A rapid microwave-assisted derivatization process for the determination of phenolic acids in brewer’s spent grains

A simple and fast method for the TMS derivatization of phenolic acids in a closed vial using microwave irradiation followed by GC/MS analysis is proposed. A full factorial design was used to investigate the effects of two independent variables, namely, time and power of microwave irradiation, on the yield of silylation reaction. The optimal conditions were tested against the classical heating derivatization procedure. No significant differences were found between the classical heating and microwave-assisted derivatization. Chromatographic separation of the nine phenolic acids examined was achieved in 16 min. The mass spectral fragmentation patterns of the derivatives obtained by the proposed method were identical to those from the classical heating. Four different batches of brewer’s spent grain were extracted and analyzed for the total phenolic acid content. Significant differences between the batches of spent grains were found for all analytes. The total phenolic acid content varied between 2688 and 4884 μg/g.     

Synthesis and characterisation of starch cuprate

The cupration of granular potato starch with ammonium tetrachlorocuprate(II) was performed by a 20 min lasting microwave-assisted process and by 40 min convectional heating. In both cases the degree of esterification (DE) did not exceed 6.4 × 10⁻³. A higher dose of cuprate had a positive effect on DE, regardless of whether the microwave irradiation or the convectional heating was applied, and on the thermal stability of the starch cuprate produced in the microwave-assisted way. As a result of cupration, the hydroxyl ligand of the starch d-glucose units was replaced by the chloride ligand of the tetrachlorocuprate(II) anion. In the starch cuprate formed in the microwave-assisted process, the cuprate anion additionally coordinated water molecules. The cupration reduced starch crystallinity, as indicated by X-ray diffractometry and differential scanning calorimetry.     

微波辐射离子液体[Bpy]HSO_4催化葵花籽油制备生物柴油

生物柴油是一种绿色可再生能源。该文报道微波辐射下离子液体[Bpy]HSO4催化葵花籽油与甲醇通过酯交换反应制备生物柴油,以正交法对制备工艺条件进行优化,考察醇油物质量比、催化剂用量、微波功率和反应时间对酯交换反应影响。实验结果表明,当醇油物质量比为10∶1、催化剂用量(催化剂与油质量比)为5%、微波功率为400W、反应时间为45min时,生物柴油转化率可达96.2%;与传统加热方式相比,采用微波辐射加热方式,反应时间明显缩短,能耗减少。     

Extraction of coffee diterpenes and coffee oil using supercritical carbon dioxide

Commercial green and roasted coffee beans were used to maximize oil extraction and conditions were studied to obtain the highest and lowest diterpene levels on green and roasted coffee oil, respectively. Thus, operational temperatures (60–90 °C) and pressure (235–380 bar) were optimized for coffee oil extraction. Oil content levels and diterpene oil concentration were compared to the results obtained with the extraction with Soxhlet apparatus, using hexane as solvent. In general, an inverse correlation was observed between the amount of extracted oil and diterpene concentration levels. As a result, different oil contents with different diterpene concentrations could be obtained. The HPLC analysis of cafestol and kahweol in the oil extracted from green coffee beans at 70 °C/253 bar resulted in the highest concentration (453.3 mg 100 g⁻¹), which was 48% lower than in the oil extracted with hexane while in the oil extracted from roasted coffee beans at 70 °C/371 bar, resulted in 71.2% reduction of diterpenes.     

Effect of dielectric microwave heating on the color and antiradical capacity of betanin

The decomposition of betanin under dielectric heating (microwave irradiation, power: 25–200 W (3–24 kJ g⁻¹)) follows first-order kinetics with a rate constant similar to that obtained during conventional conduction heating (half-life < 2 min at 100 °C). Color coordinate analysis indicates that betanin is bleached upon thermal treatment, whereas beetroot juice and spray-dried beetroot powder tend to form colored decomposition products. The antiradical capacity of betanin decreases upon heating, but is still much higher than that of standard antioxidants such as ascorbic acid and trolox. Betalamic acid, a high capacity antiradical, was detected by mass spectrometry and second-derivative absorption spectroscopy in betanin samples submitted to thermal treatment.     

