Research Article | Open Access
Atef M. K. Nassar, Stan Kubow, Danielle J. Donnelly, "High-Throughput Screening of Sensory and Nutritional Characteristics for Cultivar Selection in Commercial Hydroponic Greenhouse Crop Production", International Journal of Agronomy, vol. 2015, Article ID 376417, 28 pages, 2015. https://doi.org/10.1155/2015/376417
High-Throughput Screening of Sensory and Nutritional Characteristics for Cultivar Selection in Commercial Hydroponic Greenhouse Crop Production
Hydroponic greenhouse-grown and store-bought cultivars of tomato (cherry and beefsteak), cucumbers, bibb lettuce, and arugula were investigated to see if they could be distinguished based on sensory qualities and phytonutrient composition. Only the more dominant sensory criteria were sufficiently robust to distinguish between cultivars and could form the core of a consolidated number of criteria in a more discriminating sensory evaluation test. Strong determinants for cultivar selection within each crop included the following: mineral analysis (particularly Cu, Fe, K, Mg, and P); total carotenoids (particularly β-carotene, lycopene, and lutein); total carbohydrate (except in arugula); organic acids; total phenolics and total anthocyanins (except in cucumber). Hydroponically grown and store-bought produce were of similar quality although individual cultivars varied in quality. Storage at 4°C for up to 6 days did not affect phytonutrient status. From this, we conclude that “freshness,” while important, has a longer duration than the 6 days used in our study. Overall, the effect of cultivar was more important than the effect of growing method or short-term storage at 4°C under ideal storage conditions.
Modern agriculture aims to maximize yield and quality of plants with efficient use of resources and labor. Growing plants in a soilless environment on a supportive substrate (e.g., coir, peat, or rockwool) conserves water, nutrients, and space and is defined as hydroponics [1, 2]. Hydroponics has long been considered a powerful method of commercial vegetable and fruit production where growing conditions (light, temperature, medium composition, etc.) are fully controlled. More recently, advances in biocontrol technology have enabled pesticide-free produce. The acreage of hydroponic greenhouses ranges from 3,000 to 4,000 in the United States and Canada . Canadians consume large amounts of vegetables ~337 g/d, about 123 kg/capita/yr .
Important considerations for hydroponically grown greenhouse (GH) produce are microclimate and fertigation components that could potentially affect various plant characteristics. It is our contention that if these are held constant, selection of nutritionally optimal cultivars can be performed, based on a combination of sensory and phytonutrient characteristics. The latter includes biochemical composition and proximate traits. Fruits and vegetables are rich in phytonutrients including anthocyanins, carotenoids, minerals, organic acids, polyphenolics, and vitamins (e.g., vitamins C and E) [4, 5]. Phytonutrients contribute to the general acceptability of any food as they influence taste, color, odor, and nutritional quality. Consumption of fruits and vegetables increases the intake of antioxidant compounds. These dietary antioxidants act as scavengers of reactive oxygen and nitrogen species and prevent lipid oxidation. This decreases DNA damage related to oxidative stress, implicated in causing metabolic diseases that can lead to cancer, as well as cardiovascular and other diseases [6, 7].
Four salad crops were examined in this study including tomato, cucumber, and the leafy greens arugula and bibb lettuce. Tomato consumption in Canada in 2007 was 87 g/capita/d or 32 kg/capita/yr . Canadian consumption of cucumber and lettuce were 4.76 and 9.98 kg/capita/yr, respectively, in 2009  while statistics on Canadian arugula consumption could not be located. Tomato contains phytochemicals including carotenoids (especially β-carotene and lycopene), flavonoids, folate, minerals, polyphenols, and vitamins (C and E) . Epidemiological studies showed that tomato phytochemicals (especially lycopene) reduce cancer (e.g., prostate cancer), act as in vivo antioxidants, enhance cell-to-cell communication, modulate cell-cycle progression, and alter xenobiotic metabolism . Regular consumption of tomato has been related to reduced incidence of both prostate cancer and heart diseases (e.g., myocardial infarction) [10–12], as well as promotion of healthy eye function [13–16]. Cucumbers contain significant amounts of triterpenoid cucurbitacins that possess antioxidant, anti-inflammatory, and anticancer properties . In particular, leafy salad vegetables (arugula and lettuce) provide antioxidant compounds, fiber, polyphenolics, and vitamin C [18–20]. Green leafy vegetables are also recommended as a source of the pro-vitamin A carotenoids .
Few studies have been conducted on the phytonutrient composition of hydroponically produced vegetables. As these are grown under defined conditions of light, temperature, and fertigation, the nutritional content of specific cultivars may not vary widely from crop to crop. Although there may be some differences in nutritional composition related to season, differences are primarily related to light effects [22, 23]. Nutritional differences based on cultivar were explored for a range of crops grown in the Macdonald Campus Greenhouse and harvested fresh for comparison with store-bought produce of the same species.
Selection of the most pleasing and nutritious cultivars for each food species would contribute to maximum consumer satisfaction and health. Following sensory characterization and phytonutrient analysis, a method was developed to highlight the most revealing factors for high-throughput cultivar selection within each species (cherry and beefsteak tomato, cucumber, arugula, and bibb lettuce). A range of complex statistical methods was considered. Among the methods examined for data classification and mining were partial least squares or projection to latent structures (PLS), principal component analysis (PCA), variable clustering, classification trees, linear and quadratic discriminant analysis, neural networks, and support vector machines . Following comparative study, a decision was made to use a combination of PLS and PCA, which yield much richer results than other methods [24–26] and combine them with VARCLUS (SAS procedure for division of variables into hierarchical clusters) to verify decision-making.
