Category Archives: Proteins

Gluten: definition, structure, properties, wheat, cereal list

Gluten: contents in brief

What is gluten?

Gluten
Fig. 1 – Wheat

Gluten is not a single protein but a mixture of cereal proteins, about 80% of its dry weight (for example gliadins and glutenins in wheat grains), lipids, 5-7%, starch, 5-10%, water, 5-8%, and mineral substances, <2%.
It forms when components naturally present in the grain of cereals, the caryopsis, and in their flours, are joined together by means of mechanical stress in aqueous environment, i.e. during the formation of the dough.
The term is also related to the family of proteins that cause problems for celiac patients (see below).
Isolated for the first time in 1745 from wheat flour by the Italian chemist Jacopo Bartolomeo Beccari, it can be extracted from the dough by washing it gently under running water: starch, albumins and globulins, that are water-soluble, are washed out, and a sticky and elastic mass remains, precisely the gluten (it means glue in Latin).

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Cereals containing gluten

It is present in:

  • wheat, such as:

durum wheat (Triticum durum); groats and semolina for dry pasta making are obtained from it;
common wheat or bread wheat (Triticum aestivum), so called because it is used in bread and fresh pasta making, and in bakery products;

  • rye (Secale cereale);
  • barley (Hordeum vulgare);
  • spelt, in the three species:

einkorn (Triticun monococcum);
emmer (Triticum dicoccum Schrank);
spelta (Triticum spelta);

  • khorasan wheat (Triticum turanicum); a variety of it is Kamut®;
  • triticale (× Triticosecale Wittmack), which is a hybrid of rye and common wheat;
  • bulgur, which is whole durum wheat, sprouted and then processed;
  • seitan, which is not a cereal, but a wheat derivative, also defined by some as “gluten steak”.

Given that most of the dietary intake of gluten comes from wheat flour, of which about 700 million tons per year are harvested, representing about 30% of the global cereal production, the following discussion will focus on wheat gluten, and mainly on its proteins.

Note: the term gluten is also used to indicate the protein fraction that remains after removal of starch and soluble proteins from the dough obtained with corn flour: however, this “corn gluten” is “functionally” different from that obtained from wheat flour.

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Cereal grain proteins

Gluten
Fig. 2 – Cereal Grain Proteins

The study of cereal grain proteins, as seen, began with the work of Beccari. 150 years later, in 1924, the English chemist Osborne T.B., which can rightly be considered the father of plant protein chemistry, developed a classification based on their solubility in various solvents.
The classification, still in use today, divides plant proteins into 4 families.

  • Albumins, soluble in water.
  • Globulins, soluble in saline solutions; for example avenalin of oat.
  • Prolamins, soluble in 70% alcohol solution, but not in water or absolute alcohol.
    They include:

gliadins of wheat;
zein of corn;
avenin of oats;
hordein of barley;
secalin of rye.

They are the toxic fraction of gluten for celiac patients.

  • Glutelins, insoluble in water and neutral salt solutions, but soluble in acidic and basic solutions.
    They include glutenins of wheat.

Albumins and globulins are cytoplasmic proteins, often enzymes, rich in essential amino acids, such as lysine, tryptophan and methionine. They are found in the aleurone layer and embryo of the caryopsis.
Prolamins and glutelins are the storage proteins of cereal grains. They are rich in glutamine and proline, but very low in lysine, tryptophan and methionine. They are found in the endosperm, and are the vast majority of the proteins in the grains of wheat, corn, barley, oat, and rye.
Although Osborne classification is still widely used, it would be more appropriate to divide cereal grain proteins into three groups: structural and metabolic proteins, storage proteins, and defense proteins.

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Wheat gluten proteins

Proteins represent 10-14% of the weight of the wheat caryopsis (about 80% of its weight consists of carbohydrates).
According to the Osborne classification, albumins and globulins represent 15-20% of the proteins, while prolamins and glutelins are the remaining 80-85%, composed respectively of gliadins, 30-40%, and glutenins, 40-50%. Therefore, and unlike prolamins and glutelins in the grains of other cereals, gliadins and glutenins are present in similar amounts, about 40%.
Technologically, gliadins and glutenins are very important. Why?
These proteins are insoluble in water, and in the dough, that contains water, they bind to each other through a combination of intermolecular bonds, such as:

  • covalent bonds, i.e. disulfide bridges;
  • noncovalent bonds, such as hydrophobic interactions, van der Waals forces, hydrogen bonds, and ionic bonds.