Microwave-assisted biosynthesis of glycerol monolaurate in reverse microemulsion system: key parameters and mechanism

This study highlights the microwave-assisted biosynthesis of glycerol monolaurate catalyzed by Novozym 435 in reverse microemulsion system dispersed by Aerosol-OT. Optimization of key parameters including catalyst/surfactant loading and temperature showed that under microwave irradiation, 125.9 mol L⁻¹ (36 wt%) of glycerol monolaurate was achieved within 30 min, which had improved 150% compared with that of water bath under the same conditions (60 °C, 1 wt% enzyme, 5 wt% AOT). Also, a synergistic effect of microwave irradiation and Aerosol-OT on reaction and a decline of optimum temperature (from 70 to 60 °C) were observed in comparison with traditional heating. More importantly, further comparative studies on morphology and chemicophysical profiles of reaction system heated by different thermal modes revealed the following: (1) due to the volumetric and selective heating character of molecular heating, superiorly larger interface of reaction system was formed under microwave irradiation, directly accelerating the reaction course investigated; and (2) microwave irradiation more favored a less compact state of enzyme. And faster mobilities of lid structure of enzyme were evident under microwave irradiation. This enhancement might facilitate the transition of conformational states, which is of key importance for the activation of bioactivity of Novozym 435. Moreover, the stepwise changes in steady-state fluorescent profiles demonstrated that the thermal-induced variation was not two-state transition but involves intermediate states, which was further supported by activity measurements reflected in glycerol monolaurate yield.     

Comparison of microwave-assisted hydrodistillation withthe traditional hydrodistillation method in the extractionof essential oils from Thymus vulgaris L.

Microwave-assisted hydrodistillation (MAHD) is an advanced hydrodistillation (HD) technique utilizing a microwave oven in the extraction process. MAHD of essential oils from the aerial parts (tops) of Thymus vulgaris L. (common thyme) was studied and the results were compared with those of the conventional HD in terms of extraction time, extraction yield/efficiency, chemical composition, quality of the essential oils and cost of the operation. MAHD was superior in terms of saving energy and extraction time (75 min, compared to 4 h in HD). Scanning electron microscopy (SEM) of thyme leaves undergone HD and MAHD provided evidences as to a sudden rupture of essential oil glands with MAHD. Gas chromatography–mass spectrometry analysis of the extracted essential oils indicated that the use of microwave irradiation did not adversely influence the composition of the essential oils. MAHD was found to be a green technology.     

The impact of microwave heating of infant formula model on neo-formed contaminant formation, nutrient degradation and spore destruction

Heat treatment of milk results in the development of a large range of Maillard products, especially in infant formulas due to their high content in Maillard reaction substrates. The Maillard reaction should be minimized because some of those products, called neo-formed contaminants (NFC), are currently suspected of promoting deleterious health effects. The objective of this work was to study the impact of microwave heating on the formation of Maillard compounds, degradation of nutrients and destruction of spores of an infant formula model. An infant formula model based on pure-whey proteins was used as experimental model. A spore suspension of Geobacillus stearothermophilus was prepared and inoculated in the formulated milk before microwave treatment. The treatment was performed on a laboratory device running at 2450 MHz and equipped with a magnetron (power range 0-1860 W) and the temperature was controlled by three optical fibers. Heating kinetics were established at several microwave specific powers (5, 7, 10, 12 W/mL). The impact of the treatment on NFC formation was evaluated by 3 indicators: the FAST index (indicator of global advancement of Maillard reaction), carboxymethyllysine (CML) and furosine. All these NFC indicators seemed to follow an exponential model as a function of time. The thermal sensitive nutrient vitamin C was chosen as indicator of the nutritional quality. Like in conventional heating, an exponential model was observed for spore inactivation and vitamin C degradation. Furthermore, laws similar to those used for conventional heating could be used to express heat resistance regarding germ viability, vitamin C concentration and Maillard products. As a result, D and z-values relative to specific microwave powers were determined for each indicator. Finally, it was found that the best way to minimize NFC formation and nutrient degradation was to use high specific power for a short time, similar to what is obtained with the UHT process for conventional heating.     

The effect of microwave assisted extraction on the isolation of anthocyanins and phenolic acids from sour cherry Marasca (Prunus cerasus var. Marasca)

The aim of this study was to evaluate the influence of microwave assisted extraction on the isolation of anthocyanins and phenolic acids from sour cherry Marasca. A general factorial design was used to study the effect of temperature (from 50 to 70 °C), irradiation time (5–12 min) and microwave power (350–500 W) on individual, total anthocyanins and individual and total phenolic acids. Optimal microwave assisted extraction conditions differed for anthocyanins and phenolic acids, especially in terms of temperature and irradiation time, so lower temperature (60 °C) and shorter time (6–9 min) was more convenient for anthocyanins extraction, while phenolic acids gave higher extraction yield at higher temperatures (70 °C) and longer irradiation time (10 min). The optimal microwave power did not differ significantly for studied compounds, ranging about 400 W. Compared to conventional extraction, microwave assisted one showed higher efficiency for all studied compounds.     

