Starch

What is starch?

Starch or amylum is a polysaccharide, a digestible complex carbohydrate made of thousands of glucose molecules [1].

Starch structure image

Picture 1. Starch structure:
Glucose molecules linked by glycosidic bonds

Starch is a mixture of two polysaccharides: amylose and amylopectin [1]:

  • Amylose is a linear chain of glucose molecules; it is poorly soluble in water and slowly digestible.
  • Amylopectin is a branched chain of glucose molecules; it is more soluble in water and more easily digestible than amylose.

Nutrition Facts:

  • Calories per gram = 4.2
  • Glycemic index (GI) = 40-110 (depends on the food)
  • Net carbs = 100%

Functions of Starch in the Human Nutrition

  • Energy. Starch can provide 4.2 Calories per gram [3,4].
  • Starch provides carbon atoms for the synthesis of other substances in the body.

Starch is not an essential nutrient so, theoretically, you do not need to get it from food to be healthy [2].

Starch Sources

Starch naturally occurs only in plant foods.

Starchy foods image

Picture 2. Examples of starchy foods

Chart 1. List of foods High in Starch (>5 g/serving)

CEREALS Amaranth, barley, buckwheat, bulgur, corn or maize (cornflakes, cornmeal, popcorn, sweet corn, tortilla), couscous, energy bars (granola), farina, kamut, millet, oats (oatmeal), quinoa, ready-to-eat cereals, rice, rye, sorghum, semolina, spelt, triticale, wheat (bagels, biscuits, cakes, cookies, crackers, muffins, pancakes, pasta–macaroni, noodles, spaghetti–, pastries, pies, pizza, pretzels, waffles)
LEGUMES  Beans, chickpeas, soybeans
VEGETABLES
  • Root vegetables: Arracacha, arrowroot, canna [5], cassava (manioc, tapioca, yuca), colocasia, gourd, kudzu, malanga [8], oca, potatoes (potato chips), sago, sweet potato (yam), taro
  • Other vegetables: winter squash (acorn, butternut): ~5 g/serving
PALM TREES Sago palm starch [7]
NUTS Cashew nuts, chestnuts, water chestnuts
FRUITS Banana, breadfruit, plantain, sapote
SWEETS and DESSERTS Acorn jelly, puddings
FOODS CONTAINING 1-5 g STARCH/SERVING Breaded meats, canned soups, carob candies, chocolate milk, edamame, figs, gravies, jackfruit, lentils, peanuts, peas, prairie turnips (Navajo), raisins, salad dressings, sauces, spaghetti squash, tofu

 Chart 1 sources: US Department of Agriculture [1], Fineli.fi [9]

Starch Added to Foods

  • As a thickener, starch may be added to soft drinks, confections, savory snacks, dairy products, gravies, soups, salad dressings.
  • As a water binder (humectant), starch may be added to processed meats.

Starch can be also used as a binder or diluent in pills.

Effect of Cooking on Starch

Cooked starch can be digested easier than raw starch [11].

Starch Digestion

Starch digestion begins in the mouth and continues in the small intestine; the end product of the digestion is glucose [12]:

The enzyme alpha-amylase in the saliva, and in pancreatic juice delivered to the duodenum breaks the amylose part of starch to maltose and maltotriose, and the amylopectin part to alpha-limit dextrins and isomaltose, which are then all broken down by the enzymes sucrase-isomaltase and maltase-glucoamylase (commonly referred as alpha-glucosidases) on the surface of the small intestinal lining into glucose, which is absorbed.

Rapidly and Slowly Digestible Starches

  • Rapidly digestible starches (RDS) are digested within 20 minutes of enzymatic action. They are common in fresh white bread, white rice, and freshly cooked potatoes. They greatly increase blood glucose level [24].
  • Slowly digestible starches (SDS) are digested within 120 minutes. They are common in cooked and cooled potatoes and pasta, wholegrain bread, waxy maize starch (high in amylopectin), sorghum and legumes. They have a smaller effect on blood glucose than rapidly digestible starches [24].
  • Resistant starches (RS) are not digested in the small intestine within 120 minutes, but are broken down (fermented) by normal bacteria in the large intestine, so they act as a dietary fiber. They are common in beans, high-amylose cornstarch. They do not increase blood glucose level [22,24].

