Changes in optical properties of freeze-dried systems above the glass transition temperature

NURIA ACEVEDO; CAROLINA SCHEBOR; MARÍA DEL PILAR BUERA

Churchill Collage, Cambridge

Congreso; AMORPH 2006. MOLECULAR BASIS OF STABILITY IN PHARMACEUTICAL AND FOOD GLASSES; 2006

Light scattering in freeze-dried systems is the result of reflection and refraction of radiation at optical interfaces. An increase of water content causes structural modifications (such as those derived from glass transitions) which promote changes in the light scattering properties of the material. During storage of many freeze-dried systems, different degrees of opacity develop, parallel to brown pigments formed in the Maillard reaction. The color of these translucent materials is thus affected by internal light transmission in the turbid media. Direct reflectance measurements are not satisfactory for translucent materials because of the complicated interaction of transmittance, absorption, reflectance and light loss through internal scattering. The purpose of this work was to characterize changes in optical properties of freeze-dried model systems and vegetables as a result of structural modifications. The relative contributions of light scattering and absorption were taken into account during storage above their glass transition temperatures promoted by water content increase. Thin layer samples of freeze-dried powdered apple tissue and freeze-dried solutions of polyvinylpyrrolidone (PVP), xylose and lysine (98:1:1) were equilibrated over saturated salt solutions between 11 and 84%R.H., and incubated at 43 and 70ºC. The glass transition temperature (Tg) of the systems was determined by differential scanning calorimetry. Color functions in the CIELab color space were calculated from colorimetric measurements with a tristimulus spectrocolorimeter employing white background. The Kubelka-Munk scattering (S) and absorption (K) coefficients and K/S were calculated from reflectance measurements of thin layers of the materials with white and black backgrounds. S markedly decreased in apple systems above 33% RH, reflecting a sharp decrease in opacity as increasing water content when the apple samples were 40°C above Tg. Simple color functions (either with white or black background) were not adequate parameters to follow pigment formation due to non enzymatic browning in the apple system because of the change in opacity which influenced the measurement. The Kubelka-Munk coefficient K/S was a better estimator of pigment formation. A drastic decrease in the light scattering properties was also observed in PVP systems, but in this case in the vicinity of the glass transition temperature, which was also well represented by the scattering coefficient S. When the opacity of the samples changed as it occurred above the glass transition temperature the color formation of the sample was more sensitively represented by the Kubelka-Munk coefficients than by simple color functions.  The maximum change in light scattering properties of apple powders was observed when the samples were well above Tg at the storage temperature (high T-Tg values). Contrastingly, in PVP matrices those changes occurred at low T-Tg values. This different response of optical properties of samples to structural changes is caused by the material constituents: apple tissues contain water insoluble biopolymers and a more stable structure than water soluble PVP matrices. These water-time-temperature-composition dependent changes in translucent media should be taken into account when evaluating quality parameters of amorphous media, and should be adequately quantified. The Kubelka-Munk theory for turbid media employed in present work demonstrated to be a powerful tool to evaluate those properties.