What is Gluten Free Covering the A to Z of Gluten Free
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Moreover, it is less resistant to enzyme activity and can be more easily degraded than intact starch granules [ 80 ]. Damaged starch also plays an important role during the fermentation process due to its susceptibility to enzymatic breakdown. The amylases present in starch generate maltose, which can be directly used for the production of carbon dioxide by the yeast, which in turn gives a rise to the dough [ 32 ].
Amylases are important enzymes, which can be detected in different starch sources [ 80 ]. Amylases can have a big influence on baked breads, due to the generation of reducing sugars maltose , which can be transformed by the added yeast into carbon dioxide and alcohol [ 80 ]. A further beneficial effect of the addition of amylases is the retardation of retrogradation of amylose and, hence, decrease in the rate of staling of baked breads [ 82 ].
This could be of particular interest for gluten-free breads, as these products are based on refined starches and therefore stale faster than conventional breads.
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Enzymatic degradation of amylopectin. Reducing d -glucose residue black filled ; non-reducing d -glucose residue white filled. Arrows indicate the 1,6-branch points in the starch molecule. Adapted from Antranikian [ 83 ]. These small amounts are found in the starch granules and form complexes with amylose Figure 3. Based on the structure of the lipids, it is possible for lipids to align in the core of the amylose helix [ 85 ].
Hence, the lipids content in native starch usually correlates well with the amylose content [ 34 ].
Erica Drew (Author of What is Gluten Free Covering the A to Z of Gluten Free)
Nevertheless, there are also surface active lipids mainly triglycerides , which are bound by ionic or hydrogen bonds to the hydroxyl groups of the starch [ 86 ]. The lipid content of a starch is dependent on various factors, such as source, polysaccharide composition, physical structure of the grain endosperm and the amylose content [ 31 , 34 ]. Despite representing a minor component in starch lipids can alter the properties of starch significantly [ 84 ].
Jane [ 32 ] stated that the presence of lipids in starches increases the gelatinisation temperature, retarding granule swelling and preventing amylose from leaching out. It was further reported by Copeland et al. These interactions, leading to changes in the function of starch are of great benefit in the food industry [ 34 ].
The interaction of lipids in starch and its influence on the normal baking process has been comprehensively reviewed by Pareyt et al. Similar to the lipids, proteins form only a minor component of commercial starches [ 84 ]. The protein content is very much dependent on the source of starch as well as the extraction procedure. It has been reported by Jane et al. The morphology of starch granules can be analysed by using scanning electron microscopy SEM or confocal laser scanning microscopy CLSM.
The shape and size of starch granules depends on the origin of starch [ 31 ]. Figure 4 shows micrographs of wheat, potato, tapioca, corn and rice starches. The A granules of wheat starch have a non-uniform, lenticular or disk-like shape, while the B granules are spherical or ellipsoidal granules. Potato starch also has big and small granules. They show a smooth surface and a round, oval shape.
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Furthermore, growth rings can be seen in the potato starch when observed under CLSM [ 33 ]. The granules have both a polygonal shape and a round shape with a plain surface [ 89 ]. Rice starch has granules which are agglomerated like tapioca starch granules. The polygonal shape of the granules is similar to the shape of granules in corn starch [ 92 ]. Factors which have an influence on the rheology, functional and structural properties of starch-based foods are the different granule sizes, and their shape [ 33 ].
Scanning electron micrographs of A potato starch; B tapioca starch; C corn starch; D rice starch, E wheat starch.
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The digestion of starch is an important process with respect to dietary requirements [ 93 ]. Factors which influence the digestibility of starch are the compositional and morphological properties and the physical access of enzymes to the starch [ 94 , 95 ]. Starches are grouped according to their digestibly, as follows: rapidly digestible starch RDS , slowly digestible starch SDS and resistant starch RS [ 96 ]. The blood glucose response to food is classified by the concept of the glycaemic index GI [ 98 ].
