## 3 Results and Discussion
### Birefringence of Cassava Starches in HPP-treated Cassava Flours
#### 3.3.4 Digestive Enzyme Susceptibility of Starches from HPP-Treated Cassava Flours  
There was no three-way interaction found for %RDS, %SDS, and %RS; however, two-way interaction was found between pressure and either flour concentration (%SDS) or holding time (%RDS and %RS). Both %RDS and %RS had a significant simple main effect for pressure within each level of holding time (all \(p<0.001\)) and a simple main effect of holding time at 500 MPa only (\(p<0.001\)). Simple comparisons between pressures revealed that 600 MPa was significantly higher in %RDS than other pressures at 10 min HT by 3.42-3.76%, whereas at 30 min HT, both 500 and 600 MPa were significantly higher than other pressures but with 500 MPa significantly lower by 2.43% than 600 MPa. This means at higher HT, 500 MPa can already significantly increase the %RDS of cassava flour. An increase in digestibility has been reported in HPP-treated rice and waxy rice starches [46,47] and buckwheat flour [44], owing to microstructural damage in starch granules from gelatinization leading to easier access of enzyme to the inner structure. Despite this, starch digestibility of any HPP-treated starches was lower than their thermally treated counterpart [48,49]. The difference in starch granular integrity was postulated to be a factor. When starch is thermally gelatinized in excess water, it involves side-by-side dissociation of helices, followed by helix coil transitions and eventually a complete loss of granular structure [50]. Under pressure, the starch granule remains intact, as disintegration is incomplete since van der Waals and hydrogen bonds are stabilized, which favors the helix structure [8]. On the other hand, the simple comparisons between holding times showed that 500-30 min was 1.14% significantly higher in %RDS than 500-10 min, and yet, no HT dependency was observed for 600 MPa. This could mean that maximum hydrolysis can be achieved at 54% \(DG\) or 18% RC (600-30%-10), and no further pressure-induced gelation can increase its susceptibility to digestive enzymes. This is contrary to the results of [PERSON] et al. [46], wherein an increase in %RDS from 39 to \(\approx\)67% was observed for waxy rice starch with 0-58% \(DG\) or 0-29% RC. Potential reasons for this difference in values are the different in vitro starch digestibility protocols used and the starch substrate. The same trend of significant treatments was observed for %RS, albeit with receding values. The 600 MPa was significantly lower than other pressures at 10 min HT and 500 MPa at 30 min. The 500-30 min was significantly lower by 1.35% than 500-10 min and there was no difference between 600-10 min and 600-30 min. These interactions are illustrated in Figure 7A,B and evidently demonstrates increased susceptibility to digestive enzymes.  
In the case of %SDS, a significant simple main effect between pressures within each level of flour concentration was found (10% \(p=0.021\), 20 and 30% \(p<0.001\)). The %SDS in 0.1 MPa or untreated flour was not different from higher pressures at 10% FC but was significantly lower at 20% FC than 300-500 MPa, and only significantly lower than 500 MPa at 30% FC. Across the three flour concentrations, the %SDS in 500 MPa was significantly higher than 600 MPa. From the general trend and statistical data, it can be inferred that with an increase in pressure to 500 MPa, a significant decrease in %RS is coupled with an increase in %SDS and %RDS. A further reduction in %RS and an increase in %RDS can be obtained at 600 MPa but with the reduction of %SDS (Figure 7C). Similarly, an increase in %SDS at 500 MPa and a decline when increased to 600 MPa was exhibited by waxy rice starch [46] and sweet potato flour [51]. SDS delivers a slow release of glucose in the blood; hence, HPP-treated cassava flours can be utilized as an ingredient in novel products exhibiting a lower glycaemic index.