They are smaller than baby greens and they are harvested later than sprouts. Due to their immaturity,they tend to have concentrated flavors, tender textures, vibrant color, and densely-packed nutrients. Various micro-greens, such as broccoli, kale, celery, opal basil,and red beet, are available in the market. Because of their potent flavors and appealing sensory qualities,in the past few years,micro-greens have gained popularity and are often used by high-end chefs for garnishing soups and sandwiches. Their functional benefits have also drawn attention from nutrition researchers and have opened the door for use in the field of nutrition and health. This functionality is attributed to their high content of vitamins and minerals, as well as other bio-active compounds. It has been reported that many species of micro-greens are more saturated with micro-nutrients than the adult versions of the same plants. For instance, micro-greens have shown to be high in vitamins or their precursors, including carotenoids, ascorbic acid, to copherols and tocotrienols, phylloquinone and folate. Other phytochemicals found to be high in micro-greens include chlorophyll, phenolic compounds, anthocyanins, and glucosinolates. Results from Huang et al. showed that red cabbage micro-greens reduced high-fat diet induced weight gain and significantly lowered circulating LDL levels as well as expression of hepatic inflammatory cytokines in mice fed a high-fat diet.
Polyphenols, a class of phenolic compounds, were indicated to be a major contributor of the aforementioned effects of red cabbage micro-greens, partially owing to their antioxidant and anti-inflammation properties. Phenolic compounds, also seem to have an influence on sensory qualities of micro-greens. Xiao et al. reported that the total phenolic concentration was strongly correlated with the overall eating quality and several aspects of sensory qualities,ebb flow table including intensity of astringency, sourness, bitterness, and sweetness of micro-greens. Nowadays, products of micro-greens are available to consumers in both chain grocery stores and local farms. The growth environments and harvesting methods, however, are quite different. Farms grow micro-greens in soil or hydroponically. On the other hand, commercial micro-greens are usually hydroponically grown, which increases the productivity of micro-greens but may compromise their nutritional and sensory quality. In addition, micro-greens purchased from local farms are usually fresher than those from grocery stores due to shorter transportation time, which may further affect the nutrition and sensory properties of the plants. In spite of the increase of research on micro-greens, no study has evaluated the nutritional and sensory qualities of these specialty vegetables grown differently or from different sources. Such information is important for consumers in purchasing or for health professionals in conducting research or giving dietary advice. Therefore, the main objective of this study was to investigate the nutritional qualities and sensory properties of micro-greens grown and harvested in a commercial setting versus a local farm setting. The nutritional analysis will focus on chlorophyll content, phenolic compounds content, vitamin C content, and antioxidant capacity, which are known to contribute to the health benefits or sensory attributes of micro-greens. Since broccoli micro-greens is the only species available to researchers in both local grocery stores and local farms, it was selected for the comparison in nutritional and sensory qualities.
In addition, six other micro-greens that available in local farms were also analyzed for nutritional quality. We hypothesized that micro-greens grown in soil in a farm setting would possess higher nutritional qualities and produce better sensory properties as compared to those grown hydroponically and those from a commercial source.Commercial broccoli micro-greens samples which were hydroponically grown, were purchased from the Fresh Marketin Tuscaloosa, Alabama. These samples were designated as CH in this research. Farm grown broccoli micro-greens samples were purchased from the Alabama micro-greens . To elucidate the effects of growth environment on the qualities of micro-greens, micro-greens samples both grown in soil and hydroponics were obtained from the Alabama micro-greens. Six other micro-greens, including amaranth, kale, kohlrabi, pea, spicy broccoli, and wasabi, were also purchased from the Alabama micro-greens. The commercial micro-greens were packaged in plastic clamshell containers with unknown harvest date, while the farm samples were harvested on the same day of delivery and packaged in paper clamshell containers. Samples were used for laboratory analysis on the day of purchase and stored at 4 ◦C for the sensory study during following three days. The optical images of the three broccoli micro-greens samples are shown in Fig. 1.The sensory study was conducted on campus at the University of Alabama to evaluate the sensory qualities of micro-greens samples. Data collection was carried out in the student center during weekday lunch time . Participants were recruited by being asked whether they were interested in evaluating micro-greens and completing a survey. After being explained with the purpose of the study and showed the consent form, participants were provided the three samples of broccoli micro-greens in a random order, each in a small cup, for testing. Information about the source and growth conditions of the samples was blind to participants. Participants were asked to observe, smell, and taste each sample and evaluate their liking of smell, appearance, taste, and overall liking in a 7 Likert scale from very poor to excellent .
Participants were asked to drink water between samples. Upon completion of the survey, participants received a $3 Amazon gift card as an incentive. The study procedure and survey questionnaire were reviewed and approved by the Institutional Review Board at the University of Alabama.As shown in Fig. 2A, broccoli micro-greens samples from the local farm had significantly higher TCC than that from the commercial source . The TCC of hydroponically grown samples from the local farm was higher than that of the soil grown samples, but the difference did not reach statistical significance. Chlorophyll is a green pigment found in most plants and is essential for photosynthesis. A higher concentration of chlorophyll might indicate the production of more energy nutrients in the plants. Chlorophyll cannot be synthesized by animal tissues and must be obtained from plant foods. Recently, chlorophyll and chlorophyll-rich diets have been reported to play roles as cancer-preventive agent attributed to the ability of chlorophyll to form complexes with specific carcinogens, as well as its antioxidant and antimutagenic properties. Some other preventive or therapeutic properties of chlorophyll were also reported in literature, such as stimulating immune system, detoxifification of the liver, and normalizing blood pressure. Although more research is needed to elucidate the mechanisms under these functions of chlorophyll, it could be a valuable dietary compound for human nutrition and health. It was reported that mature broccoli contains about 0.02 mg/g of TCC, while the results showed that TCC in farm grown broccoli micro-greens was about 15 times higher. The TCC in those farm grown broccoli micro-greens was also higher than that of many other species of mature vegetables, such as celery, lettuce, and artichoke. The incorporation of these micro-greens in the diet can be a promising way to provide the health-beneficial chlorophyll and warrants more investigation. As compared to the high level of chlorophyll in farm grown broccoli micro-greens, commercial samples had a much lower TCC. Several factors might contribute to this difference.
