1. Introduction
Consumers value the presence of fresh vegetables in their daily diet, because their consumption has been established to be beneficial for human health [
1,
2]. Leafy vegetables are important and popular because they offer, apart from a high nutritional value, a diversity of crops with different characteristics (color, taste, and texture). Recently, most leafy vegetables have become more popular in the market as fresh-cut or baby leaves than as intact, fresh commodities [
3,
4]. The most important advantages for these products are their convenience, the variety of mixed combinations with attractive presentation, their freshness, and their contents of bioactive compounds [
3,
5].
It is well established that the appearance of the horticultural products is the primary criterion for consumer purchasing [
4,
6]. Previous studies show that both fresh-cut as well as baby leafy vegetables are more perishable than intact products [
7,
8]. As a result, the shelf life of these products is shorter because of changes in their sensory quality and not the microbial spoilage [
9,
10].
Lettuce (
Lactuca sativa L.) is an important leafy vegetable crop with the great advantage of availability all over the world for 12 months per year [
11]. Although lettuce is one of the major fresh-cut vegetables, it is difficult to process because of its high perishability [
11,
12,
13]. The major disorder of fresh-cut lettuce is the browning of cut surfaces, which significantly affects the visual quality and limits the shelf life of the product [
14,
15,
16]. Therefore, it is important to control the browning of cut surfaces [
13,
17]. Even though most of the research has focused on browning control with different postharvest treatments [
11,
14,
15,
18], some selection can be done to choose appropriate plant material which, as a fresh-cut product, could exhibit less browning in order to prolong its shelf life with the above treatments. In the case of fresh-cut lettuce, different pre-harvest factors were found that could affect postharvest quality such as lettuce type, cultivar, and climatic and environmental conditions [
15,
16,
19,
20]. Apart from cultivar, the position of the leaf on the plant as well as the position of the fresh-cut leaf area could have an effect on the quality of the fresh-cut product, since the whole plant is used for processing. However, there is limited literature on the above factors since Fukumoto et al. [
21] found different browning potential in photosynthetic and vascular tissues from the inner and outer leaves of iceberg lettuce.
Spinach (
Spinacia oleracea L.) is a leafy, cool-season vegetable popular worldwide with an extensive cultivation area [
22]. Baby spinach is characterized as a leafy vegetable with a very high perishable rate, which results in a marketability of only seven days when it is stored at 7 °C [
7,
23]. In the literature, the influence of different pre-harvest factors on the quality and shelf life of baby spinach, such as cultivar, physiological age, harvesting date, and growing season is well documented [
24,
25,
26]. Although the effect of growing season has been previously studied, the range of the tested period focused mainly on the winter season, where slight changes were found, probably due to differences in environmental factors [
24]. On the other hand, in recent years spinach cultivation has been successful during the warm season under nethouses [
27]. However, no information is currently available on the effect of a wide range of cultivation seasons on the postharvest quality and shelf life of baby spinach.
‘Rocket’, or ‘arugula’, represents a worldwide variety of the
Brassicaceae family species, that is well known for its spicy flavor and has a continuously increasing demand [
28,
29]. Especially, baby rocket leaves are a major economic product [
30], as they are a basic ingredient of ready-to-eat or ready-to-use salads in combination with other leafy vegetables, such as lettuce and spinach. Among the rocket species, the most common are ‘cultivated’ or ‘garden’ rocket (
Eruca sativa Mill.) and ‘wild’ rocket (
Diplotaxis tenuifolia L.), which are considered cool-season crops because of wilting during long days with high temperatures. An important quality characteristic of rocket is the green color of the leaves [
31]. Although for other leafy vegetables there are studies in the literature about the effect of pre-harvest factors on postharvest quality, for rocket there are only a few data regarding genotype, soilless cultivation systems, and typical growing seasons (autumn–winter and winter–spring) [
32].
