## 3 Results and Discussion  
Lactobacillus reuteri CECT 925 Growth in Lulo Juices. Effect on Physicochemical and Antioxidant Properties  
A multifactorial analysis of variance revealed a significant effect of pH (\(p\leq 0.05\)). Viable cell counts in the juices showed values above 8 (log CFU/mL) in all samples and above 9.5 in the juices at pH 5.5. HPH treatment had no significant effect (\(p\leq 0.05\)) on the viable cell counts when compared to that achieved in the non-homogenized samples. Results are shown in Figure 2.  
In relation to physicochemical properties such as pH and soluble solids content (Brix), the influence of HPH depends on the food product. [PERSON] et al. [35] found no significant changes in pH and soluble solids content of mango juice after HPH at pressures up to 190 MPa. [PERSON] et al. [20] found no effect of HPH at 100 MPa on pH of carrot juice, while a decrease of pH of apricot juice after the same HPH treatment was observed. In this research, the application of the HPH treatment significantly decreased the Brix of the non-fermented juices (Table 1). As expected, fermentation was the most important variable decreasing both pH and soluble solids content. This decrease was more pronounced in the juices at pH 6 where pH rectification provided a higher initial value for Brix. The growth of _Lactobacillus reuteri_ was the only variable that significantly influenced the density of the juices, slightly raising its value. Pulp sedimented and transmittance of supernatant at two centrifugation speeds were measured to evaluate physical stability. According to Stokes Law, the sedimentation velocity of spherical and rigid particles is proportional to the particle size (diameter) and inversely proportional to the dispersed medium viscosity [36]. Therefore, particle size reduction due to HPH contributes to the delay of juice cloud loss by decreasing the particle's Stokes diameter and, thus, slowing down the pulp sedimentation and improving physical stability [37]. It is important to note that the possibility of increasing physical stability by employing HPH is very interesting for fruit juices since there will be no need for hydrocolloids or other stabilizer addition. Figure 2: Impact of pH value and high homogenization pressures on survival of _Lactobacillus reuteri_ CECT 925 in lulo juices after fermentation. Different letters denote significant differences (\(p\leq 0.05\)). The results of pulp content and transmittance at the two centrifugation speeds show a higher percentage of pulp at 365\(\times\)\(g\) and a higher transmittance at 3000\(\times\)\(g\). The lower centrifugation speed allows the less stable and larger particles to settle while keeping the smaller particles in suspension. The smaller particles will sediment at 3000\(\times\)\(g\), considerably increasing the transmittance of the supernatant. Fermentation did not affect the percentage of pulp but did increase the transmittance especially in the samples treated at 150 MPa (e.g., 22.7 \(\pm\) 0.4 \(>\) 13 \(\pm\) 2 or 83.8 \(\pm\) 1.1 \(>\) 74 \(\pm\) 2). In the samples homogenized at 100 MPa, the effect was small at low centrifugation speeds and non-existent at higher speeds. Both pH and homogenization pressure had a significant effect on suspended pulp and turbidity at the two centrifugation speeds. However, the way in which the HPH affected both variables was conditioned by the pH of the juice. At pH 5.5, the centrifugation speed of 365\(\times\)\(g\) did not allow to detect the changes that the application of 100 MPa caused in the suspended pulp and turbidity (e.g., 2.79 \(\pm\) 0.17 \(\approx\) 2.8 \(\pm\) 0.3). However, increasing the centrifugation speed did detect an increase in the percentage of settled pulp (1.13 \(\pm\) 0.03 \(<\) 1.41 \(\pm\) 0.07) and in the transmittance of the supernatant (51.3 \(\pm\) 0.6 \(<\) 64 \(\pm\) 2). The application of a HPH of 150 MPa did induce detectable changes at both centrifugation speeds.