Subcritical Water Research Articles

Phytochemical Screening, Antimicrobial and In-vitro Cytotoxic Activities of Several Extracts of Six Medicinal Plants (Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens) from Central Africa Region

The qualitative study of secondary metabolites of the root and stem bark of Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens obtained by maceration were estimated by tub reaction. Antimicrobial and cytotoxic activities of the root and stem bark of the same plants obtained by maceration and subcritical water extraction were respectively tested whit antibiogram disc test on Pseudomonas Aeruginosa (ATCC 27853), Enterobacter Hormaechei (ATCC700323), Bacillus Spizizenii (ATCC6633), Enterococcus Casseliflavus (ATCC700327), Staphylococcus aureus subsp. Rosenbach (ATCC 6538), and Salmonella Typhus (ATCC 6539). The Tetrazolium test (MTT test) was realized by using the mouse fibroblast cells (NIH3T3). Extracts based on the bark of Terminalia glaucescens and Crossopteryx febrifuga Benth induce sensitivity to 4/6 bacterial strains, in particular Enterobacter Hormaechei (ATCC700323): 3±2cm; Bacillus Spizizenii (ATCC6633): 5±2 cm; Staphylococcus aureus subsp. Rosenbach (ATCC 6538): 5±2; and Salmonella Typhus (ATCC 6539): 3±2cm. Extracts based on the bark of Khaya anthotheca and those of the bark of Gardenia ternifolia induce only the sensitivity on 2/6 bacterial strains specially Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633) respectively 4±2cm and 5±2cm. Extracts based on the barks and roots of Khaya anthotheca and those of the roots and barks of Gardenia ternifolia only induce sensitivity on 2/6 bacterial strains tested, in particular the strain of Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633). Only extracts based on the bark of Crossopteryx febrifuga Benth and Terminalia glaucescens show sensitivity against Salmonella Typhus (ATCC 6539). All the plant extracts tested show very low cytotoxic power. Plant extracts obtained by the maceration method (Hydro Methanol 90%) induce 19% inhibition of cell growth compared to extracts obtained by supercritical water 125°C 60ATM and 125°C80 ATM. The extracts obtained using the supercritical water 125°C 60ATM method are the least offensive on cell proliferation. The most concentrated extracts show the greatest inhibitions. The extracts of plant E show the greatest inhibition at a concentration of 1 g/ml.

Subcritical water conversion of biomass to biofuels, chemicals and materials: a review

Subcritical water conversion of biomass to biofuels, chemicals and materials: a review

Hydrothermal hydrolysis of triglycerides: Tunable and intensified production of diglycerides, monoglycerides, and fatty acids

This study focuses on utilizing subcritical and supercritical water technology to produce intensive and selective diglycerides, monoglycerides, and fatty acids. The water properties can be easily modified by adjusting pressure and temperature, which promotes an improving solubility of the triglycerides in water and a selective production of glycerides. Triacetin and sunflower oil were used as models for short and long-chain components, exploring a range of temperatures (250 to 400 °C), pressures (22.0 to 25.0 MPa), and reaction times (0.7 to 29 s). A conversion of 98.5 % of sunflower oil is reached at 375 °C and 25.0 MPa in 29 s. The resulting products are 16.3 % diglycerides, 27.9 % monoglycerides, 48.9 % fatty acids, and 5.3 % glycerol. On the other hand, only 13 % of the oil is hydrolyzed at 300 °C, 22.0 MPa, with a reaction time of 23.6 s. The main product at this condition was fatty acids with a yield of 3.7 %. Furthermore, a kinetic model was developed to obtain comprehensive kinetic parameters for both raw materials to evaluate their reactivity during hydrolysis. This study provides valuable insights into the hydrolysis kinetics and the most promising conditions to make valuable products like fatty acids, mono and diglycerides, and glycerol.

Differential Extraction and Preliminary Identification of Polyphenols from Ugni candollei (White Murta) Berries.

Ugni candollei, commonly known as white murta, is a native Chilean berry with a polyphenol composition that has been underexplored. This study aimed to establish a comprehensive profile of white murta polyphenols using ultra-performance liquid chromatography electrospray ionization Orbitrap mass spectrometry (UPLC-ESI-ORBITRAP MS). Additionally, it compared the efficacy of conventional extraction methods with emerging techniques such as deep eutectic solvent (DES) extraction and hot pressurized water extraction (HPWE). The analysis tentatively identified 107 phenolic compounds (84 of them reported for the first time for this cultivar), including 25 phenolic acids, 37 anthocyanins, and 45 flavonoids. Among the prominent and previously unreported polyphenols are ellagic acid acetyl-xyloside, 3-p-coumaroylquinic acid, cyanidin 3-O-(6'-caffeoyl-glucoside, and phloretin 2'-O-xylosyl-glucoside. The study found HPWE to be a promising alternative to traditional extraction of hydroxybenzoic acids, while DES extraction was less effective across all categories. The findings reveal that white murta possesses diverse phenolic compounds, potentially linked to various biological activities.