Thanks to the formation of these intermolecular bonds, a three-dimensional lattice is formed. This structure entraps starch granules and carbon dioxide bubbles produced during leavening, and gives strength and elasticity to the dough, two properties of gluten widely exploited industrially.
In the usual diet of the European adult population, and in particular in Italian diet that is very rich in derivatives of wheat flour, gliadin and glutenin are the most abundant proteins, about 15 g per day. What does this mean? It means that gluten-free diet engages celiac patients both from a psychological and social point of view.

Note: the lipids of the gluten are strongly associated with the hydrophobic regions of gliadins and glutenins and, unlike what you can do with the flour, they are extracted with more difficulty (the lipid content of the gluten depends on the lipid content of the flour from which it was obtained).

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Gliadins: extensibility and viscosity

Gluten
Fig. 3 – Wheat Grain Proteins

Gliadins are hydrophobic monomeric prolamins, of globular nature and with low molecular weight. On the basis of electrophoretic mobility in low pH conditions, they are separated into the following types:

  • alpha/beta, and gamma, rich in sulfur, containing cysteines, that are involved in the formation of intramolecular disulfide bonds, and methionines;
  • omega, low in sulfur, given the almost total absence of cysteine and methionine.

They have a low nutritional value and are toxic to celiac patients because of the presence of particular amino acid sequences in the primary structure, such as proline-serine-glutamine-glutamine and glutamine-glutamine-glutamine-proline.
Gliadins are associated with each other and with glutenins through noncovalent interactions; thanks to that, they act as “plasticizers” in dough making. Indeed, they are responsible for viscosity and extensibility of gluten, whose three-dimensional lattice can deform, allowing the increase in volume of the dough as a result of gas production during leavening. This property is important in bread-making.
Their excess leads to the formation of a very extensible dough.

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Glutenins: elasticity and toughness

Glutenins are polymeric proteins, that is, formed of multiple subunits, of fibrous nature, linked together by intermolecular disulfide bonds. The reduction of these bonds allows to divide them, by SDS-PAGE, into two groups.

  • High molecular weight (HMW) subunits, low in sulfur, that account for about 12% of total gluten proteins. The noncovalent bonds between them are responsible for the elasticity and tenacity of the gluten protein network, that is, of the viscoelastic properties of gluten, and so of the dough.
  • Low molecular weight (LMW) subunits, rich in sulfur (cysteine residues).
    These proteins form intermolecular disulfide bridges to each other and with HMW subunits, leading to the formation of a glutenin macropolymer.

Glutenins allow dough to hold its shape during mechanical (kneading) and not mechanical stresses (increase in volume due to both the leavening and the heat of cooking that increases the volume occupied by gases present) which is submitted. This property is important in pasta making.
If in excess, glutenins lead to the formation of a strong and rigid dough.

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Properties of wheat gluten

From the nutritional point of view, gluten proteins do not have a high biological value, being low in lysine, an essential amino acid. Therefore, a gluten-free diet does not cause any important nutritional deficiencies.
On the other hand, it is of great importance in food industry: the combination, in aqueous solution, of gliadins and glutenins to form a three-dimensional lattice, provides viscoelastic properties, that is, extensibility-viscosity and elasticity-tenacity, to the dough, and then, a good structure to bread, pasta, and in general, to all foods made with wheat flour.
It has a high degree of palatability.
It has a high fermenting power in the small intestine.
It is an exorphin: some peptides produced from intestinal digestion of gluten proteins may have an effect in central nervous system.

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Gluten-free cereals

The following is a list of gluten-free cereals, minor cereals, and pseudocereals used as foods.

  • Cereals

corn or maize (Zea mays)
rice (Oryza sativa)

  • Minor cereals
    They are defined “minor” not because they have a low nutritional value, but because they are grown in small areas and in lower quantities than wheat, rice and maize.

Fonio (Digitaria exilis)
Millet (Panicum miliaceum)
Panic (Panicum italicum)
Sorghum (Sorghum vulgare)
Teff (Eragrostis tef)
Teosinte; it is a group of four species of the genus Zea. They are plants that grow in Mexico (Sierra Madre), Guatemala and Venezuela.

  • Pseudocereals.
    They are so called because they combine in their botany and nutritional properties characteristics of cereals and legumes, therefore of another plant family.

