Evaluation of the Storage Characteristics in Maintaining the Overall Quality of Whole and Fresh-Cut Romaine Lettuce during MA Storage

Abstract

This study aimed to examine the effect of modified atmosphere (MA) storage on the quality and storability of romaine lettuce. Whole and fresh-cut romaine lettuce were packaged with 1300; 10,000; 50,000; 100,000 cc m⁻²·day⁻¹·atm⁻¹ O₂ transmission rate (OTR) films and stored at 2 or 8 °C for 15 days. The respiration and ethylene production rates before pre-cooling were higher in fresh-cut lettuce than in whole romaine lettuce. The reduction in fresh weight for whole and fresh-cut romaine lettuce during storage was less than 0.5%. The CO₂ concentration was less than 2%, except for the 1300 cc treatment (at 8 °C for whole and fresh-cut lettuce) and 10,000 cc (in fresh-cut lettuce). At the end date of storage, the overall quality of the whole lettuce was good, the off-odor was lower in the 10,000 cc treatment, and the 50,000 cc treatment at 8 °C led to good overall quality in the fresh-cut lettuce. The chlorophyll content of the whole lettuce was maintained at a higher level in the treatment of 10,000 cc at 8 °C, while it was higher in the 50,000 cc treatments at 2 and 8 °C in the fresh-cut lettuce. Correlation and PCA confirmed that the main factors affecting the storability and quality of romaine lettuce were fresh weight loss rate, off-odor, and the concentration of CO₂. In addition, the storage characteristics of whole and fresh-cut lettuce were opposite to 0 in PC1 and PC2, indicating that the storage characteristics were different depending on the processing type.

1. Introduction

Romaine lettuce is a crop cultivated year round, and the market size is gradually increasing according to consumer demand for fresh-cut lettuce. However, in the case of fresh-cut agricultural products, they are prone to secondary quality degradation, which speeds up browning or microbial growth due to the respiration caused by cutting after harvest and the rapid increase in ethylene [1,2].

Improved packaging technology as well as preprocessing for storage are required to prevent deterioration and the subsequent phenomena. Studies on romaine lettuce have focused on topics such as cultivation [3,4], the quality of baby leaves [5,6], and comparisons of freshness maintenance according to packaging type for each type of lettuce [7].

Studies on fresh-cut romaine lettuce have focused on the inhibition of browning caused by cutting [3,8] and on storability [9,10]. In the case of loose-leaf lettuce, it has been reported that in low storage temperatures, inherent factors, such as variety, are suppressed, resulting in no difference in storability, and the extent of this reduction increases depending on the permeability when stored in PP films [6,11].

Even for the same crop, depending on whether Lactuca indica L. is a whole or baby leaf variety and whether tomatoes are whole or fresh cut, the loss rate of fresh weight during storage, the changes in the overall quality, and the O₂, CO₂, and ethylene concentrations in the films during film packaging showed differences in storability [12,13]. It was reported that when baby spinach leaves were packaged in a modified atmosphere at a ratio of 5% O₂ to 15% CO₂, high levels of antioxidant activity were maintained, and the shelf life was extended compared to the untreated atmosphere [14]. The modified atmosphere packaging of rocket leaves has a beneficial effect on the quality; this packaging method can cause off-odor, but it retains its typical flavor for up to 6 days [15]. When soft kale is MAP with a 3.0 × 10³ OTR pouch, it is suitable for maintaining its green color and bioactive compounds [16].

Accordingly, various statistical analysis methods have been applied to examine different factors affecting the quality of crops, and the relevant studies are as follows: One study analyzed the correlation with antioxidant activity for butterhead lettuce after storage [17], and another report analyzed the quality and storability factors in accordance with the growing period and growing season of iceberg and romaine lettuces [18].

Choi [19] reported a high correlation between fruit quality factors and shelf-life when comparing the quality and storability during MA storage for a large number of paprika varieties over two years, and Lee [20] reported a high correlation between the storage period and the chlorophyll content of cut hydrangea flowers. In addition, the authors of [21] used principal component analysis (PCA) and reported that the total number of bacteria increased with increasing temperature and humidity when fresh-cut head lettuce was wrapped with Ny/PE film and stored at 5 °C.

