Enolization and racemization reactions of glucose and fructose on heating with amino-acid enantiomers and the formation of melanoidins as a result of the Maillard reaction

Development and standardization of spectrophotometric assay for quantification of thermal hydrolysis-origin melanoidins and its implication in antioxidant activity evaluation

Melanoidins are brown recalcitrant polymers originating from the thermal hydrolysis pretreatment (THP) of organic solid waste (OSW). Owing to their various formation pathways and complex structures, there is currently no reliable method to quantify melanoidins. In this study, a spectrophotometric method was developed to determine melanoidins concentration in different OSW. Three typical model Maillard reaction systems (glucose-glycine, glucose/fructose-20 amino acids, and dextran-bovine serum albumin) were used to acquire the characteristic peaks and establish standard curves. The results showed that a standard curve using glucose/fructose-20 amino acids model melanoidins at 280 nm was the optimal quantification method, because it had the best correlation with the physicochemical indicators of melanoidins and semi-quantification results calculated by excitation-emission matrix fluorescence. In addition, the applicability of the proposed method was evaluated using multiple real melanoidins samples extracted from thermally pretreated OSW under different THP conditions and food-derived melanoidins as well, demonstrating its validity and advantages. This study is the first to provide a simple, effective, and accurate method for quantifying THP-origin melanoidins from different sources. Remarkably, as a specific and important application scenario, the proposed quantification method was employed to investigate the concentration dependence of melanoidins antioxidation in thermally pretreated OSW.

Characterization of melanoidins in thermal hydrolysis sludge and effects on dewatering performance

Thermal hydrolysis pretreatment (THP) of sludge can form the refractory brown melanoidins due to the occurrence of the Maillard reaction, which adversely involves the subsequent sludge anaerobic digestion (AD) process. However, details of the generation pattern of melanoidins and how they affect the sludge dewaterability remain largely unknown. This work aims to determine an approach to characterize and quantify the melanoidins created by THP of sludge. On this basis, the effect of melanoidins on sludge dewatering performance was revealed by adding synthetic melanoidins to the mixed sludge. Experimental results showed that three-dimensional fluorescence-region integration (3DEEM-FRI) could effectively distinguish melanoidins from other organic substances and achieve semi-quantitative characterization in sludge. The melanoidins significantly deteriorated the sludge dewaterability, and the lowest solids content of the filter cake (TS) was only 17.78% at the addition of 480 mg (g TS)-1, which was a drop of about 20% compared to the control group. The mechanism investigations indicated that the internal structure of sludge becoming particularly complicated and the opportunities for molecules to collide with each other enlarged because of the contribution of melanoidins, resulting in the increment of the sludge apparent viscosity and consistency coefficient (k), a decline of the flow behavior index (n) and a weakening of flowability. Melanoidins could capture massive water molecules and carry negative charges with the decrease of sludge particle size and zeta potential value, which enhanced the electrostatic repulsion between sludge particles and abated the flocculation ability, thus further aggravating the sludge dewatering performance.

Conversion of Enantiomers during the Separation of Acetoin from Fermentation Broth

Acetoin (AC) is an important platform compound with two enantiomers (R)-AC and (S)-AC. Due to its unique spatial structure, optically pure AC has particularly high application in asymmetric synthesis. Highly optically pure AC could be produced from glucose using biofermentation technology. In this paper, we have observed that the recovered AC product from the fermentation broth containing (R)-AC was a racemic mixture. The changes of the enantiomeric excess (e.e.) of (R)-AC enantiomers in the feed solution during the recovery process were then investigated, confirming that the racemization occurs during solvent distillation. Further studies showed that high temperature is the main factor affecting the conversion of the two enantiomers, while low temperature significantly prevents this conversion reaction. Therefore, we optimized the solvent recovery process and used vacuum distillation to reduce the distillation process temperature, which effectively prevented the racemization: obtains AC products with more than 98% purity and successfully maintained the proportion of (R)-AC above 96%. To our knowledge, this is the first report on the factors affecting the enantiomeric purity in the downstream extraction process of AC production by fermentation.

Evaluation of the Effect of a Combined Chemical and Thermal Modification of Wood though the Use of Bicine and Tricine

The effects of thermal modification of wood have been well established, particularly in terms of reductions in mechanical performance. In recent years, there has been an increase in studies related to the Maillard reaction. More commonly associated with food chemistry, it involves the reaction of amines and reducing sugars during cooking procedures. This study has attempted to combine the use of amines and thermal modification, with subsequent properties investigated for the treatment of spruce (Picea abies (L.) H. Karst) and beech (Fagus sylvatica L.). In this initial study, the combined effects of chemical treatments by tricine and bicine were investigated with thermal modification. Along with some preliminary data on mechanical properties, the modifications which appeared in the wood structure were evaluated by infrared spectroscopy and biological studies according to EN113 and EN117 methodologies. The hierarchal study interpretation of FTIR suggested interactions between the bicine or tricine and the wood, which was partly supported by the analysis of volatile organic compounds (VOC), though other tests were not as conclusive. The potential of the method warrants further consideration, which will be described.