So the objectives of this research were to (1) utilize taste panels to assess the appearance, taste, and texture acceptability of different cultivars of GH tomato (cherry and beefsteak types), cucumber, and two leafy green crops (arugula and bibb lettuce), (2) compare the phytonutrient composition of different cultivars of these fresh GH crops with store-bought (SB) produce, and (3) identify the most revealing factors (including sensory and phytonutrient data) that could be used in a high-throughput screening of a large number of cultivars, within each crop species, to expediently identify the most appealing and nutritious ones.
2. Materials and Methods
2.1. Plant Materials
Six cherry tomato cultivars including ‘Apero’, ‘Favorita’, and ‘Juanita’ were grown hydroponically in the greenhouse. Seeds of ‘Apero’ and ‘Favorita’ were bought from Johnny’s Selected Seeds (ME, USA) while seeds of ‘Juanita’ were bought from De Ruiter Seeds (Monsanto; Montreal, QC, Canada). Fruit of three cherry tomato cultivars were purchased from local stores and tested under their trade names: Jardino (product of EU; from an IGA store on Louis-Menard St.), Fruiterie (product of Mexico; from Inter-Marche on Cote Vertu Blvd.), and Cherries (product of Mexico; from Inter-Marche on Saint Laurence St.).
Four cultivars of beefsteak tomato including ‘Arbason’, ‘Caramba’, ‘Geronimo,’ and ‘Trust’ were grown hydroponically in the greenhouse. Seeds of ‘Arbason,’ ‘Geronimo,’ and ‘Trust’ were bought from Johnny’s Selected Seeds and seeds of ‘Caramba’ were bought from De Ruiter Seeds. Fruit of three beef steak tomatoes were bought from local grocery stores and tested under their trade names: Kaliroy (produced in Mexico; from IGA on Louis-Menard St.), BionatureL (produced in Mexico; from Loblaws on Jean-Talon St.), and BionatureP (produced in Mexico; from Provigo on St. Urbain St.). ‘Diva’ Mini Cucumber was hydroponically grown in the greenhouse from seed bought from Johnny’s Selected Seed. Three SB cultivars were tested under their trade names: Cool Cukes, Lebanese, and Mini cucumber.
Four cultivars of arugula and three of bibb lettuce were evaluated. The arugula ‘Astro’ was grown hydroponically from seeds bought from Johnny’s Selected Seeds. Three arugula were SB and tested under their trade names: ADO (bought from Adonis) and BW and PRO (bought from Provigo). The bibb lettuce ‘RexMT0’ was grown in the greenhouse from seeds purchased from Johnny’s Selected Seeds. Two store-bought bibb lettuce were tested under their trade names: ADO (bought from Adonis) and IGA (bought from IGA).
Hydroponically grown cherry tomato, cucumber, and leafy vegetables were stored in a household fridge (4°C) for 0, 3, and/or 6 d (based on produce availability). This enabled comparison of “fresh” and cold-stored produce.
2.2. Panelists and Taste Tests
Taste tests were carried out at McGill University in a taste test laboratory (McGill University Research Ethics Board Approval # 953-1110). Fifty persons were recruited and went through two rounds of training, and then individuals with consistent ratings were selected. Panelists included 25 males and females from 18 to 52 years of age with no known allergies to any of the tested fruits and vegetables. They tested hydroponic greenhouse grown and store-purchased produce, including tomato, cucumber, arugula, and lettuce. Before each taste test session, panelists were instructed regarding terminology, test procedures, and the nature of the samples. The sensory analysis taste test for each crop was conducted over several sessions during 1 d, with a variable number of 10–20 panelists per session and results were subjected to statistical analysis.
2.2.1. Sample Preparation for Sensory Evaluation
Hydroponically grown and SB tomato, cucumber, arugula, and bibb lettuce were rinsed well under tap water and air-dried for at least 15 min. Tomatoes were cut into thin round slices about 6 mm thick and then into halves, just before serving. Cucumber samples were cross-sliced (2-3 mm) just before serving. Panelists were served a plate containing, in the following order, 2-3 whole fruit of each cherry tomato cultivar, 2-3 slices of each cucumber cultivar, 2-3 half slices of each beefsteak tomato cultivar, and finally 2-3 leaves of each leafy green cultivar. Panelists were individually supplied with plain soda crackers and directed to have a bite after sampling and then sip some water to rinse their palate between each sample. More samples were given upon request and retasting was conducted when necessary.
2.2.2. Panelist Responses
Panelists scored their responses onto a print-out paper sheet that was prepared specifically for each crop. After sensory evaluation, panelists were asked to answer demographic questions (multiple choice), which included age, gender, and frequency of fresh vegetable consumption. Tomato ripeness, sweetness/saltiness, juiciness, and general acceptability of cherry or beefsteak varieties were scored on 9-point hedonic scales, where 9: ripe, 5: neutral, 1: unripe. The same scale was used for sweetness/saltiness and responses were scored: 1: salty; 5: not salty or sweet; 9: sweet. Juiciness was scored on a 9-point score where 1: dislike; 5: neutral; and 9: like. General acceptability also was given scores from 1 to 9, where 1: dislike and 9: like.
Cucumber sensory characteristics including flesh color and firmness, aroma of the fruit, and flesh taste sweetness/bitterness traits were scored on a 9-point scale, where 1: dislike and 9: like for flesh color, firmness, and fresh aroma of the fruit. Sweetness/bitterness were scored on a 9-point scale, where 1: bitter, 5: not bitter or sweet, and 9: sweet. Arugula sensory characteristics including green/grassy, intensity of bitterness, astringent, and overall liking were rated on a 9-point hedonic scale, where 1 was the lowest and 9 was the strongest taste and liking. Bibb lettuce was evaluated for color, sogginess/freshness, off-odor, and overall quality. Sensory evaluation of lettuce was done using the same hedonic 9-point scale.
