, a reduced ethylene sensitivity, a decline in the rate of respiration as well as the content of adenine nucleotides and limited free fatty acids as precursors for aroma volatiles biosynthesis are proposed. A hypothesis about how this sequence of events leads to reduced volatile production is given and finally some suggestions of how to improve volatile synthesis are discussed. Aroma volatiles are an important attribute of flavor in almost all tree fruit. Their significance was recognized by fruit physiologists quite early (Power and Chestnut, 1920), but wholesale and retail personal and even horticulturists have only recently discovered volatiles and aroma as important quality indices to satisfy consumers’ demands and/or promote sales. As a result, more pressure is now put on fruit producers and suppliers to improve aroma volatile production in fresh-marketed as well as stored fruit. As mentioned, it was the physiologists and biochemists who were first interested in aroma volatiles, but they were more concerned about the composition and chemical structure of these compounds rather than their impact on fruit flavor and perception by the consumer. Despite the early work in the 1920s, significant progress was only made after the invention of gas chromatography (GC) in the second half of the last century. It was the more physiologically oriented working groups such as Drawert’s in Germany (Drawert, 1975; Drawert et al., 1968) and Paillard’s in France (Paillard, 1979) who achieved real progress concerning the chemical composition and the biosynthetic pathways of aroma volatiles. Since then, it has been established that the metabolism of fatty acids and branched amino acids may serve as precursors for the biosynthesis of aroma volatiles in fruit (Fellman et al., 2000). With the help of GC and combined GC/mass spectroscopy, the list of identified aroma volatile compounds grew rapidly to several hundred. Furthermore, the establishment of ‘‘olfactory thresholds’’ provided the opportunity to categorize these compounds according to their organoleptic importance. This reduced the number of compounds that were considered important organoleptically, but it raised the demands for high-resolution and sensitive instrumentation because the olfactory thresholds and production rates of some of these compounds appeared to be very low. For apples, more than 300 volatile compounds have been identified, but only 30–40 compounds are considered as flavor impact compounds (Cunningham et al., 1986). Fortunately, at least with apples, the impact of various preand postharvest treatments on the production rates of most of these volatile compounds is affected to a similar degree, and this allows judgments to be made without analyzing precisely each single small peak. This chemical behavior suggests that many volatile compounds are derived from similar biochemical pathways. There are exceptions to this general behavior, e.g., Brackmann et al. (1993) showed that branched and unbranched ester production in ultralow oxygen (ULO) stored apple were differentially affected by different CO2 partial pressures in the storage atmosphere. Provided with this knowledge and experimental methodologies, it was then possible to investigate further the effects of production, storage, and handling procedures on volatile aroma production in various fruit. Volatile Production Impairment by Ripening Inhibition and Storage Handlings Apple. Patterson et al. (1974) were among the first investigators to use the mentioned sophisticated GC methods to determine the negative influence of controlled atmosphere (CA) storage on aroma volatile production. They realized that volatile aroma production recovered nicely after regular air (RA) storage but was considerably repressed after ‘‘prolonged’’ CA storage, although fruit firmness was preserved much better compared with fruit stored in RA. At that time, it was known that CA reduced not only aroma volatile production, but also poststorage fruit respiration and it was speculated by Anderson and Penney (1973) and Hatfield and Patterson (1975) that reduced respiration might be one reason for the depressed volatile production. In more detailed studies conducted by Bangerth and Streif (1987), Shatat et al. (1978), and Streif and Bangerth (1988), apples were held either in low-pressure storage (LPS), or CA with O2 partial pressures ranging from 1 to 21 kPa in combination with 0.8 to 9 kPa CO2 and different ethylene concentrations. These experiments showed a progressive decrease in respiration when CA conditions became more efficient in maintaining firmness and skin color and had a greater effect on reducing volatile formation than on other quality attributes. In addition, it became evident that the depression in respiration and volatile production after prolonged storage could not be overcome by addition of ethylene either applied directly into the storage container or by ethylene treatment during shelf life (Bangerth, 1984; Brackmann et al., 1993, 1995; Shatat et al., 1978). The most severe reduction in volatile production was observed with LPS at a partial O2 pressure of 1.4 kPa, which almost completely eliminated volatile emanation during as well as after LPS, whereas other quality parameters were maintained (Fig. 1; Bangerth, 1984). However, most surprisingly when aminoethoxyvinyl-glycine (AVG), an efficient inhibitor of ethylene biosynthesis, was sprayed two to three times preharvest, a similar effect in depressing volatile synthesis was observed, although these fruit were stored at ambient oxygen conditions (Bangerth et al., 1998; HalderDoll and Bangerth, 1987). Thus, low oxygen could not be the only factor repressing volatiles during storage. These early investigations into the effect of AVG were, to our knowledge, the first demonstration of a close correlation between fatty acid (FA) concentration of fruit and volatile production measured simultaneously in these fruit (Halder-Doll and Received for publication 3 Aug. 2011. Accepted for publication 2 Nov. 2011. We are grateful to the following colleagues and coworkers who contributed over the years to the results and conclusions: A. Brackmann; S.