Green extraction of purple sweet potato and innovative modification of starch for development of food packaging and delivery systems = 자색 고구마의 친환경 추출 및 전분의 혁신적 개질을 통한 식품 포장 및 전달 시스템 개발

다국어 초록 (Multilingual Abstract)

The rising demand for eco-friendly, biodegradable, and functional materials in food packaging and preservation has prompted research into bio-based alternatives to synthetic polymers. This study explored bio-based functional materials derived from purple sweet potato （PSP） and starch using green extraction methods and innovative modification techniques, aimed at applications in smart packaging, active packaging, and emulsion stabilization. Anthocyanin （AC）-rich purple sweet potato and starch were examined as synergistic, bio-based resources for developing green functional materials for food packaging and stabilizing lipophilic bioactives. The study established a conceptual framework through an overview of the composition and biological activities of PSP, new methods for anthocyanin extraction, innovative structural modification of starch, and the physicochemical principles of natural deep eutectic solvents （NADES） and subcritical water hydrolysis （SWH）. Using this framework, we enhanced ultrasound-assisted NADES extraction to optimize AC from PSP via response surface methodology. This method yielded significantly higher pigment and phenolic contents, as well as stronger radical-scavenging activities, compared to conventional acidified ethanol. The crude AC-NADES extracts were directly incorporated into starch–gelatin matrices to produce biodegradable smart films. In these films, NADES served both as the extraction medium and the plasticizer, improving tensile strength, water barrier properties, and pH- responsive color changes suitable for monitoring freshness while maintaining antioxidant capacity. Meanwhile, SWH was employed as an eco-friendly technique to modify the molecular structure of potato starch by reducing its molecular weight, increasing its apparent amylose content, and altering its crystallinity. This resulted in improved cold-water solubility, swelling, and water-holding capacity. The study developed hydrogels from starch treated with subcritical water, which were dried with supercritical CO2 to create low-density, highly porous aerogels. When loaded with phenolic-rich purple sweet potato peel extracts, these aerogels exhibited high water absorption, robust ABTS+ and DPPH radical scavenging, and sustained release of bioactives in both hydrophilic and hydrophobic food simulants. Application tests on fresh chicken breast demonstrated that these bioactive aerogels could absorb exudates while delivering antioxidant protection and enhancing food quality. This suggests that aerogels derived from subcritically hydrolyzed starch could serve as a potential absorbent insert in active packaging systems. Finally, the study examined subcritical water–hydrolyzed starches as solid particle stabilizers for Pickering emulsions of astaxanthin-rich sunflower oil. Starch fractions produced at intermediate hydrolysis temperatures （120 °C – 150 °C） had optimized molecular weight and amylose levels, resulting in finer droplet size distributions, improved rheological structuring, enhanced antioxidant activity, and better retention of astaxanthin during storage and gastrointestinal digestion compared to a control stabilized with Tween 80. Overall, the findings demonstrate that integrating ultrasound-assisted NADES extraction with SWH supports the development of multifunctional, starch- and PSP-based materials that provide antioxidant properties, moisture control, structural integrity, interfacial stabilization, and pH responsiveness within a comprehensive green chemistry framework. This research advances the fundamental understanding of process–structure–function relationships in biopolymer– polyphenol systems and their practical applications in active and intelligent packaging and delivery platforms for food, nutraceutical, and cosmetic uses. Keywords: Purple sweet potato, Anthocyanin, Natural deep eutectic solvents, Subcritical water hydrolysis, Antioxidant activity, Aerogels, Smart film, Starch modification, Pickering emulsion.

