Understanding and Controlling the Microstructure of Complex Foods

D. Julian McClements

• Publisher: Woodhead Publishing
• Year: 2007
• Print ISBN:
  9781845691516
• eBook ISBN:
  9781845693671
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

It is widely accepted that the creation of novel foods or improvement of existing foods largely depends on a strong understanding and awareness of the intricate interrelationship between the nanoscopic, microscopic and macroscopic features of foods and their bulk physiochemical properties, sensory attributes and healthfulness. With its distinguished editor and array of international contributors, Understanding and controlling the microstructure of complex foods provides a review of current understanding of significant aspects of food structure and methods for its control.Part one focuses on the fundamental structural elements present in foods such as polysaccharides, proteins and fats and the forces which hold them together. Part two discusses novel analytical techniques which can provide information on the morphology and behaviour of food materials. Chapters cover atomic force microscopy, image analysis, scattering techniques and computer analysis. Chapters in part three examine how the principles of structural design can be employed to improve performance and functionality of foods. The final part of the book discusses how knowledge of structural and physicochemical properties can be implemented to improve properties of specific foods such as ice-cream, spreads, protein-based drinks, chocolate and bread dough.Understanding and controlling the microstructure of complex foods is an essential reference for industry professionals and scientists concerned with improving the performance of existing food products and inventing novel food products. - Reviews the current understanding of significant aspects of food structure and methods for its control - Focuses on the fundamental structural elements present in foods such as proteins and fats and the forces that hold them together - Discusses novel analytical techniques that provide information on the morphology and behaviour of food materials

Part I

Microstructural elements and their interactions

1

Polysaccharides: their role in food microstructure

V.J. Morris Institute of Food Research, UK

1.1 Introduction

Polysaccharides are literally poly-sugars: polymers produced by linking together one or more sugars in a variety of ways. The types of sugars and the nature of the linkages determine the structure and function of the polymer. What are sugars and how are they linked together?

Most sugars found in food polysaccharides are hexoses containing six carbon atoms. Hexoses are generally present in the pyranose form; a heterocyclic ring containing five carbon and one oxygen atom, with the carbon atoms identified by the numbers C1–6 (Fig. 1.1(a)). Arabinose is an example of a sugar found in the furanose form: a heterocyclic ring containing one oxygen and four carbon atoms (Fig. 1.1(h)). Different locations of hydroxyl groups around the ring correspond to different simple sugars: glucose (Fig. 1.1(a)) differs from mannose (Fig. 1.1(c)) and galactose (Fig. 1.1(g)) by the location of hydroxyls at C2 and C4 respectively. In addition, one can encounter more complex sugars, such as deoxy-sugars (e.g., 6-deoxy mannose or rhamnose Fig. 1.1(f)) and uronic acids (e.g., mannuronic acid Fig. 1.1(e)).

Fig. 1.1 Simple sugars found in food polysaccharides: (a) α(1→4) linked D Glucose (Glc), (b) β (1→4) linked D Glc, (c) D Mannose (Man), (d) L Man, (e) D Mannuronic acid (ManA), (f) 6-deoxy L Man or L Rhamnose (Rha), (g) D Galactose (Gal) and (h) L Arabinose (Ara) in the furanose form.

Polysaccharides are formed by linking sugars together by elimination of water between C1 (the reducing end) and suitable positions on other sugars. Any such linkage has two anomeric forms, as illustrated for α(1→4) linked (Fig. 1.1(a)) and β(1→4) linked (Fig. 1.1(b)) glucose, corresponding to two possible locations of the hydroxyl group at C1. Most of the sugars shown (Fig. 1.1) are D optical isomers generally present in food polysaccharides. The equivalent L isomers are mirror image structures and L mannose and L rhamnose are shown as examples (Fig. 1.1(d) and (f)).

Food polysaccharides contain a limited number of sugars. If they contain just one sugar they are called homo-polysaccharides and they can be linear or branched (Fig. 1.2(a) and (b)). Hetero-polysaccharides contain one or more sugars. The structures can be irregular and may be linear (Fig. 1.2(c)) or branched (Fig. 1.2(d)) or show some degree of regularity. Regular structures can be block copolymers (Fig. 1.2(e)), irregularly branched (Fig. 1.2(f)), or possess structural repeat units (Fig. 1.2(g) and (h)). The influence of the type of sugar(s), the glycosidic linkages and the arrangement of sugars within the polymer on polysaccharide structure has been discussed by Rees.¹

Fig. 1.2 Schematic pictures of food polysaccharides: (a) linear homopolymer, (b) branched homopolymer, (c) irregular linear hetero-polysaccharide, (d) irregular branched hetero-polysaccharide, (e) block copolymer, (f) irregularly branched heteropolysaccharide, (g) linear repeat unit structure and (h) branched repeat unit structure.

