Ebner Azuara

Enthalpy-Entropy compensation in food vapor adsorption

Abstract Enthalpy-entropy compensation was analysed for sorption isotherms of potatoes, macadamia nuts, apricots, figs, currants, prunes and raisins. Plots of (ΔHdif)T vs (ΔSdif)T for potatoes and macadamia nuts presented two isokinetic temperatures: TB1 = 272.0 ± 57.7 K (− 1 °C) for potatoes, TB1 = 265.0 ± 18.8 K (− 8 °C) for macadamia nuts and TB2 = 382.5 ± 7.3 K (109.5 °C) for both products. The first isokinetic temperature (TB1) appeared only at the upper portion of the temperature range tested (50, 60 and 70 °C for potatoes and 50 and 60 °C for macadamia nuts). The two isokinetic temperatures observed for potatoes and macadamia nuts suggested that during the initial stages at low aw TB1 is controlled by changes in the entropy of water, whereas the second isokinetic temperature (TB2) is considered to be enthalpy-controlled. Dried fruits presented only one isokinetic curve TB = 315.7 ± 3.5 K (42.7 °C), for raisins, currants and figs (75.2–82.3% d.b. sugars) and TB = 317.7 ± 4.6 K (44.7 °C) for prunes and apricots (51.5–54.5% d.b. sugars), indicating an enthalpy-controlled adsorption process for the whole range of moisture contents covered.

Effect of the centrifugal force on osmotic dehydration of potatoes and apples

Abstract Cylindrical slices (2 cm diameter, 0.5 cm thickness) of potatoes and apples were immersed in three different osmotic solutions: 70% (w/w) sucrose, 30% (w/w) NaCl, and 15% (w/w) NaCl and 45% (w/w) sucrose. Osmotic solutions and food samples were placed into stainless steel containers and centrifuged at 64 × g using a tabletop centrifuge at 30 °C, to perform osmotic dehydration. Results obtained showed that solids uptake by the food samples was always lower when centrifugal force was applied to the system. Under centrifugal force application, and with the osmotic solution containing both NaCl and sucrose, potato slices lost ca. 70% of their original moisture after 6 h of osmodehydration, whereas only 2% of the solutes were taken by the samples. In contrast, osmotic dehydration of static samples for the same time removed ca. 65% of the moisture, but 15% of their original weight was gained by absorption of solids from the osmotic solution. For apple slices, similar results were obtained: 83% water lost, and 6% solids gained for the dynamic system, compared with 75 and 12% respectively, for the static system.

Enthalpic and Entropic Mechanisms Related to Water Sorption of Yogurt

Moisture sorption isotherms of plain, concentrated, freeze-dried, and freeze-dried concentrated yogurts at 20, 35, and 50°C were used to calculate integral properties. Enthalpy-entropy compensation for all yogurt preparations showed two isokinetic temperatures. The first isokinetic temperature was observed at low moisture contents and was controlled by changes in the entropy of water, whereas the second isokinetic temperature was considered to be enthalpy-driven and was of the same magnitude for all yogurt products (TB2 = 327.4 ± 0.7 K). Application of the Dubinin-Radushkevich equation suggested that the moisture zone controlled by entropy was produced when adsorption ocurred in the micropore range.

THERMODYNAMIC BEHAVIOR OF GREEN WHOLE AND DECAFFEINATED COFFEE BEANS DURING ADSORPTION

ABSTRACT Differential and integral thermodynamic parameters were estimated for whole green coffee beans decaffeinated with either ethyl acetate (EADB) and methylene chloride (MCDB). Adsorption isotherms were prepared at 25, 35, and 45°C. Changes in thermodynamic properties were estimated by the method of Othmer. Variations in the integral enthalpy with respect to moisture indicated that MCDB have a heterogeneous distribution of binding forces at the active sites. EADB showed smaller enthalpy values at lower moisture contents as compared to MCDB, and maintained a distribution of binding energy at the active sites similar to that of whole beans. We suggest storing MCDB at 25°C with a moisture content of 0.9 g H2O/100 g d.m., and of 2.3-4.0 g H2O/100 g d.m. for EADB and whole beans, to obtain maximum stability.

Effect of wall material on the antioxidant activity and physicochemical properties of Rubus fruticosus juice microcapsules

Abstract Blackberry (Rubus fruticosus) juice possesses compounds with antioxidant activity, which can be protected by different biopolymers used in the microencapsulation. Therefore, the effects of cell wall material including maltodextrin (MD), Arabic gum (GA) and whey protein concentrate (WPC) were evaluated on the physicochemical and antioxidant properties of encapsulated blackberries using a spray-drying technique. Anthocyanin concentration, polymeric colour, total polyphenols, radical scavenging activity of the 1,1-diphenyl-2-picrilhydrazil radical, reducing power and the stability at different storage conditions were evaluated. GA and MD conferred a similar protection to the antioxidant compounds when the microcapsules were stored at low water activities (aw < 0.515) in contrast to at a high moisture content (aw > 0.902), whereas WPC presented a high protection. Therefore, the selection of the best wall material for blackberry juice encapsulation depends of the conditions of storage of the powder.