Inhibitors of Starch Digestion

Factors that slow starch digestion and thus reduce the blood glucose spikes after eating starchy foods include poor starch gelatinization (spagetti al dente), starch entrapped in a fibrous coat (legumes, whole grains), soluble fiber (rolled oats), added fats (potato chips) [1].

White bean extract added to carbohydrate meals inhibits starch digestion and hence glucose absorption and blood glucose levels after meals [13]. There is insufficient evidence about the effectiveness of white bean extract in treating diabetes mellitus or promoting weight loss, though.

An antidiabetic drug acarbose inhibits the digestion of starch to glucose, which results in slower glucose absorption and lower blood glucose spikes after starchy meals [14].

Stimulators of Starch Digestion

The pancreatic enzyme alpha-amylase is available as a drug. It helps to digest starch in individuals with pancreatitis.

Disorders of Starch Digestion

  • In advanced chronic pancreatitis, pancreatic cancer, a stone in the common bile duct or pancreatic duct, and in cystic fibrosis, starch may be poorly digested due to lack of the pancreatic enzymes alpha-amylase.
  • Individuals with viral gastroenteritis, food poisoning, infestation with Giardia or other parasites, celiac disease, Crohn’s disease, tropical sprue, lymphoma, carcinoid and a rare genetic disorder congenital sucrase-isomaltase deficiency [15] may also have problem to digest starch due to lack of the small intestinal enzyme sucrase-isomaltase.

Starch Safety

The following starches are Generally Recognized As Safe (GRAS) by the US Food and Drug Administration (FDA): arrowroot starch, bleached starch, corn starch, high-amylose corn starch, hydroxypropyl starch, milo starch, potato starch, pregelatinized starch, rice starch, sodium hydroxide gelatinized starch, sodium hypochlorite oxidized starch, tapioca starch, waxy maize starch and wheat starch [16].

Possible Harmful Effects of Starch

Dental caries. Starch can promote tooth decay but probably less than sucrose or other sugars [17]. According to the European Food Information Council, starch from pasta, rice and bread can promote tooth decay, so they do not recommend replacing sugary foods by starches [18].

Celiac disease. Wheat- and, possibly, corn-derived starch as a food additive may contain gluten and may triggers symptoms in individuals with celiac disease.

Congenital Sucrase-Isomaltase Deficiency (CSID) or Congenital Sucrose Intolerance. Individuals with CSID cannot digest starch, so they need to avoid it.

Starch and Diabetes Mellitus

The ability of starch to raise blood glucose levels after a meal (glycemic index) in individuals with diabetes mellitus depends mainly on the rate of starch digestion and therefore on the type of starchy food [1].

Glycemic index of common starchy foods [19,20]:

  • Potatoes: 50-110
  • Bread:
    • white wheat and whole-wheat, barley, buckwheat, pita: 70 (60-90)
    • Oat bran, rye, multiseed, pumpernickel: ~50-60
  • White rice: 60 (40-90)
  • Cookies: 55-87
  • Pasta (wheat): ~50
  • Beans: 30-50

Edible Starch Powder for Cooking

Edible starch powder is used as a thickener for sauces, gravies, pie fillings and soups. It is commercially available as a corn starch, potato starch, sweet potato starch, wheat starch, oat starch, tapioca starch, kudzu root starch (“kuzu starch” or “kuzuko” in Japan, “ge gen” in China), arrowroot starch, water chestnut starch or sago starch.

Physical properties:

  • A white, odorless, tasteless powder [23]
  • Gelatinization temperature = 158-167 °F (70-75 °C); above this temperature starch becomes soluble in water [21].

Frequently Asked Questions

1. Why can humans digest starch but not cellulose?

Because humans have digestive enzymes that can break down starch but do not have enzymes that can break cellulose.

