Traitements thermiques de rétentat (X2) d'ultrafiltration de lait dans un échangeur à plaques : aspects thermiques, hydrodynamiques et encrassement par comparaison au lait et au lactosérum

Abstract - Heat treatment of x2 milk ultrafiltration retentate with a plate heat exchanger: thermal, hydrodynamic and fouling aspects compared with milk and whey
Utilization of x2 milk ultrafiltration retentate and prospect of development of this raw material led us to study its behaviour during heat treatment particularly from the point of view of its fouling potential. The study was carried out in comparison with milk and whey. In the V7 VICARB plate heat exchanger flow rate and fluid processing temperature were bound together. It was equipped with temperature and pressure sensors to follow hydrodynamic and thermal disturbances related to fouling. Fouling was also evaluated by weighing deposits on each plate in purpose to determine deposit mass profile along the exchanger.
Fluids, on which physicochemical composition was analysed, were raw whole bulk milk, x2 milk ultrafiltration retentates and sweet whey. Heat treatments were carried out using a 300 l.h^-1 flow rate at 95°C processing temperature with 20 or 146 s mean holding time. Generally, duration of experiment was 1.66 h.
During short time experiments, pressure drop and heat transfer rate measurements had not enough significant variation for fouling to be followed. Only weighing of heat exchange plates allowed to follow fouling kinetics.
In three experiments with milk, a maximum of deposit mass was observed at about 85° C ; fouling began at about 70° C. With x2 milk retentate fouling location was similar to that of milk. Quantitatively, fouling by retentate was slightly higher. Whey gave more fouling than the previous products ; deposit began at about 60°C and was maximum at 76°C.
Fouling by milk seemed essentially related to β-lactoglobulin heat denaturation and calcium salts precipitation. Qualitatively, phenomena were not very different with milk retentate ; only their relative importance varied and led to a slightly higher fouling potential. But, heat treatment of this product with a plate heat exchanger presented no more problems than the one applied to milk.
Whey fouling potential might be related to absence of caseins which fixes calcium in milk.

Résumé - Ce travail décrit des moyens de mesure de l'encrassement d'un échangeur de chaleur à plaques basés sur les perturbations hydrodynamiques ou thermiques et sur la mesure de la masse de dépôt et évalue leur validité pour des traitements thermiques de courte durée (1,66 h en général).
Sur la base des mesures de masses de dépôt, le lait, le rétentat x2 et le lactosérum sont comparés du point de vue pouvoir encrassant et faisabilité du traitement thermique. Le rétentat x2, qui se révèle avoir un comportement très peu différent de celui du lait sur le plan thermique et hydrodynamique, est, par contre, légèrement plus encrassant que le lait. Le lactosérum présente, au contraire, dans les mêmes conditions de traitement thermique, un fort pouvoir encrassant comparé à celui des fluides précédents.
La contribution respective des protéines solubles et des sels minéraux dans l'encrassement est discutée.