Article pubs.acs.org/JAFC

Effect of Ethylene and Temperature Conditioning on Sensory Attributes and Chemical Composition of ‘Comice’ Pears Warangkana Makkumrai,†,¶ Hanne Sivertsen,‡ David Sugar,§ Susan E. Ebeler,# Florence Negre-Zakharov,† and Elizabeth J. Mitcham*,† †

Department of Plant Sciences, ‡Department of Food Science and Technology, and #Department of Viticulture and Enology, University of California, Davis, California 95616, United States § Southern Oregon Research and Extension Center, Oregon State University, Medford, Oregon 97502, United States ABSTRACT: ‘Comice’ is among the pear varieties most difficult to ripen after harvest. Ethylene, cold temperature, and intermediate (10 °C) temperature conditioning have been successfully used to stimulate the ability of ‘Comice’ pears to ripen. However, the sensory quality of pears stimulated to ripen by different conditioning treatments has not been evaluated. In this study, a descriptive sensory analysis of ‘Comice’ pears conditioned to soften to 27, 18, and 9 N firmness with ethylene exposure for 3 or 1 days, storage at 0 °C for 25 or 15 days, or storage at 10 °C for 10 days was performed. Sensory attributes were then related to changes in chemical composition, including volatile components, water-soluble polyuronides, soluble solids content (SSC), and titratable acidity (TA). The sensory profile of fruit conditioned with ethylene was predominant in fibrous texture and low in fruity and pear aroma. Fruit conditioned at 0 °C was described as crunchy at 27 and 18 N firmness and became juicy at 9 N firmness. Fruit conditioned at 0 °C produced the highest quantity of alcohols and fewer esters than fruit conditioned at 10 °C, and they had higher fruity and pear aroma than fruit conditioned with ethylene, but lower than fruit conditioned at 10 °C. Fruit held at 10 °C were predominant in fruity and pear aroma and had the highest concentration of esters. Water-soluble polyuronides were strongly, positively correlated (r > 0.9) with sensory attributes generally associated with ripeness, including juiciness, butteriness, and sweetness and negatively correlated (r > −0.9) with sensory attributes generally associated with the unripe stage, such as firmness and crunchiness. However, water-soluble polyuronides were not significantly different among conditioning treatments. Sensory sweetness was not significantly correlated with SSC, but TA and SSC/TA were significantly correlated with sensory tartness. However, there were no significant differences among the conditioning treatments in sweet or sour taste perception when the fruit fully softened. The results indicate that the various methods of conditioning ‘Comice’ pear fruits for ripening had different effects on their sensory and chemical properties that may influence their sensory quality. KEYWORDS: alcohol, aldehyde, aroma, conditioning, ester, Pyrus communis, sensory descriptive analysis, volatile compounds



polyuronides decrease.5 Buttery and juicy texturex in ripe ‘Marguerite Marillat’ pears were correlated to high WSP (200 mg uronic acid (100 g fw)−1), whereas fruit with less WSP (100−150 mg uronic acid (100 g fw)−1) had a dry and coarse texture.6 Changes in cell wall metabolism during ripening are regulated by ethylene.6−8 Both temperature and ethylene conditioning can induce ethylene biosynthesis in pears, and the ethylene produced subsequently regulates the expression of genes encoding enzymes involved in cell wall degradation. Increasing ethylene production was associated with pectin degradation and a loss of firmness in ‘Passe-Crassane’ pears as evidenced by an increase in WSP.8 Pear flavor depends on aromatic volatile compounds, which impart to the fruit its distinctive and typical character.9,10 Ethyl 2-methylbutanoate was reported to be responsible for the characteristic aroma of ‘Comice’ pears.11 The aroma of ‘Comice’ pears that ripened optimally after cold storage for 3 months was dominated by esters, such as hexyl acetate and butyl acetate, accounting for >92% of total volatiles.12 However, ‘Comice’

INTRODUCTION To increase consumption of pear fruit, consumer satisfaction must be increased by reliable ripening to good eating quality.1 ‘Comice’ pears resist ripening after harvest due to their lack of capacity to produce ethylene at harvest maturity.2 Cold and intermediate (10 °C) temperature conditioning and ethylene conditioning protocols have been developed to stimulate fruit ethylene production and ripening. For ‘Comice’, cold storage at −1 to 0 °C for 25−31 days is required when fruits are harvested in the early season (corresponding to a firmness of ∼53−58 N).2 Exposure of ‘Comice’ fruit harvested at ∼58 N to an intermediate temperature of 10 °C required only 12 days to stimulate ripening of the fruit.3 Additionally, treatment with 100 μL L−1 ethylene gas for 3 days at 20 °C following 3 days of cold storage was also effective in stimulating ripening of the fruit harvested at 50−58 N.2 Despite the development of alternative techniques for conditioning ‘Comice’ pears, the effect of these conditioning treatments on the sensory quality of pears has not been investigated. Pear fruit sensory quality can be determined from its color, texture, aroma, and taste, the most important sensory properties that consumers recognize.4 Texture is partially related to changes in cell wall composition during ripening; as pears soften, watersoluble polyuronides (WSP) increase while water-insoluble © 2014 American Chemical Society

