Study on formation of acrylamide in asparagine–sugar microwave heating systems using UPLC-MS/MS analytical method

Microwave heating can be regarded as a possible way to produce a considerable amount of acrylamide. The present study investigated the formation of acrylamide in asparagine–glucose, asparagine–fructose and asparagine–sucrose microwave heating systems by the response surface methodology (RSM) and the orthogonal array methodology (OAM). The acrylamide content was rapidly quantified by a validated ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method. Results of RSM study indicated that in the asparagine–glucose system, the acrylamide content increased in the combined condition of high temperature accompanying with short heating time (>190 °C, <20 min) or low temperature accompanying with long heating time (<180 °C, >30 min). In the asparagine–fructose system, the similar conclusion was made in the combined condition of high temperature accompanying with short heating time (>175 °C, <20 min) or low temperature accompanying with long heating time (<170 °C, >25 min). In the asparagine–sucrose system, the amount of acrylamide enhanced with the increase of both heating temperature and heating time. The fitted mathematic models were successfully applied to the quantification of acrylamide formation when the heating temperature and heating time fell into the ranges of 120–240 °C and 5–35 min simultaneously. OAM study showed that acrylamide is readily formed via heating binary precursors 5 min at 180 °C in the asparagine–glucose and asparagine–fructose systems. However, acrylamide is readily generated when the binary precursors are heated 15 min at 180 °C in the asparagine–sucrose system.     

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.     

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.     

Effect of cooking on the antioxidant properties of coloured peppers

Pepper (Capsicum annum L.) has long been recognized as an excellent source of antioxidants, being rich in ascorbic acid and other phytochemicals. This study was conducted to investigate the effect of different cooking methods on the antioxidant properties of coloured peppers. Six varieties of peppers were subjected to different cooking methods, such as microwave heating, stir-frying and boiling in water, for 5 min individually. The cooked and raw peppers were analyzed for radical-scavenging activity (RSA) and total polyphenol content (TP) using 1,1-diphenyl-2-picrylhydrazyl–high-pressure liquid chromatography (DPPH)–HPLC and Folin-Ciocalteu methods, respectively. The samples were also evaluated for ascorbic acid content (AsA) by HPLC. Total carotenoid content was determined spectrophotometrically. Results suggest that there is no significant (P > 0.05) difference in RSA, TP, AsA and total carotenoid contents between the cooked and raw peppers when processed for 5 min. However, the cooked peppers show marked differences (P < 0.05) in the RSA, TP and AsA when cooked for 5 min in boiling water with further reduction observed after boiling for 30 min. This may be due to the leaching of antioxidant compounds from the pepper into the cooking water during the prolonged exposure to water and heat. Therefore, it is vital to use less water and cooking time and also to consume the water used for boiling so as to obtain the optimum benefits of bioactive compounds present in peppers. It is concluded that microwave heating and stir-frying without using water are more suitable cooking methods for pepper, to ensure the maximum retention of antioxidant molecules.     

Difructose anhydrides as quality markers of honey and coffee

Difructose anhydrides (DFAs) are pseudodisaccharides produced by condensation of two fructose molecules by means of caramelization reaction which takes place during heating of sugars or sugar-rich foodstuffs. The aim of this research was to evaluate the feasibility of DFAs as chemical markers of honey authenticity and sugar-roasted torrefacto coffee. DFAs were analysed by gas chromatography coupled to mass spectrometry after conversion to their trimethylsilyl (TMS) derivatives. α-d-fructofuranoside-1,2′:2,1′-α-d-fructofuranoside (DFA7) and α-d-fructofuranoside-1,2′:2,1′-β-d-fructopyranoside (DFA9) can be used as quality markers of honey and coffee. DFA7 and DFA9 were detected in honey added with 5% fructose and sucrose caramels and 15% of glucose caramels. Torrefacto coffees showed DFAs values ranged from 0.195 to 0.570 g/100 g whereas only traces were found in natural roasted coffees. Quantities from 0.073 to 0.189 g/100 g were measured in blends of natural and torrefacto roasted coffees. A relationship between DFAs content in torrefacto coffees and roasting conditions was observed. In conclusion, this study indicated that DFAs are useful chemical indicators to control honey authenticity and torrefacto coffee roasting.     

Microwave-assisted methanolysis of green coffee oil

Optimisation of a microwave-assisted methanolysis was performed to obtain cafestol and kahweol directly from green coffee oil (Coffea arabica). A two-factor (the methanolysis period and temperature), three-level, factorial experimental design (3²) was adopted. The methanolysis procedure was performed under microwave irradiation, using closed vessel and accurate fast responding internal fibre-optic temperature probe. The effects on the responses were measured by HPLC. After 3 min of microwave irradiation (hold time) at 100 °C, with 500 mg of green coffee oil, a yield higher than 99% was obtained. The yield of this reaction is 26% after 2 h when working under conventional heating. The methods described in the literature lead to long reaction times, poor yields and formation of side products. The microwave-assisted technique proved to be faster, avoided undesired side products and gave better conversion, when compared to conventional heating process.     

