Trial Protocol for Hydroponically Cultivated Microgreens

Trial Protocol for Hydroponically Cultivated Microgreens: History

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Subjects: Food Science & Technology

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Microgreens are young plants that are consumed at the seedling stage, which have a short production cycle (about 14 days) and require little space for growth. The hydroponic production of microgreens has potential to develop, at both an industrial, and a family level, due to the improved production platforms.

microgreens
hydroponic
trial protocol

1. Introduction

Microgreens are young plants that are consumed at the seedling stage, which have a short production cycle (about 14 days) and require little space for growth [1]. Microgreens are emerging functional foods of the 21st century [2] that are gaining interest for their sustained nutraceutical properties and are an optimistic prospect for expanding especially for the consumption of the population in large urban areas and in terms of food security. Production of microgreens using hydroponic systems must be planned and controlled with care for controlling environmental factors in order to increase quality parameters [3,4,5,6]. This is in comparison to more conventional production methods using soil, considering all the controllable factors in hydroponic systems that have been shown to influence the accumulation of bioactive substances [7,8], the harvest timeframe [9,10,11], and the quality of the finished product [12,13,14]. Furthermore, the lack of a soil’s microbiome in hydroponic systems is also important to consider, as unsuccessful parameterization leaves the plants vulnerable to harmful spoilage by microorganisms [15,16,17].

However, the advantages of hydroponic platforms and the development of evaluation protocols can lead to a positive influence on the quality of microgreens with higher concentrations of active substances [18,19] and nutrients valuable for human health [20,21,22]. This is why it is necessary to standardize certain cultivation protocols to ensure their quality [23,24,25]. For instance, there is a wide range of environmental impact factors and variation in their relationships to downstream microgreens outputs, which means that there is no single prescription that will guarantee perfect results [26]. The literature review has demonstrated that there are optimal ranges within which one can begin the task of designing effective prescriptions for successful microgreen production [3,21,27].

The time from sowing to harvest is 7–21 days for microgreens [28], a period in which the control of vegetation factors is very important. Nutritional solution, temperature, and light regime have the most important role in seed germination [29,30] and development [31], while also summarizing the recent research on the many promising research trends in refining microgreen production to achieve optimal outputs along its phenological stages [32]. The nutritional solution, air, and water temperature, light regime, pH, electrical conductivity, dissolved oxygen, CO₂ concentration, and relative humidity are all important factors which influence secondary metabolism from an incipient phase [33,34], which in the final stages increases both the perceived and actual value of the plants by contributing to human health and nutritional fortification [35,36].

Microgreen producers must integrate specific systematic hydroponic strategies to obtain high-quality microgreens and high quantity [37] and quality bioactive substances [38], while also avoiding the potential for spoilage and low-quality production [20,39] when moving too far beyond the noted parameter ranges [3].

Many authors in the literature review have noted that best practices have not been developed [40,41], which means that although there are many guidelines for producing microgreens, we do not have very clearly defined standards; this literature review has therefore gathered critical information regarding hydroponically grown microgreen production that can be used by researchers and producers to improve the protocol for testing platforms used to obtain microgreens [18,42].

Microgreens are currently considered among the five most profitable crops, along with mushrooms, ginseng, saffron and goji berries [43]. Therefore, developing species-specific growth media to support year-round production and to enhance valuable antioxidant components is affordable and of utmost importance for the microgreens industry [19,22,43]. It is particularly important that the fundamental research into ensuring the safety and quality of this new addition to healthy diets, microgreens, is carried out so that the produce industry can avoid some of the problems that have challenged the mature produce and sprout industries during the past several decades [44,45].

2. Trial Protocol for Evaluating Platforms for Growing Microgreens

The trial protocol for evaluating platforms for growing microgreens in hydroponic conditions includes the procedures to be followed and the parameters considered useful for calibrating the platform.

Hydroponic GoHydro systems (https://gohydro.org, accessed on 18 April 2022). have specific characteristics, such as the layer of crop used (nutrient solution), type of irrigation (closed), method of irrigation (immersing), irrigation level (root level) [47]. Plants are cultivated in a substrate membrane, over which the nutrient solution passes periodically [48].

The high-capacity tank helps to maintain a constant pH. The color of the tank must be white on the outside to maintain a constant temperature of the nutrient solution, and it is not affected by solar radiation [49]. The water pump is in the tank, and the nutrient solution reaches the surface of containers through a pipe system. The pump recirculates the whole solution within 30 min of a fertilization regime, and the result is the mixing of the solution in the system [50].

Microgreens can germinate and grow without any fertilizer application, up to the capacity of the specific seed’s capacity [39]. However, providing mineral nutrients to microgreens will increase yields and secondary metabolite concentration [51].

2.1. Setting the Optimal Ranges, in Controlled Settings

Setting the optimal ranges for microgreens, in controlled settings, between the limits of favorability for each species, aims to highlight the effects of the hydroponic platform [3]. As reported in the literature, special attention must be addressed to the choice of growth medium, which represents one of the key factors in the production process and could influence microgreens yield and quality [52]. Parameters defined and optimal ranges for different species of microgreens continuously monitored and controlled are presented in Table 1 [3,19]. The spectral output of the lighting system must be quantified using a spectrometer, at various points of growth of the trays of the platform [53].

