Many different classes of antioxidants are present in vegetables and it is hard to elucidate which ones are more associated with the benefits. Also, the synergistic effect and interaction of different antioxidants in one food leads to the fact that the level of a single antioxidant is not a good indicator of the total antioxidant capacity of the food item. Therefore, measuring the total antioxidant capacity, which is the cumulative capacity of food components to scavenge free radicals, has become an effective way to evaluate the potential benefits of various vegetables in preventing or managing chronic diseases. The average TAC of themicro-greens samples evaluated in this study ranged from 1.06 to 1.18 mg/g, which are comparable to previous results on micro-greens and mature vegetables and fruits. Since the TAC assay was conducted using the methanolic extract used for TPC analysis, it was not surprising that there was no significant difference in both values among samples, as phenolic compounds could be the main contributor to the TAC in this experiment.A total of 150 participants’ data was collected and analyzed in the sensory study. The descriptive information of participants is shown in Table 1. The participants were asked about their prior experience in purchasing and consuming micro-greens. Eighty-two percent had not purchased and 69% had not consumed micro-greens before participating in this study.According to the results of the sensory study , scores for the smell, appearance,taste, and overall liking of micro-greens were all significantly higher for those from the local farm as compared to those from commercial.
A higher average of scores was noted for soil-grown farm samples as compared to water-grown farm samples but did not reach statistical significance. The average scores for all the sensory attributes of farm micro-greens were in the range of 4.54–5.38 out of 7 , while those for the commercial micro-greens were in the range of 3.09–3.68 . Several factors may contribute to the better sensory quality of the micro-greens samples from the farm. Firstly, the greatly higher level of chlorophyll as aforementioned gave the farm samples a more vibrant color as compared to the commercial ones,flood tray and therefore contribute to the higher evaluation on the appearance. Appearance of a food product, especially fruits and vegetables, is the initial quality that attracts consumers, and affects their first time purchase intention. Secondly, samples from the local farm were delivered the same day of harvest and were used for the sensory test on the following3d.However,the exact harvest time of the for transportation and storage until they reached the consumers. The freshness of vegetables may significantly affect the evaluation on all the sensory attributes, including smell, taste, and appearance. Thirdly, it was reported that total sugar content is a factor that can greatly affect the sweetness, bitterness, and sourness, and thus the taste of vegetables. The higher level of chlorophyll in the farm samples as observed may result in a higher production of sugar owing to greater capability of photosynthesis, which may contribute to the higher scores on the taste of those samples as compared to the commercial ones. Future analysis can be conducted to measure the sugar contents of micro-greens samples from different sources. To explore the impact of smell, taste, and appearance on the overall liking for micro-greens, the correlations among sensory attributes were analyzed. It was found that the overall liking was most strongly correlated with the taste of micro-greens.
Scores of overall liking were also strongly correlated with the appearance and the smell of micro-greens . The results indicated that taste, smell, and appearance all contributed to consumers’ perception of micro-greens but taste may be the best predictor. This is consistent with previous findings that flavor-related characteristics best predicted consumer preferences for overall eating quality, although visual quality characteristics also contributed.The population is increasing but the farmland is reducing in China. The supply of fresh vegetables has become an urgent problem. Therefore, plant factories are being increasingly used to solve this problem. Plant factory is an efficient agricultural system which can realize the annual continuous production of crops through environmental control in facilities.Hydroponic lettuce, a vegetable with a short growth cycle and high yield, has been widely cultivated in the plant factories. Hydroponic lettuce is mainly planted on a planting board, and a deep flow technique, a hydroponic technique in which the plant roots grow in a deep and flowing nutrient layer, was used to cultivate lettuce. The hydroponic lettuce is pulled out from the planting board when it is mature, and the roots are cut as required. Then the lettuce is sold in packaging. At present, harvesting of hydroponic lettuce is carried out manually, with high labor costs and low efficiency. Therefore, an automatic harvester that can process leafy vegetables is required. Study of the physical and mechanical properties of vegetables plays a vital role in the development of automatic equipment.
Knowing the physical and mechanical properties of hydroponic lettuce will be useful for the development of harvesting machinery. One of the basic and most important experiments in the study of mechanical properties of lettuce is the tensile experiment. It can reflect some of the mechanical properties of the lettuce stem and whole lettuce. This paper will use similar vegetables as a reference to carry out research due to a lack of research on the mechanical properties of leafy vegetables. The researches on pulling force of similar vegetables were mainly focused on root crops such as radish , carrot , and garlic , etc. Several studies including Chen et al., Li et al. and, Fu et al. have used the tensile experiment to obtain the pulling force of different varieties of radishes. They concluded that the maximum pulling force to harvest green radishes and red radishes were 90 N and 110 N, respectively. Xin et al. also obtained that the main distribution range of garlic pulling force was from 24 to 27 N under the normal harvest condition. The cabbage harvester designed by Du et al. can pull cabbages out of the soil with a success rate of 86.7%, but it can only be used in the field environment. At present, there are many studies on the pulling force of vegetables in the field, but fewer studies on the pulling force of hydroponic leafy vegetables in plant factories. Therefore, the pulling force of hydroponic leafy vegetables needs to be studied to develop automatic machinery in plant factories. In addition to the tensile experiment, a shear experiment can be carried out to determine the root cutting force of hydroponic lettuce. Kanamitsu and Yamamoto and Li et al. have used shear experiments to obtain the root cutting force of cabbage. They found that the root cutting force can be affected by cutting position and cutting speed, while cutting position had a more significant impact on the root cutting force.
