Physicochemical Properties of Whole Fruit and Sensory Quality of Fresh-Cut Apples Pre-Treated with 1-Methylcyclopropene (1-MCP)

Abstract

Apple fruit quality is an important factor in minimally processed or fresh-cut slices. Ethylene action inhibitor, 1-methylcyclopropene (1-MCP) is used commercially to extend the shelf life of selected fruits, including apples. The present study was designed to assess the selected physiochemical properties of whole fruit and sensory attributes of fresh-cut slices from three 1-MCP treated apple cultivars (Jonagold, Empire, and Mutsu) that were stored for 3 months in air or 6, 9, and 12 months in controlled atmosphere. Apples and fresh-cut slices were evaluated for selected quality characteristics at 3-month intervals. Generally, the 1-MCP treatment resulted in firmer fruit and lower pH, whereas it had no or minimal effect on fruit skin color and total soluble solids. Titratable acidity values decreased in both control and 1-MCP treated apples, the only exception being 12-month controlled atmosphere stored apples that showed an increase. At each 3-month interval, fresh-cut slices (after 10-day storage at 4°C) were evaluated by a 9-member trained panel for “difference from control” on a 0–8 scoring scale; the results of these evaluations showed relatively small “difference from control” scores for the degree of browning, texture, and flavor. At the 9-month storage, a 114-member consumer panel evaluated the appearance, texture, flavor, and overall acceptability of fresh-cut slices on a 1–9 hedonic scale. Except for Jonagold slice flavor, slices from 1-MCP treated fruit exhibited similar or higher sensory scores. The overall acceptability scores were 6.78, 7.12, and 7.18 for slices from Jonagold, Empire, and Mutsu apples, respectively.

Keywords:

• Fresh-cut slices
• Apple
• 1-MCP
• Color
• Texture
• Sensory quality

INTRODUCTION

Postharvest treatment of fruits with 1-methylcyclopropene (1-MCP), which blocks ethylene production or action and hence helps retain firm texture during storage, has gained wide application in recent years.[1 ⁻ 3] Among fruits, apple (Malus x domestica Borkh.) is one of the commodities on which 1-MCP has shown significantly better results in maintaining quality during postharvest storage.[4,5] The whole fruit quality plays an important role in the shelf life of minimally processed or fresh-cut products. In the case of fresh-cut apple slices, many factors contribute to the final quality including, but not limited to, cultivar type, use of fresh versus stored apples as raw material, postharvest storage treatments like 1-MCP, processing treatments (anti-browning agents), and the type of packaging.[6]

Fresh-cut produce is one of the fastest growing segments of the food industry.[7,8] However, the fresh-cut products undergo color degradation, resulting from the action of mainly two enzymes, polyphenol oxidase and peroxidase. In addition to visible color changes, these enzymes impair not only the other sensory properties, hence, the marketability of the product, but often lower its nutritive value as well.[8,9] From the consumer's perspective, color or appearance of fresh-cut produce is one of the most important quality attributes. If the color of a fresh-cut product is not attractive and of acceptable quality, the consumer is less likely to purchase it regardless of its excellent flavor, taste, or other quality attributes.[10]

Fresh-cut slices from 1-MCP treated apples have been evaluated for objective quality previously,[11,12] but none of these studies reported on any of the sensory attributes; e.g., degree of browning, flavor, firmness, or overall acceptability. Fruit cutting method has an impact on the texture of the fresh-cut products.[13] Other past studies on slice quality from fresh apples, have focused on the determination of instrumental color, sensory or microbial quality as affected by anti-browning or sanitizer treatments,[9,14,15] modified atmosphere (MA) storage,[16 ⁻ 18] or edible coatings.[19,20] However, there is limited, if any, documentation on the sensory evaluation of fresh-cut slice prepared from 1-MCP treated apples that were stored long-term. The main objectives of the present study were to compare the selected physiochemical properties of whole fruit and sensory attributes of fresh-cut slices from three 1-MCP treated apple cultivars (Jonagold, Empire, and Mutsu) that were stored for up to 12 months. Whole fruit and fresh-cut slices were evaluated for selected quality characteristics at 3, 6, 9, and 12 months.

