Heat Treatment of Whole Milk by the Direct Joule Effect--Experimental and Numerical Approaches to Fouling Mechanisms

Heat Treatment of Whole Milk by the Direct Joule Effect—Experimental and Numerical Approaches to Fouling Mechanisms

L. Fillaudeau^*^,1, P. Winterton^*, J. C. Leuliet, J. P. Tissier, V. Maury, F. Semet, P. Debreyne, M. Berthou and F. Chopard

^* Institut National des Sciences Appliquées (INSA), Laboratoire de Biotechnologie–Bioprocédés, Centre National de la Recherche Scientifique (CNRS) Unité Mixte de Recherche (UMR) 5504, Institut National de la Recherche Agronomique (INRA) UMR 792-135, avenue de Rangueil, F-31077 Toulouse Cedex 4, France
INRA–Laboratoire de Génie des Procédés et Technologie Alimentaires (LGPTA) 369, rue Jules Guesde, 59650 Villeneuve D’Ascq Cedex, France
EDF-R&D, Les Renardières, 77818 Moret Sur Loing Cedex, France
Alfa-Laval Vicarb, rue du Rif Tronchard, 38120 Fontanil-Cornillon, France

¹ Corresponding author: Luc.Fillaudeau{at}insa-toulouse.fr

The development of alternative technologies such as the directJoule effect to pasteurize and sterilize food products is ofgreat scientific and industrial interest. Our objective was1) to gain insight into the ability to ensure ultra-high-temperaturetreatment of milk and 2) to investigate the links among thermal,hydraulic, and electrical phenomena in relation to fouling ina direct Joule effect heater. The ohmic heater [OH; ${bot}$ (where E is theelectrical field and v is the velocity); P (power) = 15 kW]was composed of 5 flat rectangular cells [e (space between theplate and electrode) = 15 mm, w (wall) = 76 mm, and L (lengthof the plate in plate heat exchanger or electrode) = 246 mm]—3active cells to ensure heating and 2 (at the extremities) forelectrical insulation and the recovery of leakage currents.In the first step, the thermal performance of the OH was investigatedvs. the flow regimen [50 < Re (Reynolds number) < 5,000],supplied power (0 < P < 15 kW), and electrical conductivityof fluids (0.1 < ${sigma}$ _20°C < 2 S/m) under clean conditionswith model fluids. This protocol enabled a global thermal approach(thermal and electrical balance, modeling of the temperatureprofile of a fluid) and local analysis of the wall temperatureof the electrode. An empirical correlation was established toestimate the temperature gradient, T_w – T_b (where T_w isthe wall temperature and T_b is the product temperature) underclean conditions (without fouling) and was used to define operatingconditions for pure-volume and direct-resistance heating. Inthe second step, the ability of OH to ensure the ultra-high-temperaturetreatment of whole milk was investigated and compared with aplate heat exchanger. Special care was taken to investigatethe heat transfer phenomena occurring over a range of temperaturesfrom 105 to 138°C. This temperature range corresponds tothe part of the process made critical by protein and mineralfouling. The objectives were 1) to demonstrate the ability ofan OH to ensure heat treatment of milk, 2) to study the thermaland hydraulic performance with an increasing power and temperaturedifference between the inlet and outlet of the OH, 3) to defineand validate a criterion to follow heat dissipation efficiency,and 4) to compare the fouling propensity with the differentconfigurations. A heat dissipation coefficient, Rh_CO, was definedand validated to monitor the fouling propensity through globalelectrical and thermal parameters. Finally, a numerical simulationwas developed to analyze heat profiles (wall, deposit, bulk).Because of an increasing Joule effect in the static deposit,the simulation showed how wall overheating would definitivelycause fouling to spiral out of control.

Key Words: ohmic heating • wall overheating • fouling • whole milk