Amaranth; the most common species are:

Amaranthus caudatus;
Amaranthus cruentus;
Amarantus hypochondriacus.

Buckwheat (Fagopyrum esculentum)
Quinoa (Chenopodium quinoa), a pseudocereal with excellent nutritional properties, containing fibers, iron, zinc and magnesium. It belongs to Chenopodiaceae family, such as beets.

  • Cassava, also known as tapioca, manioc, or yuca (Manihot useful). It is grown mainly in the south of the Sahara and South America. It is an edible root tuber from which tapioca starch is extracted.

It should be noted that naturally gluten-free foods may not be truly gluten-free after processing. Indeed, the use of derivatives of gliadins in processed foods, or contamination in the production chain may occur, and this is obviously important because even traces of gluten are harmful for celiac patients.

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Oats and gluten

Oats (Avena sativa) is among the cereals that celiac patients can eat. Recent studies have shown that it is tolerated by celiac patients, adult and child, even in subjects with dermatitis herpetiformis. Obviously, oats must be certified as gluten-free (from contamination).

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References

Beccari J.B. De Frumento. De bononiensi scientiarum et artium instituto atque Academia Commentarii, II. 1745:Part I.,122-127

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Berdanier C.D., Dwyer J., Feldman E.B. Handbook of nutrition and food. 2th Edition. CRC Press. Taylor & Francis Group, 2007

Phillips G.O., Williams P.A. Handbook of food proteins. 1th Edition. Woodhead Publishing, 2011

Shewry P.R. and Halford N.G. Cereal seed storage proteins: structures, properties and role in grain utilization. J Exp Bot 2002:53(370);947-958. doi:10.1093/jexbot/53.370.947

Yildiz F. Advances in food biochemistry. CRC Press, 2009


Digestion of starch and alpha-amylase

Factors affecting relationship between starch and alpha-amylase

alpha-amylase
Fig. 1 – Spaghetti

Amylose and amylopectin, the two families of homopolysaccharides constituting starch, during their biosynthesis within vegetable cells, are deposited in highly organized particles called granules.
Granules have a partially crystalline structure and diameter ranging from 3 to 300 µm.
The access of the alpha-amylase, the enzyme that catalyzes the breakdown of amylose and amylopectin into maltose, maltotriose, and alpha-dextrins or alpha-limit dextrins, to carbohydrates making up granules varies as a function of:

  • amylose-amylopectin ratio;
  • temperature and packaging of amylose and amylopectin;
  • granules-associated proteins;
  • presence of fibers.

Amylose-amylopectin ratio

Starch for foodstuff use is obtained from various sources, the most important of which are corn (normal, waxy or high amylose content), potatoes, rice, tapioca and wheat.
Depending on botanical origin, molecular weight, degree of branching, and amylose-amylopectin ratio will vary.
Generally, there is 20-30% amylose and 70-80% amylopectin, even if there are starches with high amylose or amylopectin content (e.g. waxy corn). These differences justify the existence of starches with different chemical-physical characteristics and, to a certain extent, different digestibility.

  • corn: 24% amylose, 76% amylopectin;
  • waxy corn: 0,8% amylose, 99.2% amylopectin;
  • Hylon VII corn: 70% amylose, 30% amylopectin;
  • potatoes: 20% amylose, 80% amylopectin;
  • rice: 18.5 amylose, 81.5% amylopectin;
  • tapioca: 16.7% amylose, 83.3% amylopectin;
  • wheat: 25% amylose, 75% amylopectin.

Temperature and packaging of amylose and amylopectin

The chains of amylose, and to a lesser extent ramifications of amylopectin, thanks to the formation of hydrogen bonds with neighboring molecules and within the same molecules, have the tendency to aggregate. For this reason, pure amylose and amylopectin are poorly soluble in water at below 55 °C (131°F), and are more resistant to alpha-amylase action (resistant starch).
However, in aqueous solution, these granules hydrate increasing in volume of about 10%.
Above 55°C (131°F), the partially crystalline structure is lost, granules absorb further water, swell and pass to a disorganized structure, that is, starch gelatinization occurs, by which starch assumes an amorphous structure more easily attachable by alpha-amylase.

