# Ultrasound for Meat Processing: Effects of Salt Reduction and Storage on Meat Quality Parameters
## 3 Results and Discussion
### Bacterial Enumeration on Treated Pork  
Understanding bacterial growth is an important element to assess the spoilage and safety profile of a product, especially in sodium-reduced foods where the impact of salt reduction can be unclear. The addition of ultrasound processing can cause diverse effects on bacterial populations depending on control parameters such as power, frequency and treatment time [43; 44]. At the moment, not many studies have been looking at the use of ultrasound for bacteria inactivation on solid foods and, the majority of the literature  
Figure 4: Hardness values of meat samples with standard sodium content (NS) or reduced sodium content (RS) at Day 0 and after 60 days storage at 4 and 10 \({}^{\circ}\)C. Values represent means \(\pm\) standard deviation (SD, N = 3). Different letters are significantly different at \(p\) < 0.05.  
available provides report on liquid/semi solid application [45]. At the conditions used in this study, the application of US processing showed no significant effects on bacterial inactivation on the meat; no changes in the growth of the inoculated bacteria populations were observed among samples containing high or low sodium content (>0.88 g and 0.65 g/100 g of meat, respectively) between all the conditions used (data not shown). A similar lack of antimicrobial effects of ultrasound was previously described by [PERSON] et al. [46] who observed that ultrasound exposures reduced the total plate count during storage, but it was impossible to produce significant difference from the control muscle samples on chicken skin. The hypothesis is that the meat layers can provide some degree of protection from cavitation for bacteria [38]. Similarly, the use of a mix of NaCl/KCl as a salt replacer, did not have a negative impact on the samples (Control NS vs. Control RS), showing no differences in the microbiological profile of SR and NS and supporting previous studies highlighting the antimicrobial efficacy of KCl [8; 47]. An increase in bacterial population following US treatment was observed in the samples stored at 10 \({}^{\circ}\)C, showing significantly higher counts for TVC and LAB (\(p\) = 0.001 and 0.002, respectively) in NS-US meat in comparison to the control, Figure 5.  
Lactic acid bacteria (LAB), at maximum numbers of \(\sim\)log\({}_{10}\) 8 CFU/g, are usually the predominant bacteria present on meat stored under anaerobic conditions such as vacuum-packed or modified atmosphere [48]. In general, for all the treatments used in this study, the population of lactic acid bacteria remained below log\({}_{10}\) 8 CFU/g, Figure 5. The only significantly higher counts (\(p\) = 0.026) were observed in NS meat samples after 60 days of storage at 4 \({}^{\circ}\)C in comparison to the other samples at the same salt level. Similar observations were reported [49], where it was shown that low-frequency ultrasound (70 kHz) increased the growth rate of bacterial cells on polyethylene surface compared to growth without ultrasound; the authors suggested that by increasing the rate of transport of oxygen and nutrients to the cells, US processing could influence the growth of bacteria [49]. The authors reported that a treatment of 10 min was the best US condition (37 kHz, 90 W) to achieve microbial reduction in beef, while increased treatment time led to increased bacterial growth [50]. On the other hand, [PERSON] et al. [17] have reported a significant (\(p\) < 0.05) increase in _E. coli_ cell numbers in beef samples after US processing, suggesting that extended ultrasound treatment could be needed to inactivate the microorganisms  
Figure 5: Total viable counts (TVC) and lactic acid bacteria (LAB) counts at day 0 and after 60 days of storage at 4 \({}^{\circ}\)C and 10 \({}^{\circ}\)C. Comparison between samples contained normal sodium levels (NS) and reduced sodium levels (RS) and processed with ultrasound (US) or without (control). Differences between treatments are significant at \({}^{\ast}\)\(p\)-value < 0.05, \({}^{\ast\ast}\)\(p\)-value < 0.01.  
present on the meat surface.