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.     

Production of feruloylated arabinoxylo-oligosaccharides from maize (Zea mays) bran by microwave-assisted autohydrolysis

Maize bran was treated with microwave irradiation (160–200 °C for 2–20 min) to release feruloylated arabinoxylo-oligosaccharides (AXOS). Lower temperatures and shorter treatment times were consistent with low AXOS yields, while higher temperatures and longer reaction times also resulted in low yields, which were contaminated with increasing levels of monosaccharides, free ferulic acid, and furfural. Maximum release of AXOS, accompanied by low production of monosaccharides, free ferulic acid, and furfural, occurred with treatment at 180 °C for 10 min or 200 °C for 2 min. Under these conditions, about 50% of the initial arabinoxylan content could be released as AXOS, containing a wide variety of molecular weights. AXOS were highly feruloylated, containing 6.62 and 8.00 g of esterified ferulate/100 g of AXOS. These feruloylated AXOS may provide health benefits, including prebiotic effects and prevention of detrimental oxidation reactions. The evaluation of these benefits will be the subject of future research.     

Phenolic content and antioxidant activity of extracts from whole buckwheat (Fagopyrum esculentum Möench) with or without microwave irradiation

The purpose of this study was to extract phenolic compounds and antioxidant activity from buckwheat with water, 50% aqueous ethanol, or 100% ethanol using microwave irradiation or a water bath for 15 min at various temperatures (23–150 °C). Phenolic content of extracts increased with increasing temperature. In general, phenolic contents in microwave irradiated extracts were higher than those heated with a water bath. The highest phenolic content, 18.5 ± 0.2 mg/g buckwheat, was observed in the extract that was microwave irradiated in 50% aqueous ethanol at 150 °C. The highest antioxidant activities, 5.61 ± 0.04–5.73 ± 0.00 μmol Trolox equivalents/g buckwheat, were found in the 100% ethanol extracts obtained at 100 and 150 °C, independent of heat source. These results indicate that microwave irradiation can be used to obtain buckwheat extracts with higher phenolic content and similar antioxidant activity as extracts heated in a water bath.     

Enhanced extraction of flavanones hesperidin and narirutin from Citrus unshiu peel using subcritical water

Flavanones including hesperidin and narirutin constitute the majority of the flavonoids that occur naturally in citrus fruits. The main purpose of this study was to extract valuable natural flavanones from agricultural by-products such as citrus peels using subcritical water extraction (SWE). Thus, the application of SWE to extraction of flavanones hesperidin and narirutin from Citrus unshiu peel was evaluated, and the effect of key operating conditions was determined by varying the extraction temperature (110–200 °C) and time (5–20 min) under high pressure (100 ± 10 atm). The maximum yields of hesperidin (72 ± 5 mg/g C. unshiu peel) and narirutin (11.7 ± 0.8 mg/g C. unshiu peel) were obtained at an extraction temperature of 160 °C for an extraction time of only 10 min. These yields accounted for approximately 99% of the total amount of these flavanones in the original material. The SWE was compared with three conventional extraction methods in terms of the extraction time and recovery yields for hesperidin and narirutin. The hesperidin yield by SWE was more than 1.9-, 3.2-, and 34.2-fold higher than those obtained by extraction methods using ethanol, methanol, or hot water, respectively, and the narirutin yield was more than 1.2-, 1.5-, and 3.7-fold higher.     

Prediction of the degree of starch gelatinization in wheat flour dough during microwave heating

In order to produce quick-boiling noodles, we made partially pre-gelatinized wheat flour dough by the microwave heating method. A cylindrical-shaped piece of wheat flour dough containing 3% NaCl was heated by 117 W microwave oven operating at 2450 MHz. The sample dough was intermittently heated to allow sufficient gelatinization of the starch granules of the dough. The resultant changes in the internal temperature profiles of the sample dough were measured and compared with numerical prediction, which is the two-dimensional heat conductive equation with a term for internal heat generation based on Lambert’s law. In order to calculate the internal heat generation during microwave heating, we measured the dielectric properties of wheat flour dough at 2450 MHz from 10 to 70 °C by the open-ended coaxial probe method. The calculated temperature history could describe the feature of the experimental one during intermittently microwave heating. Furthermore, the progress of starch gelatinization according to the heat transfer in the sample dough was predicted by the Runge–Kutta gel method. A tendency that the total gelatinization degree increased slowly during the microwave intermittent heating was obtained in the calculated result.     

