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Native, modified and clean label starches in foods and beverages
7 Native, modified and clean label starches in foods and beverages P. McDonagh, Healy Group, Ireland
Abstract: This chapter discusses native starches, modified starches from various botanical sources, and clean label starches. For native starches and modified starches, the chapter outlines their production process, where grown, global consumption/usage, some indications of applications and functionality. Major differences between the starches, and their strengths and weaknesses, are noted. Clean label starches and the continuing trend to create starches with similar functional properties to their chemically modified counterparts are then considered. Key words: native starches, modified starches, clean label starches, applications, functionality, chemically modified counterparts.
7.1
Introduction
Starches have been used since ancient times and there are many references in history to the use of starch in food and non-food applications. The extraction of starch, for example, was described in the Natural History of Pliny the Elder in around AD 77–79 (Maningat et al. 2009). The word ‘starch’ is thought to have derived from the Anglo-Saxon ‘stearc’ and has the meaning of strength or stiffness. Starch can be extracted from many of the plants containing it and thus today we find starch derived from many botanical sources. The most common sources and their starch structures are set out in Fig. 7.1. Starches can be categorised mainly into two groups, particularly from a labelling perspective, as either native or modified. Native starches are produced through the separation of naturally occurring starch from grain or root crops (such as tapioca, rice, corn and potato) and can be used directly in producing certain foods, such as noodles. The starches produced contain on average 19–22% moisture with their original structures intact. Modified starch is produced from
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Fig. 7.1 Appearance of various starches.
native starch by modification through one or more of the processes set out in Section 7.7. The modifications are carried out to improve the starch’s functionality (e.g. its ability to withstand low pH conditions and high temperatures), as native starches are typically not ‘process friendly’. Native starches are considered clean label ingredients, whereas chemically modified starches carry an E number designation and are not perceived as natural. Incidentally, the term native is applied to extracted non-processed starches, rather than natural, as this implies untouched by human hand. In terms of starch, this would mean the use in foods of the whole vegetable or botanic plant from which the starch is derived and reliance on the starch being extracted upon cooking (i.e. addition of a whole potato to soup rather than potato starch). There has, therefore, been a drive in recent years to modify native starches using physical processes to make them as functional as their chemically modified counterparts, thus retaining the label declaration ‘native’ which prefers a commercial advantage. This chapter set outs to explain what starches are, their sources, how they are extracted, their characteristics, reasons for modification, typical applications and functional properties. It summarises the types of modification employed and discusses the creation of clean label functional starches.
7.2
Manufacture of starch in plants
Green leaves of plants contain chlorophyll, which is able to absorb light quanta and utilise the energy to catalyse the formation of glucose and oxygen from carbon dioxide and water. This process is known as photosynthesis and can be written as follows (simplified): 6H2O
+ 6CO2
water
+
light/chlorophyll
carbon dioxide
C6H12O6 +
6O2
glucose
oxygen
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Starch is formed in the leaves of plants by condensation polymerisation of glucose with the aid of starch-synthesising enzymes. This process can be written as follows (simplified): nC6H12O6 glucose
enzymes
(C6H10O5)n +
nH2O
starch
water
+
[7.2]
During active photosynthesis (during the day), the starch is accumulated in the leaves in the form of tiny granules of about 1 μm in diameter (leaf or transitory starch). During the night, this leaf starch is partly broken down by enzymes and transported in the form of sugars (mainly sucrose) to other parts of the plant. Some of these sugars are re-converted to starch in the seeds, tubers and roots of various plants (storage starch). It is from these sources that commercial starch is obtained. Starch molecules are synthesised in plants from sugars but the true mechanism for the biosynthesis of amylose and amylopectin is not entirely clear. The enzymes phosphorylase (P-enzyme), starch synthase and a branching enzyme are, or may be, involved in starch biosynthesis (Smith 2001). Nowadays most starch scientists believe that starch synthase is the true chain-lengthening enzyme in normal starch biosynthesis (Fujita 2006). The branching enzyme is responsible for the synthesis of the branching points in the amylopectin molecules. The mechanism that prevents amylose from branching in the obvious presence of the branching enzyme is still unresolved. The development of starch granules commences with the accumulation of poorly organised material of unknown chemical composition. At a certain point the deposition of a minute amount of insoluble polysaccharide takes place, which acts as a nucleus for further starch deposition. This nucleus is the botanical centre (hilum) of the granule, around which the granule is grown. Initial growth gives nearly spherical granules and as the granules are enlarged they often become elongated or flattened. The starch molecular chains grow in an orientation perpendicular to the growing surface of the starch granule. As the dissolved glucose units are linked to the growing starch polymer they simultaneously solidify. During the growing of the starch granule there is an increase in the proportion of amylose and an increase in molecular size of both amylose and amylopectin Starch is a polymeric carbohydrate composed of anhydroglucose units and is extracted in granular form from the organs of certain plants. Starch granules are deposited in the seeds, tubers, roots and stem piths of plants, as a reserve food supply during periods of dormancy, germination and growth. The microscope reveals that starch is composed of tiny, white granules, ranging from about 1 to 100 μm in diameter. After cellulose, starch is the next most abundant compound synthesised by plant cells. It is a renewable substance and a new supply of starch is grown annually. The size and shape of the granules are peculiar to starch from specific botanical sources. The structures of the most common starches are set out in Fig. 7.1.
