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The Gluten Proteins: Glutenin and Gliadin

Gluten is a complex storage protein in wheat made up of mainly gliadins and glutenins. The wheat kernel itself contains about 8-15% protein and of that 85-90% is gluten. These two proteins are known together as prolamins, which refers to seed proteins that are not soluble in water and are classified by their high levels of glutamine (38%) and proline (20%) amino acid residues . Gluten can also be narrowed into more specific classes depending on its molecular weight and sulfur content. Further classifications can be determined depending by their structures such as alpha, beta, gamma, and omega. All together, these structural and physical properties of the proteins give rise to characteristic functions of different gluten proteins .

The rheological properties of gluten are heavily dependent upon the specific ratio of glutenins to gliadin and how they interact with each other . Each protein imposes a different function when it comes to trapping carbon dioxide during the leavening process of bread product development, which gives rise to viscoelastic properties. Hydrated gliadins will contribute more heavily to extensibility and viscosity of the dough, while hydrated glutenins are cohesive and produce strong elastic dough.

Gliadin, one of two main gluten proteins, is resistant to proteolytic digestion in the gastrointestinal tract due to its unique peptide sequences. Other storage proteins such as secalin in rye, hordein in barley, and avenins in oats are also referred to as ‘gluten’ as they are very similar to gliadin found in wheat . The networks of the varying proteins are different due to their size, components, how they were grown, and processing. Although these proteins have different properties, they are just as indigestible as wheat gluten.

Gluten matrixes give rise to the functional properties of baked products such as bagels and bread. It is heat stable, meaning it does not lose its function in the presence of heat. Gluten also holds properties such as moisture retention, adds extendibility to foods and acts as a binding agent . Due to its wide range of functionality, gluten is often used as a texture and flavor enhancer in many uncommon food items. This includes processed meats, reconstituted seafood, meat substitutes, emulsifiers, dressings, sauces, coatings, ice cream, thickeners and more. Gluten can also be isolated and separated from the seed to be used solely as a functional additive; this product is known as vital wheat gluten and can be purchased commercially .

Native Flours and Starch:

Starches and flours provide the backbone for the structure and texture of gluten free bagels. Choosing the right blend of these ingredients is important to the end products volume, taste and appearance. Different materials attribute to different properties such as chewiness, body, dough elasticity, volume, moistness, and crumb structure. Rice flours are the main bulk flour used in most gluten free bagel recipes. This is due to their colorless nature, very bland taste, and they contain small amounts of protein, sodium, fat, and is easily digestible . Although it has many advantages, rice flour has many negative functional properties and should be used in combination with other gluten free starches such as potato and tapioca. Rice flours form poor viscoelastic dough due to their hydrophobicity causing them to be insoluble. This leads to low bake volume, a harder texture, a short shelf life and rapid staling . To improve the viscoelastic properties that are lacking when solely rice starch is used, structuring agents such as hydroxypropylmethylcellulose, (HPMC), guar gum, xanthan gum, dairy products and psyllium husk are used .

The lipid content of the different starches plays a large role in the volume and finer crumb of the bagels. Starches have mainly polar lipids, which are surface-active components that act to stabilize the gas-liquid interface. While mixing and baking the bagels, the gas cells are preserved by this interface. Furthermore, this integrity produces a sturdy cell wall that allows for higher gas retention during fermentation. This process is especially prevalent in corn based starch blends due to their high amount of polar lipids .  In conjunction with lipid content, the granule size also plays a large part in the volume of gluten free bagels. Large granule sizes produce breads with a better crumb and a larger overall volume. Potato starch has one of the largest granule sizes of the starches by far, being 36.7 μm in diameter. Tapioca starch is half the size at 18.1 μm, corn 12.9 μm, and one of the smallest starches is rice, measuring 12.3 μm (Fig 3). 

Hydrocolloids and Gums:

In bagels using a base of mainly rice flour with lower percentages of potato starch and tapioca starch; xanthan gum, guar gum, psyllium husk and HPMC are added to improve the sensorial properties of the product. The hydrocolloids interact with the starch, affecting the bread structure and texture to be more like that of its gluten-containing counterpart. HPMC has been show to delay the rate of staling as well as reducing the crumb hardness . Micrographs taken of gluten free bakery products also show with the addition of HPMC, there is a continuous matrix between the starch and the structure is more aerated rather than collapsed (without HPMC). The HMPC will orient between the leeched amylose and amylopectin within the granule to prevent re-crystalization (Schaich, Fall 2017). The use of HPMC can also result in larger volume, uniform and fine crumb structure. This is also due to the way the HPMC orients within the molecule and keeps the amylose and amylopectin oriented open and more branched (Schaich, Fall 2017). Xanthan gum, guar gum and psyllium husk will not contribute to the volume of gluten free bagels, but they all contribute to controlling dough viscosity as they have high affinities for moisture retention, especially xanthan gum.

Protein:

When gluten is removed from gluten free bagels and replaced with rice flour, tapioca starch and potato starch, protein and other important nutrients are also lost. Dairy/protein powders also have functional properties that contribute to the color, improve crumb structure, and enhance dough handling . Milk proteins, such as whey, increase the water absorption in the system, which leads to less sticky dough and easier handling. In gluten free bagels, whey protein isolate increases the protein content of the product. In a study done in 2016, it was seen that the addition of milk proteins to rice starch based bakery products results in lower cell per square centimeter, or a more open aerated structure . This provides a softer texture in the mouth and shows less resistance when tested on a texture analyzer . In the same study, it was also observed that a protein network was formed in the microstructure when using whey protein, which may heavily contribute to the stable crumb .

