Effect of pH and temperature on the formation of volatile compounds in cysteine/reducing sugar/starch mixtures during extrusion cooking

Mixtures of cysteine, reducing sugar (xylose or glucose), and starch were extrusion cooked using feed pH values of 5.5, 6.5, and 7.5 and target die temperatures of 120, 150, and 180 degrees C. Volatile compounds were isolated by headspace trapping onto Tenax and analyzed by gas chromatography--mass spectrometry. Eighty and 38 compounds, respectively, were identified from extrudates prepared using glucose and xylose. Amounts of most compounds increased with temperature and pH. Aliphatic sulfur compounds, thiophenes, pyrazines, and thiazoles were the most abundant chemical classes for the glucose samples, whereas for xylose extrudates highest levels were obtained for non-sulfur-containing furans, thiophenes, sulfur-containing furans, and pyrazines. 2-Furanmethanethiol and 2-methyl-3-furanthiol were present in extrudates prepared using both sugars, but levels were higher in xylose samples. The profiles of reaction products were different from those obtained from aqueous or reduced-moisture systems based on cysteine and either glucose or ribose.     

Heterocyclic volatiles formed by heating cysteine or hydrogen sulfide with 4-hydroxy-5-methyl-3(2H)-furanone at pH 6.5

The reaction of 4-hydroxy-5-methyl-3(2H)-furanone (HMF) with cysteine or hydrogen sulfide at pH 6.5 for 60 min at 140 degrees C produced complex mixtures of volatile compounds, the majority of these containing either sulfur or nitrogen. Of the 68 compounds detected, 63 were identified, some tentatively, by GC-MS. Among the identified compounds were thiophenes (10), thiophenones (6), thienothiophenes (5), thiazoles (5), trithiolanes (4), pyrazines (6), and oxazoles (4). More compounds were produced in the reaction of HMF with cysteine (63) than were formed in the reaction with hydrogen sulfide (33). In both systems, thiophenones were major reaction products, accounting for 25-36% of the total volatiles formed. Possible reasons for the differences in the composition of the two systems are discussed. The contributions of these reactions, and their products, to the flavor of heated foods are considered.     

Effect of pH, temperature, and moisture on the formation of volatile compounds in glycine/glucose model systems

Mixtures of glycine, glucose, and starch were extrusion cooked using sodium hydroxide at 0, 3, and 6 g/L of extruder water feed, 18% moisture, and 120, 150, and 180 degrees C target die temperatures, giving extrudates with pH values of 5.6, 6.8, and 7.4. Freeze-dried equimolar solutions of glucose and glycine were heated either dry or after equilibration to approximately 13% moisture at 180 degrees C in a reaction-tube system designed to mimic the heating profile in an extruder. Volatile compounds were isolated onto Tenax and analyzed by gas chromatography-mass spectrometry. For the extrudates, total yields of volatiles increased with decreasing pH at 180 degrees C, reached a maximum at pH 6.8 at 150 degrees C, and increased with increasing pH at 120 degrees C. Amounts increased with temperature at all pH values. Pyrazines were the most abundant class for all sets of conditions (54-79% of total volatiles). Pyrroles, ketones, furans, oxazoles, and pyridines were also identified. Yields of volatiles from the reaction-tube samples increased by >60% in the moist system. Levels of individual classes also increased in the presence of moisture, except pyrazines, which decreased approximately 3.5-fold. Twenty-one of the compounds were common to the reaction-tube samples and the extrudates.     