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Starch composition
Starch granules usually contain 10–20% moisture and small amounts of proteins, lipids and traces of inorganic materials, in addition to the carbohydrate content. 7.3.1 Moisture The moisture content of starch products depends on the relative humidity (RH) of the atmosphere in which they have been stored. If this humidity decreases, the starches will give up moisture; if the RH increases, they will absorb moisture. The equilibrium moisture content of starch is also dependent on the type of starch product. Under normal atmospheric conditions, most commercial native starches contain 10–20% moisture. The equilibrium moisture content of all starches is low at a low RH of the atmosphere. At an RH of zero, the moisture content of the starches approaches zero. At a RH of 20%, the moisture content of all starches is about 5–6%. 7.3.2 Lipids (fatty substances) Tuber (potato) and root (tapioca) starches contain only a very small percentage of lipids (about 0.1%), compared with the common cereal starches (maize, wheat, rice, sorghum), which contain 0.8–1.0% lipids. The fatty substances in the cereal starches are predominantly free fatty acids (in maize and waxy maize starch) or phospholipids (in wheat starch). The free fatty acids consist mainly of palmitic, linoleic and oleic acids. The presence of lipids in the common cereal starches has a profound effect on the physical properties of these starches. The lipids exist as an amylose–lipid inclusion complex in the granules. The linear fraction of the starch molecules (amylose) forms helical clathrates with polar fatty substances such as the higher fatty acids. The amylose–lipid complexes are insoluble, but dissociate when heated in water above a given temperature. The dissociation temperature is indicative of the strength of bonding and depends on the type of complexing agent. The amylose–lipid complexes tend to repress the swelling and solubilisation of the cereal starch granules. Elevated temperatures (above 125°C) are required to disrupt the organised native amylose– lipid structure in the cereal starch granules and to solubilise the amylose fraction. The presence of fatty substances can create problems in the use of maize and wheat starch products because of the tendency to become rancid on storage. 7.3.3 Proteins References to protein in starches include macromolecular proteins, but also indicate the peptides, amino acids, nucleic acids and enzymes that may be present in the starch granules. Tuber (potato) and root (tapioca) starches contain only a small amount of proteins (about 0.1%) compared with the cereal starches (maize, wheat, waxy maize), which contain 0.2–0.4% proteins. Because of the residual protein, the cereal starches may have a mealy flavour and odour, and also a
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tendency to foam. The small granules of wheat starch contain much more protein (1.6%) than the large granules (0.1%). 7.3.4 Phosphorus The phosphorus in the cereal starches is mainly present as phospholipids. The root starches (tapioca) contain a very low amount of phosphorus compounds. Potato starch is the only commercial starch that contains an appreciable amount of chemically bound phosphate ester groups. 7.3.5 Flavour and odour substances The pre-gelatinised common cereal starches (maize, wheat) have a relatively raw cereal flavour. These starches impart cereal-type flavours to the foods in which they are incorporated. Potato and tapioca starches contain only a low amount of flavour substances and this may be due to their low lipid and protein content.
7.4 Amylose and amylopectin Starch can be considered to be a condensation polymer of glucose, consisting of anhydroglucose units. The glucose units are linked to one another through the C-1 oxygen in what is known as a glucoside bond. The glucoside linkage is stable under alkaline conditions and hydrolysable under acid conditions. The glucose unit at the end of the polymeric chain has a latent aldehyde group and is known as the reducing end group. Most starches are a mixture of amylose and amylopectin, each having a wide range of molecular sizes. Starches of different origin have different amylose to amylopectin ratios (Table 7.1). Table 7.1 also shows the average degree of polymerisation (DP) of both fractions in various starches. In summary, starch is considered to be a condensation polymer of glucose. The glucose units in the starch polymer are present as anhydroglucose units (AGU). Most starches contain two types of glucose polymers: amylose and amylopectin. Amylose is a linear chain of glucose units while amylopectin is a branched polymer Table 7.1 Amylose and amylopectin contents and degree of polymerisation of various starches Starch Rice Wheat Maize Potato Tapioca (cassava)
Percentage content
Average DP
Amylose
Amylopectin
Amylose
Amylopectin
22 26 28 21 17
88 74 72 79 83
4000 1000 1000 4000 4000
2 000 000 2 000 000 2 000 000 2 000 000 2 000 000
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of glucose units. These two polymers occur in differing amounts in starches from various botanical sources. Amylose is responsible for gelling properties and shear resistance, while amylopectin is responsible for stability. 7.4.1 Amylose Amylose is a linear polymer containing up to 6000 glucose units, connected by 1,4-linkages (Fig. 7.2). The ratio of amylose to amylopectin is fairly constant for a given species of starch. Maize and sorghum starch have a much higher amylose content (about 28%) than the tuber and root starches (potato, tapioca, arrowroot), which contain only about 20% amylose. The waxy starches contain no amylose fraction. Amylomaize starch, a maize starch, which has been selectively bred so that the resultant starch has a high amylose content, may contain up to 80% amylose. It is largely used in edible films, coatings and biodegradable packaging. Amylose covers a range of degrees of polymerisation, depending upon the source of the starch. The amylose molecules of potato and tapioca starch have a substantially higher molecular weight than maize and wheat starch amylose. The amylose fraction of potato starch has a DP ranging from 840 to 22 000 glucose units. The amylose fraction of maize starch has a DP of about 400–15 000 glucose units. Amylose forms inclusion complexes with iodine and various organic compounds such as butanol, fatty acids, various surfactants, phenols and hydrocarbons. These complexes are essentially insoluble in water. It is believed that amylose complexes by forming a helix coil around the complexing agent. The complex of amylose with iodine gives a characteristic blue colour, which is used to establish the presence of amylose-containing starch.
Fig. 7.2
Linear structure of amylose molecule.
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Fig. 7.3
Structure of amylopectin branching points.
7.4.2 Amylopectin Amylopectin has a highly branched structure, consisting of short linear chains with a DP ranging from 10 to 60 glucose units. The average DP of these chains is about 22. They are connected to each other by α-1,6-linkages (Fig.7.3).
