Food Stabilisers, Thickeners and Gelling Agents E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Stabilisers, thickeners and gelling agents are extracted from a variety of natural raw materials and incorporated into foods to give the structure, flow, stability and eating qualities desired by consumers. These additives include traditional materials such as starch, a thickener obtained from many land plants; gelatine, an animal by-product giving characteristic melt-in-the-mouth gels; and cellulose, the most abundant structuring polymer in land plants. Seed gums and other materials derived from sea plants extend the range of polymers. Recently-approved additives include the microbial polysaccharides of xanthan, gellan and pullulan. This book is a highly practical guide to the use of polymers in food technology to stabilise, thicken and gel foods, resulting in consistent, high quality products. The information is designed to be easy to read and assimilate. New students will find chapters presented in a standard format, enabling key points to be located quickly. Those with more experience will be able to compare and contrast different materials and gain a greater understanding of the interactions that take place during food production. This concise, modern review of hydrocolloid developments will be a valuable teaching resource and reference text for all academic and practical workers involved in hydrocolloids in particular, and food development and production in general.
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Contents
Preface
Acknowledgements
Contributors
1 Introduction
1.1 INTRODUCTION
1.2 FUNCTIONAL PROPERTIES
1.3 REGULATORY ENVIRONMENT
1.4 COMMERCIAL ENVIRONMENT
1.5 FUTURE DEVELOPMENTS
2 Acacia Gum (Gum Arabic)
2.1 INTRODUCTION
2.2 ORIGIN AND PURIFICATION PROCESS
2.3 CHEMICAL STRUCTURE
2.4 APPLICATIONS
2.5 HEALTH BENEFITS
2.6 FUTURE DEVELOPMENTS
DISCLAIMER
References
3 Agar
3.1 INTRODUCTION
3.2 RAW MATERIALS
3.3 PRODUCTION
3.4 COMPOSITION AND STRUCTURE
3.5 FUNCTIONAL PROPERTIES
3.6 APPLICATIONS
3.7 FUTURE DEVELOPMENTS
Acknowledgements
Dedication
References
4 Alginates
4.1 INTRODUCTION
4.2 PRODUCTION
4.3 CHEMICAL COMPOSITION
4.4 FUNCTIONAL PROPERTIES
4.5 GEL FORMATION TECHNIQUES
4.6 APPLICATIONS
4.7 THICKENING AND STABILISING
4.8 DAIRY PRODUCTS
4.9 FILM FORMATION
4.10 ENCAPSULATION
4.11 OTHER APPLICATIONS
4.12 SUMMARY
References
5 Carrageenan
5.1 INTRODUCTION
5.2 RAW MATERIALS
5.3 MANUFACTURING
5.4 REGULATION
5.5 STRUCTURE
5.6 FUNCTIONAL PROPERTIES
5.7 FOOD APPLICATIONS
5.7.1 Water gelling applications
5.7.3 Applications summary
References
6 Cellulose Derivatives
6.1 INTRODUCTION
6.2 RAW MATERIALS AND PROCESSING
6.3 COMPOSITION AND CHEMISTRY
6.4 FOOD APPLICATIONS
6.5 FUTURE DEVELOPMENTS
References
7 Gelatine
7.1 INTRODUCTION
7.2 MANUFACTURING PROCESS
7.3 REGULATIONS: EUROPEAN UNION AND THE USA
7.4 CHEMICAL STRUCTURE AND REACTIVITY
7.5 PHYSICOCHEMICAL PROPERTIES
7.6 FOOD APPLICATIONS
7.7 FUTURE DEVELOPMENTS
References
8 Gellan Gum
8.1 INTRODUCTION
8.2 MANUFACTURE
8.3 CHEMICAL COMPOSITION
8.4 FUNCTIONAL PROPERTIES
8.5 REGULATORY STATUS
8.6 APPLICATIONS
8.6.1.3 Asian foods
8.6.5 Confections
8.7 FUTURE DEVELOPMENTS
References
9 Gum Tragacanth and Karaya
9.1 GUM TRAGACANTH
9.2 GUM KARAYA
References
10 Inulin
10.1 INTRODUCTION
10.2 RESOURCES AND RAW MATERIALS
10.3 PRODUCTION
10.4 CHEMICAL STRUCTURE
10.5 PHYSICAL AND CHEMICAL PROPERTIES
10.6 PRINCIPLE OF FAT REPLACEMENT
10.7 PHYSIOLOGICAL PROPERTIES
10.8 APPLICATIONS
References
11 Konjac Glucomannan
11.1 INTRODUCTION
11.2 RAW MATERIALS
11.3 PROCESSING
11.4 STRUCTURE
11.5 FUNCTIONAL PROPERTIES
11.6 FOOD APPLICATIONS
11.7 NUTRITIONAL APPLICATIONS
11.8 FUTURE DEVELOPMENTS
References
12 Microcrystalline Cellulose
12.1 INTRODUCTION
12.2 MCC PRODUCT TECHNOLOGIES
12.3 MANUFACTURING PROCESS
12.4 COLLOIDAL MCC PRODUCT LINE EXTENSIONS
12.5 PHYSICAL MODIFICATION – THE ALLOYING
12.6 PHYSICAL AND FUNCTIONAL PROPERTIES
12.7 LEGISLATION AND NUTRITION
12.8 FOOD APPLICATIONS
12.9 FUTURE DEVELOPMENTS
References
13 Pectin
13.1 INTRODUCTION
13.2 RAW MATERIALS
13.3 PROCESSING
13.4 COMPOSITION
13.5 CHEMICAL PROPERTIES
13.6 APPLICATIONS
13.7 FUTURE DEVELOPMENTS
References
14 Pullulan
14.1 INTRODUCTION
14.2 RAW MATERIALS
14.3 PRODUCTION
14.4 FUNCTIONAL PROPERTIES
14.5 FOOD APPLICATIONS
14.6 FUTURE DEVELOPMENTS
References
15 Seed Gums
15.1 INTRODUCTION
15.2 RAW MATERIALS
15.3 PRODUCTION
15.4 COMPOSITION
15.5 FUNCTIONAL PROPERTIES
15.6 FURTHER DEVELOPMENTS
15.7 DERIVATISED SEED GUMS FOR TECHNICAL APPLICATIONS
References
16 Starch
16.1 INTRODUCTION
16.2 RAW MATERIALS
16.3 PROCESSING
16.4 COMPOSITION AND STRUCTURE
16.5 THICKENING AND GELLING PROPERTIES
16.6 STARCH MODIFICATION
16.7 FOOD APPLICATIONS
16.8 CONCLUSIONS
Acknowledgements
References
17 Xanthan Gum
17.1 INTRODUCTION
17.2 PRODUCTION
17.3 CHEMISTRY
17.4 SOLUTION PREPARATION
17.5 RHEOLOGY
17.6 STABILITY AND COMPATIBILITY
17.7 INTERACTIONS
17.8 APPLICATIONS
References
Colour plates
Index
Food Science and Technology
To Katie
This edition first published 2010
© 2010 by Blackwell Publishing Ltd
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Library of Congress Cataloging-in-Publication Data
Food stabilisers, thickeners, and gelling agents/edited by Alan Imeson.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-4051-3267-1 (hardback: alk. paper)
1. Hydrocolloids. 2. Food additives. 3. Food-Analysis. 4. Food-Composition. 5. Food industry
and trade. I. Imeson, A. (Alan) TP456.H93F66 2010
664′.06-dc22
2009016433
Preface
Stabilisers, thickeners and gelling agents are inherent in almost all living organisms. They determine a number of critical functions including moisture binding and control, structure and flow behaviour that enable organisms to thrive in a natural environment. For use in food products, the functional materials are carefully extracted from various natural raw materials and incorporated into foods to give the structure, flow, stability and eating qualities desired by consumers.
