Nanotechnology in the Agri-Food Sector E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 622

Veröffentlichungsjahr: 2011

Beschreibung

Providing an overview of nanotechnology in the context of agriculture and food science, this monograph covers topics such as nano-applications in teh agri-food sector, as well as the social and ethical implications.
Following a review of the basics, the book goes on to take an in-depth look at processing and engineering, encapsulation and delivery, packaging, crop protection and disease. It highlights the technical, regulatory, and safety aspects of nanotechnology in food science and agriculture, while also considering the environmental impact.
A valuable and accessible guide for professionals, novices, and students alike.

Leseprobe

Table of Contents

Cover

Table of Contents

Related Titles

Title page

Copyright page

List of Contributors

Introduction

Part One: Fundamentals

1 Intermolecular Interactions

1.1 Introduction

1.2 Water

1.3 Hydrophobic and Hydrophilic Interactions

1.4 Dispersion Interaction

1.5 Electrostatic Interactions

1.6 Steric Interactions Involving Soluble Polymers

1.7 Epilog

2 Supramolecular Structures

2.1 Introduction

2.2 Self-Assembly

2.3 Plant Cells

2.4 Organized Self-Assembled Structures

2.5 Summary

Part Two: Basic Applications

3 Nanotechnology in Food Production

3.1 Introduction

3.2 Food Production

3.3 Nanotechnology and Food

3.4 Applications of Nanotechnology in Foods

3.5 Concerns about Using Nanotechnology in Food Production

4 Packaging

4.1 Introduction

4.2 Reasons to Package Food Products

4.3 Physical Properties of Packaging Materials

4.4 Antimicrobial Functionality

4.5 Visual Indicators

4.6 Information and Communication Technology

4.7 Discussion

5 Using Nanoparticles in Agricultural and Food Diagnostics

5.1 Introduction

5.2 Biosensors

5.3 Transduction Principles

5.4 Examples of Biosensors in Which Nanoparticles Are Being Used

5.5 Future Prospects

Part Three: Food Applications

6 Nano-Functionalized Techniques in Crop and Livestock Production: Improving Food Productivity, Traceability, and Safety

6.1 Introduction

6.2 Sensors

6.3 Enzyme Biosensors and Diagnostics

6.4 DNA-Based Biosensors and Diagnostics

6.5 Radiofrequency Identification (RFID)

6.6 Integrated Nanosensor Networks: Detection and Response

6.7 Conclusions

7 Nanotechnologies for Improving Food Quality, Safety, and Security

7.1 Introduction

7.2 Improving Quality, Safety, and Security of Agricultural Production

7.3 Improving Quality, Safety, and Security in Food Processing

7.4 Improving Quality, Safety, and Security in Packaging and Distribution

7.5 Wrapping Up

8 Food Functionality and the Physics of Bionanotechnology: Some Examples and Challenges

8.1 Introduction: How Are Foods and Bionanotechnology Related?

8.2 Physics and Structures in Food Bionanotechnology

8.3 Fibrillar Structures

8.4 Plate-Like Structures

8.5 Spherically Symmetric Structures

8.6 Bicontinuous Structures in Protein–Polysaccharide Systems

8.7 Gastronomy and the Nanodomain: Molecular Gastronomy1)

8.8 Conclusions

9 Products and Their Commercialization

9.1 Introduction

9.2 Investment in Nanotechnology Research

9.3 Innovations in Food and Agriculture Nanotechnology

9.4 Nanotechnology Commercialization

9.5 Current and Emerging Markets

9.6 Conclusions

Part Four: Nanotechnology and Society

10 Toxicology of Nanomaterials in Food

10.1 Introduction

10.2 What Makes Nanomaterials Special?

10.3 Characterization of Engineered Nanomaterials

10.4 Safety Assessment of Oral-Exposure Engineered Nanomaterials for Food Application

10.5 Conclusions

11 Nanomaterials in Food and Food Contact Materials – Potential Implications for Consumer Safety and Regulatory Controls

11.1 Background

11.2 Nanomaterials Likely to be Used in Food and Related Applications

11.3 Potential Consumer Safety Implications

11.4 Current and Projected Applications for Food

11.5 Implications for Regulatory Frameworks

11.6 Conclusions

12 Environmental Considerations of and Societal Reactions to Nanotechnology in the Food Sector

12.1 Introduction

12.2 Life Cycle of Nanotechnology Food Products

12.3 Occurrence of Engineered Nanoparticles in the Environment

12.4 How Should Society Deal with Uncertainty?

12.5 Conclusions

13 Nanotechnology and Food Allergy

13.1 Introduction

13.2 Molecules in Foods Involved in Triggering Allergies

13.3 Food Structure, Processing, and Food Allergy

13.4 Impact of Nanoscale Structures on Allergenic Potential of Foods

13.5 Conclusions

Acknowledgments

14 Communication of Risks and Benefits of Nanotechnology: the Issue of Societal Acceptance of Emerging Technologies

14.1 Introduction

14.2 Science and Society: Lessons for Nanotechnology Applied to Food Production

14.3 A Short Introduction to the Psychology of Risk–Benefit Perception

14.4 How do People Form Perceptions of New Technologies

14.5 Nanotechnology Communication in the Business Context

14.6 Conclusion

15 Public Engagement with Emerging Issues in Agri-Food Nanotechnology

15.1 Introduction

15.2 What Is “Public Engagement”?

15.3 Evaluating the Effectiveness of Public and Stakeholder Engagement

15.4 Public Engagement Examples

15.5 Recommendations for Conducting Public Engagement and Public Consultation Exercises

Appendix

16 Nano-Ethics

16.1 Introduction: Historical Background

16.2 Identifying and Avoiding Unethical Nanotechnological Products

16.3 Ensuring Ethical Nanotechnological Research, Innovation, and Production

16.4 Nano-Ethics as the Question of the Good Nanotechnology Society

16.5 Conclusion: The Ethical Challenge Ahead for the Nano-Agri-Food Sector

Acknowledgments

17 Evolving Best Practice in Governance Policy – Developing Consumer Confidence in Risk Analysis Applied to Emerging Technologies

17.1 Introduction

17.2 Introduction to Food Safety Governance

17.3 Potential Innovations to the Risk Analysis Framework as Proposed by SAFE FOODS

17.4 Risk Analysis and Nanotechnology

17.5 Recommendations

Acknowledgments

Index

Related Titles

Krug, H. (ed.)

Nanotechnology

Volume 2: Environmental Aspects

317 pages

2008

Hardcover

ISBN: 978-3-527-31735-6

Kaput, J.

Nutritional Genomics

Discovering the Path to Personalized Nutrition with Wiley Plus Stand-alone t/aNutrition: Everyday Choices Set

2006

Hardcover

ISBN: 978-0-470-13837-3

Castle, D., Cline, C., Daar, A. S., Tsamis, C., Singer, P. A.

Science, Society, and the Supermarket

The Opportunities and Challenges of Nutrigenomics

164 pages

2006

Hardcover

ISBN: 978-0-471-77000-8

Kaput, J., Rodriguez, R. L. (eds.)