Therefore, the aim of this work was to investigate the effect of cultivar, leaf position, and piece position on the leaf on the sensory quality of fresh-cut butterhead lettuce as well as the effect of a wide range of growing seasons on the postharvest quality and shelf life of baby spinach and rocket.
2. Materials and Methods
2.1. Fresh-Cut Lettuce: Material and Handling
Six butterhead lettuce cultivars were cultivated soilless in an unheated plastic greenhouse at the experimental farm of Aristotle University of Thessaloniki, Central Macedonia, Greece under usual cultivation practices. The tested cultivars were ‘Ercole’, ‘d’America rossastra’, ‘Morges’, ‘Agades’, ‘Reggina di magio’ (butterhead), and ‘Plenty’. Ten plants per cultivar were used and randomly placed in the greenhouse. Immediately after harvest, lettuce heads were washed with tap water and transferred to the laboratory within 10 min. To study the effect of leaf position on the plant on browning, leaves were divided into outer and inner and were cut with a sharp, stainless steel knife into 4 numbered pieces 4 × 2 cm, with number 1 representing the first piece from the leaf base. Then, pieces were kept in the dark at 10 °C with high humidity (>95%) for 7 days. For each treatment three replications were used, with 2 inner and outer leaves from 6 different plants per replication. The experiment was conducted twice, with very similar results, and thus the results from the second experiment are presented.
2.2. Fresh-Cut Lettuce: Determinations
The degree of browning was evaluated subjectively at the 2nd, 4th, and 7th day of storage and scored using a scale of 1 to 5, where 1 = none, 3 = moderate, and 5 = severe browning. When the pieces reached a score of 3, shelf life ended. General regression equations (normally polynomial) of the browning during storage were used to calculate the shelf life of fresh-cut lettuce. For pieces 1 and 4 of both the outer and inner leaves of the ‘d’America rossastra’ cultivar the activity of the enzyme phenylalanine ammonia lyase (PAL; EC 4.3.1.5) was assayed during storage [
33].
2.3. Baby Leaves (Spinach and Rocket): Material and Handling
Baby leaves of spinach and ‘wild’ rocket were produced under unheated plastic greenhouse conditions for winter production and under nethouse conditions for the rest of the growing season, under commercial cultivation practices at Vezyroglou Farm, Alexandria Imathia, Central Macedonia, Greece. For both species the harvesting index was the outer leaf length, maximum 9–10 cm. The production conditions as well as corresponding dates of harvest are presented in
Table 1. After harvest, all leaves were kept at 4 °C for 24 h before being mechanically packaged in packages of 100 g according to the commercial procedures. Immediately after harvest packages were transferred to the laboratory within 45 min, where they were stored at 4 °C for 12 days. For each treatment, three replications were used with one package per replication.
2.4. Baby Leaves (Spinach and Rocket): Determinations
The visual quality (appearance) of each package was subjectively evaluated using a scale of 9 to 1, where 9 = excellent, 7 = very good, 5 = good—limit of marketability, 3 = fair—limit of usability, and 1 = poor—inedible. General regression equations (normally polynomial) of the appearance score during storage were used to calculate the end of shelf life of the baby leaves (score = 5). Additionally, at each evaluation the color of the leaf surface was measured with a chromameter (CR-400 Chroma Meter, Konica Minolta Inc., Tokyo, Japan) using the colorimetric coordinates of lightness (L*), hue (h°), and chroma (C*) [
34].
2.5. Statistical Analysis
Statistical analysis was conducted using IBM SPSS software (SPSS 23.0, IBM Corp., New York, NY, USA). For fresh-cut lettuce, an ANOVA was performed considering the cultivar, leaf position, and piece position on the leaf and their interactions as variation factors in order to find out which factor greatly affects the shelf life of fresh-cut lettuce. For baby leaves, data were analyzed by applying one-way analysis of variance (ANOVA). Comparisons of the means were performed using a Tukey test at α < 0.05.