Gao et al.concluded the optimal parameters combination among cutting position, cutting mode, cutting speed, cutting angle, clamping position and clamping angle to harvest hydroponic lettuce through an orthogonal experiment. However, this study was only applicable to the way of removing the root harvest. Chen et al.and Wu et al. used orthogonal experiments to investigate the cutting forces of broccoli and rape, respectively. They both concluded that the cutting position was the key factor affecting the cutting force. Although the above-mentioned studies are not about lettuce, these studies provide support for the research of root cutting force of hydroponic lettuce. Presently, studies of hydroponic lettuce have generally been limited to the sensory attributes, general appearance, wilting, decay and physiological disorders conducted mostly during investigations on the packaging, processing and storage conditions,ebb and flow tray there are few studies on the root cutting force of hydroponic lettuce. However, the root cutting force is an important parameter in the design of the cutting device of hydroponic lettuce harvesting machinery. Therefore, the root cutting force should be investigated to develop the automatic harvester. This study is aimed to investigate some of the physical and mechanical properties of hydroponic lettuce. It is carried out by means of using a tensile experiment, shear experiment, and moisture content experiment on multiple samples. The pulling force, root cutting force, geometric characteristics, and moisture content were obtained. The moisture content and pulling force are summarized and analyzed by mathematical statistics method. Meanwhile, the response surface method and variance analysis are used to analyze the change rule of root cutting force with different cutting positions and cutting speeds.The hydroponic lettuces were randomly chosen from Yangling modern agricultural demonstration park in Shaanxi Province, China. A total of 100 samples were purchased for this study. The vegetables were ripe and defect-free.
The experiments were conducted at the College of Mechanical and Electronic Engineering, Northwest A&F University, China. Because experimenting with all samples at once was impossible, the unused vegetables were grown in homemade Petri dishes.The design requirements of leaf vegetable harvesting machinery and the planting mode of hydroponic lettuce were referenced. The physical parameters of hydroponic lettuce, such as crown diameter , overlap length , total height , plant height , total root length , main root length , stem diameter , maximum expansion diameter of the root , total weight and net weight were selected as the evaluation indexes of the geometric characteristics of hydroponic lettuce. The investigation was carried out using a digital vernier caliper , a digital ruler , and an electronic balance to measure the twelve parameters of hydroponic lettuce . The CD is the maximum diameter of the projection of hydroponic lettuce in the horizontal plane under the natural growth state. The OL is the maximum value of the overlapping part of leaves with two adjacent lettuces in nature growth. The MRL is the distance between the planting board and the maximum root diameter of less than 30 mm . The MED is the maximum diameter of the projection in the horizontal direction of the lettuce root after the root leaves nutrient solution. The net weight is the weight after cutting the root. The ruler was used to measure the PH, CD, and TRL while the digital vernier caliper was short for measuring. Thirty hydroponic lettuces were randomly selected from the 100 samples which were purchased to experiment with physical properties.The direct drying method was used in this experiment to calculate the moisture content, according to the standard of GB / T 5009.3-2010 .
The roots, stems, and leaves of hydroponic lettuce were divided into three groups because experimenting on the whole vegetable was impossible. The roots were cut into a segment with 30 mm, the stems were cut into thin slices with 2 mm, and the leaves were cut into strips with 5–10 mm. Five samples were selected randomly from each group for the experiment. The experiment was performed under room temperature by using a drying oven , drying dish, and electronic balance. The weight of the drying dish and the total weight of drying dish with sample were measured, the weights were recorded as m3 and m1, respectively. Then the drying dish with sample was put into the drying oven at a temperature of 103℃±2℃. After 4 h, the drying dish with sample was taken out and weighed for the first time. The drying dish with sample should be weighed after cooling to room temperature, and tweezers were suggested to be used when moving the drying dish. After that, the weight of them was weighed every 1 h until the weight difference between two times was less than 2 mg. The maximum force to pull the hydroponic lettuce out of the planting board was called pulling force. The power consumption of the harvesting machinery and the design of the pulling device need a reference to the pulling force. A tension meter and puller of hydroponic lettuce with a length 200 mm, a width 50 mm, a height 300 mm, and a U-shaped groove with a width of 25 mm was developed to determine the pulling force of hydroponic lettuce. The stem of hydroponic lettuce was placed in the Ushaped groove of the puller of hydroponic lettuce.