Nutritional Genomics

Discovering the Path to Personalized Nutrition

496 pages

2005

Hardcover

ISBN: 978-0-471-68319-3

Brennan, J. G. (ed.)

Food Processing Handbook

607 pages with 189 figures and 41 tables

2006

Hardcover

ISBN: 978-3-527-30719-7

Schmidt, R. H., Rodrick, G. E.

Food Safety Handbook

approx. 864 pages

Hardcover

ISBN: 978-0-471-21064-1

The Editors

Prof. Dr. Lynn J. Frewer

Wageningen University

Marketing & Consumer B. Group

Hollandseweg 1

6700 EW Wageningen

Niederlande

Prof. Dr. Willem Norde

Wageningen University

Dept. Physical Chemistry

Postbus 8038

6700 EK Wageningen

Niederlande

Arnout Fischer

Wageningen University

Marketing & Consumer B. Group

Hollandseweg 1

6700 EW Wageningen

Niederlande

Dr. Frans Kampers

University of Wageningen

Agrotechnology & Food Sc. Gr.

Dreijenplein 8

6703 HB Wageningen

Niederlande

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany

All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.

ISBN: 978-3-527-33060-7

ISBN: 978-3-527-63480-4 (Epub)

ISBN: 978-3-527-63482-8 (Mobi)

List of Contributors

Yuri Aleexev

Institute of Food Research

Norwich Research Park, Colney

Norwich NR4 7UA

UK

Aart van Amerongen

Wageningen University and Research Centre

Food and Biobased Products

Biomolecular Sensing and Diagnostics

Bornse Weilanden 9

6708 WG Wageningen

The Netherlands

Remko M. Boom

Wageningen University and Research Centre

Food Process Engineering Group

Agrotechnology and Food Sciences Department

P.O. Box 8129

6700 EV Wageningen

The Netherlands

Hans Bouwmeester

Wageningen University and Research Centre

RIKILT – Institute of Food Safety

Akkermaalsbos 2

6708 WB Wageningen

The Netherlands

Betty Bugusu

Institute of Food Technologists

525 West Van Buren

Suite 1000

Chicago, IL 60607

USA

Laurence Castle

The Food and Environment Research Agency

Sand Hutton

York YO41 1LZ

UK

Qasim Chaudhry

The Food and Environment Research Agency

Sand Hutton

York YO41 1LZ

UK

Enda Cummins

University College Dublin

Biosystems Engineering

UCD School of Agriculture

Food Science and Veterinary Medicine

Belfield

Dublin 4

Ireland

Arnout R.H. Fischer

Wageningen University and Research Centre

Department of Social Sciences

Marketing and Consumer Behaviour Group

Hollandseweg 1

6706 KN Wageningen

The Netherlands

John D. Floros

The Pennsylvania State University

Food Science Department

Food Science Building

University Park

PA 16802

USA

Lynn J. Frewer

Food and Society

Centre for Rural Economy

School of Agriculture, Food and Rural Development

Newcastle University

Agriculture Building

Newcastle upon Tyne NE1 7RU

UK

Wageningen University and Research Centre

Department of Social Sciences

Marketing and Consumer Behaviour Group

Hollandseweg 1

6706 KN Wageningen

The Netherlands

Frans W.H. Kampers

Wageningen University and Research Centre

Wageningen Bionanotechnology Centre (BioNT)

P.O. Box 8026

6700 EG Wageningen

The Netherlands

Hans Kastenholz

EMPA

Environmental Risk Assessment and Management Group

Lerchenfeldstrasse 5

9014 St. Gallen

Switzerland

Gijs A. Kleter

Wageningen University and Research Centre

RIKILT – Institute of Food Safety

Akkermaalsbos 2

6708 WB Wageningen

The Netherlands

Ursula Vanesa Lay Ma

The Pennsylvania State University

Food Science Department

Food Science Building

University Park, PA 16802

USA

Erik van der Linden

Wageningen University and Research Centre

Food Physics Group

Agrotechnology and Food Sciences Department

Bomenweg 2

6703 HD Wageningen

The Netherlands

Alan R. Mackie

Institute of Food Research

Norwich Research Park

Colney

Norwich NR4 7UA

UK

Bernadene A. Magnuson

Cantox Health Sciences International

2233 Argentia Road

Suite 308

Mississauga

Ontario, L5N 2X7

Canada

Hans J.P. Marvin

Wageningen University and Research Centre

RIKILT – Institute of Food Safety

Akkermaalsbos 2

6708 WB Wageningen

The Netherlands

E.N. Clare Mills

Institute of Food Research

Norwich Research Park

Colney

Norwich NR4 7UA

UK

Mark Morrison

Institute of Nanotechnology

Lord Hope Building

141 St James Road

Glasgow G4 0LT

UK

Willem Norde

Wageningen University and Research Centre

Laboratory of Physical Chemistry and Colloid Science

Chemistry Building, Dreijenplein 6

6703 HB Wageningen

The Netherlands

University of Groningen

University Medical Center

Hanzeplein 1

9713 GZ Groningen

The Netherlands

University of Groningen

Antonius Deusinglaan 1

9713 AV Groningen

The Netherlands

Bernd Nowack

EMPA

Environmental Risk Assessment and Management Group

Lerchenfeldstrasse 5

9014 St. Gallen

Switzerland

Niall O’Brien

University College Dublin

Biosystems Engineering

UCD School of Agriculture

Food Science and Veterinary Medicine

Belfield

Dublin 4

Ireland

Geertruida A. Posthuma-Trumpie

Wageningen University and Research Centre

Food and Biobased Products

Biomolecular Sensing and Diagnostics

Bornse Weilanden 9

6708 WG Wageningen

The Netherlands

Douglas K.R. Robinson

Institute of Nanotechnology

Lord Hope Building

141 St James Road

Glasgow G4 0LT

UK

Gene Rowe

Wageningen University and Research Centre

Department of Social Sciences

Marketing and Consumer Behaviour Group

Hollandseweg 1

6706 KN Wageningen

The Netherlands

Michael Siegrist

ETH Zürich

Institute for Environmental Decisions (IED)

Consumer Behavior

Universitätstrasse 22

8092 Zürich

Switzerland

Roger Strand

University of Bergen

Centre for the Study of the Sciences and the Humanities

P.O. Box 7805

N-5020 Bergen

Norway

Pieter Stroeve

University of California

Davis

Department of Chemical Engineering and Materials Science

1 Shields Avenue

Davis, CA 95616

USA

J.(Hans)C.M. van Trijp

Wageningen University and Research Centre

Department of Social Sciences

Marketing and Consumer Behaviour Group

Hollandseweg 1

6706 KN Wageningen

The Netherlands

Richard Watkins

The Food and Environment Research Agency

Sand Hutton

York YO41 1LZ

UK

Meike T.A. Wentholt

Wageningen University and Research Centre

Department of Social Sciences

Marketing and Consumer Behaviour Group

Hollandseweg 1

6706 KN Wageningen

The Netherlands

Introduction

Lynn Frewer, Arnout Fischer, Willem Norde, and Frans Kampers

As a basic science, nanotechnology has advanced considerably over the past decades. It has generally been agreed that nanotechnology deals with structures of size 100 nm (nanometers) or less in at least one dimension, and involves developing materials or devices within these size limits. Research in nanotechnology has resulted in applications across a wide range of areas, perhaps more so than for most areas of fundamental research in the natural sciences. The potential applications range from those within the medical and pharmaceutical sectors, the development of new materials, personal care products, to applications in agriculture and food (the focus of this particular volume).