Effects of microwave heat-moisture treatment on properties of waxy and non-waxy rice starches

Waxy and non-waxy rice starches adjusted to 20% moisture (wet basis, w.b.) were heat-moisture treated in a microwave oven to determine the effects of the microwave heating characteristics on digestibility, pasting, and morphological properties of the heated starches. Microwave heating produced only minimal changes in digestibility as well as the physical characteristics of heated starches. Significant changes in viscosity properties after microwave heat treatment were observed for both waxy and non-waxy starches heat-treated in a microwave oven, relative to non-treated samples. Non-waxy starch heated in microwave oven showed an increase in breakdown viscosity from 29.8 RVU (non-treated starch) to 35.8 RVU after heating for 60 min. However, for waxy starch, breakdown viscosity decreased from 112.7 to 35.9 RVU after 60 min of microwave heat treatment, reflecting an increased stability of microwave heat-treated starch under cooking. The data obtained in this study indicate that there was much higher re-aggregation of starch granules in waxy starch after microwave heat treatment than occurred in non-waxy starch, suggesting a re-association of amylopectin branch chains in the heat-treated waxy starch.     

Microwave improvement of soy protein isolate–saccharide graft reactions

Soy protein isolate (SPI) was mixed with lactose, maltose, dextran or soluble starch (SS) and heated (temperature ?90 °C) by microwave irradiation. Compared to the classical heating, the microwave heating (MH) speeded up the graft reactions of SPI with sugars. From the results calculated, the rate constants of free amino groups in the grafts of SPI with lactose, maltose, dextran and SS by MH were, respectively, 6, 7, 57 and 12.3 times of those by classical heating. After heating by microwave, the lysine and arginine decreased and from, the FTIR spectroscopy of SPI and its grafts by MH, the absorbance of –C–O stretching and –OH deformation vibrations (1050–1150 cm⁻¹) and free –OH form (3643–3630 cm⁻¹) in SPI grafts increased compared to SPI. The temporal development of the graft reactions of SPI with sugars by MH was also shown by SDS–PAGE.     

Effect of microwave heating on the low-salt gel from silver carp (Hypophthalmichthys molitrix) surimi

Gels from silver carp (Hypophthalmichthys molitrix) surimi were obtained using microwave (MW) heating (15 W/g power intensity for 20–80 s) at different levels of salt (0 g/100 g, 1 g/100 g, or 2 g/100 g). And the gel heated by MW was compared with the gel obtained by conventional water-bath heating (85 °C for 30 min). The gel strength increased when the salt level was increased. The mechanical and functional properties of non-salted, low-salt and regular-salt products were improved by MW heating for 60 s and 80 s, significantly (p < 0.05), except for the cook loss. The content of TCA-soluble peptides indicated that the MW heating inhibited the autolysis of proteins significantly (p < 0.05) during gelling. The SDS-PAGE and total content of –SH group proved that MW enhanced the cross-linking of proteins effectively through disulphide bonds and non-disulphide covalent bonds. The microstructure of the samples revealed that a fine compact network, with particles of protein aggregates, was formed in the low-salt gels (1 g/100 g) heated by MW for 60 s. All of these properties might be responsible for the formation of a superior textural low-salt gel induced by MW.     

One-step microwave-assisted headspace solid-phase microextraction for the rapid determination of synthetic polycyclic musks in oyster by gas chromatography–mass spectrometry

A rapid, simple and solvent-free procedure was developed for the determination of synthetic polycyclic musks in oyster samples by using one-step microwave-assisted headspace solid-phase microextraction (MA-HS-SPME) and gas chromatography–mass spectrometry (GC–MS). Two commonly used synthetic polycyclic musks, galaxolide (HHCB) and tonalide (AHTN), were selected in the method development and validation. The parameters (microwave irradiation power, extraction time, amount of water added, pH value and addition of NaCl) affecting the extraction efficiency of analytes from oyster slurry were systematically investigated and optimised. The best extraction conditions were achieved when the oyster tissue mixed with 10-mL deionised water (containing 3 g of NaCl in a 40-mL sample-vial) was microwave irradiated at 80 W for 5 min. The limit of quantification (LOQ) was 0.1 ng/g in 5-g of wet tissue. The good precision and accuracy of one-step MA-HS-SPME coupled with GC–MS for the determination of trace level of AHTN in oyster samples was also demonstrated.     