-J. Choi; A. Saquet; T. Tan; and H. Xuan; we also thank Dr. C. Forney at AAFC, Canada, and Roy McCormick at Kompetenzzentrum Bavendorf, Germany, for their critical reading of the manuscript. To whom reprint requests should be addressed; e-mail [email protected]. 4 HORTSCIENCE VOL. 47(1) JANUARY 2012 Bangerth, 1987). There was now good evidence that long term CA and LPS storage as well as AVG treatments all repressed respiration, and research suggested that reduced volatile production may be related to a decrease in FA concentration in the fruit. Not only postharvest but also preharvest treatments can have significant impacts on aroma formation in fresh-marketed as well as in stored fruit. Song and Bangerth (1996) showed the earlier apples are harvested, the poorer their ability to produce volatiles and again this was coupled with lower respiration and depressed FA concentrations compared with later harvested fruit (Song and Bangerth, 2003). This effect of fruit ripening on volatile production seems to be related to ethylene sensitivity because their treatment with high concentrations of ethylene stimulates fruit respiration and FA and volatile production, although to different degrees and therefore deviates in some respects from long-term stored fruit where the addition of ethylene has little to no effect (Song, 1994; Fig. 2). Fruit tend to be harvested early (i.e., pre-climacteric) for better marketing and storage behavior. However, the optimal storage behavior of preclimacteric harvested fruit does not apply to aroma production. As shown by Brackmann et al. (1993), the better firmness, sugar:acid ratio, background color, shelf life, etc., is at the expense of volatile aroma production, which is considerably reduced with earlyharvested fruit. These residual effects of premature or early harvested fruit on volatiles are still evident even after 9 or more months of ULO storage (Brackmann et al., 1993; Fellman et al., 2003). Furthermore, depending on the gas composition of the CA storage, either branched-chain (at high CO2) or straightchain (at low O2) aroma compounds are more reduced. As discussed, preharvest as well as postharvest factors and procedures can reduce aroma volatiles with a similar reduction in consumer perception (Hoehn et al., 2008). The following discussion emphasizes postharvest behavior of aroma volatiles. We followed the volatile production of apples during and after storage under LPS or ULO or treatment with AVGand 1-methylcyclopropene (1-MCP). All of these treatments reduced volatile production, particularly after an extended storage period. Full or partial recovery is possible during shelf life after 4 to 5 months of storage; however, thereafter, this is increasingly difficult to achieve. As mentioned, low O2 cannot be the main or exclusive factor influencing volatile biosynthesis, because extensive treatments with AVG or 1-MCP followed by RA storage (Halder-Doll and Bangerth, 1987; Fig. 9) have similar inhibitive effects. The alternative possibility that volatile deficiency is in some way related at least in a climacteric fruit like apple to the physiology and particularly the sensitivity of the fruit to ethylene seems to be reasonable. This conclusion was based on the fact that when high ethylene concentrations were added to apples in an LPS storage container, a declining response of the fruit to ethylene measured in terms of firmness loss or increase in respiration and volatile production was observed (Bangerth, 1984; Bangerth et al., 1998). A decline in sensitivity to exogenous ethylene was also observed by Cook et al. (1985) who discovered this effect after treating carnation petals with AVG. Alternatively, investigations into substrate availability, O2 concentration, temperature, and a few other possibilities were also performed with apple fruit but no conclusive evidence could be found that these factors by themselves played a decisive role in ethylene sensitivity or volatile Fig. 1. Aroma volatile production of ‘Golden Delicious’ apple fruit during a 4 months storage period. (A) Regular air storage (Co.) at 1 C; (B) low-pressure storage (LPS) at 50 Torr ( 6.6 kPa) pressure corresponding to 1.4 kPa O2 and at a temperature of 1 C. Each single chromatogram depicts the aroma volatile production of 20 kg of fruit during a 1-month storage period (modified after Shatat et al., 1978). Fig. 2. Continuous treatment of immaturely harvested ‘Golden Delicious’ fruit with 100 mL L ethylene and its effect on respiration, fatty acids, and aroma volatiles. Total fatty acids were measured only once 35 d after fruits were harvested and exceeded control (control =100) values by 50 times (modified after Song and Bangerth, 1996, 2003; Tan, 1999). HORTSCIENCE VOL. 47(1) JANUARY 2012 5 | REVIEW