Apple juice particles: Fractal dimension and mechanism of aggregation

BENÍTEZ ELISA INÉS; LOZANO JORGE ENRIQUE; GENOVESE DIEGO BAUTISTA

Chicago, Illinois, USA

Congreso; IFT (Institute of Food Technologists) -Annual Meeting and Food Expo; 2007

Institute of Food Technologists

Turbidity of expressed apple juice is produced by a polidisperse suspension of particles coming from the cellular tissue. After precipitation of the coarse particles by gravity, only the fine-colloidal particles remain in suspension. Brownian motion of these particles promotes collisions between them, while interparticle forces determine if two colliding particles aggregate or not. The aggregation of colloidal particles leads to the formation of fractal structures. Objectives: to determine the morphology of these aggregates in terms of their fractal dimension (Df), and to relate it with their mechanism of aggregation. Cloudy apple juice was depectinized by enzymatic treatment, and most soluble solids removed by diafiltration. The average radius of gyration (Rg) of the aggregates was obtained by photon correlation spectroscopy. Digitalized images of the particles/aggregates were obtained by scanning electron microscopy (SEM). The Df of the aggregates was determined with the variogram method by analyzing 26 SEM images with the software FERImage (Figure). The individual particles constituting the aggregates were dispersed by sonication for 30 min. Their average radius (a) was obtained by analyzing SEM images of 500 particles. The average number of particles per aggregate was determined as N = Kg(Rg/a)Df, where Kg is a constant of the system, determined experimentally. The main results are shown in the Table. Values of Df were independent of the maximum radius of the aggregates (CV < 5%), indicating the same configuration at different sizes, characteristic of fractals. The average Df is typical of aggregates formed by slow flocculation or reaction limited aggregation. This is attributed to the presence of an energy barrier between particles, which slows down the collision frequency, in agreement with our previous studies. This work contributes to understand the structure, aggregation, and stability of apple juice particles, which is of paramount importance for the production of cloudy and clarified juices.