7.5
Starch: extraction and manufacture
Their naturally high starch content and ready availability means that the most commonly available sources of starch are derived from potato, rice, tapioca, corn (maize) and wheat. The starch industry uses a combination of wet purification techniques, milling and drying to manufacture native starch with a purity of about 98–99.5%. In the manufacturing process, starch is separated from the other constituents of the milled raw material such as fibres, proteins, sugars and salts. Figure 7.4 shows a typical production flow diagram for the production of native starch.
Fig. 7.4 Typical native starch production flow diagram.
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7.6
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Starches from different sources
Starch-rich plant sources are grown worldwide. An indication of the volumes of these plant sources is set out in Table 7.2. When selecting a starch for a particular application, it should not be forgotten that the supply of starch can be volatile. Coming from natural crop sources, it is highly sensitive to environmental factors and weather conditions. This was no better demonstrated than in 2010 when fires devastated large tracts of wheat in Russia; late cold springs in Europe, combined with drought during the growing season and flooding at harvest, led to greatly reduced potato crop yields and drought, flooding and insect damage resulted in sharp declines in harvest yields in Asia. In Europe the potato crop was down by at least 33%, and in Russia the wheat crop yield was said to be at least 30% down. 2010 was also a year of significant changes in market conditions. With global economies slowly coming out of recession and growing wealth among middle and upper classes across Asia and South America, demand for processed food and starch in particular increased greatly. Conditions such as these can potentially lead to shortages and price increases. 7.6.1 Characteristics of native starches from various botanical sources Starches are very versatile, and have supported and permitted many innovations in processing within the food industry. Perhaps less well-known is the fact that starch provides functionality in non-food applications. The starch industry refers to these non-food uses as industrial uses. Examples of industrial uses are adhesives, paper, cardboard, detergents, edible film, biodegradable packaging, oil drilling, water treatment, construction and mining industries. Food applications include soups, sauces, canned foods, cereal and snacks, beverage emulsions, dairy, meat and baked goods. With such a wide variety of starches and wide variety of functional properties it is difficult to give a comprehensive account of all relevant properties in this chapter. Key properties of starch include: heat and freeze-thaw stability; dispersability (i.e. the ability of the starch particles to disperse homogenously into
Table 7.2 Volume of starch-producing plant sources grown worldwide, 2008 Crop
Amount (million tonnes/year)
Rice Wheat Maize Potato Tapioca (cassava)
685 690 823 314 233
Average yield (tonnes/hectare) 4.30 3.00 5.10 17.30 12.50
Source: FAOSTAT, Crops worldwide (http://faostat.fao.org/site/567/default.aspx#ancor).
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a liquid or powder media and deliver their functionality homogenously throughout); viscosity (hot and cold); and abilities with regard to film formation, gel formation, oil retention, moisture-binding, suspension of solids, volume control and crispness promotion. Other parameters to consider include: impacts on food texture clarity, opacity and sheen; emulsion-stabilising capacity; adhesiveness; and tolerance of processing conditions. Few other ingredients offer the range and versatility of characteristics as effectively and as economically as starch. When selecting a starch for a particular application, it is advisable to obtain the technical data required to make a comparison of properties and an informed choice. Table 7.3 sets out some of the key characteristics of the starches from potato, rice, tapioca, corn (maize) and wheat.
7.7
Modification of starches
Starches are modified so as to give them certain useful properties (required by particular foods or processes), to make them suitable for use within certain production processes, and to retain their functionality in foods so as to allow production of safe, long-life foods. The various types of modification of native starch are designed to change one or more of the following properties:
• • •
Pasting temperature (the temperature at which initial swelling of starch granules takes place when suspended in water). Solids–viscosity relationships. Gelatinisation and cooking characteristics.
Table 7.3 Key characteristics of the starches from potato, rice, tapioca, corn (maize) starch and wheat Potato starch Rice starch
Corn starch
Tapioca starch Wheat starch
Size (μm) Shape Colour
15–80 Oval White
2–8 Round Very white
10–15 Oval White
Taste
Potato taste
Neutral
15–25 Hexagonal Yellowish, white Protein taste
Odourlessness Moderate Presence of Allergen allergens traces
Gel structure Supply stability
Sticky Volatile
20–40 Oval Greyish white Cereal taste
Slight to neutral Good Fair Allergen-free Gluten
Excellent Fair Allergen free Risk of allergen introduction due to genetic modification Creamy Firm Sticky Volatile Stable Volatile
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Resistance of starch pastes to breakdown. Viscosity by acids, heat and/or mechanical shear. Retrogradation tendencies. Ionic character. Hydrophilic character.
Taking pasting temperature as an example, this is an important characteristic in the processing of starch and starch products. If starch granules in suspension in water are heated, water penetrates the granules to hydrate them with resulting swelling. If the temperature of the starch suspension or slurry is heated above its pasting temperature, a viscous mass is produced and the starch granules lose their unique microscopic appearance or shape, which may not be regained upon cooling to room temperature. As starch granules are heated in water they hydrate and swell, the refractive index of the granules approaches that of water, and the initially opaque slurry becomes more transparent. In general, the pasting temperatures of modified starches are lower than those of native starches. This is important in some applications, for example, to keep fruit pieces suspended in a filling while it is cooking. 7.7.1 Types of modification While it is not possible to detail in full every type of modification, some of the more common modifications are set out in Table 7.4. The modification of native starch may involve a change in physical form, a controlled degradation and/or the introduction of chemical groups. Table 7.5 sets out the modified starches permitted for use in EU in foods together with their applicable E numbers, which must be present on the label. Table 7.4 Types of modification Type of modification
Reason for modification
Treatment
Dextrin
Adhesion, low viscosity
Acid modified starch
Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability High shear, acid stability Improved viscosity stability Specific properties
Dry heat and treatment with acid Acid hydrolysis
Alkaline modified starch Bleached starch Oxidised starch Enzymatically modified starch Cross-linked starch Stabilised starch Combinations of above
Treatment with sodium or potassium hydroxide Treatment with hydrogen peroxide Oxidation with sodium hypochloride Treatment with alpha amylase Hydroxyl bonding Esterification –
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Natural food additives, ingredients and flavourings Table 7.5 Modified starches permitted for use in EU (as of 26 November 2010) E number
Starch name
E1404 E1410 E1412 E1413 E1414 E1420 E1422 E1440 E1442 E1450 E1451
Oxidised starch Monostarch phosphate Distarch phosphate Phosphated distarch phosphate Acetylated distarch phosphate Acetylated starch Acetylated distarch adipate Hydroxyl propyl starch Hydroxyl propyl distarch starch Starch sodium octenyl succinate Acetylated oxidised starch
Source: www.food.gov.uk/safereating/chemsafe/ additivesbranch/enumberlist
Other types of modified starches include:
• • • • • •
Dextrin (E1400), starch roasted with hydrochloric acid. Alkaline-modified starch (E1402) with sodium hydroxide or potassium hydroxide. Bleached starch (E1403) with hydrogen peroxide. Enzyme-treated starch (INS: 1405), maltodextrin, cyclodextrin. Monostarch phosphate (E1411) with phosphorous acid or the salts sodium phosphate, potassium phosphate, or sodium triphosphate to reduce retrogradation. Carboxymethylated starch with monochloroacetic acid adding negative charge.