2. What are different types of starches?

There are several categorizations of starches:

  • By origin: wheat, corn, potato starch, etc.
  • By processing: starches naturally occurring in foods, natural starches extracted from foods and commercially available as powders, natural starches or chemically modified starches added to other foods as thickeners.
  • By digestibility: rapidly and slowly digestible and resistant starches (indigestible)

3. Glucose is to starch as…

Glucose is to starch as amino acids to proteins or fatty acids to fats.

4. Examples of low-starch vegetables?

Most vegetables, except root vegetables (potatoes, cassava, etc.) and winter squash have less than 2 grams of starch per serving.

5. Is cauliflower starch?

Cauliflower contains almost no starch.

6. Are wheat and corn starch gluten-free?

Wheat starch may not be gluten-free. Corn starch should be gluten-free, but there is a minimal risk of cross-contamination with wheat when corn and wheat are processed using the same equipment. Potato, tapioca, arrowroot and starches from other root vegetables are gluten-free [10].

7. What are possible substitutes for starch if I am allergic to it?

Allergy to starch is very rare, but you can be allergic to other substances in corn, potatoes or other plants from which the starch is made. If you are allergic to corn, you can use wheat- or potato-derived starch, for example.

8. What is a purpose of a low or no-starch diet?

Individuals with diabetes mellitus may want to limit starch intake to avoid high blood glucose spikes after meals. Individuals on a low-carb diet also want to avoid starch.

9. Is corn starch the same as corn flour?

Corn flour and other types of flour contain starch, but also proteins, fats, minerals and vitamins. Corn starch and other types of starches, as commercial products, contain only starch and small amount of water.

10. What is animal starch?

“Animal starch” is not starch but a nickname for glycogen, a polysaccharide made of glucose, found mainly in animal human liver. Starch does not naturally occur in animal foods; it can be added to commercial foods as a thickener, though.

Related Nutrients

  1. Dietary carbohydrates: sugars and starches  US Department of Agriculture
  2. Westman EC, 2002, Is dietary carbohydrate essential for human nutrition?  The American Journal of Clinical Nutrition
  3. Elia M et al, 2007, Energy values of macronutrients and specific carbohydrates in foods  European Journal of Clinical Nutrition
  4. Calculation of the energy content of foods ─ energy conversion factors  Food and Agriculture Organization of the United Nations
  5. Tanaka N et al, 2006, Edible Canna and its Starch: An Under-Exploited Starch-Producing Plant Resource  The Japan Food Chemical Research Foundation
  6. Kudzu starch  CooksInfo
  7. Ehara H eta al, Sustainable production of sago palm and its utilization for strengthening food security  Food and Agriculture Organization of the United Nations
  8. King M et al, Malanga  University of Florida
  9. List of foods high in starch  National Institute for Health and Welfare
  10. Starch  thickeners  FoodSubs
  11. Does Cooked Food Contain Less Nutrition?  Beyond Vegetarianism
  12. Christian M et al, 1999, Starch Digestion in Infancy  Journal of Pediatric Gastroenterology and Nutrition
  13. Barrett ML et al, 2011, A proprietary alpha-amylase inhibitor from white bean (Phaseolus vulgaris): A review of clinical studies on weight loss and glycemic control  PubMed Central
  14. Acarbose  Drugs.com
  15. Congenital Sucrase-Isomaltase Deficiency (CSID) symptoms  CSIDinfo.com
  16. SCOGS (Select Committee on GRAS Substances)  US Food and Drug Administration
  17. Moynihan P et al, 2004, Diet, nutrition and the prevention of dental diseases  World Health Organization
  18. 2006, Dental health, European Food Information Council
  19. International table of glycemic index and glycemic load values: 2002  The American Journal of Clinical Nutrition
  20. Glycemicindex.com
  21. Coral DF et al, 2009, Determination of the gelatinization temperature of starch presented in maize flours  IOP Science
  22. Sajilata MG et al, 2006, Resistant Starch – a Review  Wiley Online Library
  23. Starch Encyclopaedia Britanica
  24. Digestibility and starch structure: the key to tailored energy release  Artbeitgemeinschaft Getreideforschung

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