2.3. Mineral Analysis Using ICP-OES
Greenhouse and SB produce were hand-rinsed under running tap water and then blotted onto paper towels and air-dried for 1-2 h. Samples consisted of 1 g cross-sectional slices from the middle of each of 7–12 whole fruit or up to 20 leaves of leafy vegetables. These were incubated overnight in 3 mL nitric acid (trace metal analysis grade, Fisher Scientific Co., ON, Canada) in a 10 mL Oak Ridge centrifuge tube (Thermo Scientific, NY, USA) placed in a fume hood. On the following day, samples were digested using a heating block (Thermolyne heater type 16500 Dri Bath model DB16525; Thermolyne, Dubuque, IA 52001, USA). The samples were heated to 105°C until no nitrous oxide gases (brown gases) were evolved. Samples were diluted (1 : 4) with Type-1 water (18 Ωcm) and mixed thoroughly (flipping tubes over) prior to injection into the ICP-OES apparatus for analysis. Control elemental stock standard solution (J. T. Baker, St. Louis, MO, USA) was used to calibrate the instrument before sample injection. The inductively coupled argon plasma optical emission spectrometer (ICP-OES) used for mineral analysis in this study was a model VISTA-MPX CCD Simultaneous ICP-OES (Varian Australia PTY Ltd., Australia). The settings were as follows: power 1.2 kW, plasma flow 15 Lmin−1, argon pressure 32 L min−1 (600 kPa), nebulizer flow 0.75 Lmin−1, auxiliary flow 1.5 L min−1, pump rate 15 rpm, viewing height 10 mm, replicate reading time 10 s, and instrument stabilization delay 15 s . Mineral contents were expressed as g or mg of macro- or microminerals, respectively, per 100 g DW based on the dry matter content of each variety.
2.4. Sample Preparation for Phytonutrient Analyses
2.4.1. Freeze-Drying and Grinding of Plant Samples
Plant samples (about 1 kg of beef-steak, cherry tomato, or cucumber and 0.5 kg of arugula and bibb-lettuce leafy vegetables; all randomly selected, 3 replicates/variety) were homogenized and cut into thin slices (0.5–1 cm) and fast-frozen on aluminum plates (diameter of 25 cm) using liquid nitrogen and then freeze-dried at −60 to −70°C for up to 4 d in a freeze-dryer (Christ Freeze-Dryer, Gamma 1-16 LSC, Osterode, Germany). Freeze-dried samples (2 samples/replicate) were ground into a fine powder in liquid nitrogen and stored in a −80 C Freezer (Thermo Electron Corporation, OH, USA). Sample weights were recorded before (fresh weight) and after (dry weight) freeze-drying to calculate the sample dry matter content. Freeze-dried samples were used for phytonutrient analyses.
2.4.2. Preparation of Crude Extracts for Phytonutrient Measurements
About 100 mg of freeze-dried powder was extracted with 2 mL of 90% methanol (MeOH). Samples were vortexed at maximum speed for 60 s, sonicated (Branson 2200, Branson Ultrasonics Corporation, CT, USA) for 30 min, and centrifuged at 3,500 rpm for 15 min at 4°C. Supernatants were collected into 15 mL Falcon tubes. The remaining pellet was reextracted with 1 mL of 90% MeOH and supernatants were combined. Crude extract was used to measure total soluble phenolics (Folin Ciocalteu or FC test), the antioxidant scavenging capacity using 2,2-diphenyl-1-picrylhydrazyl (DPPH), and 2,2′-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS) (hydrophilic phase) . The remaining pellet was reextracted twice, each time with 1 mL hexane, and supernatants were combined. This extract was dried in a Speed-Vac (Thermo Savant, Waltham, MA, USA) and the residues resolubilized in 2 mL of 95% ethanol and used for the evaluation of antioxidant scavenging capacity using ABTS (lipophilic phase).
2.5. Total Phenolics
Total extractable phenolic contents of GH or SB produce were evaluated with Folin-Ciocalteu (FC) reagent according to  using gallic acid (GA) as a standard. A 100 μL sample aliquot of extract or standard dilution was mixed with 2 mL water followed by 200 μL FC reagent (2 N). Tubes were vortexed and incubated at room temperature (RT) for 5 min; then 1 mL of aqueous sodium carbonate solution (20%) was added. Samples were vortexed and kept at RT for 1 h. Absorbance was measured at 765 nm in a Beckman DU 640 spectrophotometer (Beckman Instruments, Fullerton, CA, USA) using 1 cm disposable cells. All measurements were replicated 2 times. Total phenolic content was expressed as milligrams of gallic acid equivalent (mg GAE) per 100 g DW.
2.6. Antioxidant Capacity via ABTS (Hydrophilic and Lipophilic Phases)
ABTS (7 mM) stock solution was prepared in 18 Ωcm−1 water. Radical cation of ABTS () was produced by reacting ABTS solution with potassium persulfate (K2S2O8) (2.45 mM) in the dark at RT for 12–16 h before use for complete oxidation of ABTS . Oxidation of ABTS starts immediately after adding the K2S2O8 but absorbance is not maximal or stable until after period more than 6 h. The radical is stable in this form for more than 2 d when stored in the dark at room temperature. The solution was diluted with 95% ethanol to an absorbance of 0.70 ± 0.02 at 734 nm. About 1.2 mL of diluted solution (A734 nm = ) was added to 100 μL of sample extracts (hydrophilic or lipophilic) or Trolox standards (0–15 μM) in ethanol and absorbance was reported 1 min after initial mixing and up to 5 min. Antioxidant scavenging activity was expressed as μg Trolox equivalent (TE)/100 g DW .
2.7. Antioxidant Capacity Using DPPH Radical Scavenging Capacity
Radical scavenging capacity assay was performed as described by . About 100 μL of methanolic crude extract was added to 1.5 mL of DPPH (2.5 μM in methanol) and shaken vigorously. After incubation at RT for 30 min, the absorbance of the remaining DPPH molecules was determined at 517 nm in a Beckman DU 640 spectrophotometer using 1 cm disposable cells. The mean values were obtained from triplicate determinations. Antioxidant activity was expressed as mg GAE/100 g DW.
