## 4 Discussion
### DPPH Radical Scavenging Activity  
Liver is an important source of peptides with antioxidant activities [4; 37]. This was reflected in the DPPH radical scavenging activity observed in the hydrolysates obtained in the present study from the enzymatic hydrolysis of porcine liver.  
As mentioned before, the enzymes used for their extraction conditioned their activity [4]. The values found were in accordance with those found by other authors in porcine liver [10], where alcalase hydrolyses exhibited better DPPH values than papain (42.3% vs. 40.3%, respectively). Similar trends and DPPH activity were observed by [PERSON] et al. [7] in porcine liver proteins hydrolyzed with the aforementioned enzymes (42.0% and 37.0%, respectively). Alcalase also showed good DPPH values when it was compared with those found by other enzymes, such as pepsin and pancreatin [38]. On the contrary, [PERSON] et al. [39] observed that venison hydrolyses obtained with papain displayed higher antioxidant activity than the hydrolyses produced with alcalase. Moreover, the DPPH ability depends on the material used to carry out the hydrolysis, since higher DPPH activity could be obtained using the same enzyme in different matrices. This trend was observed when pepsin was used to hydrolyze chicken and porcine liver, with higher antioxidant activities observed in hydrolyses obtained from chicken liver (95.0% vs. 55.0%, respectively).  
Regarding hydrolysis time, except for bromelain, DPPH values increased as the process time increased, with increases of up to 53.0% and 25.0% observed for papain and alcalase, respectively. These findings were previously found by other authors [10; 40]. In this way, the antioxidant activity of protein hydrolysate extracted from porcine liver was affected by the different hydrolyzing periods, increasing from 23.2% to 42.3% for alcalase and from 21.1% to 40.3% for papain during the six hour of hydrolysis [10]. This could be due to relevant roles of the peptide structure, molecular weight, and amino acid sequence in the radical scavenging activity [10].  
Ultrafiltration with MWCO membranes is the primary perm-selective barrier commonly used to separate proteins from meat processing by-products, with pore sizes ranging from 4 to 30 kDa being the most effective for the separation of macro- and microsolutes, such as protein hydrolyses [41]. Previous studies also confirmed that low molecular weight peptides were stronger as antioxidants than high molecular weight peptides [38; 39]. This behavior was observed in the present study for papain and bromelain hydrolyses, suggesting that lower molecular weight peptides (10 kDa) aremore effective as potent DPPH radical scavengers than high molecular weight peptides (30 kDa). Moreover, in the case of papain at 4 h, the activity values increased as the pore size decreased (249, 228, and 145 ug Trolox/g for 5, 10, and 30 kDa, respectively). This outcome agrees with data observed by [PERSON] et al. [42], who noticed that fractions with a small molecular weight (<3 kDa) possessed better antioxidant potential, with evidence of higher DPPH. On the other hand, the alcalase values were in contrast to the current observations, since an increase in the DPPH scavenging activity was obtained when the peptide size of the protein hydrolysate increased. The same trend was observed in the results obtained from bovine liver sarcoplasm, poultry viscera, and yellowstripe trevally (_Selaroides leptolepis_) protein hydrolysates [37; 43; 44].