3.1.1. Colour

The PLS–R revealed a good correlation between pressure and L\*, C\*, h\* values. They can be observed according to axis 1: P5 projected on the positive side while P100 is projected on the negative side. The external and near position of P5 to L\* and P100 to C\* denotes a significant e ffect of pressure on colour attributes, so that low pressure leads to a high L\* value (see red circle in Figure 1) while high pressure leads to a high C\* value. Despite h\* also being positively projected on axis 1 and so a ffected by pressure, its projection stays in the inner circle of the PLS-R although near the external limit. This indicates a lowered impact of the pressure on the hue angle when compared with that observed for C\* and L\*. It can also be notice that the projection of C\*on the PLS-R, in the negative part of axis 1 and axis 2, is higher when the interaction between P100 and T50 is considered (see green circle in Figure 1). The shorter freeze-drying process carried out at 50 ◦C contributes to promoting freeze-dried products with a higher value of chroma. In addition, C\* is projected in the same side of the fast freezing rate (FR-F), the last one being in the inner circle (poorly correlated according to axis 2).

These observations were confirmed by the ANOVA, as values of L\*, C\* and h\* of the samples were significantly a ffected by working pressure (*p* < 0.05). Further, C\* was also a ffected by the interaction between shelf temperature and freezing rate (*p* < 0.05), but with a low F-Value (7.86). Taking into account the significances shown by the PLS-R analysis and the F-Values of the ANOVA, Figure 2 was constructed, showing L\* and C\* values of the samples obtained at the di fferent pressure and shelf temperature and considering the mean value at both freezing rates. When working with the highest pressure during freeze-drying (P100), the samples showed lower values of L\* and higher C\*, which means a darker and saturated colour. In this case, the chroma is specially enhanced at a higher temperature in the freeze-drier shelves, either 40 ◦C or 50 ◦C (Figure 2, *p* < 0.05). The hue angle, with values between 80.3 and 82.6, showed a lower value when working with high pressure and freeze-drier shelves temperature below 50 ◦C (*p* < 0.05). Similar results have been reported by Hammami et al. [22], who noted a slight L\* decrease when working at higher pressures ( *P* > 108 Pa) for strawberries pieces, which was related to the pronounced shrinkage observed under these conditions. Di fferent authors found that the operating working pressure should be lower than 50 Pa to avoid shrinkage for strawberries pieces and banana slides [22]. With regards to shelf temperature, increasing temperature may cause some slight sugar browning reactions like non-enzymatic or Maillard reactions, which means a reinforcement of the colour shown by the increase in C\*. Nevertheless, it is better detected in shrunken samples due to its di fferent optical light reflection capacity [22].

**Figure 2.** Values (mean and Tukey's HSD) of L\* in left axis and C\* in right axis of the freeze-dried purees according to the interaction between shelf temperature (30, 40 or 50 ◦C) and pressure (P5: 5 Pa and P100: 100 Pa) factors. Different letters for each attribute indicate different homogeneous groups for the Temperature\*Pressure interaction (*p* < 0.05). Data of both freezing rates are considered in the mean values.

According to the PLS-R and the ANOVA, pressure and shelf temperature were the factors that had a significant impact on the colour. The total differences in colour were calculated in order to evaluate the impact of both factors on the colour. Regarding the shelf temperature, values of ΔE\* between 1.11 and 5.51 were obtained, while the range was 4.45–9.98 for the pressure influence. According to Bodart et al. [39], total differences in colour are not obvious to the human eyes when ΔE\* < 1, minor colour differences could be appreciated by the human eye depending on the hue when 1 < ΔE\* < 3 and visually obvious changes for human eyes occur when ΔE\* > 3. Therefore, it seems that pressure has a greater effect than the other factors on the colour.
