## 4 Discussion  
Alternating low and high sodium concentrations within a food product can reduce taste adaptation, meaning that sensations are more intensely perceived compared to a continuous stimulus [35]. Our study design involved beef burger samples of varied salt contrast intensities between salted and unsalted meat, as well as their varied proportions or location. The majority of studies investigating the impact of layered salt distribution on saltiness were performed on starch- or dairy-based products. In the study by [PERSON] et al. [16], a heterogeneous salt distribution showed a significant saltiness enhancement in two snack types (a bi-layer cereal product and four cream-based layers). A high heterogeneity of salt located on the outermost layer of the four-layer cream-based products enhanced saltiness perception. The snack was evaluated to be 25% saltier than the reference sample. This finding implies that the salt content in the salted layer is a crucial component in enhancing saltiness in such a heterogeneous food product. In further research [16; 17], in which the odour-induced saltiness enhancement and special salt distribution strategies were evaluated, the saltiness enhancement was significantly greater for products with only one layer of added salt, especially when the salt was concentrated in an external layer. Moreover, a four-layer cream-based snack with a heterogeneous distribution of salt was significantly saltier than a snack with a homogeneous distribution, regardless of the aroma distribution (including the sample without added aroma) [7]. The data obtained by [PERSON] et al. [35] indicated that a heterogeneous spatial sodium chloride distribution in starch-based 3D food snacks increased saltiness as long as the sensory contrast was strong enough and the overall NaCl concentration was low. The saltiness enhancement in bread, which was studied by [PERSON] et al. [15], was the most significant at a low salt content, with a 117% increase due to heterogeneity. The magnitude increased with sensory contrast. The spatial heterogeneity of NaCl allowed for a 28% salt reduction in bread without a loss of saltiness intensity or the use of sodium substitutes, taste, or aroma additives. Beyond the heterogeneous distribution of NaCl, the improvement in saltiness can be further extended by utilising lactic acid [11]. Our sensory profiling results supported the hypothesis that the heterogeneity of salt content in meat products, through the layering of different salt contents, stimulated a perception of increased intensity of saltiness in all meat products where it did not exist. Therefore, our first hypothesis can be accepted. With regard to the second hypothesis, our findings were not consistent with the proposal that a higher sensory perception would reside in products with higher salt contents in the outer layers of the foodstuff. Instead, the trained panel analysis and the ranking analysis of the consumer study provided supporting evidence that increasing the salt content in the inner layers of the product enhanced the perception of saltiness, indicating that burger samples with internally located salt resulted in a higher perception of saltiness. The hardening effect on the surface coupled with cooking perhaps explains why the heterogeneity did not have a high impact on the external  
Figure 3: Consumer saltiness perception depending on habitual dietary salt intake level. **(a)** Mean rank-based saltiness depending on habitual dietary salt intake level. **(b)** * An asterisk indicates the result of MANOVA, wherein sample and level of habitual dietary salt intake were fixed factors. high salt layer formulations. The current study contrasts with previous research in this aspect [7]. Discrepancies between our results and those previously reported are likely due to the specificity of the different food matrices utilised in the different studies in this area. An alternative reason could be the salt diffusion phenomenon. [PERSON] et al.