Preharvest factors influencing flavor of fresh fruit and vegetables

Preharvest factors influencing flavor of fresh fruit and vegetables

Postharvest Biology and Technology 15 (1999) 227 – 232 Preharvest factors influencing flavor of fresh fruit and vegetables James P. Mattheis a,*, Joh...

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Postharvest Biology and Technology 15 (1999) 227 – 232

Preharvest factors influencing flavor of fresh fruit and vegetables James P. Mattheis a,*, John K. Fellman b a

US Department of Agriculture, Agricultural Research Ser6ice, Tree Fruit Research Laboratory, 1104 N. Western A6enue, Wenatchee, WA 98801, USA b Department of Horticulture and Landscape Architecture, Washington State Uni6ersity, Pullman, WA 99164, USA Received 30 June 1998; accepted 11 November 1998

Abstract Perception of fruit and vegetable flavor is a composite of sensory responses in the nose and mouth to aroma and taste. A diverse array of fruit and vegetable constituents including acids, sugars, volatiles and many other compounds individually elicit sensory responses that are recognized in total as flavor. Accumulation of these compounds during development as well as dynamic changes during ripening and/or senescence are determined in large part by the genetics of each species as well as developmental stage at harvest. However, other factors that influence development prior to harvest subsequently impact flavor. For horticultural crops, environment, cultural practices, agrichemicals and nutrition are some of the factors impacting flavor through effects on plant development. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Flavor; Fruit; Perception; Preharvest factors; Vegetable

Flavor is a combination of the basic tastes (salty, sweet, sour and bitter), mouthfeel and aroma (Meilgaard et al., 1991). A diverse array of compounds contribute to the characteristic flavor of fresh fruit and vegetables. Total soluble solids and titratable acidity are often evaluated in analyses of produce and these components are critical to overall sensory quality. In addition, many * Corresponding author. Tel. + 1-509-664-2280; fax: + 1509-664-2287; e-mail: [email protected]

other compounds contribute to flavor via stimulation of taste receptors in the mouth or by retronasal stimulation in the nose during chewing. The complexity of interactions between many compounds that contribute to fresh produce flavor makes arriving at meaningful conclusions for research based solely on instrumental analysis difficult. Sensory analysis provides a critical additional means to evaluate the flavor of produce. The horticultural literature contains numerous reports describing effects of preharvest factors on

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postharvest quality of fruits and vegetables. Much of this information documents the impact of various cultural practices and environmental influences on accumulation during development and/or production during ripening of compounds that may contribute to flavor. Instrumental analyses of compounds contributing to flavor are usually interpreted for impact on ‘quality’ rather than flavor. The use of instrumental analyses to identify factors that alter produce composition in ways that may impact flavor is valuable and serves as an initial phase of fruit and vegetable flavor research. Many preharvest factors are known to impact produce development and quality. Genetic, environmental, cultural and developmental influences are well documented. In many cases, sensory analysis has also shown impacts on flavor. Criteria for selecting the examples to follow focused on evaluation of a preharvest factor for impact on produce flavor determined using sensory analysis. Accompanying instrumental analysis of produce flavor components is presented where available; however, not all of the factors have both types of results available in the literature. The literature cited herein is not intended to be a comprehensive reference of all reports of preharvest factors influencing flavor but rather an illustration of how the preharvest environment can influence flavor of fresh fruit and vegetables. The influence of climate on carrot (Daucus carota L.) flavor was demonstrated using plants grown in controlled environments that simulated climatic conditions of locations in California, Florida and Wisconsin (Simon et al., 1982). Diurnal temperature, humidity, daylength and light intensity were controlled using growth chambers. Flavor and compositional differences were demonstrated by sensory and objective analyses of raw carrots. Carrots grown under simulated Wisconsin conditions rated higher for harsh flavor and lower for sweetness and preference. The amounts of carotenoids and terpenes were high and total sugars low in carrots grown under simulated Wisconsin conditions. The shift in the contents of these flavor constituents may have contributed to the poorer flavor ratings.