Optimizing headspace temperature and time sampling for identification of volatile compounds in ground roasted Arabica coffee

Equilibration time and temperature were the factors studied to choose the best conditions for analyzing volatiles in roasted ground Arabica coffee by a static headspace sampling extraction method. Three temperatures of equilibration were studied: 60, 80, and 90 degrees C. A larger quantity of volatile compounds was extracted at 90 degrees C than at 80 or 60 degrees C, although the same qualitative profile was found for each. The extraction of the volatile compounds was studied at seven different equilibration times: 30, 45, 60, 80, 100, 120, and 150 min. The best time of equilibration for headspace analysis of roasted ground Arabica coffee should be selected depending on the chemical class or compound studied. One hundred and twenty-two volatile compounds were identified, including 26 furans, 20 ketones, 20 pyrazines, 9 alcohols, 9 aldehydes, 8 esters, 6 pyrroles, 6 thiophenes, 4 sulfur compounds, 3 benzenic compounds, 2 phenolic compounds, 2 pyridines, 2 thiazoles, 1 oxazole, 1 lactone, 1 alkane, 1 alkene, and 1 acid.     

Effects of Calcium Hydroxide and Processing Conditions on Corn Meal Extrudates

The effects of added calcium hydroxide (0.0, 0.15, 0.25, and 0.35%) and processing conditions, feed moisture content (mc) (16, 18, and 20%) and barrel temperature (130 and 150°C) on characteristics of corn meal extrudates were studied. Extruder screw speed was maintained at 130 rpm. Corn meal was extruded with a single-screw extruder (Brabender model GNF 1014/2) with a screw compression ratio of 3:1. The highest values (P < 0.05) for radial expansion and the lowest values for density and breaking force of extrudates were found for the treatment with 0.00% calcium hydroxide extruded at 16% feed mc and 130°C barrel temperature. This treatment was statistically different from the other treatments. Best values for radial expansion of samples extruded with added calcium hydroxide were for the samples with 0.15% calcium hydroxide at 18% feed mc and 130°C barrel temperature, followed by the sample with 0.35% calcium hydroxide at 16% feed mc and 130°C barrel temperature. Water absorption index and water solubility index were affected by calcium hydroxide and extrusion conditions evaluated. Extrudates had large numbers of flattened and sheared granules. Increases in calcium hydroxide increased extrudate yellowness. The combined action of calcium hydroxide and extrusion conditions completely modified the organized structure of the starch and suggest the formation of a starch-calcium complex (crystalline region). The texture of the extruded products was crispy after puffing.     

Effects of plant sulfur nutrition on acrylamide and aroma compounds in cooked wheat

Wheat flour from plants deficient in sulfur has been shown to contain substantially higher levels of free amino acids, particularly asparagine and glutamine, than flour from wheat grown where sulfur nutrition was sufficient. Elevated levels of asparagine resulted in acrylamide levels up to 6 times higher in sulfur-deprived wheat flour, compared with sulfur-sufficient wheat flour, for three varieties of winter wheat. The volatile compounds from flour, heated at 180 degrees C for 20 min, have been compared for these three varieties of wheat grown with and without sulfur fertilizer. Approximately 50 compounds were quantified in the headspace extracts of the heated flour; over 30 compounds were affected by sulfur fertilization, and 15 compounds were affected by variety. Unsaturated aldehydes formed from aldol condensations, Strecker aldehydes, alkylpyrazines, and low molecular weight alkylfurans were found at higher concentrations in the sulfur-deficient flour, whereas low molecular weight pyrroles and thiophenes and sugar breakdown products were found at higher concentrations in the sulfur-sufficient flour. The reasons for these differences and the relationship between acrylamide formation and aroma volatile formation are discussed.     

Changes in volatile compounds and overall aroma profile during storage of coffee brews at 4 and 25 degrees C

In this work, the chemical changes occurring in the volatile fraction of Arabica coffee brews during storage at 4 and 25 degrees C for 30 days have been characterized for the first time by means of HS-GC-MS. A total of 47 compounds were identified and quantified: 2 sulfur compounds, 7 aldehydes, 3 esters, 15 furans, 5 ketones, 1 alcohol, 2 thiophenes, 4 pyrroles, 1 pyridine, 5 pyrazines, 1 alkene, and 1 acid. No new volatile compounds were detected at the end of the storage time. The changes observed are, in general, slower and less pronounced at refrigeration temperature. Storage also affects the sensory characteristics of the stored coffee brews, which lose part of their aroma intensity and freshness, acquiring some nondesirable notes such as rancid aroma, mainly during storage at 25 degrees C. Furthermore, seven aroma indices have been proposed as indicators of coffee brew staling, which show a good correlation with some sensory attributes, not only for aroma but also overall sensory quality. Consequently, they could be considered useful to monitor both the "age" and the sensory quality of stored coffee brews.     