In common with other emerging technologies, as well as existing technologies that are being re-evaluated, the opportunities for technological advancement are potentially profound. However, many scholars in the risk assessment community have raised concerns about the toxicity in regard to both human health and the environment. As a consequence, there is an ongoing discussion regarding whether specific measures regarding the regulation of nanoparticles are required. This is as true in the agri-food sector as in other nanotechnology application areas. Some academics have called for stricter application of the precautionary principle, with delayed marketing approval, enhanced labeling, and additional safety data development requirements in relation to certain forms of nanotechnology. From this, there have been discussions of the need to invoke the precautionary principle with regard to the application of nanotechnology. The precautionary principle states that, if an activity – for example, the application of an emerging technology – is potentially associated with health or environmental risks, the burden of proof that it is not harmful falls on those who advocate taking the action, if there is no evidence to suggest otherwise. On the one hand, the application of the precautionary principle allows policy-makers to take discretion in the absence of complete scientific proof of safety. On the other hand, this means that delays occur in marketing approval, and additional safety data may be needed in relation to specific applications of nanotechnology in the agri-food sector. In addition, appropriate labeling strategies may be needed to ensure informed consumer choice, given the emerging societal debate about nanotechnology and its applications to the agri-food sector.

The editors have attempted to address these (and related) issues in the current volume. In Part One, the fundamentals of nanotechnology applied to the agri-food sector are discussed, in particular, intermolecular interactions and self-assembly of macromolecules. In Part Two, the basics applications of nanotechnology in the agri-food sector are identified. Novel techniques such as encapsulation, diagnostics and sensing, and packaging are presented in detail, and their applications to food production are described. Part Three, on specific applications to food, deals with application in crop and livestock production, the application to improving the food supply (in terms of quality, safety, and security), discussion of functionality, and commercialization. Finally, Part Four, which deals with nanotechnology and society, focuses not only on the potential benefits of nanotechnology, but also on potentially emerging risks and what needs to be done to ensure safety. As well as a chapter focusing on the toxicology of nanomaterials in the agri-food sector, there is additional consideration of what this implies in terms of putative changes to risk regulation and governance. As part of this, it is essential to take account of the views and preferences of society, in terms of risk–benefit perceptions and preferences for co-development. These issues are addressed from both a theoretical and a practical perspective. The question that is asked is how should consumers and citizens be effectively involved in the societal debate about the development, application, and commercialization of food nanotechnology. As part of this, emerging ethical issues need to be addressed, and a chapter has been dedicated to discussion of these.

Part One: Fundamentals

1

Intermolecular Interactions

Willem Norde

1.1 Introduction

Nanotechnology may be broadly defined as the study, fabrication, and application of systems by manipulating structures or objects having nanoscale dimensions (say, between 1 nm and 100 nm). Of course, molecular scientists, in both chemistry and biology, have been dealing with nanoscopic (polymer) molecules and biological cell components for decades. So, what’s new? New is that, with the advent in the 1980s of new instrumentation, in particular scanning probe microscopes – for example, atomic force microscopy (AFM) – individual nano-objects can be observed and manipulated (see Figure 1.1).

Figure 1.1 Atomic force microscopy. (a) The topography of a surface is scanned with subnanometer resolution, so that nano-sized objects can be (b) observed and (c) manipulated. (d) Atomic force microscopy may also be applied to determine the interaction between two objects.

Using AFM, the positions of molecules and nanoparticles, relative to each other, may be rearranged in a controlled way. AFM furthermore allows the measurement of interaction forces between nanoparticles as well as between nanoparticles and macroscopic objects. Other recently developed devices, the so-called optical tweezers and magnetic tweezers, also enable the controlled motion of, and the determination of forces between, nanoparticles.

Manipulation on the nanoscale may be done in two “directions”, referred to as top-down and bottom-up. In the top-down approach, structures are made increasingly smaller by progressively removing matter, usually by etching. Perhaps the most well-known example of a top-down structure is the electronic chips present in various devices. Another example is the micro- or nano-sieve, a solid wafer punctured with equally sized micro- or nanopores. Nano-sieves are in particular relevant for food processing and water treatment. Because various agricultural and dairy products are of heterodisperse particulate nature, that is, emulsions, foams, and dispersions of solid particles, they may be fractionated using a series of sieves of varying pore size. The separate components thus obtained may be recombined to give newly composed products of superior quality. Also, nano-sieves could be used in (cold) sterilization by filtering out microbial cells.

In the agri-food sector, however, bottom-up nanostructures are more often encountered. Bottom-up implies that atoms or molecules are distributed and rearranged to build new, functional nano-objects. Nature itself is full of bottom-up nanostructures, especially in living species. Think of viruses, where nucleic acids and proteins are arranged and interact such that viral activity results. Think of microbial, plant, and animal cells in which the various nano-sized organelles and membranes are complex bottom-up assemblies of precisely arranged building blocks (Figure 1.2).

Figure 1.2 Cartoon of a biological cell showing a variety of nano-sized subcellular structures.

Although nature is capable of making structures far more complicated and sophisticated than the ones that scientists can – for the time being – achieve in their laboratories, it may not be a surprise that nano-engineers are strongly inspired by nature. A few examples come to mind: in making addressable biocompatible nanoparticles to be used for the encapsulation and delivery of nutriceuticals and pharmaceuticals, nature provides clues as to how the surface of such particles should look; viruses may serve as a model in the design of particles carrying deoxyribonucleic acid (DNA) fragments to be used in gene therapy; non-fouling surfaces may be mimicked from the outer composition and structure of cell surfaces; and the texture of foodstuff may be optimized by imitating nanostructures as they occur in nature, for example, fibrillar protein aggregates in meat replacers and three-dimensional polymer networks in mousses.

To achieve the specific architectures related to the desired (biological) function of the nanostructure, the physicochemical interactions between the building blocks should be tuned with high accuracy. Needless to say, understanding the mechanisms underlying the various types of interaction is a prerequisite for successful tuning.

In this chapter an introduction to the main types of interactions that may play a role in bottom-up nanotechnology is given. These are physicochemical interactions more or less sensitive to changing environmental conditions and therefore result in the formation of annealed, responsive structures. The discussion here may not be the most rigorous one, as, in view of the scope of this book, the scientific language of chemistry and physics that involves formulas and equations will be avoided as much as possible.

In natural systems, including those of the agri-food sector, most nano-objects exist by virtue of their interaction with an aqueous environment. Not only their existence but also their shape and spatial structure are to a large extent determined by their interaction with water. It is, therefore, essential first to pay attention to some physicochemical properties of water.