These additives include traditional materials such as starch, a thickener obtained from many land plants, and gelatine, an animal by-product giving characteristic melt-in-the-mouth gels. Cellulose, the most abundant structuring polymer in land plants, seed gums and other materials derived from sea plants extend the range of polymers. Recently approved additives include the microbial polysaccharides of xanthan, gellan and pullulan. With stringent regulations in place governing the use of additives, it is unlikely that many new polymers will be approved and researchers must employ the current range of products to deliver the range of attributes needed for their particular food products.
Hydrocolloids have a profound impact on food properties when used at levels ranging from a few parts per million for carrageenan in heat-treated dairy products to high levels of Acacia gum, starch or gelatine in jelly confectionery. The correct application of these materials is a fascinating topic that continues to engage the attentions of many expert researchers. Over recent years, investigative techniques have shed more light on the fine structure of the polymers to enhance the understanding of network formation and how they combine with other polymers. These structures determine a number of properties in finished foods, such as emulsion stability, the long-term suspension of fortified beverages using ‘fluid gels’ and for giving rich, creamy eating qualities.
Calorie-dense materials such as fats and oils may be replaced with ‘structured water’ to give healthy, reduced-calorie foods with excellent eating quality. Some fibres are currently being studied for their effects on satiety and the reduction of daily energy intake. In addition to the functional attributes, future acceptance and, possibly, positive endorsement may derive from the recognition that soluble and insoluble fibres contribute many physiological benefits to the natural function and well-being of the body.
This book is highly practical and directed to all those involved in various sectors of the food industry. Although it is particularly valuable for product and process developers, marketing personnel will appreciate the value of these highly functional materials and it will help people involved in ingredient procurement appreciate that these materials are often complex functional additives. The information is easy to read and assimilate. New students will find chapters presented in a standard format, enabling key points to be located quickly.
Those with more experience will be able to compare and contrast different materials and gain a greater understanding of the interactions that take place during food production. This concise, modern review of hydrocolloid developments will be an invaluable teaching resource and reference text for all academic and practical workers involved in hydrocolloids in particular and food development and production in general.
Acknowledgements
In commending this book to readers, I must pay tribute and thank all the authors and contributors to the chapters in this book. Their great enthusiasm and commitment over the long period needed to complete this project has produced an excellent series of chapters linking the structure and function of the polymer in nature to a wide range of properties needed for high-quality foods.
These excellent contributions summarise the current state of knowledge on the use of these materials in food. The authors have used data, diagrams and figures made available by many suppliers to the hydrocolloid industry. The cooperation and support from major manufacturers and suppliers have been essential in producing this book. These companies continue to enhance and extend their product ranges, to actively investigate new applications for their products, to provide detailed support and direction to new customers and to contribute to new publications.
I would also like to acknowledge the support of the publishers, Wiley-Blackwell, who have continued to encourage and support this project, despite several setbacks, before this successful conclusion.
Finally, I must recognise the great inspiration, encouragement, support and tolerance from my family, particularly my wife, Hazel, whilst I have nurtured this project to a successful outcome.
Alan Imeson
Contributors
Peder Andersen
Technologist, FMC BioPolymer, Sandvika,
Norway
William R. Blakemore
Principal, Celtic Colloids Inc., Topsham,
ME, USA
Sarah M. Brejnholt
Research Scientist, CP Kelco, Lille
Skensved, Denmark
Greg Buliga
Technology Growth Leader, FMC
BioPolymer, FMC Corp., Princeton, NJ,
USA
Sandra J. Caputo
Technical Service, Regulated Industries,
Ashland Aqualon Functional Ingredients,
Wilmington, DE, USA
Mary Jean Cash
Technical Service, Regulated Industries,
Ashland Aqualon Functional Ingredients,
Wilmington, DE, USA
Hiroto Chaen
Plant Manager, Okayama Plant II,
Hayashibara Co., Ltd., Okayama-shi, Japan
Ross Clark
Distinguished Research Fellow, CP Kelco,
San Diego, CA, USA
Therese Fjæreide
Project Leader, FMC BioPolymer,
Sandvika, Norway
Olav Gåserød
Team Leader Alginate Commercial
Technology, FMC BioPolymer, Sandvika,
Norway
Alan R. Harpell
Senior Scientist, FMC BioPolymer, FMC
Corp., Princeton, NJ, USA
Trond Helgerud
Alginate Commercial Technology
Manager, FMC BioPolymer, Sandvika,
Norway
Alan Imeson
Account Manager, FMC BioPolymer,
Epsom, Surrey, UK
Gregory R. Krawczyk
Senior Research Scientist, FMC
BioPolymer, FMC Corp., Princeton, NJ,
USA
Christian Klein Larsen
Senior Development Chemist, FMC
BioPolymer, Sandvika, Norway
Jenny M. Mayes
Technical Manager, Arthur Branwell & Co.
Ltd., Epping, Essex, UK
Jean-Marc Parry
Reasearch & Development Manager, Kalys
SA., Bernin, France
Dennis Seisun
President, IMR International, San Diego,
CA, USA
Paul Sheldrake
Market Manager, Avebe Food, Sheffield,
UK
Paul Stevens
Technical Service and Application
Development Manager, Rousselot NV,
Gent, Belgium
Graham Sworn
Group Manager Gums and Systems
Development, Danisco, Paris, France
Francis Thevenet
Technical Director, Colloides Naturel
International, Rouen, France
Domingo C. Tuason
Senior Engineering Associate, FMC
BioPolymer, FMC Corp., Princeton, NJ,
USA
Raymond Valli
Senior Research Scientist, CP Kelco,
San Diego, CA, USA
Willem Wielinga
Consultant, Taegerwilen,
Switzerland
Rudy Wouters
Head of Food Application Technology,
BENEO Group, Tienen, Belgium
1
Introduction
Dennis Seisun
ABSTRACT
Food stabilisers, thickeners and gelling agents are obtained from a wide range of natural raw materials including microorganisms, land and sea plants and animal connective tissues. They control moisture and provide structure, flow, stability and eating qualities to food products. Approvals for food use and purity criteria are closely controlled by regulation. Commercial applications are determined by the combination of properties provided by these materials including the current significant market drivers of price and availability coupled with consumer and retailer preferences. Future developments with hydrocolloids will recognise the value of nutritional and therapeutic benefits in addition to the functional attributes.
1.1 INTRODUCTION
‘Little known and yet ubiquitous in virtually all processed foods’. This statement summarises the role of stabilisers, thickeners and gelling agents in today’s food industry. It is virtually impossible to list the multitude of functions that these additives have in making foods look, feel and taste like they do. Such a list is virtually impossible, not only because it is so long and all encompassing, but also because it is changing and growing all the time. New uses and functions for these unique ingredients are constantly being found.
