joi, 8 iulie 2010

Factorii Care Modifica Indicele Glicemic

Most of the carbohydrates that we commonly consume are complex carbs essentially made up of starches belonging to the amylose category which is divided into four families:
The different amylose families
Tender wheat

Coarse wheat








Sweet Potato


Tropical yam -US/ yam -UK


Malanga, Tania
String beans


Chick peas




In order for all of these starches to be absorbed and enter our bloodstream, they have to be broken down into glucose (the smallest of the sugar molecules of which starches are composed). This decomposing process is the work of our digestive enzymes (more precisely, of alpha-amylases).

Digestion of starch normally begins in the mouth where an enzyme, salivary amylase, is secreted, catalyzing the break up of the starch by hydrolysis. After a quick passage through our stomachs, additional breakdown of starch occurs in the small intestine with amylase secreted from the pancreas.
Glycemia indicates glucose absorption rates, namely, the digestibility of certain starches.

The Glycemic Index scale measures starch digestibility through comparison. Observation shows that, for similar portions of carbohydrates from one foodstuff to another, the postprandial Glycemic response can vary immensely since there are fractions of starches which cannot be digested and this is what determines their absorption rate.

Several factors can cause these variations and the purpose of GIs is precisely to classify starches according to this variation in their digestibility. Glycemic Indexes.

Starch structure:
Starch granules are made up of two types of molecular components: amylose and amylopectin. These can be associated to lipids, proteins, fibers and micronutrients (vitamins, salts, minerals …)

The amount of amylose in proportion to amylopectin is what basically determines the physical-chemical nature of amylase foods and their nutritional impact on the human organism.
The proportion of amylose / amylopectin can vary from one botanic family to the other as well as from one variety to the other within the same family of plants.
Cereal starches normally contain 15 to 28% amylose.

Certain varieties of corn contain less than 1% (waxy corn whose extract is used by the food industry as thickener.)

Other varieties, on the other hand, contain from 55 to 80% but they are not commonly grown since the higher the amylose, the lower their productivity. Tuber starches (still called “flour starches”), as in the case of potatoes, have a much lower amylose content (from 17% to 22%).

Starch in pulses (lentils, chick peas, shellouts…) contain much more amylose (from 33 to 66%)
Glycemic Index Variations
An amylose food’s Glycemic Index is determined by several parameters::
  • How much amylose there is in proportion to amylopectin
Extreme boiling temperatures modify starch structure. When an aqueous suspension of starch is heated, water is absorbed, and the starch granules swell and a fraction of the amylopectin becomes part of the substance. When the heating process is prolonged, a fraction of amylose also becomes component of the substance.
This process conditions the substance’s degree of viscosity and it is commonly called gelatinization because the solution formed has a gelatinous, highly viscous consistency.  
The degree of gelatinization is proportional to the amount of amylose; the less amylose there is, the greater the degree of gelatinization and vice-versa.

There is evidence to the fact that the greater the degree of gelatinization suffered by starches (as a result of low amylose levels), the greater the chances of it being hydrolyzed by alpha-amylase (starch digestive enzymes), the greater its propensity to become glucose and, naturally, the greater its tendency to raise blood sugar levels.

In other words, starches with lower amylose content will have higher Glycemic Indexes. Inversely, starches with a higher amylose content will be less susceptible to gelatinization, that is, to breaking down into glucose, that which makes for low Glycemic Indexes.
This is why potatoes, which have an extremely low amylose level, have a high Glycemic Index while lentils, which are high in amylose, have a very low GI.
Corn is also an illustrative example of this phenomenon.

 « Waxy » corn, which is almost totally lacking in amylose, is a favorite of the food industry precisely because its starch is particularly viscous. It is commonly used as a thickening agent for fruit jellies and as texturizing agent for canned or frozen foods. It is labeled as “cornstarch” and its Glycemic Index is one of the highest (near the 100 value). Cornstarch is thus one of the ingredients which cause industrial food preparations to evoke high blood sugar responses.

This does not have to be the rule and an experiment carried out in Australia proves that the food industry can also promote healthy foods and eating habits. An Australian industrial bread maker decided to use a special variety of corn which is high in amylose (>80) with the aim of lowering his bread’s Glycemic Index. This bread has apparently sold quite well and children, who do not generally like whole-wheat bread, seem to particularly like this bread which is the equivalent of the bread popularly sold in supermarkets.
  • How the food is technically and thermally processed
Hydration and heat raise food’s Glycemic Indexes. Carrots, for example, have a 35 GI when raw. The moment they are boiled, their GI rises to 85 as a result of the gelatinization of it starch content.

Certain industrial processes take gelatinization to the extreme. This is true for mashed potatoes and cornflakes as well as for binding agents such as modified starches and dextrinized starches.
These processes noticeably increase foodstuffs Glycemic Indexes (85 for cornflakes, 95 for mashed potatoes, 100 for modified starches.)

Likewise, exploding corn grains to make pop-corn or rice grains to make puffed rice increases the original food’s GY by 15 to 20%.
« Pastification », on the other hand, reduces Glycemic Indexes
Comparatively, there is a natural technical process which tends to block starch hydration: Pastification of coarse wheat. Extruding wheat paste through a drain heats the food in such a way that it produces a protective coating which slows down starch gelatinization.

While this applies to spaghetti and certain tagliatelles which are “pastified” (extruded under great pressure), it does not hold for raviolis nor lasagna and not even for fresh pasta which are hand cut and thus have a much higher Glycemic Index even if they are also made from durum wheat flour.
As we can see, we can use the same flour and end up producing foods with quite different Glycemic Indexes, at times they can be twice as high: raviolis 70, spaghettis 40.
Cooking at home also affects our food’s Glycemic Indexes.
Cooking al dente (5 to 6 minutes), for example, allows us to keep spaghettis GIs as low as possible while prolonged cooking (from 15 to 20 minutes) will raise GIs since it accelerates starch gelatinization.

