The different amylose families
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Cereals
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Tubers
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Pulses
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Fruit
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Tender wheat
Coarse wheat Rice Corn Oats Barley Rye Sorghum Millet |
Potatoes
Sweet Potato Cassava Tropical yam -US/ yam -UK Taro Malanga, Tania |
String beans
Peas Chick peas Lentils Beans |
Bananas
Mangoes Apples |
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 20 GI when raw. The moment they are boiled, their GI rises to 50 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.