Received: Revised: Accepted: Published: 4988

November 23, 2013 March 30, 2014 April 8, 2014 May 20, 2014 dx.doi.org/10.1021/jf405047v | J. Agric. Food Chem. 2014, 62, 4988−5004

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for use in the experiments. The fruits were stored at 20 °C and >90% relative humidity prior to the initiation of treatments on the following day. Conditioning Treatments. The pears were randomly divided into six groups (five conditioning treatment groups and one control, nonconditioned group) with 200−350 fruits per group. Three groups were exposed to different storage temperatures and times; 0 °C (low temperature) for 15 or 25 days and 10 °C (intermediate temperature) for 10 days with >90% relative humidity (RH). Two groups were exposed to 100 μL L−1 ethylene gas at 20 °C in two 300 L stainless steel tanks for 1 or 3 days. Humidified air (95% RH) containing 100 μL L−1 ethylene was passed through the tank at 4 L min−1 to keep CO2 concentration 1 (Table 6).

1-heptanol, 1-hexanol, 1-octanol) as compared to fruit from other conditioning treatments. These compounds have a sweet, green note except 1-butanol, which is described as fermented and fruity (Table 6). These alcohols described fruit conditioned at 0 °C and softened to 27 N; a reduced number of these compounds described fruit softened to 18 N, and only 1-butanol described fruit softened to 9 N firmness. Our results showed that fruit conditioned at 10 °C for 10 days developed higher levels of volatiles in general and esters in particular at each ripeness stage evaluated, even though they had lower ethylene production at some firmness levels. The volatile composition of ripe fruit conditioned at 0 °C for 15 days was predominant in aldehydes, whereas fruits conditioned at 0 °C for 25 days were predominant in esters and alcohols. Fruit conditioned with ethylene for 1 or 3 days had high concentrations of aldehydes, but fruit conditioned with ethylene for 3 days also had high concentrations of esters. It appears that the longer treatments with ethylene or at 0 °C, both of which represent recommended treatments for maximum initiation of ripening, resulted in greater production of fruity esters. It will be interesting to further investigate gene expression and enzyme activities involved in alcohol formation (alcohol dehydrogenase, ADH) and ester formation (alcohol acyltransferase, ATT) in each conditioning treatment to determine how ADH and AAT are regulated in ‘Comice’ pears and the role of ethylene. Most compounds detected in our study by headspace sorptive extraction (HSSE) have been previously detected in pears using other extraction techniques such as dynamic headspace concentration on carbon traps, simultaneous distillation and extraction, and solid phase microextraction (SPME). However, HSSE can be much more sensitive than those previous methods. Tienpont et al.29 showed that HSSE was 100 times more sensitive than SPME, with high enrichment factors for highly volatile solutes. Limits of detection were on the order of 0.1 ppb in the full scan MS mode in Tienpont’s study. Moreover, HSSE has a higher sensitivity than SPME for nonpolar and some medium polar compounds.30,31 Using the dynamic headspace method, Lara et al.32 detected ∼150 and ∼50 μg kg−1 butyl acetate and hexyl acetate, respectively, in ‘Comice’ pears after 2 months of storage at −1 °C and ripening. Suwanagul and Richardson12 found butyl acetate and hexyl acetate in the amounts of ∼9 and ∼30 μg kg−1 100 L−1, respectively, after the fruits were stored at −1 °C for 3 months and softened. In our study using the HSSE technique, we detected much higher amounts of butyl acetate and hexyl acetate. Fruit conditioned at 0 °C for 25 days and softened to 9 N had 225 and 91 μg kg1− of butyl acetate and hexyl acetate, respectively, whereas fruit conditioned at 10 °C for 10 days had more, with 404 and 412 μg kg−1, respectively. Several compounds not previously identified in pears were detected in our samples, including four aldehydes (decanal, 2-decanal, tetrdecanal, and a tentative identification of pentadecanal), one ester (methyl benzoate), two ketones (2-pentanone, β-damascenone, and limonene) and nonanonic acid (tentative). Even though the pears used in sensory testing were selected with an instrument (penetrometer) to have the same firmness, panelists perceived significant differences in firmness and crunchiness attributes among the various conditioning treatments. Crunchiness and firmness characterized cold-temperature treatments when the fruits were not fully ripe (27 and 18 N), whereas in 9 N fruit these attributes became prominent in fruit conditioned with ethylene. Juiciness was not significantly different among the various conditioning treatments in firmer 5001

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treatment at 0 °C resulted in fruit with higher concentrations of alcohols, the odor units for these alcohols were low, and therefore their contribution to aroma perception is uncertain. Ester levels were also elevated in fruit conditioned at 0 °C for 25 days compared with other treatments, although not as high as in fruit conditioned at 10 °C. Ethylene treatments produced fruit described predominantly as fibrous in texture and having sweet, green aroma notes. The concentrations of hexanal and (E)-2-hexenal were high in fruit conditioned with ethylene for 1 day. When the same type of conditioning treatments were compared, fruit conditioned at 0 °C for 25 days had higher sweetness and juiciness scores than fruit conditioned at 0 °C for 15 days, and fruits conditioned with ethylene for 3 days were juicier than fruits conditioned with ethylene for 1 day. It is essential to determine whether the differences in sensory attributes observed among these conditioning treatments will influence consumer liking before the industry can determine the best conditioning treatment to use to maximize consumer satisfaction. This remains for a future study.

