Daily cycles of light and darkness also promoted maintenance of glucosinolate content during post harvest storage of cabbage . Total glucosinolate content in the cabbage leaf disks stored under light/dark cycles remained stable with no significant fluctuation in levels over the 21 days of analysis . In comparison to total glucosinolate levels in light/dark-stored cabbage, the glucosinolate levels were significantly lower by 7 days when cabbage leaf disks were stored under constant darkness and by 21 days when stored under constant light . Total glucosinolate levels declined by 70% and 88%, respectively, in cabbage disks stored at 22°C under constant light or constant darkness. Remarkably, glucosinolate levels of the cabbage leaf disks stored at 4°C also showed a significant decrease by 21 days, with a loss of 50% of the initial glucosinolate levels , indicating that storage under cycle of light/darkness led to enhanced retention of this valuable phytochemical even relative to refrigeration.In this work, hydroponic bucket we examined whether kale, cabbage, lettuce and spinach leaf tissue maintain the ability to respond to light/dark cycles during post harvest storage and whether under these conditions that better mimic the natural light cycles of the environment tissue deterioration would be reduced.
Our goal was to expose plant tissues to diurnal conditions known to maintain the functioning of the circadian clock and thereby capitalize on physiological enhancements conferred by robust circadian rhythms. Plants grown under light/dark cycles that match the endogenous cycling of their internal circadian clock have a growth and reproductive advantage over plants exposed to light/dark cycles that do not match their internal oscillator . Furthermore, phasing of circadian rhythms so as to be synchronized with the external environment promotes biotic stress resistance. We found that storing green leafy vegetables in cycles of 12 hours of light followed by 12 hours of darkness improved several post harvest performance markers compared to post harvest storage of the leaf tissues under constant light or constant darkness. Similarly, a modest reduction in senescence was noted for post-harvest broccoli stored under natural light/dark cycles. Perhaps surprisingly, we found that storage in light/dark cycles resulted in several aspects of post harvest performance being comparable to storage under refrigeration, a commonly practiced method of post harvest storage thought to slow down cellular breakdown. The longevity of kale and lettuce leaf color, chlorophyll levels, and tissue integrity, which are important contributors to the appeal of green leafy vegetables to consumers, were largely indistinguishable whether the kale and lettuce leaf samples were stored at 22°C under light/dark cycles or were stored under refrigeration in constant darkness .
Spinach leaf samples also maintained green coloration and chlorophyll levels under light/dark cycles at 22°C as well as when refrigerated, but refrigeration was more successful at preventing spinach leaf tissue breakdown. Significant improvement of green coloration and chlorophyll content was seen when cabbage leaves were stored under light/dark cycles at 22°C compared to refrigeration, demonstrating that light may not only be important for clock entrainment but also can provide the additional benefit of promoting continued photosynthesis during post harvest storage. Promotion of photosynthesis and/or chlorophyll levels was previously observed in post-harvest crops stored under light. However, constant light during post harvest storage can also cause detrimental physiological activity, such as respiration leading to browning and transpiration contributing to weight loss. Therefore, cycling of light treatment with darkness periods may not only maintain clock function but may also avoid physiological damage that may occur in plant tissues under too much light. In addition to improvement of green leafy vegetable appearance by post harvest storage under light/dark cycles, we found that this post harvest storage treatment of plant crops may improve human health benefits through maintenance of phytochemical content .
Chlorophyll, responsible for the visual appeal of green leafy vegetables, also has beneficial impact upon human health upon ingestion. Chlorophyll can limit efficacy of carcinogens, such aflatoxin B1 and can activate Phase II detoxifying enzymes. Glucosinolates, sulfurcontaining compounds that play a major role in Brassicaceae plant herbivore defense, also underlie the human health benefits attributed to Brassicaceae vegetable consumption. For example, the glucosinolate glucoraphanin has potent anticancer activity. Previous studies have shown that glucosinolate levels can be maintained by refrigeration or exposure to radiation; here we find that post-harvest storage under light/dark cycles can also lead to sustained glucosinolate levels .Fruits are a good source of compounds with phenolic functionality including phenols, lignins, lignans, coumarins, tannins, phenolic acids, and Žavonoids which are important in the human diet. ese phenolic secondary metabolites comprise of an aromatic benzene ring with one or more hydroxyl groups that can exist as a simple monomeric phenolics or a complex polymerized polyphenolic molecules. ese molecules generally play a critical role in plant defense mechanisms including pathogen or insect attack, ultraviolet light, and mechanical damage in plants. ey also play an important role in human health, protecting against damage induced by reactive oxygen or nitrogen species. Phenolics are classified based upon their carbon skeleton and in accordance with the number of phenol units present. Phenolics arise from either phenylalanine or tyrosine amino acids that are deaminated into cinnamic acids and are primarily derived from the phenylpropanoids. Simple phenolic acids are divided into two groups: hydroxybenzoic acids and hydroxycinnamic acids. ese molecules play a vital function to protect the organism against to biotic or abiotic stress factors. Most phenolic compounds are highly reactive, are therefore not stabile, and can degrade into a wide variety of products. In the last decade, many epidemiological studies indicate a direct relationship between consumption of fruits and the prevention of diseases such as cancer and those related to inŽammation and oxidative stress. Monomeric phenolic compounds can polymerize to form more complex phenolic compounds. ese include Žavonoids, tannins, and lignin. Complex phenolics have a range of biological activities and functional characteristics and are of great interest to the food industries, cosmetic industry, and allied health fields.Flavonoids are complex phenolics that can be classified into six subclasses that include flavonols , flavanones , flavones , isoflavones , anthocyanins , and flavanols and are present principally as glycosylated, esterified, and polymerized derivative forms in fruits. Flavonols play an important role as antioxidants; for example, they protect ascorbic acid from autoxidation in juices and which can lead to juice discoloration. Although flavonoids are abundant in fruit, and fruits or beverages can be a significant source of dietary flavonoids, levels will vary depending on the varieties, environmental conditions, soil, and climatic factors. Berries are a good source of quercetin and its derivatives , stackable planters whereas the most abundant dietary flavanone glycoside is hesperetin-7-O-rutinoside present in citrus fruits. Peterson et al. reported that the most prevalent dietary flavanone aglycones are naringenin, hesperetin, isosakuranetin, and eriodictyol. ,e same authors demonstrate that a citrus fruit is also a primary source of narirutin, eriocitrin, didymin, neohesperidin, naringin, hesperidin, neoeriocitrin, and poncirin. ,e ratio of these compounds to each other can vary. For example, narirutin and naringin were detected in grapefruit in high ratios, while the levels of hesperidin and narirutin in oranges and eriocitrin in lemons were even higher. In addition, some flavanone glycosides such as 7-rutinoside are tasteless, in contrast to neohesperidin , naringin, and hesperetin which have an intense bitter taste isolated from bitter oranges and grapefruit. Apigenin is another key flavone found in fruits, vegetables, spices, and herbs and is abundant in grapefruit, beverages, some vegetables, and herbal plants such as chamomile.