Therefore, in this study, we examined the effects of MA storage on quality and storability and the main factors of whole and fresh-cut romaine lettuce.

2. Materials and Methods

2.1. Plant Materials

Romaine lettuce (Lactuca sativa L. cv. Parris Island) was cultivated in Samcheok-si, Gangwon-do, South Korea, and harvested in October 2020. The weight of the whole lettuce was 284 ± 68 g, and immediately after harvest, 2–3 outer leaves were removed and packaged, while the fresh-cut lettuce was prepared from the whole romaine lettuce by cutting 3.0 ± 0.5 pieces and putting them into 25 g packages each. The whole and fresh-cut romaine lettuce were pre-cooled in a low-temperature chamber (2 ± 1 °C, 85%RH).

2.2. Packaging Materials and Storage Conditions

The pre-cooled whole and fresh-cut romaine lettuce were packaged with oxygen transmission rate (OTR) films (Dae Ryung Precision Industry Co., Ltd., Gwangju-si, Korea), i.e., laser-processed polypropylene (PP) films (21 × 30 cm) to control the O₂ transmission rate. The whole lettuce was packaged with 1300; 10,000; 50,000 cc·m⁻²·day⁻¹·atm⁻¹ OTR films, while the fresh-cut lettuce was packaged with 1300; 10,000; 50,000; 100,000 cc·m⁻²·day⁻¹·atm⁻¹ OTR films. The control was packaged with a perforated film (PF) (∅ 8 mm × 8 hole). After packaging, the samples were stored at 85% relative humidity at two temperatures (i.e., 2 ± 1 °C and 8 ± 1 °C) for 15 days.

2.3. Atmosphere Analysis and Sensory Qualities

Putting whole romaine and fresh-cut romaine lettuce in gas-tight containers (1140 mL) and leaving it at room temperature (25 °C) for 3 h, carbon dioxide and ethylene gas were measured with an infrared sensor (Checkmate, PBI, Ringsted, Denmark) and gas chromatography (GC-2010, Shimadzu, Kyoto, Japan) to derive the respiration rate and ethylene generation rate. The concentrations of O₂, CO₂, and ethylene gas in the films were measured at 3 day intervals using GC and infrared sensors during storage [22]. The fresh weight loss is expressed as a percentage of the weight lost from the initial weight, and the oxygen and carbon dioxide concentrations were measured using an infrared sensor through a needle in the film. Ethylene gas was measured by inserting a syringe (81330, Hamilton, Reno, NV, USA) in the film, collecting a 1.0 mL gas sample and injecting it into the GC equipped with a fused silica capillary column (Supelcowax 10, 30 m × 0.25 mm × 0.25 μm, Supleco Analytical, Bellefonte, PA, USA) and a flame ionization detector (FID). The detector and injector operated at 200 °C, the oven at 50 °C, and the carrier gas (He) flow rate was 1.76 mL⁻¹·min.

At the end date of storage, we examined the overall quality, off-odor, and chlorophyll. A sensory test for overall quality and off-odor was conducted by five skilled panels. The overall quality grade was assumed as follows: 5 points for the best condition before storage, 3 points for maintaining a marketable value, and 1 point for a condition needing disposal [22]. The off-odor was defined as follows: 1 point for not being able to smell the odor and 5 points for a very strong odor. The chlorophyll content was measured before storage and at the end of storage with a chlorophyll meter (SPAD-502 plus, Konica Minolta, Tokyo, Japan) and shown as a SPAD value.

2.4. Statistical Analysis

All storage experiments (i.e., respiration rate, ethylene production rate, oxygen concentration, carbon dioxide concentration, ethylene concentration, and fresh weight loss rate) were repeated five times, and quality surveys (i.e., overall quality, off-odor, and chlorophyll) were repeated 10 times, and the standard deviation was expressed in Microsoft Excel 2017. Then, using SPSS v.24 (IBM, New York, NY, USA), PCA, Pearson’s correlation analysis, and an independent sample t-test were performed to examine the correlation between the storability and storage characteristics of the lettuce.

3. Results and Discussion

3.1. Respiration Rate and Ethylene Production Rate

The respiration and ethylene generation rates at room temperature before pre-cooling were higher in the fresh-cut lettuce than the whole lettuce, but they were not statistically significant (

Table 1. The respiration rate and ethylene production rate of whole and fresh-cut romaine lettuce in room temperature before storage.