Yeast and Sugar:

The specific aroma that is associated with baked goods such as gluten free bagels arises from three steps, kneading, fermentation and baking. Each of these is significantly affected by temperature and time of baking . When yeast, Saccharomyces cerevisiae, is added to the dough, fermentation is initiated. The yeast uses the sugars/simple carbohydrates to mainly produce carbon dioxide and methanol. These are the air bubbles that cause the bagels to rise and increase in volume, providing a ‘fluffy’ and soft texture . In gluten free bagels, bakers yeast is used, which releases secondary metabolites such as higher alcohols, aldehydes, sulfur containing compounds, esters, phenols, carbonyl compounds, and organic acid. These metabolites effect the sensory of the product by releasing the aroma associated with fermented baked goods . During the baking process, the sugars added to the formula that are used as a precursor to the fermentation reaction, go through a browning reaction. This is known as the Maillard reaction and caramelization. The Maillard reaction is induced by heat and can only occur in the presence of reducing sugars and free or protein bound amino acids. The final products of this reaction are brown macromolecules known as melanoidins . At temperatures higher than 140°C, multiple volatile compounds are produced that contribute to crust color and flavor of the bagel .

Butter:

Gluten free bagels tend to be dry and have short shelf lives if left at ambient temperatures. Butter or hard fat can be added to formulations to improve rheological properties that improve overall mouthfeel and perception of moistness. Bagels made with wheat flour are usually made without any fat at all. The inclusion of fat into gluten free breads and bagels has shown to reduce their hardness even with lower levels of moisture . The fat creates pockets when it melts that provides more openness inside the bagel in addition to the air-liquid interface created by the yeast. The fat also orients itself between the fringe regions of the amylopectin within the starch granules, preventing them from collapsing. The lipids will also help to trap moisture within the amylopectin, preventing dehydration. The branched or fringe regions of amylopectin have a rigid texture to them, contributing to hardness in food products after processing; therefore potato starches high percentage of amylopectin creates a more tough texture in the bagel. By adding fat, staling can be delayed and a softer texture can be achieved. This allows for a light and airy texture rather than a dense bagel that is often associated with gluten free bakery products.

Methods of Manufacturing

Most bagels are produced through a two-step process of boiling then baking. After the dough is prepped according to specific formulation instructions, it is divided and formed into the conventional bagel shape. This is done by rolling the dough under a pressure plate into a log like forn that rolls around a round mold to make the bagel. A manufacturing machine can produce about one bagel per second . After the bagels are formed, they then undergo a proofing step where the yeast raises the bagels. After the bagels proof for the allotted time, usually about an hour), they are ready to boil. In traditional bagels containing gluten, the boiling water contains malt to form a crunchy crust on the outter layer of the bagel . Malt is an ingredient that those with celiac disease or gluten intolerance cannot consume, so an alternative ingredient must be used to form this crisp outer layer. Baking soda can be added to replace the malt to form the crust on the bagel. The baking soda causes a pH difference in the water, making it more alkali. The boiling water gelatinizes the starch on the outer later of the bagel, which begins the cooking process . This makes the gluten free bagel slightly chewy and will contribute to it’s brown crust and distinct mouthfeel associated with traditional bagels. This process promotes the Maillard reaction and carmamelization, which gives rise to color and flavor . After a short time in the rolling boil (90 seconds), the bagels are removed and transferred to a hot oven to dry out and bake until golden.

Shelf Life

The reasoning behind the molecular changes in regular bread are not yet fully understood, and the research into the staling of gluten free breads has just begun. The decline in fresh flavor of gluten containing bread is thought to be due to many physiochemical changes such as the recrystallization of amylopectin, moisture diffusion, loss of gluten plasticity, and the interactions between starch and gluten . Some of these changes also carry over to gluten free bread products such as amylopectin recrystallization. This leads to the increase in firmness of the breads texture. Although this is a similar process of staling, it occurs at a much faster rate due to the increased amount of starch in gluten free formulations; with potato starch being especially high in amylopectin .  With the lack of gluten in gluten free bagels, there is a high amount of water used in the formation to combat dryness. This water is mostly free water rather than bound water, which makes it susceptible to migration through the product. This is a large problem in gluten free breads and bagels as the moisture migrates from the crumb to the crust, making it tough to chew . In a study done on the moisture contents of fresh gluten free breads, the moisture of the crumb was over ~10% higher in fresh gluten free breads than in standard gluten containing breads . The higher moisture levels will lead to faster staling and spoilage due to the reasons previously mentioned. The fresh gluten free breads had an average moisture content of about 52%, while the standard fresh gluten containing bread had average moisture content of about 42% .  Due to the faster staling process of gluten free bread products, bagels should be sold and stored frozen following production to ensure a longer shelf life and freshness.

In order to prevent staling, multple methods can be used to interfere with the process to inhibit it. Mono and diglycerides are often used in gluten free bread systems as they act in a similar manner to lipids in which they prevent the fringe regions from re-associating . Instead of having trigclyderides, the mono and diglycrides induce helical regions that orient at the outer fringes of the amylopectin. These helices are hydrophobic and will inhibit retrogradation and recrystalization that leads to the staling of bagels and bread products. Stearic acid is also another retrogradation inhibiting agent that produces helical regions and prevents staling in the same manor as mono and diglycerides (Fig.6) .