Seasonal variation in tangerine (Citrus reticulata)(Harding and Sunday, 1949), tangelo (Citrus× Tangelo) (Harding et al., 1959) and grapefruit (Citrus×paradisi Macf.) (Harding and Fisher, 1945) flavor were determined by sensory analysis to be related, in part, to variation in sugar and acid accumulation. Variation in onion (Allium cepa L.) pungency, estimated by enzymatic determination of pyruvic acid (Schwimmer and Weston, 1961; Wall and Corgan, 1992) was evaluated for ten cultivars planted at two locations during three seasons (Hamilton et al., 1996). Season was the most significant source of pungency variation followed by cultivar. Rootstock effects on crop quality have been noted for a number of citrus fruit. Harding et al. (1940) determined through extensive study of sweet orange (Citrus sinensis) cultivars grown in Florida over three seasons that the best flavor was found in fruit from trees grown on sour-orange (Citrus aurantium), sweet-orange, grapefruit or Citrus reticulata ‘Cleopatra’ rootstocks. Lower flavor ratings for fruit grown on other rootstocks were accompanied by lower fruit titratable acidity. Rootstock has been shown to influence flavor of grapefruit (Cooper and Lime, 1960) and tangerine (Harding and Sunday, 1949; Hodgson, 1967) Differences in ‘Valencia’ oranges on two rootstocks were reported by Baldry et al. (1982). Fruit from trees with Citrus 6olkameriana rootstock were judged inferior in flavor and juiciness, and also had lower ascorbic acid content compared to the same scion grafted onto sour-orange rootstock. Volkameriana rootstock also resulted in fruit with lower titratable acidity and total soluble solids. Citrus fruit and juice bitterness has also been shown to be influenced by rootstock (Marsh, 1953; Kefford and Chandler, 1961; Chandler and Kefford, 1966). Crop nutrition can influence plant development with subsequent effects on fruit and vegetable flavor. Accumulation of the precursor for the onion lacrimatory factor increases with increased sulfur availability (Randle, 1997). This increase is accompanied by a decrease in the amounts of precursors for volatiles imparting ‘green’ and ‘cabbage’ notes to fresh onion. Sensory analysis of ‘Granex 33’ onions grown under increasing sulfur

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availability demonstrated a reduction in green aroma until excess sulfur was present, while pungency and total sulfur flavor increased with sulfur availability. Fertilizer treatments can influence production of volatile compounds during apple (Malus syl6estris Mill., Malus pumila Mill., Malus domestica Borkh.) fruit ripening. High nitrogen treatments resulted in increased volatile production by ‘McIntosh’ apples (Smogyi et al., 1964). Volatile production by ‘Cox’s Orange Pippin’ increased when foliar sprays of urea and MgSO4 were used to supplement soil-applied Ca(NO3)2. Alcohol production relative to esters was higher in fruit with low phosphorus (P) content, indicating a possible limitation of low P content on ester production (Brown et al., 1968b). Flavor ratings of ‘d’Anjou’ pear fruit decreased with increased nitrogen fertilization (Raese, 1977). Fruit flavor ratings conducted after 7 months’ storage were negatively correlated with leaf nitrogen (N) although lower N treatments also resulted in smaller fruit size. This example illustrates the grower’s dilemma in that, although fruit flavor is an important attribute of overall quality, size is much easier to evaluate and is one of the parameters determining grower returns. Although the influence of cropping and crop load on a number of aspects of fruit quality has been extensively reported, little information exists regarding crop load impact on compounds contributing to aroma. Poll et al. (1996) examined how ‘Jonagored’ apple fruit quality and volatile production were influenced affected by crop load. Apples grown with the lowest fruit/leaf ratio had the highest soluble solids content and titratable acidity throughout a late season harvest period. Production of butyl and hexyl acetate were highest for the lowest fruit/leaf ratio at the later harvests, indicating a possible beneficial relationship between low crop load and fruit aroma. Water availability is another preharvest factor that impacts flavor. Sensory ratings for sweet potatoes (Ipomoea batatas Lam.) were highest when the amount of irrigation maximized yield (Thompson et al., 1992). Flavor ratings decreased as irrigation exceeded 111% of pan evaporation. The response to drip or furrow irrigation applied