1.2 Water

Water is one of the most abundantly occurring chemical compounds on Earth (although very unevenly distributed). Because of its ubiquity, we are inclined to think of water as a trivial, common, and normal liquid. However, from a physicochemical point of view, water is a highly extraordinary substance. By virtue of its unique properties, water is the medium in which life has evolved and is sustained. Which properties make water so special, and how can these properties be explained and understood at the molecular level?

Water, H2O, has a molar mass of 18 g mol−1. Under ambient conditions, water boils at 100 °C. Among other components of comparable molar mass, this is an exceptionally high temperature. For instance, the boiling points of methane (14 g mol−1) and ethane (30 g mol−1) are −162 °C and −88 °C, respectively. Also, the heat of vaporization, which is essentially the energy required to separate the molecules when they go from the condensed liquid phase to the gas phase, is extremely high for water, 2255 J g−1, whereas for methane and ethane this is a little more than 500 J g−1. Another interesting property is the heat capacity, the amount of heat needed to increase the temperature of a substance by one degree Celsius. For the sake of fairness, equal amounts of the substances should be compared and at the same temperature and pressure. While the heat capacity of water at 20 °C and 1 atm amounts to 4.18 J K−1 g−1, the values for other liquids are much lower (cf. for chloroform it is 0.90 J K−1 g−1 and for ethanol 2.49 J K−1 g−1). These anomalously high values for the boiling point, heat of vaporization, and heat capacity (and, in this context, further extraordinary characteristics of water could be presented) originate from the phenomenon that water molecules attract each other. They attract each other so much that they strongly attach to one another. In scientific terms, water shows a strong internal coherence. Hence, to evaporate the liquid, favorable interactions between the water molecules have to be disrupted (which explains the large heat of vaporization), and this does not occur before the molecules have attained strong thermal motion (explaining the high boiling point). The large heat capacity reflects not only that heat is used for increasing thermal motion (corresponding to a one degree temperature rise) per se, but also that loosening of the internal coherence is necessary to increase thermal motion, which is the major energetic cost.

To understand the strong internal coherence, we zoom in on the molecular structure of water. Figure 1.3a shows a model of the molecular architecture of an H2O molecule. The hydrogen (H) atoms are very small relative to the oxygen (O) atom. Hence, the H2O molecule is nearly spherical, having a radius of about 0.14 nm. Atoms consist of a positively charged nucleus around which negatively charged electrons are orbiting. Hydrogen atoms have the tendency to donate their electrons for sharing with oxygen, which eagerly accepts that donation. Hydrogen is an electron donor and oxygen is an electron acceptor. Because of the positions of the H atoms relative to the O atom, the charge in the (overall electrically neutral) H2O molecule is not evenly distributed. Positive charges (+q) are centered on each H atom, and compensating negative charges (–q) are on the opposite sides of the O atom.

Figure 1.3 Water. (a) Model of a water, H2O, molecule showing the positive charges +q on the hydrogen atoms and the negative charges –q on the oxygen atom. (b) Polar interaction occurs through so-called hydrogen bonds between water molecules. (c) The three-dimensional structure of a water lattice in which all potential hydrogen bonds are realized (i.e., ice).

Because of those positive and negative sides, H2O is said to be a polar molecule. It is the polar character of the molecules that causes the strong internal coherence in water: the positive and negative poles attract each other (Figure 1.3b), so that each H2O molecule tends to be connected to four other H2O molecules via so-called hydrogen bonds. In its solid state, ice, the water molecules are in more or less fixed positions, with all four hydrogen bonds realized. Owing to the positions of the poles on the H2O molecules this results in a relatively open structure (Figure 1.3c) with an H2O volume density of 55%. When put under pressure some hydrogen bonds may become disrupted and the regular ice structure will be distorted: ice melts under pressure, and in the liquid state H2O has a somewhat less open structure or, in other words, a higher density than in the solid state. This is another peculiarity of water. In the liquid state the H2O molecules are still strongly associated in clusters and participate in about three out of the potential four bonds per molecule. Contrary to what one would expect for an associated liquid, the viscosity, that is, the fluidity, of water is not strikingly different from that of other, non-polar, liquids. The mobility of the individual water molecules in the clusters, underlying the macroscopic fluidity of the liquid, is retained, because the molecules readily rotate and hop about every 10−11 s from one partner to another, while having most, but not all (as in ice), hydrogen bonding potentialities satisfied.

Another property of water that deserves attention is the dielectric constant. Without going into detail, the dielectric constant is a measure of the ability to screen the electrostatic interaction between two charges at a given separation distance. Water has a high dielectric constant: in water, electrostatic interaction is 20 times weaker than in chloroform and about five times weaker than in ethanol. It is for this reason that salts in water dissociate into their oppositely charged ions. For the same reason, (bio)polymers, such as proteins, DNA, and polysaccharides, as well as synthetic polymers that contain ionizable groups, acquire charge in an aqueous medium. And so do the surfaces of (solid) materials when they are exposed to water.

Now, having gained some insight into some relevant characteristics of water, we may be able to understand the crucial role of water in shaping bottom-up nanostructures.

1.3 Hydrophobic and Hydrophilic Interactions

It has been illustrated and discussed in Section 1.2 that water is a strongly associated liquid because of favorable polar intermolecular interactions. Addition of another substance (referred to as “solute”) will disturb the coherence between the water molecules (Figure 1.4). If the solute molecules are also polar or have a net charge (ions), the polar water molecules interact favorably with the solute as well, just as they do with other water molecules. In that case the solute readily dissolves in water. The polar substance is called hydrophilic. Salts, sugar, and alcohol are examples of hydrophilic substances. However, if the solute is uncharged and non-polar (i.e., does not have an uneven charge distribution over its molecule), the water molecules prefer to stay attached to each other rather than to the non-polar solute molecules. This results in the non-polar molecules being expelled from the water and driven together to form a separate phase. Oils and fats therefore do not mix with water. For the same reason, the surfaces of plastics, Teflon, and so on, are poorly wetted by water. Such substances, disliked by water, are referred to as hydrophobic.

Figure 1.4 (a) Polar and (b) non-polar molecules immersed in water.

The description given above of water bordering other substances is highly simplified, especially in the case of non-polar, hydrophobic materials. There are still controversial issues to be solved. Nevertheless, theoretical and experimental studies indicate that, at hydrophobic surfaces, reorientation of water molecules imposes a higher degree of structural order in the adjacent water layer (the so-called hydration layer). Obviously, water molecules bordering non-polar surfaces tend to arrange themselves in a preferred orientation that allows them to form as many H bonds as possible with water molecules in the nearest-neighboring layer, and so on. As nature strives for disorder (in scientific terms, for maximum entropy), hydration of non-polar material is unfavorable and, consequently, the contact area between water and non-polar substances tends to be as small as possible. As a result, non-polar substances spontaneously associate in water because this leads to reduction of the contact area between water and the non-polar substance. Such association between non-polar, hydrophobic substances in an aqueous environment is known as hydrophobic interaction. Clearly, the hydrophobic interaction scales with the water-accessible surface area of the non-polar moieties involved. Hydrophobic interaction is one of the major types – if not the major type – of interaction occurring in biological systems. If interference by any other type of interaction does not occur, hydrophobic interaction leads to a featureless two-phase system: an “oil” (popular indication of non-polar) phase separates out from an aqueous phase. However, a different and more interesting pattern presents itself when solute molecules possess a non-polar and a polar part. Such molecules are referred to as amphiphilic. They occur abundantly in nature, for instance in lipids and proteins.