2.8. Total Anthocyanin Content (TAC)
Total anthocyanin content was extracted following the method previously described [31–33]. One hundred micrograms of each freeze-dried sample was mixed with 4 mL acetone and the extracted material was separated from the cake by filtration on a Buchner funnel. The filter cake remnant was reextracted with 70% (v/v) aqueous acetone twice. Filtrates were combined and partitioned with chloroform (1 : 2 acetone : chloroform, v/v) and stored overnight at 1°C. The aqueous portion containing the TAC was recovered and separated from the residual acetone at 40°C and resolubilized in 0.01% HCl. Extractions were repeated two times. All extracts were stored at −70°C until being analyzed. The TAC in plant produce extracts were determined by using the pH differential method . Absorbance was measured at 510 and 700 nm in a Beckman DU 640 spectrophotometer in potassium chloride (0.025 M, pH 1.0) and sodium acetate (0.4 M, pH 4.5) buffers. A molar extinction coefficient of 26,900 L cm−1 mol−1 and a molecular weight of 449.2 were used for calculation of total monomeric anthocyanin. Results were expressed as mg of cyanidin-3-glucoside equivalents (mg CGE)/100 g DW.
2.9. Total Carotenoids in Hexane and Ethanol Extracts
2.9.1. Hexane Extraction
Total carotenoids were extracted using hexane as described by . A freeze-dried sample of 0.2 g was weighed into an amber vial and 5 mL of 0.05% (w/v) butylated hydroxyl toluene (BHT) in acetone and then 5 mL of 95% ethanol and 10 mL of hexane were added. Samples were stirred on a magnetic stirring plate, placed on ice, and transferred onto an orbital shaker (Lab-Line Instruments Inc., Illinois, USA) at 180 rpm for 15 min. After shaking, 3 mL of deionized water was added and shaking was resumed for an additional 5 min (still on ice). Sample vials were left at RT for 5 min to allow for phase-separation. Absorbance of the upper hexane layer was recorded at 450 nm in a 1 cm path length quartz cuvette with a hexane blank in a Beckman DU 640 spectrophotometer. Total carotenoid content was calculated for sample weight using the absorbance at 450 nm based on a standard β-carotene curve .
2.9.2. Ethanol Extraction
Total carotenoids were assayed following the method of  to exclude the chlorophyll content. Approximately 0.2 mg of freeze-dried sample was extracted in 5 mL of 95% ethanol. Extracts were vortexed for 60 s and sonicated for 30 min and then centrifuged at 3,700 rpm for 10 min at 4°C before transfer to 1 cm disposable cells. Absorbance of supernatants was read at 470, 648.6, and 664.1 nm in a Beckman DU 640 spectrophotometer. Concentrations of total carotenoids (including xanthophylls) were determined by the following equations:
2.10. β-Carotene, Lutein, and Lycopene Measurements
The carotenoid compounds in freeze-dried produce samples were extracted using a modified method from . A freeze-dried sample of 0.1 g was extracted with 5 mL of extraction solvent (hexane/acetone/ethanol: 50 : 25 : 25 v/v/v) in dark glass vials. The extracts were vortexed for 30 min and 1 mL of water was added. The upper layer was collected and the extract was evaporated to dryness in a Speed-Vac. The residue was dissolved to a final volume of 0.5 mL of ethanol/acetone 65 : 35 (v/v). The extract was passed through a 0.20 μm nylon membrane filter and 20 μL was injected for HPLC analysis.
Solvents and the HPLC running method under isocratic conditions were as described by . Analysis was performed using a reverse phase Varian 9012 HPLC Gemini-NX (5 μm, 100 mm × 4.6 mm) C-18 column equipped with a tertiary pump, refrigerated autosampler, and single variable UV wavelength detector. The mobile phase consisted of methanol (solvent A) and acetonitrile with triethylamine (9 μM) (solvent B) at 90 : 10 at a flow rate of 0.9 mL min−1. The column temperature was 30°C and the absorbance was read at 475 nm.
2.11. Measurement of Organic Acids
The organic acids, ascorbic, citric, fumaric, and malic, were determined following  using a HPLC (Varian 9012, Varian Chromatography Systems, CA, USA) equipped with a tertiary pump, refrigerated autosampler, and single variable UV wavelength detector. A 100 mg freeze-dried sample was extracted with 5 mL of 0.008 N H2SO4 in type-1 water (18 Ωcm). Samples were shaken for 1 min on an orbital shaker (Lab-Line Instruments Inc., Illinois, USA) and centrifuged at 3,000 rpm for 10 min at 4°C. Supernatant was collected and filtered through a 20 μm nylon syringe filter (Fisher brand, Fisher Scientific, ON, Canada). Extracts were analyzed using a reverse phase HPLC Gemini-NX (5 μm, 100 mm × 4.6 mm) C-18 column (Phenomenex Inc., CA, USA) and a 4.6 mm × 2.0 mm guard column. The mobile phase was 0.008 N H2SO4 in water at 1.0 mL min−1 under isocratic conditions at 245 nm for ascorbic acid and at 210 nm for malic, citric, and fumaric acids. Extraction was repeated twice for each sample. The data obtained was expressed as mg of organic acid per 100 g DW.
2.12. Determination of Total Carbohydrates
About 20 mg of freeze-dried sample was added to 5 mL of 1 M HCl and placed into a preheated shaker water bath at 100°C for 2 h . Digested supernatant was collected and used for total carbohydrate measurements. Total carbohydrates were determined using the phenol-sulfuric acid colorimetric method described by Nelsen . One mL of 5% w/w phenol and 5 mL of concentrated sulfuric acid were added to 1 mL of acid-digested sample in a 15 mL glass tube. Tubes were allowed to stand for 30 min in a shaker (180 rpm) water bath at 30°C. The absorption was read at 485 nm in a 1 cm quartz cell against a blank consisting of 1 mL of 5% w/w phenol, 5 mL of concentrated sulfuric acid, and 1 mL of deionized water. Standard solutions of glucose from 10–100 mg L−1 were used for preparation of a standard curve.