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close to harvest was evaluated for effects on flavor and other muskmelon (Cucumis melo L.) fruit quality attributes (Lester et al., 1994). All plots were watered 8 days prior to harvest followed by an additional irrigation at 1, 2 or 4 days prior to harvest or no additional irrigation prior to harvest. Melons harvested from plots drip-irrigated 4 days prior to harvest had lower sweetness and preference ratings compared to those having the last water application 8 days prior to harvest. Furrow irrigated plots watered 1 and 2 days prior to harvest also had lower preference ratings relative to other treatments. Water applied by both irrigation methods 4 days prior to harvest resulted in lower soluble solids concentration, greater fruit volume and greater moisture compared to treatments where the last irrigation was 8 days prior to harvest. This example illustrates the lack of simple relationships between sugar content and sensory analysis of flavor and preference. The quantity and quality of sunlight during development also impacts fruit and vegetable flavor and composition. Plastic mulches used to increase water use efficiency and weed control were found to also impact turnip (Brassica rapa L.) flavor (Antonious et al., 1996). Plants grown with blue, green or white mulch produced turnips sensory panelists rated having ‘sharp’, ‘mild’ or ‘less distinct’ flavor, respectively. Glucosinolates, compounds considered to contribute to flavor of cruciferous vegetables, had the highest concentration in turnips grown with blue mulch. Reducing sugar was highest in roots grown under green mulch. Manipulation of sunlight availability during fruit development resulted in altered postharvest volatile production by ‘Delicious’ apple fruit (Miller et al., 1998). Knitted plastic shade cloth was used reduce light intensity but not light quality incident on the fruit. Acetate ester production for apples harvested 179 days after full bloom increased with light exposure up to 53% of full sun. Acetate ester production was lower for fruit with more or less exposure than 53% full sun. A canopy position effect on ester production was also observed—fruit with a western or southern aspect having higher ester emission rates compared to apples with northern or eastern exposure.

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Bagging of apples and other tree fruit during development on the tree is a commercial practice used for pest control, to reduce physiological disorders and to alter fruit appearance (Mink, 1973; Hofman et al., 1997). In addition to exclusion of direct sunlight, bagging also alters fruit temperature (Andrews and Johnson, 1996). The flavor of bagged apples is considered inferior to that of non-bagged fruit (Mink, 1973), although no difference in consumer preference was found for bagged and non-bagged mango (Mangifera indica L.) fruit (Hofman et al., 1997). Bagged apple fruit has lower soluble solids content (Kume and Kudo, 1982) and titratable acidity and ester production is reduced at harvest and throughout subsequent cold storage (J.P. Mattheis, unpublished). Reduced soluble solids content, titratable acidity, watercore and ester production by apple fruit bagged during development may all contribute to the lack of flavor mentioned by Mink (1973). Plant growth regulators are powerful horticultural production tools and effects of several materials on flavor and fruit composition have been reported. Ethephon (2-chloroethyl phosphonic acid), an ethylene releasing compound, applied 3 days prior to harvest of muskmelon, resulted in reduced sensory scores for sweetness and flavor when fruit were analyzed 1 day after harvest (Yamaguchi et al., 1977). The same sensory analysis conducted 2 days after harvest indicated a difference only for sweetness, and no differences were detected 5 days after harvest. ‘Jonathan’ apples treated with ethephon had higher sensory scores for flavor and general acceptability while the same treatments applied to ‘Delicious’ apples did not result in enhanced flavor or acceptability ratings (McBride and Faragher, 1978). Part of the difference may have been due to the lack of an effect of ethephon on ‘Jonathan’ texture while the same treatments accelerated texture changes in ‘Delicious’ fruit. The ethylene synthesis inhibitor aminoethyoxyvinylglycine (AVG) delays fruit ripening and reduces volatile production (Romani et al., 1983; Child et al., 1984). This effect of AVG can be overcome if fruit are exposed to ethylene immediately after harvest (Bangerth et al., 1984) or during cold storage (Bangerth and Streif, 1987).