	Respiration Rate (CO2 mL·kg−1·h−1)	Ethylene Production Rate (C2H4 μL·kg−1·h−1)
Whole	29.5 ± 3.2 z	0.011 ± 0.002
Fresh Cut	35.3 ± 6.7	0.015 ± 0.003
	t = −1.948	t = −3.100
	p = 0.087 y	p = 0.015

^z Values are presented as means ± SE (n = 5). ^y Statistically non-significant (t-test).

). The respiration rates of the whole and fresh-cut romaine lettuce were 29.5 ± 3.2% and 35.3 ± 6.7%, respectively, showing similar results to those reported for romaine lettuce at 30–38 mL CO₂∙kg⁻¹∙h⁻¹ at 20 °C [23]. The findings are similar to Kays and Paull [24] and Cantwell and Suslow [2], who reported that the ethylene production rate of romaine lettuce was very low, less than 0.02 C₂H₄ μL∙kg⁻¹∙h⁻¹.

3.2. Atmosphere Analysis

The O₂ concentrations in the film-packed whole lettuce were 17% and 15%, which were the lowest values, in the 1300 cc treatment at 2 and 8 °C, respectively, and the remaining MA storage and PF treatments were similar to the atmospheric conditions.

The O₂ concentration in the films of the fresh-cut lettuce decreased steadily in the 1300 cc MA storage treatment at 2 °C, and it showed the lowest values of 9% and 15% in the 10,000 cc treatment at the end date of storage, respectively; meanwhile, the rest of the MA storage treatments and the control were similar to the atmospheric conditions. At 8 °C, the O₂ in the films steadily decreased in the 1300 and 10,000 cc conditions, showing low concentrations of 1.5% and 6%, respectively, while the PF treatment showed a level of 18–20% for the 50,000 and 100,000 cc treatments (Figure 1). The O₂ concentration in the films of the fresh-cut lettuce exceeded the minimum permissible concentration by 2% or less at the end date of storage in the 1300 cc treatment at 8 °C. At 2 and 8 °C, the CO₂ concentrations in the films in the MA storage treatments of 50,000 and 100,000 cc of the fresh-cut lettuce showed the most similar values to those of previous reports (Figure 1). After the “heating” variety, the main lettuce was freshly cut and packaged with low-density polyethylene (LD-PE), medium density polyethylene (MD-PE), and polyvinyl chloride (PVC) film and stored for 15 days at 0 and 5 °C; the CO₂ concentration in the film was less than 4% at 0 °C and less than 7% at 5 °C [10]. However, the fresh-cut lettuce in this study was higher in the 1300 and 10,000 cc films at 2 and 8 °C, and it is believed that the CO₂ concentration accumulated in the film also increased, because the respiration rate increased as the storage temperature increased.

The CO₂ concentration in the film of the whole lettuce was higher as the O₂ transmission rate of the film was lower, but the 1300 cc, which had the lowest transmission rate, maintained a 2% CO₂ concentration at 2 °C until the storage end date. At 8 °C, the 1300 cc treatment of the whole lettuce maintained a CO₂ concentration of approximately 4.5% until the end date of storage, while in the 10,000 cc treatment, the concentration was less than 3.0%, and in the 50,000 cc treatment, it was less than 2.3% (Figure 2). The CO₂ concentration in the film-packed, fresh-cut lettuce showed a high concentration in the 1300 and 10,000 cc treatments at 2 °C at the end date of storage, which were 7.4% and 5.2%, respectively, and less than 1% in the remaining treatment. At 8 °C, the 1300 and 10,000 cc treatments were high at the end date of storage (i.e., 12.2% and 10.7%, respectively) and less than 3% in the other treatments (Figure 2). The controlled atmosphere (CA) conditions for lettuce were reported to be 2–5% O₂ + 0% CO₂, with a minimum permissible O₂ concentration of 2% and a maximum permissible CO₂ concentration of 2% [25]. The concentrations of CO₂ in the films of all of the MA storage treatments at 2 °C and the 10,000 and 50,000 cc treatments at 8 °C were similar to or lower than the maximum permissible concentration.