2.13. Total Dietary Fiber
Freeze-dried samples were analyzed for soluble and insoluble fiber using the method of AOAC (1995) [42, 43]. The method includes enzymatic hydrolysis with α-amylase (heat stable, Sigma A3306), protease (from Bacillus licheniformis, Sigma P3910), and amyloglucosidase (from Aspergillus niger, Sigma A9913). A 1 g quantity of freeze-dried sample was suspended in 50 mL phosphate buffer and a series of enzymes were added in the following sequence in a water bath (Reciprocal Shaking Bath Model 25, Precision Scientific, Il, USA): (1) 50 μL of thermo-resistant α-amylase at 95–100°C for 35 min, (2) 100 μL of protease at 60°C for 30 min, and then following pH correction to 4.0–4.7, (3) 300 μL of amyloglucosidase at 60°C for 30 min. After soluble fiber precipitation with ethanol (95% v/v) at 60°C, the sample was filtered. The crucibles containing the residues were dried at 105°C, then cooled in a desiccator, and weighed. Residue was divided into two portions. The first part was used for estimation of protein content using a LECO nitrogen analyzer and a %N × 6.25 conversion factor to estimate the total protein content. The second part was incinerated at 525°C overnight to measure the ash content. Total dietary fiber was calculated using the following equation:
2.14. Statistical Analyses
Statistical analysis was performed using the statistical analysis systems (SAS) software (version 9.2, 2009). Analysis of variance (ANOVA) was used to detect significant differences between varieties for individual attributes. Sensory evaluations were analyzed using the general linear model (GLM) procedure of SAS (Version 9.2, Cary, NC, USA). The data were first analyzed to determine panelist’s age and gender effects in a randomized complete block design (RCBD). Least square means of panelists’ responses were separated when effects were significant in the ANOVA table (). Panelist, panelist’s age, gender, and crop varieties were analyzed as main factors and were studied for their interactions. When effects of panelists were not significant they were pooled. The means were compared using Scheffe’s Multiple Comparison Procedure ().
Phytonutrients were analyzed using the GLM procedure of SAS (Version 9.2, Cary, NC, USA, 2011). Analysis of variance (ANOVA) was used to differentiate between main factors for cultivars within each crop and between crops for individual phytonutrient characteristics. The data were first analyzed for normality using PROC UNIVARIATE. When effects were significant in the ANOVA table () least square means of measurements were separated and compared using Scheffe’s Multiple Comparison Test ().
2.14.1. Identification of the Dominant (Useful) Variables
Identification of the dominant variables was done after running the variable cluster analysis (VARCLUS), principal component analysis (PCA), and partial least square (PLS) statistical analysis. Based on the hierarchical clustering patterns, classification of variables in nonoverlapping clusters was estimated by analyzing the correlation or the covariance matrices subsequently.
Cluster analysis is a separation method that divides the data set into a number of groups or clusters (tree dendrogram; tree leaves and leaflets) based on similarity. The data points within each group (cluster; one major leaf with one or several leaflets) are more similar to one another than they are to the data points within other clusters. The distance between clusters on the -axis (horizontal axis) indicates how similar to one another the elements of a cluster are. The more distinct a cluster is, the longer the branch along the -axis that connects it to a larger cluster is.
Then PCA and PLS were run to confirm and visualize in space the cluster (group) classification of variables based on the weight of each variable in relation to the variance (PLS) and covariance (PLS and PCA) matrices [24, 26, 44]. Clustering of variables based on their contribution to the overall variance was done using the VARCLUS procedure. VARCLUS output mainly classified variables based on their values. Three values were compared for better selection of variables: with own (the greater the better clustering), with nearest cluster (the lower the better), and ratio (the lower the better).
Correlation loading plot is a circular graph showing the variation accounted for by each extracted factor (of one or more variable(s)) and is generated by partial least square (PLS) analysis. The amount of variation in the data points is relative to their distance from the origin. So, the outside circle shows the data points that are most similar and grouped together (highly correlated). Each successive circle working from the outside to the inside indicates decreasing levels of explained variation. The correlation between any two variables is relative to the length of the projection of the point corresponding to one variable on a line through the origin passing through the other variable. The sign of the correlation (negative or positive) corresponds to which side of the origin the projected point falls on.
Multidimensional preference analysis is another mapping technique for the two principal components that show most of the variance and is generated by the principal component analysis (PCA). A biplot displays the independent and dependent variables in a single plot by projecting them onto the plane that accounts for the most variance. Points that are tightly clustered and point in the same direction share the most attributes in common. Pointing in the opposite direction suggests negative correlation. The longer the projection, the relatively more important the variable.
For data mining and discriminant analysis, statistical analysis was done on two steps: (1) significant variation among the 34 variables was identified (based on MANOVA table) and the nondiscriminant variables were identified for a drop-list. (2) Variable clustering analysis was done according to the contribution of each variable to the overall variance using the VARCLUS procedure. Then, their possible interrelationships were studied through visualization of a network structure using both principal component analysis (PCA) and partial least squares regression (PLS).
3. Results and Discussion
3.1. Sensory Traits of Fruits and Vegetables
3.1.1. Sensory Traits of Cherry and Beef Steak Tomatoes
Data presented in Table 1 show sensory traits of cherry and beefsteak tomato from the greenhouse and Montreal area grocery stores. Tomato ripeness was expressed by fruit firmness and color. Unripe tomatoes are firmer. As they ripen, they soften until they reach the fully ripe state where their color (red or orange) is most intense and the fruit are slightly soft to the touch .