Granules-associated proteins

In granules, starch is present in association with proteins, many of which are hydrophobic, that means with low affinity for water. This association have the effect to hinder the interaction, in the intestinal lumen, between alpha-amylase, a polar protein, and the polysaccharides making up starch granules.
The physical processes to which cereals undergo before being eaten, such as milling or heating for several minutes, change the relationship between starch and the associated proteins, making it more available to α-amylase action.

Fibers

Alpha-amylase activity may also be hindered by the presence of nondigestible polysaccharides, the fibers: cellulose, hemicellulose and pectin.

Conclusions

The presence of inhibitors, of both chemical and physical type, hinders starch digestion, even when pancreatic α-amylase secretion is normal. This means that a part of starch, ranging from 1% to 10%, may escape the action of the enzyme, being then metabolized by colonic bacteria.
Refined starch is instead hydrolyzed efficiently, even when there is an exocrine pancreatic insufficiency (EPI), condition in which alpha-amylase concentration in gut lumen may be reduced to 10% of the normal.

References

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009

Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

Osorio-Dıaz P., Bello-Perez L.A., Agama-Acevedo E., Vargas-Torres A., Tovar J., Paredes-Lopez O. In vitro digestibility and resistant starch content of some industrialized commercial beans (Phaseolus vulgaris L.). Food Chem 2002;78:333-7 [Abstract]

Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Daily protein requirements for athletes

Daily protein requirements and sports

Proteins Requirements
Fig. 1 – Food High in Proteins

It is now accepted by athletes, coaches and athletic trainers that proper diet is one of the cornerstones for achieving better athletic performance. Despite this widely spread assumption, many, even at the highest levels, still believe that an high protein intake is fundamental in the athlete’s diet. This opinion is not new and is deeply rooted in the imaginary of many people almost as if, eating meat, even of big and strong animals, we were able to gain their strength and vitality too.
The function of proteins as energy-supplier for working muscle was hypothesized for the first time by von Liebig in ‘800 and it is because of his studies if, even today, animal proteins, and therefore meats, are often believed having great importance in the energy balance in the athlete’s diet, despite nearly two centuries in which biochemistry and sports medicine have made enormous progress.
Really, by the end of ‘800 von Pettenkofer and Voit and, at the beginning of ‘900, Christensen and Hansen retrenched their importance for energy purposes, also for the muscle engaged  in sport performance, instead bringing out the prominent role played by carbohydrates and lipids.
Of course we shouldn’t think that proteins are not useful for the athlete or sedentary people. The question we need to answer is how many proteins a competitive athlete, engaged in intense and daily workouts, often two daily sessions (for 3-6 hours), 7/7, for more than 10 months a year, needs per day. We can immediately say that, compared to the general population, and with the exception of some sports, (see below) the recommended amount of protein is greater.

Metabolic fate of proteins at rest and during exercise

In a healthy adult subject engaged in a non-competitive physical activity, the daily protein requirements is about 0.85 g/kg desirable body weight, as shown by WHO.
Proteins turnover in healthy adults, about 3-4 g/kg body weight/day (or 210-280 g for a 70 kg adult), is slower for the muscle than the other tissues and decreasing with age, and is related to the amount of amino acids in the diet and protein catabolism.
At rest the anabolic process, especially of synthesis, uses about 75% amino acids while the remaining 25% undergoes oxidative process, that will lead to CO2 and urea release (for the removal of ammonia).
During physical activity, as result of the decreased availability of sugars, i.e. muscle glycogen and blood glucose used for energy purposes, as well as the intervention of cortisol, the percentage of amino acids destinated to anabolic processes is reduced while it increases that of amino acids diverted to catabolic processes, that is, it occurs an increase in the destruction of tissue proteins.
At the end of physical activity, for about two hours, anabolic processes remain low whereupon it occurs their sharp increase that brings them to values ​​higher than basal ones, so, training induces an increase in protein synthesis even in the absence of an increase in proteins intake.

What determines the daily protein requirements?

There are many factors to be taken into account in the calculation of the daily protein requirements.