1.1.1 Scope
Within the food industry, stabilisers, thickeners and gelling agents are often more simply referred to as food hydrocolloids. The hydrocolloids traditionally used in food thickening and gelling include, but are not limited to, the following: agar, alginates, arabic, carrageenan, cassia tora, carboxymethyl cellulose, gelatin, gellan, guar, karaya, konjac, locust bean gum, methyl cellulose, hydroxypropylmethyl cellulose, microcrystalline cellulose, pectin, starches, tara, tragacanth and xanthan.
This book deals with all of these materials in a structured fashion starting with raw materials, followed by the production process and ending with application-related information. Chemical structure and conformation, viscosity and gelation charts and many food formulations are included for each ingredient in separate chapters. Readers will obtain a good overview of scientific, technical and commercial aspects for each material.
1.1.2 Definition
All the stabilisers, thickeners and gelling agents covered in this book are also known and described as ‘food hydrocolloids’ implying that functional properties are obtained by mixing them with water. A strict definition of a hydrocolloid is, however, difficult. Ask ten scientists what is a hydrocolloid and it is likely that ten different answers will be obtained. These could include statements as follows:
A colloidal substance obtained from…A material that hydrates in water…A colloid forming a suspension and not a true solution in water…A synonym for gum (e.g. guar gum, locust bean gum, gum arabic…)A macromolecule, such as a carbohydrate polymer or a protein, that is water soluble…Many of these ingredients are carbohydrates but at least one important hydrocolloid, gelatin, is a protein. Most are agricultural derivatives but some are biotechnology derived, and gelatin, of course, is an animal product. This volume presents some scientific information but focuses more on the ‘real world’ of application-related data that will be of most benefit to food technologists and food formulators. Issues of functional properties, synergy, production and raw materials are most relevant to readers, but molecular structure and chemical definition have also been covered.
1.1.3 Classification
The hydrocolloids are treated in alphabetical order in this book, but readers should bear in mind that there are several methods of grouping them. Raw material origin has been used to classify them, for example as seaweed extracts, seed gums, fermentation products or plant exudates. Their general functional properties may also be used to classify them as thickeners, stabilisers or gelling agents. More recently, their commercial availability and price stability have been used as a differentiating factor. Those that are commercially steady in availability and price include the cellulose derivatives and fermentation products, such as xanthan and gellan gum. On the other hand, notorious for their instability in terms of price and availability are guar gum, locust bean gum and gum arabic. More recent developments may allow a further aspect of classification and differentiation, namely nutritional and therapeutic function. Future research may uncover yet more functions for these versatile food wonders.
1.1.4 Differentiated grades
Many, if not all, of the thickening and gelling agents for food are available in a wide range of differentiated grades. Starch is a good example. There are literally hundreds of different food starches based on different raw materials and production process conditions. A doublederivatised, waxy maize starch is totally different from, for example, a pregelatinised potato starch. Cellulose derivatives, such as carboxymethyl cellulose, microcrystalline cellulose, methyl cellulose and hydroxypropylmethyl cellulose, come in a virtually limitless range of differentiated grades depending on the degree of substitution and other processing factors. A large number of ‘new’ and ‘differentiated’ properties have been and continue to be developed for hydrocolloids that fall under an ‘umbrella category’; for example methyl cellulose, hydroxypropylmethyl cellulose and microcrystalline cellulose can be produced in a multitude of grades to suit a wide range of specific functional needs. Xanthan is offered in different mesh sizes, rapidly hydrating, brine tolerant and/or as a clarified grade. New versions are constantly being developed and assure the specialty future of at least part of this market.
1.2 FUNCTIONAL PROPERTIES
The following is a brief overview of the key functional properties for which these ingredients are used. Nutritional properties are relatively new and nutraceutical or health-enhancing properties are even more recent. Further work is sure to advance the use of hydrocolloids beyond modification of the rheology of foods.
1.2.1 Viscosity
Viscosity is probably one of the most widely used properties. In this respect, hydrocolloids are often used in systems where the oil or fat content has been reduced or eliminated through substitution with water. The hydrocolloid thickens water, which, in turn, replaces the fat or oil to give a product with similar properties to the full-fat food. A typical application for this function is reduced-fat salad dressings. In other cases, the thickened water simply adds body, texture and mouthfeel to a food such as table syrups, particularly low-calorie syrups.
1.2.2 Stability
If oil or fat is partially removed from a formulation and is replaced with thickened water, an emulsion is usually formed. Often the function of the hydrocolloid is to stabilise the emulsion, to prevent separation and, in the case of frozen foods, to control ice crystal formation. New technology and new ingredients have been developed specifically to address the problem of ice crystals in frozen foods, but hydrocolloids will continue to play a role. Virtually every ice cream product sold in retail outlets is stabilised with carrageenan, locust bean gum and/or guar gum. Low-fat salad dressings, discussed above, also benefit from emulsion-stabilising properties.
1.2.3 Suspension
If insoluble particles are included in the thickened product then separation and settling should be eliminated or at least minimised. Some hydrocolloids create solutions with a ‘yield point’ that will keep particles immobilised in suspension. Salad dressing is a good example of this and xanthan gum is the typical hydrocolloid to supply this functionality.
1.2.4 Gelation
One of the key texturising aspects of hydrocolloids is the ability to gel and solidify fluid products. For example, in gelled milk desserts, even low levels of carrageenan will form a solid milk gel. Other classic gelling agents are pectin, gelatin and agar. Many others, however, will form a gel under specific conditions. Certain grades of alginates form gels with calcium ions. Xanthan and locust bean gum do not gel individually but together they display synergy and form a strong cohesive gel. Methyl cellulose and hydroxypropylmethyl cellulose are unusual in forming solutions that reversibly thicken or gel when heated. The food industry has a myriad of gelling applications ranging from soft, elastic gels to hard and brittle gels.
1.2.5 Nutritional and nutraceutical
There is already a wide use of some hydrocolloids, arabic and guar gum, for example, as sources of soluble dietary fibre. Much research has been conducted in the nutraceutical benefits of hydrocolloids. Potential benefits range from cholesterol reduction to cancer risk prevention. Their use in weight loss programmes is already widespread and likely to expand further.
1.3 REGULATORY ENVIRONMENT
1.3.1 Background
The use of hydrocolloids in food has been steadily evolving. Pectin, agar, starches and gelatin have been used for centuries. They are amongst the few hydrocolloids that are sold directly to consumers at the retail level. These are the hydrocolloids with which the consumer is most familiar and comfortable. Until the 1980s and 1990s, gelatin was amongst the preferred and most label-friendly hydrocolloids, but this changed rapidly and dramatically with the advent of bovine spongiform encephalopathy (BSE), otherwise known as mad cow disease. Gelatin provides an example of the changing fortunes for individual hydrocolloids in the marketplace.
Many of the hydrocolloids in use today were developed long before regulatory approvals and constraints were imposed on use levels or in specific applications. Alginates, agar and carrageenan, for example, were extracted from seaweeds some of which were eaten as a basic food. Red seaweeds have long been used as a food. In Ireland, ‘carrageen’ was used to gel dairy products centuries ago. Extracts from such seaweeds were therefore deemed safe for use in food. This principle of ingredients and extracts thereof that are ‘generally recognised as safe’ (GRAS) is still in use today albeit under more rigorous review. Many of the more recent texturising options offered to food technologists derive from differentiated hydrocolloids based on materials that are currently approved. Carrageenan, for example, can be modified in many ways. There is lambda, kappa and iota carrageenan. There is a refined version produced through alcohol precipitation or specific precipitation with potassium chloride (the ‘gel press’ process) and there is a semi-refined carrageenan produced through a much simpler process. Semi-refined carrageenan made its way into the texturising world only in the 1980s and 1990s, whereas refined carrageenan has been offered for 60–70 years or more.