국문 초록 (Abstract)

식품 포장 및 보존 분야에서 친환경적이고 생분해 가능하며 기능성을 갖춘 소재에 대한 수요가 증가함에 따라, 합성 고분자를 대체할 수 있는 바이오 기반 소재에 대한 연구가 활발히 이루어지고 있다. 본 연구에서는 자색 고구마（purple sweet potato, PSP）와 전분으로부터 유래한 바이오 기반 기능성 소재를 탐색하였으며, 친환경 추출법과 혁신적인 개질 기술을 적용하여 스마트 포장, 능동 포장 및 에멀전 안정화에의 활용을 목표로 하였다. 안토시아닌（anthocyanin, AC）이 풍부한 자색 고구마와 전분은 식품 포장용 친환경 기능성 소재 및 지용성 생리활성 물질의 안정화를 위한 상호보완적인 바이오 자원으로 평가되었다. 본 연구는 PSP의 조성 및 생리활성, 안토시아닌 추출의 최신 방법, 전분의 혁신적 구조 개질, 그리고 자연유래 심층 공융용매（natural deep eutectic solvents, NADES）와 아임계수 가수분해（subcritical water hydrolysis, SWH）의 물리화학적 원리에 대한 고찰을 통해 개념적 프레임워크를 구축하였다. 이러한 프레임워크를 바탕으로, 초음파 보조 NADES 추출 공정을 반응표면분석법（response surface methodology）을 통해 최적화하여 PSP로부터 안토시아닌의 추출 효율을 향상시켰다. 이 방법은 기존의 산성 에탄올 추출법에 비해 색소 및 페놀 함량이 유의하게 높았으며, 라디칼 소거 활성 또한 우수한 결과를 보였다. 추출된 AC-NADES 원액은 전분–젤라틴 매트릭스에 직접 도입되어 생분해성 스마트 필름을 제조하는 데 활용되었다. 이 과정에서 NADES는 추출 용매이자 가소제로 동시에 작용하여 인장 강도, 수분 차단성, pH 반응성 색 변화 특성을 향상시켰으며, 항산화 활성을 유지하면서 식품의 신선도 모니터링에 적합한 특성을 부여하였다. 한편, SWH는 감자 전분의 분자량을 감소시키고 겉보기 아밀로스 함량을 증가시키며 결정 구조를 변화시키는 친환경 개질 기술로 활용되었다. 그 결과, 저온 수용해도, 팽윤도 및 수분 보유 능력이 향상되었다. 또한 아임계수 처리 전분으로 하이드로겔을 제조하고, 이를 초임계 이산화탄소（supercritical CO2）로 건조하여 저밀도·고다공성 에어로겔을 제조하였다. 이러한 에어로겔에 페놀 화합물이 풍부한 자색 고구마 껍질 추출물을 탑재한 결과, 높은 수분 흡수력과 함께 ABTS⁺ 및 DPPH 라디칼 소거 활성이 우수하였으며, 친수성 및 소수성 식품 모사 용액에서 생리활성 물질의 지속 방출 특성을 나타냈다. 신선한 닭 가슴살에 대한 적용 실험에서는, 해당 바이오활성 에어로겔이 유출액을 효과적으로 흡수하는 동시에 항산화 보호를 제공하고 식품 품질을 향상시키는 것으로 나타났다. 이는 아임계수 가수분해 전분 기반 에어로겔이 능동 포장 시스템에서 흡수 패드로 활용될 수 있는 잠재력을 시사한다. 마지막으로, 본 연구에서는 아임계수로 가수분해된 전분을 아스타잔틴（astaxanthin, AST）이 풍부한 해바라기유 피커링 에멀전（Pickering emulsion）의 고체 입자 안정제로 적용하였다. 중간 온도（120 °C – 150 °C）에서 처리된 전분 분획은 최적화된 분자량과 아밀로스 함량을 나타내었으며, 그 결과 더 작은 액적 크기 분포, 향상된 유변학적 구조, 높은 항산화 활성, 그리고 Tween 80을 사용한 대조군에 비해 저장 및 위장 소화 과정에서 아스타잔틴의 안정성이 개선되었다. 종합적으로, 본 연구는 초음파 보조 NADES 추출과 아임계수 가수분해의 통합 공정이 항산화 기능, 수분 조절 능력, 구조적 안정성, 계면 안정화 및 pH 반응성을 동시에 제공하는 다기능성 전분 및 PSP 기반 소재의 개발에 효과적임을 입증하였다. 또한 본 연구는 바이오고분자–폴리페놀 시스템에서 공정–구조–기능 간의 상관관계에 대한 기초적 이해를 확장하고, 식품, 기능성 식품 및 화장품 분야에서의 능동 및 지능형 포장과 전달 시스템으로의 실용적 적용 가능성을 제시한다.