The concentration of ethylene in the film-packed whole lettuce repeated an increase and a decrease in all of the films while being maintained at approximately 4.8 μL/L at 2 °C and 5.6 μL/L at 8 °C. However, there was no statistical significance in the difference between film treatments depending on the O₂ transmission rate. Meanwhile, the ethylene concentration in the film-packed, fresh-cut lettuce was maintained at approximately 6 μL/L, and a difference between film treatments depending on the O₂ transmission rate was not found (Figure 3). In this study, the ethylene concentration of all MA treatments was 4–6 μL/L, which was higher than the ethylene concentration of 126 μmol·m⁻³ (approximately 2.82 μL/L) of iceberg lettuce, where russet spots appeared [26]. However, the whole and fresh-cut romaine lettuce of all MA treatments did not show russet spots because of the high CO₂ concentrations in the films that suppressed the ethylene response. It has already been reported that russet spots on lettuce can be alleviated by high CO₂ conditions [27].

3.3. Biochemical Traits and Sensory Qualities

When the pre-cooled whole romaine lettuce was packaged with an OTR film and stored for 15 days, the loss rate of the fresh weight was 16% and 23% for the PF treatment at both storage temperatures until the end date of storage. The loss rate of the fresh weight for the MA storage treatment, excluding the PF treatment, was as low as or less than 0.3%. For the fresh-cut lettuce, which was cut into a certain size from whole romaine lettuce, the loss rate of the fresh weight during the storage period was 0.1% in the MA storage treatments at 2 °C and less than 0.5% in the MA storage treatments at 8 °C. The PF treatment showed a 10% fresh weight loss rate at 2 °C and a 12% loss rate at 8 °C at the end date of storage (Figure 4). The maximum permissible loss rate of the fresh weight at which lettuce can maintain its marketable value is 3.7% [24], but there was no overall quality degradation due to the moisture loss.

In terms of the overall quality of the whole lettuce examined via a panel test at the end date of storage, the 50,000 cc treatment at 2 °C had the best condition, obtaining three points, which was the limit for the marketable value, while the 10,000 cc treatment at 8 °C was good (

Table 2. The overall quality, off-odor, and chlorophyll content of whole romaine lettuce in OTR film in MA storage at 2 and 8 °C for 15 days.

	Overall Quality		Off-Odor		Chlorophyll (SPAD)
	2 °C	8 °C	2 °C	8 °C	2 °C	8 °C
Initial	5.0 ± 0.0 a	5.0 ± 0.0 a	1.0 ± 0.0 c	1.0 ± 0.0 c	50.2 ± 1.4 a	50.2 ± 1.4 a
Perforated	2.5 ± 0.0 c	1.4 ± 0.1 c	1.0 ± 0.0 c	1.0 ± 0.0 c	50.1 ± 0.7 a	43.4 ± 1.8 b
1300 cc	2.9 ± 0.1 b	1.5 ± 0.3 c	2.8 ± 0.2 a	4.1 ± 0.1 a	46.3 ± 2.2 a	44.8 ± 1.9 b
10,000 cc	3.0 ± 0.2 b	2.2 ± 0.2 b	1.9 ± 0.1 b	2.9 ± 0.2 b	50.9 ± 3.4 a	47.3 ± 1.9 ab
50,000 cc	2.9 ± 0.2 b	2.1 ± 0.3 b	1.9 ± 0.3 b	2.8 ± 0.1 b	50.1 ± 2.7 a	42.2 ± 3.7 b

Values are presented as means ± SE (n = 10) and each letter in the same column indicate the significant differences by Duncan’s Multiple Range Test (α < 0.05).

). The overall quality of the baby leaf lettuce during storage at 8 °C was best maintained when packaged with a 20,000 cc·m⁻²·day⁻¹·atm⁻¹ OTR film compared to the perforated and 1300; 20,000; 40,000; 100,000 cc films, because the loss of fresh weight was less than 0.2%, and the CO₂ concentration was 1.55–4.90% of the 20,000 cc treatment [28]. In this study, the PF treatment showed the lowest overall quality. At the end of storage, the overall quality of the fresh-cut lettuce as per the panel test was best with the 1300 and 10,000 cc treatments at 2 °C and 1300 cc treatment at 8 °C, but it was below the marketable limit of three in all treatments.

