Integrated One-Pot Process to Fabricate and Impregnate Starch Aerogels in Supercritical Carbon Dioxide

Summary

This protocol describes the development of a one-pot strategy for the fabrication and impregnation of starch aerogels. Some modifications were made within the traditional fabrication process, which allowed the integration of the three critical steps (gelatinization, retrogradation, and drying) of aerogel fabrication into a single step.

Abstract

The goal of this work was to develop a one-pot strategy for the fabrication and impregnation of starch aerogels with green coffee oil (GCO) in supercritical carbon dioxide (scCO₂). For that purpose, different modifications were made to the production process to improve the integration of the three essential steps of aerogel fabrication. A strategy based on supercritical extraction (SCE) was proposed to address the conventional drying process, as well as the utilization of CO₂ during the aerogel fabrication steps. The development of a novel drying approach was the most challenging task of this work as it should be performed without any solvent-exchange step. The results show that aerogels with high surface area (95 m².g^-1) could be produced using a continuous flow of CO₂/ethanol (20 MPa, 40 °C, 2 mL.min^-1 CO₂ with 11% v/v ethanol). The next step comprised the formation of aerogels in the presence of CO₂. The optimal surface area was 185 m².g^-1. Finally, the integration of all the above steps was achieved, and gelatinization, retrogradation, and drying happened sequentially in the same vessel under CO₂ atmosphere. This one-pot fabrication was followed by the impregnation with GCO using a high-pressure injection step. An impregnation efficiency of 24% was obtained with this one-pot strategy confirming that a fully integrated process for the fabrication and impregnation of starch aerogels could be attained.

Introduction

The fabrication of starch aerogels is often described in three major steps: formation of hydrogels, formation of alcohol gel through solvent exchange step, and drying^1,2. The formation of hydrogel involves two steps: gelatinization followed by retrogradation. The gelatinization promotes irreversible physical changes^3,4,5 in the structure of starch and results from the swelling and denaturation of starch granules. Retrogradation promotes the restructuring of free amylose, leading to the formation of a three-dimensional....

Protocol

1. Development of a supercritical drying method by using a mixture of supercritical carbon dioxide and ethanol

1. Prepare a hydrogel in the traditional way by heating 10 mL of a 15% w/v starch solution in water in a beaker at 120 °C for 20 min with magnetic stirring (600 rpm), as described previously^1,2,22, to promote starch gelatinization.
2. For the retrogradation step, transfer 2 mL of this solution to a .......

Discussion

One of the critical steps of this protocol is the gradual removal of water in the supercritical drying step. To obtain materials with a good surface area, the flow should be maintained between 2 and 3 mL.min^-1, otherwise an extensive pore-collapsed material will be obtained. Thus, judicious control of the extraction conditions is essential to obtain materials with the desirable properties for impregnation. Moreover, the size of the hydrogel is also a critical point. Large hydrogels could be difficult to dry an.......

Materials

Name	Company	Catalog Number	Comments
Equipment
6-Way high pressure valve	Rheodyne
Field Emission Scanning Electron Microscopy	JOEL	JSM-7401 F
High pressure cell 10 mL	Citua (Campinas, Brazil)	Custom made	Set Up construction done by Citua (Campinas, Brazil)
High pressure cell 20 mL	Citua (Campinas, Brazil)	Custom made	Set Up construction done by Citua (Campinas, Brazil)
High pressure p-series Pump	Thar  Technologies	Model Thar SFC, P-50A
HPLC-type pump	Shimadzu	Model LC-10AD Vp
Polypropylene molds (eppendorf)	Eppendorf, Brazil	Safe-Lock Tube 2.0 mL
Surface Area Analyzer	Quantachrome Instruments	Model Nova 1220
X-Ray Diffraction  Instrument	Rigaku	Model Miniflex	Powder X-Ray Diffraction (XRD) Instrument
Chemicals
Ethanol anhydrous, ≥99.5%	Sigma-Aldrich, Brazil		CAS # 64-17-5
Liquid carbon dioxide, 99.98%	Oxilumen, Brazil		CAS # 124-38-9
Native potato starch	Vetec, Brazil		CAS # 9005-25-8