As we all know, ethanol with different concentration is the most commonly extracted solvent, because of its strong ability to penetrate the plant cell wall, lower boiling point (related to industrial energy consumption and solvent recovery), low cost and non-toxicity. The ethanol concentration is an important factor, because the solubility biological small molecules (DHQ) in ethanol solution determine the extraction yield of DHQ. L. gmelinii wood flour 30.0 g was extracted by ultrasound or microwave for 30 min with different ethanol concentration (10%–100%), the solid-liquid ratio was 1:10. The experiments were repeated three times and averaged. As can be seen from Figure 1a, with the ethanol concentration increased, the yield of DHQ was showed an upward trend, and the yield of DHQ was up to the maximum value when the ethanol concentration was 60%. While the ethanol concentration was greater than 60%, the yield of DHQ decreased slightly. So, 60% ethanol is selected for the extraction of DHQ.

The role of infiltration in the extraction process is to make the raw materials fully run-up under the condition of non-energy consumption, which can help the dissolution of small molecules in a solution. The treatment time of infiltration is longer and the run up is better. However, long infiltration may cause moldy materials (if the solvent is water), long extraction cycles and low extraction efficiency. In this experiment, 30.0 g wood flour of different particle size was soaked with 60% ethanol for 0, 1, 2, 3, 8, 12 and 24 h, respectively. The solid-liquid ratio was 1:10, and the effect of soaking time and the particle size of raw materials on the yield of DHQ was studied. In Figure 1b, the DHQ extraction yields of UAE and MAE increased with the soaking time for the first 3 h, and were almost unchanged after 3 h. However, in Figure 1(b2), the DHQ extraction yield of MAE increased dramatically with soaking time as it increased from 12 h to 24 h. This is because a long infiltration time is conducive to full dissolution of DHQ molecule under the condition of being heated. However, after a long infiltration time, the wood flour is difficult to filter. In order to save time and maximize infiltration, the soaking time is 3 h for further experiments.

Large solvent content may cause complex procedures and unnecessary waste, increasing the energy consumption of recycling, while small solvent content may cause incomplete extraction. To evaluate the effect of the solid-liquid ratio, a series of extractions were carried out with different solid-liquid ratios (1:8, 1:10, 1:12, 1:14, 1:16, 1:18 and 1:20 g/mL). Figure 1c indicated that the extraction yield of DHQ increased following the increase of the solvent volume before the solid-liquid ratio reached 1:12, and was then not significantly influenced by further increase of the solvent amount. Therefore, a solid-liquid ratio of 1:12 was determined and then used in the further studies.

Extraction cycles are a crucial factor in increasing the extraction yield of DHQ, but too many times, will lead to excessive dissolution of impurities, resulting in undue burden to the subsequent separation and purification process. From Figure 1d, the total four-times extraction yield of UAE and MAE were 64.06 and 80.66 mg/g, respectively, and the total extraction yield of four times was defined as 100%. Each of the extraction yields of DHQ were then compared with the total extraction yield. Each-time extraction yield of UAE were 33.95, 21.23, 7.60, and 1.27 mg/g, respectively. The extraction yield sum of 2 times had reached more than 85%, and the extraction yield sum of 3 times was about 98%. Each-time extraction yield of MAE were 56.16, 18.65, 4.32, and 1.53 mg/g, respectively. The extraction yield sum of 2 times had reached to 92.74%, and the extraction yield sum of 3 times was more than 98%. Considering the relevant factors, including time, solvent and energy consumption, extraction 3 times was more reasonable.

In order to investigate the effect of extraction time on the extraction yield of DHQ with UAE and MAE, respectively, the process was performed in ultrasound unit and microwave oven under the optimized conditions. Thirty grams of dried sample was mixed with 60% ethanol soaked for 3.0 h, and then extracted for 60 min (UAE) and 30 min (MAE), respectively. The power of UAE and MAE were 250 W and 700 W, respectively. The solid-liquid ratio was 1:12, and the yields of DHQ were tested every 10 min and 5 min, respectively.

From Figure 2a, it can be seen that the yield of DHQ with MAE was much higher than that with UAE. As the extraction time increased, the yield of DHQ increased at first. Then, after UAE 40 min, the extraction yield of DHQ was almost unchanged, and after MAE 20 min, the yield had even declined. This may result in DHQ degradation with long microwave irradiation times. Therefore, 40 min ultrasound treatment and 20 min microwave irradiation time were selected as the optimization conditions for extraction DHQ.