Except for the PF treatment, the whole lettuce in the 50,000 cc treatment with the highest transmission rate had the lowest off-odor at both temperatures, while the highest off-odor was found in the 1300 cc treatment with the lowest transmission rate (Table 2). On the last day of storage, the off-odor of the fresh-cut lettuce treated with MAP at 2 °C was similar to that of the PF treatment, which did not have any off-odor, but all MA treatments showed an off-odor of three points that lost marketability at 8 °C (

Table 3. The overall quality, off-odor, and chlorophyll contents of fresh-cut romaine lettuce in OTR film in MA storage at and 8 °C for 15 days.

	Overall Quality		Off-Odor		Chlorophyll (SPAD)
	2 °C	8 °C	2 °C	8 °C	2 °C	8 °C
Initial	5.0 ± 0.0 a	5.0 ± 0.0 a	1.0 ± 0.0 a	1.0 ± 0.0 c	50.2 ± 1.4 a	50.2 ± 1.4 a
Perforated	1.8 ± 0.3 c	1.3 ± 0.3 e	1.0 ± 0.0 a	1.0 ± 0.0 c	39.8 ± 2.5 b	30.6 ± 3.1 d
1300 cc	2.7 ± 0.3 b	1.9 ± 0.4 c	1.0 ± 0.1 a	3.3 ± 0.1 a	43.7 ± 3.1 ab	39.6 ± 2.7 b
10,000 cc	2.6 ± 0.2 b	1.8 ± 0.3 cd	1.5 ± 0.2 a	2.9 ± 0.1 b	39.1 ± 3.0 b	37.8 ± 2.0 bc
50,000 cc	2.6 ± 0.2 b	2.2 ± 0.6 b	1.1 ± 0.1 a	2.9 ± 0.1 b	44.2 ± 1.5 ab	41.4 ± 2.1 b
100,000 cc	2.5 ± 0.4 b	1.5 ± 0.4 de	1.3 ± 0.2 a	2.8 ± 0.1 b	44.3 ± 2.4 ab	33.2 ± 2.0 bc

Values are presented as means ± SE (n = 10) and each letter in the same column indicate the significant differences by Duncan’s Multiple Range Test (α < 0.05).

). These findings are similar to a previous report that confirmed that the lower the film transmission rate and the higher the storage temperature, the higher the off-odor [26,29].

The chlorophyll content of the whole lettuce was similar to the initial value and remained the highest in the 10,000 cc treatment, without statistical significance in the different treatments (Table 2). The chlorophyll content of the fresh-cut lettuce was similar to the initial value of the 1300; 50,000; 100,000 cc treatments at 2 °C, while at 8 °C, the 1300 and 50,000 cc treatments were the highest (Table 3). In a previous storage experiment on lettuce leaves, it was also reported that the chlorophyll content decreased during storage, but there was no difference among treatments depending on the storage temperature and packaging method [6]. Compared to the fresh-cut PF treatment, the ethylene concentrations were higher for the MA storage treatments. Ethylene is known to accelerate aging and cause yellowing by reducing the chlorophyll content in green vegetables [25], but it is thought to be suppressed by high concentrations of CO₂ in the films [30]. As in this experiment, the elevated CO₂ and reduced O₂ atmospheres inhibited the activity of 1-aminocyclopropane-1-carboxylic acid synthase, a key enzyme of ethylene biosynthesis [31]. In a recent study, chlorophyll degradation was retarded in CO₂-treated strawberry fruits by inhibiting the activity of chlorophyllase and downregulating the expression of FaChl b reductase, FaPAO, and FaRCCR [32].

In asparagus, chromaticity is an indicator used to easily judge the quality degradation visually due to the yellowing caused by chlorophyll, which is rapidly reduced when exposed to ethylene [33]. In this study, as the ethylene concentration in the whole and fresh-cut romaine lettuce films did not show a difference among film treatments depending on the transmission rate, there was no clear trend in the chlorophyll content.