A typical lipid molecule has a non-polar tail and a polar head-group, as depicted schematically in Figure 1.5a. In water, such molecules show dual behavior. The hydrophobic tails, rejected by the water, assemble together, but the polar heads are preferably hydrated. The exposure of the heads to the aqueous medium prevents phase separation on a macroscopic scale. Instead, microscopic or nanoscopic non-polar phases, surrounded by the polar, hydrated head-groups, are spontaneously formed. Depending on the dimensions of the tail and the head, relative to each other, the amphiphilic molecules aggregate in spherical, worm-like or bilayer structures that close to form spherical objects called vesicles or liposomes. Such structures that are spontaneously formed are named self-assembled structures (see Figure 1.5b–d).

Figure 1.5 (a) The typical structure of an amphiphilic (= “lipid”) molecule consisting of a polar and a non-polar part. (b)–(d) In an aqueous environment, amphiphilic molecules aggregate to form supramolecular structures of various geometries.

Self-assembly plays an important role in shaping the spatial structure of proteins as well. Proteins may be viewed as hundreds of units (the amino acids) linked together to form a long chain, illustrated in Figure 1.6a. There are 22 different amino acids available and, therefore, an almost infinite number of sequences of amino acids along the chain is possible. Some amino acids are polar, others are non-polar. This makes the protein amphiphilic. In an aqueous environment, the chain will try to fold such that the non-polar groups are shielded from contact with water and the polar ones are exposed to water. Of course, other types of interaction, such as electrostatic ones (to be discussed in Section 1.5) may interfere with these tendencies. Still, globular proteins possess a relatively non-polar interior and a polar exterior (see Figure 1.6b). It may be clear that the exact folding pattern depends on the composition and, even more so, on the sequence of the various amino acids along the chain. Many different proteins are active in biological systems, each one having its own specific structure and function.

Figure 1.6 Folding of (a) a poly(amino acid) chain, containing polar (black) and non-polar (gray) amino acids, into (b) a compactly structured globular protein molecule, where the non-polar amino acids prefer to reside in the protein’s interior, shielded from contact with water, and the polar amino acids prefer the periphery, exposed to the aqueous environment.

It should be emphasized that the lipid assemblies shown in Figure 1.5 and the folded protein structure in Figure 1.6 represent highly ordered structures. Their spontaneous formation demonstrates the constructive power of chaos: such ordered structures exist because of increased disorder in the water that is released from hydrophobic hydration.

It is no surprise that (bio)nano-engineers are strongly inspired by the phenomenon of self-assembly. They design and synthesize tailor-made amphiphilic molecules to build their desired supramolecular assemblies and nano-objects.

1.4 Dispersion Interaction

Dispersion interaction is the most generic one among the different types of physical interactions. Wherever there is matter, irrespective of its properties, there are dispersion interactions.

The origin of dispersion interaction between two atoms may be understood as follows. In any atom, negatively charged electrons orbit around a positively charged nucleus. Although the time-average position of the electrons with respect to the nucleus may be concentric, at any point in time their positions deviate from that average, which gives rise to a small dipole moment. This instantaneous dipole generates an electric field, which, in turn, induces a dipole moment in a neighboring atom, and so on. Thus dispersion interactions are larger between atoms that have a larger polarizability. When the interaction occurs across a medium, the excess polarizabilities (i.e., the polarizability of the atoms in vacuum minus that of the atoms of the medium in vacuum) should be taken into account. Dispersion interaction across a medium is therefore strongly reduced, as compared to vacuum. Dispersion interaction between two atoms diminishes very strongly with separation distance (say, within 0.5 nm), but between particles, containing a large ensemble of atoms, they are more long-ranged and may be effective over (tens of) nanometers.

The strength and the range of dispersion interaction between particles depend not only on the polarizability of the constituent atoms but also on the density (number of atoms per unit volume) and the size of the particles. Dispersion interactions may play significant roles in aggregation of particles, in coating of surfaces with particles, and in folding polymeric molecules into condensed structures, such as, for example, globular proteins. Because dispersion interactions are non-directional, do not require special properties of the interacting species, and occur always and everywhere, they hardly provide a handle to direct bottom-up fabrication of nanostructures.

1.5 Electrostatic Interactions

Electrostatic interactions occur between electric charges. We may distinguish between interactions involving (i) atoms and small molecules, and (ii) polymers, particles, and surfaces.

1.5.1 Atoms and Small Molecules

Atoms and small molecules interact electrostatically when they have a net charge (i.e., when they are ions), and also when they are uncharged but polar. Obviously, interaction between positive and negative charges is attractive, whereas charges having the same charge sign repel each other. In Section 1.2 it was mentioned that interaction between charges across a medium is inversely proportional to the dielectric constant of that medium. It was further pointed out that the dielectric constant is directly related to the polarity of the molecules of the medium.

Charged groups are almost always surrounded by water, but sometimes they reside in a non-aqueous environment. This occurs, for instance, when ionic amino acids become trapped in the interior of a globular protein molecule. In such a low-dielectric environment, ions can only exist when they form a pair: two oppositely charged ions close together, as depicted in Figure 1.7.

Figure 1.7 Ion-pair disruption in an unfolding protein molecule.

The question arises whether ion pairing would contribute to the folding of the protein molecule. An ion pair in the compact structure is electrostatically favored over a wider separation in the unfolded hydrated chain (where the dielectric constant is almost that of water), but the pairing goes at the expense of favorable hydration of the ions in the unfolded structure. These two effects more or less balance each other. Hence, ion pairing as a factor promoting either a compact or unfolded structure in an aqueous medium is highly uncertain. Yet, if ionic groups are forced into a non-aqueous, low-dielectric environment – for instance, due to hydrophobic bonding of adjacent non-polar moieties – pairing between ions of opposite sign is strongly promoted.

Similar reasoning applies to the weaker electrostatic interactions between an ion and a dipole, and to the even weaker dipole–dipole interaction.

Because of the relative strength of electrostatic interactions, inserting charged groups at selective locations in a polar and/or non-polar environment can help to direct construction of a desired nanostructure.

1.5.2 Polymers, Particles, and Surfaces

Polymers, particles, and (macroscopic) surfaces in an aqueous environment are often charged, in most cases due to groups that dissociate or associate with hydrogen ions. The charge is balanced by an uneven, diffuse distribution of counter-ions in the adjoining solution in a so-called electrical double layer (Figure 1.8). Electrostatic interactions involving polymers, particles, and surfaces are therefore referred to as electrical double layer interactions.

Figure 1.8 Electrostatic potential profile across an electrical double layer.