Optimization of Ultrasound-enhanced Subcritical Water Hydrolysis of Oil Palm Empty Fruit Bunch for the Production of Fermentable Sugar

To enhance the hydrolysis to produce fermentable sugar, oil palm empty fruit bunch (OPEFB) has undergone ultrasonication pretreatment prior to subjection to subcritical water hydrolysis. This work aims to optimize the effect of temperature, reaction time, and the concentration of sodium dodecyl sulfate (SDS) as the surfactant, with the primary aim of maximizing sugar production in the subcritical water hydrolysis process applied to oil palm empty fruit bunch (OPEFB). The pretreatment process conditions were optimized using response surface methodology of the central composite design (RSM–CCD). The experimental design includes three factors and levels, with a range of 180–220 °C temperature (X1), 60–80 minutes process time (X2), and 1–5% w/w SDS concentration (X3), an α value of 1.68, and reducing sugar concentration (g/L) as response (Y1). The optimum condition for subcritical water hydrolysis of OPEFB was obtained at 208 °C, 78 minutes, and 2.6% w/w SDS concentration with an expected yield of 6.09 g/L. As a result, reducing sugar produced by enzymatic hydrolysis increased by 324.7% compared to raw OPEFB, with sugar yield of 45.64% after 36 hours. Along these, changes in crystallinity, chemical composition, lignocellulosic functional groups, and morphology were analyzed to determine the impact of the pre-treatment on OPEFB.

Turbulent kinetic energy budget of sediment-laden open-channel flows: bedload-induced wall-roughness similarity

New experiments in highly turbulent, steady, subcritical and uniform water open-channel flows have been carried out to measure the mean turbulent kinetic energy (TKE) budget of sediment-laden boundary layer flows with two sizes (dp = 3 mm and 1 mm) of Plexiglas particles $({\rm relative\ density}\ = 1.192)$ . The experiments covered energetic sediment transport conditions (Shields number of $0.35 < \theta < 1.2$ ) ranging from non-capacity to full-capacity flows in bedload-to-suspension-dominated transport modes (suspension number of $0.5 < {w_s}/{u_\ast } < 1.3$ where ${w_s}$ is the settling velocity and $u_*$ is the friction velocity) and for weakly to highly inertial, finite size turbulence-particle conditions (Stokes number of 0.1 < St < 3.5 and dp/η > 10 where $\eta$ is the Kolmogorov length scale). It was shown that the effects of sediments on the TKE budget are very pronounced in all large particle experiments for which a bedload layer of several grain diameter thickness is developed above the channel bed. When compared with the corresponding reference clear-water flows, the TKE shear-production rate for the 3 mm particle flows is strongly reduced in the wall region corresponding to the bedload layer. This turbulence damping is seen to increase with sediment load until full capacity for flows with constant Shields value, as well as with Shields number value. Inside this damped TKE shear-production zone, a distinct peak of maximal turbulence production appears to coincide with the upper edge of the bedload layer delimited by a sharp gradient in mean sediment concentration. This vertically upshifted peak of TKE production is accompanied by an enhanced net downward oriented TKE flux when compared with the reference clear-water flows. The downward diffused TKE is found to act in the bedload layer as a local energy source in reasonable balance with the sediment transport term. The mechanism behind this downward TKE transport was further analysed on the basis of coherent flow structure dynamics controlled by ejection- and sweep-type events. The agreement between the height of downward directed mean TKE flux and the height below which sweep-type events dominate the Reynolds shear-stress contribution over ejections, revealed the leading role played by sweeps in mean TKE transport. This agreement holds for all reference clear-water flows supporting the well-known wall-roughness-induced dominance of the sweep contribution in turbulent, rough clear-water boundary layer flows. Furthermore, for all 3 mm particle flows, the two referred to transition levels were significantly and similarly upshifted to the upper edge of the bedload layer. Only for these sediment-laden flows, the bedload layer thickness is seen to exceed the wall-roughness sublayer of the reference clear-water flows. This supports a strong analogy between wall-roughness effects in clear-water flows and bedload layer effects in sediment-laden flows, on the mean TKE budget induced by a similarly modified coherent flow structure dynamics. The bedload layer-controlled wall roughness is finally confirmed by the good prediction of the wall-roughness parameter ks of the logarithmic velocity distribution. An empirical formulation fitting the presented measurements is presented, valid over the range of Shields number values covered herein.