To examine the effect of the ultrasound power on the extraction yield of DHQ, extractions were carried out with a constant time for ultrasound treatment of 40 min at 50, 100, 150, 200, and 250 W, respectively. Figure 2b indicated that the average extraction yield was significantly influenced when the ultrasound power was increased. Therefore, the 250 W ultrasound powers (the maximum value of ultrasound unit) satisfied the conditions for extracting of DHQ.

In order to study the effect of the microwave power on the extraction yield, extractions were carried out with 20 min for microwave treatment at 100, 250, 400, 550 and 700 W, respectively. Figure 2b indicated that the average extraction yield was increased with the irradiation power increasing. Thus, the 700 W (the maximum value of microwave oven) microwave power satisfied the conditions for DHQ extraction.

As can be seen from Table 1, the extraction yield of DHQ with item F was 119.6 ± 2.9 mg/g, better than other comparison methods. Conversely, reflux extraction is a time-saving method, but it has energy consumption in the process of heating. The extraction yield of DHQ with 60% ethanol reflux extraction 4 h was 90.0 ± 2.8 mg/g. Apparently, water is the most common and inexpensive solvent, therefore pure water is always selected as the co-solvent in various extraction processes. The extraction yield of DHQ with water stirring extraction for 8 h at 50 °C was only 35.6 ± 1.8 mg/g, which was similar to the extraction yield of DHQ with water reflux extraction for 4 h (35.0 ± 1.4 mg/g). Compared to above extraction methods, alternant digestion by UAE and MAE was a better extraction method of DHQ.

The most common method for extraction of DHQ from L. gmelinii is reflux extraction using 60% ethanol. The energy required to perform the reflux extraction method is 1.00 kW. Table 1 shows that, not only is the extraction yield of DHQ by UMAE (119.6 ± 2.9 mg/kg) higher than that by reflux extraction (90.0 ± 2.8 mg/kg), but also the total time of energy consumption dramatically reduces (from 4 h to 1 h) and the carbon dioxide rejected in the atmosphere reduces (from 3200 g CO₂ to 320 g CO₂ rejected). The reduced cost of DHQ extraction is clearly advantageous for the UMAE method in terms of time and energy. The energy consumption time required by reflux extraction is 4 h. The energy powers required are 1.00 kW/h for reflux extraction, 0.7 kW/h for MAE, and 0.25 kW/h for UAE, respectively. Regarding environmental impact, the calculated quantity of carbon dioxide rejected in the atmosphere of reflux extraction (3200 g CO₂) is much higher than that of UMAE (320 g CO₂). These calculations have been made according to the literature [32]: to obtain 1 kW·h from coal or fuel, 800 g of CO₂ will be rejected in the atmosphere during combustion of fossil fuel. Therefore, UMAE can be suggested as an environmentally friendly extraction method, which avoids the use of organic solvents. UMAE can also be offered for the production of larger quantities of DHQ by applying the existing large-scale ultrasound-microwave extractors instead of the conventional reflux extractors, with the improvement and development of ultrasonic and microwave devices.

POV is an important parameter of fats and oils, and DHQ as a kind of food additive that shows a good antioxidant effect. The changes in POV during storage of soy bean oil at 25 °C and 60 °C for 20 days after addition of DHQ were investigated and given in Tables 2,3. Lower POV shows a better resistance to oxidation; by increasing the dosage of DHQ, the antioxidant activity also increased. However, excessive dosage will result in unnecessary waste. It is evident from these results that, as the concentration of DHQ increased, inhibitory effects on POV also increased considerably, but when the concentration of DHQ was higher than 0.08‰, the inhibitory effects on POV was not increased significantly, and the optimal addition dosage of DHQ is 0.08‰ (the mass ratio of DHQ and soy bean oil). The addition of 0.08‰ DHQ caused significant reduction in POV from 0.090 ± 0.002 (control) to 0.146 ± 0.003 and 0.559 ± 0.007 meq/kg, during 20 days, storage at 25 °C and 60 °C, respectively.