Benefits of SDS are a distinct hormonal and metabolic profile. Furthermore, a lower GI is linked with a reduced risk of diabetes and cardiovascular diseases [ 99 ].
Differences in starch characteristics and extraction processes have an influence on its digestibility as shown in Table 2 and Table 3 , respectively. Starch digestibility plays an important role in gluten-free products, as they rely on starch as a main ingredient. Based on this, many studies have been carried out on the digestibility of gluten-free products [ 47 , 94 , , , , , , , ].
Table 2 shows intrinsic factors influencing starch digestibility. As described in 4. This results in amylopectin having a greater surface area, which makes it more susceptible to amylolytic attacks.
This susceptibility results in faster digestion to amylose. This indicates that starch with a high amylopectin content is digested more rapidly than starch with a lower content.
The lower digestibility of amylose is due to the glucose chains, which are more bound to each other by hydrogen bonds [ 94 ]. This makes amylose less available to amylolytic attacks.
However, as stated above other factors influence the digestibility. Minor components of starch, such as protein and fat, can form complexes with amylose and affect the enzymatic susceptibility [ 94 ]. As presented in Table 3 , many treatments cause an increase in RDS. Most of these processes promote hydrolysis of amylose and amylopectin by gelatinisation, in excess water.
In the case of baking, for example, the decreased amount of SDS is explained by reduced gelatinisation of starch, due to the lack of accessible water.
In general, gelatinised starch is more easily digested, while retrograded and recrystallized starch is less digestible. To produce gluten-free products with decreased digestibility, modified starches with a higher amylose content could be employed. However, a higher amylose content will also ultimately change the process-ability and product quality parameters.
Starch plays a major role in food products with a variety of applications [ ]. During the processing of food products, starch undergoes physico-functional changes, due to different heating and cooling steps. Major factors affecting the functionality include, amongst others, granule size and shape, starch crystallinity, amylose-amylopectin ratio, packing density, presence of fat, encapsulated starch granules, swelling power and solubility, gelatinisation, retrogradation and rheological properties [ ]. Since every starch differs with respect to these properties, the selection of starch depends on the desired food properties and its production and processing [ ].
Native starch is insoluble in cold water; in this state starch, does not have an effect on food characteristics. However, when heat is applied to starch in excess water, the starch undergoes changes, such as gelatinisation, pasting and retrogradation Figure 5. Gelatinisation, pasting and retrogradation of starch influenced by heat and time, where AM is amylose and AP amylopectin. Adapted from Schirmer et al. Native starch granules have a complex architecture.
Therefore, the processing of starch granules usually involves the disruption of the structural order within the granules during heating in water [ ]. This collapse or disruption of molecular order within the starch granule leads to irreversible changes in properties such as granular swelling, native crystallite melting, loss of birefringes and starch solubilisation.
These changes lead to an increase in viscosity of the medium and are termed gelatinisation [ ]. When starch is heated in water, amylose leaches from the granules [ ]. The leaching of amylose and the increase of swollen granules increase the viscosity of the medium [ 34 ]. Gelatinisation is a key functional property of starch granules which determines its use in food [ ].
The desired thickening property is achieved when the starch granule reaches its maximum swelling, but is not disrupted yet [ ]. The temperature at which point the starch granules start to swell is referred to gelatinisation temperature. This temperature varies amongst different starch sources [ 41 ]. In general, the gelatinisation temperatures of root and tuber starches are reported to be lower than those of cereal starches [ 4 ]. Additional heating or shear at the stage of maximal swollen granules will destruct them by disrupting hydrogen bonding between polymer chains.
When this happens, a dispersion of amylose and amylopectin and granule fragments is formed, which results in loss of viscosity of the paste [ ]. The point between maximal swelling and disruption leading to viscosity loss, is referred to the peak viscosity. A differentiation of pasting from gelatinization was proposed by Atwell [ ].
The author defined pasting as phenomena following gelatinisation in the dissolution of starch. It involves granule swelling and eventual total disruption of the starch granule.