Asparagus officinalis L. , also known as sparrow grass, is a perennial dioecious plant with branched stem belonging to the Asparagaceae family. This plant is cultivated for the shoots coming out and consumed in spring. These are full in nutrients and contain sugars, zinc, magnesium, calcium, phosphorous, potassium, iron, selenium, chromium, vitamins A, B, K and C, flavonoids such as rutin, proteins, dietary fibers, beta carotene, some nitrogenous substances including the asparagine, asparagusic acid, and saponins among other compounds. The plant should not be consumed in case of nephritis, due to the presence of sulfur-containing compounds (such as asparagusic acid), and the subsequent presence of mercaptans in urine.
Asparagus is considered the “kind of the vegetables” due to their potent
Among the Asparagus active compounds, saponins are particularly relevant for their wide field of application and will be discussed in a separate section in order to fully understand their potential in the pharmaceutical, cosmetic and agri-food industries. Other important Asparagus responsible are flavonoids. These compounds are responsible for the antioxidant activity of the plant.
Saponins and their biological effect
Asparagus saponins are constituted by a steroidal aglycone and one or more sugar units. The sugar moiety is linked to the aglycone by a covalent bond at one or two glycosylation sites. Saponins are surfactants meaning that, thanks to their molecular composition they are able to decrease the surface tension between two liquids, a liquid and a gas or a liquid and a solid. Surfactants can be employed as emulsifiers, foaming agents, wetting agents and dispersants.
The kind of saponins present in asparagus species are directly responsible for the biological activity but are also responsible for the flavor. [1] The growing interest about asparagus-derived saponins is due to the array of biological effects that they can exert like immunity enhancement, antitumor, anti-inflammatory, antifungal, and antiviral actions, blood glucose and lipid reduction, antioxidation, cardiovascular function improvement etc.
Among these we will explain the saponin action for their:
- Hypocholesterolemic action
- Cytotoxic and antitumor action
- Antifungal action
Hypocholesterolemic action
Asparagus derived saponins are capable of reducing cholesterol levels in blood. Steroidal saponins can improve the lipid profile by decreasing the total level of cholesterol, low-density lipoproteins (LDL) and triglycerides.
Cytotoxic and antitumor action
The cytotoxic capacity of saponins is dependent on their molecular structure. Steroidal saponins have cytotoxic effect on some tumors and the steroidal saponins like the one found in certain asparagus species such as the “triguero”, once in contact with cancer cells is capable of blocking signaling pathways, arrest the cell cycle and induce cell death through apoptosis. [1]
Antifungal action
Steroidal saponins have been studied for their antifungal action. Nevertheless it seems that furostanol derivatives are inactive while spirostanol derivatives are active. The total saponin fraction extracted from Asparagus officinalis L. has activity against various kinds of fungi such as Microsporum, Trichophyton, Candida, Cryptococcus, Epidermophyton. The mechanism of antifungal action is not yet well understood but it might be due to loss of membrane integrity.
Potential pest and pathogen control using saponins
The significant abundance of saponins in asparagus processing and cultivation by-products makes these compounds particularly interesting in the context of circular economy.
The observed effects of saponins on insect pests are reduced food intake, indigestion, weight reduction, developmental retardation, decrease in the rate of reproduction, and mortality. [2] [3]
For this reason the inclusion of asparagus-derived saponins in the development of a new kind of phytosanitary product has the potential of showing great activity against pests, improvement of bio-resistance of crops and the possible bio-stimulant activity. [4]
In particular the saponin fraction and other asparagus-derived extraction products such as flavonoids could face the main diseases of asparagus plants, regardless if caused by fungi or pests including the Fusarium stem and crown rot caused by Fusarium moniliforme, Fusarium wilt and root rot caused by Fusarium oxysporum, the purple spot, Stemphylium vesicarium, the gray mold shoot blight, Botrytis cinerea, the Cercospora blight fungus, and the asparagus rust caused by Puccinia asparagi, also the main insect pests such as the asparagus beetle, Crioceris asparagi, the spotted asparagus beetle, Crioceris duodecimpunctata, the asparagus fly, Platyparea poeciloptera, the asparagus aphid, Brachycolus asparagus, some cutworm species and other asparagus pests. [5]
Extraction methods
Extraction is a critical step in order to recover the bioactive compounds contained in asparagus plants and many factors can influence the solid-liquid extraction including:
- Kind of solvent used
- Extraction temperature
- pH
- solid to liquid ratio
- particle size
- Extraction time
The extraction yield is highly affected by the solvent composition. Some study reports that in order to maximize the concentration of flavonoids responsible for the antioxidant activity of Asparagus extracts, 50% ethanol as a solvent with liquid-solid ratio of 30:1 appears to be the best option. [6] Among the compounds responsible for the antioxidant activity there are: ferulic acid, quercetin, rutin, isorhamnetin and kaempferol. A recent research reports the comparison of different extraction methods to maximize the final flavonoid content and different solvents such as ethanol, methanol and water are compared, as well as the procedure following single-step extraction at room temperature and at 50 °C as well as three-step extraction (one h for each step) at 50 °C. [7]
Ethanol and methanol are so far the most widely used solvents for Asparagus extraction and new methods including ultrasonic assisted extraction (UAE) probes showed great results in terms of final yields, improving extraction time and efficiency. [8] However ethanol gives best results compared to methanol in terms of extraction yield and final antioxidant activity.
Supercritical fluid extraction (SFE) using carbon dioxide (CO2) as solvent is among the newest extraction methods used in order to maximize the therapeutic potential of Asparagus extracts.
Future perspectives
There is still a lack of new sustainable, green and scalable extraction process and purification methods in order to isolate saponins along with other bioactive compounds from asparagus processing and cultivation byproducts. More studies should be done on extraction technologies in order to open the possibilities of including saponins in other food matrices to improve the shelf life, reducing the risk of mold formation and improving soup or juices foaming capacity. Moreover, if advanced and efficient extraction methods will be developed it will be possible the large scale isolation and standardization of plant-derived saponins for human consumption and pharmaceutical applications in order to exploit their potential therapeutic effect in decreasing blood lipids, lowering cancer risks and lowering blood glucose response.
References:
[1] https://digital.csic.es/bitstream/10261/176566/1/NFSIJ_2018_V7_555704.pdf
[2] https://doi.org/10.1016/j.lwt.2017.08.077
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8879102/
[4] https://doi.org/10.3390/separations9070163
[5] https://doi.org/10.17660/ActaHortic.2008.776.15
[6] https://pubmed.ncbi.nlm.nih.gov/25892766/
[7] https://www.sciencedirect.com/science/article/abs/pii/S0308814623006325
[8] https://www.sciencedirect.com/science/article/abs/pii/S0308814619304650