Formation of Taste-Active Pyridinium Betaine Derivatives Is Promoted in Thermally Treated Oil-in-Water Emulsions and Alkaline pH

The oil-water interface can be used as an efficient reaction controller in foods by carrying specific reactants and products in either the hydrophobic or hydrophilic phase. The formation of the taste-active compounds N-(1-carboxyethyl)-6-hydroxymethyl-pyridinium-3-ol inner salt (alapyridaine) and 1-(1-carboxyethyl)-3-hydroxy-pyridinium inner salt is influenced by the presence of a dispersed saturated triglyceride oil phase and by the pH of the aqueous phase. At pH 6.5, the formation of both betaines was 1.24 and 6 times higher in emulsions than in aqueous solution after 4 min at 140 °C. In alkaline emulsions (pH = 9.5, 4 min), the concentrations of alapyridaine and 1-(1-carboxyethyl)-3-hydroxy-pyridinium ion were 6.2 and 3.8 times higher, respectively, than in unbuffered emulsions as a result of the interaction between the polar head group of the surfactant and pyridinium rings. Here, we reported for the first time the effects of multiphase systems on the formation of nonvolatile, taste-active end products.

Recalcitrant dissolved organic nitrogen formation in thermal hydrolysis pretreatment of municipal sludge

Thermal hydrolysis pretreatment (THP) has been considered as an advanced approach to enhance the performance of anaerobic digestion treating municipal sludge. However, several drawbacks were also identified with THP including the formation of brown and ultraviolet-quenching compounds that contain recalcitrant dissolved organic nitrogen (rDON). Melanoidins produced from the Maillard reaction between reducing sugar and amino group have been regarded as a representative of such compounds. This review presented the state-of-the-art understanding of the mechanism of melanoidin formation derived from the research of sludge THP, food processing, and model Maillard reaction systems. Special attentions were paid to factors affecting melanoidin formation and their implications to the control of rDON in the sludge THP process. These factors include reactant availability, heating temperature and time, pH, and the presence of metallic ions. It was concluded that efforts need to be focused on elucidating the extent of the Maillard reaction in sludge THP. This paper aims to provide a mechanistic recommendation on the research and control of the THP-resulted rDON in municipal wastewater treatment plants.

Characterization, Variables, and Antioxidant Activity of the Maillard Reaction in a Fructose⁻Histidine Model System

Fructose and its polysaccharides are widely found in fruits and vegetables, with the Maillard reaction of fructose affecting food quality. This study aimed to investigate the Maillard reaction of fructose using a fructose–histidine model system. The reaction process was characterized using fluorescence spectroscopy and ultraviolet spectroscopy. The effects of temperature, initial reactant concentration, initial fructose concentration, initial histidine concentration, and initial pH value on the different stages of the Maillard reaction were studied. Reactant reduction, ultraviolet and fluorescence spectra, acetic acid content, 5-hydroxymethylfurfural (5-HMF) content, and browning intensity were evaluated. The results showed that increasing the temperature and reactant concentration promoted the condensation reaction of fructose and amino acid in the early stage, the formation of intermediate products with ultraviolet absorption and fluorescence in the intermediate stage, and the formation of pigment in the final stage. The 5-HMF concentration decreased with increasing histidine concentration and initial pH value. Changes in the shape of ultraviolet and fluorescence spectra showed that the initial pH value affected not only the reaction rate, but also the intermediate product types. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging rate of the Maillard reaction products increased with increasing temperature, reactant concentration, and initial pH value.

