# Investigation of Salt and Water Diffusion During Dry Salting, Wet Curing, and Ultrasonic Wet Curing
## 1 Introduction  
Curing, as a technological operation, has been present in food production throughout history, but the phenomena influencing the curing process are still the subject of scientific research today. The changes occurring during curing require time, which, on the one hand, determines the \"turnover rate\" of the product, i.e., the financial profit-making ability, and, on the other hand, as a bottleneck of the plant, limits the product volume that can be released. Therefore, during curing operations, efforts should be made to minimize the time while maintaining the quality. The classic and simple method of curing is dry salting. Its disadvantage is that the distance between the \"curing core\" and the brine is typically large. Unfortunately, the diffusion factor for salt migration in the meat cannot be improved. Another problem isthat the surface salt concentration is very high; so, to achieve the appropriate enjoyment value, equalization must be solved in a separate operation (e.g., through storage, washing, or soaking), which are space- and time-consuming procedures and involve microbiological risks [1]. Despite the fact that its production is less economical, it remains an important product, and its raison d'etre is undeniable among customers looking for and consuming \"traditional products\". Wet curing is a widely used technology in the industry. It is similar to dry salting in many respects, but the surface salt concentration can be better controlled, the product shrinkage is less, and the yield is more favorable. The factors influencing diffusion (curing core-surface distance and diffusion coefficient) cannot be influenced either. Due to the lower surface concentration, the concentration gradient decreases, which is not favorable for the curing time [2]. The permeability of the cell membrane can be improved by using active ultrasound [3]. According to [PERSON] [4] \"...sounds with a frequency higher than 20 kHz are ultrasound\". [PERSON] et al. [5] define the concept of ultrasound from the human hearing threshold, 16 kHz. They also define the concept of active ultrasound, which refers to acoustic waves with an energy higher than 0.1 W/cm\({}^{2}\) and a frequency range of 16-100 kHz (i.e., high energy but relatively low frequency). The characteristic of active ultrasound is that it causes a change in the structure or other characteristics of the treated material or product. [PERSON] et al. [6] showed that, in the case of bean samples soaked in water, sonication at a frequency of 20/40 kHz significantly accelerated the diffusion processes. According to [PERSON] [7], the diffusion of salt in pork loin is favorably influenced by active ultrasound treatment with an intensity between 2 and 4 W/cm\({}^{2}\), and in this case, the following exponential relationship exists between the diffusion coefficient and the ultrasound intensity:  
\[\mathrm{D}\left[\mathrm{m}^{2}/\mathrm{s}\right]=2.7\times 10^{-10}\times e^{0.477 \times 1}, \tag{1}\]  
where I [W/cm\({}^{2}\)] is the intensity of the ultrasound. [PERSON] et al. [8] also found that the effect of active ultrasound on a product depends not only on the intensity of the treatment but also on the geometry of the product. Research on the industrial applicability of active ultrasound is ongoing. [PERSON]. [PERSON] et al. [9] reviewed several research results, from the 1990s onward. In their summary, they drew attention to the fact that the field of US treatments is controversial in both food preservation and product modification. The contradictory research results suggest that the application of active ultrasound in the food industry may primarily gain legitimacy as part of combined treatments, for example, chemical treatment + active ultrasound [10; 11] or supplementing traditional treatment with active ultrasound [12]. So, further clarifying scientific work is needed to ensure wide-ranging industrial applicability. A notable example of combined processes is ultrasonic curing supplemented with microbubble treatment.