Effect of temperature on molecular mobility,oxygen permeability,and dynamic site heterogeneity in amorphous α-lactalbumin films

We investigated molecular mobility and oxygen diffusion in amorphous solid films of α-lactalbumin (α-La) using phosphorescence from the xanthene probe erythrosin B in order to elucidate the molecular mechanism(s) that control oxygen permeability in amorphous solid proteins. Emission peak energy and bandwidth were determined by fitting spectra to a lognormal bandwidth function; intensity decays were fit to a stretched exponential function to determine the lifetime and the stretching exponent. Peak emission energy decreased gradually over the range from −20 to over 120 °C, indicating a gradual increase in the matrix dipolar relaxation rate. Bandwidths were constant at low temperature but increased dramatically above ∼50 °C, indicating that the dynamic heterogeneity of the protein matrix increased at high temperature. Emission lifetimes decrease gradually at low and more steeply at high temperature, indicating that the rate of matrix collisional quenching increased with temperature. Arrhenius analysis of the rate constant for non-radiative decay showed a gradual increase in quenching indicative of matrix softening. Comparison of lifetimes in air and N₂ (±oxygen) monitored oxygen permeability. Oxygen permeability became detectable at about 0 °C and appeared to correlate with matrix mobility. The emission spectrum shifted to higher energy as a function of time following excitation, whereas the phosphorescence lifetime decreased with increasing emission wavelength; both behaviors provided strong evidence for distinct sites within the protein matrix varying in molecular mobility. Phosphorescence spectroscopy thus provides a simple tractable tool for monitoring conditions that activate oxygen diffusion in amorphous solid foods.     

Non-enzymatic browning kinetics analysed through water–solids interactions and water mobility in dehydrated potato

Non-enzymatic browning (NEB) development was studied in dehydrated potato at 70 °C. It was related to the macroscopic and molecular properties and to water–solid interactions over a wide range of water activities. Time resolved ¹H NMR, thermal transitions and water sorption isotherms were evaluated. Although non-enzymatic browning could be detected in the glassy state; colour development was higher in the supercooled state. The reaction rate increased up to a water content of 26 g/100 g of solids (a_w = 0.84) and then decreased at higher water contents, concomitantly with the increase of water proton mobility. The joint analyses of NEB kinetics, water sorption isotherm and proton relaxation behaviour made it evident that the point at which the reaction rate decreased, after a maximum value, could be related to the appearance of highly mobile water. The results obtained in this work indicate that the prediction of chemical reaction kinetics can be performed through the integrated analysis of water sorption, water and solids mobility and the physical state of the matrix.     

Enhanced survival of spray-dried microencapsulated Lactobacillus rhamnosus GG in the presence of glucose

The survival of spray dried Lactobacillus rhamnosus GG (LGG) preparations encapsulated in whey protein isolate (WPI)-maltodextrin, WPI-maltodextrin-glucose, WPI-inulin, and WPI-inulin-glucose mixtures during storage at 25 °C (11%, 57% and 70% relative humidity, RH) was examined. The glass transition temperature of each encapsulant formulation was also assessed. RH was most important for maintaining viability over time; the inclusion of glucose improved viability, irrespective of when all formulations were in a glassy or rubbery state. When LGG microcapsule powders were stored at the same RH, the addition of glucose in the encapsulant formulation had a greater influence on survival of LGG during storage than the maintenance of a glassy state. Both the maintenance of a glassy state during storage and the incorporation of glucose in the encapsulant formulation were required for optimal survival of probiotic microcapsule powders prepared from fresh cultures.

Practical significance

The incorporation of glucose into the encapsulant formulation prior to spray drying of protein-carbohydrate based LGG formulations improves the survival of LGG during long term storage.     

Original article: Encapsulation of ascorbic acid in amorphous maltodextrin employing extrusion as affected by matrix/core ratio and water content

Ascorbic acid (AA) was encapsulated in glassy low‐dextrose equivalent maltodextrin matrix by extrusion. The effects of formulation parameters, i.e., core/matrix ratio and water content were mainly investigated on T_g of extrudate. The AA yield, AA content and water content of the products together with extrusion parameters were also determined and compared for different formulations. The T_g of extrudates containing water content from 7.860% to 10.430% ranged from 43.17 to 27.48 °C, and the T_g of extrudates which core to matrix from 1:4 to 1:8 ranged from 35.79 to 41.64 °C. AA yield of all extrudates is above 96%, and with increasing water content, there was a slight decrease in the AA yield. The increased water level and core/matrix ratio reduce specific mechanical energy and die head pressure. X‐ray diffraction and scanning electron microscopy suggested that AA was most likely molecularly dispersed within the maltodextrin indicating the miscibility of AA and maltodextrin.     

Kinetics of non-enzymatic browning in amorphous solid systems: distinguishing the effects of water activity and the glass transition

Debates have emerged recently on whether water activity or the state of the system as dictated by the glass transition temperature (T_g) impacts the rates of chemical reactions in reduced-moisture solid systems. The objective of this study was to evaluate the kinetics of brown pigment formation in polyvinylpyrrolidone model systems of different molecular weights so that the effects of water activity and the glass transition could be distinguished. Browning rates at different T_gs, but constant water activity, were significantly different except when all were in the glassy state. As the system changed from a glassy state to a rubbery state, the rate of browning increased 7-fold. The rate of browning also increased as water activity increased from 0.33 to 0.54, but then appeared to plateau with further increases in water activity. Thus, the rate of brown pigment formation is influenced significantly by the glass transition temperature of the system and less by the water activity. In addition, the concentration of reactants in the aqueous microenvironment had a significant impact on the rate of brown pigment formation.     

Food stability determination by macro–micro region concept in the state diagram and by defining a critical temperature

In the 1950s the concept of water activity was proposed for determining food stability. This concept is now being used although it has some limitations. Indeed, these limitations mean that the concept is not universally applicable and in fact is invalid under certain conditions. In order to address the limitations of the water activity concept, the glass transition concept was proposed in the 1960s, although significant application of the concept only started in the 1980s. Recently, it has become evident that the glass transition concept is also not universally valid for stability determination in all types of foods when stored under different conditions. Currently in the literature the need is emphasized to combine the water activity and glass transition concepts since both concepts could complement each other. The glass transition concept was used to develop the state diagram by drawing another stability map using freezing curve and glass transition line. In this paper an attempt is made to review the published methods used to combine both concepts. These approaches are graphical plot of glass transition conditions and water content as a function of water activity, and macro–micro region concept in the state diagram. In addition, a new approach is proposed in this paper by defining a critical temperature for stability and then relating it with water content, and other hurdles affecting food stability. The water mobility concept is also reviewed to provide another dimension of food stability in order to determine a more complete picture.