Kinetic Study on Peptide-Bound Pyrraline Formation and Elimination in the Maillard Reaction Using Single- and Multiple-Response Models

Pyrraline, an advanced glycation end product (AGE), is related to some chronic diseases and can be employed as an indicator for heat damage in food processing. In this study, the impact of changing the reactant concentration and ratio on the kinetic parameters describing peptide-bound pyrraline (pep-pyr) formation and elimination was evaluated in the Lys-Gly/glucose model systems, with microwave heating treatment ranging from 120 to 200 °C. The maximum pep-pyr concentration increased as follows: 200 °C ˂ 180 °C ˂ 160 °C ˂ 120 °C ˂ 140 °C. First, the pep-pyr formation and elimination was modeled by using a single-response modelling. The formation rate constant (k_F ) of pep-pyr was independent of the initial concentration of the reactants and ratios. However, the elimination rate constant of pep-pyr (k_E ) increased with increasing reactant concentrations. Second, a multiresponse modelling was performed to illustrate the pathways of pep-pyr formation and elimination. Two adapted models can fit to the experimental data with the goodness-of-fit ranging from 0.663 to 0.920. Glucose-to-fructose isomerization rather than glucose-to-mannose epimerization was detected in an equimolar model system and the model system with an excess of any of the reactants. The caramelization reaction was negligible in the equimolar systems and the model systems with an excess of peptide. The reaction rate constant of glucose-to-fructose isomerization was independent of the initial reactant ratios. It was more difficult for pep-pyr elimination in the model system with an excess of peptide than that in the other 2 model systems (the equimolar system and the system with an excess of glucose), whereas a reverse result in pep-pyr formation was obtained.

Formation and reduction of 5-hydroxymethylfurfural at frying temperature in model system as a function of amino acid and sugar composition

5-Hydroxymethylfurfural (HMF) is formed during heat treatment of carbohydrate-containing foods, especially in a deep-fat frying process. This study aimed to investigate the effect of amino acids on the formation and reduction of HMF from glucose, fructose and sucrose at frying temperature in model systems containing binary mixtures of an amino acid and a sugar in equal concentrations (0.3M). The results revealed that the formation of HMF from sugars accelerated in the presence of acidic amino acids (i.e. glutamic and aspartic acids). Conversely, the presence of basic amino acids (i.e. lysine, arginine and histidine) led to reduced concentrations of HMF to non-detectable levels in model systems. The results showed that both pH and heating time significantly affected the formation of HMF from fructose in the presence of glutamic acid. In this regard, a higher amount of HMF was formed at lower pH.

Quality assessment of butter cookies applying multispectral imaging

A method for characterization of butter cookie quality by assessing the surface browning and water content using multispectral images is presented. Based on evaluations of the browning of butter cookies, cookies were manually divided into groups. From this categorization, reference values were calculated for a statistical prediction model correlating multispectral images with a browning score. The browning score is calculated as a function of oven temperature and baking time. It is presented as a quadratic response surface. The investigated process window was the intervals 4-16 min and 160-200°C in a forced convection electrically heated oven. In addition to the browning score, a model for predicting the average water content based on the same images is presented. This shows how multispectral images of butter cookies may be used for the assessment of different quality parameters. Statistical analysis showed that the most significant wavelengths for browning predictions were in the interval 400-700 nm and the wavelengths significant for water prediction were primarily located in the near-infrared spectrum. The water prediction model was found to correctly estimate the average water content with an absolute error of 0.22%. From the images it was also possible to follow the browning and drying propagation from the cookie edge toward the center.

Volatile profile of cashews (Anacardium occidentale L.) from different geographical origins during roasting

Volatile compounds were quantified in the headspace of Indian, Vietnamese, and Brazilian cashews, both raw and during roasting by selected ion flow tube-mass spectrometry. The optimum roasting times based on color measurements were also determined. Raw cashews were oil roasted for 3 to 9 min at 143 °C and color and volatiles measured. An excellent correlation, following a pseudo 1st-order reaction, was found between L* value and roasting time; darkness increases as roasting time increases. The optimum roasting time was 6, 8, and 9 min for Vietnamese, Indian, and Brazilian cashews, respectively. Raw cashews had lower concentrations of volatiles than roasted cashews. Most volatiles significantly increased in concentration during roasting of Brazilian, Indian, and Vietnamese cashews. Only a few volatiles significantly decreased during roasting. Ethanol and 1-heptene significantly decreased during roasting in Brazilian cashews and toluene decreased in Vietnamese cashews. Brazilian cashews had significantly higher levels of most volatiles than Indian and Vietnamese cashews. Most volatile levels in Indian and Vietnamese cashews were not significantly different. Of the volatiles, Strecker aldehydes, including methylbutanal, 2-methylpropanal, and acetaldehyde, were at the highest concentration in roasted cashews. The Maillard reaction contributed to the formation of most of the volatiles in cashews from the 3 countries. There was also degradation of sugars to form furan-type compounds and oxidation of lipids to form alkanals such as hexanal.

PRACTICAL APPLICATION

The volatile profile during roasting of cashews can be used to determine the best roasting time for each type of cashew. The rate of color development and the production of volatiles differ for the cashews from the 3 geographical locations.