Response surface methodology of subcritical water-reactive extraction of gingerol to shogaol ginger dregs through ultraviolet light pretreatment

The increase of shogaol production through reactive extraction of subcritical water depends on the stages of the pretreatment process in destroying the cellular cells of ginger pulp. Pretreatment using ultraviolet B (UVB) is predicted to be able to damage cellular cell walls and outer cell membranes (plasmalemma). In addition, to initiate the degradation of gingerols to shogaols, the reaction conversion and selectivity of shogaols are increased. This study aimed to examine the optimum process conditions for the subcritical water-reactive extraction of gingerol to shogaol from the ginger pulp by utilizing UVB pretreatment: reactive extraction temperature (110 – 130oC), ratio solvent/feed (22:1 – 32:1 b/b), and time (10 – 30 minutes). The extract of shogaol in solvent was separated from the raffinate with a separator funnel and analysed using HPLC-MS. Interpretation of experimental data to optimize the reactive extraction process used the Response Surface Methodology (RSM). The study results show that at 120°C, solvent:inlet ratio 27:1 for 20 minutes through ultraviolet B-light pretreatment for 10 minutes, the yield of shogaol increased from 177 µg/L (without UV) to 18646 µg/L. Moreover, the RSM model was very good at predicting shogaol acquisition responses with a determination coefficient of R2 to be 0.8554. The optimum condition for reactive extraction through UV-B light pretreatment was reached at a temperature of 121.46°C and an inlet solvent ratio of 27.96 at 18.36 minutes, with a shogaol concentration of 18567.1 µg /L.

Evaluation of the potential of activated sludge biomass from Nigeria as a feedstock for biodiesel production

AbstractDepleting fossil fuel reserves and the effects of greenhouse gases on climate change have led to the investigation of alternative energy sources such as biofuel, including biodiesel and renewable diesel. Unfortunately, the cost of the feedstock exceeds 70% of the total production cost. The current work investigates the potential of utilizing a waste product (activated sludge biomass) from wastewater treatment, as an alternative feedstock for biodiesel production. The activated sludge biomass was pretreated with subcritical water to enhance the lipid yield before extracting the lipid using solvent extraction. The result showed that lipid yield from the activated sludge was 4.73–8.58%. After subcritical water pretreatment with varying residence time, temperature, and biomass loading, the lipid yield increased by 158.04–400%. Gas chromatography–mass spectrometry (GC–MS) analysis of the fatty acid methyl esters (FAME) from the transesterification of the lipid extract using calcined egg shell as a catalyst identified the following lipids: triglyceride, fatty acids, sphingolipid, phospholipid, glycerolipid, glycerophospholipid, steroid, and cholesterol. The predominant lipid was triglyceride, and the high percentage of unsaturated fatty acids, including arachidonic acid (23.54%), eicosadienoic acid (8.37%), and oleic acid (6.23%) suggest that the lipid extract is a potential feedstock for biodiesel production.

Potential of Subcritical Water Hydrolysis to Valorize Low-Valued Ray-Finned Fish (Labeobarbus nedgia): Effects of Hydrolysis Temperature and Pressurization Agent.

Subcritical water (SCW) hydrolysis was applied to valorize the low-valued ray-finned fish (Labeobarbus nedgia) into valuable protein hydrolysates, employing N2 and CO2 as pressurization agents at varying temperatures (140, 160, 180, and 200 °C). The degree of hydrolysis (DH) and total free amino acid content increased with temperature for both pressurizing agents. The highest DH (54.5 ± 0.4%) and total free amino acid content (210 ± 1 mg/gprot) were observed at 200 °C when CO2 gas was used as the pressurizing agent. Predominantly, glycine and alanine were released for both pressurizing agents. The antioxidant activity, evaluated through three different assays, increased with temperature and was found to be the highest at 200 °C. This study illustrated the advantages of the intensified SCW technology by using CO2 as a pressurization agent in valorizing low-valued ray-finned fish (Labeobarbus nedgia), as animal residue rich in proteins, for the production of valuable protein hydrolysates with a high fraction of valuable free amino acids, which could offer potential applications as a functional ingredient in the food industry.