Study of colour and acrylamide formation in coffee,wheat flour and potato chips during heating

The effects of heating on colour generation measured as CIE colour space parameters of L* a* b* and acrylamide formation were studied in various food matrices including green coffee, wheat flour and potato chips at different temperatures. Changes in both the acrylamide concentration and the redness parameter a* during heating at relatively higher temperatures followed a typical kinetic pattern in which an initial increase to an apparent maximum followed by a subsequent decrease was observed. The similarities between the changes in acrylamide and redness parameter a* during heating revealed that colour may be a reliable indicator of acrylamide levels in thermally processed foods. The overall results suggest that both acrylamide and redness parameter a* form as intermediate products during Maillard reaction. Since an apparent decrease was observed in its level during prolonged heating at certain temperatures, prediction of acrylamide level in foods during processing should be based on realistic reaction mechanism, instead of simple linear regression model.     

微波作用下亚油酸转化为共轭亚油酸工艺的研究

以玉米胚芽油为原料,丙三醇为溶剂,在微波场作用下对亚油酸异构化为共轭亚油酸工艺进行了研究。通过紫外、红外吸收光谱分析,证明微波作用下生成的产物主要是共轭亚油酸,并含有顺式和反式共轭结构。与油浴加热方法比较微波作用异构化生产共轭亚油酸的方法优越可行。通过对影响共轭异构化反应的因素及正交试验的分析,确定了最佳工艺条件,获得了95%以上的共轭异构化转化率。     

Comparison of glycation in conventionally and microwave-heated ovalbumin by high resolution mass spectrometry

The glycation extent of ovalbumin under two heating conditions, conventional and microwave heating was monitored by high resolution mass spectrometry, following pepsin digestion. The sequence coverage of the unglycated and glycated ovalbumin was 100% and 95%, respectively. About 35.2% of the lysines after microwave heating and 40.8% of the lysines after conventional heating were modified by d-glucose. The glycation content increased quickly when ovalbumin–glucose mixture was incubated for 15 min, under both processing conditions. These modifications were slowed down after 30 min of heating and no obvious advanced stage products were observed. The glycated peptides exhibited varying degrees of glycation, under both conventional and microwave heating, suggesting that glycation is strongly relevant to the protein structure. The fact that some peptides showed a lower level of glycation when heated by microwave indicated that microwave radiation might be a non-thermal process. In addition, the lack of browning after microwave heating emphasised the difference between microwave and conventional heating.     

Microwave-assisted synthesis of prebiotic di-D-fructose dianhydride-enriched caramels

The synthesis of prebiotic caramels involving the use of microwaves as the activating/heating source has been achieved. The yields in di-fructose dianhydrides (DFAs) in caramels were measured. The aim of this study was twofold: first to check the feasibility of the process, and second to determine the conditions to obtain an optimum response with microwave heating. The study showed that it was possible to obtain a yield of almost 50% of DFAs in a reaction time that was 10 times shorter than a previous study; i.e. 5–10 min for microwave activation compared to 60–120 min for conventional heating. It was shown that the radiation time and the radiation power were linked. The simultaneous determination of the values of these two factors was therefore necessary to obtain significant yields. This technique demonstrates the advantage of activation for mixtures such as caramels.     

Effect of energy density and citric acid concentration on anthocyanins yield and solution temperature of grape peel in microwave-assisted extraction process

In order to verify the sensitivity of anthocyanins (Acys) yield of grape peel to the energy density (microwave power for a given unit of volume) and citric acid concentration in microwave-assisted extraction (MAE) process, the effect of energy density and citric acid concentration in MAE process were investigated. The results indicated that, Acys yield was more sensitive to energy density than to microwave power. The correlation between Acys yield and energy density follows well Gaussian function, suggested by higher R squares (R² > 0.9399). Exponential decay model can well describe the correlation between Acys yield and citric acid concentration, or between solution temperature rise and energy density. The solution temperature was a linear function of citric acid concentration and extraction time. Energy density was the most significant factor affecting the Acys yield, followed by citric acid concentration, ratio of liquid to solid and extraction time (p < 0.05).     