3.4. Pearson’s Correlation and Principal Component Analysis

The correlation between overall quality (OQ), which is reported as an important item in judging the storability of horticultural products [25], and the storage characteristics was investigated. The loss rate of fresh weight showed a highly negative correlation in all treatments (

Table 4. The values for the Pearson’s correlation between the overall quality and characteristics of romaine lettuce for different types and at different storage temperatures.

		Fresh Weight Loss Rate	Oxygen Content	Carbon Dioxide Content	Ethylene Content	Chlorophyll	Off -Odor
Whole	2 °C	−0.785 **	−0.550 *	0.586 **	0.710 **	0.299	0.522 *
	8 °C	−0.452 *	−0.093	0.145	0.400	0.190	0.060
Fresh Cut	2 °C	−0.777 **	−0.422	0.448 *	0.725 **	0.375	−0.094
	8 °C	−0.548 *	−0.319	0.287	0.435	0.842 **	0.454 *

** Significant at p < 0.01, ANOVA. * Significant at p < 0.05, ANOVA.

). When the maximum permissible reduction rate for each crop was exceeded during storage, the quality of the fresh weight appeared to deteriorate due to the fact of water loss [24]. Moreover, in this study, the OQ was low in the perforated film treatment, which had significantly higher fresh weight loss compared to the other film treatments. The OQ of the fresh-cut lettuce showed a highly negative correlation with the O₂ concentration in films at 2 °C. In addition, the CO₂ concentrations in the whole and fresh-cut lettuce showed a significantly positive correlation when stored at 2 °C (Table 4).

Horticultural crops can be maintained for the maximum number of days of storage when proper CA conditions are maintained, while O₂ decreases and CO₂ increases due to the fact of their respiration during MA storage [25]. In this experiment, it was judged that the decrease in O₂ and the increase in CO₂ concentrations in the films affected the OQ. However, there was no significant correlation between the OQ and the CO₂ concentration at 8 °C, regardless of processed type, because the respiration of lettuce at 8 °C was too high to show any of the advantages of MA.

The concentration of ethylene in films causes aging and deterioration of horticultural crops and is subject to removal [25]. However, in this experiment, ethylene was positively correlated with the OQ for the whole and fresh-cut lettuce stored at 2 °C, which is thought to be because the CO₂ accumulated in the films offsets the quality deterioration effect of ethylene gas. The chlorophyll content of the fresh-cut lettuce stored at 8 °C also showed a positive correlation with the OQ. In this study, the PF and all MA treatments decreased compared to the initial value and the leaf color changed, which became an indicator of the OQ conducted through panel testing.

The off-odor generated during MA storage is a result of the anaerobic conditions caused by high CO₂ and low O₂ concentrations in films [25]. The off-odor showed a positive correlation with the OQ in the whole lettuce at 2 °C and fresh-cut lettuce at 8 °C. The reason is that in the MA condition, there was no clear trend in the processed types and storage temperatures. The most influenced storage characteristics of processed fresh-cut or unprocessed whole romaine lettuce were the FW and the CO₂ and ethylene concentrations.

Principal component and correlation analyses were carried out to obtain a basic overview of the physiological and biochemical quality parameters characterizing the whole and fresh-cut lettuce under various O₂ transmission rates of packaging film × storage temperature treatments.

The primary main component (PC1) was 56.28%, and the secondary main component (PC2) was 26.60%, showing a contribution rate of 82.88% up to the secondary main component in the case of whole lettuce (Figure 5A).

The variability of the storage characteristics in whole romaine lettuce can be clearly described by PCA, which depicted a positive correlation between the CO₂ and ethylene concentrations and the off-odor with PC1, whereas the fresh weight loss and O₂ were negatively correlated. The overall quality and chlorophyll content correlated with one another in PC2, which were found to be opposite to the off-odor and CO₂ and ethylene concentrations in PC1 (

Table 5. Correlation of variables to the factors of PCA based on factor loadings.

Variables	Whole		Fresh Cut
	PC1	PC2	PC1	PC2
FW	−0.863	−0.391	−0.696	−0.602
O2	−0.838	0.114	−0.845	−0.027
CO2	0.926	−0.131	0.901	−0.001
C2H4	0.858	0.142	0.783	0.394
SPAD	−0.052	0.833	−0.077	0.827
OQ	−0.001	0.948	0.056	0.927
Off-Odor	0.946	−0.262	0.808	−0.292

Values in bold within the same factor indicate the variable with the largest correlation.

