Kyriacou, Marios (2021) Unravelling postharvest quality in microgreens through modulation of preharvest factors. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: Unravelling postharvest quality in microgreens through modulation of preharvest factors
Date: 5 July 2021
Number of Pages: 208
Institution: Università degli Studi di Napoli Federico II
Department: Agraria
Dottorato: Food science
Ciclo di dottorato: 33
Coordinatore del Corso di dottorato:
Rouphael, youssefUNSPECIFIED
Date: 5 July 2021
Number of Pages: 208
Keywords: antioxidant activity, ascorbate, carotenoids, flavonoids, hydroxycinnamic acids, macro-minerals, phenolic compounds, amaranth, blue-red light, cress, minerals, mizuna, Orbitrap LC-MS/MS, purslane, agave fiber, capillary mat, cellulose sponge, coriander, nitrate, pak choi, harvest maturity, isothiocyanates, ontogeny, volatile organic compounds, antinutritive agents, bioactive value, flavonol glycosides, hydroxycinnamic acids
Settori scientifico-disciplinari del MIUR: Area 07 - Scienze agrarie e veterinarie > AGR/15 - Scienze e tecnologie alimentari
Date Deposited: 20 Jul 2021 14:01
Last Modified: 07 Jun 2023 10:39

Collection description

The relatively brief growth cycle required for microgreens to reach harvest maturity renders genotype selection a key component for this expanding new industry. Important compositional differences were presently characterized across microgreens from 13 species and five botanical families. Nitrate hyper-accumulator microgreens were identified that warrants preharvest measures to suppress nitrate content. Across species, K was the most abundant macro-mineral, followed by Ca, P, Mg, S and Na. Genotypic differences in Na, K and S concentrations were wide while variation in P, Ca and Mg was narrower. Antioxidant capacity assayed in vitro was highest in brassicaceous microgreens. The levels of ascorbic acid present in microgreens were higher than corresponding levels in sprouts, plausibly owing to the presence of photosynthetic hexose precursors absent from sprouts. Genotypic variation in pigmentation was also expressed in terms of chlorophyll and carotenoid concentrations. Lamiaceae microgreens exhibited comparatively higher phenolic content, notwithstanding significant varietal differences. Moreover, alternative phenolics-rich species of microgreens, such as coriander from the Apiaceae were for the first time identified. Qualitative and quantitative determination of phenolic profiles demonstrated the predominance of flavonol glycosides, with the O-glycosides of kaempferol showing more species-related distribution. Principal Component Analysis revealed that the clustering of phenolic profiles reflected microgreens' botanical taxonomy with relative consistency. Such information is critical for selecting new species/ varieties of microgreens that satisfy demand for both taste and health. Further to genotype selection, the targeted modulation of microgreens secondary metabolism through select spectral bandwidths was assessed as a tool to produce phytochemically-enriched microgreens of high functional quality and nutritive value. Analytical data on microgreens' response to different light spectra constitutes a valuable resource for designing future crop-specific spectral management systems. Thus, variation in productivity, nutritive and functional quality of novel microgreens (amaranth, cress, mizuna, purslane) was examined in response to select spectral bandwidths (red, blue, blue-red). Growth parameters dependent on primary metabolism were found most favored by blue-red light's efficiency in activating the photosynthetic apparatus. Nitrate accumulation was higher under monochromatic light owing to the dependency of nitrite reductase on the light-driven activity of PSI, most efficiently promoted by blue-red light. Mineral composition was mostly genotype-dependent, however monochromatic red and blue lights increased K and Na and decreased Ca and Mg concentrations. Lutein, β-carotene, and lipophilic antioxidant capacity were generally increased by blue-red light putatively due to the coupling of heightened photosynthetic activity to increased demand for protection against oxidative stress. Finally, the general response to light treatments was a decrease in polyphenolic constituents, particularly flavonol glycosides, and total polyphenols under blue-red light. Notwithstanding that genotype specificity underlies some of the responses to light treatments summarized above, the current work highlights how selection of genetic background combined with effective light management can drive the production of microgreens with superior functional quality. The choice of growth substrate is critical for the production of high-quality microgreens. Therefore, understanding how the physicochemical properties of natural fiber (agave fiber, coconut fiber and peat moss) and synthetic substrates (capillary mat and cellulose sponge) impact the growth and yield attributes, the nutritive and phytochemical composition and the antioxidant potential of select microgreen species (coriander, kohlrabi and pak choi) wan imperative and novel next step in the present line of research. A key finding of this work, which advances our understanding of the current and future literature on microgreens production and potential bioactive value, is that substrates which combine optimal physicochemical properties, such as peat moss, tend to promote faster growth and higher fresh yields