Cold-temperature (0 °C) conditioning was associated with high concentrations of alcohols, including 1-butanol, 1-hexanol, 1-pentanol, 1-octanol, and 1-heptanol. These alcohols have a high odor threshold (Table 6), and their odor units in 9 N fruit held at 0 °C for 25 days (the conditioning treatment that resulted in the highest concentrations of these alcohols) were very low (1. Hence, fruits conditioned at 0 °C for 25 days were predominant in fruity, pear aroma. However, each of the above assumptions is based on concentrations of volatile compounds calculated relative to the internal standard, and therefore more accurate quantification, along with aroma extract dilution analysis, and aroma reconstitution and omission experiments,40 is needed to ascertain the precise contribution of each compound to pear aroma following each conditioning treatment. Consumer studies have found that people like juicy and sweet pears with detectable pear flavor and aroma.1,4,34 From the correlation table, the analytical variables (i.e., butyl acetate, pentyl acetate, hexyl acetate, propyl acetate, 3-methylbutyl acetate, 5-hexenyl acetate, heptyl acetate, 1-butanol, 1-hexanol, 1-hepatanol, 1-propanol, 1-octanol, total esters, total alcohols, total aroma, and WSP) all had similar positive correlations with sensory attributes including fruity aroma, pear aroma, apple aroma, fermented aroma, butteriness, juiciness, and sweetness; these compounds may therefore be indicators of ripeness associated with desired eating quality. The compounds having high correlation with fruity aroma (r > 0.85) and pear aroma (r > 0.75) were butyl acetate, pentyl acetate, and hexyl acetate. On the other hand, hexanal, (E)-2-hexenal, and pentanal were positively correlated with green stem aroma, an attribute that is generally undesirable in ripe pears.1 Water-soluble polyuronide content was the variable that best correlated with texture attributes generally associated with ripe fruit, such as butteriness (r = 0.95), juiciness (r = 0.94), and sweetness (r = 0.95), and was negatively correlated with firmness (r = −0.93) and crunchiness (r = −0.92). Our results are consistent with previous studies which showed that WSP levels increased as fruit softened6 and that the fruits with higher WSP were more buttery and juicier than those with lower WSP.6 Although there were significant differences in WSP among the firmness stages, the conditioning treatments did not affect WSP levels. This is interesting given the significant differences in perception of firmness, crunchiness, juiciness, and fibrousness that were detected among the conditioning treatments. The present study found that SSC was not a good instrumental parameter for predicting sensory sweetness, confirming prior studies.19,34,35 However, TA was a good parameter for predicting sensory sweetness and tartness, and SSC/TA was good for predicting sensory tartness. These results agree with previous studies.19,36 In conclusion, different conditioning treatments resulted in different sensory attributes of ‘Comice’ pears. The 10 °C for 10 day and 0 °C for 25 day treatments resulted in ripe fruit described as having fruity and pear aroma attributes that were associated with high ester content, especially butyl, pentyl, and hexyl acetate. These esters were much higher in fruit conditioned at 10 °C than at 0 °C. Fruity and pear aroma attributes were much lower in fruit conditioned at 0 °C for 15 days. Although



AUTHOR INFORMATION

Corresponding Author

*(E.J.M.) Phone: (530) 752-7512. Fax: (530) 752-6746. E-mail: [email protected]. Present Address ¶

(W.M.): Horticulture Research Institute, Department of Agriculture, Bangkok, Thailand.

Funding

This work was supported in part by a USDA Specialty Crops Grant (2009-51181-05783). Also, financial support from the Loyal Thai Government Scholarship is appreciated. Notes

The authors declare no competing financial interest.

■ ■ ■

ACKNOWLEDGMENTS We thank all of the members of the Mitcham laboratory group for their assistance. ABBREVIATIONS USED PLS, partial least-squares; SSC, soluble solids content; TA, titratable acidity; WSP, water-soluble polyuronides REFERENCES

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dx.doi.org/10.1021/jf405047v | J. Agric. Food Chem. 2014, 62, 4988−5004

Effect of ethylene and temperature conditioning on sensory attributes and chemical composition of 'Comice' pears.

'Comice' is among the pear varieties most difficult to ripen after harvest. Ethylene, cold temperature, and intermediate (10 °C) temperature condition...
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