1.2 Food polysaccharides

Before describing and explaining polysaccharide functionality it is useful to introduce the main food polysaccharides. Most gelling polysaccharides are hetero-polysaccharides (Fig. 1.2). They can be irregular unbranched structures, regular or irregular branched structures, block copolymers, or structures containing complex but well-defined repeat units.

Starch: Two polysaccharides can be extracted from starch.^2–3 Amylose is basically a linear polymer composed of α(1→4) D glucose (Fig. 1.1(a)). Typically amylose molecules will contain several thousand glucose units. Amylopectin^2–3 is a highly multiply-branched homo-polymer containing α(1→6) linked amylosic chains. Amylopectin molecules can contain several hundreds of thousands of glucose units. Within native starch granules the amylopectin branches are present as crystalline lamellae. Hence the branched structure of amylopectin is not random but is regulated, reflecting the location and type of crystalline structure within the granule.

Cellulose: This is a homo-polymer of β(1→4) D glucose⁴ (Fig. 1.1(b)). Pure cellulose is insoluble in water and soluble derivatives used in the food industry are prepared by introducing charge, or substituents that can block hydrogen bonding between molecules, thus reducing aggregation and crystallisation. The common forms are sodium carboxymethylcellulose (CMC), cellulose ethers and particulate microcrystalline cellulose (MCC).

Xanthan and gellan: The bacterial polysaccharides xanthan^2,5 and gellan^2,5 have regular carbohydrate repeat units. Xanthan possesses a cellulosic backbone substituted on every second sugar by a 3-linked trisaccharide sidechain βDMan(1–4)βDGlcA(1–2)αDMan(1-containing mannose (Man) and glucuronic acid (GlcA). Gellan has a linear tetrasaccharide repeat unit −3)βDGlc(1–4)βDGlcA(1–4)βDGlc(1–4)αLRha(1- containing glucose (Glc), glucuronic acid and rhamnose (Rha). The non-carbohydrate substitution pattern for the two polysaccharides are variable and it is not known why this is so: the substitution may be truly irregular and incomplete for all polymers, or extracts may be a mixture of fully substituted and unsubstituted polymers. During isolation of the principal commercial form of gellan, an alkaline treatment is used to de-esterify the material, leaving a regular polysaccharide structure.

Carrageenan, furcellaran and agar: The algal polysaccharides agar and the carrageenan family are extracted from red seaweeds. These polysaccharides have structures^2,6,7 that approximate to a simple disaccharide repeat unit of β(1→3) linked D galactose and α(1→4) linked 3,6 anhydro-D-galactose. The carrageenan family can be subdivided into three members (κ-carrageenan, ι-carrageenan and furcellaran) differing in the site and level of sulphation. The idealised repeat unit for κ-carrageenan is shown in (Fig. 1.3(a)). These algal polysaccharides are heterogeneous showing structural variation both within and between polysaccharides. Structural regularity and gelling ability can be enhanced through choice of seaweed and rational modification during extraction. An important structural defect for agar and carrageenans is replacement of (1→4) linked anhydrogalactose by galactose or galactose-6-sulphate (Fig. 1.3(b)). This alters the shape of the sugar ring and interferes with helix formation. Gelation is critically dependent on helix formation and such structural kinks¹ can seriously impair gelation. Alkali treatment converts galactose-6-sulphate residues to anhydrogalactose residues and enhances gelation. Certain species of seaweeds are richer in particular structural forms: Eucheuma spinosa and Eucheuma cottoni yield almost pure ι- and κ-carrageenans. Detailed chemical analysis has shown that these extracts are mixtures of the two forms and probably intra-molecular hybrid structures.^8–9 Agar is based on a non-sulphated carrageenan-like disaccharide^2,6 which contains 3,6 anhydro α L galactose units.

Fig. 1.3 κ-carrageenan structures: (a) idealised repeat unit for κ-carrageenan and (b) common structural defect in κ-carrageenan. Note how removal of the anhydride bridge changes the...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Introduction
• Part I: Microstructural elements and their interactions
• Part II: Novel methods to study food microstructure
• Part III: Microstructural-based approaches to design of functionality in foods
• Part IV: Microstructural approaches to improving food product quality
• Appendix: Magnetic resonance methods for the study of food microstructure
• Index