Kinetics and mechanisms of the reaction of chromium(III) and 2,4-hexanedione in aqueous solution

Producción CientíficaThe kinetics and mechanism of the reaction between Cr(III) and 2,4-hexanedione in aqueous solution have been studied. Complexation occurs by coordination of both oxygen atoms of the ligand to the chromium(III) center, with concomitant loss of a proton yielding 1:1 complex of the type [Cr(CH3COCHCOC2H5)(H2O)4]2+. When the metal ion is in pseudo first-order excess the concentration dependence of the observed rate constants agrees with a mechanism involving reversible reaction between the enol tautomer of 2,4-hexanedione and the metallic species Cr(H2O)63+ and Cr(H2O)5OH2+, with rate constants of 8.0 × 10−4 and 3.5 × 10−2 dm3 mol−1 s−1, respectively. Keywords: chromium(III), β-diketones, β-diketonates, kinetics, complexation.La cinétique et le mécanisme de la réaction de la 2,4-hexanedione avec le Cr(III) en solution aqueuse ont été étudiés. La complexation se produit par la coordination de tous deux atomes d'oxygène du ligand avec le chrome(III), avec une perte simultanée d'un proton pour produire le complexe 1:1 du type [Cr(CH3COCHCOC2H5)(H2O)4]2+. Quand l'ion métallique se trouve en pseudo premier-ordre excès, l'influence de la concentration sur la constante de vitesse observée s'accorde avec une mécanisme réversible selon lequel le tautomére énol réactionne avec les espèces Cr(H2O)63+ and Cr(H2O)5OH2+. Les constantes de vitesse qui correspondent à ces réactions ont pour valeur 8,0 × 10−4et3,5 × 10−2dm3 mol−1 s−1, respectivement. Mots clés : chromo(III), β-dicétones, β-dicétonates, cinétique, complexation

Phenols and Melanoidins as Natural Antioxidants in Beer. Structure, Reactivity and Antioxidant Activity

Producción CientíficaBeer is one of the most consumed drinks around the world, containing a variety of compounds that offer both appreciated sensorial characteristics and health advantages. Important healthy compounds in beer are those with antioxidant properties that attenuate the content of free radicals produced as by-products in the human metabolism, exerting an appreciable effect against cancers or cardiovascular diseases. This work details a study of antioxidant compounds present in beer, focusing on the two main groups: phenols (including polyphenolic forms) and melanoidins, formed specifically during brewing as Maillard products. The fundaments of the most important methods to evaluate beer antioxidant activity, the main antioxidant compounds present in beer—especially those with healthy properties—and the new trends to increase beer antioxidant activity are also discussed

Thermal deterioration of the trans-/cis-ratio of bittering agents of commercial beers

Producción CientíficaThe use of reduced isomerised hop extracts to achieve both bitterness and light stability became very popular during the last decade. Changes during storage in four hop extracts and seven commercial lager beers are studied using HPLC and spectrophotometric techniques. The degradation of the iso-α-acids and tetrahydro iso-α-acids as a function of time is represented by the ratio, in percentage, of the sum of trans-isomer concentration to the sum of the cis-isomer concentration (T/C). The results provided conclusive evidence that the gradual decreased of bitterness intensity in beer was due to the degradation of iso-α-acids, notably to the instability of the trans-iso-α-acids. When the beers were stored for 14 days at 45◦C the decrease of the T/C average varied from 8.6 to 14.0%, except in lemon beer, in which case, T/C decreased 36.9%. The results allowed us to infer that in open storage and/or warm conditions the deterioration of beer was critical above 35◦C. On the other hand, the results showed that tetrahydro iso-α-acids remained unaltered

A novel correlation for rapid lactose determination in milk by a cryoscopic technique

Producción CientíficaResidual lactose in special milk was systematically determined for people with lactose intolerance by means of a rapid on-line measurement of the cryoscopic point. A proposed cryoscopic procedure was compared to 2 conventional yet highly laborious methods: the enzymatic procedure with spectrophotometric control and the polarimetric method. Several experiments with different mixtures of both semi-skimmed and low-lactose milk were performed. A lineal relationship was found between lactose concentration and freezing point, the analytical equation for which shows a close relationship regarding the 3 methods used. The advantages of the cryoscopic procedure include speed in obtaining results and operational simplicity at a low cost, better monitoring of enzymatic hydrolysis kinetics, and greater control over the production process for delactosed milk. The equation obtained also enables prediction of the lactose percentage in commercial milk by a simple measurement of freezing point

Improving industrial full-scale production of baker's yeast by optimizing aeration control

Producción CientíficaThis work analyzes the control of optimum dissolved oxygen of an industrial fed-batch procedure in which baker's yeast (Saccharomyces cerevisiae) is grown under aerobic conditions. Sugar oxidative metabolism was controlled by monitoring aeration, molasses flows, and yeast concentration in the propagator along the later stage of the propagation, and keeping pH and temperature under controlled conditions. A large number of fed-batch growth experiments were performed in the tank for a period of 16 h, for each of the 3 manufactured commercial products. For optimization and control of cultivations, the growth and metabolite formation were quantified through measurement of specific growth and ethanol concentration. Data were adjusted to a model of multiple lineal regression, and correlations representing dissolved oxygen as a function of aeration, molasses, yeast concentration in the broth, temperature, and pH were obtained. The actual influence of each variable was consistent with the mathematical model, further justified by significant levels of each variable, and optimum aeration profile during the yeast propagation was found

Simulation and flavor compound analysis of dealcoholized beer via one-step vacuum distillation