Modified starches may be pre-gelatinised to render them readily soluble in cold water, and capable of swelling and gelling without heat. Alternatively they may be presented in an uncooked form, which must be cooked like regular starch in order to gelatinise the starch granules. Drying methods to make starches soluble in cold water are extrusion, drum drying or spray drying, and agglomeration.
7.8
Clean label starches
At present ‘clean label’ is an ill-defined term with different interpretations across the globe and no legal definition. One useful definition was devised by National Starch in the US: Free from chemical additives; simple ingredient listing (without ingredients that sound chemical or artificial); minimally processed using traditional techniques that are understood by consumers and not perceived as being artificial. It is worth noting that clean label is not a consumer term; it is in fact an industry term.
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Some trends to bear in mind that are currently fuelling the demand for clean label ingredients are discussed below. 1 Through the various celebrity chef cookery programmes and advertising by the supermarket groups, consumers are increasingly avoiding foods high in fat, salt, sugar and also foods containing additives. Modified starch is an ingredient which carries an E number and is not perceived as natural. 2 Food labels are increasingly referring to products as local, kitchen-style or home-style, etc. Modified starch is not perceived as fitting into these categories. 3 The percentage of consumers who are reading labels is thought to be increasing. 4 The terms ‘natural’ or ‘free from additives’ are increasingly appearing on food product labels. Modified starch is an additive and is not perceived as fitting into these categories. 5 Descriptive language on food packs increasingly describes products as ‘pure’ or ‘fresh’. Modified starch is not perceived as fitting into these categories. While the onset of the recession in 2008 brought some respite to food processors who were otherwise being pressurised to using clean label starches, manufacturers are now once again under pressure from supermarket groups and consumers to remove additives from food, including modified starches. This has led to reduced sales, especially in the UK and Ireland, of modified starches but increased sales of functional clean label equivalents. The issue at stake for starch manufacturers is to find innovative ways to deliver clean label solutions that offer the functionality and qualities of modified starch. This challenge is greater in the EU than the US due to the dominance of supermarkets in Europe and also discrepancies between the definition of clean label in the two continents. However, the clean label movement is thought also to be gaining appeal in the US. Clean label starches have been on offer in the marketplace for some time now – National Starch have been active in this area since the mid to late 1990s, when its Novation® starch range came into being. Starch-producing companies are employing many techniques to impart functional characteristics to native starches that are equivalent to their modified counterparts. While the first generation clean label starches were not so stable in conditions of high acid shear, the current generation clean label starches are indeed very functional. Some of the techniques which starch companies are employing include:
• • •
Physical modification through agglomeration and granulation. Selective crop breeding – we have recently seen the launch of high amylopectin waxy potato and wheat starches (by changing the ratio of amylose to amylopectin it is possible to offer differing functional performance). Thermal treatment of native flours.
The techniques and technologies applied by the starch producers are closely guarded by those companies because of their commercial sensitivity. It is not possible to discuss the processes in more detail because the information is not in the public domain.
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7.9
Conclusions and future trends
When it comes to clean label it is the retailers and the consumers who are presenting the ultimate challenge: they require clean label starches that have the same function as modified starches and cost the same. Companies most active in the field of clean label starches include National Starch, Ulrick & Short, and AVEBE. The author’s personal feeling is that clean label is not a passing fad: it is here to stay. National Starch is currently considered the market leader and is expected to maintain this position. As many existing products are protected by patents, other companies entering the market may have to rely on selective breeding to offer perhaps one or more of the attributes required in a clean label starch, and those clean label starches that they produce may not be as functional, versatile or easy to formulate into foods as those currently established in the marketplace.
7.10
• • • •
Sources of further information and advice
Emsland Group (www.emsland-group.de) AVEBE (www.avebe.com) National Starch (now a subsidiary of Corn Products International), (www. nationalstarch.com) Ulrick & Short (www.ulrickandshort.com)
7.11
References
FUJITA N, YOSHIDA M, ASAKURA N, OHDAN T, MIYAO A
et al. (2006), ‘Function and characterisation of starch synthase I using mutants in rice’, Plant Physiology, 140, 1070–1084. MANINGAT C C, SEIB P A, BASSI S D, WOO K S and LASETER G D (2009), ‘Wheat starch: Production, properties, modification and uses’, in BeMiller J and Whistler R (eds.), Starch: Chemistry and Technology, 3rd edition, Academic Press, Burlington, VA, Chapter 10, pp. 442–510. SMITH A M (2001), ‘The biosynthesis of starch granules’, Biomacromolecules, 2, 335–341.