Changes in volatile compounds of palm sap (Arenga pinnata) during the heating process for production of palm sugar

This study was carried out to determine the changes of volatile compounds composition during palm sugar production. The samples were collected at every 30 min interval for 4 h of heating process at 150 °C from a local traditional manufacturer in Kuala Pilah, Malaysia. The analyses were performed by gas chromatography–mass spectrometry after headspace solid phase micro extraction. The results showed that N-heterocyclic chemical class possessed the highest relative percentage area (RPA) 83.69%, followed by O-heterocyclic group with RPA of 14.5%. Main volatile compounds the determined were 5-methyl-6,7-dihydro-5H-cyclopenta pyrazine and 4-hydroxy-2,5-dimethyl-3(2H) furanone which were responsible for roasty and sweet caramel-like aroma notes, respectively. The pyrazine compounds increase exponentially with heating time but furan derivatives compounds were formed at a later stage, i.e. at 180 min of heating time and above 105 °C.     

High pressure–temperature degradation kinetics of polyacetylenes in carrots

The present study assessed the effect of high pressure–temperature (HPT) processing on the levels of three polyacetylenes in carrot disks immediately after processing in comparison to sous-vide processing. The degradation kinetics of these compounds following processing at HPT was also investigated. The highest pressure–temperature combination which gave maximum retention during the time 10–30 min, for falcarinol it was 400 MPa, at 50 and 60 °C for 10 min; for falcarindiol it was 400 MPa, at 50 °C for 10 min and for falcarindiol-3-acetate was 400 MPa, at 50 °C for 10 min, respectively. Falcarindiol-3-acetate was found to be most barosensitive and falcarindiol was found to be most thermosensitive of the three polyacetylenes studied. When compared with sous-vide (SV) processed carrot disks, HPT processed samples showed higher retention of polyacetylenes. The changes in the levels of polyacetylenes during HPT treatment were adequately described by Weibull model function.     

Study of nonenzymic browning in α-amino acid and γ-aminobutyric acid/sugar model systems

The reactivities, in nonenzymic browning, of γ-aminobutyric acid (GABA), a non-protein amino acid with wide natural occurrence and potential health benefits as it occurs in foods, and of the α-l-amino acids arginine, glutamic acid, glutamine, leucine, lysine, and phenylalanine were investigated by heating equimolar mixtures of glucose and the cited amino acids at 110 °C at pH 6.0 for different times (0–4 h). Linear regression analysis indicated that the colour development in a GABA/glucose mixture was slower than that of a lysine/glucose mixture and comparable to that of a phenylalanine/glucose mixture. High-performance anion-exchange chromatography (HPAEC) with integrated pulsed amperometric detection (IPAD) showed that the decrease in GABA levels (ca. 10% after heating for 4 h) as a function of heating time was smaller than that of glucose (ca. 30% after heating for 4 h). At the same time, glucose to fructose isomerisation took place. After 20 min of heating at pH 6.0, all mixtures showed a fructose peak, the area of which increased with heating time. However, after correcting for fructose isomerisation, glucose losses were still higher than amino acid losses. In contrast to its precursor glutamic acid, GABA was stable during heating of a solution containing it alone. Heating of GABA-containing d-sugar solutions (xylose, fructose, glucose, maltose and sucrose) showed that the relative order of colour yield was pentose > hexose > disaccharides. As well as glucose to fructose isomerisation, HPAEC–IPAD allowed monitoring of the different isomerisation reactions occurring, and also disaccharide hydrolysis in the different GABA/sugar mixtures.     

Dielectric properties of ground almond shells in the development of radio frequency and microwave pasteurization

To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10–1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, but increased with increasing temperature and moisture content. The absolute value of the slopes of log–log plots between loss factor and frequency increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% w.b.) and 3–9 times larger at 90 °C than 20 °C for the highest moisture content (36% w.b.). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6–24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.     

Characteristics and antioxidative activity of Maillard reaction products from a porcine plasma protein–glucose model system as influenced by pH

Maillard reaction products (MRPs) were prepared by heating the solution containing 2% porcine plasma protein (PPP) and 2% glucose adjusted to various pHs (8, 9, 10, 11 and 12) at 100 °C for different times (0, 2, 4, 6 and 8 h). The pH of all MRPs markedly decreased within 2 h of heating time. Browning and intermediate products, as monitored by absorbance at 420 nm and absorbance at 294 nm, sharply increased within 2 h (P < 0.05). Thereafter, slight increases were observed up to 8 h. Fluorescence intensity (Ex 347 and Em 415 nm) sharply increased within the first 2 h with a subsequent decrease when heating time increased (P < 0.05). Increases in browning and formation of intermediate products were concomitant with the decreases in free amino group and reducing sugar contents. Among all MRPs tested, those derived from the PPP–glucose system at pH 12 rendered the highest browning and intermediate products. However, no differences in reducing power or DPPH radical-scavenging activity of MRPs with initial pH ranges of 10–12 were noticeable. Electrophoretic study revealed that cross-linked proteins with high molecular weight were formed in the PPP–glucose model system to a greater extent at pHs 8 and 9, than at pHs 10–12. Nevertheless, heating times had no pronounced effect on protein pattern of glycated proteins.