1.3.2 Legislation
Regulatory approval of a food ingredient is critical. Without approval of the appropriate government bodies, the additive has no market or function in food. Nowadays, obtaining regulatory approval for a new ingredient is a very costly and time-consuming process. The approval process itself is evolving and has opened up opportunities for differentiated versions of existing products. This has allowed for continued improvement and innovation to be offered to food formulators.
Labelling is a key factor in marketing any food ingredient. The rules governing food label nutritional information have changed significantly. There has been little impact on hydrocolloids other than fibre claims. There are literally hundreds of differentiated food starches. On the food label, however, starches are simply segmented into modified or native starches. Dozens of grades of carboxymethyl cellulose are produced, but they are all simply called carboxymethyl cellulose or cellulose gum. In Europe, E-numbers have beene stablished for all hydrocolloids. Often, however, for marketing reasons, food processors elect to replace the E-number with the accepted name of the additive. For example, locust bean gum, guar gum and carrageenan are often declared instead of E410, E412 and E407, respectively.
Regulatory authorities strictly control the approval of food additives. Chemical modifications are generally not allowed with the exception of starches, cellulose derivatives and propylene glycol alginate. Physical and enzymatic modification, however, is allowed. Physically modified pectin, sold under the name SlendidTM by CP Kelco Division of JM Huber, is an example. Some new hydrocolloids are brought to market under the GRAS designation, such as tara gum, konjac and pullulan. Gellan gum is one of the last hydrocolloids to go through a full food additive petition on a global scale. Its approval took many years and tens of millions of dollars of research and lobbying effort. Nearly 20 years after its approval, gellan gum has not yet reached commercial volumes that justify the cost of bringing it to market. These high stakes and high risks probably mean that no new hydrocolloid will be taken through a full approval process in the foreseeable future. Cassia gum has recently been approved in France (August 2008) and more widespread approval in the EU is expected soon. Approval for human food in the USA is still pending.
Genetic engineering could offer a tremendous opportunity for new functional developments in hydrocolloids. Bio-fermentation products such as xanthan and gellan gum could be manipulated to provide specific functional properties not now available. Seaweed, seed or other agricultural raw materials could be genetically enhanced. Giant, rapidly growing kelp could be programmed to produce more than alginates. The time for a carob tree to reach maturity and give a commercially viable yield could be reduced from the required 12–15 years. All these scenarios for improved production currently have a lid tightly sealed over them by consumer concerns. This is not to say that future generations and future nutritional conditions will not radically change.
1.3.3 Consumer concerns
Readers of this book must bear in mind one fundamental concept in terms of markets for food hydrocolloids: ‘The perception of consumers is the reality of hydrocolloid producers.’ In other words, whatever the scientific facts, it is the consumers’ perceptions and resulting action or inaction that dictates the commercial future of any food ingredient, including hydrocolloids. In terms of perception and label image, pectin probably has the best and most friendly image. Seaweed extracts such as agar, carrageenan and alginates are also ‘label friendly’. Cellulose derivatives have a ‘variable’ image in the mind of consumers. Terms such as carbohydrate gum and vegetable gum, used in the USA to describe methyl cellulose and hydroxypropylmethyl cellulose, are very label friendly. On the other hand, a name such as carboxymethyl cellulose has a very chemical connotation, but its synonym, cellulose gum, is much more label friendly. This is the main reason why EU authorities were lobbied to allow cellulose gum on the label.
Until the late 1980s and early 1990s, gelatin had an excellent label image, but that changed dramatically with the advent of BSE or mad cow disease as it is more commonly known. Consumers made a perceptual link between gelatin and BSE. Gelatin consumption suffered dramatically and most large food formulators made strong efforts to replace or eliminate the use of gelatin. All the scientific evidence indicates that there is no risk of the BSE prion being found in gelatin, but consumer concerns and their effect on buying behaviour are often more emotional than rational and scientific.
1.4 COMMERCIAL ENVIRONMENT
1.4.1 Global market
The global market value for all food hydrocolloids during the period April 2007 to March 2008 is estimated at US$4.2 billion. This market value is calculated at the basic producer level. Once markups by service companies and distribution channels have been factored into the equation, the food hydrocolloid market is worth around US$5.0–5.5 billion per year. Gelatin and starches are by far the largest in terms of value, as indicated in Table 1.1.
The price of gelatin and starch is lower than the average of many other hydrocolloids. This means that, in terms of volume, gelatin and starches account for an even higher proportion of the total than indicated in Table 1.1. Pure gellan gum is one of the most expensive hydrocolloids at US$14–15/lb (US$31–33/kg). Native starches are at the other end of the scale and can be purchased for US$0.15–0.20/lb or US$0.33–0.44/kg. Based on price and value calculations, the total volume of hydrocolloids in 2007–2008 was around 1.7–1.8 million metric tonnes. Growth rates vary depending on the hydrocolloid. Not surprisingly, large volume items such as starches are growing at a low rate of 1.0–1.5%. Modified starches in the USA, however, are growing much faster at 4-5%. Overall, the average growth for hydrocolloids is estimated at 2.5–3.0%.
Table 1.1 Global hydrocolloid estimates (April 2007 to March 2008, estimated annual market).
HydrocolloidPrice (million US$)Value (%)Starches130727Gelatin82324Pectin44310Carrageenan4169Xanthan2256Agar2095Arabic1535Locust bean gum1344Carboxymethyl cellulose1463Alginates1252Guar762Microcrystalline cellulose601Methyl571cellulose/hydroxypropylmethyl celluloseOther421Total4216100Excl. starch290973Fig. 1.1 Guar price history.
1.4.2 Cost
These food ingredients are cost-effective and generally stable in price. Overall, the cost of texturising, stabilising or gelling a food formulation has been steady or even declined over the last 30–40 years. Hydrocolloid producers have improved the efficiency of production, reduced costs and even improved performance such that lower use levels are needed. The suppliers of raw materials have also tried to keep costs down. There are, however, severe fluctuations in price and availability in some of these products. Prices in 2008 have shattered the image of stability which most of these ingredients had achieved. Dramatic price increases and even shortages were experienced for several hydrocolloids in 2008 and expected to continue well into 2009.
1.4.2.1 Guar gum
Guar gum, for example, is notoriously cyclical in price as indicated in Fig. 1.1.
The main cause for the cyclical nature of guar availability and price is its concentrated geographic source. Virtually all the guar gum in the world originates in the Indian subcontinent. If the monsoon rains in that region are poor, there is a poor guar crop and tightness in world supply.
1.4.2.2 Locust bean gum
Locust bean gum is another hydrocolloid notorious for periodic tightness in supply and correspondingly high prices. The cycle (8–10 years) of locust bean gum price and availability is longer than that of guar gum (3–4 years), but more severe when it does impact the market (Fig. 1.2). It is impossible to predict when another shortage will occur, but Mother Nature will ensure that periodically there will be a poor crop. In 1994, locust bean gum prices went up to US$18–20/lb (US$40–44/kg). In 2004, prices again rose more than 100%. They have since been steadily declining, until the next locust bean gum crisis!