The diffuse distribution of ions in the electrical double layer gives rise to an electrostatic potential that drops off with increasing distance from the charged object. Without explaining the details here, it should be mentioned that the potential decays more steeply with increasing salt concentration in solution (= higher ionic strength) and the separation distance over which the two charged bodies interact decreases correspondingly. For example, in tap water that distance would be about 15 nm, in milk 1.3 nm, in blood 0.8 nm, and in sea water 0.3 nm. For nano-engineers electrical double layer interactions are convenient to work with because of the possibility to modulate their magnitude by just varying easily adjustable parameters such as pH and ionic strength.

In bottom-up nanotechnology, charged soluble polymers, named polyelectrolytes, are essential building blocks. Natural polymers, such as proteins, deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and polysaccharides, are well-known examples, but synthetic polyelectrolytes are regularly used in nanotechnological applications as well. When the polyelectrolytes have the same charge sign, they repel and try to avoid each other; but when they are oppositely charged, interesting phenomena occur. Oppositely charged polyelectrolytes form complexes mainly because of the release of counter-ions from the electrical double layers, which implies increasing disorder and therefore a higher entropy in the system. Here, too, as described for amphiphilic molecules in Section 1.3, the formation of macroscopic aggregates is prevented if at least one of the two polyelectrolytes is linked to an uncharged hydrophilic polymer in a so-called block copolymer. During complexation of the two polyelectrolytes, the uncharged polymer block tends to remain dissolved in the surrounding water, thereby preventing the complexes from growing to macroscopic dimensions. This results in the formation of polyelectrolyte micelles, as shown in Figure 1.9, that are generally referred to as polyion condensates or complex coacervate core micelles. Recently these structures have attracted a lot of attention as potential nano-containers for the encapsulation and controlled release of pharmaceuticals and nutriceuticals, as well as for their application as nano-bioreactors.

Figure 1.9 Formation of a polyelectrolyte micelle.

Complexation between oppositely charged polyelectrolytes can also be used to regulate the consistency of certain foodstuffs. The texture of dairy products such as (drink) yogurts may be optimized by adding positively charged polysaccharides that associate with, for instance, negatively charged milk proteins.

Particles dispersed in water carry their charged groups exclusively at the aqueous periphery (except, perhaps, for a few ion pairs formed inside the non-aqueous interior of the particle; see Section 1.3). When charged particles approach each other, the electrical double layers overlap, giving rise to repulsion between like-charged particles, and attraction if the particles are oppositely charged. A repulsive electrical double layer overlap helps to keep the particles apart and therefore contributes to the stabilization of the dispersion. Even when the particle surface is predominantly hydrophobic, at not too high ionic strength, electric double layer repulsion operating over a larger distance than attractive dispersion and hydrophobic interaction (see Sections 1.3 and 1.4) prevents the particles from aggregating. In media of higher salt concentration, typically beyond, say, 1% sodium chloride, electrical double layer repulsion is too short-ranged, and attractive dispersion interaction takes over and the like-charged particles aggregate. It goes without saying that oppositely charged particles readily aggregate under all conditions of ionic strength (see Figure 1.10).

Figure 1.10 Different packing patterns of aggregated particles.

The stability of nanoparticles against aggregation is a requirement in various food products, and in pharmaceutical and cosmetic formulations. Destabilization of particulate dispersions may be required when clearing, for example, wine or fruit juices and in (waste)water purification processes.

Electrostatic interactions between polymers and particles, between polymers and surfaces, and between particles and surfaces follow the same principles as described above for polymers and particles. Thus, attachment of polymers onto particles and planar surfaces, and of particles onto planar surfaces may be electrostatically favored or disfavored. As in nature, in nanotechnology, polymer–particle interactions often determine whether or not particles connect to each other. This will be further discussed in Section 1.6.

Surfaces can be covered by a layer of nanoparticles to provide the surface with special properties with respect to, for example, wetting or scratch resistance. Finally, polymers are applied at surfaces to render them resilient against (bio)fouling by suppressing the deposition of proteins, biological cells, and microbes. Non-fouling surfaces are a requirement for successful operation of equipment in food processing and water purification equipment, such as separation membranes, heat exchangers, and so on.

1.6 Steric Interactions Involving Soluble Polymers

In both naturally occurring and man-made nanoscopic constructs, polymers are present in solution and/or attached (= adsorbed) onto (particle) surfaces. The polymers could be uncharged or charged (polyelectrolytes), strongly hydrated or more compact. Polymers, either attached to a surface or not, influence the behavior of nanosystems through steric interactions. These interactions are intricate and delicate, and may change from attractive to repulsive by subtle variations of environmental conditions.

1.6.1 Depletion Aggregation of Particles by Non-adsorbing Polymers

The center of a non-adsorbing polymer molecule cannot approach a particle surface closer than its own radius. Hence, around each particle there is a zone where polymer molecules cannot be accommodated. When particles, by diffusion or otherwise, come closer than twice the radius of the polymer, the polymer will be moved aside and the gap between the particles will be depleted of polymer and just be filled with solvent. This situation is schematically illustrated in Figure 1.11. Because solvent and soluble polymers tend to mix homogeneously, solvent molecules flow from the gap into the solution, which drives the particles together. This type of aggregation induced by non-adsorbing polymers is referred to as depletion aggregation. Depletion aggregation is enhanced by increasing the concentration and the size of the polymer molecules.

Figure 1.11 Aggregation of particles driven by overlapping polymer depletion zones.

1.6.2 Bridging Aggregation of Particles by Adsorbing Polymers

Most polymers tend to adsorb at surfaces. This may be due to binding of hydrophobic segments in the polymer chain (to escape from the hostile water), to hydrogen bonding with surface groups, or, in the case of polyelectrolytes and charged surfaces, to favorable electrostatic interactions. When a polymer adsorbs onto a surface, it usually does not lie flat on that surface but adopts a “loopy” structure with loose tails at the terminal ends, as shown in Figure 1.12. If the particle surface is in excess of the polymer, the surface will only become partially covered by the polymer. This allows one and the same polymer molecule to bridge between different particles (see Figure 1.13). Similar bridging aggregation can occur if bare particles are added to particles that are covered with polymer molecules, fully or not.

Figure 1.12 Structure of a coiled polymer (a) in solution and (b) adsorbed onto a surface.

Figure 1.13 Particle aggregation by polymer bridging.

Bridging aggregation requires that the polymer adsorbed on one particle extends into the surrounding solution far enough to reach another particle. When both particles are charged and repel each other electrostatically (Section 1.5), polymer bridging can only take place if the extension of the polymer exceeds the range over which electrostatic repulsion operates. For this reason, bridging aggregation is more likely to happen with longer polymers and at higher salt concentrations in solution, where the range of electrostatic interaction is reduced. The aggregates formed are loosely structured, with the particles not in direct contact with each other. Closer approach of the particles is detrimental, as it would impose unfavorable deformation of the polymer bridges between the particles.