The commercial samples might have been harvested before the development of the cotyledon leaves, where chlorophyll accumulates. The plant does not require light for the first few days of growth. The chlorophyll might also be degraded due to long supply chain and storage time of the commercial samples, deteriorating the freshness of the vegetable. The TCC could affect sensory quality of the micro-greens, especially their appearance and the impression of freshness.Results in Fig. 2B showed that there was no significant difference in the TPC between commercial and local farm samples. Furthermore, whether it was soil grown or hydroponically grown did not affect the TPC of the samples from the local farm. Phenolic compounds are a large class of plant secondary metabolites that comprise an aromatic ring with one or more hydroxyl substituents. These compounds are important for the quality of plants and are strongly associated with the flavor properties of vegetables, such as taste and color. For instance, phenolic compounds, e.g., tannins and phenolic acids, contribute to the astringency taste, i.e., the drying, puckering, and shrinking sensation in the oral cavity. Some phenolic compounds are colorless, while some others show various colors. Phenolic compounds show numerous bio-active properties, hydroponic grow table most well-known of which are their antioxidant and anti-inflammatory activities. Furthermore, a number of phenolic compounds demonstrate inhibitory effect on the activity of enzymes to digest starch in gastrointestinal tract, e.g., amylase and glucosidase, indicating a potential of these compounds in improving glucose homeostasis and metabolic conditions. The average TPC in broccoli micro-greens samples in this study was in the range from 10.71 to 11.88 mg/g. It is about 10 times higher than the TPC in a lot of species of mature vegetables, including broccoli, brussels sprout, and kailan, which are known to be excellent sources of phenolic compounds. Therefore, these micro-greens can be regarded as great sources of this class of phytochemicals. Future research is needed to analyze the profiles of specific phenolic compounds in micro-greens as well as their in vivo bioavailability, and to explore their beneficial effects in human health. Our results indicated that different growing and harvesting conditions did not affect the TPC of the micro-greens. The TPC of plants depends on the balance between its synthesis and oxidation. Phenolic compounds are produced through the phenylpropanoid metabolic pathway, which starts with the reaction of l-phenylalanine converted to trans-cinnamic acid, and other phenolics are produced via subsequent reactions. As antioxidants, phenolic compounds can be oxidized to quinone under oxidative stress. It was previously reported that light exposure or different packaging methods did not affect the TPC of radish micro-greens. Future research on the effects of growth environment, harvesting condition, and/or post-harvest interventions on the synthetic or oxidation pathway of phenolic compounds in plants will provide insights into the key factors that determine the TPC level of micro-greens.
Results on VCC indicated that broccoli micro-greens samples grown in soil from the local farm possessed significantly higher VCC than hydroponically grown ones from the farm and the commercial ones also grown hydroponically. Vitamin C is a co-factor required by many enzymatic reactions and a critical antioxidant. Previous studies showed that higher circulating vitamin C concentrations are associated with lower risks of hypertension, coronary heart disease, and stroke. Mature raw broccoli, known as an excellent source of vitamin C, contains an average of 0.89 mg/g of total ascorbic acid according to the USDA National Nutrient Database for Standard Reference. Mature spinach, one of the most commonly consumed leaf vegetable in the United States, has about 0.28 mg/g of total ascorbic acid. The VCC of our micro-greens broccoli samples is in the range of 0.33–0.56 mg/g. Worth to note is that the VCC measured in this study is the content of free ascorbic acid so the total ascorbic acid content would be higher.Therefore,micro-greens broccoli, especially the soil-grown ones from the local farm, can be regarded as good sources of vitamin C. Comparing VCC in vegetables grown in soil and hydroponically has created mixed results. For instance, a study by Treftz et al. found significantly higher ascorbic acid content in soil grown raspberries yet lower in soil grown strawberries, compared with their hydroponically grown counterparts. Buchanan et al. reported significantly higher total ascorbic acid content in three of the four hydroponically grown lettuce varieties than their soil grown versions, while the fourth one showed higher but non-significantly total ascorbic acid content. Although not fully understood, vitamin C is synthesized in plants as a response to oxidative stresses and through the l-galactose pathway using mannose or galactose. Therefore, the growth conditions, including nutrition in soil and hydroponic growing media, and environmental stresses may all impact vitamin C biosynthesis in micro-greens and result in the VCC difference in this study.Similar to the results of TPC, no significant difference was observed in the TAC of micro-greens samples from different sources and grown conditions . It has been well established that the consumption of vegetables is inversely associated with morbidity and mortality from chronic diseases and antioxidants play a significant role in the beneficial effects.