*3.6. Stability of Antioxidant Capacity (RSA and FRAP) of Dried Pomegranate Arils*

Understanding the stability or degradation mechanisms of food products is essential to maximise the nutritional and sensory quality of products [55]. The stability of antioxidant capacity (RSA and FRAP) of pomegranate arils after hot-air and freeze-drying were evaluated based on changes in their concentrations (Table 4). This table further shows the kinetic parameters (kinetic rate constants and the half-life values) determined for the thermal degradation of the antioxidant capacity. A lower degradation rate indicates lower kinetic rate constants (*k*) and higher half-life [55].


**Table 4.** Effect of drying methods on the kinetic parameters of antioxidants (RSA and FRAP) degradation in dried pomegranate arils.

*k*, kinetic rate constants; t1/2; half-life; *R*2, coefficients of determination; RSA, radical-scavenging activity; FRAP, ferric ion reducing antioxidant power.

The RSA activity in freeze-dried arils had a lower degradation rate (*k* = 0.146; t1/<sup>2</sup> = 5.844) than hot-air dried arils (*k* = 0.151; t1/<sup>2</sup> = 5.654); however, the FRAP activity in hot-air dried arils had a lower degradation rate (*k* = 0.129; t1/<sup>2</sup> = 7.306) than the freeze-dried arils (*k* = 0.143; t1/<sup>2</sup> = 6.597). Considering the calculated degradation *k* and t1/<sup>2</sup> as an indicator of the amount of antioxidant loss, with a half-life (t1/2/week), the stability of RSA in the hot-air dried arils was approximately 3.3% lower than freeze-dried arils. However, the stability in FRAP activity in freeze-dried arils was 9.7% lower than the hot-air dried arils (Table 4). Several researchers have reported a decrease in the bioactive compounds in fruit after drying [19,59,60]. Zhou et al. [61] reported high degradation in antioxidant capacity (DPPH, FRAP and ABTS) of red pepper.

Similarly, Garau et al. [62] also found that air-drying decreased the antioxidant capacity in orange fruit matrix. This is consistent with the results of this study. Moreover, the values of coefficients of determination (*R*2) ranging from 0.85–0.99 were obtained for all linear regressions, indicating that the degradation process of these bioactive compounds for both hot-air and freeze-drying methods followed first-order reaction kinetics.

#### *3.7. Correlations among Quality Attributes for Dried Arils at 12 Weeks of Cold Storage*

Significant relationships that exist among attributes measured for dried arils are presented in Table 5.

Pearson's correlation tests indicated a strong positive relationship between TPC and TAC (*r* = 0.998) (Table 5). Additionally, there were strong negative correlations between TPC and RSA (*r* = −0.894) as well as TPC and FRAP values (*r* = −0.998); TAC and RSA (*r* = −0.910) as well as TAC and FRAP (r = −1.000). However, a positive correlation was found between RSA and FRAP (*r* = 0.919) (Table 5). A similar result was reported by Cano-Lamadrid et al. [54] between antioxidant ABTS and FRAP in osmotically dehydrated pomegranate arils cv. Mollar de Elche. Strong correlations were found between TSS and phytochemical properties (TPC and TAC), but none of the relationships seems to be applicable in practice. For instance, a strong correlation (*r* = 0.937) was found between TSS and TAC

(Table 5). In practice, no relevant prediction of dried aril flavour could be made using total anthocyanin content since soluble solids measurement technique applies only to the sweetness (◦Brix) of aril tissues. However, a moderately negative correlation was observed between TSS and TA (*r* = −0.555) (Table 5). This relationship clearly showed that the increase in total soluble solids of dried arils could also contribute to a decrease in TA. Other relationships found a moderately negative correlation between TA and TSS:TA (Table 5).

**Table 5.** Pearson's correlation coefficients among the investigated parameters of dried pomegranate arils during the 12-week storage period.


95% confidence interval. TPC, total phenolic content; TAC, total anthocyanin content; RSA, radical scavenging activity; FRAP, ferric reducing antioxidant power; TCD, total colour difference; TSS, total soluble solids; TA, titratable acidity. ns; non-significant, \* = *p* < 0.05 and \*\* = *p* < 0.001 (two-tailed).

In consideration of the noted benefits of consuming fruit with high phytochemical properties, it is therefore not surprising that antioxidant capacity (RSA and FRAP) showed a strong positive correlation with TPC. Therefore, the overall quality of dried arils investigated showed that only the interactions among the bioactive components seem promising and practicable.