Effect of extrusion temperature on the solubility and molecular weight of lentil bean flour proteins containing low cysteine residues

Lentil flour was extruded at die temperatures of 135, 160, and 175 degrees C. The soluble protein content in the extrudates decreased by 40.1% in the extracting buffer (1% sodium dodecyl sulfate in 50 mM sodium phosphate buffer, pH 6.9) as the extrusion die temperature was increased to 175 degrees C. The most insoluble proteins in the extrudates extruded at die temperatures of up to 175 degrees C could be resolubilized by using sonication. The total disulfide content and sulfhydryl content in the extrudates decreased. The SDS-PAGEs showed that the molecular weight distribution of proteins in the lentil flour changed little before and after extrusion as well as during reduction. The results from this study show that the extrusion temperature had less effect on the solubility and molecular weight of the lentil proteins, which contain a lower level of cysteine residues than wheat proteins.     

Waxy Soft White Wheat: Extrusion Characteristics and Thermal and Rheological Properties

Waxy wheat flour was analyzed for its thermal and rheological properties and was extruded to evaluate its potential for extruded products. Normal soft white wheat flour was analyzed with the same methods and same extrusion conditions to directly compare differences between the two types of flour. Through DSC analysis, waxy wheat flour was found to have a higher gelatinization peak temperature of 66.4°C than normal wheat at 64.0°C, although the transition required 2.00 J/g less energy. Rapid visco‐analysis indicated that the waxy wheat flour pasted much more quickly and at lower temperatures than the normal wheat flour. Preliminary extrusion experiments were conducted to determine the optimal screw profile for waxy wheat with respect to maximum radial expansion. The optimum screw profile was used for extrusion trials with varying flour moisture (15–25% wb) and extruder screw speed (200–400 rpm) while monitoring process conditions including back pressure and specific mechanical energy. Physical properties of the extrudates were then studied. The radial expansion ratios of the waxy wheat extrudates exceeded those of the normal wheat extrudates by nearly twice as much, and it was observed that the waxy wheat flour took less energy in the form of fewer shear screw elements to expand. The waxy wheat extrudates also exhibited significantly higher water solubility and less water absorption than the normal wheat extrudates owing to solubilizing of the extrudates. The results of our study indicate that waxy wheat flour may be a viable ingredient for creating direct expanded products with less energy.     

Investigation of the reaction between 4-hydroxy-5-methyl-3(2H)-furanone and cysteine or hydrogen sulfide at pH 4.5

Reaction of 4-hydroxy-5-methyl-3(2H)-furanone (HMF) with cysteine or hydrogen sulfide at pH 4.5 for 60 min at 140 degrees C produced complex mixtures of volatile compounds, the majority of which contained sulfur. Sixty-nine compounds were identified, some tentatively, by GC/MS. These included disulfides (26), thiols (7), dithiolanones (6), thiophenones (4), dithianones (3), and thienothiophenes (6). The main non-sulfur compounds were 2, 3-pentanedione, 2,4-pentanedione, and 3,4-hexanedione. Both systems produced approximately the same total quantity of volatile compounds, but the reaction containing cysteine gave the larger number of individual compounds, with thiols quantitatively the dominant components. By comparison, the major products formed in the reaction with hydrogen sulfide were the dithiolanones. Reaction pathways are presented for the major products and, where applicable, possible reasons for the differences in composition of the two systems are discussed. The contribution of these reactions, and their products, to the flavor of roasted foods is considered.     