Quality and microstructural changes in goat meat during heat treatment

Environmental scanning electron microscopy (ESEM) was used to study the structure of the semi-membranous muscle of goat meat during heat treatment. The four treatments examined were raw (control), conventional heating, domestic microwave heating and industrial microwave heating. The ESEM showed shrinkage and denaturation in muscle fibers after cooking by the various methods. No surface damage was observed in any image for conventional heating. More structural damage was observed in microwave heating at both domestic (700 W) and industrial (12,000 W) levels. The pattern of distribution in microwave heating caused surface damage to muscle fiber with separation of some portions and denaturation of collagen.     

Modelling heat-disinfestation of dried fruits on “biological model” larvae Ephestia kuehniella (Zeller)

The effects of heat treatment on the survival of isolated Ephestia kuehniella Zeller larvae were examined. Larvae were treated in a specific device at a temperature range of 46 to 70 °C. Statistical analysis of the data demonstrated that the thermal survival kinetics was best represented by a 2-parameter Weibull model above 50 °C. The mortality of larvae only occurs beyond a critical temperature of 41.6 °C ± 2.5 °C. A 2D axial-symmetric model of heat transfer (convection, microwave at 915 MHz) was developed for cocoa bean and date palm. It was combined with the survival rate of larvae (Weibull model) and some z-values (from literature) are used for calculating a “food quality index” degradation kinetics. The intensive heating/convection process may not be adaptable for food products at low water activity due to high surface product temperatures. The use of microwave heating for date fruits and conventional hot air treatments for cocoa beans is recommended.     

Microwave-assisted extraction of phenolic antioxidant compounds from peanut skins

A microwave-assisted extraction system was used to extract phenolic antioxidants from peanut skins. The effects of microwave power (10%, 50%, 90% nominal), irradiation time (30, 90, 150 s) and sample mass (1.5, 2.5, 3.5 g) on total phenolic content (TPC), ORAC (oxygen radical absorbance capacity) level and resveratrol content of peanut skin extracts (PSE) were investigated. Peanut skins were extracted with 37.5 ml of 30% ethanol (EtOH) in water. A response surface method was used to estimate optimum extraction conditions, based on TPC, ORAC level and resveratrol content. The maximum predicted TPC, under the optimised conditions (90% microwave power, 30 s irradiation time and 1.5 g skins), was 143.6 mg gallic acid equivalent (GAE)/g skins. The highest ORAC value was 2789 μmol trolox equivalents (TE)/g skins, which occurred at 90% power, 150 s and 1.5 g of skins. Resveratrol was identified in PSE by LC–MS–MS analysis.     

Microwave assisted solid state reaction synthesis of methionine complexes of iron (II)

Methionine complexes of iron (II) were synthesized by employing a simple and novel microwave assisted solid state route in a domestic microwave (MW) oven (2.45 GHz frequency, 700 W power) and six crucial reaction parameters were investigated on the basis of their individual effects on the yield of the synthesized compounds. Results suggest that optimal synthesis conditions are microwave irradiation time, 260 s; the airflow temperature, 100 °C; and the amount of reactant, 1.5 g. The synthesized compounds were characterized by infrared spectrometry. From the results of chemical analysis and infrared spectrometric analyses, the minimum molecular formula of the synthesized compounds could be determined as Fe(Met)₂ · H₂O, in which iron, nitrogen element and water account for 15.93%, 7.97% and 0.07%, respectively. The microwave method was found to be simple, fast and efficient, with high selectivity for the preparation of these compounds.     

Microwave steam diffusion for extraction of essential oil from orange peel: Kinetic data,extract’s global yield and mechanism

The microwave steam diffusion (MSDf) apparatus for extraction of essential oils from orange peel (by-products) was studied. MSDf has been compared with conventional steam diffusion (SDf). A response surface methodology (RSM) was realised to investigate the influence of process variables by a central composite design (CCD) approach. The statistical analysis revealed that the optimal conditions for the extraction of orange essential oil were a steam mass flow rate of 25 g min⁻¹ and a microwave power of 200 W. Cytohistological investigations using light microscopy provided evidences for rapid and pronounced cell and tissue alterations of oil glands with MSDf in comparison with SDf. The essential oils extracted by MSDf for 12 min were quantitatively (yield) and qualitatively (aromatic profile) similar to those obtained by SDf for 40 min. Extraction of essential oils from orange peel with MSDf was better than SDf in terms of energy saving, cleanliness and reduced waste water.