C.I. Beristain

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.

Effect of foaming agents on the stability, rheological properties, drying kinetics and flavour retention of tamarind foam-mats

Abstract Tamarind foam-mats were prepared using as foaming agents ovalbumin, mesquite gum and a low molecular weight surface active blend, by themselves or in combination, in accordance to a 2 3 factorial experimental design. The stability, rheological properties (flow and viscoelasticity) and drying kinetics of the different foam treatments were evaluated. Sensory evaluation of the dried powders was performed. Response surface analysis yielded polynomial models that explain the influence of the foaming agents treatments on the selected foams response variables: drainage mean time, yield stress, apparent plastic viscosity, critical drying time, instantaneous elastic modulus, and mean retardation time. In general terms, the use of binary combinations of the foaming agents at their high usage level yielded foams that exhibited longer drainage mean times; higher yield stress, apparent plastic viscosity, instantaneous elastic modulus and mean retardation times; and a shorter onset of the critical drying time, that yielded dried tamarind powders with better sensory flavour perception. A close interrelationship existed between the drainage mean time and the apparent plastic viscosity, and between the critical drying time and the mean retardation time of the foams.

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.

Evaluation of softening and associated enzyme activities during the ripening of coated ‘Manila’ mangoes

SummaryA hydrophobic coating formulated with maltodextrins, carboxymethylcellulose, propyleneglycol and a mixture of sorbitan esters was applied to pre-climacteric ‘Manila’ mangoes. The coated fruit were stored for 20 d at 25°C and 80–85% r.h. Changes in mesocarp hardness, as well as pectinesterase (PE), polygalacturonase (PG) and cellulase (Cx) activities were determined. Results during the first 8 d of storage showed evidence that fruit treated with the coating suffered less mesocarp softening along with concomitant reductions of PG and Cx activities, than did control fruit. After the initial storage period, activity of PG increased steadily during further ripening of coated fruit.

Microencapsulation of Eugenia uniflora L. juice by spray drying using fructans with different degrees of polymerisation

The objective of this work was to microencapsulate pitanga (Eugenia uniflora L.) juice by spray drying, using High Performance Agave Fructans (HPAF) and High Degree of Polymerisation Agave Fructans (HDPAF) and maltodextrin (MD), respectively, as the wall materials. The physicochemical and antioxidant properties of the capsules during storage at various temperatures were evaluated. The microparticles developed using fructans HPAF and HDPAF, exhibited similar physicochemical and flow properties to those presented by the microparticles prepared with MD. The highest yield and concentration of anthocyanins after drying and during storage were found for a 1:6 core:wall material ratio. The total color change was a good indicator of the microcapsule stability. This study showed that both fructans fraction possess similar encapsulating properties to MD and that the HDPAF were more efficacious than MD at protecting the antioxidants during drying and storage.

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.