목차 (Table of Contents)

• Chapter 1 1
• General introduction 1
• 1.1. Purple sweet potato 1
• 1.2. Extraction of PSPAs 3
• 1.2.1. Solvent Extraction 3
• 1.2.2. Ultrasound-Assisted Extraction 4
• 1.2.3. Natural deep eutectic solvents 7
• 1.3. Structure of starch 9
• 1.4. Starch modification and its application 12
• 1.5. Subcritical water hydrolysis (SWH) 14
• 1.6. Objective of the thesis 19
• 1.7. References 21
• Chapter 2 32
• Extraction of anthocyanin from purple sweet potato using ultrasound-assisted natural deep eutectic solvents and its application for smart packaging film 32
• 2.1. Introduction 33
• 2.2. Materials and methods 35
• 2.2.1. Raw materials and chemicals 35
• 2.2.2. Preparation of NADES 35
• 2.2.3. NADES ultrasound-assisted extraction of AC 36
• 2.2.4. Optimization of AC extraction using RSM 36
• 2.2.5. Determination of total AC content 40
• 2.2.6. Gas chromatography-mass spectrometry (GC-MS) analysis of AC 40
• 2.2.7. Total phenolic content (TPC) 41
• 2.2.8. In-vitro antioxidant activity 41
• 2.2.9. UV-visible spectrophotometer analysis of AC-NADES extracts 41
• 2.2.10. Preparation of smart films 41
• 2.2.11. Morphological and structural examination of prepared films 42
• 2.2.12. Thickness and color of films 42
• 2.2.13. Mechanical properties of films 42
• 2.2.14. Behavior of films to moisture 43
• 2.2.15. pH sensitivity of AC-NADES smart films. 44
• 2.2.16. TPC of the films 44
• 2.2.17. Antioxidant activity of the films 44
• 2.2.18. Biodegradation study 45
• 2.2.19. Statistical analysis 45
• 2.3. Results and discussion 46
• 2.3.1. Extraction optimization 46
• 2.3.2. GC-MS analysis of AC extracts 53
• 2.3.3. TPC and antioxidant activity 58
• 2.3.4. Color and thickness of films 62
• 2.3.5. Mechanical and behavioral properties of films to moisture 62
• 2.3.6. UV-visible spectra and color variations of AC-NADES extracts 65
• 2.3.7. Color response of smart films to pH 67
• 2.3.8. SEM and FT-IR analysis 69
• 2.3.9. TGA 71
• 2.3.10. Surface hydrophobicity 73
• 2.3.11. Biodegradation of the film 73
• 2.4. Conclusions 76
• 2.5. References 77
• Chapter 3 85
• Bioactive absorbent aerogels from subcritical water-induced high amylose content starch and antioxidant-rich purple sweet potato peel extracts for food preservation 85
• 3.1. Introduction 86
• 3.2. Materials and methods 88
• 3.2.1. Raw materials and chemicals 88
• 3.2.2. Preparation of bioactive extract from purple sweet potato peel 88
• 3.2.3. UPLC–ESI–QTOF-MS/MS analysis 89
• 3.2.4. Starch modification using the SWH 89
• 3.2.4.1. Gel permeation chromatography (GPC) 89
• 3.2.4.2. Amylose content and reducing sugar content 92
• 3.2.4.3. Physical properties of modified starch under SWH 92
• 3.2.5. Preparation of the aerogels 93
• 3.2.6. Analysis of the aerogel by modified starch and native starch 96
• 3.2.6.1. Density and porosity of aerogels 96
• 3.2.6.2. Morphological and structural examination of prepared aerogels 96