The changes in POV during storage of soy bean oil at 25 °C and 60 °C after addition of synthetic antioxidants are given in Tables 4 and 5. The addition dosage of synthetic antioxidants (BHA and BHT) is 0.08‰ (the mass ratio of antioxidants and soy bean oil), the same as the addition dosage of DHQ. It is apparent from these results that addition of DHQ, BHA and BHT retarded the development of rancidity in soy bean oil, but DHQ resulted in a slightly better protection than BHA and BHT. The POV of BHA and BHT decreased from 0.084 ± 0.002 and 0.091 ± 0.002 (control) meq/kg to 0.177 ± 0.003, 0.203 ± 0.003 meq/kg after 20 days’ storage at 25 °C, and to 1.032 ± 0.009, 1.380 ± 0.005 meq/kg after 20 days’ storage at 60 °C, respectively. Briefly, as antioxidant storage at 25 °C and 60 °C, the antioxidant activity order is DHQ > BHA > BHT.

The HPLC system consists of a Waters 717 automatic sample handling system composed of series HPLC system equipped with 1525 Bin pump, 717 automatic column temperature control box and 2487 UV-detector (Waters, USA). Chromatographic separation was performed on a HiQ sil-C18 reversed-phase column (4.6 mm × 250 mm, 5 μm, Kya Tech) for the determination of DHQ. For HPLC analysis, acetonitrile-water-acetic acid (18:82:0.1, v/v/v) was used as the mobile phase with 1.0 mL/min flow rate, 10 μL injection volume, and 25 °C column temperature. The absorbance was measured at a wavelength of 294 nm for the detection of DHQ, the run time was 30 min, and the retention time of DHQ was 24 min. Corresponding calibration curves for DHQ is Y = 3.1755 × 10⁷X + 2.5993 × 10⁴ (r = 0.9999). A good linearity was found for DHQ in the range of 0.0312–0.5000 mg/mL.

First, the common factors of UAE and MAE by single-factor experiment with ultrasonic bath (KQ-250DB, Kunshan Ultrasonic Co. Ltd., China) and microwave oven (PJ21C-AU, Glanz, China) were optimized, and then the core factors of UAE and MAE were optimized, respectively. The common factors are included the volume fraction of ethanol, soaking time, solid-liquid ratio, and extraction times. The core factors are included extraction time and energy intensity of UAE and MAE, respectively. The alternant test program is shown in Table 1A–F.

The conventional extraction methods of DHQ are including maceration extraction with 60% ethanol at room temperature for 24 h, reflux extraction with 60% ethanol for 2 h, stirring extraction with water for 8 h at 50 °C and reflux extraction with water for 4 h (in Table 1G–J). The yields of DHQ with the various extraction methods were compared.

Microstructure analysis was performed using SEM (FEI QUANTA200, The Netherlands). The sample surfaces were sputter coated with a thin layer of gold using an SCD 005 sputter coater (BAL-TEC, Switzerland), to provide electrical conductivity.

DHQ powder which was extracted by UMAE according to program F, as an antioxidant, was tested by the determination of POV during storage of soy bean oil at 25 °C and 60 °C. The POV (meq/kg of oil) was measured by titration with sodium thiosulphate, using starch as indicator, as described in Wang et al. [35,36]. Three grams of oil samples was dissolved in a blended solution of 30 mL chloroform-glacial acetic acid (2:3, v/v). A saturated solution of potassium iodide (1 mL) was added, airtight and shaken by hand for 0.5 min, and then kept in the dark for another 3.0 min. After the addition of 100 mL deionized water, the mixture was shaken, and immediately titrated against sodium thiosulphate (0.002 M) until the yellow color almost disappeared. Then, about 1.0 mL of starch indicator (0.1%) solution was added. Titration was sustained until the blue color just disappeared as the end point. A blank was also determined under similar conditions. POV was calculated as follows:

where V₁ is the reagent blank consumption volume of sodium thiosulfate standard solution, mL; V₂ is the sample consumption volume of sodium thiosulfate standard solution, mL; c is the concentration of sodium thiosulfate standard solution, M; 0.1269 is the considerable iodine mass titrated with 1.00 mL sodium thiosulfate standard solution [c (Na₂S₂O₃) = 1.00 M], g; 78.8 is conversion factor; m is the mass of oil sample, g. All determinations were carried out in triplicate and the statistical significance was evaluated using Excel and set at p < 0.05.