Isoflavones are present in plants in the glycosylated forms but are converted to aglycone forms through the action of intestinal microflora. Isoflavones are detected commonly in legumes such as green beans, fava, and soybeans, and among them, genistein -4H-1-benzopyran-4- one and daidzein -4H-1- benzopyran-4-one are the two major forms of dietary isoflavones and are consumed in soy products. Due to the structural similarities to human hormone estrogen, isoflavones have potent estrogenic properties. Anthocyanins are another important class of flanovids that are colorful water-soluble glycosides and acylglycosides of anthocyanidins. 3-O-glycosides or 3,5-di-O-glycosides of malvidin, delphinidin, pelargonidin, cyanidin, petunidin, and peonidin are known as the most common natural anthocyanins and are classified based on the number and position of hydroxyl and methoxy groups. Anthocyanins are responsible for the brilliant colors of various plant parts including flowers and leaves and especially fruits having red, blue, purple colors, particularly strawberries, blueberries, black currants, cherries, raspberries, and red and purple grapes. Anthocyanidins are also responsible for the color of red wines. ,eir color based upon the degree of methylation and with pH is discrete from other phenolics by the range of colors each forms . Color differences of anthocyanins depend on the substitutions of the B ring, the pattern of glycosylation, and the degree and nature of esterification of the sugars with aliphatic or aromatic acids, and also on the pH, temperature, type of solvent, and the presence of copigments. Berries are a good source of anthocyanins, and 100 g of berries can provide up to 500 mg of anthocyanins. Flavan-3-ols are the core structure of condensed tannins and are the most complex subclass of flavonoids. Flavan-3-ols have been previously reported as an antioxidant, chemopreventive, and immunoregulation agents. Procyanidins exist in a wide range of foods and often exist in foods in a range of galloylated forms. Most widely used techniques for phenolics are HPLC , LC/MS, GC , GC/MS, UV-Vis spectrophotometry, mass spectroscopy, electrochemical, and fluorometric methods. Liquid chromatography mass spectrometry is used to determine phenolics in both APCI and ESI techniques, ABTS+ and DPPH.Sample preparation and extraction methods varied widely based on the nature of the sample matrix of the fruit or vegetable and based on the chemical structures of the phenolic compounds being extracted. As most samples contain a mixture of simple and complex polyphenolic compounds, such as phenolic acids, flavonoids, anthocyanins, and proanthocyanins, it is critical to choose a suitable method for sample preparation and extraction. Proteins, carbohydrates, lipids, or other elements may play negative effect to extraction of phenolics. In addition, it is not always possible to extract fresh samples, and special preparation techniques such as lyophilization, nitrogen pulverization, or drying may be needed. Particle size of extracted material and solvent-to-solute ratios need to be considered. As seen in Figure 1, there are many reliable qualitative and quantitative methods available for the measurement and characterization of the phenolic content in different natural products. Moreover, the success of these techniques will depend on the most effective sample preparation and extraction methods. Extraction efficiency is greatly influenced by solvent choice and composition and plays a critical role in the extraction yield of phenolics from fruits and vegetables. Generally, for the extraction of phenolics, water, acetone, ethyl acetate, alcohols , and their various percentages of mixtures are used. In addition to the solvent type extraction conditions, parameters such as temperature and duration also influence the yield of phenolics.Khoddami et al. previously reported that recovery of phenolics varied from one sample to another sample. It is also reported that acid- or base-catalyzed hydrolysis is also an important consideration for the stability of the phenolics in extracts. Davidov-Pardo and Marn-Arroyo reported that the extraction pH plays an important role in the extraction efficiency of phenolic compounds, and the same authors implied that catechins and their isomers are detected more efficiently in alkaline conditions as compared with acidic ones. Extraction of phenolic compounds are commonly done using either liquid-liquid or solid-liquid extraction technique. However, liquid-liquid extraction has some disadvantages because of using costly and potentially toxic solvents. For this reason, improved extraction methods such as solid-phase microextraction and solid-phase extraction techniques are used to extract phenolics from liquid samples. In general, inexpensive and simple methods such as soxhlet, reflux, and maceration processes are the more conventional procedures used to recover phenolics from solid samples. In addition, ultrasound-assisted extraction , microwave-assisted extraction , ultrasound microwave-assisted extraction , supercritical fluid extraction , subcritical water extraction , and high hydrostatic pressure processing are the methods that help us to shorten extraction times and decrease the release of toxic pollutants through reducing organic solvent consumption and are relatively simple to perform.