MATERIALS AND METHODS

Jonagold, Empire, and Mutsu apples, procured from a local source, treated with 1 μL/L of 1-MCP (SmartFresh) for 16 h, were stored at 0.0 ± 0.5°C under air for 3 months or controlled atmosphere (CA) conditions (1.5 mL/100 mL O₂, 3.0 mL/100 mL CO₂) for 6, 9, and 12 months. The long-term storage period was investigated to assess the feasibility of storing apples up to12 months to ensure continuous supply of high-quality fruit for the fresh-cut industry. Apples from above storage conditions were pulled out after 3, 6, 9, and 12 months for fresh-cut slice processing and quality evaluation; at each interval, whole fruit were also analyzed for selected physicochemical quality attributes.

Whole Fruit Quality

At each slice processing period, objective evaluation of apples from three cultivars was conducted for the following quality attributes: Fruit firmness, skin color, total soluble solids (TSS), pH, and titratable acidity (TA). All these analyses were done using three replicate, unless noted otherwise.

Fruit firmness was determined using a drill-stand-mounted penetrometer (FT-327, Food Technology Corp., Sterling, VA, USA) fitted with an 11-mm diameter probe.[21] Two skin discs of approximately 2.5 cm diameter were removed from opposite sides of the fruit. The penetrometer probe was pressed into the fruit tissue to a depth of 8–9 mm in a single smooth motion. Fruit firmness, as mean of 20 measurements from 10 apples, is reported as pound force (lb-force).

The skin color, TSS, pH, and TA were analyzed according to the method of Siddiq et al.[22] Skin color, as “L”, “a” and “b” values (10 or more apples, 20 measurements), was determined using Hunter Color Meter (Model: D25 L optical sensor, Hunter Associates Lab., Reston, VA, USA). The TSS were determined as Brix, using an Abbe-3L refractometer, sensitivity 0.1% (Bausch & Lomb Optical Co., Rochester, NY, USA). The TA was determined as g/100 g malic acid equivalent, using 0.1N NaOH titration method, as TA = (mL NaOH × N NaOH × 0.067 meq × 100) / weight of sample.

Fresh-Cut Slice Processing

Apples were graded for the uniformity of size and washed under tap water and then dipped in a commercially available sanitizer, Fruit & Vegetable Wash (SC Johnson Professional, Sturtevant, WI, USA) for five minutes at a concentration to yield 160 μL/L of available chlorine. Apples were then sliced using an automatic corer/slicer (Bock Engineered Products, Inc., Toledo, OH, USA) with a 10-cut blade (i.e., 10 slices per apple). Slices were dipped in the above sanitizer for five minutes at a concentration to yield 80 μL/L of chlorine followed by a 30 s dip in 3% (w/v) NatureSeal^® (Mantrose-Haeuser Co., Inc., Westport, CT, USA), a calcium salt of ascorbic acid that is used as an anti-browning treatment commercially. After draining off excessive treatment solution, the cut slices were packaged in 2 mil polyethylene bags (U-line, Pleasant Prairie, WI, USA), with nine pin-holes each. Apple slice bags (2 lbs slices/bag) were sealed and stored at 4°C for 10 days before evaluation of sensory quality.

Sensory Evaluation

Sensory evaluation of slices was done by a small trained panel (at 3-, 6-, 9-, and 12-month storage) and a 114-member consumer panel evaluated slices prepared from 9-month stored apples. The university's IRB (Biomedical, Health Sciences Intuitional Review Board) approved the study protocol for sensory evaluation. Prior to each evaluation, both the trained and consumer panelists signed consent forms for voluntary participation in the present study.