Thermal degradation studies of glucose/glycine melanoidins

Nondialyzable and water-insoluble melanoidins, isolated from a glucose/glycine model reaction mixture, which was prepared in a standardized way according to the guidelines of the COST Action 919, were heated at different temperatures ranging from 100 to 300 degrees C. Among the volatile compounds, which were analyzed by SPME and GC-MS, pyrazines, pyridines, pyrroles, and furans were detected. In general, total amounts of volatile compounds increased with the temperature. When water-insoluble melanoidins were heated, especially at higher temperatures, this resulted in a higher diversity of isolated compounds. For furans, pyrroles, pyrazines, and carbonyl compounds a maximum was observed in the case of high molecular weight melanoidins around 200-220 degrees C. Pyridines and total oxazoles, however, were generated in higher yields with increasing temperatures. These results demonstrate the possibility of producing some flavor-significant volatiles from heated standard melanoidins at temperatures relevant to food preparation and contribute to the flavor aspects originating from melanoidins.     

Sensory and instrumental analyses of volatiles generated during the extrusion cooking of oat flours

Three batches of oats were extruded under four combinations of process temperature (150 or 180 degrees C) and process moisture (14. 5 and 18%). Two of the extrudates were evaluated by a sensory panel, and three were analyzed by GC-MS. Maillard reaction products, such as pyrazines, pyrroles, furans, and sulfur-containing compounds, were found in the most severely processed extrudates (high-temperature, low-moisture). These extrudates were also described by the assessors as having toasted cereal attributes. Lipid degradation products, such as alkanals, 2-alkenals, and 2, 4-alkadienals, were found at much higher levels in the extrudates of the oat flour that had been debranned. It contained lower protein and fiber levels than the others and showed increased lipase activity. Extrudates from these samples also had significantly lower levels of Maillard reaction products that correlated, in the sensory analysis, with terms such as stale oil and oatmeal. Linoleic acid was added to a fourth oat flour to simulate the result of increased lipase activity, and GC-MS analysis showed both an increase in lipid degradation products and a decrease in Maillard reaction products.     

Effect of extrusion parameters on conjugated linoleic acids of corn extrudates

The effects of extrusion temperature, 150-190 degrees C, and torque, 50-70%, on the content and configuration of conjugated linoleic acids (CLA) in corn extrudates were analyzed by GC and HPLC. At a temperature of 150 degrees C, CLA content increased from 1.2 mg/g of oil in feed to 7.8 mg/g of oil in corn extrudates. A decrease in total CLA (P < 0.05) was obtained when the product temperature was further increased to 190 degrees C. Alteration of CLA geometrical configuration was observed at higher extrusion temperatures. trans,trans-CLA significantly increased (P < 0.05) from 10.2% in feed to 11.9% of CLA at the extrusion condition of 190 degrees C and 70% torque. The highest expansion of extrudates was found at the product temperature of 150 degrees C and 70% torque. This extrusion condition also gave the maximum total CLA content and minimum trans,trans-CLA formation.     

Immunochemical and Electrophoretic Analysis of the Modification of Wheat Proteins in Extruded Flour Products

Antibodies specific for wheat proteins were used to identify protein fractions modified during extrusion of Hard Red Spring wheat flour (14% protein) under four different combinations of extrusion conditions (18 and 24% feed moisture and 145 and 175°C die temperature). Antibody binding was assessed on immunoblots of proteins extracted from flour and extrudates separated by SDS‐PAGE. Antibodies to high molecular weight glutenin subunits (HMW‐GS) and to B‐group low molecular weight glutenin subunits (LMW‐GS) recognized intact subunits from both flour and extrudates. Antibodies to C‐group LMW‐GS had diminished binding to extruded proteins. Glutenin‐specific antibodies also recognized protein in the extrudates migrating as a smear at molecular weights higher than intact subunits, indicating cross‐linked proteins. Antibodies recognized albumins or globulins in flour but not in extrudates, evidence that these fractions undergo significant modification during extrusion. Acid‐PAGE and antibody reaction of gliadins extracted in 1M urea and in 70% ethanol revealed total loss of cysteine‐containing α, β, γ‐gliadins but no obvious effects on sulfur‐poor ω‐gliadins, suggesting gliadin modification involves replacing intramolecular disulfides with intermolecular disulfide cross‐links. Identifying protein fractions modified during different extrusion conditions may provide new options for tailoring extrusion to achieve specific textural characteristics.     