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Superscripts not sharing the same letter within each column are significantly different (). Tomato ripeness was scored on 9-point hedonic scales, where 9: ripe, 5: neutral, 1: unripe. Sweetness/saltiness responses were scored as 1: salty, 5: not salty or sweet, and 9: sweet. Juiciness was scored on a 9-point scale, where 1: dislike, 5: neutral, and 9: like. General acceptability also was given scores from 1 to 9, where 1: dislike and 9: like.|
There was no effect of panelist gender or age on cherry tomato ripening classification. No differences occurred in ripeness score between the GH cherry tomatoes ‘Apero,’ ‘Favorita,’ ‘Juanita,’ and the SB Fruiterie while SB Jardino was scored as less ripe than the others. Sweetness or saltiness of tomato is a trait related to soluble sugars and mineral content of tomato [39, 46]. Juiciness represents the amount of juice/moisture perceived in the mouth. Mouth feeling is a measure of granularity of tomato texture. GH ‘Apero’ and ‘Favorita’ and SB Fruiterie had significantly sweeter taste, were juicier, and had a better mouth feel and greater general acceptance compared with both GH ‘Juanita’ and SB Jardino (Table 1). There was no effect of age or gender on panelist response.
No differences in sensory traits were found between GH beefsteak tomato cultivars (Table 1). The GH beefsteak tomatoes all scored similarly for juiciness and mouth feel but were significantly firmer than both the SB varieties (CHS and Plain Jane). Firmness also was reported to be highly correlated with general acceptability of field-grown tomato [45, 47]. GH ‘Arbason,’ ‘Caramba,’ and SB ‘Plain Jane’ were considered riper and more generally acceptable than the other cultivars. No differences were discerned in sweetness/saltiness and all of them were considered more sweet than salty. Sex, but not age, was important to determination of sensory traits of beefsteak tomato. Male panelists gave greater scores than females for ripeness, sweetness, juiciness, mouth feel, and general acceptability.
3.1.2. Sensory Traits of Cucumber
Results of sensory characterization of cucumber cultivars were presented in Table 2. Hydroponic greenhouse-grown ‘Diva’ was ranked above the two SB cucumber cultivars. Flesh color and fruit firmness of ‘Diva’ were more accepted compared with SB cucumbers. ‘Diva’ and Lebanese were sweeter than Mini Cucumber but all three were similar for fruit aroma. Panelists scored sensory characteristics of cucumber similarly without apparent age or gender bias.
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Superscripts not sharing the same letter within each column are significantly different (). Cucumber flesh color and firmness, aroma of the fruit, and flesh taste sweetness/bitterness traits were scored on 9-point scales, where 1: dislike and 9: like for flesh color, firmness, and fresh aroma of the fruit. Sweetness/bitterness were scored on a 9-point scale, where 1: bitter, 5: not bitter or sweet, and 9: sweet.|
3.1.3. Sensory Traits of Arugula
Results of sensory characterization of arugula (Rocket salad) cultivars are presented in Table 3. All arugula cultivars scored similarly for overall quality and greeny/grassy trait (a measure of freshness). The GH arugula cultivar had an intensely bitter and astringent taste compared with the SB cultivars. Although all participants in this test had never eaten arugula before, their gender and age significantly affected their responses. Male panelists had greater scores for greeny/grassy compared with female participants. Age of participants affected sensory traits of arugula with no obvious trend. For example, panelists, 21–41 years old, had greater scores for overall quality than panelists of the age of 19-20 (3 panelists) and ≥ 43 (2 participants) years. For the intensity of bitterness, only one panelist (a 29 years old) scored less than all other participants. Also, only 2 panelists (49 years old) had greater scores of greeny/grassy trait than 2 panelists (43 years old). Similarly, four panelists (18 years old) scored the astringent characteristic greater than 1 participant, 25 years old. Mixed responses of panelists to the flavor of arugula could be due to the presence of glucosinolates/isothiocyanates, which give both positive and negative sensory properties .
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Superscripts not sharing the same letter within each column are significantly different (). Arugula sensory characteristics including green/grassy, intensity of bitterness, astringency, and overall liking, were rated on 9-point hedonic scales, where 1 was the lowest and 9 was the strongest taste and liking.|
3.1.4. Sensory Traits of Bibb Lettuce
Overall quality and freshness were not different between GH ‘RexMT0’ and the SB bibb lettuce cultivars and all had acceptable odor (Table 4). Panelists liked the color of ‘RexMT0’ and the IGA bibb lettuce but not ADO. No age or sex bias was apparent for panelist response to overall quality of bibb lettuce. However, for lettuce color only, males had greater scores compared with female participants.
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Superscripts not sharing the same letter within each column are significantly different (). Bibb lettuce was evaluated for color, sogginess/freshness, off-odor, and overall quality. Sensory evaluation of lettuce was done using the same hedonic 9-point scales where 1 was the lowest and 9 was the strongest taste and liking.|
3.2. Mineral Content of Fruits and Vegetables
3.2.1. Mineral Content of Cherry Tomato
Least squares means results for the macro mineral (Ca, Mg, K, and P; g/100 g FW) contents of three hydroponically grown GH and three SB cherry tomato varieties are presented in Table 5. There were no clear distinctions in overall macro mineral content between GH and SB cherry tomato. However, the GH ‘Apero’ had the greatest overall content of macrominerals. In particular, it had greater K content but similar Mg content compared with ‘Juanita.’ Also, GH ‘Apero’ and ‘Juanita’ (but not ‘Favorita’) had greater Mg and P content compared to all of the SB varieties. The SB varieties ‘Cherries’ and ‘Fruiterie’ (but not ‘Jardino’) had greater Ca levels compared with the GH cultivars.