1.6.3 Stabilization of Dispersed Particles by Adsorbing Polymers

When there is an excess of polymers, each particle is saturated with a layer of adsorbed polymer. Then, upon approach of the particles, the outermost fringes of the loops and tails anchored to the respective particles begin to interpenetrate. This leads to unfavorable confinement of the polymer and, hence, to repulsion between the particles that is effective over roughly twice the thickness of the adsorbed polymer layer (Figure 1.14).

Figure 1.14 Steric stabilization of polymer-coated particles.

Such polymer coatings may be applied to stabilize dispersions under conditions where electrostatic repulsion is insufficiently strong to keep the particles apart, that is, in the case of oppositely charged and uncharged particles, but also of like-charged particles in media of high ionic strength (Section 1.5). To protect the particles from aggregation, the separation distance at which steric repulsion becomes effective (to be regulated by the thickness of the polymer coatings) should be large enough to suppress attractive dispersion interaction (Section 1.4).

Thus, the impact of polymers on interparticle interaction strongly responds to environmental conditions and can therefore readily be manipulated. For instance, by varying the quality of the solvent (e.g., by changing pH, ionic strength, temperature, additives, etc.), the polymer behavior may be adapted from adsorbing to non-adsorbing and vice versa. In this way, bridging may be eliminated, steric stabilization converted into depletion aggregation, and so on. When solvent quality is reduced to below the solubility of the polymer, steric repulsion between fully polymer-covered particles changes into attraction.

1.6.4 Polymer Brushes to Prevent Particle Aggregation and Particle Deposition at Surfaces

A very effective method of steric stabilization can be achieved by grafting polymers at one end onto a (particle) surface, leaving the rest of the molecule dangling in solution. This is best obtained by using diblock copolymers, of which one block has a strong affinity for the surface and the other for the solvent. The profile of the polymer at the surface depends on the grafting density, as shown in Figure 1.15. Obviously, steric stabilization against particle aggregation or deposition improves with increasing grafting density, and is extremely effective when the polymer molecules are stretched out from the surface in a so-called polymer brush. The density of the brush prevents indwelling particles entering, and the strong hydration and high mobility of the polymer chains cause a strong resilience against compression. According to this principle, liposomes (see Section 1.3) used in drug delivery systems may be protected against removal by the immune system, allowing them a longer circulation time in the body. Similarly, the corona polymers of polyion micelles (see Figure 1.9) form a brush that stabilizes the micelles, possibly loaded with some functional ingredient, against external attack by, for example, enzymes or immunoproteins.

Figure 1.15 End-grafted polymers in (a) a mushroom and (b) a brush conformation.

Polymer brushes may also be applied to planar surfaces to make them resistant to protein adsorption and microbial adhesion, which, in turn, suppress biofouling. An example is given in Figure 1.16. Besides use in various biomedical applications, non-fouling surfaces are of the utmost importance in the food industry to avoid – or, at least, to retard – the formation of a biofilm that will reduce heat and mass transfer and increase frictional resistance and, moreover, may be a source of microbial contamination.

Figure 1.16 Effect of applying a polymer brush (right-hand side of each image) on the adhesion of micro-organisms: (a) Staphylococcus epidermidis and (b) Candida albicans.

Nature uses its own polymers, usually polysaccharides, to keep cell surfaces clear of unwanted particles or molecules. Nature even goes a step further. At the far end of some polymer chains, receptor molecules may be attached that bind specific target molecules with high affinity. In this way, a bio-selective surface (Figure 1.17) is obtained that binds target molecules but prevents non-specific deposition of other species. Bio-nanoresearch is under way to mimic this principle for application in highly specific and sensitive biosensors and solid-state diagnostics. Such devices have great potential to be used for quality control in the agri-food sector.

Figure 1.17 Cartoon of a functionalized bio-selective surface.

1.7 Epilog

For (bio)nanostructures to be functional, they have to respond to an external trigger or signal. Internal bonds in such structures should therefore not be permanent but be sensitive to changing environmental conditions. The physical interactions discussed in this chapter provide such flexibility. For the sake of simplicity, the different types of interaction were presented separately. However, it should be realized that they rarely occur separately, but are usually interdependent. For instance, hydrophobic interaction may be accompanied by ion pairing in a non-polar environment. Conversely, like-charged ions may prevent hydrophobic interaction from occurring as in the case of stabilization of hydrophobic particles by electrical double layer interaction. Another example of interweaving interactions is the combination of electrostatic and steric effects induced by polyelectrolytes.

Hydrophobic, electrostatic, and steric interactions depend on different properties, that is, polarity, charge, solubility, and polymer adsorption behavior, whereas dispersion forces are less specific. Knowledge of the origin, characteristics, and mutual dependence of the various types of interaction provides the nano-engineer with clues to design the building blocks to be used in bottom-up nanostructuring. If tailor-made building blocks are brought together in a well-defined, usually aqueous, medium, they may self-assemble to yield the desired nano-object. Isn’t it miraculous? It is almost magic! It is like a car spontaneously emerging from the proper blend of its parts, screws, and bolts.

Further Reading

For more specific and detailed information the reader is referred to:

Norde, W. (2011) Colloids and Interfaces in Life Sciences and Bionanotechnology, 2nd edn, CRC Press, Boca Raton, FL (forthcoming).

Israelachvili, J.N. (2004) Intermolecular and Surface Forces, 3rd edn, Academic Press, New York.

2

Supramolecular Structures

Pieter Stroeve

2.1 Introduction

Well before the current fascination with nanoscience and nanotechnology, scientists were studying phenomena on the nanoscale. For example, colloid and surface scientists have been interested in colloidal dispersions, micelles, vesicles, and surface modification by a layer of molecules for more than 150 years. Cell biologists have studied the organized structures existing in living cells since the nineteenth century. These structures are now known to have intricate geometry on the nanoscale, with very specialized molecular functions such as transport, synthesis, and energy generation. Plant cells have very complex internal structures similar to living cells. Most of our food is ultimately derived from plants, and studies of plant cells on the nanoscale are giving microbiologists, plant scientists, food scientists, and engineers new information about how to modulate plant growth, plant diversification, harvesting, food processing, and food preservation. For food science, nanotechnology has a different meaning from that encountered in other disciplines, such as the fabrication of integrated circuits for high-speed computers. For food, nanotechnology can be defined as the understanding of food on the nanoscale and translating this knowledge into new processes for food modification and enhancement of food value and preservation. This approach is one of the greatest challenges in food science and engineering.

The understanding of plant cells on the nanoscale is the fundamental basis for developing the nanoscience and nanotechnology to produce new and improved foods. By way of example, pulsed electric field (PEF) processing may be mentioned. PEF processing has been used to increase the rate of dehydration of water from fruits to produce dried fruit. It is well known that for certain conditions PEF can irreversibly open nanopores in the plant cell membranes (electroporation) and water can then escape more rapidly from the plant cell. The cell membrane is made up of a lipid bilayer, and lipids have self-assembly properties. The optimal conditions for PEF to increase dehydration for different fruits are not known. Likewise, the optimum nanopore size in the cell plant membrane and the number of openings per unit area of membrane are not known. Although there is now some knowledge about the response of the lipid bilayer to PEF, in a cell membrane proteins are associated with the lipid bilayer and they change the properties of the cell membrane significantly. Thus the study on PEF and its effect on plant cell membranes on the nanoscale can give important information that would be useful for making decisions on PEF processing of plant materials used for food.