Fig. 1.2 Locust bean gum price history.
1.4.2.3 Acacia gum or gum Arabic
Acacia gum or gum arabic serves as a final example of volatile price and availability. Much of the world’s gum arabic originates in one country of the world, namely Sudan. A bad season, combined with political unrest, resulted in dramatic price increases in 2004. It must be noted that these increases were preceded by a steady period of decline for the previous 6–8 years as indicated in Fig. 1.3.
Gum arabic availability and price will remain dependent on conditions in the Sudan despite efforts in other countries, such as Nigeria and Chad, to diversify the availability of raw material.
1.4.2.4 Xanthan gum
Xanthan gum is one of the more versatile of the hydrocolloids and demand for it has grown rapidly. In contrast to gum arabic, xanthan gum has been a paragon of price predictability with a small exception in the late 1990s when demand outstripped supply for a short period (Fig. 1.4). The number of new producers of xanthan gum entering the market makes it unlikely than another tight supply situation will occur in the foreseeable future. In the last 16 years that IMR has been tracking quarterly hydrocolloid prices, the cost of xanthan gum has declined from $5.65/lb in 1991 to $2.10 in 2007. This is an average price drop of 5.7% per year. Some recovery in xanthan and several other hydrocolloid prices started at the end of 2007 and accelerated as 2008 events drove up the price of most hydrocolloids.
Fig. 1.3 Acacia gum price history or Gum arabic price history.
Fig. 1.4 Xanthan gum price history.
1.5 FUTURE DEVELOPMENTS
Processed foods are here to stay. As the world continues down the path of modernisation, consumption of processed foods will increase and the corresponding demand for texturising agents will also increase. Four key factors will drive growth in processed foods:
ConvenienceQualityNutritionCostIn developed and developing countries alike, leisure time is becoming more limited and more precious, particularly in families with two working partners. Neither person is willing or able to spend the time to cook meals ‘from scratch’. Even countries with more traditional cooking practices, such as France and Italy, are inexorably going towards higher consumption of processed foods.
Quality has always been an issue but even more so in a world where consumers approach zero tolerance for risk and/or variation in quality. Specifications for purity and performance are getting tighter. The performance of ingredients, such as hydrocolloids, becomes all the more critical when their use level is very low.
The aspect of nutrition has become paramount in the mind of consumers. This concept has been taken one level higher with the development of ‘functional foods’ which fulfil some therapeutic role at the same time as being a food. Added fibre, calcium, vitamins and various other elements have become a standard in many foods. Some hydrocolloids provide benefits of soluble or insoluble fibre. Gum arabic has achieved the status of being a food ingredient rather than a food additive in the EU. Others are being evaluated with a promising outlook for providing other therapeutic benefits.
2
Acacia Gum (Gum Arabic)
Francis Thevenet
ABSTRACT
Acacia gum, also known as gum arabic, is a natural gum exudate obtained from acacia trees in the ‘African sub-Sahelian zone’. The gum has a highly branched compact arabinogalactan structure which gives a low-viscosity solution together with a central protein fraction that provides good emulsification properties. The powder hydrates readily in water and concentrations up to 40–50% can be handled easily. Key food applications include a range of confectionery products, flavoured oil emulsions and capsules and health foods as a source of soluble fibre with prebiotic properties.
2.1 INTRODUCTION
Acacia gum, also known as gum arabic, is a natural, vegetable exudate from acacia trees known since antiquity and used for thousands of years in foods as an additive and ingredient, in the pharmaceutical industry and for technical purposes.
There are various species of acacia trees with more than 700 spread across the world in Africa, Australia, India and South America.
The botanical definition of acacia gum was specified at the 53rd Joint FAO/WHO Expert Committee session in 1999 as being ‘a dried exudate obtained from the stems and branches of Acacia senegal (L.) Willdenow or Acacia seyal (fam. Leguminosae): synonyms: Gum arabic, Acacia gum, arabic gum, INS No. 414’. This new definition has been approved by the Codex Alimentarius Commission (ALINORM 99/12.A). In Europe, acacia gum is listed as a food additive by the European Directive 98/86/CE and 2008/84/EC with the number E414. Acacia gum (gum arabic) is also listed by the US Code of Federal Regulations number 21CFR184.1330 and accorded Generally Recognised as Safe (GRAS) status. Worldwide, these natural gums from A. senegal and A. seyal species are approved for pharmaceutical applications and listed in the US National Formulary and European pharmacopeia (01/2009: 307).
To secure the supply of gum, international associations have made many technical and economic efforts to extend plantations and promote the collection from acacia trees in the sub-Saharan zone of Africa (Dondain and Phillips, 1999). In place of the original three main gum-producing countries, namely Sudan, Nigeria and Chad, gum is now collected from 12 different countries from Senegal to Ethiopia, forming the so-called ‘African Gum Belt’.
Thirty years ago, 90% of the gum arabic used in food was from A. senegal species. A. seyal grades were kept only for technical purposes, such as glue, textiles, inks and printing (Meer, 1980). As a result of the development of new purification processes, research on specific properties and the promotion of new applications, the market for gum from A. seyal species is increasing every year. With new amendments of the food and pharmaceutical regulations in favour of A. seyal gum, the worldwide consumption of gum from A. seyal species is now about the same as gum from A. senegal. This is a very important factor for the stabilisation of the gum market.
Acacia gum is the third largest hydrocolloid additive and ingredient used by industry in terms of volume: worldwide consumption was around 57000 tons in 2008. The market for acacia gum is growing every year because it is a multifunctional food additive with properties as a texturising agent, film former, emulsifier and stabiliser, with numerous health benefits as a soluble fibre with prebiotic effects, and reinforced by the natural, non-modified vegetable origin.
2.2 ORIGIN AND PURIFICATION PROCESS
All A. senegal and A. seyal gums used in the food industry as additives or ingredients are collected from native sources or plantations in Africa. Acacia trees grow in a geographical belt (gum belt) located at the border of the desert from the western part of Africa in Senegal to the eastern part in Ethiopia (Imeson, 1992).
The gum exudes naturally after tapping the tree during the dry season from December to May. Nodules are hand-collected by farmers. One tree produces only about 400 g of gum per year (Vassal, 1985; Coppen, 1995).
A. senegal and A. seyal gums are normally harvested from different areas in the same country. A. senegal and A. seyal trees are very different in terms of shape, size, colour and thorns and are not easily confused. In gum-producing countries, the main gum manufacturers have subsidiaries equipped with laboratories where the quality of the gum is checked before shipment to purification plants.
Raw gum from the same botanical origin is a blend of gum nodules with different mesh sizes, containing vegetable and mineral impurities and fluctuating bacteriological contamination. Using dry purification steps, such as kibbling, sieving and pulverisation, the level of impurities can be slightly reduced but bacteriological contamination cannot be improved. Most of the time, raw gum does not meet international food or pharmaceutical specifications. Consequently, the dry methods of purification have been substituted by purification in aqueous solution, which is much more efficient (see Fig. 2.1). The gum is fully dissolved in water and all the impurities removed by a cascade of filtration steps giving levels of insoluble matter in the finished product as low as 0.02%. Bacteriological contamination is reduced by a plate heat exchanger and the gum syrup is concentrated to between 25% and 35% and dried, giving a level of contamination in the powder as low as 500 total germs per gram or below.