Trained Panel

The trained panel consisted of nine members who had previous experience in similar sensory evaluations. Training was conducted in three orientation sessions of two hours each wherein panelists developed rating scales for degree of browning, texture, and flavor of fresh-cut slices. For training purposes and familiarization, panelists were given slices of varying degree of quality. Fresh-cut slice sensory evaluations, on post-processing day 10, were conducted under white fluorescent light. Panelists performed “difference from control” discrimination sensory test for degree of browning, texture, and flavor on a 0–8 scoring scale; 0 and 8 corresponding to “no difference” and “extremely different” from the control, respectively.[23] For each sample, three to four slices, equilibrated at room temperature for about 1 h, were placed in randomly coded small cups that were presented randomly to avoid panelist's bias. Panelists were provided with water, for rinsing between samples, and unsalted crackers to neutralize their mouthfeel.

Consumer Panel

The 114-member consumer panel consisted of a broad cross-section of adult population including faculty, staff, and graduate students. Sample preparation and presentation method was same as for trained panel evaluation. Panelists were asked to rate their likeness for color, texture, flavor, and overall acceptability on a 1–9 hedonic scale: 1 = dislike extremely, 2 = dislike very much, 3 = dislike moderately, 4 = dislike slightly, 5 = neither like nor dislike, 6 = like slightly, 7 = like moderately, 8 = like very much, and 9 = like extremely.[23] A score of five or below was considered a limit of acceptability for each of the four sensory attributes evaluated. All evaluations were conducted under white fluorescent lights on the same day. Questionnaires were prepared and provided to the panelists using SIMS Sensory Evaluation Software, version 6.0 (Sensory Computer Systems, Morristown, NY, USA).

Statistical Analysis

All data were analyzed by ANOVA using SAS software, version 9.1 (SAS Institute, Inc., Cary, NC, USA). The separation of means or significant differences was made by Tukey's HSD and the statistical significance was defined as p < 0.05.

RESULTS AND DISCUSSION

Whole Fruit Quality Evaluation

Fruit firmness

The pre-storage 1-MCP treatment of apples, generally, resulted in firmer fruits when evaluated at each storage interval (Fig. 1). However, Empire at 9- and 12-month, and Mutsu at 6-month storage did not show a significant difference (p < 0.05). Mutsu was observed to be the most vulnerable variety to firmness loss in CA storage if not treated with 1-MCP. Mutsu showed highest firmness values, followed by Empire and Jonagold, at all storage intervals for treated apples. Our results are similar to those reported by Mir and Beaudry[21] where 1-MCP was shown to effectively prevent softening of apples, however, at storage temperatures of >0°C, the benefits of 1-MCP application became less pronounced. Consumers prefer firmer slice texture as an indicator of better quality. Based on the results, it is concluded that the 1-MCP treated apples were more suitable for processing fresh-cut slices for year-round supply. While there are firmness threshold recommendations available for whole apples for the fresh market, i.e., 12-lb minimum firmness established by Washington tree fruit industry,[21] fruit firmness criteria, especially with respect to different cultivars, needs to be defined for the fresh-cut industry as well.

Figure 1 Effect of 1-MCP treatment on the firmness of Jonagold, Empire, and Mustu apples stored under air (3 months) or controlled atmosphere, CA (6, 9, and 12 months). “Ctrl” represents apples without 1-MCP treatment but stored under similar conditions.

Apple Skin Color

The results of whole fruit skin color measurements are shown in Fig. 2. Although, Hunter color “L” values increased during storage in most cases, 1-MCP treatment had little or no effect on this phenomenon. The increase in Hunter “L” values indicated that the fruit skin color turned “lighter” during extended storage. Mutsu apples’ skin color was the lightest, followed by Jonagold and Empire, at the beginning as well as at the end of 12-month storage. Regardless of the 1-MCP treatment or storage atmosphere (air or CA), in general, there were no significant differences (p < 0.05) in Hunter color “a” values, (increasing “a” value mean an increase in “redness” and decreasing “a” values correspond to increase in green tint). With respect to Hunter color “b” values, no clear pattern was observed (higher “b” values correspond to an increase in “yellowness” and lower “b” values correspond to an increase in blue tint).