Volatile compounds from potato-like model systems

The volatile reaction products of aqueous mixtures comprising combinations of methionine, glucose, linoleic acid, and starch heated in a modified Likens-Nickerson apparatus were extracted and analyzed by gas chromatography-mass spectrometry (GC-MS). The majority of volatile compounds were formed from linoleic acid degradation, hexanal, 2,4-decadienal, and 2-pentylfuran being identified in the greatest amounts. Dimethyl disulfide and dimethyl trisulfide were detected in every system containing methionine. 3-(Methylthio)propanal (methional) and other sulfur compounds were detected when methionine was heated with another precursor. No binding of volatile compounds to starch was observed; rather, starch appeared to act as an additional source of reactive carbohydrate. Almost all the components identified have been identified among the aroma components of cooked potato. No pyrazines, pyridines, or thiazoles were identified, probably due to the relatively low temperature/high moisture conditions.     

Extrusion of Wheat Gluten Plasticized with Glycerol: Influence of Process Conditions on Flow Behavior,Rheological Properties,and Molecular Size Distribution

Gluten-glycerol dough was extruded under a variety of processing conditions using a corotating self-wiping twin-screw extruder. Influence of feed rate, screw speed, and barrel temperature on processing parameters (die pressure, product temperature, residence time, specific energy) were examined. Use of flow modeling was successful for describing the evolution of the main flow parameters during processing. Rheological properties of extruded samples exhibited network-like behavior and were characterized and modeled by Cole-Cole distributions. Changes in molecular sizes of proteins during extrusion were measured by chromatography and appeared to be correlated to molecular size between network strands, as derived from the rheological properties of the materials obtained. Depending on operating conditions, extrudates presented very different surface aspects, ranging from very smooth-surfaced extrudates with high swell to completely broken extrudates. The results indicated that extrudate breakup was caused by increasing network density, and some gliadins may have acted as cross-linking agents. Increasing network density resulted in decreasing mobility of polymeric chains, and “protein melt” may no longer have been able to support the strain experienced during extrusion through the die. Increasing network density was reflected in increased plateau modulus and molecular size of protein aggregates. Increasing network structure appeared to be induced by the severity of the thermomechanical treatment, as indicated by specific mechanical energy input and maximum temperature reached.     

Physical Characteristics,Nutritional Quality,and Antioxidant Potential of Extrudates Produced with Polished Rice and Whole Red Bean Flours

In Brazil, rice (Oryza sativa L.) and beans (Phaseolus vulgaris L.) are the basis of the population's diet, and their consumption together is a good strategy to improve protein biological value. The aim of this study was to produce extruded products with whole red bean (WRBF) and polished rice (PRF) flours and to evaluate the effects of extrusion temperature (T) and feed moisture content (FM) on technological properties and total phenolic compounds content. The extrudates were elaborated in a twin‐screw extruder following a 2² central composite rotatable design with FM (15–23%) and T (120–160°C) as independent variables. WRBF and PRF were used at a 1:3 ratio. Amino acid content and profile were evaluated in the optimum extrudate (produced at FM = 19% and T = 140°C). The total phenolic content identified in extruded products was provided by the red bean seed coat, and its quantification suggested the release of bound phenolics with the extrusion process (not temperature dependent). The extrusion of PRF and WRBF, in combination, produced extruded products of high protein quality, being complete in essential amino acids for the diets of people at least 48 months old. The results indicate that legume flours such as WRBF incorporated into rice flour can cause a positive impact on technological, nutritional, and functional quality of extrudates.     

