# Physicochemical and Microbiological Changes Associated with Processing in Dry-Cured Tuna
## 3 Results and Discussion  
### Physicochemical Parameters  
The evolution of physicochemical characteristics, including NaCl content, water activity (a\({}_{w}\)), moisture content, water hold capacity (WHC), total acidity, and pH during the manufacturing process of _mojama_ is shown in Table 1.  
Concerning the salt content, the raw tuna fillets contained a low amount of salt, more specifically 0.30%. As expected, the mean values showed significant differences, mainly due to the salting process that occurs in the manufacturing process. The concentration of salt increased significantly after the salting step and decreased after the excess salt was removed during the salt-washing step (6.53% vs. 4.79%). Salting is a very common technique used for food preservation and for flavor enhancement [27]. The mechanism behind salting relies on the concept of diffusion. During the salting step, a simultaneous exchange of flows takes place, such as water loss and salt uptake. NaCl diffuses into the tuna flesh and water diffuses out of the tuna muscle, due to the osmotic pressures [15]. After the drying step, the salt content of the samples reached 5% (on average) in accordance with the range established in the specifications for the manufacturing process for the PGIs \"Mojama de Barbate\" and \"Mojama de Isla Cristina\" (3-9%) [11; 12; 13; 14]. The average a\({}_{w}\) found in the raw tuna fillets was 1.01 (Table 1). This result was similar to that of [PERSON] et al. [28] for the muscle of yellowfin tuna (0.99). During the salting step, a significant decrease was observed (0.94), mainly due to the salt bonds with the muscle tissue and the water loss that reduces the available free water. The samples analyzed after the salt-washing step showed a significant increase in a\({}_{w}\) (0.98), due to the hydration of the product in the water. The lowest values of a\({}_{w}\) (0.86) were measured in the samples obtained after the drying step. These results are in accordance with those reported by [PERSON] et al. [29] for salted fish products (0.73-0.86). The decrease in water activity is very important for the quality and safety of the _mojama_ since it is a way of controlling microbial growth, lipid oxidation, and the formation of non-desirable compounds such as biogenic amines [30]. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & & **Raw Tuna** & **Salting** & **Salt-Washing** & **Dry-Cured** & **_p_-**Value** \\ \hline Salt (\% NaCl) & average \(\pm\) SD & 0.30 \(\pm\) 0.19 c & 6.53 \(\pm\) 2.51 a & 4.79 \(\pm\) 1.52 b & 5.09 \(\pm\) 1.05 ab & *** \\  & min & 0.05 & 3.19 & 2.49 & 3.69 & \\  & max & 0.52 & 9.50 & 7.19 & 7.01 & \\ a\({}_{w}\) & average \(\pm\) SD & 1.01 \(\pm\) 0.00 a & 0.94 \(\pm\) 0.03 c & 0.98 \(\pm\) 0.02 b & 0.86 \(\pm\) 0.03 d & *** \\  & min & 1.00 & 0.90 & 0.93 & 0.82 & \\  & max & 1.01 & 1.00 & 1.00 & 0.89 & \\ Moisture (\%) & average \(\pm\) SD & 62.89 \(\pm\) 2.37 a & 56.70 \(\pm\) 6.99 a & 62.75 \(\pm\) 4.85 a & 33.03 \(\pm\) 8.87 b & *** \\  & min & 59.44 & 45.84 & 54.02 & 22.04 & \\  & max & 66.29 & 67.27 & 68.96 & 49.66 & \\ WHC (\%) & average \(\pm\) SD & 77.25 \(\pm\) 1.36 c & 84.46 \(\pm\) 3.05 b & 75.03 \(\pm\) 2.62 c & 97.29 \(\pm\) 0.87 a & *** \\  & min & 75.01 & 80.92 & 71.72 & 95.89 & \\  & max & 79.21 & 89.56 & 79.20 & 98.25 & \\ pH & average \(\pm\) SD & 6.04 \(\pm\) 0.22 & 6.07 \(\pm\) 0.18 & 6.05 \(\pm\) 0.14 & 5.93 \(\pm\) 0.18 & ns \\  & min & 5.78 & 5.65 & 5.74 & 5.49 & \\  & max & 6.38 & 6.35 & 6.35 & 6.22 & \\ Total acidity (\%) & average \(\pm\) SD & 0.55 \(\pm\) 0.03 & 0.50 \(\pm\) 0.08 & 0.51 \(\pm\) 0.03 & 0.54 \(\pm\) 0.15 & ns \\  & min & 0.51 & 0.36 & 0.47 & 0.38 & \\  & max & 0.59 & 0.59 & 0.58 & 0.84 & \\ \hline \end{tabular} Different letters indicate significant differences by [PERSON]’s test. ***, ns—significant at \(p\) \(<\) 0.001 and not significant, respectively. SD—standard deviation  
\end{table}