Hugo S. Garcia

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.

Polyphenoloxidase activity during ripening and chilling stress in 'Manila' mangoes

Summary ‘Manila’ mangoes were stored at 6 or 12°C, while a control lot was ripened at 258C. Polyphenoloxidase activity was measured in pulp and peel of mangoes during normal ripening and chilling-stress conditions. Compositional parameters (textural hardness, pH, acidity, and soluble solids) were monitored during ripening and chilling injury (CI) was assessed by visual inspection. PPO activity increased in refrigerated mangoes but fruit stored at 6°C displayed greater activity than those stored at 12°C. Both temperatures produced a peak in enzyme activity at day 16 of storage. Peel browning and acidity correlated with PPO activity (r.=.0.9073 and 0.9818) at 6°C. After the PPO peak was observed at day 16, refrigerated mangoes showed lower enzyme activities; however external browning became more pronounced.

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.

FOAM-MAT DEHYDRATION OF JAMAICA (Hibiscus sabdariffa L.) INSTANT DRINK

ABSTRACT Water extracts of Hibiscus sobdariffa L. (Jamaica) calyces were foam-dehydrated. Combinations of two emuisifiers (Sorbac 60-Polydorbac 80) were used to study the stability of foams and their suitability for further dehydration. Maltodextrins (5 % w/w)were added as drying aids. The effects of temperature and sample thickness on dehydration rates were evaluated. Drying times were found to be proportional to power thickness. Results indicated that moisture transfer is externally controlled.

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.

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.