Effects of carnosine on volatile generation from Maillard reaction of ribose and cysteine

Carnosine occurs naturally in meat and meat products in significant quantity, and it possesses strong antioxidant activity that inhibits lipid oxidation and enhances shelf life. In this study, the effects of carnosine on thermal flavor generation were investigated using the model system of cysteine and ribose, which was heated to the roasting temperature of 180 degrees C for 2 h at pH 5 and pH 8.5. The results indicated that carnosine affected volatile formation in a complex manner. Volatiles identified from the liquid phase of the reaction systems of ribose and cysteine showed that the sulfur-containing compounds such as thiophenes, thiazoles, and polysulfides were the most abundant compounds. The addition of carnosine into the reaction mixtures in general caused a reduction in contents of thiophenes and some important meaty flavor compounds such as 2-methyl-3-furanthiol, 2-furfurylthiol, and their associated dimers. On the other hand, it facilitated the generation of several important nitrogen-containing volatiles such as pyrazine, methylpyrazine, 2,6-dimethylpyrazine, and other alkyl pyrazines and thiazoles, which are known to elicit roasty and nutty flavor notes. The results suggested that carnosine acts as a nitrogenous source to facilitate the formation of nitrogen-containing compounds, possibly by degradation to form ammonia.     

Effects of Particle Size and Starch Damage of Flour and Alkaline Reagent on Yellow Alkaline Noodle Characteristics

A hard white spring wheat was milled to yield three patent flours with different starch damage levels by manipulating reduction grinding conditions, and each flour was sieved to give three different particle sizes (85–110, 110–132, 132–183 μm). Raw alkaline noodles were prepared using either 1% w/w kansui (sodium and potassium carbonates in 9:1 ratio) or 1% w/w sodium hydroxide. Noodles prepared with sodium hydroxide were significantly brighter, less red, and more yellow than those made with kansui. Differences in noodle color among flour treatments were evident but were attributable to differences in flour refinement rather to than particle size or starch damage. Noodles were rested for 1 hr after processing before cooking. Alkaline reagent was the main factor associated with cooking loss, being ≈50% greater for sodium hydroxide noodles because of higher pH compared with kansui noodles. Cooked sodium hydroxide noodles were thicker than kansui noodles, and cooked strands for both noodle types became thicker as starch damage increased and as particle size became coarser. Instrumental assessment of cooked noodle texture showed that maximum cutting stress (MCS), resistance to compression (RTC), recovery (REC), stress relaxation time (SRT), chewiness (CHE), and springiness (SPR) were influenced by the type of alkaline reagent. Flour particle size and starch damage also influenced noodle texture but the magnitude of the effects and the trends were dependent on alkaline reagent. MCS of kansui noodles was much greater than for sodium hydroxide noodles. MCS of kansui noodles increased as starch damage increased but, in contrast, MCS of sodium hydroxide noodles decreased with increasing starch damage. REC of kansui noodles increased with increasing starch damage and decreased with larger particle size, whereas for sodium hydroxide noodles REC decreased with increasing starch damage and declined dramatically with larger particle size. Kansui noodles exhibited significantly shorter SRT than sodium hydroxide noodles. SRT of kansui noodles was only moderately affected by starch damage and particle size, whereas for sodium hydroxide noodles, SRT became much shorter as flour became coarser and starch damage became higher. CHE of kansui noodles was greater than for sodium hydroxide noodles. CHE of kansui noodles increased as starch damage increased. In contrast, CHE of sodium hydroxide noodles decreased as starch damage increased and also decreased as flour became coarser. SPR of both noodle types decreased as flour became coarser and starch damage became greater. On the basis of these experiments, flour of smaller particle size is an asset to the cooking quality of sodium hydroxide noodles, but high starch damage is to be avoided. For kansui noodles, the impact of flour particle size on cooked noodle texture was less evident and low starch damage, rather than high starch damage, was an asset.