During solubilisation and purification, the thermal conditions are critical. Acacia gum contains proteins which are very important for the emulsifying properties but sensitive to heat denaturation. The selection of temperature and selection of time are key parameters for determining the quality of the gum.
Different processes are used for recovering purified, powdered acacia gum from the syrup. Roller drying is used to produce a gum in powder form with good hydration properties, but it has reduced emulsifying properties due to the drastic thermal treatment during the drying step. Regular spray drying is also used, which gives the gum good physical qualities and functional properties. Recently, spray drying has been improved by using a multi-stage drying process where fine particles of gum produced during drying are recycled at the top of the dryer. Agglomerated gum particles are obtained, keeping the entire properties of the raw gum, but containing no dust or particles below 75 µm and giving unique hydration and dissolution properties in water, without any lump formation, up to the maximum level of solubility of 45–50%.
Fig. 2.1 Purification process of acacia gum.
Such a purified gum is sold with general specifications according to the local food or pharmaceutical regulations together with specific criteria related to the proposed application of the gum, such as viscosity, colour, microbiology and functionality tests. As a result of the integration between the producing countries and processing plants, full traceability ‘from the tree to the finished product’ is guaranteed.
2.3 CHEMICAL STRUCTURE
Acacia gum is a highly branched arabinogalactan polysaccharide with a high molecular weight developing a low viscosity in water.
All molecules of acacia gum contain the same sugars: galactose, arabinose, rhamnose and glucuronic acids (Jurasek et al., 1993) partially neutralised with calcium, potassium, sodium and magnesium salts. In the 1960s and 1970s, Anderson and Farquhar (1974) conducted full taxonomy studies on many exudates from various acacia species. With new techniques of gel permeation, flow field-flow fractionation and multi-angle laser light scattering, Phillips, Fenyo and Muller have been able to clarify the complex structure of acacia gum (Connolly et al., 1987; Picton et al., 2000; Al-Assaf et al., 2005b).
Anacacia gum molecule isnot aunique structure. Gelpermeation and size-exclusion chromatography have shown that both A. senegal (Vulgares species) and A. seyal (Gummiferae species) have at least two fractions with different molecular weights. For both exudates, the highest molecular weight fraction contains the majority of the proteins but represents a minority percentage of gum.
The ‘wattle blossom’ structure (Fincher et al., 1983; Connolly et al., 1988) represents the highly branched compact structure of acacia gum from A. senegal. Arabinogalactans are attached to a protein skeleton forming the arabinogalactoprotein (AGP) fraction. The polysaccharide fraction is composed of a linear chain of β[1,3]-linked galactose. In position [1,6], this chain is branched with side chains of galactose and arabinose. Rhamnose, glucuronic acid or methyl glucuronic acid units are found as chain terminations in the Arabinogalactan (AG) fraction (Fig. 2.2) (Street and Anderson, 1983).
Fig. 2.2 Schematic representation of A. senegal gum molecule: the ‘wattle blossom’ model (Fincher et al., 1983). (Reprinted, with permission, from the Annual Review of Plant Physiology, Volume 34, © 1983 by Annual Reviews, www.annualreviews.org.)
From Table 2.1, we can compare the main physical parameters of various samples of the two species approved for food and pharmaceutical applications (Islam et al., 1997; Al-Assaf et al., 2005a; Hassan et al., 2005). A simple fingerprint of the molecule is the value of the optical rotation (Biswas and Phillips, 2003) which easily demonstrates from which acacia species the gum has been collected. A. senegal gives laevorotatory rotation of about −30° and A. seyal shows dextrorotatory rotation of about +50°. For further structural information, sugar composition is determined by HPLC after acidic gum hydrolysis. This qualitative sugar composition is now part of the Food Monograph and Pharmacopeia. The ash content results from the cations associated with the uronic acids. The presence of uronic acids in acid and salt forms gives A. senegal and A. seyal gums buffer properties which make the gum solutions pH stable after the addition of acids or bases.
The nitrogen content of A. senegal gum is about double that of A. seyal gum. It has been found that the protein fraction, which contains the same amino acids in A. senegal and A. seyal gums, is usually linked to the high-molecular-weight fraction. It has been shown that the protein in A. seyal is also much less available than in A. senegal (Anderson and McDougall, 1987). The lower content and lower availability of the protein fraction explain why A. seyal gum is less efficient for emulsion stabilisation and, consequently, it is not used for emulsions where long-term stability is required.
A comparison of molecular weight clearly shows a higher molecular weight for the A. seyal gum and, in general, for gums from the Gummiferae species. Despite this higher molecular weight, the intrinsic viscosity of A. seyal gum is lower than that of A. senegal gum at 15 mL/g compared to 20 mL/g. This indicates a more compact structure for A. seyal (Flindt et al., 2005).
Table 2.1 Comparison between the compositions of A. senegal and A. seyal gum.
Source: Al-Assaf et al. (2005b), Hassan et al. (2005), Islam et al. (1997).
Acacia senegal (Vulgares species)Acacia seyal(Gummiferae species) Sugar composition after hydrolysis Galactose (%) 44 38 Arabinose (%) 27 46 Rhamnose (%) 13 4 Glucuronic acids (%) 14.5 6.5 4–0-Methylglucuronic acid (%) 1.5 5.5 Specific rotation (degrees) −30 +51 Intrinsic viscosity (mL/g) 16–24 13–17 Nitrogen (%) 0.29 0.14 Ash (%) 3.93 2.87 Acacia senegal (Var. senegal)Acacia seyal (Var. seyal) Average molecular weight (×105) 3.3–9.4 8.4–35.6 Molecular weight peak 1 (×106) 1.6–3.0 2.06–9.28 Mass peak 1 (%) 9–17 6–29 Molecular weight peak 2 (×105) 2.6–4.0 7.2–12.8 Radius of gyration (nm) 17–30 22–36Compared to other water-soluble polysaccharides with a similar molecular weight, acacia gum exhibits very low viscosity in water. At 1% concentration, guar, xanthan and lambda carrageenan develop viscosity around 3000–5000 mPa s (Brookfield viscometer). To reach such viscosities, acacia gum has to be dissolved at concentrations of 40–45%. The low viscosity in solution is due to the globular, highly branched structure of acacia gum which hinders the formation of cross-links or hydrogen bonding with water. Rheological behaviour of acacia gum solutions is Newtonian up to 25% concentration and then becomes pseudoplastic (Sanchez et al., 2002). This highly branched structure for both acacia gums makes the product highly resistant to hydrolysis in acidic media and to degradation in extreme thermal conditions and by enzymes.
2.4 APPLICATIONS
2.4.1 Confectionery
Acacia gum is not considered a thickening hydrocolloid when dissolved in water at low concentrations up to 30%. However, when used in sucrose or sugar-free systems at high levels of dry solids, acacia gum provides a unique texture to confectionery products (Edwards, 1995). Acacia gum is used in a wide range of finished products including moulded candies, jujubes, pastilles with sucrose or polyols, for coated and non-coated chewy products, for sugar-free hard candies and in different tableting processes where binding properties are needed.
2.4.1.1 Moulded candies
For moulded candies made with sucrose, acacia gum is used at different levels depending upon the texture required.
