Porous Starch Materials via Supercritical- and Freeze-Drying

Abstract

The production of porous materials based on starch has been explored with supercritical drying-yielding aerogel-and freeze-drying. The two drying procedures were applied on the same gelling solution of amylomaize starch pasted at 140 °C and for two concentrations (5 and 10 wt.%). After gelation and retrogradation, water from the samples to be supercritically dried was exchanged to ethanol. The resulting starch aerogel presented high specific surface area (197 m²/g). Freeze-drying was assessed by investigating the effect of the gelation, retrogradation, freezing temperature, and sublimation pressure. The resulting starch materials were macroporous, with limited specific surface area and limited mechanical integrity. Cohesive open cell foam with pore size of ~20 µm was produced by quenching the hot starch melt in liquid nitrogen. The highest specific surface area obtained with freeze-drying was 7.7 m²/g for the hot starch melt frozen at -20 °C.

Keywords: aerogel; cryogel; cryogelation; freeze-drying; open cell foam; starch; supercritical drying.

Figure 1

Physical appearance of freeze-dried starch samples (“FD”) derived from 5 and 10 wt.% starch suspensions corresponding respectively to the first and second row of pictures (a–d) and (e–h). Samples gelled and retrograded at +6 °C overnight are denoted as “FD-G” (pictures (a,b,e,f)), non-gelled samples are denoted as “FD-NG” (pictures (c,d,g,h)). Prior to freeze-drying the samples were frozen at −20 °C in a freezer (“FR”—pictures (a,e,c,g)) or in liquid nitrogen (“LN”—pictures (b,f,d,h)). Sublimation was performed at 0.045 mbar (cold trap temperature −49 °C) for 96 h. The scale bar corresponds to approximatively 1 cm.

Figure 2

Physical appearance of freeze-dried starch samples (“FD”) derived from 5 and 10 wt.% starch suspensions corresponding respectively to the first and second row of pictures (a–d) and (e–h). Samples gelled and retrograded at +6 °C overnight are denoted as “FD-G” (pictures (a,b,e,f)), non-gelled samples are denoted as “FD-NG” (pictures (c,d,g,h)). Prior to freeze-drying the samples were frozen at −20 °C in a freezer (“FR”—pictures (a,e,c,g)) or in liquid nitrogen (“LN”—pictures (b,f,d,h)). Sublimation was performed at 2.38 mbar (cold trap temperature −10 °C) for 48 h. The scale bar corresponds to approximatively 1 cm.

Figure 3

Structure of the sample FD-G-FR derived from 5 wt.% starch suspension and freeze-dried at sublimation pressure of 0.045 mbar (A,C) and 2.38 mbar (B).

Figure 4

Dense structure of the sample FD-G-LN derived from 5 wt.% starch suspension and freeze-dried at sublimation pressure of 2.38 mbar. Picture (A) shows an overview of the material structure and picture (B) presents a dense pore wall with higher magnification.

Figure 5

Partially porous structure of the sample FD-G-LN derived from 5 wt.% starch suspension and freeze-dried at sublimation pressure of 0.045 mbar. Picture (C) shows an overview of the material structure. Pictures (A,B) correspond respectively to a pore wall with apparent porosity and a region with smaller pores observed with higher magnifications.

Figure 6

Structure of the sample FD-NG-FR derived from 5 wt.% starch suspension and freeze-dried at sublimation pressure of 2.38 mbar. Picture (A) shows an overview of the material structure and picture (B) a dense pore wall with higher magnification.

Figure 7

Structure of the sample FD-NG-LN derived from 5 wt.% starch suspension and freeze-dried at sublimation pressure of 0.045 mbar. Picture (A) shows an overview of the material structure and pictures (B,C) a pore wall with higher magnifications

Figure 8

Structure of the sample FD-NG-LN derived from 10 wt.% starch suspension and freeze-dried at sublimation pressure of 0.045 mbar. Picture (A) shows an overview of the material which fine structure is displayed with a higher magnification on picture (B).

Figure 9

Structure of the starch aerogel (SCD-G) derived from 10 wt.% starch suspension. Picture (A) shows an overview of the material which fine structure is displayed with a higher magnification on picture (B).

Figure 10

Envelope density comparison of freeze-dried and supercritically dried samples for starch concentration of 5 w% (hashed bars) and 10 wt.% (plain bars).

Figure 11

Porosity comparison of freeze-dried and supercritically dried samples for starch concentration of 5 w% (hashed bars) and 10 wt.% (plain bars).