Producción CientíficaThe coupled operation of vacuum distillation process to produce alcohol free beer at laboratory scale and Aspen HYSYS simulation software was studied to define the chemical changes during the dealcoholization process in the aroma profiles of 2 different lager beers. At the lab-scale process, 2 different parameters were chosen to dealcoholize beer samples, 102 mbar at 50 °C and 200 mbar at 67 °C. Samples taken at different steps of the process were analyzed by HS-SPME–GC–MS focusing on the concentration of 7 flavor compounds, 5 alcohols and 2 esters. For simulation process, the EoS parameters of the Wilson-2 property package were adjusted to the experimental data and one more pressure was tested (60 mbar). Simulation methods represent a viable alternative to predict results of the volatile compound composition of a final dealcoholized beer.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA330U13

Brewing with Starchy Adjuncts: Its Influence on the Sensory and Nutritional Properties of Beer

Producción CientíficaIn brewing, the use of cereals (wheat, barley, maize, rice, sorghum, oats, rye or millet), pseudo-cereals (buckwheat, quinoa or amaranth) and tubers (sweet potato), as starch adjuncts, is being promoted for the production of a variety of high-quality beers, from sensory and nutritional points of view. The sensory properties of the obtained beer depend on the characteristics of each adjunct but also on the forms in which the adjunct is added: whole cereal, grits, malted, extruded grains, torrefied and syrup. Among these common forms, the extruded grains (maize or rice) produce a higher content of aroma compounds in beer. From a nutritional point of view, the use of non-conventional starch adjuncts, such as black rice, buckwheat or sweet potato, leads to an increase in the polyphenol content of the beer, and thus, its antioxidant capacity. Cereals such as maize, rice, sorghum or millet are the most promising for the production of gluten-free beers. A close relationship can be developed between the use of adjuncts in the beer industry and the use of commercial enzymes. Advances made by biotechnology to design new enzymes with different functionalities could be associated to a future increase in adjunct usage in brewing

Importance of Tetrahydroiso α-acids to the microbiological stability of beer

Producción CientíficaWhile beer provides a very stable microbiological environment, a few niche microorganisms are capable of growth in malt, wort, and beer. The production of off-flavors and development of turbidity in the packaged product are due to the growth and metabolic activity of wild yeast, certain lactic acid bacteria (LAB) and anaerobic Gram-negative bacteria. Beer also contains bitter hop compounds, which are toxic to Gram-positive and Gram-negative bacteria, and contribute to preventing the spoilage of this beverage. In the boiling process, the hop -acids (humulones) are isomerized into iso -acids. These products are responsible for the bitter taste of beer, but they also play an essential role in enhancing foam stability. Antibacterial activity of iso -acids and their hydrogenated derivates (rhoiso -acids and tetrahydroiso -acids) in MRS broth and beer have been evaluated against different LAB (Lactobacillus and Pediococcus) for the determination of their beer-stabilizing capabilities. Besides this, we have determined the minimum inhibitory concentration and the bacteriostatic effect of each compound against Pediococcus. We found that tetrahydroiso-acids (added directly to beer during production processes) are the compounds that present the greatest antibacterial activity against the main agents implicated in beer spoilage

Sensory Characterization of Commercial Lager Beers and Their Correlations with Iso-alfa-acid Concentrations

[EN] A total of 40 commercial lager beers (6 dark, 28 pale and 6 alcohol free) were analysed by High-Performance Liquid Chromatography (HPLC) and evaluated by a descriptive sensorial panel. Discriminant Analysis was applied to 15 sensory descriptors to obtain a classification by types of beer. Cluster Analysis identified 4 clusters grouping the 15 sensory descriptors. The Correlation Matrix confirmed a correlation between them and the subsequent Principal Component Analysis. A stepwise discriminant analysis was used to eliminate the less significant descriptors, leading to 100% of the samples being correctly classified with a reduced number of variables (colour intensity, smell of caramel, smell of toasted, persistence and viscosity). The analysis of the correlations between iso-α-acid concentrations and sensory descriptors shows a good relationship with bitter taste. Thus, HPLC data can be used for predicting lager beer bitterness through the mathematical correlation developed in this paper.Financial support from Junta de Castilla y León. (VA332A12-2) is gratefully acknowledged.Blanco Fuentes, CA.; Nimubona, D.; Fernández Fernández, E.; Alvarez Cano, MI. (2015). Sensory Characterization of Commercial Lager Beers and Their Correlations with Iso-alfa-acid Concentrations. Journal of Food and Nutrition Research. 3(1):1-8. https://doi.org/10.12691/jfnr-3-1-1S183

Nuclear Magnetic Resonance Methodology for the Analysis of Regular and Non-Alcoholic Lager Beers

Producción CientíficaThe presence of seven main agents responsible for beer aroma and taste (n-propanol, isobutanol, 3-methylbutanol, tyrosol/tyrosine, ethyl acetate, isoamyl acetate, and acetaldehyde) is determined by different nuclear magnetic resonance (NMR) techniques (1H PRESAT, total correlation spectroscopy, heteronuclear single quantum correlation, and heteronuclear multiple bond correlation) in five regular and five low-alcoholic or alcohol-free beers. The new methodology includes the identification of the 1H and 13C NMR chemical shifts of the analytes by a standard addition method and the consequent identification of the compounds studied in regular and non-alcoholic beers. The chemical composition is different depending on whether the beer is regular or non-alcoholic, therefore affecting the organoleptic characteristics of each type of beer.MINECO (CTQ2013-41067-P)Junta de Castilla y León (VA332A12-2