  • How retrogradation inverses gelatinization
Starch, after being gelatinized when getting cold is subjected to further modifications.
With coolness gelatinized starch gradually begins to reorganize its amylose and amylopectin macro-molecules. This is what is known as retrogradation, a return (which can be more or less significant) to its former molecular structure. Retrogradation becomes more intense as time passes and temperatures go down.
Preserving amylase foods for long periods at low temperatures (41° Fahrenheit) stimulates retrogradation. Something similar occurs with food drying processes. Dry bread, for example, loses its humidity and stimulates starch retrogradation, as in the case of toasted bread.

Although retrogradation does not wholly reverse food gelatinization, it does contribute to lowering foodstuffs’ Glycemic Indexes. Spaghetti (even white refined), for example, will have a 35 Glycemic Index if cooked al dente and eaten cold (in salads).
As we can see, the same bread (made from the same flour) can have a different GI depending on how it is prepared: freshly baked and still oven hot, dried or toasted. Fresh bread when frozen and thawed out at room temperature will also have a much lower GI.
It is also interesting to note that cold green lentils (more so if they were stored in the fridge for at least 24 hours) have a much lower GI than when they are just cooked (form 10 to 15). The higher the amylose content in a starch, the greater the effectiveness of the retrogradation process.
Nonetheless, there is evidence to the fact that adding lipids to starches which have been gelatinized tends to slow down retrogradation.
It is handy to know that retrograded starches lose some of their gelatinization potential. Approximately a 10% portion of the retrograded starch becomes thermo-resistant, which indicates that reheating carbs after cold storage contributes to lowering their GI.

Lastly, it is important to point out that starches (in their raw and natural form) are not only contained in raw foods. Raw starches can also be found after cooking when water contents are not sufficient to produce gelatinization. A case in point is bread crust and shortbread, the granular structure of the starch in these foods persists after cooking and this makes their Glycemic Index lower that that of  those starches which have been gelatinized as, for example, in the case of the soft interior of bread.
This is why slow vapor or steam cooking, which does not hydrate food as much as immersion cooking, provokes less gelatinization.
  • How protein and fiber content reduce GIs

The natural protein content of certain carbohydrates might be the reason why their starches are not hydrolyzed (digested) as much as others and why they have lower Glycemic Indexes. This is what happens with cereals.
This phenomenon is particularly noticeable with pasta. Their gluten content slows down digestive amylases secretion and, consequently, limits glucose absorption. This is why coarse wheat (richer in gluten) had a lower GI than the tender wheat which is used to make bread. Generally speaking, modern wheat (which yields more per plant) has two to three times less gluten than traditional wheat. Modern cereals tend to significantly raise blood sugar levels not only because they contain less gluten to begin with but also because the refining processes to which they are subjected contribute to further reducing their gluten content.
The fiber contained in starches can also serve to block the amylase action contributing to reducing glucose absorption. Basically, the fibers that directly or indirectly contribute to reducing intestinal glucose absorption and thus to lowering the corresponding starches Glycemic Indexes are soluble fibers (generally contained in pulses and oats).

  • How GIs depend on how ripe the fruit is
Starchy fruits may increase their Glycemic Index depending on how ripe the fruit is. Bananas are particular susceptible to this phenomenon , more so than apples. Green bananas have low GIs (approximately 40) but when they are ripe they will have a much higher GI (approx 65) since as bananas ripen, their starches are transformed and become less resistant. Cooking green bananas produces basically the same effect as the ripening process.

In order to propose as much useful information as possible, I wish to point out that preserving certain foods, particularly potatoes, increases their GIs as a result of the transformation undergone by their starches. Consequently, potatoes which have been stored for months have higher GIs than freshly-harvested potatoes.
  • How particle size affects hydrolyzation and GIs
When starchy food are ground, their particles become much finer and, as this makes their hydrolyzation easier, and so raises their Glycemic Index. This is what happens to cereals when they are ground into flour. Rice flour, accordingly, has a higher GI than rice itself.
Formerly, when wheat was ground by hand with a flystone it was reduced into large particles. Even when sifted, the resulting flour remained coarse. What at the time was called “white bread” had a 60 to 65 GI, which was fairly reasonable. The modern equivalent of this bread is the famous « Poilâne » bread. Poilâne bread is even more attractive if we consider the fact that it is made with natural sourdough yeast, that which contributes to further reducing its GI.

In olden times, the bread of the people, was made out of coarse flour which retained the wheat grains, thus the name “integral bread”. Since the particles were coarse, it was rich in fibers and proteins and was made with natural yeast to boot, its Glycemic Index was even lower, from 35 to 45).
The Montignac “integral bread”, to be found in Québec’s Première Moisson Bakeries, fits these conditions and needs.

Whole-wheat flour / 100g
White flour (T55) /100g
12 g
8 g
2.5 g
1 g
60 g
74 g
10 g
3 g
15.5 g
14 g
Particle size
Glycemic Index
The invention of the cylinder mill in 1870, generalized white flour production, first in the West and later, throughout the world. This technical process, then considered a sign of progress, turned out to be a step in the wrong direction as far as people’s health was concerned.
Later, thanks to increasingly sophisticated mills, flour became more and more refined. At a nutritional level this implied that they lost fibers, proteins and micronutrients (vitamins, minerals, essential fatty acids..) and were broken down into increasingly smaller particles. All of these transformations have contributed to raising the Glycemic Index  of those foods made from these hyper-refined flours...

Nota: Traducerea acestui material va fi adaugata in curand, desi sunt multe pagini de web in limba romana care fac referire la conceptul Montignac - Indicele Glicemic.

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