For a hard texture, acacia gum from A. senegal is used alone at high concentrations in the finished confectionery. A typical formula contains 35% acacia gum, 30% sucrose, 25% glucose and about 10% water plus flavouring and colouring. Because of the occasional shortage of acacia gum, modified starch has partially substituted the natural gum. And yet, compared to modified starch, it is recognised that hard candy made with acacia gum lasts longer in the mouth, does not stick to the teeth and provides a unique flavour release.
To produce moulded candies with a softer texture, acacia gum is used with other gelling agents, such as gelatine. The combination of the hard texture from acacia gum with the flexible, soft gel from gelatine gives a wine gum-type texture. A typical formula is based on 15% acacia gum from A. seyal or A. senegal with 5% gelatine, 24% sucrose, 44% glucose, colour and flavour, and the remaining part being 12% moisture.
It is important to note that since acacia gum is all natural, vegetable in origin and a source of fibre, it benefits from a unique marketing image which is used in claims on packaging. In Scandinavian countries, sugar-free gumdrops have always been popular by claiming reduced calorific values and anti-cariogenic properties. Acacia gum is the only ingredient used to guarantee these claims. A formula contains 45–50% of acacia gum (A. senegal), with the remaining components being polyols, sorbitol, mannitol or maltitol, sweeteners, liquorice or flavours and, sometimes, ammonium chloride.
The production of moulded candies needs specific equipment which involves the following:
Efficient stirring/dissolving systems to prepare a 70–75% dry solids syrup of acacia gum and sugars.A thermal exchanger followed by a vacuum chamber to fully deaerate the thick syrup.An automatic depositing system (Mogul) to mould the syrup at 72–75% dry solids into printed starch trays.Ovens for drying the trays at 35–45°C for 2–3 days.Machines for removing pastilles from the moulds and for cleaning and glazing the pastilles with sugar or oil.2.4.1.2 Chewy confectionery
A chewy product is a slightly whipped, soft confection containing 74% sucrose and glucose, 5% hydrogenated vegetable fat, flavour and acid. Specific textures are obtained by including 1% gelatine for aeration and 1% gum from A. seyal for a long-lasting, cohesive chew. These chewy products can also be used as centres and then sugar coated.
2.4.1.3 Sugar-free hard candies
Because of the water-binding properties of acacia gum, the addition of a low level of 2–5% of gum in the formulation of a sugar-free hard candy, based on sorbitol, maltitol or mannitol, slightly increases the amount of residual water by 1–3% after cooking and, therefore, decreases the cooking temperature between 5 and 15°C. Hygroscopicity of the candy is reduced, recrystallisation of polyols is avoided and wrapped sweets are not sticky.
2.4.1.4 Tableting
Tableting includes different techniques: direct compression, wet granulation and making lozenges which are a type of tablet. Agglomerated acacia gum from A. seyal and A. senegal is used as a binder in these different processes to make food and pharmaceutical products.
For direct compression, purified and agglomerated acacia gum is mixed with the other powders having the same mesh size before filling the die.
In wet granulation, a solution of acacia gum is added to the powders to make a slurry which is dried and sieved to produce a free-flowing material which is then compressed.
For lozenges, two binders are combined: acacia gum and gelatine are dissolved in water and used to bind the flavoured icing sugar. The paste is then sheeted, cut to shape and dried in an oven. The lozenges are usually flavoured with mint and have an old-fashioned traditional appearance with a rough surface.
2.4.2Coating and panning
Coated confections, also called dragées, are one of the oldest forms of confectionery. Different types of centres, such as chocolate, lentils, almonds, nuts, jellies, liquor centres and chewing gum, are coated with sugar, polyols or chocolate (Lynch, 1992). Depending upon the centre, the nature of the coating and the required texture, three different processes are used: hard coating, soft coating and chocolate coating (see Fig. 2.3). These processes involve many different steps. Numerous layers of syrup are applied and dried with more than 100 for a hard sugar coating. The complete coating process may last more than 1 day.
Acacia gum is used for its film-forming properties to improve the physical and mechanical parameters of the centres and make the hard- and soft-coating layers more effective. Gum from A. seyal is used for this application.
In sugar-coated products, acacia gum is mainly used during the gumming step. An almond or a chocolate lentil contains between 30% and 55% fat. If the centre is not sealed with a flexible film, fat migration or blooming will occur, with cracking and an oxidised taste on the surface of the sugar-coated dragées. In a rotating-pan coating system, by applying a syrup of acacia gum, sucrose and glucose to the centre, fat migration and shell cracking is avoided. In addition, the mechanical resistance of the centre is improved, especially at the corners which tend to break during the rotation of the pan. The surface of the centres will be more even, allowing further sugar layers to adhere more easily. An example gumming syrup formula contains 60% water, 20% acacia gum, 15% sugar and 5% glucose. This syrup is sprayed over the centre and dried by dusting with granulated sugar and then a 50:50 blend of icing sugar and fine-mesh acacia gum. After complete crystallisation overnight, the centre is perfectly sealed and ready to be hard or soft coated.
For multiple-layered sugar coating, the hard-coating layer obtained after forced-air drying will be softer and less brittle if acacia gum is added at low levels of between 3% and 5% to the 80% dry solids content sucrose syrup. Acacia gum is known to act as a plasticiser for sucrose by reducing the formation of large sugar crystals.
In soft coating, a 70% dry solids syrup of a sugar–glucose mix is sprayed to wet the centres and then dried by dusting with icing sugar. Acacia gum can also be added in powder form to reinforce the coating layers.
For chocolate coating, the addition of 30–35% acacia gum solution is used to polish and shine the final layers of chocolate. This shiny effect is sensitive to air humidity and is protected by further layers of shellac.
Recently, sugar-free coated products appeared on the market and they continue to grow rapidly. Consumers are looking for sugar-free dragées because of their low calorific value, non-cariogenic effect and unique release of flavour due to the cooling effect of the polyols. The most popular sugar-free product is polyol-coated chewing gum in which the gum base is used as a carrier for an active principle suitable for tooth health. Sorbitol, maltitol and xylitol can be used for hard coating. Each of these polyols has its own specific behaviour in terms of stability, hygroscopicity, cooling effect, crunchiness and sweetness. Xylitol and maltitol are the main polyols used for panning sugar-free chewing gum. Acacia gum is used in a hard-coating syrup containing about 65% polyol, 3% acacia gum, 1% titanium dioxide and 31% water. This is sprayed onto the surface of the chewing gum centre and then dried by air at 30°C and 40% humidity. One role of acacia gum is to decrease the crystallisation temperature of the polyol in order to apply the syrup at a lower temperature of 65–70°C so as not to damage the shape of the chewing gum centres. Other benefits of including acacia gum in the coating syrup are improving resistance of the polyol layers and increasing shelf life by decreasing hygroscopicity.
Fig. 2.3 Preparation of pan-coated products.
For salted products, such as dry roasted peanuts, acacia gum syrup is used as a glue to adhere salt and spices around the nut before roasting. In a rotating-pan coating cylinder, or in a tunnel equipped with helical brushes and nozzles, a 30% solution of acacia gum is used to coat the nuts. Afterwards, salt and spices are dusted over the wet and sticky surface and the nuts are roasted in an oven. The salt and spices remain well fixed around the nut after packaging. Film-forming properties of the gum prevent fat exudation, maintaining the original non-oxidised taste.