Table 1  Effect of 1-MCP treatment on the TSS, pH, and TA of Jonagold, Empire, and Mustu apples stored under 3-month air or 6-, 9-, and 12-month CA storage

	Storage conditions
	3-Month (Air)		6-Month (CA)		9-Month (CA)		12-Month (CA)
	Control	1-MCP	Control	1-MCP	Control	1-MCP	Control	1-MCP
TSS (Brix):
Jonagold	14.00 ± 0.00a1	14.00 ± 0.00a	14.20 ± 0.00a	14.20 ± 0.00a	13.90 ± 0.00a	13.90 ± 0.00a	13.20 ± 0.00b	14.30 ± 0.00a
Empire	14.00 ± 0.00a	14.00 ± 0.00a	14.00 ± 0.00a	14.00 ± 0.00a	13.50 ± 0.00a	13.60 ± 0.00a	13.60 ± 0.00a	13.60 ± 0.00a
Mutsu	15.40 ± 0.00a	15.40 ± 0.00a	14.60 ± 0.00a	14.60 ± 0.00a	14.40 ± 0.00b	16.20 ± 0.01a	14.40 ± 0.00a	15.40 ± 0.01a
pH:
Jonagold	3.82 ± 0.01a	3.61 ± 0.00b	3.62 ± 0.01a	3.61 ± 0.01a	3.61 ± 0.01a	3.63 ± 0.00a	2.80 ± 0.01a	2.49 ± 0.01b
Empire	3.70 ± 0.01a	3.697 ± 0.01a	3.53 ± 0.01a	3.53 ± 0.01a	3.62 ± 0.01a	3.58 ± 0.01b	2.77 ± 0.00b	3.06 ± 0.01a
Mutsu	3.85 ± 0.00a	3.64 ± 0.01b	3.58 ± 0.01b	3.65 ± 0.01a	3.77 ± 0.00a	3.72 ± 0.01b	3.25 ± 0.01a	2.69 ± 0.00b
Titratable acidity (g/100 g):
Jonagold	0.29 ± 0.01b	0.35 ± 0.01a	0.38 ± 0.00a	0.36 ± 0.00b	0.36 ± 0.00a	0.34 ± 0.00b	0.31 ± 0.00b	0.44 ± 0.01a
Empire	0.42 ± 0.01b	0.48 ± 0.01a	0.49 ± 0.01a	0.42 ± 0.01b	0.39 ± 0.01a	0.37 ± 0.01a	0.37 ± 0.01a	0.43 ± 0.01a
Mutsu	0.36 ± 0.01b	0.52 ± 0.01a	0.41 ± 0.00a	0.41 ± 0.00a	0.32 ± 0.00b	0.38 ± 0.01a	0.20 ± 0.00b	0.38 ± 0.00a
^1Means with different letters in rows for individual storage intervals are significantly different (p < 0.05).

Table 2  Trained panel sensory evaluation1 of browning, texture, and flavor of fresh-cut slices that were prepared from control or 1-MCP treated apples stored under air or CA storage

	Storage conditions
	3-Month	6-Month	9-Month	12-Month
	(Air)	(CA)	(CA)	(CA)
Degree of browning:
Jonagold	0.46 ± 0.04b2	0.40 ± 0.10a	0.66 ± 0.12ab	0.38 ± 0.04b
Empire	0.46 ± 0.04b	0.31 ± 0.03a	1.04 ± 0.08a	1.50 ± 0.12ab
Mutsu	1.23 ± 0.16a	0.38 ± 0.06a	0.49 ± 0.03b	2.33 ± 0.08a
Texture:
Jonagold	2.67 ± 0.16ab	1.03 ± 0.10a	1.94 ± 0.20ab	1.70 ± 0.12b
Empire	1.63 ± 0.12b	0.96 ± 0.12a	1.15 ± 0.16b	1.46 ± 0.10b
Mutsu	3.62 ± 0.20a	1.08 ± 0.16a	3.20 ± 0.12a	3.19 ± 0.20a
Flavor:
Jonagold	2.63 ± 0.18a	2.44 ± 0.16a	1.75 ± 0.12a	2.07 ± 0.18a
Empire	1.65 ± 0.12b	1.26 ± 0.12a	1.60 ± 0.10a	2.56 ± 0.12a
Mutsu	3.23 ± 0.28a	2.07 ± 0.10a	1.98 ± 0.14a	2.16 ± 0.12a
^1Difference from control or non-1-MCP treated fruit slices (0 = no difference, 8 = extremely different).
^2Means for individual quality attributes across columns with different letters are significantly different (p < 0.05).