In addition to the drying of fruits and vegetables, PEF is also useful in killing bacteria in process water streams by making the membranes of bacteria leaky. Another application of PEF that has been explored is enzyme deactivation. Further, reversible electroporation may provide an opportunity to introduce desirable components (color, flavor, nutrients, antioxidants) or remove valuable components while damage to the plant is reversible.

Another process that is receiving considerable interest is high-pressure processing of foods. In this process, food is treated at elevated pressures of the order of 6000 atm. The purpose of the treatment is to inactivate bacteria and to change the food quality. The precise effects of high-pressure processing on plant cell structures and properties are not known but are currently under study by a number of groups.

The study of the properties and functions of nanostructures in plant cells and their changes due to processing is of utmost importance to develop the database for devising new food processes. The molecules in plant cells inherently can self-assemble into structures and this process of self-assembly will be treated in this chapter.

2.2 Self-Assembly

Self-assembly is the process in which a disordered system of molecules spontaneously forms an organized structure or pattern that is at equilibrium or in a quasi-equilibrium state. A typical example is when surfactant molecules dissolved in water self-assemble to form micelles. A typical surfactant is sodium dodecyl sulfate (SDS), shown in Figure 2.1, which is an anionic surfactant. A surfactant contains both an “oil-loving” hydrocarbon chain and a “water-loving” hydrophilic head-group. This gives the surfactant molecule amphipathic (“being of two kinds”) properties, in that the surfactant can be in either an aqueous or a hydrophobic environment. In an aqueous environment, SDS can self-assemble its hydrocarbon chains into a micellar aggregate.

Figure 2.1 The chemical structure of sodium dodecyl sulfate.

In self-assembly, the organized structure or pattern formed has a reduced free energy compared to the initial state of the disorganized molecules. The specific molecular interactions existing between the molecules, before and after the spontaneous change, lead to the lowering of the overall free energy. Self-assembly processes in plant cells often take place at constant temperature and ambient pressure in an aqueous environment. For a spontaneous process to take place at constant temperature and pressure, the second law of thermodynamics states that the Gibbs free energy change between the initial state of a mixture of molecules, and the final state of some organized molecules, must be equal to or less than zero:

(2.1)

Since the Gibbs free energy is related to the change of enthalpy and entropy, Equation 2.1 can be changed to

(2.2)

where H is the enthalpy, S is the entropy, and T is the absolute temperature. It can be inferred from Equation 2.2 that a decrease in enthalpy and an increase in entropy favor the occurrence of the process. However, if one of the terms, that is, ΔH or TΔS, opposes the process, the other one must have an (over)compensating favorable contribution to allow for a spontaneous process. The process of self-assembly of molecules to form organized structures does lead to an overall increase of entropy if both the organizing molecules and the solvent molecules are considered. This phenomenon can be further understood by the fact that, for the initial state, the water molecules in contact with an individual organic molecule are organized around the organic molecule like a cage. The water cage is similar to a clathrate and has a crystal-like structure. Upon self-assembly of the organic molecules (with themselves), the initial state and order of the water molecules around the individual organic molecules is changed, and the water molecules now in their final state are more random than before and are more like bulk water. Thus, even though the organic molecules have increased order and decreased entropy, the net entropy of the system has increased due to the increase in disorder and greater increase in entropy of the water molecules.

The spontaneous and reversible organization of molecular units into ordered structures occurs by non-covalent molecular interactions. The molecular interactions include van der Waals forces, hydrogen bonding, π–π bonds, and ionic interactions. These molecular interactions are often called weak interactions because their energies are considerably weaker (by a factor of 10 or more) than covalent or other bonds. Nevertheless, weak interactions play a very important role in nature. Weak interactions are responsible for the state of a pure component, such as the liquid or solid state versus the gaseous state. Obviously, weak interactions are of great importance in biological systems, such as the self-assembly of organized structures in plant cells. Examples of organized structures in biology are self-assembled monolayers, lipid bilayers, micelles, and vesicles. The folding of peptide chains into functional proteins and enzymes is another example of structures in biology and occurs in plant cells. The weak interactions are also responsible for the possibility that the organized structures can undergo changes due to a change in thermodynamic variables, and return to the original structure if the thermodynamic conditions return to the original values, that is, the structures can show a degree of reversibility. The weak forces allow the structure to change, effectively seeking a new minimum free energy, depending on the existing thermodynamic conditions. Thus the properties of organized structures can change depending on the thermodynamic conditions. In terms of applications, such as food processing using plant materials, it makes external control of induced changes by processing steps such as temperature, pressure, electric field, and so on more difficult and troublesome because one needs to fully understand not only the structural changes that occur on the nanoscale but also the change in the properties on the nanoscale level and the change in the overall properties on the macroscopic level. On the other hand, it opens exciting new opportunities to change the properties of foods and the possibility to create new value-added foods.

It is possible for chemical reactions to cause molecules to self-assemble. An example is the chemisorption of molecules on a surface to form an organized monolayer. The driving force for self-assembly is the change in enthalpy due to the chemical transformation. This type of self-assembly is not reversible.

Before considering plant cells and self-assembled structures, one should understand that the complexity of molecules that can self-assemble is of great importance. The chemical composition of molecules, their size and shape play important roles in what type of organized structures can be created. Because of this complexity, a wide variety of nano- and mesoscopic structures can be formed depending on the type of molecules involved. Further, it is not necessary that the molecules are all the same: mixtures of different molecules (composition, molecular weight, shape, size, and charge) can self-assemble into organized structures. Properties associated with the structures depend on the nature of the structure and the types of molecule that make up the structure. Nature has learned to exert fine control on the formation of cellular structures with specific functions and properties, by choosing the appropriate precursors to self-assemble into the desired structure. These structures can be called supramolecular assemblies.

From a historical point of view, researchers in the discipline of chemistry were the first to explore the field of supramolecular chemistry, the assembly of synthesized molecules that can arrange into precise, well-organized structures. The synthesis of special molecules that can give rise to supramolecular assemblies is known as supramolecular chemistry. Supramolecular assemblies include molecular self-assembly, folding, molecular recognition, and host–guest chemistry (enzyme–substrate). The principles of supramolecular chemistry and supramolecular assembly are similar to what is done by nature in biological systems. In both situations, molecules can interact because of weak molecular interactions. The folding of two single-stranded deoxyribonucleic acid (DNA) chains into a double helix is a supramolecular assembly and it is a consequence of weak interactions. The base-pairing in the DNA double helix formation is an example of molecular recognition that is solely due to weak molecular interactions. The study of non-covalent interactions is important in many biological systems. Self-assembly is crucial to the function of cells. An example is the self-assembly of lipids to form membranes, the formation of double helical DNA, and the assembly of proteins.

2.3 Plant Cells

Plant cells are eukaryotic cells that have distinctive, organized structures. A plant cell is shown in Figure 2.2