that favor high production turnover; however, this is achieved at the expense of reduced phytochemical content, foremost of polyphenols. Therefore, controlled stress applications (e.g., osmotic stress) on microgreens growing on such media warrants investigation as a means of enhancing phytochemical composition without substantial compromise in crop performance and production turnover. Substrates promoting fast growth (e.g., peat moss) also tend to promote nitrate accumulation in microgreens, especially in brassicaceous species that are known nitrate hyperaccumulators. Therefore, nitrate deprivation practices should be considered for microgreens grown on such substrates in order to minimize consumer exposure to nitrates. Although microgreens have become acclaimed as novel gastronomic ingredients that combine visual, kinesthetic and bioactive qualities, the definition of the optimal developmental stage for their harvesting remains fluid. The ontogenetic stages for harvesting microgreens range from the cotyledonary stage to the emergence of the second true leaf. Their superior phytochemical content against their mature counterparts fueled the subsequent work hypothesis that significant changes in their compositional profile likely take place during the brief interval of ontogeny from the appearance of the first (S1) to the second true leaf (S2). Elucidating this hypothesis will contribute towards the standardization of harvest maturity for the microgreens industry. Microgreens of four brassicaceous genotypes (Komatsuna, Mibuna, Mizuna and Pak Choi) thus grown under controlled conditions, harvested at S1 and S2. They were appraised for yield traits and subsequently examined for mineral, volatile organic compounds, polyphenols, ascorbate as well as hydrophilic and lipophilic pigment concentrations. Analysis of compositional profiles revealed genotype as the principal source of variation for all constituents. The absence of significant growth stage effect on many of the phenolic components identified is consistent with previous findings that post-germination differences in phenolic composition between S1 microgreens and baby leaves are minimal. The response of mineral and phytochemical composition and of antioxidant capacity to growth stage was also limited and largely genotype-dependent. It is, therefore, questionable whether delaying harvest from S1 to S2 would significantly improve the bioactive value of microgreens while the cost-benefit analysis for this decision must be genotype-specific. In terms of yield, the lower-yielding genotypes (Mizuna and Pak Choi) registered higher relative increase in fresh yield between S1 and S2, compared to the faster-growing and higher-yielding genotypes. Although the optimal harvest stage for specific genotypes must be determined considering the increase in yield against reduction in crop turnover, harvesting at S2 seems advisable for the lower-yielding genotypes. As reiterated above, microgreens constitute rudimentary leafy greens that impart gastronomic novelty and sensory delight, but are also packed with nutrients and phytochemicals. As such, they comprise an upcoming class of functional foods. However, apart from bioactive secondary metabolites, microgreens also accumulate antinutritive agents such as nitrate, especially under conducive protected cultivation conditions. As stated above, commercially favorable substrates such as peat moss promote fast growth but also tend to promote nitrate accumulation in microgreens, warranting nitrate deprivation practices in order to minimize consumer exposure to nitrates. In this perspective, nutrient deprivation before harvest (DBH) was examined as a plausible strategy, applied by replacing nutrient solution with osmotic water for six and twelve days, on different species (lettuce, mustard and rocket) of microgreens. DBH impact on major constituents of the secondary metabolome, mineral content, colorimetric and yield traits was appraised. Nutrient deprivation was found effective in reducing nitrate content, however effective treatment duration differed between species with decline being more precipitous in nitrate hyperaccumulating species such as rocket. DBH interacted with species for phenolic constituents. It increased the phenolic content of lettuce, decreased that of rocket and did not affect mustard. Further research to link changes in phenolic composition to the sensory and in vivo bioactive profile of microgreens might be warranted. However, it may be safely concluded that brief (≤ 6 days) DBH can be applied across species with moderate or no impact on the phenolic, carotenoid and mineral composition of microgreens. Such brief nutrient deprivation applications also have limited impact on microgreens' yield and colorimetric traits hence on the commercial value of the product. They can therefore be applied for reducing microgreen nitrate levels without significantly impacting key secondary metabolic constituents and their potential bioactive role. Through step-wise examination and appraisal of critical preharvest factors – ranging from genotype and substrate selection, to spectral management, ontogenetic stage at harvest and nutrient deprivation schemes – the current project contributes to the advancement of our understanding on the role and potential utility of these factors in configuring microgreens' yield, sensory, safety, nutritive and bioactive profile.


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