© Woodhead Publishing Limited, 2012
Abstract: This chapter discusses native starches, modified starches from various botanical sources, and clean label starches. For native starches and modified starches, the chapter outlines their production process, where grown, global consumption/usage, some indications of applications and functionality. Major differences between the starches, and their strengths and weaknesses, are noted. Clean label starches and the continuing trend to create starches with similar functional properties to their chemically modified counterparts are then considered. Key words: native starches, modified starches, clean label starches, applications, functionality, chemically modified counterparts.
7.1
Introduction
Starches have been used since ancient times and there are many references in history to the use of starch in food and non-food applications. The extraction of starch, for example, was described in the Natural History of Pliny the Elder in around AD 77–79 (Maningat et al. 2009). The word ‘starch’ is thought to have derived from the Anglo-Saxon ‘stearc’ and has the meaning of strength or stiffness. Starch can be extracted from many of the plants containing it and thus today we find starch derived from many botanical sources. The most common sources and their starch structures are set out in Fig. 7.1. Starches can be categorised mainly into two groups, particularly from a labelling perspective, as either native or modified. Native starches are produced through the separation of naturally occurring starch from grain or root crops (such as tapioca, rice, corn and potato) and can be used directly in producing certain foods, such as noodles. The starches produced contain on average 19–22% moisture with their original structures intact. Modified starch is produced from
© Woodhead Publishing Limited, 2012
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163
Fig. 7.1 Appearance of various starches.
native starch by modification through one or more of the processes set out in Section 7.7. The modifications are carried out to improve the starch’s functionality (e.g. its ability to withstand low pH conditions and high temperatures), as native starches are typically not ‘process friendly’. Native starches are considered clean label ingredients, whereas chemically modified starches carry an E number designation and are not perceived as natural. Incidentally, the term native is applied to extracted non-processed starches, rather than natural, as this implies untouched by human hand. In terms of starch, this would mean the use in foods of the whole vegetable or botanic plant from which the starch is derived and reliance on the starch being extracted upon cooking (i.e. addition of a whole potato to soup rather than potato starch). There has, therefore, been a drive in recent years to modify native starches using physical processes to make them as functional as their chemically modified counterparts, thus retaining the label declaration ‘native’ which prefers a commercial advantage. This chapter set outs to explain what starches are, their sources, how they are extracted, their characteristics, reasons for modification, typical applications and functional properties. It summarises the types of modification employed and discusses the creation of clean label functional starches.
7.2
Manufacture of starch in plants
Green leaves of plants contain chlorophyll, which is able to absorb light quanta and utilise the energy to catalyse the formation of glucose and oxygen from carbon dioxide and water. This process is known as photosynthesis and can be written as follows (simplified): 6H2O
+ 6CO2
water
+
light/chlorophyll
carbon dioxide
C6H12O6 +
6O2
glucose
oxygen
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+
[7.1]
164
Natural food additives, ingredients and flavourings
Starch is formed in the leaves of plants by condensation polymerisation of glucose with the aid of starch-synthesising enzymes. This process can be written as follows (simplified): nC6H12O6 glucose
enzymes
(C6H10O5)n +
nH2O
starch
water
+
[7.2]
During active photosynthesis (during the day), the starch is accumulated in the leaves in the form of tiny granules of about 1 μm in diameter (leaf or transitory starch). During the night, this leaf starch is partly broken down by enzymes and transported in the form of sugars (mainly sucrose) to other parts of the plant. Some of these sugars are re-converted to starch in the seeds, tubers and roots of various plants (storage starch). It is from these sources that commercial starch is obtained. Starch molecules are synthesised in plants from sugars but the true mechanism for the biosynthesis of amylose and amylopectin is not entirely clear. The enzymes phosphorylase (P-enzyme), starch synthase and a branching enzyme are, or may be, involved in starch biosynthesis (Smith 2001). Nowadays most starch scientists believe that starch synthase is the true chain-lengthening enzyme in normal starch biosynthesis (Fujita 2006). The branching enzyme is responsible for the synthesis of the branching points in the amylopectin molecules. The mechanism that prevents amylose from branching in the obvious presence of the branching enzyme is still unresolved. The development of starch granules commences with the accumulation of poorly organised material of unknown chemical composition. At a certain point the deposition of a minute amount of insoluble polysaccharide takes place, which acts as a nucleus for further starch deposition. This nucleus is the botanical centre (hilum) of the granule, around which the granule is grown. Initial growth gives nearly spherical granules and as the granules are enlarged they often become elongated or flattened. The starch molecular chains grow in an orientation perpendicular to the growing surface of the starch granule. As the dissolved glucose units are linked to the growing starch polymer they simultaneously solidify. During the growing of the starch granule there is an increase in the proportion of amylose and an increase in molecular size of both amylose and amylopectin Starch is a polymeric carbohydrate composed of anhydroglucose units and is extracted in granular form from the organs of certain plants. Starch granules are deposited in the seeds, tubers, roots and stem piths of plants, as a reserve food supply during periods of dormancy, germination and growth. The microscope reveals that starch is composed of tiny, white granules, ranging from about 1 to 100 μm in diameter. After cellulose, starch is the next most abundant compound synthesised by plant cells. It is a renewable substance and a new supply of starch is grown annually. The size and shape of the granules are peculiar to starch from specific botanical sources. The structures of the most common starches are set out in Fig. 7.1.