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Small case superscripts within species and large case superscripts between species not sharing the same letter are significantly different (). Produce (based on availability) was fridge-stored (4°C) for 3 and/or 6 days.|
Levels of microminerals Cu, Fe, and Na (Table 5) and Al, As, Cr, Se, and Zn (mg/100 g FW) (Table 6) varied widely in GH and SB cherry tomato. As with the macrominerals, there were no clear distinctions in overall micro mineral content between GH and SB cherry tomato. However, ‘Apero’ had the greatest Fe and Na contents compared with other GH and SB varieties but similar Cu levels to ‘Cherries.’ Store-bought cherry tomato also showed less Na content compared with GH varieties. Cd was detected only in the GH ‘Apero’ and ‘Juanita’ (but not ‘Favorita’) with no differences between them. Hydroponically grown GH cherry tomato showed consistently less Al content compared with SB varieties while the SB variety cherries were uniquely high in Al content among all of the cherry and beefsteak tomato cultivars. Overall mineral content was not affected by fridge-storage of cherry tomato cultivars for 6 d (Tables 5 and 6). However, storage resulted in increased Fe levels in ‘Juanita’ and As content in ‘Apero’ while levels of Se were dramatically reduced in ‘Favorita’ and ‘Juanita.’
|LS means were compared using Scheffe’s Multiple Comparison Procedure. Small case superscripts within species and large case superscripts between species not sharing the same letter are significantly different (). Produce (based on availability) was fridge-stored (4°C) for 3 and/or 6 days. —: not detected.|
3.2.2. Mineral Content of Beefsteak Tomato
Concentrations of macrominerals (Ca, K, Mg, and P; g/100 g FW) of the four GH beefsteak tomato cultivars (Arbason, Caramba, Geronimo, and Trust) and the three SB varieties (Kaliroy, BionatureL, and BionatureP) are presented in Table 5. Overall, the GH cultivars were superior in macromineral content to SB varieties. However, clear cultivar differences were apparent among both the GH and the SB beefsteak tomato groups. The cultivars with the greatest tissue concentrations of all macrominerals were ‘Caramba’ (greatest in all except Ca and P) and ‘Geronimo’ and ‘Trust’ (greatest in Ca, and less than ‘Caramba’, but greater than the other cultivars for other macrominerals).
Microminerals (mg/100 g FW), Cu, Fe, and Na (Table 5) and Al, As, Cd, Cr, Pb, Se, and Zn (Table 6), varied between GH and SB beefsteak tomato varieties. ‘Caramba’ had the greatest Cu, Fe, and Na compared with other beefsteak tomato cultivars. The GH group had the least Al concentrations and As and Cd were detected only in ‘Arbason.’ ‘Geronimo’ showed the least Cr content but this was similar in the other GH cultivars. Also, ‘Geronimo’ had the greatest Pb level but this was similar in several other cultivars, both GH and SB. Selenium was detected, at similar concentrations, only in two cultivars, Caramba and Geronimo. Very high Zn content occurred in three of the four GH beefsteak cultivars. These Zn levels were much greater than in the other crops tested. ‘Caramba’ had the greatest Zn content followed by ‘Geronimo’ and ‘Trust.’ The other beefsteak cultivars (Arbason, Kaliroy, BionatureL, and BionatureP) had relatively low tissue levels of Zn.
3.2.3. Mineral Content of Cucumber
Results of least square means of Ca, K, Mg, and P (g/100 g FW) of one GH cultivar (Diva) and three SB cultivars of cucumber are presented in Table 5. No differences were found for Ca and Mg levels among GH and SB cucumber. Similar K concentrations were reported for ‘Diva,’ ‘Lebanese,’ and ‘Mini Cucumber.’ ‘Diva’ had greater P content than ‘Mini Cucumber’ and ‘Cool Cukes’ but was not different from ‘Lebanese.’ Hydroponically grown GH ‘Diva’ led the SB varieties in Cu content but had similar Fe content to ‘Cool Cukes’ (Tables 5 and 6). Na level was greater in ‘Diva’ compared with cultivars Mini Cucumber and Cool Cukes but was not different from ‘Lebanese.’ ‘Diva’ had greater concentrations of Zn and Al compared to the SB varieties and Cr and Pb concentrations were similar for GH and SB cucumbers. Overall, it was apparent that fridge-storage of the cucumber ‘Diva’ for 3 or 6 d somewhat affected its mineral content (Table 5). While storage did not affect Ca, Cu, K, and Fe, it increased Mg, Na, P, and Zn and reduced Al levels, compared with fresh cucumber (Table 6). Also, both As and Cd were detected, at similar concentrations for both storage intervals, only after 3 and 6 d storage but were not detected in the fresh sample. The rationale for differences in mineral content before and after storage is unclear, but the results may reflect reduced moisture content after storage, difficulty in complete extraction of minerals from fresh cucumber tissue, and/or potential matrix interferences.
3.2.4. Mineral Content of Arugula
Arugula from the GH (1 cultivar) and SB (3 cultivars) were similar in both macro- and micromineral content, except for Fe, Na, and Al, where all of the SB cultivars had consistently greater levels (Tables 5 and 6). Fridge-storage for 6 d of GH ‘Astro’ did not have a great impact on mineral content (Tables 5 and 6). Fresh ‘Astro’ was similar in mineral content to Astro after 6 d storage except for greater Ca and Fe (levels reduced by 34.39 and 34.83%, resp.; Table 5) and Cr and Pb (not detected in stored produce) (Table 6).