Table 1. Parameters defined for different species of microgreens continuously monitored and controlled.

No.	Parameter	Unit of Measurement	Average Value of Parameters for Example Species ***
No.	Parameter	Unit of Measurement	Basil	Lettuce	Brussels Sprouts
1	Light	W	400	400	400
1.1	Photoperiodicity	h	06:30–21:30 (15 h) (10–20 h)	07:00–20:00 (12 h)	07:00–20:00 (12 h)
1.2	Light intensity	μmolm⁻²s⁻¹	300 (200–400)	500	300 ± 15
1.3	Color spectrum	nm	440–460 (260–780)	440–460	400–700
1.4	Distance from light	cm	150—Lamps HPS * 40—Lamps LED *	150—Lamps HPS 40—Lamps LED	150—Lamps HPS 40—Lamps LED
2	Ambient air temperature	°C	21 ± 2 Day/17 Night	20 ± 2	24 Day/18 Night ± 2
3	Relative humidity	%	65 ± 5 (50–60)	80 ± 5	70/80% ± 5
4	Nutrient concentration	N-P-K: 3-2-3 (%)	changed every 10 days **	changed every 10 days	changed every 10 days
5	pH	pH units	6.8 ± 0.4	6.3 ± 0.4	6.0 ± 0.2
6	Electrical conductivity	mS	1.2 ± 0.2	1.8 ± 0.2	1.8 ± 0.2
7	Dissolved oxygen	mgL⁻¹	6.5	6	6
8	Solution temperature	°C	20 ± 2	18 ± 2	20 ± 2

Note: * HPS-High Pressure Sodium; LED-Light emitting diodes. ** 8 o’clock in the morning; *** monitor daily at 8 o’clock in the morning in 3 repetitions.

The vegetation chamber is controlled by a system operated through a software program. The environmental factors (temperature, humidity, light) are controlled and monitored throughout the entire experimental period for the controlled experiments. Thus, for example, in the case of basil, the environment factors from the vegetation chamber shall be set as follows: temperature 21 ± 2 Day/17 Night; humidity 65 ± 5%, additional light by lamps of 400 W, photoperiodism: 06:30–21:30 (15 h), automatic airing at ±2 °C, compared to the programmed temperature. Table 2 presents measurement units, methods and possible equipment to be used.

Table 2. Recommended measurement methods and equipment.

No.	Parameter	Unit of Measurement	Methods	Equipment for Measuring (Example)
1	Light	W	HPS/LED	Parameter specific
1.1	Photoperiodicity	h	Soft setting	Clock
1.2	Light intensity	μmolm⁻²s⁻¹	Number of photons	Digital device (Luxmeter, spectroradiometer)
1.3	Color spectrum	nm	Light spectrum	Spectrometer
1.4	Distance from light	cm	Adjustment	Ruler
2	Ambient temperature	°C	Temperature sensor	Temperature sensor
3	Humidity	%	Relative humidity	Hygrometer sensor
4	Nutrient	N-P-K: 3-2-3 (%)	Type of solution	Standard
5	pH	pH units	Solution reaction	Laboratory pH meter
6	Electrical conductivity	mS	Electrical conductivity in water	Digital electrical conductivity measurement water conductivity sensor
7	Dissolved oxygen	mgL⁻¹	Oxygen level as % of Saturation	Oxygenometer
8	Solution temperature	°C	Temperature sensor	TMCx-HD Water Temperature Sensor

The nutrient solution shall be changed every 10 days (at 8 o’clock in the morning) in order to satisfy the need for macro- and micro-elements [49]. After each change, the systems and all the devices used shall be disinfected [54]. The nutrient solution shall be monitored daily and manipulated in order to be maintained at the best parameters for the development of plants [55]. The level of the nutrient solution must be kept constant [19].

Measurements of the oxygen dissolved into water shall be made daily [55,56]. These measurements shall record the quantity of oxygen dissolved into water, the temperature of the solution, the date, the time and the temperature from the atmosphere.

Artificial lighting shall be measured on all the experimental surface, in different points [57], both from the point of view of intensity, and from the point of view of the quality of light. Light intensity shall be measured with a digital device which determines the number of photons relatively to surface and time (μmol m⁻² s⁻¹). The light spectrum shall be determined by using a spectrometer. These measurements shall be made above each tray [58] and at differences of 10, 20, 30 and 40 cm above their canopy.

In the case of measurements related to water losses, the crop trays shall be measured daily by using the digital scales [59].

2.2. Weather Conditions in the Area/Testing Period

In order to determine whether the weather conditions in the area/testing period have an influence on the operation of the hydroponic platform, important atmospheric data will be recorded daily [46]. The growing medium plays a very important role in determining the microgreens’ yield and quality [52], and the sustainability of the production process. The determined parameters will be: temperature min/max/medium (°C); atmospheric humidity min/max/mean (%).

This entry is adapted from the peer-reviewed paper 10.3390/foods11091327

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