Moisture loss, gain and migration in foods

International audienceThe loss, gain and transfer of moisture often affect food materials. Whether arising from interaction with the atmosphere or with another component of the food, such changes always cause deterioration of the overall quality of the food through softening, toughening, breakdown, swelling or shrinkage due to phase transitions or dissolution. In most cases, water migration leads to organoleptic or microbiological changes in the food. With a view to better understanding the physical deterioration of food and to providing a tool for better control of food quality (and therefore of longer shelf life), this chapter reviews the water relationships in foods with particular attention to, and illustration of, glass transition-related phenomena. It also considers examples of foods affected by moisture exchanges with the atmosphere or within the product itself. The mechanisms controlling these migrations are presented together with some experimental approaches (measurements of moisture content, water activity and migration and modelling)

New insights on the thermal analysis of low moisture composite foods

International audienceLow moisture baked products were investigated with a view to characterising the effect of both formulation and humidity on their physical stability. At the end of the baking process, the samples were in the amorphous state as a result of starch gelatinization and sugar melting. Their thermal properties were analyzed with differential scanning calorimetry and their glass transitions were studied. The DSC thermograms were thoroughly studied through a Gaussian deconvolution of the first derivative of their heat flow. This approach evidenced a multiple phase behavior with different glass transitions in composite systems. They were associated with either a polymer-rich phase and/or a plasticizer (sugar)-rich phase whose behavior depended on the sample water content.This novel approach of thermal properties suggested new insights: considering the phase behavior of complex systems and thus the properties of their individual phases could contribute to a better understanding of the physical stability of the products

Positronium as a probe in natural polymers: decomposition in starch

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I3, remains constant at 26.2% while its lifetime, $\tau$3, is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, Eact = (1.52 ± 0.05) eV, and a frequency factor, ln(k0, s-1) = 25.3 ± 0.4. In the decomposition region, the PALS data show the unexpected correlation ($\tau$3n)3 = I3n, linking the normalised values of $\tau$3, $\tau$3n, and of I3, I3n. This is explained by considering that the changes in I3 with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in 3 reflect the shrinking of the radius of the free volumes, the latter decreasing in proportion to p1/3. Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, Eact = (1.53 ± 0.03) eV, and frequency factor, ln(k0, s-1) = 25.4 ± 0.2, in excellent agreement with the values derived from TGA

Moisture sorption characteristics and dynamic mechanical thermal analysis of dried petiole and rhizome of red water lily (Nymphaea x rubra)

International audienceThis research aimed to experimentally determine moisture sorption characteristics and mechanical thermal properties of different parts of red water lily (Nymphaea x rubra). The data obtained from dynamic vapor sorption (DVS) were modeled with six sorption isotherm models. The shape of sorption isotherms of dried petiole and rhizome was classified as Type III and II, respectively. Peleg model was the best fit with the experimental data. GAB and BET models were used to estimate monolayer moisture content (M-0) of the samples and M-0 of petiole ranged between 7.17 to 8.291% d.b. and 10.455 to 10.588% d.b. for GAB and BET models, respectively and M-0 of rhizome ranged between 6.208 to 7.741% d.b. and 3.566 to 3.669% d.b. for GAB and BET models, respectively. Blahovec-Yanniotis model was used to describe the amount of bounded water and solution water in material and the contribution of solution water played an important role in both adsorption and desorption processes of dried petiole and rhizome. Dried red water lilies were equilibrated at different relative humidity levels. Dynamic mechanical thermal analysis (DMTA) was used to estimate the glass transition of the samples at different water activities. Increasing the solicitation frequency shifted the temperature of the relaxation to a higher temperature and Arrhenius equation described well the frequency dependency of the transition temperature. The apparent activation energies (E-a) of dried petiole and rhizome were in the range from 217.98 to 248.49 kJ/mol and 187.34 to 230.30 kJ/mol, respectively

Changes in properties of palm sugar syrup produced by an open pan and a vacuum evaporator during storage

International audienceThe aim of this study was to monitor the changes in the properties of palm sugar syrup produced by an open pan and a vacuum evaporator at 70°C and 80°C during storage under 4°C and room temperature (30°C) for 12 months at monthly intervals. During storage, Maillard reaction took place in samples stored under 4°C lower than those stored under 30°C. This was shown by lower a* values, intermediate browning products (IBP), browning intensity (BI) and HMF content, and higher L* values, fructose, glucose and free amino group contents during storage for 12 months. HMF, a possible mutagen formed by nonenzymatic browning during the heating and storage of sugar based products, seems of particular interest and concern. Only the sample produced by an open pan and stored under 30°C contained HMF content (50.58 mg/kg) higher than the permitted maximum limit (40 mg/kg as recommended by the Codex Alimentarious). However, other samples still contained HMF in agreement with this standard. In addition, the microbiological quality and total soluble solids of all samples complied with Thai legislation standards for palm sugar syrup during 12 months of storage