3.2.5. Mineral Content of Bibb Lettuce
Data presented in Tables 5 and 6 showed no differences between GH and SB bibb lettuce in K (g/100 g FW) and As (mg/100 g FW) mineral concentrations. However, the SB variety ADO had greater Mg and P contents compared with both GH ‘RexMT0’ and the SB variety IGA. Similar Ca content was found in GH ‘RexMT0’ and the SB ‘ADO’ and IGA. Store-bought variety ADO had greater Al, Cd, Cu, Fe, and Na levels compared with both GH ‘RexMT0’ and the SB variety IGA. Few differences were observed in mineral content between fresh and fridge-stored (6 d) GH ‘RexMT0.’ Storage did not alter the concentrations of Ca, Cu, K, Fe, Na, and P but reduced Al, Cd, Mg, and Zn levels.
3.3. Dry Matter (%) of Fruit and Vegetables
Cherry tomato cultivars had the greatest dry matter percentages (9.22%) followed by arugula (7.79%), then beefsteak tomato (5.11%), cucumber (4.91%), and bibb lettuce (4.71%) (Table 7). For cherry tomato, the GH ‘Apero’ and ‘Juanita’ but not ‘Favorita’ had greater dry matter percentages than the three SB cultivars. There were no differences in dry matter content between GH and SB cultivars for the other crops. There was no effect of storage duration (3 or 6 d) on the dry matter content of any crop.
|LS means were compared using Scheffe’s Multiple Comparison Test. Small case superscripts within species and large case superscripts between species not sharing the same letter are significantly different (). Produce (based on availability) was fridge-stored (4°C) for 3 and/or 6 days.|
3.4. Total Carbohydrates
Total carbohydrates of GH and SB crops were expressed as mg glucose equivalent/100 g DW (Table 7). Total carbohydrate levels varied with crop. Cucumber (74.44) had the greatest total carbohydrate content followed by cherry tomato (61.00) and then beefsteak tomato (51.94) and bibb lettuce (36.69), while arugula (13.67) had the least content. Hydroponically grown GH and SB crops were similar in their carbohydrate content. No differences in total carbohydrate contents occurred between fresh GH cherry tomato, bibb lettuce, and cucumber compared with produce that was fridge-stored for 6 d. However, storage of arugula ‘Astro’ for 6 d decreased the total carbohydrate levels compared with fresh ‘Astro.’
3.5. Total Dietary Fiber (%)
Percentages of total dietary fiber (TDF) of GH and SB produce are presented in Table 7. TDF% ranged from 2.07 to 3.43 (arugula), 2.03 to 2.19 (bibb lettuce), 1.64 to 2.68 (cucumber), 1.65 to 2.14 (beefsteak tomato), and 1.37 to 1.88 (cherry tomato), with no differences between GH or SB produce. Fridge-storage (6 d) had no effect on produce TDF%.
3.6. Antioxidant Assays for Fruits and Vegetables
3.6.1. Antioxidant Scavenging Activity (ABTS (Hydrophilic and Lipophilic) and DPPH)
Antioxidant scavenging capacity of GH and SB vegetables was evaluated using two oxidizing agents: 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) (Table 8). Hydroponically grown GH and SB cherry tomato cultivars had similar ABTS and DPPH antioxidant activity. Similarly, no differences were found between GH and SB beefsteak varieties in the hydrophilic and total ABTS and the DPPH scavenging activities except for GH ‘Geronimo’ that showed less ABTS lipophilic antioxidant activity than the SB variety BionatureL. However, ‘Geronimo’ showed similar antioxidant capacity to all other varieties. The GH cucumber ‘Diva’ had the greatest hydrophilic and total ABTS activity compared with the three SB varieties. No differences were found between Mini Cucumber, Cool Cukes, and Lebanese cucumber in antioxidant scavenging capacity. There were no differences in antioxidant capacity observed between GH arugula ‘Astro’ and the three SB varieties. For bibb lettuce, the SB variety ADO had greater hydrophilic ABTS antioxidant activity (TE/100 g DW) compared with all other lettuce varieties. Also, GH and SB bibb lettuce showed similar antioxidant activity based on lipophilic and total ABTS and DPPH assays (Table 8).
|LS means were compared using Scheffe’s Multiple Comparison Test. Small case superscripts within species and large case superscripts between species not sharing the same letter are significantly different (). Produce (based on availability) was fridge-stored (4°C) for 3 and/or 6 days.|
Antioxidant activity based on ABTS (hydrophilic) (mg TE/100 g DW) showed that, overall, arugula, beefsteak tomato, and cherry tomato had greater activity than bibb lettuce and cucumber. Bibb lettuce showed greater antioxidant activity than cucumber (Table 8). Cherry tomato was reported to contain higher antioxidant scavenging activity compared to beefsteak tomato . ABTS (lipophilic) scavenging activity was greatest for arugula followed by bibb lettuce and beef-steak tomato, cherry tomato, and cucumber varieties. In direct contrast, the DPPH free radical test (mg AAE/100 g DW) showed that, overall, cucumber had greater antioxidant activity than all other crops, while beefsteak tomato, bibb lettuce, and arugula had greater activity than cherry tomato but arugula and cherry tomato were similar in their antioxidant scavenging capacity. Fridge-storage of three GH cherry tomato cultivars, one arugula, and one bibb lettuce cultivar for 6 d did not affect their antioxidant scavenging activity based on either method used (Table 4). For cucumber, fresh samples of ‘Diva’ had greater hydrophilic and total ABTS activity but similar lipophilic ABTS and DPPH antioxidant scavenging activity compared with fridge-stored (3 or 6 d) produce. Using the ORAC technique, Song et al.  reported no differences in the antioxidant capacity among tomato, cucumber, and lettuce.
3.6.2. Total Phenolic Content
Total phenolic content of GH and SB vegetables was expressed as mg of gallic acid equivalent (GAE) per 100 g DW (Table 9). Most cultivars of cherry tomatoes (Apero, Favorita, Juanita, and Jardino), whether GH or SB, had similar total phenolic levels. However, the SB variety Cherries had the least content compared with all other cherry tomato cultivars. Total phenolic content of the GH beefsteak tomatoes was similar to that of the SB varieties.