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7.3
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Starch composition
Starch granules usually contain 10–20% moisture and small amounts of proteins, lipids and traces of inorganic materials, in addition to the carbohydrate content. 7.3.1 Moisture The moisture content of starch products depends on the relative humidity (RH) of the atmosphere in which they have been stored. If this humidity decreases, the starches will give up moisture; if the RH increases, they will absorb moisture. The equilibrium moisture content of starch is also dependent on the type of starch product. Under normal atmospheric conditions, most commercial native starches contain 10–20% moisture. The equilibrium moisture content of all starches is low at a low RH of the atmosphere. At an RH of zero, the moisture content of the starches approaches zero. At a RH of 20%, the moisture content of all starches is about 5–6%. 7.3.2 Lipids (fatty substances) Tuber (potato) and root (tapioca) starches contain only a very small percentage of lipids (about 0.1%), compared with the common cereal starches (maize, wheat, rice, sorghum), which contain 0.8–1.0% lipids. The fatty substances in the cereal starches are predominantly free fatty acids (in maize and waxy maize starch) or phospholipids (in wheat starch). The free fatty acids consist mainly of palmitic, linoleic and oleic acids. The presence of lipids in the common cereal starches has a profound effect on the physical properties of these starches. The lipids exist as an amylose–lipid inclusion complex in the granules. The linear fraction of the starch molecules (amylose) forms helical clathrates with polar fatty substances such as the higher fatty acids. The amylose–lipid complexes are insoluble, but dissociate when heated in water above a given temperature. The dissociation temperature is indicative of the strength of bonding and depends on the type of complexing agent. The amylose–lipid complexes tend to repress the swelling and solubilisation of the cereal starch granules. Elevated temperatures (above 125°C) are required to disrupt the organised native amylose– lipid structure in the cereal starch granules and to solubilise the amylose fraction. The presence of fatty substances can create problems in the use of maize and wheat starch products because of the tendency to become rancid on storage. 7.3.3 Proteins References to protein in starches include macromolecular proteins, but also indicate the peptides, amino acids, nucleic acids and enzymes that may be present in the starch granules. Tuber (potato) and root (tapioca) starches contain only a small amount of proteins (about 0.1%) compared with the cereal starches (maize, wheat, waxy maize), which contain 0.2–0.4% proteins. Because of the residual protein, the cereal starches may have a mealy flavour and odour, and also a
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tendency to foam. The small granules of wheat starch contain much more protein (1.6%) than the large granules (0.1%). 7.3.4 Phosphorus The phosphorus in the cereal starches is mainly present as phospholipids. The root starches (tapioca) contain a very low amount of phosphorus compounds. Potato starch is the only commercial starch that contains an appreciable amount of chemically bound phosphate ester groups. 7.3.5 Flavour and odour substances The pre-gelatinised common cereal starches (maize, wheat) have a relatively raw cereal flavour. These starches impart cereal-type flavours to the foods in which they are incorporated. Potato and tapioca starches contain only a low amount of flavour substances and this may be due to their low lipid and protein content.
7.4 Amylose and amylopectin Starch can be considered to be a condensation polymer of glucose, consisting of anhydroglucose units. The glucose units are linked to one another through the C-1 oxygen in what is known as a glucoside bond. The glucoside linkage is stable under alkaline conditions and hydrolysable under acid conditions. The glucose unit at the end of the polymeric chain has a latent aldehyde group and is known as the reducing end group. Most starches are a mixture of amylose and amylopectin, each having a wide range of molecular sizes. Starches of different origin have different amylose to amylopectin ratios (Table 7.1). Table 7.1 also shows the average degree of polymerisation (DP) of both fractions in various starches. In summary, starch is considered to be a condensation polymer of glucose. The glucose units in the starch polymer are present as anhydroglucose units (AGU). Most starches contain two types of glucose polymers: amylose and amylopectin. Amylose is a linear chain of glucose units while amylopectin is a branched polymer Table 7.1 Amylose and amylopectin contents and degree of polymerisation of various starches Starch Rice Wheat Maize Potato Tapioca (cassava)
Percentage content
Average DP
Amylose
Amylopectin
Amylose
Amylopectin
22 26 28 21 17
88 74 72 79 83
4000 1000 1000 4000 4000
2 000 000 2 000 000 2 000 000 2 000 000 2 000 000
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of glucose units. These two polymers occur in differing amounts in starches from various botanical sources. Amylose is responsible for gelling properties and shear resistance, while amylopectin is responsible for stability. 7.4.1 Amylose Amylose is a linear polymer containing up to 6000 glucose units, connected by 1,4-linkages (Fig. 7.2). The ratio of amylose to amylopectin is fairly constant for a given species of starch. Maize and sorghum starch have a much higher amylose content (about 28%) than the tuber and root starches (potato, tapioca, arrowroot), which contain only about 20% amylose. The waxy starches contain no amylose fraction. Amylomaize starch, a maize starch, which has been selectively bred so that the resultant starch has a high amylose content, may contain up to 80% amylose. It is largely used in edible films, coatings and biodegradable packaging. Amylose covers a range of degrees of polymerisation, depending upon the source of the starch. The amylose molecules of potato and tapioca starch have a substantially higher molecular weight than maize and wheat starch amylose. The amylose fraction of potato starch has a DP ranging from 840 to 22 000 glucose units. The amylose fraction of maize starch has a DP of about 400–15 000 glucose units. Amylose forms inclusion complexes with iodine and various organic compounds such as butanol, fatty acids, various surfactants, phenols and hydrocarbons. These complexes are essentially insoluble in water. It is believed that amylose complexes by forming a helix coil around the complexing agent. The complex of amylose with iodine gives a characteristic blue colour, which is used to establish the presence of amylose-containing starch.
Fig. 7.2
Linear structure of amylose molecule.
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Fig. 7.3
Structure of amylopectin branching points.
7.4.2 Amylopectin Amylopectin has a highly branched structure, consisting of short linear chains with a DP ranging from 10 to 60 glucose units. The average DP of these chains is about 22. They are connected to each other by α-1,6-linkages (Fig.7.3).