).

For the fresh-cut lettuce, PC1 was 49.63% and PC2 was 27.95%, showing a contribution rate of 77.58% up to PC2. Among the storage characteristics used for PCA, the CO₂ and ethylene concentrations highly contributed to the positive direction of PC1, and PC2 had a higher chlorophyll content and overall quality (Figure 5B).

Although the proper OTR films for MA storage were different with whole and fresh-cut lettuce, the loading plots of the storage characteristics were located in the same position at the end of storage analyzed by PCA.

The loading and scoring plots by PCA were interpreted as shown in Figure 6. When PCA was conducted on the date of examining the fresh weight loss rate and O₂, CO₂, and ethylene concentrations of the film-packaged whole and fresh-cut lettuce during storage, for the whole lettuce, PC1 was 84.20% and PC2 was 14.36%, and the cumulative contribution rate was 98.56%. The ethylene concentration in the film of PC1 and the O₂ concentration of PC2 showed a high contribution rate. Meanwhile, PC1 of the fresh-cut lettuce was 71.40% and PC2 was 21.79%, and the cumulative contribution rate was 93.19%. The loss rate for the fresh weight of PC1 and the O₂ concentration of PC2 showed a high contribution rate. The PCA results, according to the evaluated days during storage, did not show a consistent trend. Moreover, PC1 and PC2 of the whole and fresh-cut lettuce were located opposite of the zero point, indicating that the effects of the storage characteristics on the storage properties were different depending on the processing type, even for the same crop (Figure 6A,B). The fresh weight loss and the overall quality were located on the opposite side of one another, because the higher the fresh weight loss, the lower the overall quality by water loss; minimally processed lettuce leaves and rocket also showed the same results [34].

The distinction of the various combinations of packaging treatments for the whole and fresh-cut lettuce for storage periods was capable of being visualized in the plots of the scores, where two clusters could apparently be distinguished (Figure 6B). Although the scores for the whole and fresh-cut lettuce on each stored day moved in the same direction, from the negative side to the positive side in PC1, the positions of the scores were different. The scores for the whole lettuce were positioned on the negative side of PC1 and clearly separated from those of the fresh-cut lettuce, which were located on the positive side of PC1 (Figure 6C).

Among the factors affecting the storage characteristics of whole and fresh-cut lettuce, the O₂, CO₂, and ethylene concentrations in the film, the loss rate of fresh weight, the appearance quality, the off-odor, and the chlorophyll content at the end of storage were evaluated by PCA. The three-dimensional PCA plots of different processing types (i.e., whole and fresh cut) of romaine lettuce represent the differences among storage characteristics at end of storage.

The factors of the storage characteristics were located in the same positions on the loading plots for both whole and fresh-cut lettuce analyzed at end of storage, whereas they were located in different plots when analyzed during storage (Figure 5 and Figure 6). These results indicate that in the case of factor analysis of the characteristics involved in crop physiology through PCA, the results may be different depending on the data generation period. It also provides basic information on desirable storage conditions for maintaining the quality of various romaine lettuce products and PCA.

4. Conclusions

For whole romaine lettuce, it is considered desirable to package it with a 10,000 cc OTR film, as our research showed that it kept it within the permissible range of CO₂, resulting in less yellowing. In the case of fresh-cut romaine lettuce, it is desirable to package it with a 50,000 cc OTR film, which, in this study, kept a CO₂ content within the permissible range, a higher chlorophyll content, and a lower off-odor. The OQ of the romaine lettuce showed a highly negative correlation with fresh weight loss rate regardless of processing type and storage temperatures. In addition, the characteristics showed a positive correlation with CO₂ and ethylene concentration in films at 2 °C for both whole and fresh-cut types. As a result of PCA at the end of storage, it was found that the main component factors affecting the storability and quality of romaine lettuce were off-odor and CO₂ concentration in the film for whole and fresh-cut romaine lettuce. The storage characteristics of the whole and fresh-cut lettuce during storage were located opposite of the zero points in the PC1 and PC2. These results indicate that the effects of the storage characteristics on the storage properties were different depending on the processing type, even for the same crop.