Interest of modelling heat transfer inside a reactor to estimate kinetic parameters

A space-and-time dependent model was developed for a reactor. The model, which combines heat transfer and chemical reaction, was intended to identify kinetic parameters under non-isothermal conditions for lowintermediate moisture foodstuff. In particular, the model was used to evaluate the effect of high temperature heat treatments (100-140 °C) on anthocyanins degradation in blackberry juice - quartz sand system at low water activity (aw 0.34). The coupled transport equations were solved using Comsol® calculation code. Kinetic and thermal parameters were identified with a nonlinear regression method. The relevance of this approach was assessed by comparison between the kinetic parameters estimated as described before (heterogeneous assumption) and the parameters identified by considering a homogeneous temperature distribution inside the reactor. The results showed no significant difference between the anthocyanins degradation reaction rate constant (k120°C) for the heterogeneous (3.29±0.05×10-3 s-1) and the homogeneous (3.5±0.3×10-3 s-1) assumption. And a slight difference was found between the activation energies (Ea) for the heterogeneous (95±5 kJ mol-1) and the homogeneous (108±7 kJ mol-1) identification. Therefore, the use of the developed model allowed a slight improvement in the identification of the kinetic parameters. However, the assumption of a heterogeneous distribution of temperature inside the reactor is valid for the identification of kinetic parameters with high orders of magnitude. (Résumé d'auteur

Impact of Ohmic Heating on Coconut Water Volatile Compounds

Immature coconut water (CW) is a low acid fruit juice mainly composed of sugars and minerals. Beside its healthy feature, it can also be a pleasant refreshing drink especially when coming from aromatic coconut varieties. Unlike conventional thermal processes, ohmic heating is an innovative technology using volumetric heating technique to pasteurize or sterilize food products. As such, it is known to overcome the overheating problem in fruit juices and to improve aroma preservation. This work aimed at obtaining a commercially safe CW beverage by ohmic heating while looking at the volatile compounds kinetic evolution. Coconut water from an aromatic Thailand Green Dwarf variety was submitted to different ohmic heating time-temperature treatments ranging from 100°C to 140°C and from 0 to 600 seconds. Volatile compounds from the fresh and heated samples were extracted by headspace-solid phase microextraction before being identified by gas chromatography coupled to mass spectrum analysis. Volatile compounds variations were described thanks to principal component analysis and chemical kinetics. Sixty volatile compounds were identified. Even after high temperature ohmic heating treatment, flavor compounds responsible for the typical CW aroma remained in samples headspace. None of the Strecker degradation molecules was detected in the GC analysis of CW after a 5s or 10s treatment at 140°C. The variations of the volatile compounds composition and levels during ohmic treatment confirmed that the higher the temperature is, the less the impact on the chemical reactions thus on flavor quality. At least two volatile molecules were apparently good indicators of the heating treatment level: 3-penten-2-one and ethyloctanoate. During the isothermal stage, the kinetic approach lead to Ea=67.7 kJ.mol-1 for the 3-penten-2-one increase. These results proved that ohmic HT-ST treatments could ensure a commercially safe high quality beverage thanks to a better retention of the original volatile compounds of immature coconut water. (Résumé d'auteur

Optimization of extraction and concentration of anthocyanins from roselle (Hibiscus sabdariffa l.) by aqueous extraction and nanofiltration

To prepare roselle extract, dried calyces of Hibiscus sabdariffa cultivar CLT 92 from Senegal, were mixed with plain water To optimize the aqueous extraction of anthocyanins from Roselle calyces, five ratios calyces/water (kg/kg) were studied (1/5, 1/10, 1/15, 1/20, 1/25). By identifying the effect of various operative parameters, we found that an appropriate sieve of the calyces and the extraction temperature improved both the kinetics and yield of extraction of anthocyanins from Roselle calyces. By studying extraction kinetics, we identified and quantified the effective diffusivity, with ranged from 1.71 and to 4.10 x 10-11 m2.s-1 and the yield of extraction of anthocyanins from Roselle calyces were between 67 and 74 % at extraction temperatures between 25 and 40 ° C. The ratio 1/5 with an extraction yield of 72 % at 35 ° C was selected to produce the Roselle extract to be concentrated. The aqueous extract obtained is characterized by a high content of anthocyanins (1.5 g/100 g DM), high acidity (14 meq/100g) and low sugar content (2.9 g/100g). This extract is enriching in anthocyanins by nanofiltration using a laboratory pilot of 133 cm2. Membranes from three different suppliers and with similar cut-off thresholds (200-500 Da) were tested: NF270 Dow, GE Osmonics DL and Nadir NP030. Permeate flux and physico-chemical characteristics were determined for each membrane by varying the transmembrane pressure from 5 to 20 bar. The flux densities were ranging between 1 and 37 kg.h-1.m-2. DL membrane from GE Osmonics brand presents the best permeate flux at 20 bar (37 kg.h-1.m-2). All the membranes have retention rates in anthocyanins between 92 and 100 %. The membrane Nadir NP030 shows the least important retention rate in anthocyanins to 20 bar (97 %). These results were validated on a semi-industrial nanofiltration pilot featured with a membrane DL of 2.5 m2. The nanofiltration permitted to concentrate the roselle extract to 50 and 250 g kg-1 of total soluble solids with a volumetric reduction ratio (VRR) of 4 and an average permeate flux of 20 kg.h-1.m-2 at a transmembrane pressure of 20 bar and a temperature of 30 ° C. (Résumé d'auteur