Enthalpy-Entropy Compensation and Water Transfer Mechanism in Osmotically Dehydrated Agar Gel

Enthalpy-entropy compensation and water transfer in osmotically dehydrated agar gel were studied by carrying out experiments at 30, 40, and 50°C in a 60% (w/w) sucrose solution. An additional experiment was carried out at the isokinetic temperature (TB = 14°C) to confirm the physical meaning of TB. When osmotic dehydration (OD) was carried out at the isokinetic temperature, the diffusion coefficient remained constant (≈0.54 × 10−10 m2/s) during the entire process and the weight loss reached a limit (≈0.277 g/g) when the process was performed at TB. Lefflers criterion indicated that diffusion mechanism was entropically controlled given the internal resistance developed during OD. Results were confirmed by the linear relationship found between the relaxation time and entropy variation according to the Adam and Gibbs equation.

Oleoresins from Capsicum spp.: Extraction Methods and Bioactivity

Capsicum spp. fruit is one of the most produced vegetables around the world, and it is consumed both as fresh vegetable and as a spice like a food additive for their characteristic red color and, in many cases, its pungency. In addition to its economic importance, the bioactivity of some important compounds such as capsaicinoids and carotenoids has promoted its research. The use of Capsicum oleoresins has been increased due to its advantages comparing with the traditional dry spice. These include obtaining higher quality products with the desired content of bioactive and flavored substances. The wide diversity of extraction methods including water extraction, organic solvent extraction, microwave-assisted extraction, and ultrasound assisted extraction as well as supercritical fluid extraction among others are discussed in the present review. Moreover, pretreatments such as chemical treatments, osmotic dehydration, sun and oven drying, and freeze-drying commonly used before the extraction are also presented. Due to its importance, Capsicum oleoresins produced with “green” solvents and the improvement of fractional extraction techniques that allow to obtain separately the various bioactive fractions will continue under research for further development.

Thermodynamic analysis of the sorption process of mesquite gum

Determination of the thermodynamic parameters, allows for a more thorough interpretation of sorption isotherms and provides a better insight into sorption mechanisms. In this work, the adsorption and desorption isotherms of mesquite gum were determined at 25, 35 and 45 degrees C. All isotherms were fitted using the GAB model and the thermodynamic properties were estimated by Othmers method. The hysteresis decreased when temperature increased. However the effect of temperature was higher on the desorption isotherms, indicating the existence of metastable states. The adsorption process showed smaller enthalpy values during the hysteresis as compared to desorption process. The minimum integral entropy for the adsorption and desorption was located around 14 gH2O/100 g dry solids. This suggests that it is possible to determine the most suitable conditions for storage, using adsorption or desorption isotherms.

Physicochemical and antioxidant properties of honey from Scaptotrigona mexicana bee

The physicochemical properties of honey from the stingless bee Scaptotrigona mexicana harvested at four, three, two, and actual years did not show significant differences in most of their physicochemical and antioxidant properties. Some properties, such as: color, pH, and titratable acidity were affected by storage temperature, being most affected at 45 °C. The concentration of polyphenols was correlated with the percentage of radical 2,2′-diphenyl-1-picrylhydrazyl scavenging activity (R2 = 0.930). Honey showed an antibacterial activity against gram (+) and gram (–) bacteria, not showing inhibition of yeast and mold tested. The sensory evaluation showed that this honey had a high level of preference by consumers. Honey from S. mexicana bee can be stored for at least three years without being significantly affected by their physicochemical and antioxidant properties, and this is acceptable for marketing.

DETERMINATION OF MOISTURE DESORPTION ENTHALPY OF MESQUITE GUM WITH DIFFERENTIAL SCANNING CALORIMETRY AND THERMOGRAVIMETRIC ANALYSIS

ABSTRACT Moisture sorption isotherms were determined for mesquite gum at 25, 35 and 45 ° C. The water desorption enthalpy was calculated from the isotherms with the Clausius-Clapeyron equation, and additionally by both different scanning calorimetry and thermogravimetric analysis. The two calorimetric techniques showed greater degree of water binding al the product surface at the lower moisture contents. There was a decrease in desorption enthalpy when moisture conlenl was increased. Negative enthalpy values suggested an endothermic reaction probably attributed to biopotymer solution. Results indicated that calorimetric and thermogravimetric analyses are suitable physical methods to estimate isosteric heats of mesquite gum.