• 3.2.6.3. Water absorption capacity 96
• 3.2.6.3. Antioxidant Capacity 96
• 3.2.6.4. In-vitro release assay of phenolic compounds from aerogels 97
• 3.2.7. Utilization of aerogel for the absorption of exudates from chicken breast fillets 97
• 3.2.8. Statistical analysis 98
• 3.3. Results and discussion 99
• 3.3.1. Properties of MS 99
• 3.3.2. Amylose, molecular weight, and reducing sugar analysis 104
• 3.3.3. Density, porosity, and water absorption of aerogel 107
• 3.3.4. Characterization of the bioactive aerogels (FT-IR, XRD & SEM) 110
• 3.3.5. Phenolic compound profiling of PSPp by UPLC–ESI–QTOF-MS/MS 119
• 3.3.5.1. Phenolic acids and their glycoside derivatives. 119
• 3.3.5.2. Flavonoids and their glycoside derivatives 125
• 3.3.5.3. Other polyphenols 125
• 3.3.6. Antioxidant activity of the bioactive aerogels 127
• 3.3.7. In-vitro release of PSPp from bioactive aerogels 128
• 3.3.8. Aerogel application as an absorbent of chicken breast exudate 131
• 3.3.9. Limitations and future directions 133
• 3.4. Conclusions 133
• 3.5. References 135
• Chapter 4 142
• Subcritical water-hydrolyzed potato starch as a Pickering stabilizer for astaxanthin-loaded sunflower oil emulsion 142
• 4.1. Introduction 143
• 4.2. Materials and methods 145
• 4.2.1. Raw materials and chemicals 145
• 4.2.2. SWH of starch 145
• 4.2.3. Extraction of shrimp waste AST by Supercritical CO2 with sunflower oil as cosolvent 146
• 4.2.4. Gel permeation chromatography (GPC) 146
• 4.2.5. Amylose content and oil absorption capacity 146
• 4.2.6. Preparation of AST-loaded Pickering emulsion 147
• 4.2.7. Determination of turbidity, viscosity, and color measurement 147
• 4.2.8. Droplet size and ζ-potential 148
• 4.2.9. Fourier transform infra-red spectroscopy (FT-IR), Cryo-scanning electron microscope (SEM), and thermogravimetric analysis (TGA) analysis 148
• 4.2.10. In-vitro gastrointestinal digestion 149
• 4.2.11. In-vitro antioxidant analysis 149
• 4.2.12. Rheological properties of emulsion 149
• 4.2.13. Stability assessment of the selected emulsion 150
• 4.2.13.1. The effect of pH, ionic strength, and thermal treatment 150
• 4.2.14. Determination of AST concentration and encapsulation efficiency 150
• 4.2.15. Statistical analysis 151
• 4.3. Results and discussion 152
• 4.3.1. Molecular weight, amylose content, and OAC 152
• 4.3.2. Turbidity, viscosity, color, and AST content 156
• 4.3.3. Particle size distribution and ζ-potential 161
• 4.3.4. FT-IR spectroscopy, Cryo-SEM, and TGA 164
• 4.3.5. Antioxidant activity followed by in-vitro gastrointestinal digestion 169
• 4.3.6. Rheological properties of Pickering emulsion 174
• 4.3.7. Effects of temperature, pH, and ionic conditions 178
• 4.3.8. AST stability and particle size distribution 182
• 4.3.9. Pearson’s correlation coefficients 186
• 4.4. Conclusions 190
• 4.5. References 191
• Summary 198
• Abstract (In Korean) 201
• Acknowledgement 203