  • The age of the subject (if, for example, he/she is in the age of development).
  • Gender: female athletes may require higher levels as their energy intake is lower.
  • An adequate carbohydrate intake reduces their consumption.
    During physical activity, glucogenic amino acids may be used as energy source directly in the muscle, after their conversion to glucose in the liver through gluconeogenesis.
    An adequate carbohydrate intake before and during prolonged exercise lowers the use of body proteins.
  • The amount of carbohydrates stored in muscles and liver (glycogen) (see above).
  • The energy intake of the diet.
    A reduced energy intake increases protein requirements; conversely, the higher energy intake, the lower the amount of protein required to achieve nitrogen balance; usually there is a nitrogen retention of 1-2 mg per kcal introduced.
    If the athlete is engaged in very hard competition/workouts, or if he requires an increase in muscle masses (e.g. strength sports) nitrogen balance must be positive; a negative balance indicates a loss of muscle mass.
    The nitrogen balance is calculated as difference between the nitrogen taken with proteins (equal to: g. proteins/6.25) and the lost one (equal to: urinary urea in 24 hours, in g., x0.56]; in formula:

Nb (nitrogen balance) = (g. protein/6.25) – [urinary urea in 24 hours, in g., x0.56)]

  • The type of competition/workouts that the athlete is doing, either resistance or endurance, as well as the duration and intensity of the exercise itself.
    Resistance training leads to an increase in protein turnover in muscle, stimulating protein synthesis to a greater extent than protein degradation; both processes are influenced by the recovery between a training and the next one as well as by the degree of training (more training less loss).
    In the resistance and endurance performances the optimal protein requirements in younger people as for those who train less time are estimated at 1.3 to 1.5 g protein/kg body weight, while in adult athletes who train more time is slightly lower, about 1-1.2 g/Kg of body weight.
    Why?
    In subjects engaged in a hard physical activity, proteins are used not only for plastic purposes, which are incremented, but also for energy purposes being able to satisfy in some cases up to 10-15% of the total energy demand.
    Indeed, intense aerobic performances, longer than 60 minutes, obtain about 3-5% of the consumed energy by the oxidation of protein substrates; if we add to this the proteins required for the repair of damaged tissue protein structures, it results a daily protein demand about 1.2 to 1.4 g/kg body weight.
    If the effort is intense and longer than 90 minutes (as it may occur in road cycling, running, swimming, or cross-country skiing), also in relation to the amount of available glycogen in muscle and liver (see above), the amount of proteins used for energy purposes can get to satisfy, in the latter stages of a prolonged endurance exercise, 15% of the energy needs of the athlete.
  • The physical condition.
  • When needed, the desired weight.
    Athletes attempting to lose weight or maintain a low weight may need more proteins.

From the above, protein requirements don’t exceed 1.5 g/kg body weight, also for an adult athlete engaged in intense and protracted workouts, while if you consider the amount of protein used for energy purposes, you do not go over 15% of the daily energy needs.
So, it’s clear that diets which supply higher amounts (sometimes much higher) of proteins aren’t of any use, stimulate the loss of calcium in bones and overload of work liver and kidney. Moreover, excess proteins don’t accumulate but are used to fat synthesis.

How to meet the increased protein requirements of athletes

Protein Requirements
Fig. 2 – Road Cycling

A diet that provides 12 to 15% of its calories from protein will be quite sufficient to satisfy the needs of almost all of the athletes, also those engaged in exhausting workouts.
In fact, with the exception of some sports whose energy expenditure is low, close to that of sedentary subject (for example: shooting, or artistic and rhythmic gymnastics), athletes need a high amount of calories and, for some sports such as road cycling, swimming or cross-country skiing, it may be double/triple than that of a sedentary subject.
The increase in food intake is accompanied by a parallel increase in protein intake, because only a few foods such as honey, maltodextrin, fructose, sugar and vegetable oils are protein-free, or nearly protein-free.

Calculation of protein requirements of athletes

If you consider an energy demand of 3500 kcal/die, with a protein intake equal to 15% of total daily calories, you have:

3500 x 0.15 = 525 Kcal

As 1 gram of protein contains 4 calories, you obtain:

525/4 = 131 g of proteins

Dividing the number found by the highest protein requirements seen above (1.5 g/kg body weight/day), you obtain:

131/1.5 = 87 kg

that is, the energy needs of a 87 kg athlete engaged in intense workouts are satisfied.
Repeating the same calculations for a caloric intake of 5000 , you obtain  187 g of protein; dividing it by 1.5 the result is 125 kg, that is, the energy needs of a 125 kg athlete are satisfied.
These protein intakes can be met by a Mediterranean-type diet, without protein or amino acids supplements.

References

Giampietro M. L’alimentazione per l’esercizio fisico e lo sport. Il Pensiero Scientifico Editore. Prima edizione 2005

Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation. 2002 (WHO technical report series ; no. 935) [PDF]