Figure 2 Effect of 1-MCP treatment on the fruit skin Hunter color values of Jonagold, Empire, and Mustu apples stored under air (3 months) and CA (6, 9, and 12 months).

Our results agree with Dauny and Joyce,[4] who reported no significant advantage for fruit color in 1-MCP treated apples. However, Watkins[24] reported in a review that loss of greenness of the background or ground color of the fruit skin, usually considered a negative attribute in commercial marketing, is inhibited by 1-MCP. For the acceptability of the fresh-cut slices by the consumers, it may be noted that the outer skin color of the apple may not be as significant an attribute as the color or appearance of the slice cut surfaces that are susceptible to discoloration.

TSS, pH, and TA

TSS, did not change significantly (p < 0.05) for any apple cultivar regardless of 1-MCP treatment or type of storage (air vs. CA), as shown in Table 1. Generally, lower pH values were observed in 1-MCP treated apples, though these differences were not always significant. Though no clear pattern could be established for storage intervals, 9- and 12-month stored control Mutsu apples showed a large difference in pH as compared to those treated with 1-MCP.

Across all apple cultivars, in most cases, there was a significant difference (p < 0.05) in TA between air and CA and 1-MCP stored apples for each storage interval. The TA values decreased in control apples for all three cultivars; the same was true for 1-MCP treated fruit, except for 12-month storage. The TA values increased for apples treated with 1-MCP and stored in air (3-month); however, 1-MCP showed a negative effect (lower acidity) in apples stored under CA condition. These results are similar to those reported by Watkins et al.,[3] who reported that 1-MCP gave mixed response during air or CA storage. However, Watkins[24] reported that, in general, 1-MCP delays loss of TA concentration, whereas complete absence of treatment effect was shown by Mir et al.[25]

Sensory Evaluation-Apple Slices

Trained panel-difference from control

A small trained panel (n = 9) performed sensory evaluation of fresh-cut apple slices on a 0–8 point “different from control” scale (“0” and “8” representing “no” and “extreme” difference from control, respectively). Control slices were prepared from the apples that were not treated with 1-MCP but were stored under similar conditions.

Degree of Browning

From 3-month air-stored apples, a significant (p < 0.05) difference in the degree of browning was observed in slices from Mutsu control and 1-MCP treated samples as compared to other two cultivars, Empire and Jonagold (Table 2). No significant differences in browning were found by the panelists in slices from 6-month CA-stored apples where the difference from control scores ranged from 0.31 to 0.40. There were significant differences when slices from 1-MCP treated and control apples were compared after 9 and 12 months of storage. After 9-month CA storage, slices processed from Mutsu showed the smallest difference from control (0.49) while the Empire had the largest (1.04); among the three cultivars, Mutsu differed significantly than the other two. At 12 months, slices from Jonagold and Empire showed a difference from control score of 0.38 and 1.50, respectively, and were significantly different from Mutsu (2.33). There are benefits of using 1-MCP with CA storage in apples. Tanprasert et al.[26] studied the effect of 1-MCP and CA storage on fresh-cut slices from Jonagold apples stored up to nine months. They found that the internal ethylene content of whole apples was kept lower with 1-MCP treatment alone compared to CA storage alone through 5-month storage. For all of the storage times they evaluated, the 1-MCP treatment in combination with CA storage provided very low levels of internal ethylene. Blockage of ethylene action slows down the senescence process by reducing the respiration rate,[27] which results in a lower degree of tissue softening and browning.