7.5
Starch: extraction and manufacture
Their naturally high starch content and ready availability means that the most commonly available sources of starch are derived from potato, rice, tapioca, corn (maize) and wheat. The starch industry uses a combination of wet purification techniques, milling and drying to manufacture native starch with a purity of about 98–99.5%. In the manufacturing process, starch is separated from the other constituents of the milled raw material such as fibres, proteins, sugars and salts. Figure 7.4 shows a typical production flow diagram for the production of native starch.
Fig. 7.4 Typical native starch production flow diagram.
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7.6
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Starches from different sources
Starch-rich plant sources are grown worldwide. An indication of the volumes of these plant sources is set out in Table 7.2. When selecting a starch for a particular application, it should not be forgotten that the supply of starch can be volatile. Coming from natural crop sources, it is highly sensitive to environmental factors and weather conditions. This was no better demonstrated than in 2010 when fires devastated large tracts of wheat in Russia; late cold springs in Europe, combined with drought during the growing season and flooding at harvest, led to greatly reduced potato crop yields and drought, flooding and insect damage resulted in sharp declines in harvest yields in Asia. In Europe the potato crop was down by at least 33%, and in Russia the wheat crop yield was said to be at least 30% down. 2010 was also a year of significant changes in market conditions. With global economies slowly coming out of recession and growing wealth among middle and upper classes across Asia and South America, demand for processed food and starch in particular increased greatly. Conditions such as these can potentially lead to shortages and price increases. 7.6.1 Characteristics of native starches from various botanical sources Starches are very versatile, and have supported and permitted many innovations in processing within the food industry. Perhaps less well-known is the fact that starch provides functionality in non-food applications. The starch industry refers to these non-food uses as industrial uses. Examples of industrial uses are adhesives, paper, cardboard, detergents, edible film, biodegradable packaging, oil drilling, water treatment, construction and mining industries. Food applications include soups, sauces, canned foods, cereal and snacks, beverage emulsions, dairy, meat and baked goods. With such a wide variety of starches and wide variety of functional properties it is difficult to give a comprehensive account of all relevant properties in this chapter. Key properties of starch include: heat and freeze-thaw stability; dispersability (i.e. the ability of the starch particles to disperse homogenously into
Table 7.2 Volume of starch-producing plant sources grown worldwide, 2008 Crop
Amount (million tonnes/year)
Rice Wheat Maize Potato Tapioca (cassava)
685 690 823 314 233
Average yield (tonnes/hectare) 4.30 3.00 5.10 17.30 12.50
Source: FAOSTAT, Crops worldwide (http://faostat.fao.org/site/567/default.aspx#ancor).
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a liquid or powder media and deliver their functionality homogenously throughout); viscosity (hot and cold); and abilities with regard to film formation, gel formation, oil retention, moisture-binding, suspension of solids, volume control and crispness promotion. Other parameters to consider include: impacts on food texture clarity, opacity and sheen; emulsion-stabilising capacity; adhesiveness; and tolerance of processing conditions. Few other ingredients offer the range and versatility of characteristics as effectively and as economically as starch. When selecting a starch for a particular application, it is advisable to obtain the technical data required to make a comparison of properties and an informed choice. Table 7.3 sets out some of the key characteristics of the starches from potato, rice, tapioca, corn (maize) and wheat.
7.7
Modification of starches
Starches are modified so as to give them certain useful properties (required by particular foods or processes), to make them suitable for use within certain production processes, and to retain their functionality in foods so as to allow production of safe, long-life foods. The various types of modification of native starch are designed to change one or more of the following properties:
• • •
Pasting temperature (the temperature at which initial swelling of starch granules takes place when suspended in water). Solids–viscosity relationships. Gelatinisation and cooking characteristics.
Table 7.3 Key characteristics of the starches from potato, rice, tapioca, corn (maize) starch and wheat Potato starch Rice starch
Corn starch
Tapioca starch Wheat starch
Size (μm) Shape Colour
15–80 Oval White
2–8 Round Very white
10–15 Oval White
Taste
Potato taste
Neutral
15–25 Hexagonal Yellowish, white Protein taste
Odourlessness Moderate Presence of Allergen allergens traces
Gel structure Supply stability
Sticky Volatile
20–40 Oval Greyish white Cereal taste
Slight to neutral Good Fair Allergen-free Gluten
Excellent Fair Allergen free Risk of allergen introduction due to genetic modification Creamy Firm Sticky Volatile Stable Volatile
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Resistance of starch pastes to breakdown. Viscosity by acids, heat and/or mechanical shear. Retrogradation tendencies. Ionic character. Hydrophilic character.
Taking pasting temperature as an example, this is an important characteristic in the processing of starch and starch products. If starch granules in suspension in water are heated, water penetrates the granules to hydrate them with resulting swelling. If the temperature of the starch suspension or slurry is heated above its pasting temperature, a viscous mass is produced and the starch granules lose their unique microscopic appearance or shape, which may not be regained upon cooling to room temperature. As starch granules are heated in water they hydrate and swell, the refractive index of the granules approaches that of water, and the initially opaque slurry becomes more transparent. In general, the pasting temperatures of modified starches are lower than those of native starches. This is important in some applications, for example, to keep fruit pieces suspended in a filling while it is cooking. 7.7.1 Types of modification While it is not possible to detail in full every type of modification, some of the more common modifications are set out in Table 7.4. The modification of native starch may involve a change in physical form, a controlled degradation and/or the introduction of chemical groups. Table 7.5 sets out the modified starches permitted for use in EU in foods together with their applicable E numbers, which must be present on the label. Table 7.4 Types of modification Type of modification
Reason for modification
Treatment
Dextrin
Adhesion, low viscosity
Acid modified starch
Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability Low viscosity, strong viscosity stability High shear, acid stability Improved viscosity stability Specific properties
Dry heat and treatment with acid Acid hydrolysis
Alkaline modified starch Bleached starch Oxidised starch Enzymatically modified starch Cross-linked starch Stabilised starch Combinations of above
Treatment with sodium or potassium hydroxide Treatment with hydrogen peroxide Oxidation with sodium hypochloride Treatment with alpha amylase Hydroxyl bonding Esterification –
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Starch name
E1404 E1410 E1412 E1413 E1414 E1420 E1422 E1440 E1442 E1450 E1451
Oxidised starch Monostarch phosphate Distarch phosphate Phosphated distarch phosphate Acetylated distarch phosphate Acetylated starch Acetylated distarch adipate Hydroxyl propyl starch Hydroxyl propyl distarch starch Starch sodium octenyl succinate Acetylated oxidised starch
Source: www.food.gov.uk/safereating/chemsafe/ additivesbranch/enumberlist
Other types of modified starches include:
• • • • • •
Dextrin (E1400), starch roasted with hydrochloric acid. Alkaline-modified starch (E1402) with sodium hydroxide or potassium hydroxide. Bleached starch (E1403) with hydrogen peroxide. Enzyme-treated starch (INS: 1405), maltodextrin, cyclodextrin. Monostarch phosphate (E1411) with phosphorous acid or the salts sodium phosphate, potassium phosphate, or sodium triphosphate to reduce retrogradation. Carboxymethylated starch with monochloroacetic acid adding negative charge.
