2.1.3. Cast Films

Corn starch (water content 15 wt %) and glycerol (water content 14 wt %) were purchased from Carl Roth (Karlsruhe, Germany). Pure starch-glycerol films without nanofillers were tested as reference material.

#### 2.1.4. Karl Fischer Titration

Formamide (99.5 wt %), iodine solution (Roti®hydroquant C5; 5 mg H2O mL−1, free of pyridine), methanol (Roti®hydroquant D; dry), and a water standard (Roti®hydroquant; 10 mg H2O g−1) were purchased from Carl Roth (Karlsruhe, Germany).

#### 2.1.5. Pinhole Testing

Pinholes were determined with peanut oil containing Sudan Red III (Merck, Darmstadt, Germany) in a concentration of 1 part per thousand.

### *2.2. Nanoparticle Preparation*

CNC were prepared by sulfuric acid hydrolysis and subsequent basic neutralization. The procedure was derived from the method described by Müller et al. [31]. 2 mol of sulfuric acid (64 wt %) was added to cut cotton linters in a mass ratio of 15:1 and transferred to a heated water bath at 50 ◦C. The raw cellulose was hydrolyzed for 3 h under vigorous stirring. Following hydrolysis, the reaction solution was diluted to 54 wt % H2SO4 to decrease the viscosity and then decanted to 3 mol of sodium hydroxide (7 mol kg−1) in a cooled water bath. After homogenization for 15 min, the CNC were separated from the reaction solution by consecutive centrifugation steps until a pH of ~2 was reached. Excess ions were removed from the hydrolyzed product by decantation of the supernatant after each washing step and successive redispersion with H2O. Eventually, the dispersion was ultrasonicated (8 kJ g−<sup>1</sup> CNC) with a homogenizer (Sonoplus HD 3400 with the sonotrode VS 70 T, Bandelin, Berlin, Germany) and stored at 6 ◦C until further use.

The hydrolysis conditions to produce SNP was derived from the method demonstrated by Angellier et al. [40]. 1 mol of sulfuric acid (18 wt %) was added to corn starch in a mass ratio of 7:1 and then hydrolyzed for 120 h at 40 ◦C. Subsequently, the reaction mixture was decanted to 1.5 mol of sodium hydroxide (5 mol kg−1) for neutralization. Similar to the extraction of CNC, SNP were separated from the salt solution by consecutive precipitation and redispersion until the pH stabilized at ~2.7. After ultrasonication, 0.01 g sodium azide L−<sup>1</sup> was added as an antimicrobial agent before storing the product at 6 ◦C.