Efficient Mineralization of Lithium Bis(pentafluoroethanesulfonyl)imide and Related Electrolyte Fluorochemicals Using Superheated Water.

Lithium bis(pentafluoroethanesulfonyl)imide, Li[N(SO2C2F5)2], a typical fluorochemical aimed at better electrochemical performance of battery electrolytes, in superheated water was studied for its waste treatment. When Li[N(SO2C2F5)2] was reacted in pure superheated water at 300 °C, little F- ions were produced. In contrast, complete mineralization of the fluorine, sulfur, and nitrogen atoms in Li[N(SO2C2F5)2] was achieved when the reaction was performed in the presence of KMnO4. Specifically, when Li[N(SO2C2F5)2] was treated for 18 h with 158 mM of KMnO4, the F- and SO42- yields were 101 and 99%, respectively, and the sum of the NO3- and NO2- yields was 101%. In the gas phase, trace CO2 was detected and no CHF3, which has high global warming potential, was formed. Furthermore, the fluorine, sulfur, and nitrogen atoms in the analogues K[N(SO2C4F9)2] and K[N(SO2CF2)2CF2] also underwent complete mineralization using the same approach.

Adsorption study on the formation of interfacial layers based on birch glucuronoxylans

Glucuronoxylans (GX), particularly crude fractions obtained by pressurized hot water extraction of birch wood, act as potent emulsifiers and stabilizers against physical separation and lipid oxidation. Herein, we studied the adsorption of GX on hydrophobic interfaces to correlate their multicomponent character towards the formation of interfacial layers in emulsions. Dynamic interfacial tension (DIFT) and quartz crystal microgravimetry with dissipation monitoring (QCM-D) were applied to various GX fractions and the results compared with those from cellulose-based emulsifiers. The roles of residual lignin and polysaccharides are discussed considering the formation of interfacial layers during emulsification. The DIFT of the different GXs reached quasi-equilibrium faster as the lignin concentration increased, implying a correlation between the rate of adsorption and the residual lignin content. The effect of NaCl addition was more pronounced in polysaccharide-rich fractions, indicating that the polysaccharide fraction modulated the effect of ionic strength. QCM-D showed that despite the fast adsorption exhibited by the lignin-rich GX extract in the DIFT curves, the adsorbed materials were lightweight, suggesting that the polysaccharide fraction built the bulk of the interfacial layer. These results provide a foundation towards understanding the role of GX in interfacial stabilization beyond traditional plant-based counterparts.

Catalytic Conversion of Oil Shale over Fe or Ni Catalysts under Sub-Critical Water

Sub-critical water is an environment-friendly solvent. It is widely used for the extraction of various organic compounds. It can be used to dissolve and transport organic matter in oil shale. In this study, the conversion of oil shale was synergistically catalyzed by the addition of Fe or Ni to the Fe inherent in samples under sub-critical water conditions. Oil shale can be converted to gas, oil and residues of oil. Thermogravimetric (TG) analysis results presented that the weight loss of raw oil shale was up to 15.85%. After sub-critical water extraction, the weight loss rate of the residues was reduced to 8.41%. With the application of a metal catalyst, Fe or Ni, the weight loss of residues was further reduced to 7.43% and 6.57%, respectively. According to DTG curves, it was found that there were two weight-loss rate peaks. The decomposition process of kerogen in oil shale could be divided into two cracking processes. One is decomposed at a high velocity at around 420 °C, and another is decomposed at a low velocity at around 515 °C. Gas chromatography (GC) results of gas products indicated that Fe or Ni could contribute to producing normal alkanes, such as methane, ethane, propane, etc., which are produced by the hydrogenation of alkenes via hydrogen transfer during the conversion process of kerogen. Gas chromatography-mass spectrometry (GC–MS) was conducted to analyze the components of the liquid products. The results showed that n-alkanes, iso-alkane, oxygenated hydrocarbons and aromatic compounds were the major components of the kerogen cracking products. When Ni was introduced as a catalyst, the contents of aromatic compounds and oxygenated hydrocarbons in the liquid products were increased from 19.55% and 6.87% to 22.38% and 13.77%, respectively. This is due to the synergistic effect of the addition of Ni with the inherent Fe in oil shale under sub-critical water which ensures kerogen is more easily cracked to produce aromatic compounds and oxygenated hydrocarbons.