Materials used for pest and disease management also result in changes in fruit flavor and composition. A possible indirect effect of insect control on strawberry (Fragaria× ananassa, Duch.) fruit flavor was demonstrated by Podoski et al. (1998). In addition to biological controls, abamectin (avermectin B1) and fenbutatin oxide [hexakis(2-methyl-2-phenylpropyl)distannoxane] were used to maintain spider mite density at 10–50 motile forms per leaflet. Mite control resulted in improved sweetness and flavor intensity of ‘Sweet Charlie’ strawberries but had no effect on fruit color or firmness. ‘Golden Delicious’ apples receiving multiple applications of preharvest sterol synthesis inhibiting fungicides had reduced production of several classes of volatile compounds (Aubert, 1997). The production of aldehydes, alcohols, sesquiterpenes and esters decreased with increased number of fungicide applications. Differences in sensory flavor and concentrations of compounds contributing to flavor attributable to various environmental and cultural practices are evident from the previous examples. The influence of cultivar and maturity at harvest also are critical determinants of flavor. Cultivar differences and the complexity of the relationship between sensory and instrumental analyses is illustrated by research reported for tomato (Lycopersicon esculentum Mill.) fruit (Stevens et al., 1977). Cultivar differences in soluble solids, titratable acidity, pH and concentrations of individual sugars, acids and volatile compounds were detectable but analytical values did not correlate well with sensory scores for all cultivars. The importance of interactions between sugars and acids to sweetness, sourness and overall flavor was apparent as was the lack of simple relationships between volatile compounds and aroma. Although aroma differences between cultivars were not detected by the sensory panel, large intercultivar variation in volatile content was determined by gas chromatography and significant correlations between flavor scores for ‘overall flavor intensity’ and ‘tomato-like’ were present for several volatile compounds. Cultivars with a range of flavor descriptors have been shown to differ in amounts of volatile compounds in a manner that

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allows cultivar discrimination (Langlois et al., 1996). However, the relationship between volatiles, aroma and flavor is complex and often only general conclusions as to consumer preference can be made from extensive sensory comparisons of cultivars (Daillant-Spinnler et al., 1996). Maturity at harvest plays a central role in flavor development, particularly for climacteric fruits where ripening is regulated by ethylene. Production of volatile compounds that contribute to aroma and flavor change dramatically as ripening progresses. However, fruit harvested at an immature stage produce many of these compounds at rates too low to achieve characteristic flavor. Early harvest is advantageous commercially to maintain texture during storage, handling and transport but comes at the expense of flavor development during storage. Early harvest of apple and tomato fruit results in a time lag until ripening related volatile production begins, and the amount of volatiles produced does not approach that of fruit harvested at a more mature stage (Brown et al., 1968a; Dirinck et al., 1989; Baldwin et al., 1991). Tomato fruit picked at a relatively immature stage and allowed to ripen are less sweet, more sour, less ‘tomato-like’ and have more off-flavor compared to vine ripened fruit (Kader et al., 1977). Perceived flavor results from the interaction of numerous components of fruits and vegetables. Many preharvest factors impact flavor by influencing plant growth and development, but the influence may be insignificant if produce is harvested too early or the cultivar is not genetically capable of developing desirable flavor. There are many examples of differences in flavor and/or amounts of compounds contributing to flavor attributable to environmental and cultural preharvest factors. General seasonal effects, particularly temperature during development, impact flavor and composition as does rootstock, cropping and pruning. Irrigation, plant nutrition, pest management practices and the use of plant growth regulators during production all can impact flavor. Much of the research that has evaluated the impacts of cultural practices on fruit and vegetables has focused on chemical analysis of produce com-

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ponents that are known or assumed to be important for flavor development. Accompanying sensory assessment is often not conducted, perhaps limiting the overall impact of the research. Sensory analysis provides its own set of analytical, logistic and economic challenges, yet it remains the ultimate evaluation for flavor of fresh produce.

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