Modified starches may be pre-gelatinised to render them readily soluble in cold water, and capable of swelling and gelling without heat. Alternatively they may be presented in an uncooked form, which must be cooked like regular starch in order to gelatinise the starch granules. Drying methods to make starches soluble in cold water are extrusion, drum drying or spray drying, and agglomeration.
7.8
Clean label starches
At present ‘clean label’ is an ill-defined term with different interpretations across the globe and no legal definition. One useful definition was devised by National Starch in the US: Free from chemical additives; simple ingredient listing (without ingredients that sound chemical or artificial); minimally processed using traditional techniques that are understood by consumers and not perceived as being artificial. It is worth noting that clean label is not a consumer term; it is in fact an industry term.
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Some trends to bear in mind that are currently fuelling the demand for clean label ingredients are discussed below. 1 Through the various celebrity chef cookery programmes and advertising by the supermarket groups, consumers are increasingly avoiding foods high in fat, salt, sugar and also foods containing additives. Modified starch is an ingredient which carries an E number and is not perceived as natural. 2 Food labels are increasingly referring to products as local, kitchen-style or home-style, etc. Modified starch is not perceived as fitting into these categories. 3 The percentage of consumers who are reading labels is thought to be increasing. 4 The terms ‘natural’ or ‘free from additives’ are increasingly appearing on food product labels. Modified starch is an additive and is not perceived as fitting into these categories. 5 Descriptive language on food packs increasingly describes products as ‘pure’ or ‘fresh’. Modified starch is not perceived as fitting into these categories. While the onset of the recession in 2008 brought some respite to food processors who were otherwise being pressurised to using clean label starches, manufacturers are now once again under pressure from supermarket groups and consumers to remove additives from food, including modified starches. This has led to reduced sales, especially in the UK and Ireland, of modified starches but increased sales of functional clean label equivalents. The issue at stake for starch manufacturers is to find innovative ways to deliver clean label solutions that offer the functionality and qualities of modified starch. This challenge is greater in the EU than the US due to the dominance of supermarkets in Europe and also discrepancies between the definition of clean label in the two continents. However, the clean label movement is thought also to be gaining appeal in the US. Clean label starches have been on offer in the marketplace for some time now – National Starch have been active in this area since the mid to late 1990s, when its Novation® starch range came into being. Starch-producing companies are employing many techniques to impart functional characteristics to native starches that are equivalent to their modified counterparts. While the first generation clean label starches were not so stable in conditions of high acid shear, the current generation clean label starches are indeed very functional. Some of the techniques which starch companies are employing include:
• • •
Physical modification through agglomeration and granulation. Selective crop breeding – we have recently seen the launch of high amylopectin waxy potato and wheat starches (by changing the ratio of amylose to amylopectin it is possible to offer differing functional performance). Thermal treatment of native flours.
The techniques and technologies applied by the starch producers are closely guarded by those companies because of their commercial sensitivity. It is not possible to discuss the processes in more detail because the information is not in the public domain.
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7.9
Conclusions and future trends
When it comes to clean label it is the retailers and the consumers who are presenting the ultimate challenge: they require clean label starches that have the same function as modified starches and cost the same. Companies most active in the field of clean label starches include National Starch, Ulrick & Short, and AVEBE. The author’s personal feeling is that clean label is not a passing fad: it is here to stay. National Starch is currently considered the market leader and is expected to maintain this position. As many existing products are protected by patents, other companies entering the market may have to rely on selective breeding to offer perhaps one or more of the attributes required in a clean label starch, and those clean label starches that they produce may not be as functional, versatile or easy to formulate into foods as those currently established in the marketplace.
7.10
• • • •
Sources of further information and advice
Emsland Group (www.emsland-group.de) AVEBE (www.avebe.com) National Starch (now a subsidiary of Corn Products International), (www. nationalstarch.com) Ulrick & Short (www.ulrickandshort.com)
7.11
References
FUJITA N, YOSHIDA M, ASAKURA N, OHDAN T, MIYAO A
et al. (2006), ‘Function and characterisation of starch synthase I using mutants in rice’, Plant Physiology, 140, 1070–1084. MANINGAT C C, SEIB P A, BASSI S D, WOO K S and LASETER G D (2009), ‘Wheat starch: Production, properties, modification and uses’, in BeMiller J and Whistler R (eds.), Starch: Chemistry and Technology, 3rd edition, Academic Press, Burlington, VA, Chapter 10, pp. 442–510. SMITH A M (2001), ‘The biosynthesis of starch granules’, Biomacromolecules, 2, 335–341.
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