Water in Dairy Products | Analysis and Measurement of Water Activity

International audienceWater activity of a system (aw) is a thermodynamic concept used to characterize the contained water. It has been considered, in spite of some limitations, as the most important parameter in food technology for the past 50 years. It can be measured by the ratio (p/p0) of the water vapor pressure in the atmosphere at equilibrium with the material to the saturated vapor pressure of pure water at the same temperature, or by the physical properties related to it (freezing point depression, mechanical/electrical properties). Although the physical/chemical mechanisms responsible for the depression of aw as compared to that of pure water are more or less identified, the expressions describing the relations between composition and aw remain mainly empirical for real foods. Water activity is only one of the several water-related factors controlling quality characteristics of food,with molecular mobility being equally important. It is, however, an essential parameter to consider in situations involving microbial activity (shelf life, production of aroma, etc.). It is also an essential tool in food technology, permitting to predict the transfer of water between two compartments of a product or between a food product and its environment during storage or a drying operation. Thus, aw controls the water content of the product, and in turn the quality characteristics such as texture of cheese, and the kinetics of physical (crystallization of lactose) or chemical/enzymatic reactions

Characterization of palm sap harvested in Songkhla province, Southern Thailand

International audienceThe purpose of this study was to characterise the quality of palm sap after harvested in Songkhla province, southern Thailand. Ten palm sap samples were analyzed. The results showed differed in physical and chemical quality among samples (P<0.05). The results showed range of L*, a* and b* values between 61.49 to 87.53, 1.46 to 3.52 and 12.41 to 19.31, respectively. The turbidity was ranged from 39.56% to 79.95%. The pH value was varied from 4.19 to 5.23, while total acidity was ranged from 0.27% to 0.93%. The total soluble solids ranged from 10.80 to 17.40OBrix. Total and reducing sugars were varied in a range of 10.36% to 16.94% and 0.88% to 3.56%, respectively. The sucrose, glucose and fructose contents were found vary in a range from 9.29% to 17.44%, 0.50% to 1.85% and 0.50% to 1.81%, respectively. Protein content varied from 0.31-0.39 mg/g. Ethanol was also found in all samples that indicating the fermentation. All results indicated a large variation quality of palm sap although they harvested in the same production area in Songkhla province. The different quality of palm sap was mainly due to the fermentation of sugars by the activity of microorganisms during palm sap collecting time

Identification of complex glass transition phenomena by DSC in expanded cereal-based food extrudates: Impact of plasticization by water and sucrose

International audienceThe physical state and mechanical properties of extruded cereal based products were studied as a function of sucrose content and relative humidity (RH) to evaluate how the presence of sucrose affects glass transition temperature (), sorption isotherm, and texture parameters. Extrudates were prepared with different sucrose content (0–20 %wt). Sorption isotherm showed the water content of extrudates decreased when product contains high sucrose at low aw range and the inverse effect was observed at high aw. s were determined using differential scanning calorimetry (DSC) and two transitions were detected on the heat flow first derivative curve. Addition of sucrose or water decreased both s in extrudates. Young's modulus showed water acts as anti-plasticizer at low aw, while shows plasticizing effect at high aw. A stability map can explain the brittle-ductile transition occurred while it was below

Encapsulation and Oxidative Stability of PUFA-Rich Oil Microencapsulated by Spray Drying Using Pea Protein and Pectin

International audienceThis study aimed at evaluating the potential of pectin combination with pea protein isolate (PPI) in the microencapsulation of polyunsaturated fatty acids (PUFA)-rich oil by spray drying, in order to maximize encapsulation efficiency and minimize lipid oxidation. The feed emulsions used for particle production consisted of PUFA-rich oil droplets coated by either PPI (primary emulsion) or PPI–pectin (secondary emulsion). Dry emulsions characteristics and oxidative stability of microencapsulated oil as a function of relative humidity (RH; from 11 % to 75 %) were determined. Scanning electron microscopy (SEM) images revealed considerable structural changes. Oil droplets retained their shape upon drying and reconstitution. However, a shift in oil droplet size upon reconstitution indicated that oil droplet coalescence occurred within the process. Oxidation of microencapsulated oil in secondary emulsion was delayed from that of primary emulsion but followed the same pattern with regards to humidity. A high rate of oxidation was found for intermediate RH conditions (33 % and 57 % RH). The lowest rate of oxidation as followed by hydroperoxide and thiobarbituric acid reactive substances values was found at 75 % RH, a condition that is likely to diverge significantly from the monolayer moisture value. The oxidative stability of encapsulated oil was influenced by both physical state of the emulsions and the different constituents at the oil-in-water interface with PPI–pectin secondary emulsion giving the best protection of the oil

Water and temperature contribution to the structuration of starch matrices in the presence of flavour

International audienceThe effect of modulating the gelatinisation extent by hydration (50/50 and 80/20 water to starch ratio) and temperature (65 or 85 degrees C) on various properties of wheat starch in presence of flavours has been studied. The hydrothermal treatments resulted in samples with different properties. The lowest residual flavour content was found in samples treated at the highest hydration and temperature (85 degrees C) while the other treatment conditions led to samples with similar residual flavour content. Ethyl hexanoate significantly increased the characteristic pasting viscosities compared to starch +/- 2-hexanone; suggesting a greater structuration with ethyl hexanoate. Heating starch in excess water caused amylopectin melting, but promoted an incomplete granular swelling as revealed by RVA. This study suggested that lowering the hydration upon treatment could limit both crystal melting (with a residual crystalline content up to 38% in the most extreme conditions) and granular swelling but increased granule organisation like following annealing