Texture

Since consumers prefer to eat apples that are firm and crisp, firmness retention is one of the most important attributes that 1-MCP contributes to the apple slices. Sensory texture scores of fresh-cut slices are shown in Table 2. For slices from 3-month air stored apples, the difference from control was the largest (3.62), which was significantly different (p < 0.05) from Empire that had a score of 1.63. Slices from 1-MCP treated apples showed minimal differences (firmer texture) regardless of the cultivar. Six-month CA storage showed the smallest difference from control (0.96 to 1.08) for slices from treated apples. Tanprasert[5] examined the texture of fresh-cut slices from Jonagold apples stored for five or seven months and reported higher instrumental texture values for slices from apples treated with 1-MCP; however no sensory evaluation was done in that study. Texture results for fresh-cut slices from 9- and 12-month storage for the three apple varieties showed that Mutsu differed significantly (p < 0.05) than Jonagold and Empires. Though not analyzed across storage, difference from control scores for Mutsu and Jonagold were greater for the 9- and 12-month storage than for the 6-month storage. In most cases, slices from 1-MCP treated apples exhibited better texture. The results of this study demonstrated that fresh-cut apple slices from the 6-, 9-, or 12-month CA stored and 1-MCP treated apples maintained better texture than CA alone.

Flavor

The largest slice flavor difference from control (3.23) was found for three-month air storage of Mutsu followed by Jonagold (2.63), while Empire had the lowest difference (1.65) and differed significantly with the other two cultivars (Table 2). A similar trend was observed for slices from 6-month storage; however, there was no difference among cultivars. For slices from 9- and 12-month storage, the flavor differences were minimal among three cultivars with no significant difference. Panelists’ comments for 12-month stored Mutsu were that control slices had slight “old apple” flavor and off-flavor. A greater degree of off-flavor was detected for both the control and 1-MCP treated Empire apple samples than the shorter storage duration.

Consumer Panel

At 9-month storage, a consumer panel evaluation of fresh-cut slices was also conducted. Consumer panel results showed no significant (p < 0.05) difference in slice appearance between control and 1-MCP treatment. The score range across all cultivars was from 7.05 to 7.28 (Fig. 3), indicating good acceptability. Supporting data from the trained panel looking at difference in browning for slices only found small difference at 9-month storage (0.49–1.04), which indicated that the 1-MCP treatment did not have a large effect on the degree of slice browning. Texture scores of slices from 1-MCP treated Mutsu and Jonagold were significantly higher than the control samples. Trained panel data had also shown that the Mutsu and Jonagold “degree of texture difference” was larger than that found in the Empire apples. This was consistent with the higher texture scores of the consumer panel for the slices from 1-MCP treated Mutsu and Jonagold apples. For slice flavor, only the Empire treated sample's degree of liking was significantly higher than the control. For overall acceptability, highest scores were observed for slices from Mutsu, followed by Empire and Jonagold. The only significant (p < 0.05) difference among control and 1-MCP treated apples was found in slices from Empire. Panelists were asked some additional questions about fresh-cut apple slices; “nutritious” and “tasty” were selected by over 80% of the respondents as reasons to purchase fresh apple slices, while about 40% chose “good value.” Results of this study indicated that 1- MCP treatment combined with CA storage of apples could be successfully used for year-round processing of fresh-cut apple slices.

Figure 3 Consumer panel (N = 114) sensory evaluation of fresh-cut slices prepared from 1-MCP treated apples that were CA stored for nine months (1–9 point hedonic scale: 1 = dislike extremely, 9 = like extremely).

CONCLUSIONS

This study evaluated the fresh-cut slice potential of apples treated with 1-MCP and stored in air (3 month) or CA (up to 12 months). The 1-MCP treatment was helpful in imparting a firmer texture to apples, whereas, some of the other physiochemical attributes for whole fruit showed a mixed trend. Sensory evaluation by a trained and consumer panel showed that fresh-cut slices from 1-MCP treated apples had better quality attributes of appearance, texture, flavor and overall acceptability. The results of this study demonstrated that CA stored and 1-MCP treated apples could be utilized for year-round processing of fresh-cut slices with good consumer acceptability.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support extended by Michigan Apple Research Committee and Michigan State University's Project GREEEN for support of this research.

Notes

Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the Michigan State University and does not imply its approval to the exclusion of other products or vendors that also may be suitable.