Investigation for flavanone synthesis in subcritical water with La-ZrO2, Mg-ZrO2, and Ca-ZrO2 as solid base catalysts

In this study, flavanone was synthesized via a two-step process, Claisen–Schmidt condensation between 2’-hydroxyactophenone and benzaldehyde followed by isomerization, in subcritical water with ZrO2 based catalysts. Kinetic analysis was conducted to quantitatively show the catalytic activity of different modified ZrO2. The activity of ZrO2 was improved upon incorporating with active metal species. Compared with La-ZrO2 and Mg-ZrO2, Ca-ZrO2 showed the highest activity attributed to strong basicity, but substantial formation of side product and loss of product were observed. La-ZrO2 showed weaker activity than Mg-ZrO2, though La-ZrO2 had higher basicity. One explanation was the dissociation of water to provide OH− on the catalyst surface was more challenging since containing significantly more active sites with strong water affinity. Moreover, such active sites contributed less to reaction due to dense water surrounding the catalyst surface. The reaction catalyzed by Mg-ZrO2 reached flavanone selectivity of 72.2 % in 3 hours at 150 °C.

Obtaining Extracts and Hydrolysates from Cambuci Peel Through Subcritical Water: An In-line Detection Approach

Obtaining Extracts and Hydrolysates from Cambuci Peel Through Subcritical Water: An In-line Detection Approach

Industrial byproduct pine nut skin factorial design optimization for production of subcritical water extracts rich in pectic polysaccharides, xyloglucans, and phenolic compounds by microwave extraction

Pine nut skin (Pinus pinea L.) is a poorly explored industrial byproduct with potential to be utilized as a food ingredient. Subcritical water extraction (SWE), an eco-friendly extraction technique with higher efficiency than hot-water extraction (HWE), was studied to evaluate its suitability in producing extracts rich in soluble fiber and phenolic compounds. For this, a factorial design was developed considering temperature (120–180 °C), time (2–10 min), and mass/volume ratio (1–3 g/60 mL) under microwave irradiation. This design aimed to maximize the extraction of carbohydrates, while achieving the highest content of phenolic compounds and antioxidant activity. SWE produced higher yields (2.6-fold in relation to HWE) of extracts rich in polysaccharides, determined by methylation analysis, oligosaccharides, determined by GC-qMS as alditol acetates, and phenolic compounds, determined by HPLC-ESI-MS. SWE increased the recovery of pectic oligosaccharides (10-fold) and xyloglucans (2-fold), and allowed to recover pectic polysaccharides, type II arabinogalactans and insoluble-bound phenolic compounds. Mono-, oligo- and polysaccharides were not hydrolyzed during in vitro gastrointestinal digestion, showing potential prebiotic functionality. Although phenolic compounds suffered a 23 % (gallic acid equivalents) decrease, phenolic acids and aldehydes were released or conserved upon intestinal phase. These results highlight the potential of PNS valorization as functional food ingredients through the subcritical water solubilization of polysaccharides and phenolic compounds.

Features of heat/mass transfer and explosive water boiling at the laser fiber tip

Experimentally, as well as using numerical simulation methods, the features of the processes of heat and mass transfer and explosive boiling of water, initiated by continuous laser radiation with λ = 1.94 μm, emerging from the tip of the laser fiber, are studied. The trajectories of the microvolume with the maximum temperature of the superheated liquid under laser heating, in which the probability of explosive boiling up is maximum, are determined. The calculations took into account changes in the density of superheated water and the dependence of the absorption coefficient of laser radiation on temperature. Using acoustic methods and high-speed video recording, it is shown that during explosive boiling up, the duration of the leading front of the acoustic signal is ∼300 ns. Such a long duration of pressure rise at the receiving point is explained by the fact that explosive boiling occurs in a wide area of superheated liquid near fiber tip, when the nuclei located in it are stimulated by an initially arising pressure pulse.

Effects of synergistic action on rheological and thermal properties of potato starch complexes co-gelatinized with caffeic acid and squash polysaccharides extracted with water and subcritical water