2.4.3 Emulsions
Acacia gum is used as an emulsifier and stabiliser in preparing oil-in-water emulsions. Acacia gum is not considered an actual emulsifier which contains a lipophilic and hydrophilic part in its molecule. Acacia gum is a water-soluble polysaccharide. However, it is possible to give it a hydrophilic–lipophilic balance value (Chun et al., 1958). The protein contained in the AGP fraction of the molecule gives surface-active behaviour to the molecule and allows formation of a colloidal film around the oil droplets as shown in Fig. 2.4 (Randall et al., 1998).
To guarantee a long shelf life of a concentrated or diluted emulsion (normally 1 year is required), it is necessary to ensure stability to avoid creaming, flocculation and coalescence (Dickinson and Galazka, 1991). Stabilisation of the emulsion is obtained by steric hindrance due to the high-molecular-weight fraction of the gum molecule and electric repulsion due to the uronic acids on each dispersed oil droplet.
Stability of the emulsion, expressed as the destabilisation velocity, V, follows Stokes’ law. Different parameters are determined either by formulation or by the process of making the
Fig. 2.4 Mechanism of emulsion stabilisation using acacia gum.
emulsion:
where (d1 − d2) is the difference in specific gravity between the dispersed and continuous phases. To get it as low as possible, oil with high specific gravity close to 1.0 will be used or oil with lower specific gravity, such as citrus oil with a value of d1 ∼ 0.80 will be weighted by a food-approved weighting agent, such as Estergum or sucrose acetate isobutyrate (Tan, 1998).
Here r is the radius of the oil droplet. The emulsion will be processed using highshear mixing and homogenisation under pressure at 100–300 kg/cm2 in order to reduce the size of the droplets to between 1 and 0.4 µm (Pandolfe, 1981). This size distribution avoids destabilisation by coalescence and provides a strong clouding effect after dilution (Fig. 2.5).
In the equation, η is the viscosity and has to be in the range of 30–100 mPa s to ensure efficient homogenisation and easy dispersion of the emulsion in syrup.
A comparison between gum from A. senegal and A. seyal shows that the amount of protein is double in A. senegal: 2% for A. senegal compared to 1% for A. seyal. Since there is more protein available in A. senegal than in A. seyal, it is obvious that if a stable concentrated or diluted emulsion is needed, gum from A. senegal is the one to use. An emulsion processed with A. seyal gum will give a slightly bigger oil droplet distribution and the stability will be limited to a few days.
Both acacia gums are very resistant to acidic media with no hydrolysis down to pH 2, which contributes to the guarantee of stability in acidic products, such as citrus and cola beverages.
The level of acacia gum used to stabilise emulsions depends upon the type, dosage and specific gravity of oil (Buffo and Reineccius, 2000). Generally, the gum level is between 12% and 20% of the total formula. Gum is dissolved in water at room temperature and then the oil phase is added under high-shear mixing to prepare the pre-emulsion with an oil droplet size around 5 µm. The pre-emulsion is homogenised under pressure to produce oil droplets below 1 µm. For pressure homogenisation, two passes are usually applied in a two-step system at pressures up to 300 kg/cm2 (4300 psi) (Fig. 2.6).
Fig. 2.5 Oil droplet size distribution for a homogenised emulsion obtained by laser granulometry.
Fig. 2.6 Typical process for beverage emulsions.
Acacia gum is the main emulsifier used for preparing concentrated emulsions for soft drinks. All formulations are compatible with acacia gum including the following:
Citrus oils, artificial flavours, cola oils, neutral vegetable oils, triglycerides, etc.Artificial colours, such as sunset yellow and tartrazine, or natural colours including carotene and oleoresins.Concentrated emulsions for beverages, containing about 10–20% oil phase, are normally diluted at a ratio of either 10–20 g/L in high Brix syrup or 1–2 g/L in carbonated water to produce soft drinks (Thevenet, 2002). Such stabilised emulsions could also be diluted for flavouring alcoholic drinks, containing up to 20% alcohol, without any gum flocculation.
Forecasting emulsion stability is essential. Most methods used are based on oil droplet size distribution measurements by laser granulometry or microscopy applied to heated and non-heated emulsions and creaming observations made visually or by transmittance and back scattering. Of course, these two accelerated tests are supplemented by long-term observation of beverages.
Apart from beverage emulsions, acacia gum is used for producing water-dispersible natural colours in liquid form, such as carotene and oleoresin emulsions. For natural nutraceutical emulsions, acacia gum stabilises oil-soluble vitamins or unsaturated fatty acids using the same process as described previously.
2.4.4 Encapsulation
Encapsulation is a general term that includes many different techniques using different carriers including acacia gum (Risch, 1995). The main reasons for encapsulating an active principle are:
protection against oxidation, water migration or internal reactions;production of a water-dispersible, free-flowing powder;production of a controlled release powder;reduction of hygroscopicity or reduction of dust pollution.The encapsulation process is generally classified into two groups:
Matrix encapsulation. The dispersion of oil droplets inside the matrix in which acacia gum may be used. Such a system allows 10–30% oil content in the carrier. The matrix system is normally produced by spray drying.Membrane encapsulation. The formation of membrane around a ‘big’ oil droplet from 20 to 500 µm in size. Such a system allows encapsulation of up to 90–95% oil.Processes used to form the membrane system are mainly:
complex coacervation where acacia gum could be involved, orextrusion where the membrane could be gelatine, agar or alginate.The different technologies used for protecting sensitive products are:
spray drying,spray coating,adsorption,coacervation,extrusion,hot-melt coating,molecular inclusion.Each technology involves different equipment, gives a different quality of encapsulation and, of course, has different costs.
Acacia gum is used as a carrier for encapsulation mainly with two different technologies:
complex coacervation (membrane encapsulation), andspray drying (matrix encapsulation).Complex coacervation involves the reaction between two polymers, a polyanion and a polycation. Since A. seyal or A. senegal gums have uronic acids as constituents of their molecules, they are negatively charged with a zeta potential around −20 mV. Depending upon the isoelectric point, gelatine may be negatively charged above the isoelectric point or positively charged below the isoelectric point. Complex coacervation occurs when the oil dispersed in a dilute gelatine solution is mixed together with acacia gum solution and the pH reduced to 4, below the isoelectric point. The coacervate is formed around the oil droplet. It is then hardened by adding tannins or glutaraldehyde which reacts with the gelatine. Coacervates in which oil is entrapped can be used as a suspension in water or filtered and dried.
In addition to being the basic patented process for carbonless paper, this technology is used to encapsulate flavours used in food products where the heating or cooking treatment is very drastic, such as microwave cooking, retorting and products that are UHT treated.
Coacervation is also used for perfume encapsulation, for example in ‘scratch and sniff’ advertising or for contact glues.
Encapsulation using spray-drying technology is used frequently in the food industry because it is an efficient and economical technique and spray driers are available in many factories. This type of encapsulation is summarised in Fig. 2.7. Basically, the oil to be encapsulated is emulsified as small oil droplets below 1 µm using acacia gum as an emulsifier and carrier. The emulsification is performed by high-shear mixing and homogenisation. Then, the emulsion is dried by regular spray drying or by a multi-stage drying unit to produce powder with better water-dispersion properties. A typical formulation to produce powder containing 20% orange oil is as follows:
After homogenisation, this emulsion contains 35% dry substance and is spray dried to provide a powder having a composition as follows:
