Mass Transfer Effects during Meat Ultrasonic Brining

Mass transfer processes in meat brining can be enhanced by taking advantage of the effects produced by high intensity ultrasound. Pork loin slices were immersed in a saturated solution of NaCl at 2 oC during 45 minutes. Different conditions of agitation of the solution and different levels of ultrasound intensity were applied during brining. Ultrasounds were applied with a probe system and the intensity was measured by means of a calorimetric method. The water and NaCl content of samples after treatments showed a high influence of the ultrasound intensity on the mass transfer. Above a threshold of ultrasonic intensity, NaCl and water transport were affected. NaCl content was higher in sonicated than non sonicated samples. At intensities higher than this threshold, water content of samples was higher than initial water content of meat. Introduction Brining is a common operation in some meat processes. In this operation, meat is immersed in saturated brine and two main mass transfer processes take place. The water migrates from meat to brine and the solutes migrate from brine to meat. As a consequence the changes in solid composition due to the mass transfer processes produces a decrease of the amount of water available for degradation reactions by enzymes and microorganisms. The flows of matter should overcome several resistances. In particular, the meat has itself an internal resistance to water and solutes transfer. On the other hand, an external resistance to mass transfer appears between the meat surface and the brine. Any factor that affects these resistances, can affect the mass transfer processes. These processes are characterized by the relatively slowness of the mass transfer. Several methods such as the application of vacuum [1], [2], [3], centrifugal forces [4], electric pulses [5], [6] or high intensity ultrasound [7], [8] have been tested to accelerate the operation The ultrasounds produce a series of effects when traveling across a medium that can affect mass transfer processes [9]. The main effects that can affect the resistance to mass transfer are the implosion of bubbles produced by cavitation [10], heating of materials due to thermoacustic effects [11], microstirring, mainly at interfaces [9] and structural effects [12], [13]. For these reasons ultrasound has been used to accelerate mass transfer in several food systems, products and processes. Examples of these are brining of meat [14] and cheese [15], drying of rice [12], carrots [15] or onions [16], osmotic dehydration of apples [9], [7] and several extraction processes [17], [18], [19], [20]. The aim of this work was to study the effect of the intensity of ultrasound in the water and solute transfer during meat brining. Materials and Methods Pork loin slices of 5 x 40 x 50 mm were cut from a fresh whole piece of Longissimus dorsi (pH 5.5). Individual samples were wrapped in a plastic film and frozen at -20 oC. Before the salting experiments, samples were slowly thawed at a constant temperature of 2 oC during 12 h to avoid the influence of the thawing conditions in the meat brining treatments. Brining experiments were carried out in 800 mL of saturated brine of NaCl into a 1000 mL glass beaker. During the experiments brine temperature was kept constant at 2 ± 0.5 C placing the glass beaker into a temperature controlled bath. The saturation of the brine was visually monitored by the existence of solid NaCl at the bottom of the beaker during brining experiments. In all experiments, the time of brining was 45 min. This time was chosen from results reporte in the literature, [7] and [8]. In these cases, this time allowed to observe differences between treatments since it was apart enough from the initial and the equilibrium conditions of the samples. Several types of brining experiments were carried out: without agitation of brine, with mechanical agitation and with ultrasound application. In the last case, experiments were performed without mechanical agitation of brine. The brine was stirred by using an agitator Heidolph RZR1 (Heidolph Instruments GMBH & Co., Schwabach, Germany) provided with a marine type impeller that described a circle of 55 mm diameter. Samples of meat were placed in the zone of maximum agitation. 2000 r.p.m. was the maximum possible velocity of the impeller. Higher levels of agitation produced the overflow of the brine. Ultrasound energy was applied to the brine using a probe system (100 W and 20 kHz, SONICS & WCU 2003, Paris, september 7-10, 2003