In order to broaden the application range of squash polysaccharide (WESP/SWESP) and caffeic acid (CAA) and improve the quality of potato starch (PS) products, the effects of WESP/SWESP and CAA on the gelatinization, rheology, thermodynamics, microstructure and in vitro digestion of PS were investigated. Meanwhile, the synergistic effect of WESP/SWESP and CAA on PS was further analyzed. Differently, due to WESP and SWESP had different monosaccharide composition and structure, they had different effects on the system. Pasting properties results showed that the presence of WESP/SWESP and CAA significantly reduced the peak viscosity, trough viscosity, breakdown viscosity and final viscosity of PS, especially under the combined action. In rheological tests, all sample gels belonged to the pseudoplastic fluids and weak gel system (tan δ < 1). Besides, thermodynamic properties revealed that WESP/SWESP and CAA synergistic effect had better retrogradation delay effect. In the ternary system, WESP/SWESP, CAA and PS can form a new network structure and improve the stability of the gel system. In addition, the results of infrared spectroscopy, Raman spectroscopy, x-ray diffraction and scanning electron microscopy exhibited that the ternary system can promote the accumulation and winding of the spiral structure of PS chain, and make the structure of PS gel network more orderly and stable. Furthermore, compared with PS gel, the ternary system had lower RDS and higher SDS and RS content, suggesting that the addition of WESP/SWESP and CAA at the same time was more conducive to reducing the hydrolysis rate of PS. This work revealed the interaction between WESP/SWESP, CAA and PS, which improved the physicochemical and digestive properties of PS. It will provide a theoretical basis for improving the quality of potato starch-related products and developing functional foods.

Protein and Polysaccharide Recovery from Shrimp Wastes by Natural Deep Eutectic Solvent Mediated Subcritical Water Hydrolysis for Biodegradable Film.

Environmental pollution is a significant problem due to the improper disposal of plastics and shrimp shells outdoors. Therefore, the synthesis of biodegradable film from waste materials is highly important. The novelty of this research lies in the extraction of protein hydrolysates and chitosan from shrimp shells, as well as the fabrication of biodegradable film from these materials. In this study, the composite films were produced using the solution casting method. Moreover, the combined effect of ultrasound pretreatments (UPT) and natural deep eutectic solvents (NADES) was investigated as extraction media, to determine their potential impact on shrimp waste subcritical water hydrolysis (SWH). Shrimp shells were submitted to UPT in NADES solution, followed by SWH at different temperatures ranging from 150 to 230°C under 3MPa for 20min. Then, the physiochemical properties and bioactivities of the hydrolysates were assessed to determine their suitability for use in biodegradable packaging films. Additionally, the physiochemical properties and bioactivities of the resulting hydrolysates were also analyzed. The highest amount of protein (391.96 ± 0.48mg BSA/g) was obtained at 190°C/UPT/NADES, and the average molecular size of the protein molecules was less than 1000Da with different kinds of peptide. Overall, combined UPT and SWH treatments yielded higher antioxidant activity levels than individual treatments. Finally, the application of composite films was evaluated by wrapping fish samples and assessing their lipid oxidation. The use of higher concentrations of protein hydrolysates significantly delayed changes in the samples, thereby demonstrating the film's applicability.

Exploring the green frontier: Subcritical water chromatography for sustainable analytical practices.

Water in the subcritical state is characterized by properties significantly different from water under standard conditions. These include low viscosity, low surface tension, and a much lower dielectric constant, increasing the solubility of nonpolar substances. For this reason, it can provide an alternative solvent and be used in chromatographic techniques-subcritical water chromatography (SBWC). SBWC appears to be one of the greenest analytical techniques until we unravel chromatography with pure water at room temperature. The versatility of SBWC is explored through its applications in the separation and analysis of a wide range of compounds, including pharmaceuticals, natural products, etc. The use of subcritical water as a mobile phase requires suitable stable stationary phases and special apparatus. Still, it makes it possible to conduct analyses without using organic solvents. When using this technique, it is important to remember that it suits the analysis of thermally stable substances. The following work is a critical review of developments in SBWC.

Antioxidant and antimicrobial extracts from grape stalks obtained with subcritical water. Potential use in active food packaging development

In order to valorise winemaking grape stalks, subcritical water extraction at 160 and 180 °C has been carried out to obtain phenolic-rich extracts useful for developing active food packaging materials. Red (R) and white (W) varieties (from Requena, Spain) were used, and thus, four kinds of extracts were obtained. These were characterised as to their composition, thermal stability and antioxidant and antibacterial activity. The extracts were incorporated at 6 wt% into polylactic acid (PLA) films and their effect on the optical and barrier properties of the films and their protective effect against sunflower oil oxidation was analysed. Carbohydrates were the major compounds (25–38%) in the extracts that contained 3.5–6.6% of phenolic compounds, the R extracts being the richest, with higher radical scavenging capacity. Every extract exhibited antibacterial effect against Escherichia coli and Listeria innocua, while PLA films with extracts preserved sunflower oil against oxidation.